Alpha 2-adrenergic binding and action in human adipocytes. Comparison between binding to plasma membrane preparations and to intact adipocytes.

Binding of the alpha 2-adrenoceptor antagonist [3H]yohimbine was demonstrated on intact human adipocytes and on human adipocyte membranes. Specific binding was rapid, reversible, saturable and of high affinity in both preparations. [3H]Yohimbine binding was inhibited by various adrenergic agents in a manner which suggests that the labeled sites probably represent the alpha 2-receptor both in intact adipocytes and in the membrane fraction. In adipocyte membranes the maximal binding capacity (Bmax) was 463 +/- 38 fmol/mg protein and the binding was of high affinity with a Kd of 2.1 +/- 0.4 nM. In intact human adipocytes, Bmax was 903 +/- 139 fmol/10(6) cells (or 342 +/- 21 fmol/100 cm2) and the binding affinity was 6.6 +/- 0.8 nM. Adrenergic antagonists bound to a homogeneous class of receptors (linear Scatchard plots) both in isolated membranes and in intact adipocytes. However, agonist binding was heterogeneous in both preparations. The affinity of agonist binding was 5-10 times higher in membranes than in intact adipocytes. The physiological relevance of the binding data was evaluated by correlating the binding of yohimbine with the antilipolytic effect of clonidine. A positive and significant correlation was found between Bmax and maximal antilipolytic effect of clonidine (r = 0.70) in membranes. Furthermore, the binding affinity (Kd) was positively correlated to the sensitivity (IC50) of the clonidine-induced antilipolysis (r = 0.65). A positive and significant correlation was also found in intact adipocytes between Bmax and the maximal antilipolytic effect of clonidine (r = 0.79). However, there was no significant correlation between Kd in intact adipocytes and the IC50 of clonidine. It is concluded that both intact human adipocytes and membrane fractions are useful models to investigate the properties and regulation of alpha 2-adrenoceptors. However, it appears from our correlation studies that the binding data obtained with the membranes are the best related to the physiological effects mediated by alpha 2-receptors in human adipocytes.