Normal Physiology of Growth Hormone in Normal Adults

Excerpt

Growth hormone (GH) is an ancestral hormone, secreted episodically from somatotroph cells in the anterior pituitary. Since the recognition of its multiple and complex effects in the early 1960s, the physiology and regulation of GH has become a major area of research interest in the field of endocrinology. In adulthood its main role is to regulate metabolism. Pituitary synthesis and secretion of GH is stimulated by episodic hypothalamic secretion of GH releasing factor and inhibited by somatostatin. Insulin-like Growth Factor I (IGF-I) inhibits GH secretion by a negative loop at both hypothalamic and pituitary levels. In addition, age, gender, pubertal status, food, exercise, fasting, sleep, and body composition play important regulatory roles. GH acts both directly through its own receptors and indirectly through the induced production of IGF-I. Their effects may be synergic (stimulate growth) or antagonistic as for the effect on glucose metabolism: GH stimulates lipolysis and promotes insulin resistance, whereas IGF-I acts as an insulin agonist. The bioactivity of IGF-I is tightly controlled by a multitude of IGF-I binding globulins. The mechanisms to explain the insulin antagonist effect of GH in humans are causally linked to lipolysis and ensuing elevated levels of circulating free fatty acids (FFA). The nitrogen retaining properties of GH predominantly involve stimulation of protein synthesis, which could be either direct or mediated through IGF-I, insulin, or lipid intermediates. In this chapter the normal physiology of GH secretion and the effects of GH on intermediary metabolism throughout adulthood, focusing on human studies, are presented. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.