Term Paper on Pituitary Gland | Endocrine Glands | Biology

Here is a term paper on ‘Pituitary Gland’. Find paragraphs, long and short term papers on ‘Pituitary Gland’ especially written for school and college students.

Term Paper # 1. Introduction to Pituitary Gland:

The pituitary gland (hypophysis) is enclosed in the hypophyseal fossa and is attached to the base of the brain by a thin stalk emerging from the tuber cinereum. In, the adult human the gland weighs about 0.5 to 0.6 g, being somewhat larger in the female than in the male.

In certain mammals, the pituitary can readily be separated into two main arts the “anterior lobe” and the “posterior lobe,” and from these parts hormonal principles have been isolated. This deceptively simple terminology for the hypophysis should probably be replaced by a more realistic one. The anterior lobe, adenohypophysis, consists of three regions the pars distalis pars tuberalis, and the pars intermedia.

The posterior lobe or neurohypophysis, is separable into the median eminence, the infundibular stem and the infundibular process. The posterior lobe of the older terminology is formed by the infundibular process and the pars intermedia. The pars distalis has many of the characteristics of a secretory gland while neural lobe does not. The precise role of the nervous system in the control of the neurohypophysis and the function of the neurosecretions of this tissue are not as yet clear.

The remoteness, and the small size of the gland- early led to much erroneous speculation as to its physiological function. It was not until the early 1900’s that this structure’s true function as an endocrine organ became apparent, and it was not until 1940 that a pure hormone was finally isolated from the gland.

All the recognized home of the pituitary are proteins. Because of the great difficulties associated; with the fractionation and purification of proteins, and also because of the small size of the gland and its low hormone content, our knowledge of the chemistry of the pituitary hormones has developed slowly.

Although the, chemistry of these hormones remains to be clarified, considerable information is available with respect to the physiological functions of the pituitary fractions.

Term Paper # 2. Anterior Pituitary Hormones:

1. Adrenocorticotropic Hormone (ACTH):

i. Synthesis:

The synthesis of ACTH in the anterior pituitary proceeds via precursor intermediates that are glycoproteins with molecular weights in the range 10,000 to 30,000. ACTH, when split out hydrolytically from these precursors, is a single-chain peptide of 39 amino acid residues. Only the linkage of the first 24 residues, whose sequence appears to be identical in many species, is essential for hormonal activity.

ii. Secretion:

The release of ACTH from the adenohypophysis is triggered by releasing factors delivered to this gland from the hypothalamus. Present evidence indicates that there are two adrenocorticotropic hormone releasing factors (α and β) made in the hypothalamus and that both are polypeptides. As with the other hypothalamic regulatory factors, the secretion of these polypeptides can be a response to external stimuli such as trauma, stress, and drugs.

The nervous signals resulting from these stimuli are delivered by the autonomic nervous system to the hypothalamus. Secretion of the ACTH releasing factors can also be stimulated hormonally (i.e., by substances already present in the body and having access to the hypothalamus via the circulation). These agents include other hormones, such as insulin, thyroxine, vasopressin, and epinephrine.

Typical of a tropic hormone, the secretion of ACTH by the anterior pituitary is subject to negative feedback control by hormones produced in and secreted from its target organ, the adrenal cortex. These hormones, the adrenal cortical steroids, inhibit ACTH secretion by suppressing the release of the ACTH releasing factor from the hypothalamus.

Accordingly, the rate of ACTH secretion is found to vary inversely with the level of adrenal cortical steroids in the plasma. As would also be expected from these relationships, the adrenal cortex will undergo atrophy in individuals treated for extensive periods with adrenal cortical steroids.

Mode of Action of ACTH:

ACTH not only stimulates synthesis and secretion of the adrenal cortico steroids but also enhances the growth of the cortex. The effect on steroid production is rapid in the intact animal, occurring 1 to 3 hours after administration.

The trophic action on protein synthesis in the cortex is much slower. Since ACTH triggers the synthesis and release of a mixture of cortico steroids, administration of ACTH to an animal results in all of the physiological and biochemical responses attributable to these hormones.

How does ACTH promote the synthesis of the adrenocortical steroids? The rate-limiting process in steroid biosynthesis is the initial oxidative removal of the six-carbon side chain of cholesterol to yield pregnenolone.

It will be recalled that this process, as well as subsequent mixed-function oxidation reactions, require NADPH, a product of the phosphogluconate pathway.

ACTH is responsible for promoting the formation of NADPH at levels required by the steroid hydroxylations and accomplishes this by triggering the following sequence of reactions:

ACTH → binding to membrane receptors of the adrenal cortex → stimulation of adrenal cortical adenylate cyclase → short-lived protein(s) ← cyclic AMP → activation of glycogen phosphorylate → (involved in steroidogenesis) production of glucose-6-phosphate → phosphogluconate pathway → NADPH

The level of the second dehydrogenase of the pentose phosphate shunt, 6-phosphogluconolactone dehydrogenase, can be increased several-fold upon administration of ACTH.

There is no stimulation of steroid synthesis by ACTH in the adrenal if protein synthesis in the cortex is inhibited at the translational level. Thus, steroidogenesis is blocked in the presence of puromycin but not by actinomycin D, an inhibitor of RNA synthesis. The increase in cyclic AMP elicited by ACTH is not affected by the presence of these inhibitors.

Except for aldosterone, whose secretion is less dependent on the hypophysis, ACTH stimulates the production of all of the other adrenal steroids- aldosterone, the glucocorticoids, and androgens. As expected, therefore, administration of ACTH to an individual will elicit all of the responses attributable to these hormones— increased excretion of total nitrogen, uric acid, phosphate, and potassium; and increased concentrations of free fatty acids and of fasting blood glucose in the plasma.

In experimental animals, the stimulation of the adrenal cortex by ACTH can be monitored by measuring the ascorbate content of the cortex. The decrease in adrenal ascorbic acid elicited by stress is not observed in animals pretreated with ACTH.

Effects of ACTH on Other Tissues:

There is evidence from in vitro studies that ACTH can stimulate glucose utilization and lipolysis in adipose tissue, an effect comparable to that observed with adrenaline. Similar effects of ACTH are observed in vivo. Thus, administration of the hormone to adrenalectomized rats causes release of fatty acids from adipose tissue into the plasma. These effects of ACTH on non-adrenal tissue may involve a sequence of reactions beginning with an interaction of the hormone with a specific membrane receptor, followed by a cyclic AMP-mediated activation of a lipase.

2. Thyrotropin or Thyroid-Stimulating Hormone (TSH):

The structure of bovine TSH has been determined. The hormone is a glycoprotein composed of two subunits, designated as α and β whose molecular weights are 13,600 and 14,700, respectively. The amino acid sequence in the subunit has been found to be the same as in the corresponding a subunits of the gonadotropins—FSH, LH, and chorionic gonadotropin-of the same animal species. The biologic specificities of these hormones must be dictated, therefore, by the structure of their β chains.

Synthesis and Release:

The synthesis and release of TSH from the anterior pituitary can be initiated independently by the hypothalamic thyrotropin releasing factor. Release of thyrotropin can be detected within a minute. The releasing factor is specific and is Ca²⁺ dependent, affecting primarily the thyrotropin releasing cells of the adenohypophysis by stimulating adenylate cyclase.

Inhibition of thyrotropin release is dependent on a negative feedback relationship between the target gland (i.e., the thyroid and the anterior pituitary). The contribution of this mechanism is demonstrable by the marked reduction in thyrotropin observed following administration of either thyroxine or triiodothyronine.

Biochemical and Physiologic Effects:

Interaction of TSH with the thyroid triggers hyperactivity in the gland and an intact animal administered the hormone will therefore show all of the signs of hyperthyroidism. In addition to increased growth the thyroid exhibits enhanced glucose oxidation, oxygen uptake, and synthesis of RNA, protein, and phospholipids. Not only is thyroxine synthesis increased, the thyroglobulin already there is also broken down and thyroxine and T₃ are released.

For a broader perspective of the biochemical and physiologic action of TSH on the thyroid, it may be helpful to first review some of the features of the structure of the gland itself. The fully developed thyroid weighs approximately 30 g and consists of two lobes connected by an isthmus. The lobes, wrapped around the trachea just below the larynx, resemble the wings of a butterfly. The secretory units of the thyroid, the follicles, are vesicular spheres about 300 µm in diameter.

The wall of each follicle consists of a monolayer of cuboidal epithelial cells. The lumen of the vesicle is filled with colloid, the material containing the thyroglobulin. Twenty to forty follicles are grouped as lobules, each served by an artery, and groups of lobules in turn form the lobes of the thyroid.

The morphology of the follicles changes markedly with activity of the gland. When the thyroid is hyperactive, the follicle contains little colloid and its epithelium is tall and columnar. In contrast in its resting state, the follicle has a large store of colloid and its epithelium is flattened.

The stimulation of the thyroid by TSH begins with an interaction of the hormone with specific receptors in the membranes of the follicle epithelial cells. The receptor appears to be a glycoprotein containing sialic acid. More specifically, the receptor has the properties of a glycoprotein-ganglioside. Following its binding to the receptor, which is Ca²⁺-dependent, the TSH molecule undergoes a conformational change.

As a result, the adenylate cyclase in the membrane of the epithelial cells is stimulated and the concentration of cyclic AMP in the cells rises. This event triggers an increased metabolic activity in the cell-an enhanced uptake of iodide and conversion to iodotyrosine, increased oxidative metabolism, and synthesis of thyroglobulin. The biologic half-life of TSH in the plasma is about 10 minutes, but its effect on iodide uptake lasts up to 100 times longer.

Further details of thyroxine and thyroglobulin formation, secretion, and function will be presented in the discussions of the thyroid gland. In addition to its specific action on the thyroid, TSH may have effects on other tissues as well. Thus, as has been demonstrated with ACTH, TSH is found to stimulate lipolysis in adipocytes in vitro.

3. Gonadotropins:

There are three gonadotropic hormones secreted by the anterior pituitary-follicle stimulating hormone (FSH), luteinizing or interstitial cell-stimulating hormone (LH or ICSH) and prolactin. All three are functional in the female; only FSH and LH/ICSH are active in the male.

Molecular Structure:

Similar to TSH, both FSH and LH are glycoproteins with two non-covalently bound subunits, termed α and β. The amino acid sequences are identical in the chains of the three enzymes. The biological activities and immunological specificities are determined by the structures of the β subunits. The molecular weights of human FSH and LH are 34,000 and 28,500, respectively. Their carbohydrate content accounts for almost one-sixth of their weight and includes galactose, mannose, fucose, N-acetylglucosamine. N-acetylgalactosamine, and sialic acid.

Prolactin is not a glycoprotein. Its synthesis, like that of ACTH, entails precursors of high molecular weight.

Secretion:

The secretion of the FSH and LH/ICSH is controlled by the concerted action of their hypothalamic regulatory factors and the feedback mechanism involving the two gonadotropins themselves, as well as the circulatory hormones produced from their final target glands and tissues.

Prolactin does not cause the release of any hormone from its terminal target tissue, the mammary gland. Stimulation of secretion of prolactin from the adenohypophysis is triggered by its specific hypothalamic releasing factor and suppression of secretion is effected by the release-inhibiting factor.

Additional aspects of the control of gonadotropin secretion will be considered in the discussion of regulatory functions of circulating androgens and estrogens.

Mode of Physiologic Action of the Gonadotropins in the Female:

Although the nature of its “clock” mechanism is unknown, the hypothalamus, integrated with the hypophysis, determines the rhythmic release of the gonadotropins characteristic of sexual activity in the female. However, a regulating influence on gonadotropin release is also exerted by the final target organs, the ovaries.

The repetitive sequence of gametogenic and endocrine functions of the ovaries in an adult female are most apparent in the ovarian cycle. A brief overview of the morphologic and physiologic changes characteristic of a normal cycle may be helpful.

The development of the mature ovum from the primordial germ cell in the human female involves a succession of mitotic and meiotic divisions. The production of primordial cells capable of such divisions ceases before birth, with a final number of approximately 200,000 per ovary.

Some of these germ cells will become surrounded by a single layer of somatic cells, to give structures referred to as primordial follicles. Of the original 400,000 such potential germ cells in the two ovaries, only about 0.1 percent will ever reach the stage of ovulation (i.e., being discharged from a mature follicle).

Little change occurs in the ovaries from birth to puberty. However, each month thereafter, waves of primordial follicles experience a cycle of maturation and regression. Changes occur in the two cell layers surrounding the germ cell, the granulosa layer and the theca interna.

As these cells proliferate, fluid accumulates between the granulosa cells, forming a small cavity. Only one follicle, occasionally two, continue(s) to develop in the ovary. The others regress (atresia). As the volume of follicular fluid grows larger, the germ cell is pushed to one side of the follicle, to lie in a mass of granulosa cells termed the cumulus.

The follicle continues to increase in size, the theca interna cells enlarging and becoming vascularized. Compared with the original primordial follicle, which has a diameter of 30 µm, the mature follicle is 10 to 30 mm in diameter when it ruptures to discharge the ovum.

After ovulation, the granulosa cells in the ruptured follicle wall proliferate and become much vascularized. The granulosa component of this mass becomes the major component of the corpus luteum. The corpus luteum, which is intensely yellow due to the lipids in the luteinized cells, synthesizer steroids for 8 to 10 days.

The progesterone produced is important for making the endometrium of the uterus ready for implantation of the fertilized’ egg. If the egg becomes implanted, the corpus luteum remains active in its secretory function. If the ovum is hot fertilized, the corpus luteum degenerates and its hormone secretions rapidly stop.

The endocrine interrelationships associated with the menstrual cycle can be summarized as follows:

The feedback effects of the estrogens and the progestagens manifest during the sexual cycle reflect the close interrelationships among the hypothalamus, adenohypophysis, and ovaries. The adenohypophysis is under the domination of the hypothalamus. During menstruation, the hypophysis is stimulated to release predominantly FSH, which in turn promotes follicular growth. Estrogen secretion then increases during the follicular phase of the cycle and this inhibits FSH release and at the same time stimulates secretion of LH and prolactin.

As a result of this synergism of hormonal effects, ovulation ensues and luteinization begins. These events are approximately midway into the cycle. Progesterone secretion from the corpus luteum now increases and oppresses LH and prolactin release. As the corpus luteum degenerates, its production of progesterone drops. With resumption of FSH secretion, another sexual cycle begins.

Progesterone is also produced and secreted by the placenta in the terminal stages of pregnancy. As a major precursor of the steroids, progesterone is found in all tissues that synthesize these hormones, including the testes and the adrenal cortex.

The third gonadotropic hormone active in the female is prolactin or lactogenic hormone. Its target is the mammary gland, causing development during pregnancy and milk production following parturition. The physiological action of prolactin is accomplished synergistically with estrogens, the progestagens, the adrenocortical steroids, thyroxine, and growth hormone. Although prolactin helps to maintain the corpus luteum in rats, there is no evidence for such a function in the human female.

There is some question at the present as to whether prolactin and growth hormone are the same in humans. Prolactin inhibits luteinization by LH. This anti-ovulation effect may be synergistic with the action of progesterone on the corpus luteum.

Gonadotropin Effects in the Male:

FSH stimulates spermatogenesis by a direct action in the seminiferous tubules in the testes, a process also facilitated by testosterone. The seminiferous tubules have basement membranes containing germinal cells in various stages of maturation and also sustentacular cells (Sertoli cells). FSH causes these cells to synthesize the androgen-binding protein required to transport testosterone and dihydrotestosterone to the germinal cells in the tubules.

The interrelationships among the hypothalamus, anterior pituitary, and testes can be formulated as shown below:

The seminiferous tubules are embedded in connective tissue comprised of interstitial cells, the Leydig cells. These cells, some of which are swollen with lipids, are the site of testicular androgen (testosterone) synthesis and secretion. LH (ICSH) stimulates the Leydig cells to produce testosterone.

Castration causes an increase in the urinary excretion of gonadotropins. This indicates that any inhibition of these hormones from the adenohypophysis is modulated by a feedback mechanism involving the testes. The implication of the hypothalamus in this control mechanism is evident from two observations in experimental animals.

First, infliction of a hypothalamic lesion abolishes the castration effect. Second, implantation of some testosterone in the hypothalamus can cause testicular atrophy. Atrophy is not observed when such an implant is made in the anterior pituitary.

Biochemical Actions of the Gonadotropins:

As in the case of the other tropic hormones produced by the adenohypophysis, the initial biochemical reactions of the gonadotropins with their target cells involve specific receptors in the membranes. This reaction is followed by an enhanced activity of the membrane-bound adenylate cyclase.

In the testis FSH triggers an increase in protein kinase activity and nuclear RNA synthesis in rapid succession, followed by the other cellular events that characterize initiation of protein synthesis. One of these proteins, produced in the Sertoli cells, is the androgen-binding protein.

Of the testosterone synthesized in the Leydig cells under the stimulus of LH, a portion enters the blood and lymph to become associated with “binding” proteins for transport to target tissues. The remaining testosterone is bound to the androgen binding protein in the fluid of the seminiferous tubules. Following its diffusion into the seminiferous epithelial cells, the testosterone is reduced to its dihydro form, 5(α)-dihydrotestosterone.

This compound becomes bound to a receptor protein in the cytoplasm. When this complex has undergone a conformational change, it diffuses into the nucleus and binds to and activates the chromatin. The entire process is then culminated with transcription, translation, and protein synthesis.

LH can also promote synthesis of the E group of prost­aglandins in luteal tissues. Current evidence suggests that the LH enhances the activities of a cholesterol esterase and cholesterol acyl transferase, both of which are needed for the synthesis of the prostaglandin precursor, arachidonic acid.

The stimulatory effect of prolactin on protein synthesis needed for the development of the mammary gland is at the level of translation. Subsequent differentiation of the secretory cells of the gland requires Cortisol and insulin. The apparent cooperativity of these two hormones with prolactin in the gland’s development is due to their stimulation of the transcription process.

4. Growth Hormone (GH, Somatotropin):

Human growth hormone contains 191 amino acids, giving a molecular weight of 21,500 –

The overlap in hormonal action of growth hormone and prolactin is undoubtedly a reflection of the extensive homology in their amino acid sequences. The physiologically active “core” of human growth hormone appears to be the amino-terminal peptide fragment comprised of the first 134 amino acids.

Synthesis and Secretion:

The synthesis of growth hormone (GH) (and prolactin) in the acidophil cells of the anterior pituitary involves high-molecular-weight intermediates.

The formation and release of the growth hormone is regulated by a multi-control mechanism requiring four hypothalamic factors. Two of these are specific for GH, one being the releasing promoter and the other, a release-inhibiting factor.

GH release from the adenohypophysis can also be promoted by the thyrotropin releasing factor and by β-endorphin, a 16-amino acid peptide fragment derived from β-lipotropin. This compound is one of a group of peptides termed opioids because of their ability to bind opiate receptors.

Plasma levels of growth hormone show wide variations, irrespective of age of the individual. Beginning at a concentration barely detectable, a burst of secretion may result in a level of 60 ng ml^-1. The biologic half-life following secretion is short-25 to 30 minutes.

In addition to the more direct feedback controls of GH release modulated through the hypothalamus, secretion and clearing of the hormone will also be determined by the physiological state of the individual, particularly by food intake and energy expenditure.

For example, the rise in blood glucose subsequent to a meal clears the plasma of the hormone, whereas hypoglycemia promotes release. Another stimulus for reaction is exercise in the fasting state.

Biochemical and Physiologic Effects:

As a growth promoter, GH is an “anabolic” hormone. Insofar as normal growth and development are the composite of highly integrated processes, it is difficult to implicate growth hormone as affecting any specific or isolated reactions. However, with tissue preparations it can be demonstrated that GH stimulates membrane-bound adenylate cyclase.

As would be expected, administration of GH to an experimental animal results in the stimulation of RNA and protein synthesis in liver and peripheral tissues. These effects are manifest as a positive nitrogen balance. The somatotrophic action of GH complements the comparable anabolic effects of the androgens.

Administration of the hormone to an Experimental animal results first in an acute hypoglycemia, presumably because of a stimulation of insulin release from the cells of the pancreas. Further administration of GH leads to a hyperglycemia and glucosuria. The hyperglycemic effect is due to inhibited insulin-induced uptake of glucose by muscle as well as to a stimulation of glucagon release from the cells of the pancreas.

Both skeletal muscle and heart glycogen stores are increased under these circumstances. GH also stimulates mucopolysaccharide and collagen synthesis in connective tissue. In addition, the hormone promotes chondrogenesis and osteogenesis by effecting chondroitin sulfate synthesis.

Concomitant with its hyperglycemic effects, GH has a lipolytic action on adipose tissue resulting in a ketogenesis. Therefore, in the broad sense GH may be considered to be a diabetogenic hormone. The growth-enhancing activity of GH appears to be mediated by other anabolic factors found in the serum. These secondary factors are called somatomedins.

5. Melanotropin or Melanocyte-Stimulating Hormone (MSH):

Although the pars intermedia lobe of the pituitary is not well defined in humans, it has been established that this is the site of synthesis and secretion of melanocyte- or melanophore-stimulating hormone (MSH). Two forms of the hormone, α and β, are produced in mammals.

In monkey, horse, beef, pig, sheep, and camel, α-MSH contains 13 amino acids and all in the same sequence. This sequence is homologous with the order of the first 13 amino acids of ACTH from each of these species. It is reasonable, therefore, that ACTH should have MSH activity.

The β-MSH from these species contains 18 amino acids, whereas the human hormone has 22 residues –

Ala-Glu-Lys-Lys-Asp-Glu-Gly-Pro-Tyr-Arg-Met-Glu-His-Phe-Arg-Trp-Gly-Ser-Pro-Pro-Lys-Asp

The sequence of a seven-amino acid core of the β-MSH is also found in α-MSH, ACTH, and in the β and g forms of lipotropin.

Secretion:

The synthesis of α- and β-MSH in the adenohypophysis and their release are under the dual control of a releasing factor and a release-inhibiting factor, both produced in the hypothalamus. Hyper-secretion of MSH (and ACTH) will be seen in any hypofunctional condition of the adrenal cortex, as in Addison’s disease with lowered levels of circulating adrenal corticoids, feedback inhibition of MSH (and ACTH) release is impaired.

Biochemical Function of MSH:

The melanin-producing melanocytes are found between the cells of the stratum basale at the junction of the epidermis and dermis. The melanocytes have many fine dendritic processes penetrating the surrounding basal cells and these provide the means for melanin transport.

The sequence of synthetic reactions that is stimulated by MSH is the following:

Although in isolated cells it can be demonstrated that the administration of β-MSH causes an increase in intracellular cyclic AMP and tyrosinase activity, the precise site of MSH action has not been determined.

Native skin colour is not dependent on the number of melanocytes per unit area of skin but instead on the differences in rates of melanin production. For example, blacks and whites have the same number of melanocytes.

Term Paper # 3. Neurohypophysis or Posterior Pituitary:

As a functional unit, the neurohypophysis should be regarded as including the supraoptic and paraventricular nuclei of the hypothalamus, the nerve fibers of the pituitary stalk, and the posterior lobe of the pituitary gland. The two neurohypophysial hormones, vasopressin and oxytocin, are synthesized in the perikaryon of the nerve cells comprising the supraoptic and paraventricular nuclei.

After combining with a protein carrier to form granules, they move down the axons of the stalk to be accumulated and stored in the posterior lobe of the pituitary. Hence, in contrast to the anterior pituitary, the neurohypophysis is not itself an endocrine gland but instead serves as a reservoir for the two hormones to be secreted. Upon appropriate external stimuli, nerve impulses originate in the hypothalamus and move down the same axons that carry the hormone granules.

These nervous impulses, upon reaching the ends of the axons, trigger the release of the hormones into the bloodstream. Therefore, the mode of release of the posterior pituitary hormones is different from the mechanism of release of the anterior pituitary hormones. It will be recalled the secretion of the latter is regulated by soluble hypothalamic factors that reach the anterior pituitary via a portal system of blood vessels.

i. Vasopressin:

Human vasopressin is a nonapeptide with the following structure:

Synthesis:

The synthesis of vasopressin in the hypothalamus begins with the production of a protein with a molecular weight of approximately 20,000. As transport of this protein via the axons to the posterior lobe proceeds, it gives rise to smaller peptides, about 10,000 in molecular weight, which have been termed neurophysins. These proteins are the precursors of both vasopressin and oxytocin, and it is believed that a specific neurophysin is associated with each hormone.

Completion of the synthesis of vasopressin is accomplished as its specific neurophysin is transported down the axon of the pituitary stalk to the neurohypophysis. The derivation of a nonapeptide from precursors or prohormones with molecular weights of 10,000 to 20,000 may involve proteolytic excision(s) analogous to the conversion of proinsulin to insulin.

Secretion:

The secretion of the hormone is stimulated when the osmolarity of the extracellular fluid is increased. As a result, water reabsorption in the kidney is also increased and urine becomes more concentrated and is excreted in smaller volumes, hence the synonym “antidiuretic” hormone.

The original name, vasopressin, came into use following the discovery that crude extracts of neurohypophysis can raise blood pressure. It is now thought that this action of the hormone may not be physiologically important since it requires concentrations much higher than the levels that elicit the diuretic effect.

In experimental animals it can be demonstrated that an elevation of plasma osmolarity by only 1 to 2 percent will give an antidiuretic response. This does not happen when the neurohypophysial system has been injured. Changes in plasma volume also affect vasopressin release and can actually bring about an antidiuresis even when the plasma osmolarity is low.

For example, with an acute hemorrhage there is usually an antidiuresis. These results suggest that there are volume receptors or osmoreceptors in the area of the lobe (the supraoptic nuclei) served by the internal carotid artery and that these receptors trigger vasopressin release.

Secretion of vasopressin can be strongly influenced by drugs. For example, the diuresis associated with consumption of alcohol is believed to be due to the inhibition of hormone release. On the other hand, secretion can be stimulated by agents such as morphine and nicotine.

Biochemical and Physiologic Action of Vasopressin:

As in the case of the other peptide hormones, the biochemical action of vasopressin in the kidney begins with its binding to specific receptors on the target cells. The molecular events that follow result in an increase in the permeability of the cells of the collecting ducts and enhanced reabsorption of water.

One plausible postulate for the antidiuretic activity of vasopressin is that its —S—S— residue is capable of oxidizing two —SH groups in the cell membrane, thereby causing a conformational change and increased permeability. Arguing against this hypothesis is the finding that synthetic analogs of vasopressin lacking an —S—S— linkage also have antidiuretic activity.

Presently it is believed that vasopressin activity depends on its ability to enhance synthesis of cyclic AMP in the cells of the distal tubules. Obviously, the consequences of this activation of the adenyl cyclase at the contra luminal cell surface must be transmitted to the luminal side of the tubule, where water reabsorption occurs. It can be postulated that phosphorylation of the luminal membrane changes its structure and, as a consequence, its permeability to water.

Any decrease in the level of vasopressin seen in impaired neurohypophysial function results in production of large volumes of dilute urine. This condition is called diabetes insipidus.

ii. Oxytocin:

Like vasopressin, oxytocin is a nonapeptide with an —S—S— linkage –

It will be noted that the structures of the two hormones differ in only two amino acid residues, numbers 3 and 8. The overall similarity between the two hormones is also reflected by common structural requirements for their activity –  for example, the cyclic —S—S— structure, the proline in position 7, and the amide groups on the glutamate and aspartate. With synthetic organic techniques now available, it has been possible to correlate physiologic activities and structures of a number of analogs.

Secretion:

Stimuli that would be expected to effect selective release of vasopressin and oxytocin instead appear to cause their simultaneous release from the posterior pituitary.

The factors causing oxytocin release coincident with cervical dilatation and the onset of labour are not yet fully understood. The ejection of milk from the mammary gland involves a neurohumoral reflex response to suckling. The afferent side of this reflex is nervous and the efferent side is triggered by oxytocin.

Biochemical and Physiologic Action:

The myometrium, the uterine muscle, shows an increased sensitivity to oxytocin in the last few weeks of pregnancy preceding labour. The molecular events associated with influence of oxytocin on myometrial fibers are not yet delineated. The same is true for the effects of oxytocin on the smooth muscle, the myoepithelium, in the breast.