Peptide Hormones of the Posterior Pituitary

Oxytocin (OT) is a nonapeptide in which six amino acids form a ring closed by a disulfide bridge, while the ring itself forms an antiparallel pleated sheet. The tail portion of the peptide, composed of Pro-Leu-Gly-NHj, is also rigidly held in a folded conformation. Oxytocin causes the powerful contraction of some smooth muscles and plays a vital role in milk ejection (not to be confused with milk secretion, which is regulated by prolactin). It also has uterotonic action, contracting the muscles of the uterus, and is therefore used clinically to induce childbirth. 

Vasopressin occurs in two variations arginine-vasopressin (AVP) and lysine-vasopressin (LVP), in which Arg is replaced by Lys. The conformation of these hormones is almost identical to that of oxytocin, except that the terminal tail is con-formationally free and not held by the ring. The physiological role of the vasopressins is the regulation of water reabsorption in the renal tubules (i.e., an antidiuretic action). In high doses, they promote the contraction of arterioles and capillaries and an increase in blood pressure hence the name of these hormones. Because of their very similar structures, OT and VP overlap in a number of effects. 

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Oxytocin is a peptide hormone of the posterior pituitary gland. It stimulates the contractions of the pregnant uterus, which becomes much more sensitive to it at term. Patients with posterior pituitary disease (diabetes insipidus) can, however, go into labour normally. 

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