Posterior Pituitary Disorders

Arginine vasopressin (AVP) and oxytocin are the principal hormones secreted from the posterior pituitary. The two major stimuli of oxytocin secretion are suckling during lactation and dilation of the cervix during labor. Although not essential for initiation of labor, oxytocin can be used pharmacologically to initiate labor or control postpartum hemorrhage and uterine atony. Rarely, it has been used to induce milk ejection. The physiologic role of oxytocin in males is not known. AVP differs from oxytocin by only one amino acid. AVP is found in all mammals except pigs and related species, in which lysine vasopressin replaces AVP. In humans and many mammals, AVP and oxytocin are associated with two neurophysins, the exact roles of which are not known, except as carrier proteins in storage and transport of posterior pituitary hormones (Mooradian and Morley, 1988).

Antidiuretic hormone (vasopressin) is synthesized in the hypothalamus and migrates down into the posterior lobe of the pituitary to be stored and later secreted. Some ADH is secreted directly into the cerebrospinal fluid (CSF) rather than the posterior pituitary. Thus, pathologic lesions affecting the hypothalamus below the median eminence may preserve some functional ADH that migrates from the CSF into the systemic circulation. The half-life of AVP in circulation is only 20 minutes because of its susceptibility to peptidases. Loss of the terminal amino group in position 1 makes this peptide resistant to degradation, whereas substitution of the levo analog of arginine for dextroarginine in position 8 reduces presser effect without altering its antidiuretic properties. The resultant peptide deamino-8-D-argenine vasopressin (DDAVP) is currently the treatment of choice for central diabetes insipidus.

The biologic effects of AVP are initiated at two receptors, V1 and V2. The V1 receptors are located in the vascular system, and their stimulation results in vasoconstriction. The V2 receptors are located in the kidneys, and their stimulation results in free-water reabsorption (Korbonits and Carlsen, 2009). Plasma osmolality, blood volume, and blood pressure are the most important physiologic stimuli of AVP secretion. Other factors that modulate AVP secretion include pain, stress, nausea, hypoglycemia, hypercapnea, angiotensin II, atrial natriuretic hormone, and drugs. Many stimuli of AVP release also promote thirst. Thirst is less sensitive than AVP release in response to these stimuli and therefore is a second-line defense against dehydration.

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