NORMAL CONTROL OF GROWTH-HORMONE SECRETION

Growth hormone (GH) is secreted by the anterior pituitary gland in a pulsatile fashion under the regulation of two hypothalamic peptides: GH-releasing hormone ( GHRH) stimulates GH synthesis and secretion while somatostatin inhibits GH release. Studies in rats, sheep and humans indicate that whereas GHRH is required for the initiation of GH pulses, the amplitude of GH pulses is modulated by somatostatin. In humans, these interactions result in a pattern of volleys of GH-secretory pulses with intervening periods of relative secretory quiescence. The amplitude and frequency of GH-secretory pulses are regulated by a complex array of external and internal stimuli including age, gender, menstrual cycle phase, pubertal status, nutrition, sleep, body composition and exercise. Changes in plasma concentrations of gonadal hormones, insulin and insulin-like growth factor-I likely mediate the effects of several of these factors. A greater understanding of the physiology of GH secretion will enable the development of future strategies to enhance GH secretion in GH-deficient states including the use of GH secretagogues and modification of nutrition and exercise habits.