DIFFERENTIAL ACTIVATION OF BETA(1)-ADRENOCEPTOR, BETA(2)-ADRENOCEPTORAND BETA(3)-ADRENOCEPTOR BY CATECHOLAMINES IN WHITE AND BROWN ADIPOCYTES

This review summarizes the experiments performed by various groups to determine how the activation of the different beta-adrenoceptors (beta -ARs) is ordinated when they are present in the same fat cell and invo lved in the same biological event. When expressed after the transfecti on of their genes in Chinese hamster ovary cell (CHO cells), beta(1) a nd beta(2)-ARs present a higher affinity for catecholamines than beta( 3)-ARs. In vitro, the lipolytic effect induced by low concentrations o f catecholamines in dog and rat white fat cells is due to the selectiv e activation of beta(1)- and/or beta(2)-ARs. Higher concentrations onl y are able to activate beta(3)-ARs. Similar results have been obtained in rat brown adipocytes. On the other hand, the lipolytic effect of c atecholamines in human and primate adipocytes does not involve a beta( 3)-AR component whatever the concentration used. In vivo experiments i n the dog have also shown that lipomobilization induced by low doses o f isoprenaline only involved beta(1)- and beta(2)-AR activation, this effect being blocked by beta(1)-/beta(2)-antagonist pretreatment. Howe ver, in the same blockade conditions, perfusion of a 10-fold higher do se of isoprenaline revealed a beta(3)-AR contribution in the lipomobil izing effect. These data showed that brown and white adipocyte beta(3) -ARs possess a lower affinity for catecholamines than beta(1)- and bet a(2)-ARs and are only recruited by high concentrations of the amines. Thus, even if the beta(3)-AR plays an indisputable role in the white a nd brown adipose tissue of some species, its physiological status and its expression are still subject to debate in human white fat cells.