Growth Hormone in the Aging Process

GH is the most abundant pituitary hormone. Although GH and prolactin are closely related, GH secretion depends upon hypothalamic stimulation, without which GH secretion falls to low levels and somatotrophs atrophy.Growth hormone-releasing hormone (GHRH), in full sequence a 44-amino-acid peptide, is the principal identified hypothalamic stimulator of pituitary GH synthesis and secretion, activating specific GHRH receptors on the surface of pituitary somatotrophs. GHRH and GHRH-related peptides have a very restricted distribution in the CNS but are also synthesized in gut, pancreas, and gonads, where their physiological roles are still uncertain. Somatostatin [somatotropin- release-inhibiting factor, (SRIF)], a group of 14- and 28-amino-acid peptides, is a potent noncompetitive inhibitor of GH secretion. As with LH and other pituitary hormones, the pattern of GH secretion is episodic,with six to eight pulses per day and very low levels between pulses. Some of these pulses are associated with meals, stress, exercise, or slow-wave sleep. The traditional view has been that the pattern of episodic GH secretion arises from the interaction of GHRH and SRIF secretion modulated by peripheral feedback by circulating IGF-I and other factors.