alpha-Adrenergic receptors in human adipocyte membranes: direct determination by [3H]yohimbine binding

The presence of alpha 2-adrenergic receptors in membranes derived from human sc adipose tissue was directly demonstrated with a new alpha 2-selective ligand, [3H]yohimbine. Binding of this radiolabeled antagonist to adipocyte membranes was of high affinity (Kd = 3.9 +/- 2.4 nM) and saturable. Computer modelling of [3H]yohimbine saturation curves demonstrated that it binds to a homogeneous class of sites with a density of 145.0 +/- 33.8 fmol/mg protein. Adrenergic agonists competed with [3H]yohimbine in the order expected of alpha-receptors, and their binding was strongly influenced by guanine nucleotides. Competition of alpha-antagonists with this radioligand demonstrated yohimbine to be more potent than prazosin, indicative of alpha 2-receptors. Antagonist binding was unaffected by guanine nucleotides. Paired saturation curves in these adipocyte membranes with the alpha 2-selective [3H]yohimbine and the nonsubtype selective alpha-antagonist [3H]dihydroergocryptine demonstrated similar receptor concentrations. [3H]Dihydroergocryptine has been previously shown to label both alpha 3- and alpha 2-receptors with equal affinity. Therefore, these data indicate that the vast majority of alpha-receptors in human sc fat are of the alpha 2-subtype. [3H]Yohimbine with its alpha 2-selectivity and high specific binding will provide an excellent tool for the clinical investigation of human adipocyte alpha-receptor mechanisms in both normal and pathological states.