Fat cell adrenoceptors: inter- and intraspecific differences and hormone regulation

The present review summarizes recent data on fat cell adrenoceptors with the aim of clarifying the role played by catecholamines in the regulation of adipocyte metabolism. Part of the review is focused on the possible interest of animal models for the study of catecholamine-mediated effects in human fat cells. It is now clearly demonstrated that human, hamster, dog and rabbit fat cells possess three basic types of adrenoceptor: the beta 1-, alpha 2- and alpha 1-adrenoceptors identified in biological assays or binding studies with selected radioligands. The rat is an exception in the species commonly studied as catecholamines exert an exclusive lipolytic effect through beta-adrenoceptor stimulation, there are no alpha 2-adrenoceptors in rat white fat cells although an alpha 1-adrenoceptor does exist. In human fat cells, physiological amines are lipolytic or antilipolytic. Binding studies have revealed that alpha 2-adrenoceptors are three to four times more numerous than beta 1-adrenoceptors. Moreover physiological amines, in particular epinephrine, have a higher affinity for alpha 2-sites than for beta 1-sites. Dose-response studies of the effect of epinephrine on adenosine-deaminase or isoproterenol-stimulated fat-cells demonstrate an inhibitory effect of epinephrine on lipolysis promoted by stimulation of alpha 2-adrenoceptors which occurs before the commonly described beta 1-adrenergic effect which promotes stimulation of lipolysis. This aspect and its putative physiological interest is described and discussed. Intraspecific variations in adrenergic responses of adipocytes have been briefly analysed. The appearance and disappearance of alpha 2-adrenoceptors according to the extent of adipose tissue and increment of fat cell size are discussed. Variations of adrenergic responsiveness during fasting, calorie restriction or chronic stimulation of the adipocytes by physiological amines are also discussed.