ENDOCRINE SYSTEM

Hormones are regulatory molecules secreted into the blood by endocrine glands. Chemical categories of hormones include steroids, amines, polypeptides, and glycoproteins. Interactions between the various hormones produce effects that may be synergistic, permissive, or antagonistic.

Endocrine glands lack the ducts that are present in exocrine glands. The endocrine glands secrete their products, which are biologically active molecules called hormones, into the blood. The blood carries the hormones to target cells that contain specific receptor proteins for the hormones, and which therefore can respond in a specific fashion to them. Many endocrine glands are organs whose primary functions are the production and secretion of hormones. The pancreas functions as both an exocrine and an endocrine gland; the endocrine portion of the pancreas is composed of clusters of cells called the pancreatic islets (islets of Langerhans). The concept of the endocrine system, however, must be extended beyond these organs, because many other organs in the body secrete hormones. These organs may be categorized as endocrine glands even though they serve other functions as well. It is appropriate, then, that a partial list of the endocrine glands should include the heart, liver, adipose tissue, and kidneys. Some specialized neurons, particularly in the hypothalamus, secrete chemical messengers into the blood rather than into a narrow synaptic cleft. In these cases, the chemical that the neurons secrete is sometimes called a neurohormone.

In addition, a number of chemicals—norepinephrine, for example—are secreted both as a neurotransmitter and a hormone. Thus, a sharp distinction between the nervous system and the endocrine system cannot always be drawn on the basis of the chemicals they release. Hormones affect the metabolism of their target organs and, by this means, help regulate total body metabolism, growth, and reproduction.

Chemical Classification of Hormones

Hormones secreted by different endocrine glands vary widely in chemical structure. All hormones, however, can be divided into a few chemical classes.

1. Amines. These are hormones derived from the amino acids tyrosine and tryptophan. They include the hormones secreted by the adrenal medulla, thyroid, and pineal glands.

2. Polypeptides and proteins. Proteins are large polypeptides, so the distinction between the two categories is somewhat arbitrary. Antidiuretic hormone is a polypeptide with eight amino acids, too small to accurately be called a protein. If a polypeptide chain is larger than about 100 amino acids, such as growth hormone with 191 amino acids, it can be called a protein. Insulin blurs the two categories, because it is composed of two polypeptide chains derived from a single, larger molecule.

3. Glycoproteins. These molecules consist of a protein bound to one or more carbohydrate groups. Examples are follicle stimulating hormone (FSH) and luteinizing hormone (LH).

4. Steroids. Steroid hormones are derived from cholesterol after an enzyme cleaves off the side chain attached to the five-carbon “D” ring. Steroid hormones include testosterone, estradiol, progesterone, and cortisol. In terms of their actions in target cells, hormone molecules can be divided into those that are polar, and therefore water-soluble, and those that are nonpolar, and thus insoluble in water. Because the nonpolar hormones are soluble in lipids, they are often referred to as lipophilic hormones. Unlike the polar hormones, which cannot pass through plasma membranes, lipophilic hormones can gain entry into their target cells. These lipophilic hormones include the steroid hormones and thyroid hormones.  Steroid hormones are secreted by only two endocrine glands: the adrenal cortex and the gonads. The

gonads secrete sex steroids; the adrenal cortex secretes corticosteroids (including cortisol and aldosterone) and small amounts of sex steroids.

The major thyroid hormones are composed of two derivatives of the amino acid tyrosine bonded together. When the hormone contains 4 iodine atoms, it is called tetraiodothyronine (T4), or thyroxine. When it contains 3 atoms of iodine, it is called triiodothyronine (T3). Although these hormones are not steroids, they are like steroids in that they are relatively small, nonpolar molecules. Steroid and thyroid hormones are active when taken orally (as a pill). Sex steroids are the active agents in contraceptive pills, and thyroid hormone pills are taken by people whose thyroid is deficient (who are hypothyroid). By contrast, polypeptide and glycoprotein hormones cannot be taken orally because they would be digested into inactive fragments before being absorbed into the blood. Thus, insulin-dependent diabetics must inject themselves with this hormone. Polar, water-soluble hormones include polypeptides, glycoproteins, and the catecholamine hormones secreted by the adrenal medulla, epinephrine and norepinephrine. These hormones are derived from the amino acid tyrosine. Thus, like the polypeptide and glycoprotein hormones, the catecholamines are too polar to pass through the phospholipid portion of the plasma membrane. The hormone secreted by the pineal gland, melatonin, is different; derived from the nonpolar amino acid tryptophan, melatonin pills can be effective because (like steroids and thyroxine) this hormone can pass through plasma membranes. Melatonin, however, also has some similarities to the polar hormones in terms of its effects on cells.

Prohormones and Prehormones

Hormone molecules that affect the metabolism of target cells are often derived from less active “parent,” or precursor, molecules. In the case of polypeptide hormones, the precursor may be a longer chained prohormone that is cut and spliced together to make the hormone. Insulin, for example, is produced from proinsulin within the beta cells of the islets of Langerhans of the pancreas. In some cases, the prohormone itself is derived from an even larger precursor molecule; in the case of insulin, this molecule is called preproinsulin. The term prehormone is sometimes used to indicate such precursors of prohormones. In some cases, the molecule secreted by the endocrine gland (and considered to be the hormone of that gland) is actually inactive in the target cells. In order to become active, the target cells must modify the chemical structure of the secreted hormone. Thyroxine (T4), for example, must be

changed into T 3 within the target cells in order to affect the metabolism of these cells. Similarly, testosterone (secreted by the testes) and vitamin D 3 (secreted by the skin) are converted into more active molecules within their target cells. In this text, the term prehormone will be used to designate those molecules secreted by endocrine glands that are inactive until changed by their target cells.  When two or more hormones work together to produce a particular result, their effects are said to be synergistic. A hormone is said to have a permissive effect on the action of a second hormone when it enhances the responsiveness of a target organ to the second hormone, or when it increases the activity of the second hormone.  In some situations, the actions of one hormone antagonize the effects of another. Lactation during pregnancy, for example, is inhibited because the high concentration of estrogen in the blood inhibits the secretion and action of prolactin.

The mechanism of steroid hormone action. (1) Steroid hormones, transported bound to plasma carrier proteins, dissociate from their plasma carriers and pass through the plasma membrane of their target cell. (2) The steroid hormone binds to receptors, which may be in the cytoplasm. (3) The hormone-bound receptor translocates to the nucleus, where it binds to DNA. (4) This stimulates genetic transcription, resulting in new mRNA synthesis. (5) The newly formed mRNA codes for the production of new proteins, which (6) produce the hormonal effects in the target cell.

The mechanism of thyroid hormone action. (1) Thyroxine ( T4 ), carried to the target cell bound to its plasma carrier protein, dissociates from its carrier and passes through the plasma membrane of its target cell. (2) In the cytoplasm, T4 is converted into T3 (triiodothyronine), which (3) uses binding proteins to enter the nucleus. (4) The hormone-receptor complex binds to DNA, (5) stimulating the synthesis of new mRNA. (6) The newly formed mRNA codes for the synthesis of new proteins, which (7) produce the hormonal effects in the target cell.

The adenylate cyclase–cyclic AMP second-messenger system. (1) The hormone binds to its receptor in the plasma membrane of the target cell. (2) This causes the dissociation of G-proteins, allowing the free α (alpha) subunit to activate adenylate cyclase. (3) This enzyme catalyzes the production of cAMP (cyclic AMP), which (4) removes the regulatory subunit from protein kinase. (5) Active protein kinase phosphorylates other enzyme proteins, activating or inactivating specific enzymes and thereby producing the hormonal effects on the target cell.

GROWTH HORMONE

BIOSYNTHESIS & CHEMISTRY

The long arm of human chromosome 17 contains the growth hormone-hCS cluster that contains five genes: one, hGH-N, codes for the most abundant (“normal”) form of growth hor­mone; a second, hGH-V, codes for the variant form of growth hormone; two code for human chorionic somato­mammotropin (hCS); and the fifth is prob­ably an hCS pseudogene.  Growth hormone that is secreted into the circulation by the pituitary gland consists of a complex mixture of hGH-N, peptides derived from this molecule with varying degrees of posttranslational modifications, such as glycosylation, and a splice variant of hGH-N that lacks amino acids 32–46. The physiologic significance of this complex array of hormones has yet to be fully understood, particularly since their structural similarities make it difficult to assay for each species separately. Nevertheless, there is emerging evidence that, while the vari­ous peptides share a broad range of functions, they may occa­sionally exert actions in opposition to one another. hGH-V and hCS, on the other hand, are primarily products of the placenta, and as a consequence are only found in appreciable quantities in the circulation during pregnancy.

SPECIES SPECIFICITY

The structure of growth hormone varies considerably from one species to another. Porcine and simian growth hormones have only a transient effect in the guinea pig. In monkeys and humans, bovine and porcine growth hormones do not even have a transient effect on growth, although monkey and human growth hormones are fully active in both monkeys and humans. These facts are relevant to public health discussions surrounding the presence of bovine growth hormones (used to increase milk production) in dairy products, as well as the popularity of growth hormone supplements, marketed via the Internet, with body builders. Controversially, recombinant human growth hormone has also been given to children who are short in stature, but otherwise healthy (ie, without growth hormone deficiency), with apparently limited results.

PLASMA LEVELS, BINDING, AND METABOLISM

A portion of circulating growth hormone is bound to a plasma protein that is a large fragment of the extracellu­lar domain of the growth hormone receptor. It appears to be produced by cleavage of receptors in humans, and its concentration is an index of the number of growth hormone receptors in the tissues. Approximately 50% of the circulating pool of growth hormone activity is in the bound form, providing a reservoir of the hormone to compensate for the wide fluctuations that occur in secretion.  The basal plasma growth hormone level measured by radioimmunoassay in adult humans is normally less than 3 ng/mL. This represents both the protein-bound and free forms. Growth hormone is metabolized rapidly, at least in part in the liver. The half-life of circulating growth hormone in humans is 6–20 min, and the daily growth hormone output has been calculated to be 0.2–1.0 mg/d in adults.

GROWTH HORMONE RECEPTORS

The growth hormone receptor is a 620-amino-acid protein with a large extracellular portion, a transmembrane domain, and a large cytoplasmic portion. It is a member of the cytokine receptor superfamily. Growth hormone has two domains that can bind to its receptor, and when it binds to one receptor, the second binding site attracts another, producing a homodimer.  Dimeriza­tion is essential for receptor activation.  Growth hormone has widespread effects in the body, so even though it is not yet possible precisely to cor­relate intracellular and whole body effects, it is not surprising that, like insulin, growth hormone activates many different intracellular signaling cascades. Of particular note is its activation of the JAK2–STAT pathway. JAK2 is a member of the Janus family of cytoplasmic tyrosine kinases. STATs (for signal transducers and activators of transcrip­tion) are a family of cytoplasmic transcription factors that, upon phosphorylation by JAK kinases, migrate to the nucleus where they activate various genes. JAK–STAT pathways are known also to mediate the effects of prolactin and various other growth factors.

EFFECTS ON GROWTH

In young animals in which the epiphyses have not yet fused to the long bones, growth is inhibited by hypophysectomy and stimulated by growth hormone. Chondrogenesis is accelerated, and as the cartilaginous epiphysial plates widen, they lay down more bone matrix at the ends of long bones. In this way, stature is increased. Prolonged treatment of animals with growth hormone leads to gigantism.  When the epiphyses are closed, linear growth is no longer possible. In this case, an overabundance of growth hormone produces the pattern of bone and soft tissue deformities known in humans as acromegaly. The sizes of most of the viscera are increased. The protein content of the body is increased, and the fat content is decreased.

EFFECTS ON PROTEIN & ELECTROLYTE HOMEOSTASIS

Growth hormone is a protein anabolic hormone and produces a positive nitrogen and phosphorus balance, a rise in plasma phosphorus, and a fall in blood urea nitrogen and amino acid levels. In adults with growth hormone deficiency, recombinant human growth hormone produces an increase in lean body mass and a decrease in body fat, along with an increase in metabolic rate and a fall in plasma cholesterol. Gastrointes­tinal absorption of Ca2+ is increased. Na+ and K+ excretion is reduced by an action independent of the adrenal glands, prob­ably because these electrolytes are diverted from the kidneys to the growing tissues. On the other hand, excretion of the amino acid 4-hydroxyproline is increased during this growth, reflective of the ability of growth hormone to stimulate the synthesis of soluble collagen.

Some of the principal signaling pathways activated by the dimerized growth hormone receptor (GHR). Solid arrows indicate established pathways; dashed arrows indicate probable pathways. The details of the PLC pathway and the pathway from Grb2 to MAP K are discussed in Chapter 2. The small uppercase letter Ps in yellow hexagons represent phosphorylation of the factor indicated. GLE-1 and GLE-2, interferon γ-activated response elements; IRS, insulin receptor substrate; p90^RSK, an S6 kinase; PLA2, phospholipase A2; SIE, Sis-induced element; SRE, serum response element; SRF, serum response factor; TCF, ternary complex factor.

EFFECTS ON CARBOHYDRATE AND FAT METABOLISM

At least some forms of growth hormone are diabetogenic because they increase hepatic glu­cose output and exert an anti-insulin effect in muscle. Growth hormone is also ketogenic and increases circulating free fatty acid (FFA) levels. The increase in plasma FFA, which takes several hours to develop, provides a ready source of energy for the tissues during hypoglycemia, fasting, and stressful stimuli. Growth hormone does not stimulate β cells of the pancreas directly, but it increases the ability of the pancreas to respond to insulinogenic stimuli such as arginine and glucose. This is an additional way growth hormone promotes growth, since insulin has a protein anabolic effect.

DIRECT AND INDIRECT ACTIONS OF GROWTH HORMONE

The understanding of the mechanism of action of growth hormone has evolved. It was originally thought to produce growth by a direct action on tissues, and then later was believed to act solely through its ability to induce somato­medins. However, if growth hormone is injected into one proximal tibial epiphysis, a unilateral increase in cartilage width is produced, and cartilage, like other tissues, makes IGF-I. A current hypothesis to explain these results holds that growth hormone acts on cartilage to convert stem cells into cells that respond to IGF-I. Locally produced as well as circulating IGF-I then makes the cartilage grow. However, the independent role of circulating IGF-I remains important, since infusion of IGF-I in hypophysectomized rats restores bone and body growth. Overall, it seems that growth hor­mone and somatomedins can act both in cooperation and independently to stimulate pathways that lead to growth. The situation is almost certainly complicated further by the exis­tence of multiple forms of growth hormone in the circulation that can, in some situations, have opposing actions. However, growth hor­mone probably combines with circulating and locally pro­duced IGF-I in various proportions to produce at least some of the latter effects.

Feedback control

THYROID CELL

ANATOMIC CONSIDERATIONS

The thyroid is a butterfly-shaped gland that straddles the tra­chea in the front of the neck. It develops from an evagination of the floor of the pharynx, and a thyroglossal duct marking the path of the thyroid from the tongue to the neck sometimes persists in the adult. The two lobes of the human thyroid are connected by a bridge of tissue, the thyroid isthmus, and there is sometimes a pyramidal lobe arising from the isthmus in front of the larynx. The gland is well vascu­larized, and the thyroid has one of the highest rates of blood flow per gram of tissue of any organ in the body.

The portion of the thyroid concerned with the produc­tion of thyroid hormone consists of multiple acini (follicles). Each spherical follicle is surrounded by a single layer of polar­ized epithelial cells and filled with pink-staining proteinaceous material called colloid. Colloid consists predominantly of the glycoprotein, thyroglobulin. When the gland is inactive, the colloid is abundant, the follicles are large, and the cells lining them are flat. When the gland is active, the follicles are small, the cells are cuboid or columnar, and areas where the colloid is being actively reabsorbed into the thyrocytes are visible as “reabsorption lacunae.  Microvilli project into the colloid from the apexes of the thyroid cells and canaliculi extend into them. The endoplas­mic reticulum is prominent, a feature common to most glan­dular cells, and secretory granules containing thyroglobulin are seen. The individual thyroid cells rest on a basal lamina that separates them from the adjacent capillaries. The capillaries are fenestrated, like those of other endocrine glands.

FORMATION AND SECRETION OF THYROID HORMONES

THYROID HORMONE SYNTHESIS AND SECRETION

At the interface between the thyrocyte and the colloid, iodide undergoes a process referred to as organification. First, it is oxidized to iodine, and then incorporated into the carbon 3 position of tyrosine residues that are part of the thyroglobu­lin molecule in the colloid. Thyroglobulin is a glycoprotein made up of two subunits and has a molecular weight of 660 kDa. It contains 10% carbohydrate by weight. It also contains 123 tyrosine residues, but only 4–8 of these are normally incorporated into thyroid hormones. Thyroglobulin is synthesized in the thyroid cells and secreted into the col­loid by exocytosis of granules. The oxidation and reaction of iodide with the secreted thyroglobulin is mediated by thyroid peroxidase, a membrane-bound enzyme found in the thyro­cyte apical membrane. The thyroid hormones so produced remain part of the thyroglobulin molecule until needed. As such, colloid represents a reservoir of thyroid hormones, and humans can ingest a diet completely devoid of iodide for up to 2 months before a decline in circulating thyroid hormone levels is seen. When there is a need for thyroid hormone secre­tion, colloid is internalized by the thyrocytes by endocytosis, and directed toward lysosomal degradation. Thus, the peptide bonds of thyroglobulin are hydrolyzed, and free T4 and T3 are discharged into cytosol and thence to the capillaries (see below). Thyrocytes thus have four functions: They collect and transport iodine, they synthesize thyroglobulin and secrete it into the colloid, they fix iodine to the thyroglobulin to gener­ate thyroid hormones, and they remove the thyroid hormones from thyroglobulin and secrete them into the circulation.

Thyroid hormone synthesis is a multistep process. Thyroid peroxidase generates reactive iodine species that can attack thyroglobulin. The first product is monoiodotyrosine (MIT). MIT is next iodinated on the carbon 5 position to form diio­dotyrosine (DIT). Two DIT molecules then undergo an oxi­dative condensation to form T4 with the elimination of the alanine side chain from the molecule that forms the outer ring. There are two theories of how this coupling reaction occurs. One holds that the coupling occurs with both DIT molecules attached to thyroglobulin (intramolecular coupling). The other holds that the DIT that forms the outer ring is first detached from thyroglobulin (intermolecular coupling). In either case, thyroid peroxidase is involved in coupling as well as iodination. T3 is formed by condensation of MIT with DIT. A small amount of RT3 is also formed, probably by condensation of DIT with MIT. In the normal human thyroid, the average distribution of iodinated compounds is 3% MIT, 33% DIT, 35% T4, and 7% T3. Only traces of RT3 and other components are present.

The human thyroid secretes about 80 μg (103 nmol) of T4, 4 μg (7 nmol) of T3, and 2 μg (3.5 nmol) of RT3 per day. MIT and DIT are not secreted. These iodin­ated tyrosines are deiodinated by a microsomal iodotyrosine deiodinase. This represents a mechanism to recover iodine and bound tyrosines and recycle them for additional rounds of hormone synthesis. The iodine liberated by deiodination of MIT and DIT is reutilized in the gland and normally provides about twice as much iodide for hormone synthe­sis as NIS does. In patients with congenital absence of the iodotyrosine deiodinase, MIT and DIT appear in the urine and there are symptoms of iodine deficiency.

Iodinated thyronines are resistant to the activity of iodotyrosine deiodinase, thus allowing T4 and T3 to pass into the circulation.

MECHANISM OF ACTION

Thyroid hormones enter cells and T3 binds to TR in the nuclei. T4 can also bind, but not as avidly. The hormone–receptor complex then binds to DNA via zinc fingers and increases (or in some cases, decreases) the expression of a variety of different genes that code for proteins that regulate cell func­tion. Thus, the nuclear receptors for thyroid hormones are members of the superfamily of hormone-sensitive nuclear transcription factors.There are two human TR genes: an α receptor gene on chromosome 17 and a β receptor gene on chromosome 3.  By alternative splicing, each forms at least two different mRNAs and therefore two different receptor proteins. TRβ2 is found only in the brain, but TRα1, TRα2, and TRβ1 are widely dis­tributed. TRα2 differs from the other three in that it does not bind T3 and its function is not yet fully established. TRs bind to DNA as monomers, homodimers, and heterodimers with other nuclear receptors, particularly the retinoid X receptor (RXR). The TR/RXR heterodimer does not bind to 9-cis reti­noic acid, the usual ligand for RXR, but TR binding to DNA is greatly enhanced in response to thyroid hormones when the receptor is in the form of this heterodimer. There are also coactivator and corepressor proteins that affect the actions of TRs. Presumably, this complexity underlies the ability of thy­roid hormones to produce many different effects in the body.  In most of its actions, T3 acts more rapidly and is three to five times more potent than T4. This is because T3 is less tightly bound to plasma proteins than is T4, but binds more avidly to thyroid hormone receptors. As previously noted, RT3 is inert. 

CALORIGENIC ACTION

T4 and T3 increase the O2 consumption of almost all metaboli­cally active tissues. The exceptions are the adult brain, testes, uterus, lymph nodes, spleen, and anterior pituitary. T4 actu­ally depresses the O2 consumption of the anterior pituitary, presumably because it inhibits TSH secretion. The increase in metabolic rate produced by a single dose of T4 becomes mea­surable after a latent period of several hours and lasts 6 days or more.

Some of the calorigenic effect of thyroid hormones is due to metabolism of the fatty acids they mobilize. In addi­tion, thyroid hormones increase the activity of the membrane-bound Na, K ATPase in many tissues.

EFFECTS ON THE CARDIOVASCULAR SYSTEM

Large doses of thyroid hormones cause enough extra heat pro­duction to lead to a slight rise in body temperatures, which in turn activates heat-dissipating mechanisms. Periph­eral resistance decreases because of cutaneous vasodilation, and this increases levels of renal Na+ and water absorption, expanding blood volume. Cardiac output is increased by the direct action of thyroid hormones, as well as that of catechol­amines, on the heart, so that pulse pressure and cardiac rate are increased and circulation time is shortened.

T3 is not formed from T4 in cardiac myocytes to any degree, but circulatory T3 enters the myocytes, combines with its receptors, and enters the nucleus, where it promotes the expression of some genes and inhibits the expression of others. Those that are enhanced include the genes for α-myosin heavy chain, sarcoplasmic reticulum Ca2+ ATPase, β-adrenergic receptors, G-proteins, Na, K ATPase, and certain K+ chan­nels. Those that are inhibited include the genes for β-myosin heavy chain, phospholamban, two types of adenylyl cyclase, T3 nuclear receptors, and NCX, the Na+–Ca2+ exchanger. The net result is increased heart rate and force of contraction.

The two myosin heavy chain (MHC) isoforms, α-MHC and β-MHC, produced by the heart are encoded by two highly homologous genes located on the short arm of chromosome 17. Each myosin molecule consists of two heavy chains and two pairs of light chains. The myosin containing β-MHC has less ATPase activity than the myosin containing α-MHC. α-MHC predominates in the atria in adults, and its level is increased by treatment with thyroid hormone. This increases the speed of cardiac contraction. Conversely, expres­sion of the α-MHC gene is depressed and that of the β-MHC gene is enhanced in hypothyroidism.

EFFECTS ON THE NERVOUS SYSTEM

In hypothyroidism, mentation is slow and the cerebrospinal fluid (CSF) protein level is elevated. Thyroid hormones reverse these changes, and large doses cause rapid mentation, irrita­bility, and restlessness. Overall, cerebral blood flow and glu­cose and O2 consumption by the brain are normal in adult hypothyroidism and hyperthyroidism. However, thyroid hor­mones enter the brain in adults and are found in gray matter in numerous different locations. In addition, astrocytes in the brain convert T4 to T3, and there is a sharp increase in brain D2 activity after thyroidectomy that is reversed within 4 h by a single intravenous dose of T3. Some of the effects of thyroid hormones on the brain are probably secondary to increased responsiveness to catecholamines, with consequent increased activation of the reticular activating system. In addition, thyroid hormones have marked effects on brain development. The parts of the central nervous system (CNS) most affected are the cerebral cortex and the basal ganglia. In addition, the cochlea is also affected. Consequently, thyroid hormone deficiency during development causes mental retar­dation, motor rigidity, and deaf–mutism. Deficiencies in thy­roid hormone synthesis secondary to a failure of thyrocytes to transport iodide presumably also contribute to deafness in Pendred syndrome.

Thyroid hormones also exert effects on reflexes. The reac­tion time of stretch reflexes is shortened in hyperthyroidism and prolonged in hypothyroidism. Measure­ment of the reaction time of the ankle jerk (Achilles reflex) has attracted attention as a clinical test for evaluating thyroid function, but this reaction time is also affected by other dis­eases and thus is not a specific assessment of thyroid activity.

RELATION TO CATECHOLAMINES

The actions of thyroid hormones and the catecholamines nor­epinephrine and epinephrine are intimately interrelated. Epi­nephrine increases the metabolic rate, stimulates the nervous system, and produces cardiovascular effects similar to those of thyroid hormones, although the duration of these actions is brief. Norepinephrine has generally similar actions. The toxicity of the catecholamines is markedly increased in rats treated with T4. Although plasma catecholamine levels are normal in hyperthyroidism, the cardiovascular effects, tremu­lousness, and sweating that are seen in the setting of excess thyroid hormones can be reduced or abolished by sympathec­tomy. They can also be reduced by drugs such as propranolol that block β-adrenergic receptors. Indeed, propranolol and other β-blockers are used extensively in the treatment of thy­rotoxicosis and in the treatment of the severe exacerbations of hyperthyroidism called thyroid storms. However, even though β-blockers are weak inhibitors of extrathyroidal con­version of T4 to T3, and consequently may produce a small fall in plasma T3, they have little effect on the other actions of thyroid hormones. Presumably, the functional synergism observed between catecholamines and thyroid hormones, par­ticularly in pathologic settings, arises from their overlapping biologic functions as well as the ability of thyroid hormones to increase expression of catecholamine receptors and the signal­ing effectors to which they are linked.

EFFECTS ON SKELETAL MUSCLE

Muscle weakness occurs in most patients with hyperthyroid­ism (thyrotoxic myopathy), and when the hyperthyroidism is severe and prolonged, the myopathy may be severe. The muscle weakness may be due in part to increased protein catabolism. Thyroid hormones affect the expression of the MHC genes in skeletal as well as cardiac muscle. However, the effects produced are complex and their relation to the myopa­thy is not established. Hypothyroidism is also associated with muscle weakness, cramps, and stiffness.

EFFECTS ON CARBOHYDRATE METABOLISM

Thyroid hormones increase the rate of absorption of car­bohydrates from the gastrointestinal tract, an action that is probably independent of their calorigenic action. In hyperthy­roidism, therefore, the plasma glucose level rises rapidly after a carbohydrate meal, sometimes exceeding the renal threshold. However, it falls again at a rapid rate.

EFFECTS ON CHOLESTEROL METABOLISM

Thyroid hormones lower circulating cholesterol levels. The plasma cholesterol level drops before the metabolic rate rises, which indicates that this action is independent of the stimula­tion of O2 consumption. The decrease in plasma cholesterol concentration is due to increased formation of low-density lipoprotein (LDL) receptors in the liver, resulting in increased hepatic removal of cholesterol from the circulation. Despite considerable effort, however, it has not been possible to pro­duce a clinically useful thyroid hormone analog that lowers plasma cholesterol without increasing metabolism.

EFFECTS ON GROWTH

Thyroid hormones are essential for normal growth and skel­etal maturation. In hypothyroid children, bone growth is slowed and epiphysial closure delayed. In the absence of thyroid hormones, growth hormone secretion is also depressed. This further impairs growth and develop­ment, since thyroid hormones normally potentiate the effect of growth hormone on tissues.

ADRENAL GLANDS

There are two endocrine organs in the adrenal gland, one surrounding the other. The main secretions of the inner adrenal medulla are the catecholamines epinephrine, norepinephrine, and dopamine; the outer adrenal cortex secretes steroid hormones.  The adrenal medulla is in effect a sympathetic ganglion in which the postganglionic neurons have lost their axons and become secretory cells. The cells secrete when stimulated by the preganglionic nerve fibers that reach the gland via the splanchnic nerves. Adrenal medullary hormones work mostly to prepare the body for emergencies, the so-called “fight-or-flight” responses.  The adrenal cortex secretes glucocorticoids, steroids with widespread effects on the metabolism of carbohydrate and protein; and a mineralocorticoid essential to the maintenance of Na+ balance and extracellular fluid (ECF) volume. It is also a secondary site of androgen synthesis, secreting sex hormones such as testosterone, which can exert effects on reproductive function. Mineralocorticoids and the glucocorticoids are necessary for survival. Adrenocortical secretion is controlled primarily by adrenocorticotropic hormone (ACTH) from the anterior pituitary, but mineralocorticoid secretion is also subject to independent control by circulating factors, of which the most important is angiotensin II, a peptide formed in the bloodstream by the action of renin.

ADRENAL CORTEX: STRUCTURE & BIOSYNTHESIS OF ADRENOCORTICAL HORMONES

The hormones of the adrenal cortex are derivatives of choles­terol. Like cholesterol, bile acids, vitamin D, and ovarian and testicular steroids, they contain the cyclopentanoperhydro­phenanthrene nucleus. Gonadal and adreno­cortical steroids are of three types: C21 steroids, which have a two-carbon side chain at position 17; C19 steroids, which have a two-carbon side chain at position 17; C 19 steroids, which have a keto or hydroxyl group at position 17; and C 18 steroids, which, in addition to a 17-keto or hydroxyl group, have no angular methyl group attached to position 10. The adrenal cortex secretes primarily C 21 and C 19 steroids. Most of the C 19 steroids have a keto group at position 17 and are therefore called 17-ketosteroids. The C 21 steroids that have a hydroxyl group at the 17 position in addition to the side chain are oft en called 17-hydroxycorticoids or 17-hydroxycorticosteroids. The C 19 steroids have androgenic activity. The C 21 steroids are classified, using Selye’s terminology, as mineralocorticoids or glucocorticoids. All secreted C 21 steroids have both mineralocorticoid and glucocorticoid activity; mineralocorticoids are those in which effects on Na + and K + excretion predominate and glucocorticoids are those in which effects on glucose and protein metabolism predominate.

Hormone synthesis in the zona glomerulosa. The zona glomerulosa lacks 17α-hydroxylase activity, and only the zona glomerulosa can convert corticosterone to aldosterone because it is the only zone that normally contains aldosterone synthase. ANG II, angiotensin II.

Outline of hormone biosynthesis in the zona fasciculata and zona reticularis of the adrenal cortex. The major secretory products are underlined. The enzymes for the reactions are shown on the left and at the top of the chart. When a particular enzyme is deficient, hormone production is blocked at the points indicated by the shaded bars.

EFFECTS OF MINERALOCORTICOIDS ACTIONS

Aldosterone and other steroids with mineralocorticoid activity increase the reabsorption of Na+ from the urine, sweat, saliva, and the contents of the colon. Thus, mineralocorticoids cause retention of Na+ in the ECF. This expands ECF volume. In the kidneys, they act primarily on the principal cells (P cells) of the collecting ducts. Under the influence of aldosterone, increased amounts of Na+ are in effect exchanged for K+ and H+ in the renal tubules, producing a K+ diuresis and an increase in urine acidity.

MECHANISM OF ACTION

Like many other steroids, aldosterone binds to a cytoplasmic receptor, and the receptor–hormone complex moves to the nucleus where it alters the transcription of mRNAs. This in turn increases the production of proteins that alter cell func­tion. The aldosterone-stimulated proteins have two effects—a rapid effect, to increase the activity of epithelial sodium chan­nels (ENaCs) by increasing the insertion of these channels into the cell membrane from a cytoplasmic pool; and a slower effect to increase the synthesis of ENaCs. Among the genes activated by aldosterone is the gene for serum-and glucocorticoid reg­ulated kinase (sgk), a serine-threonine protein kinase. The gene for sgk is an early response gene, and sgk increases ENaC activity. Aldosterone also increases the mRNAs for the three subunits that make up ENaCs. The fact that sgk is activated by glucocorticoids as well as aldosterone is not a problem because glucocorticoids are inactivated at mineralocorticoid receptor sites. However, aldosterone activates the genes for other pro­teins in addition to sgk and ENaCs and inhibits others. There­fore, the exact mechanism by which aldosterone-induced proteins increase Na+ reabsorption is still unsettled.  Evidence is accumulating that aldosterone also binds to the cell membrane and by a rapid, nongenomic action increases the activity of membrane Na+–K+ exchangers. This produces an increase in intracellular Na+, and the second mes­senger involved is probably IP3. In any case, the principal effect of aldosterone on Na+ transport takes 10–30 min to develop and peaks even later, indicating that it depends on the synthesis of new proteins by a genomic mechanism.

OTHER STEROIDS THAT AFFECT Na⁺ EXCRETION

Aldosterone is the principal mineralocorticoid secreted by the adrenals, although corticosterone is secreted in sufficient amounts to exert a minor mineralocorticoid effect. Deoxycorticosterone, which is secreted in appre­ciable amounts only in abnormal situations, has about 3% of the activity of aldosterone. Large amounts of progesterone and some other steroids cause natriuresis, but there is little evidence that they play any normal role in the control of Na+ excretion.

EFFECT OF ADRENALECTOMY

In adrenal insufficiency, Na+ is lost in the urine; K+ is retained, and the plasma K+ rises. When adrenal insufficiency devel­ops rapidly, the amount of Na+ lost from the ECF exceeds the amount excreted in the urine, indicating that Na+ also must be entering cells. When the posterior pituitary is intact, salt loss exceeds water loss, and the plasma Na+ falls. However, the plasma volume is also reduced, resulting in hypotension, circulatory insufficiency and, eventually, fatal shock. These changes can be prevented to a degree by increas­ing dietary NaCl intake. Rats survive indefinitely on extra salt alone, but in dogs and most humans, the amount of supple­mentary salt needed is so large that it is almost impossible to prevent eventual collapse and death unless mineralocorticoid treatment is also instituted.

ROLE OF MINERALOCORTICOIDS IN THE REGULATION OF SALT BALANCE

Variations in aldosterone secretion is only one of many fac­tors affecting Na+ excretion. Other major factors include the glomerular filtration rate, ANP, the presence or absence of osmotic diuresis, and changes in tubular reabsorption of Na+ independent of aldosterone. It takes some time for aldoste­rone to act. When one rises from the supine to the standing position, aldosterone secretion increases and Na+ is retained from the urine. However, the decrease in Na+ excretion devel­ops too rapidly to be explained solely by increased aldosterone secretion. The primary function of the aldosterone-secreting mechanism is the defense of intravascular volume, but it is only one of the homeostatic mechanisms involved in this regulation.

ESTROGENS

Chemistry, Biosynthesis, & Metabolism of Estrogens

The naturally occurring estrogens are 17β-estradiol, estrone, and estriol. They are C18 steroids that do not have an angular methyl group attached to the 10 position or a ê4-3-keto configuration in the A ring. They are secreted primarily by the granulosa cells of the ovarian follicles, the corpus luteum, and the placenta. Their biosyn­thesis depends on the enzyme aromatase (CYP19), which converts testosterone to estradiol and androstenedione to estrone. The latter reaction also occurs in fat, liver, muscle, and the brain. Theca interna cells have many LH receptors, and LH acts via cAMP to increase conversion of cholesterol to andro­stenedione. The theca interna cells supply androstenedione to the granulosa cells. The granulosa cells make estradiol when provided with androgens, and it appears that the estradiol they form in primates is secreted into the fol­licular fluid. Granulosa cells have many FSH receptors, and FSH facilitates their secretion of estradiol by acting via cAMP to increase their aromatase activity. Mature granulosa cells also acquire LH receptors, and LH also stimulates estradiol production.  Two percent of the circulating estradiol is free, and the remainder is bound to protein: 60% to albumin and 38% to the same gonadal steroid-binding globulin (GBG) that binds testosterone.  In the liver, estradiol, estrone, and estriol are converted to glucuronide and sulfate conjugates. All these compounds, along with other metabolites, are excreted in the urine. Appre­ciable amounts are secreted in the bile and reabsorbed into the bloodstream (enterohepatic circulation).

Secretion

The concentration of estradiol in the plasma during the men­strual cycle is shown in Figure 22–14. Almost all of this estro­gen comes from the ovary, and two peaks of secretion occur: one just before ovulation and one during the midluteal phase. The estradiol secretion rate is 36 μg/day (133 nmol/day) in the early follicular phase, 380 μg/day just before ovulation, and 250 μg/day during the midluteal phase. After menopause, estrogen secretion declines to low levels.  As noted previously, the estradiol production rate in men is about 50 μg/day (184 nmol/day).

Effects on the Female Genitalia

Estrogens facilitate the growth of the ovarian follicles and increase the motility of the uterine tubes. Their role in the cyclic changes in the endometrium, cervix, and vagina has been discussed previously. They increase uterine blood flow and have important effects on the smooth muscle of the uterus.

In immature and castrated females, the uterus is small and the myometrium atrophic and inactive. Estrogens increase the amount of uterine muscle and its content of contractile pro­teins. Under the influence of estrogens, the muscle becomes more active and excitable, and action potentials in the individ­ual fibers become more frequent. The “estrogen-dominated” uterus is also more sensitive to oxytocin.

Chronic treatment with estrogens causes the endome­trium to hypertrophy. When estrogen therapy is discontin­ued, sloughing takes place with withdrawal bleeding. Some “breakthrough” bleeding may occur during treatment when estrogens are given for long periods. Prolonged exposure to estrogen alone (unopposed by progesterone) has been indicated as a risk factor in the development of endometrial cancer.

Effects on Endocrine Organs

Estrogens decrease FSH secretion. Under some circum­stances, they inhibit LH secretion (negative feedback); in other circumstances, they increase LH secretion (positive feedback). Women are sometimes given large doses of estro­gens for 4–6 days to prevent conception after coitus during the fertile period (postcoital or “morning-after” contraception). However, in this instance, pregnancy is probably prevented by interference with implantation of the ovum rather than changes in gonadotropin secretion.

Estrogens cause increased secretion of angiotensinogen and thyroid-binding globulin. They exert an important pro­tein anabolic effect in chickens and cattle, possibly by stimulat­ing the secretion of androgens from the adrenal, and estrogen treatment has been used commercially to increase the weight of domestic animals. They cause epiphysial closure in humans.

Effects on the Central Nervous System

The estrogens are responsible for estrous behavior in animals, and they increase libido in humans. They apparently exert this action by a direct effect on certain neurons in the hypothala­mus. Estrogens also increase the proliferation of dendrites on neurons and the number of synaptic knobs in rats.

Effects on the Breasts

Estrogens produce duct growth in the breasts and are largely responsible for breast enlargement at puberty in girls; they have been called the growth hormones of the breast. They are respon­sible for the pigmentation of the areolas, although pigmenta­tion usually becomes more intense during the first pregnancy than it does at puberty.

Female Secondary Sex Characteristics

The body changes that develop in girls at puberty—in addition to enlargement of breasts, uterus, and vagina—are due in part to estrogens, which are the “feminizing hormones,” and in part simply to the absence of testicular androgens. Women have nar­row shoulders and broad hips, thighs that converge, and arms that diverge (wide carrying angle). This body configuration, plus the female distribution of fat in the breasts and buttocks, is seen also in castrate males. In women, the larynx retains its pre­pubertal proportions and the voice stays high-pitched. Women have less body hair and more scalp hair, and the pubic hair gen­erally has a characteristic flat-topped pattern (female escutch­eon). However, growth of pubic and axillary hair in both sexes is due primarily to androgens rather than estrogens.

Other Actions

Normal women retain salt and water and gain weight just before menstruation. Estrogens cause some degree of salt and water retention. However, aldosterone secretion is slightly elevated in the luteal phase, and this also contributes to the premenstrual fluid retention.  Estrogens are said to make sebaceous gland secretions more fluid and thus to counter the effect of testosterone and inhibit formation of comedones (“black-heads”) and acne. The liver palms, spider angiomas, and slight breast enlarge­ment seen in advanced liver disease are due to increased circu­lating estrogens. The increase appears to be due to decreased hepatic metabolism of androstenedione, making more of this androgen available for conversion to estrogens.  Estrogens have a significant plasma cholesterol-lowering action, and they rapidly produce vasodilation by increasing the local production of NO.

Mechanism of Action

There are two principal types of nuclear estrogen receptors: estrogen receptor α (ERα) encoded by a gene on chromo­some 6; and estrogen receptor β (ERβ), encoded by a gene on chromosome 14. Both are members of the nuclear recep­tor super-family. After binding estrogen, they form homodimers and bind to DNA, altering its transcrip­tion. Some tissues contain one type or the other, but overlap also occurs, with some tissues containing both ERα and ERβ. ERα is found primarily in the uterus, kidneys, liver, and heart, whereas ERβ is found primarily in the ovaries, prostate, lungs, gastrointestinal tract, hemopoietic system, and central nervous system (CNS). ERα and ERβ can also form heterodimers. Male and female mice in which the gene for ERα has been knocked out are sterile, develop osteoporosis, and continue to grow because their epiphyses do not close. ERβ female knockouts are infertile, but ERβ male knockouts are fertile even though they have hyperplastic prostates and loss of fat. Both receptors exist in isoforms and, like thyroid receptors, can bind to vari­ous activating and stimulating factors. In some situations, ERβ can inhibit ERα transcription. Thus, their actions are complex, multiple, and varied.  Most of the effects of estrogens are genomic, that is, due to actions on the nucleus, but some are so rapid that it is difficult to believe they are mediated via production of mRNAs. These include effects on neuronal discharge in the brain and, possibly, feedback effects on gonadotro­pin secretion. Evidence is accumulating that these effects are mediated by cell membrane receptors that appear to be structurally related to the nuclear receptors and pro­duce their effects by intracellular mitogen-activated pro­tein kinase pathways. Similar rapid effects of progesterone, testosterone, glucocorticoids, aldosterone, and 1,25-dihy­droxycholecalciferol may also be produced by membrane receptors.

TESTOSTERONE

Chemistry & Biosynthesis of Testosterone

Testosterone, the principal hormone of the testes, is a C19 ste­roid with a hydroxyl group in the 17 position. It is synthesized from cholesterol in the Leydig cells and is also formed from androstenedione secreted by the adrenal cortex. The biosynthetic pathways in all endocrine organs that form steroid hormones are similar, the organs differing only in the enzyme systems they contain. In the Leydig cells, the 11- and 21-hydroxylases found in the adrenal cortex are absent, but 17α-hydroxylase is present. Pregnenolone is therefore hydroxylated in the 17 position and then sub­jected to side chain cleavage to form dehydroepiandros­terone. Androstenedione is also formed via progesterone and 17-hydroxyprogesterone, but this pathway is less prominent in humans. Dehydroepiandrosterone and androstenedione are then converted to testosterone.  The secretion of testosterone is under the control of LH, and the mechanism by which LH stimulates Leydig cells involves increased formation of cyclic adenosine monophosphate (cAMP) via the G-protein–coupled LH receptor and Gs. Cyclic AMP increases the formation of cholesterol from cholesterol esters and the conversion of cholesterol to pregnenolone via the activation of protein kinase A.

Secretion

The testosterone secretion rate is 4–9 mg/d (13.9–31.33 μmol/d) in normal adult males. Small amounts of testosterone are also secreted in females, with the major source being the ovary, but possibly from the adrenal as well.

Actions

In addition to their actions during development, testoster­one and other androgens exert an inhibitory feedback effect on pituitary LH secretion; develop and maintain the male secondary sex characteristics; exert an important protein-anabolic, growth-promoting effect; and, along with FSH, maintain spermatogenesis.

Secondary Sex Characteristics

The widespread changes in hair distribution, body configu­ration, and genital size that develop in boys at puberty is called as the male secondary sex characteristics. The prostate and seminal vesicles enlarge, and the seminal vesicles begin to secrete fructose. This sugar appears to function as the main nutritional supply for the spermatozoa. The psychological effects of testosterone are difficult to define in humans, but in experimental animals, androgens provoke boisterous and aggressive play. Although body hair is increased by androgens, scalp hair is decreased. Hereditary baldness often fails to develop unless dihydrotestosterone (DHT) is present.

Anabolic Effects

Androgens increase the synthesis and decrease the breakdown of protein, leading to an increase in the rate of growth. It used to be argued that they cause the epiphyses to fuse to the long bones, thus eventually stopping growth, but it now appears that epiphysial closure is due to estrogens. Secondary to their anabolic effects, androgens cause moder­ate Na+, K+, H2O, Ca2+, SO4–, and PO4– retention; and they also increase the size of the kidneys. Doses of exogenous testoster­one that exert significant anabolic effects are also masculin­izing and increase libido, which limits the usefulness of the hormone as an anabolic agent in patients with wasting dis­eases. Attempts to develop synthetic steroids in which the ana­bolic action is separated from the androgenic action have not been successful.

Mechanism of Action

Like other steroids, testosterone binds to an intracellular receptor, and the receptor/steroid complex then binds to DNA in the nucleus, facilitating transcription of various genes.  In addition, testosterone is converted to DHT by 5α-reductase in some target cells, and DHT binds to the same intracellular receptor as testosterone. DHT also circulates, with a plasma level that is about 10% of the testosterone level. Testosterone–receptor complexes are less stable than DHT–receptor complexes in target cells, and they conform less well to the DNA-binding state. Thus, DHT for­mation is a way of amplifying the action of testosterone in tar­get tissues. Humans have two 5α-reductases that are encoded by different genes. Type 1 5α-reductase is present in skin throughout the body and is the dominant enzyme in the scalp. Type 2 5α-reductase is present in genital skin, the prostate, and other genital tissues.  Testosterone–receptor complexes are responsible for the maturation of Wolffian duct structures and consequently for the formation of male internal genitalia during develop­ment, but DHT–receptor complexes are needed to form male external genitalia (Figure 23–8). DHT–receptor complexes are also primarily responsible for enlargement of the prostate and probably of the penis at the time of puberty, as well as for the facial hair, the acne, and the temporal recession of the hair­line. On the other hand, the increase in muscle mass and the development of male sex drive and libido depend primarily on testosterone rather than DHT.

MENOPAUSE

The human ovaries become unresponsive to gonadotropins with advancing age, and their function declines, so that sex­ual cycles disappear (menopause). This unresponsiveness is associated with and probably caused by a decline in the number of primordial follicles, which becomes precipitous at the time of menopause

THE MENSTRUAL CYCLE

The reproductive system of women, unlike that of men, shows regular cyclic changes that teleologically may be regarded as periodic preparations for fertilization and preg­nancy. In humans and other primates, the cycle is a menstrual cycle, and its most conspicuous feature is the periodic vaginal bleeding that occurs with the shedding of the uterine mucosa (menstruation). The length of the cycle is notoriously vari­able in women, but an average figure is 28 days from the start of one menstrual period to the start of the next. By common usage, the days of the cycle are identified by number, starting with the first day of menstruation.

Ovarian Cycle

From the time of birth, there are many primordial fol­licles under the ovarian capsule. Each contains an immature ovum. At the start of each cycle, several of these follicles enlarge, and a cavity forms around the ovum (antrum formation). This cavity is filled with follicular fluid. In humans, usually one of the follicles in one ovary starts to grow rapidly on about the 6th day and becomes the dominant follicle, while the others regress, forming atretic follicles. The atretic process involves apoptosis. It is uncertain how one fol­licle is selected to be the dominant follicle in this follicular phase of the menstrual cycle, but it seems to be related to the ability of the follicle to secrete the estrogen inside it that is needed for final maturation. When women are given human pituitary gonadotropin preparations by injection, many fol­licles develop simultaneously. The primary source of circulating estrogen is the granulosa cells of the ovaries; however, the cells of the theca interna of the follicle are necessary for the pro­duction of estrogen as they secrete androgens that are aroma­tized to estrogen by the granulosa cells. 

At about the 14th day of the cycle, the distended follicle ruptures, and the ovum is extruded into the abdominal cav­ity. This is the process of ovulation. The ovum is picked up by the fimbriated ends of the uterine tubes (oviducts). It is transported to the uterus and, unless fertilization occurs, out through the vagina.  The follicle that ruptures at the time of ovulation promptly fills with blood, forming what is sometimes called a corpus hemorrhagicum. Minor bleeding from the follicle into the abdominal cavity may cause peritoneal irritation and fleeting lower abdominal pain (“mittelschmerz”). The granulosa and theca cells of the follicle lining promptly begin to proliferate, and the clotted blood is rapidly replaced with yellowish, lipid-rich luteal cells, forming the corpus luteum. This initiates the luteal phase of the menstrual cycle, dur­ing which the luteal cells secrete estrogen and progesterone. Growth of the corpus luteum depends on its developing an adequate blood supply, and there is evidence that vascular endothelial growth factor (VEGF) is essen­tial for this process.

If pregnancy occurs, the corpus luteum persists and usu­ally there are no more periods until after delivery. If pregnancy does not occur, the corpus luteum begins to degenerate about 4 days before the next menses (24th day of the cycle) and is eventually replaced by scar tissue, forming a corpus albicans.  The ovarian cycle in other mammals is similar, except that in many species more than one follicle ovulates and multiple births are the rule. Corpora lutea form in some submamma­lian species but not in others. In humans, no new ova are formed after birth. During fetal development, the ovaries contain over 7 million primor­dial follicles. However, many undergo atresia (involution) before birth and others are lost after birth. At the time of birth, there are 2 million ova, but 50% of these are atretic. The mil­lion that are normal undergo the first part of the first mei­otic division at about this time and enter a stage of arrest in prophase in which those that survive persist until adulthood. Atresia continues during development, and the number of ova in both of the ovaries at the time of puberty is less than 300,000. Only one of these ova per cycle (or about 500 in the course of a normal reproductive life) normally reaches maturity; the remainder degenerate. Just before ovulation, the first meiotic division is completed. One of the daughter cells, the secondary oocyte, receives most of the cytoplasm, while the other, the first polar body, fragments and disappears. The secondary oocyte immediately begins the second meiotic divi­sion, but this division stops at metaphase and is completed only when a sperm penetrates the oocyte. At that time, the second polar body is cast off and the fertilized ovum proceeds to form a new individual. The arrest in metaphase is due, at least in some species, to formation in the ovum of the protein pp39mos, which is encoded by the c-mos protooncogene. When fertilization occurs, the pp39mos is destroyed within 30 min by calpain, a calcium-dependent cysteine protease.

Uterine Cycle

At the end of menstruation, all but the deep layers of the endo­metrium have sloughed. A new endometrium then regrows under the influence of estrogens from the developing follicle. The endometrium increases rapidly in thickness from the 5th to the 14th days of the menstrual cycle. As the thickness increases, the uterine glands are drawn out so that they lengthen, but they do not become convoluted or secrete to any degree. These endometrial changes are called prolifera­tive, and this part of the menstrual cycle is sometimes called the proliferative phase. It is also called the preovulatory or follicular phase of the cycle. After ovulation, the endometrium becomes more highly vascularized and slightly edematous under the influence of estrogen and progesterone from the corpus luteum. The glands become coiled and tortuous and they begin to secrete a clear fluid. Consequently, this phase of the cycle is called the secretory or luteal phase. Late in the luteal phase, the endo­metrium, like the anterior pituitary, produces prolactin, but the function of this endometrial prolactin is unknown.

The endometrium is supplied by two types of arteries. The superficial two-thirds of the endometrium that is shed during menstruation, the stratum functionale, is supplied by long, coiled spiral arteries, whereas the deep layer that is not shed, the stratum basale, is supplied by short, straight basilar arteries.  When the corpus luteum regresses, hormonal support for the endometrium is withdrawn. The endometrium becomes thinner, which adds to the coiling of the spiral arteries. Foci of necrosis appear in the endometrium, and these coalesce. In addition, spasm and degeneration of the walls of the spiral arteries take place, leading to spotty hemorrhages that become confluent and produce the menstrual flow.  The vasospasm is probably produced by locally released prostaglandins. Large quantities of prostaglandins are present in the secretory endometrium and in menstrual blood, and infusions of prostagladin F2α(PGF2α) produce endometrial necrosis and bleeding.

From the point of view of endometrial function, the pro­liferative phase of the menstrual cycle represents restoration of the epithelium from the preceding menstruation, and the secretory phase represents preparation of the uterus for implantation of the fertilized ovum. The length of the secre­tory phase is remarkably constant at about 14 days, and the variations seen in the length of the menstrual cycle are due for the most part to variations in the length of the prolifera­tive phase. When fertilization fails to occur during the secre­tory phase, the endometrium is shed and a new cycle starts.

Normal Menstruation

Menstrual blood is predominantly arterial, with only 25% of the blood being of venous origin. It contains tissue debris, prostaglandins, and relatively large amounts of fibrinolysin from endometrial tissue. The fibrinolysin lyses clots, so that menstrual blood does not normally contain clots unless the flow is excessive.  The usual duration of the menstrual flow is 3–5 days, but flows as short as 1 day and as long as 8 days can occur in normal women. The amount of blood lost may range nor­mally from slight spotting to 80 mL; the average amount lost is 30 mL. Loss of more than 80 mL is abnormal. Obviously, the amount of flow can be affected by various factors, including the thickness of the endometrium, medication, and diseases that affect the clotting mechanism.

INSULIN

BIOSYNTHESIS AND SECRETION

Insulin is synthesized in the rough endoplasmic reticulum of the B cells. It is then transported to the Golgi apparatus, where it is packaged into membrane-bound granules. These granules move to the plasma membrane by a process involving microtubules, and their contents are expelled by exocytosis. The insulin then crosses the basal lamina of the B cell and a neighbor­ing capillary and the fenestrated endothelium of the capillary to reach the bloodstream. Like other polypeptide hormones and related proteins that enter the endoplasmic reticulum, insulin is synthe­sized as part of a larger preprohormone. The gene for insulin is located on the short arm of chromosome 11 in humans. It has two introns and three exons. Prepro­insulin originates from the endoplasmic reticulum. The remainder of the molecule is then folded, and the disulfide bonds are formed to make proinsulin. The peptide segment connecting the A and B chains, the connecting peptide (C peptide), facilitates the folding and then is detached in the granules before secretion. Two proteases are involved in processing the proinsulin. Normally, 90–97% of the prod­uct released from the B cells is insulin along with equimo­lar amounts of C peptide. The rest is mostly proinsulin. C peptide can be measured by radioimmunoassay, and its level in blood provides an index of B cell function in patients receiving exogenous insulin.

FATE OF SECRETED INSULIN

INSULIN & INSULIN-LIKE ACTIVITY IN BLOOD

Plasma contains a number of substances with insulin-like activity in addition to insulin. The activity that is not sup­pressed by anti-insulin antibodies has been called non­suppressible insulin-like activity (NSILA). Most, if not all, of this activity persists after pancreatectomy and is due to the insulin-like growth factors IGF-I and IGF-II. These IGFs are polypeptides. Small amounts are free in the plasma (low-molecular-weight fraction), but large amounts are bound to proteins (high-molecular-weight fraction).  One may well ask why pancreatectomy causes diabetes mellitus when NSILA persists in the plasma. However, the insulin-like activities of IGF-I and IGF-II are weak compared to that of insulin and likely subserve other specific functions.

METABOLISM

The half-life of insulin in the circulation in humans is about 5 min. Insulin binds to insulin receptors, and some is internal­ized. It is destroyed by proteases in the endosomes formed by the endocytotic process.

EFFECTS OF INSULIN

The physiologic effects of insulin are far-reaching and com­plex. They are conveniently divided into rapid, intermediate, and delayed actions. The best known is the hypo­glycemic effect, but there are additional effects on amino acid and electrolyte transport, many enzymes, and growth. The net effect of the hormone is storage of carbohydrate, protein, and fat. Therefore, insulin is appropriately called the “hormone of abundance.” 

GLUCOSE TRANSPORTERS

Glucose enters cells by facilitated diffusion or, in the intestine and kidneys, by secondary active trans­port with Na+. In muscle, adipose, and some other tissues, insulin stimulates glucose entry into cells by increasing the number of glucose transporters (GLUTs) in the cell membranes.

The GLUTs that are responsible for facilitated diffu­sion of glucose across cell membranes are a family of closely related proteins that span the cell membrane 12 times and have their amino and carboxyl terminals inside the cell. They differ from and have no homology with the sodium-glucose linked transporters (SGLT-1 and SGLT-2), which are responsible for the secondary active transport of glu­cose in the intestine and renal tubules, although the SGLTs also have 12 transmem­brane domains.  Seven different GLUTs, named GLUT 1–7 in order of discovery, have been characterized. They contain 492–524 amino acid residues and their affinity for glucose varies. Each transporter appears to have evolved for special tasks. GLUT-4 is the transporter in muscle and adi­pose tissue that is stimulated by insulin. A pool of GLUT-4 molecules is maintained within vesicles in the cytoplasm of insulin-sensitive cells. When the insulin receptors of these cells are activated, the vesicles move rapidly to the cell membrane and fuse with it, inserting the transporters into the cell membrane. When insulin action ceases, the transporter-containing patches of membrane are endocytosed and the vesicles are ready for the next expo­sure to insulin. Activation of the insulin receptor brings about the movement of the vesicles to the cell membrane by activating phosphatidylinositol 3-kinase. Most of the other GLUT transporters that are not insulin-sensitive appear to be constitutively expressed in the cell membrane.

In the tissues in which insulin increases the number of GLUTs in cell membranes, the rate of phosphorylation of the glucose, once it has entered the cells, is regulated by other hor­mones. Growth hormone and cortisol both inhibit phosphor­ylation in certain tissues. Transport is normally so rapid that it is not a rate-limiting step in glucose metabolism. However, it is rate-limiting in B cells.  Insulin also increases the entry of glucose into liver cells, but it does not exert this effect by increasing the num­ber of GLUT-4 transporters in the cell membranes. Instead, it induces glucokinase, and this increases the phosphorylation of glucose, so that the intracellular free glucose concentration stays low, facilitating the entry of glucose into the cell.  Insulin-sensitive tissues also contain a population of GLUT-4 vesicles that move into the cell membrane in response to exercise, a process that occurs independent of the action of insulin. This is why exercise lowers blood sugar. A 5’-adenosine monophosphate (AMP)–activated kinase may trigger the insertion of these vesicles into the cell membrane.

RELATION TO POTASSIUM

Insulin causes K+ to enter cells, with a resultant lowering of the extracellular K+ concentration. Infusions of insulin and glucose significantly lower the plasma K+ level in normal individuals and are very effective for the temporary relief of hyperkalemia in patients with renal failure. Hypokalemia often develops when patients with diabetic acidosis are treated with insulin. The reason for the intracellular migra­tion of K+ is still uncertain. However, insulin increases the activity of Na, K ATPase in cell membranes, so that more K+ is pumped into cells.

OTHER ACTIONS

The action on glycogen synthase fosters glycogen storage, and the actions on glycolytic enzymes favor glucose metabolism to two carbon fragments, with resulting promotion of lipogenesis. Stimulation of protein synthesis from amino acids entering the cells and inhibition of protein degradation foster growth.  The anabolic effect of insulin is aided by the protein-spar­ing action of adequate intracellular glucose supplies. Failure to grow is a symptom of diabetes in children, and insulin stimulates the growth of immature hypophysectomized rats to almost the same degree as growth hormone.

MECHANISM OF ACTION

INSULIN RECEPTORS

Insulin receptors are found on many different cells in the body, including cells in which insulin does not increase glucose uptake.  The insulin receptor, which has a molecular weight of approximately 340,000, is a tetramer made up of two α and two β glycoprotein subunits. All these are syn­thesized on a single mRNA and then proteolytically separated and bound to each other by disulfide bonds. The gene for the insulin receptor has 22 exons and in humans is located on chromosome 19. The α subunits bind insulin and are extracel­lular, whereas the β subunits span the membrane. The intracel­lular portions of the β subunits have tyrosine kinase activity. The α and β subunits are both glycosylated, with sugar resi­dues extending into the interstitial fluid.  Binding of insulin triggers the tyrosine kinase activity of the β subunits, producing autophosphorylation of the β subunits on tyrosine residues. The autophosphorylation, which is necessary for insulin to exert its biologic effects, triggers phosphorylation of some cytoplasmic proteins and dephosphorylation of others, mostly on serine and threonine residues. Insulin receptor substrate (IRS-1) mediates some of the effects in humans but there are other effector systems as well. For example, mice in which the insulin receptor gene is knocked out show marked growth retarda­tion in utero, have abnormalities of the central nervous sys­tem (CNS) and skin, and die at birth of respiratory failure, whereas IRS-1 knockouts show only moderate growth retar­dation in utero, survive, and are insulin-resistant but other­wise nearly normal.  The growth-promoting protein anabolic effects of insu­lin are mediated via phosphatidylinositol 3-kinase (PI3K), and evidence indicates that in invertebrates, this pathway is involved in the growth of nerve cells and axon guidance in the visual system.  It is interesting to compare the insulin receptor with other related receptors. The insulin receptor is very similar to the receptor for IGF-I but different from the receptor for IGF-II. Other receptors for growth factors and recep­tors for various oncogenes also are tyrosine kinases. How­ever, the amino acid composition of these receptors is quite different.  When insulin binds to its receptors, they aggregate in patches and are taken up into the cell by receptor-mediated endocytosis. Eventually, the insulin–receptor complexes enter lysosomes, where the receptors are broken down or recycled. The half-life of the insulin receptor is about 7 h.

Reference Books:

1. Ganong’s Review of Medical Physiology, 25edition, 2016.

2. Guyton_Hall_Textbook_of_Medical_Physiology, 13^th edition, 2016.

3. Vander’s Human Physiology, 14^th edition, 2015.

4. Essentials of Medical Physiology, 6^th edition, K Sembulingam.

5. Fox_S._Human_physiology, 12^th edition.

6. Hansen KoeppenNetter's Atlas of Human Physiology, Saunders, 2002.

7. Krieger_A_Visual_Analogy_Guide_to_Human_Anatomy_&_Physiology

8. Lauralee_Sherwood_Introduction_to_Human_Physiology 8^th edition.

9. RoddieWallace_MCQs_and_EMQs_in_Human_Physiology 6^th edition.