Growth Hormone Biologic Actions
hGH has well-defined growth promoting and meta-bolic actions. hGH stimulates the growth of cartilage and bone directly, through hGH receptors in those tissues, and indirectly, via local increases in IGF-1 (Fig. 3) (Bouillon 1991; Isaksson et al., 2000). Metabolic actions, which may be directly controlled by hGH, include the elevation of circulating glucose levels (diabetogenic effect) and acute increases in circulating concentrations of free fatty acids (lipolytic effect). Other hGH anabolic and metabolic actions believed to be mediated through increases in local or systemic IGF-1 concentrations include: increases in net muscle protein synthesis (anabolic effect); modulation of reproduction in both males and females; maintenance, control and modulation of lymphocyte functions; increases in glomerular filtration rate and renal plasma flow rate (osmoregulation); influences on the release and metabolism of insulin, glucagon, and thyroid hormones (T3, T4); and possible direct effects on pituitary function and neural tissue developmentLe Roith et al. 1991; Casanueva, 1992; Strobl and Thomas, 1994).