Physiology Of The Posterior Pituitary

The posterior pituitary gland secretes two hormones. They are oxytocin, which means "rapid birth'' in reference to its action to increase uterine contractions during parturition, and vasopressin, in reference to its ability to contract vascular smooth muscle and thus raise blood pressure. Because the human hormone has an arginine in position 8 instead of the lysine found in the hormone that was originally isolated from pigs, it is called arginine vasopressin (AVP). Both oxytocin and AVP are nonapeptides and differ from each other by only two amino acid residues. Similarities in the structures of their genes and in their post-translational processing make it virtually certain that these hormones evolved from a single ancestral gene. The genes that encode them occupy adjacent loci on chromosome 20, but in opposite transcriptional orientation.

Each of the posterior pituitary hormones has other actions in addition to those for which it was named. Oxytocin also causes contraction of the myoepithelial cells that envelop the secretory alveoli of the mammary glands and thus enables the suckling infant to receive milk. AVP is also called antidiuretic hormone (ADH) for its action to promote reabsorption of ''free water'' by renal tubules (see Chapter 28). These two effects are mediated by different heptihelical receptors that are coupled to different G-protein-dependent second messenger systems. V1 receptors signal vascular muscle contraction by means of the inositol-trisphosphate-diacylglycerol pathway (Chapter 2), whereas V2 receptors utilize the cyclic AMP system to produce the antidiuretic effect in renal tubules. Oxytocin acts through a single class of G-protein receptors that signals through the inositol-trisphosphate-diacylglycerol pathway. Physiologic actions of these hormones are considered further in Chapters 28 and 47.

Oxytocin and AVP are stored in and secreted by the neurohypophysis, but are synthesized in magnocellular neurons whose cell bodies are present in both the supraoptic and paraventricular nuclei of the hypothalamus. Cells in the supraoptic nuclei appear to be the major source of neurohypophysial AVP, whereas cells in the paraventricular nuclei may be the principal source of oxytocin. After transfer to the Golgi apparatus, the oxytocin and AVP prohormones are packaged in secretory vesicles along with the enzymes that cleave them into the final secreted products. The secretory vesicles are then transported down the axons to the nerve terminals in the posterior gland where they are stored in relatively large amounts. It has been estimated that sufficient AVP is stored in the neurohypophysis to provide for 30-50 days of secretion at basal rates or 5-7 days at maximal rates of secretion. Oxytocin and AVP are stored as 1:1 complexes with 93-95 residue peptides called neurophysins, which are actually adjacent segments of their prohormone molecules. The neuro-physins are co-secreted with AVP or oxytocin, but have no known hormonal actions. The neurophysins, however, play an essential role in the post-translational processing of the neurohypophysial hormones. The amino acid sequence of the central portion of the neurophysins is highly conserved across many vertebrate species, and mutations in this region of the preprohor-mone are responsible for hereditary deficiencies in AVP that produce the disease diabetes insipidus (see Chapter 28) even though expression of the AVP portion of the preprohormone is normal.

As already discussed, AVP is also synthesized in small cells of the paraventricular nuclei and is delivered by the hypophysial portal capillaries to the anterior pituitary gland where it plays a role in regulating ACTH secretion. AVP is produced in considerably larger amounts in the magnocellular neurons and is carried directly into the general circulation by the veins that drain the posterior lobe. It is unlikely that AVP that originates in magnocellular neurons acts as a hypophy-siotropic hormone, but it can reach the corticotropes and stimulate ACTH secretion when its concentration in the general circulation increases sufficiently. Oxytocin, like AVP, may also be synthesized in parvocellular neurons at other sites in the nervous system and be released from axon terminals that project to a wide range of sites within the central nervous system. Oxytocin may also be produced in some reproductive tissues where it acts as a paracrine factor.

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