title: Growth hormone: its physiology and control.
abstract: Growth hormone (GH) is a protein hormone produced by the somatotrophs of the anterior pituitary gland of birds and other vertebrates. The secretion of GH in birds is under hypothalamic control; it involves three peptidergic releasing factors: growth hormone-releasing factor (GRF) (stimulatory); thyrotropin-releasing hormone (TRH) (stimulatory); and somatostatin (SRIF) (inhibitory). In addition, there is evidence for effects of biogenic amines (including serotonin and norepinephrine) and prostaglandins at the level of the hypothalamus and possibly also the pituitary gland. In all avian species examined, plasma concentrations of GH are high in young posthatching chicks but low in the adult and embryo. The difference in plasma concentrations of GH between young and adult birds is due to both greater GH secretion and reduced clearance. The lower secretion of GH in adult birds reflects fewer somatotrophs in the pituitary, changes in somatotroph structure, and reduced GH responses to TRH or GRF administration. There is only limited data on the role of GH in birds. GH appears to be required for normal growth; acting at least in part by increasing somatomedin production. However, plasma concentrations of GH do not necessarily correlate with growth rate. For instance, in chicks with reduced growth rate owing to either goitrogen or protein deficiency in the diet, plasma concentrations of GH are elevated. GH also can influence lipid metabolism by increasing lipolysis, decreasing lipogenesis, and stimulating the uptake of glucose by adipose tissue. The physiological significance of these actions is, however, not established. In addition, GH affects the secretion of other hormones, the immune system, and perhaps also the reproductive system.
mesh_terms: D000375:Aging; D000818:Animals; D001717:Birds / Q000254:growth & development / Q000502:physiology*; D055598:Chemical Phenomena; D002621:Chemistry; D004777:Environment; D013006:Growth Hormone / Q000032:analysis / Q000097:blood / Q000378:metabolism / Q000502:physiology*; D007031:Hypothalamus / Q000502:physiology; D050356:Lipid Metabolism; D008970:Molecular Weight; D018377:Neurotransmitter Agents / Q000502:physiology; D010903:Pituitary Gland, Anterior / Q000378:metabolism; D011453:Prostaglandins / Q000502:physiology
chemical_list: D018377:Neurotransmitter Agents; D011453:Prostaglandins; D013006:Growth Hormone