Adrenergic regulation of adipocyte metabolism

Adipocytes can be readily isolated from intact adipose tissue. In adipocytes from hamster and human white adipose tissue it is possible to demonstrate beta, alpha 1, and alpha 2 adrenoceptors. Alpha 2 adrenoceptor activation inhibits while beta adrenoceptor activation stimulates cyclic AMP accumulation and lipolysis. The effects of catecholamines on cyclic AMP accumulation are mediated through regulation of adenylate cyclase activity, which is activated through beta adrenoceptors and inhibited through alpha 2 adrenoceptors. Activation of alpha 1 adrenergic receptors has been shown to be associated with elevations of cytosol calcium and increased turnover of phosphatidylinositol. In white adipocytes, the only known alpha 1 adrenergic effects are inhibition of glycogen synthase and stimulation of glycogen phosphorylase via mechanisms distinct from those by which cyclic AMP produces similar end effects. In brown adipocytes, alpha 1 adrenoceptor activation stimulates respiration. Thyroid hormones primarily regulate the sensitivity of adipocytes to beta-adrenergic amines while having little effect on alpha adrenoceptor sensitivity.