Determination of β 3 ‐Adrenoceptor Mediated Lipolysis in Human Fat Cells

The existence and relative importance of β 3 ‐adrenoceptors in man is still controversial. The aim of the present study was 1) to find further evidence for the existence of functional β 3 ‐adrenoceptors in human fat, and 2) to investigate factors that may influence this β 3 ‐adrenoceptor function. Fifty individuals were examined. Lipolysis mediated by the selective β 3 ‐adrenoceptor agonist CGP 12177 in omental fat cells correlated with the response in subcutaneous fat cells. However, lipolysis was more pronounced in omental as compared to subcutaneous adipocytes, the intrinsic activity for CGP 12177 was 41% and 33%, respectively, while dobutamine, terbutaline and norepinephrine were full agonists. Both the lipolytic response and the sensitivity to CGP 12177 correlated with the effects of norepinephrine in the omental fat cells (r 2 = 0. 68 and 0. 50, respectively, p=0. 0001). The β 2 ‐adrenoceptor mediated lipolytic response did also correlate with the responses induced by β 1 ‐ and β 2 ‐agonists and by postreceptor acting agents. The antagonistic properties (pA 2 ) of the β‐adrenoceptor subtypes were also investigated. The pA 2 for the selective β 1 ‐ and β 2 ‐adrenoceptor antagonists versus CGP 12177‐induced lipolysis were 2 to 3 log units lower than those for the β 1 ‐antagonist versus dobutamine or for the β 2 ‐antagonist versus terbutaline. Furthermore, bupranolol had a significantly better antagonistic effect (pA 2 7. 17, p<0. 001) on the CGP 12177‐induced lipolysis than had the β 1 ‐ and β 2 ‐adrenoceptor antagonists (pA 2 6. 26 and 6. 05, respectively). These data clearly support the existence of a third human β‐adrenoceptor. Several factors may contribute to the contradictory β 3 ‐adrenoceptor results in man. The sensitivity of the different lipolytic systems vary considerably. Omental fat cells are preferable to subcutaneous cells for β 3 ‐adrenoceptor studies in man. The β 3 ‐responses are more attenuated in isolated fat cell preparations than in tissue fragments. Furthermore, as the β 3 ‐adrenoceptor activity correlates to the norepinephrine activity, more pronounced effects will be expected in catecholamine sensitive subjects. At present, the number of tools available for β 3 ‐adrenoceptor studies are limited, and the receptor is hard to study, why it is essential to perform β 3 ‐adrenoceptor studies under optimal conditions in order to obtain conclusive effects.