Determination of adrenoceptors of the alpha 2-subtype on isolated human fat cells.

Binding state for the selective alpha 2-adrenergic antagonist (3H)-yohimbine on isolated intact human subcutaneous abdominal fat cells is described in the present study. Specific binding was rapid, reversible, saturable and stereospecific. Adrenergic antagonists competed with (3H)-yohimbine binding in the order indicative of alpha-receptors. Competition studies with adrenergic antagonists revealed that (3H)-yohimbine binding sites displayed properties consistent with those of alpha 2-subtype. Saturation experiments demonstrated that (3H)-yohimbine specific binding was of high affinity with a dissociation constant (Kd) of 3.7 +/- 0.4 nmol/l and a maximum occupancy (BO) of 10.5 +/- 2.1 pmol/10(7) cells. The latter corresponded to approximately 600,000 alpha 2-receptors/fat cell. A significant (P less than 0.002) linear relationship was found between the fat cell volume and BO. A complete saturation experiment for the determination of Kd and BO could be performed using 0.5 g adipose tissue (0.5 ml isolated fat cells), which is 30 times less required when adipocyte membranes are used for these studies. It is concluded that the method can be used for the study of alpha 2-receptor properties and regulation in subcutaneous adipose tissue in a given individual.