Heterogeneity and subcellular localization of hamster adipocyte α‐adrenergic receptors

IntroductionHamster white fat cells are known to display, like the human adipocytes [ 11, a mixed (Y-and fl-adrenergic sensitivity [2,3], which set these cells apart from the more frequently used and almost exclusively P-sensitive rat white adipocytes [4].Methods measuring the binding of the labeled antagonist [3H]dihydroergocryptine have been successfully used for the direct identification of cr-adrenergic receptors in different tissues [5--g].Applying these binding techniques to a 'crude' membrane fraction prepared from hamster adipocytes, we have identified and characterized specific binding sites for [3H]DHEC in this preparation [9].Although most of the characteristics expected of true physiological a-adrenergic receptors were fulfilled by these binding sites [9], some of their properties suggested that these sites may be heterogeneous or that only a part of them may represent the true a-receptor sites.This has led us to reexamine in detail some of the properties of these [3H]DHEC binding sites.We report here the subcellular localization of these sites, the results of competition experiments using two highly selective at-and olz-adrenergic antagonists, prazosin and yohimbine [lo-111 and finally binding data obtained with membranes treated under conditions shown to induce an irreversible block of the fat cell physiological a-responsiveness.We show that [3H]DHEC binding sites are almost exclusively