The human placenta--a rich source of beta-adrenergic receptors: characterization of the receptors in particulate and solubilized preparations.

A crude particulate fraction of human placenta possesses a high concentration of beta-adrenergic receptors, as determined by (-)-[3H]dihydroalprenolol binding (approximately 240 fmol/mg protein; Kd approximately 2 nM). The sites display all the typical characteristics of beta-adrenergic receptors, including rapid and reversible kinetics, saturability, and appropriate specificity and stereospecificity. Computer modelling of ligand binding data indicate that the binding of (-)-[3H]dihydroalprenolol to these sites conforms closely to the pattern anticipated for interactions of the ligand with a homogeneous class of receptors according to the law of mass action. The rejeptors are readily solubilized with digitonin, retaining their typical beta-adrenergic characteristics. The human placenta is likely to be a particularly useful source of beta-adrenergic receptors for purification because of its high receptor content and its ready availability in substantial quantities.