[Adrenergic lipolysis in human fat cells: properties and physiological role of alpha-adrenergic receptors (author's transl)].

Lipolytic activity of human isolated fat cells from different fat deposits was studied. The purpose of the present investigations was to determine the epinephrine responsiveness, with regard to alpha- and beta-adrenergic receptor site activity, of omental and subcutaneous adipocytes (abdominal or from the lateral part of the thigh). Adipocytes were obtained from normal subjects or from obese subjects on iso- or hypocaloric diets. The lipolytic effect of epinephrine varied according to the fat deposits, while the beta-lipolytic effect of isoproterenol was more stable (Fig. 1). We explored the possible involvement of adrenergic alpha-receptors, in order to explain these results. The potentiating action of phentolamine on epinephrine-induced lipolysis, and the antilipolytic effect of alpha-agonists on basal or theophylline--induced lipolysis, were found to be a good indication of alpha-adrenergic activity. The alpha-adrenergic antilipolytic effect was most prominent in adipose tissue from the lateral part of the thigh, and less noticeable in omental adipocytes. In conclusion, the inability of epinephrine to induce lipolysis, and the epinephrine-induced inhibition of lipolysis observed when the basal rate of FFA release was spontaneously increased in subcutaneous fat-cells of the thigh, could be explained by an increased alpha adrenergic responsiveness (Fig. 2). Moreover, various alpha-adrenergic agonists (phenylephrine, noradrenaline and adrenaline) showed a clear inhibiting effect on theophylline-stimulated adipocytes from the thigh. The pharmacological study of the antilipolytic effect of epinephrine on theophylline-induced lipolysis showed that the inhibition was linked to a specific stimulation of the alpha-receptors of the subcutaneous adipocytes (Fig. 4). From the different sets of experiments, it is shown that the modifications in the lipolytic effect of epinephrine on adipocytes of different areas could be explained by the occurrence of a variable alpha-adrenergic effect initiated by catecholamine. Furthermore, theophylline stimulation of lipolysis provides an accurate system to investigate the alpha-inhibiting effect of catecholamines. Our study was completed by the investigation of the lipolytic activity of subcutaneous fat cells from obese subjects submitted to a hypocaloric diet (800-1 000 Cal/day). An increased alpha-inhibitory effect of epinephrine was shown on the increased basal lipolytic activity observed in the fat cells of obese subjects on a hypocaloric diet (Fig. 5); a similar effect was observed when these adipocytes were stimulated by theophylline. To conclude, these investigations allow the alpha-adrenergic effect to be considered as a regulator mechanism of the in vitro lipolytic activity in human adipose tissue, since the antilipolytic effect is operative whenever the basal rate of lipolysis is increased (spontaneously, after caloric restriction, or with a lipolytic agent such as theophylline).