α-Adrenergic Receptors in Human Adipocyte Membranes: Direct Determination by [3H]Yohimbine Binding

Abstract

The presence of α₂-adrenergic receptors in membranes derived from human sc adipose tissue was directly demonstrated with a new α₂-selective ligand, [³H]yohimbine. Binding of this radiolabeled antagonist to adipocyte membranes was of high affinity (K_d =3.9 ± 2.4 nin) 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 α-receptors, and their binding was strongly influenced by guanine nucleotides. Competition of α-antagonists with this radioligand demonstrated yohimbine to be more potent than prazosin, indicative of α₂-receptors. Antagonist binding was unaffected by guanine nucleotides. Paired saturation curves in these adipocyte membranes with the α₂-selective [³H]yohimbine and the nonsubtype selective α-antagonist [³H]dihydroergocryptine demonstrated similar receptorconcentrations. [³H]Dihydroergocryptine has been previously shown to label both α₁-and α₂-receptors with equal affinity. Therefore, these data indicate that the vast majority of α-receptors in human sc fat are of the α₂-subtype. [³H]Yohimbine with its α₂-selectivity and highspecific binding will provide anexcellent tool for the clinical investigation of human adipocyte α-receptor mechanisms in both normal and pathological states.