ACTION OF GROWTH-HORMONE IN ADIPOSE-TISSUE

Growth hormone (GH) has diverse and still not fully elucidated effects on adipose tissue. Both preadipocytes and mature adipocytes possess s pecific GH receptors. GH may mediate its actions via these receptors, but some effects are indirectly mediated through the GH-mediated secre tion of insulin-like growth factor-I (IGF-I). IGF-I may then act back on the adipose tissue in an autocrine/paracrine manner. In primary cul tures of adipose precursor cells obtained from human or rat adipose ti ssue, GH is found to stimulate the proliferation of these immature cel ls and reduce their differentiation to mature adipocytes. During long- term incubation and in in vivo studies, GH has a pronounced lipolytic effect. Whether this lipolytic effect is a direct effect of GH or more indirectly mediated, for instance, via inhibition of the action of an tilipolytic compounds (e.g. adenosine, prostaglandins and insulin) is presently unknown. Finally, GH produces a pronounced inhibition of adi pose tissue lipoprotein lipase activity. This enzyme plays a main role for hydrolysing triglycerides in the blood circulation in the adipose tissue and then for triglyceride accumulation in adipose cells. Thus, GH inhibits adipocyte differentiation, reduces triglyceride accumulat ion and increases lipolyses - all mechanisms which reduce adipose tiss ue mass.