NEUROENDOCRINE CONTROL OF GROWTH-HORMONE SECRETION IN HUMANS

The episodic secretion of growth hormone (GH) depends on the rhythmic alternation in the hypothalamic release of GHRH and somatostatin (SS) into the hypophyseal portal system. In turn, GH appears to maintain this rhythm by stimulating SS and inhibiting GHRH secretion. Central adrenergic pathways, by modulating SS secretion, seem to be the final mediator for most stimuli, including other neurotransmitters, modifying GH release. Glucocorticoids enhance GH gene transcription and facilitate GH gene expression; the latter effect also depends on physiologic plasma levels of thyroid hormones. Sex steroids mainly act on SS neurons, most likely by affecting the alpha-2-adrenergic transmission to them. Metabolic intermediates can also affect GH secretion: arginine and hypoglycemia inhibit SS release, whereas hyperglycemia and free fatty acids (FFA) stimulate it. In addition, a strong inhibitory effect of FFA on the somatotrophs also occurs.