CHARACTERIZATION OF RADIOLABELED AGONIST BINDING TO BETA-ADRENERGIC RECEPTORS IN MAMMALIAN-TISSUES

Abstract

The molecular interactions of agonists with .beta.-adrenergic receptors differ from those of antagonists. Most of this evidence has come from studies of agonist inhibition of radiolabeled antagonist binding. Agonist binding was examined directly in rat lung membranes using radiolabeled hydroxybenzylisoproterenol (3H-HBI). Specific binding of 3H-HBI was stereoselective and was inhibited by catecholamines with a potency order characteristic of .beta.2-adrenergic receptors. Gpp(NH)p [5''-guanylylimidodiphosphate] increased the rates of association and dissociation of 3H-HBI from the receptor. In the absence of Gpp(NH)p, Scatchard plots were curvilinear suggesting a complex interaction of the agonist with the receptor. The total number of 3H-HBI binding sites was similar to that of 125I-IHYP [iodohydroxy benzyl pindolol] binding sites. In the presence of increasing concentrations of Gpp(NH)p, the affinity of 3H-HBI was decreased and Scatchard plots became linear. NaCl mimicked the effect of Gpp(NH)p in lowering the affinity of the receptor for 3H-HBI. MgCl2 had the opposite effect in that it promoted high affinity binding. The effect of NaCl was largely overcome by the presence of MgCl2.