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SECOND HORMONES OF THE POSTERIOR PITUITARY GLAND
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The posterior pituitary is an extension of the
hypothalamus and contains the axon terminals of magnocellular neurons located
in the Supraoptic and paraventricular nuclei. These neurons generate and
propagate action potentials, producing membrane depolarization and exocytosis
of the contents of their secretory granules. The neuropeptides oduced by the
magnocellular neurons, and consequently released from posterior pituitary, are
oxytocin and arginine vasopressin
1. Oxytocin :
The neuropeptide oxytocin is synthesized by
magnocellular neurons in the supraoptic and paraventricular nuclei of the
hypothalamus and is released from the posterior pituitary into the peripheral
circulation. The release of oxytocin is stimulated during breast-feeding
(lactation)
and childbirth (parturition)(42).
The two main target organs for oxytocin's
physiologic effects are the lactating breast and the uterus during pregnancy.
In the lactating breast, oxytocin stimulates milk ejection by producing
contraction of the myoepithelial cells that line the alveoli and ducts in the
mammary gland. In the pregnant uterus, oxytocin produces rhythmic contractions
to help induce labor and to promote regression of the uterus following
delivery. Oxytocin is used clinically during labor and delivery to promote
uterine contractions and during the postpartum period to help decrease bleeding
and return the uterus to its normal size (uterine involution) (42).
2. Arginine Vasopressin (AVP)
Arginine vasopressin, also known as antidiuretic
hormone (ADH), the other neuropeptide
produced by magnocellular neurons of the hypothalamus and released from the
posterior pituitary. The principal effect of AVP is to increase water
reabsorption by enhancing permeability to water in the distal convoluted
tubules and the medullary collecting ducts in the kidney. The result is the
production of smaller volumes of concentrated urine. In addition, AVP increases
vascular resistance.
This function of AVP may be important during periods of severe lack of
responsiveness to other vasoconstrictors, as may occur during severe blood loss
(hemorrhagic shock) or systemic infection (sepsis)(43).
The
importance of AVP is better understood in terms of the total amount of urine
that would be. excreted in its absence. For example, in a healthy individual an
average of 180 L of glomerular filtrate is formed per day. Thus, without
AVP-mediated reabsorption of 10% of filtered water in the distal collecting
ducts, urine output would be close to 18 L/d. This is 10-fold higher than the
volume of urine output (1.5-2 L/d) under normal conditions(43). 
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