Two important hormones are secreted from the posterior pituitary in both males and females. These are vasopressin (VP), the antidiuretic hormone, and oxyto cin (OT), which in females acts as the milk ejection factor. Both are nonapeptides, closely related in structure, and apparently derived from the same ancestral gene.

Although neuronal cells that produce OT and VP share the same locations in the brain, OT neurons predominate in one region and VP neurons predominate in another. Also, OT and VP are the secretions of separate neurons. OT and VP are transported to the nerve ending of a specific neuron, stored there, and released upon appropriate differential stimulation. The nerve endings that store these secretions are located in the posterior pituitary.

The primary recognized function of VP is to stimulate the reabsorption of water from the distal tubular kidney. This is clearly established because in the absence of VP up to as much as 25 liters of diluted urine can be excreted each day. Accordingly, the release of VP is generated by the need to maintain the blood osmolarity of plasma within strict limits (homeostasis), especially with reference to increased Na+ concentrations in blood produced by the ingestion of NaCl. VP secretion also is increased when blood volume or blood pressure is decreased. The sites of VP synthesis in the hypothalamus appear to be close to the osmoreceptor sites, which sense changes in electrolyte (solute) concentrations in circulation and signal release of the hormone from neuronal terminals in the posterior pituitary. The osmoreceptor is close to the thirst center in the hypothalamus and also interacts with the renin-angiotensin system. Collectively, these systems appear to be the primary elements for regulation of water balance (see also Chapter 15).


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