CHARACTERIZATION OF ALPHA-1 ADRENERGIC-RECEPTORS IN THE HEART USING [H-3]WB4101 - EFFECT OF 6-HYDROXYDOPAMINE TREATMENT

Abstract

To identify and characterize the cardiac .alpha.-adrenoceptors, a radioreceptor binding assay using the potent .alpha. adrenergic antagonist, [3H]WB4101 was performed in rat hearts. Specific [3H]WB4101 binding to rat left ventricular homogenates was saturable, reversible and of high affinity (Kd = 0.18 nM) with a Bmax [maximum number of binding sites] of 2.57 f[femto]mol/mg tissue (27.7 fmol/mg protein). Adrenergic agonists competed for specific [3H]WB4101 binding in the order: (-)-epinephrine > (-)-norepinephrine .mchgt. (-)-isoproterenol. Stereospecificity of the [3H]WB4101 binding sites was demonstrated with (-)-epinephrine, (Ki [inhibition constant] = 90) nM being 270 times as potent as (+)-epinephrine, (K1 [association constant] = 24 .mu.M). Adrenergic antagonists competed for the binding in the order WB4101 = prazosin .mchgt. yohimbine > (-)-propranolol. WB4101 and prazosin exhibited a markedly greater (2000 times) affinity for [3H]WB4101 binding sites than yohimbine. The affinities (pKi) of .alpha. agonists and antagonists for [3H]WB4101 binding sites in the rat heart closely correlated with their pharmacological potencies in the heart. Scatchard analysis for [3H]WB4101 binding, performed in 5 regions from control and 6-hydroxydopamine-treated rat hearts, revealed specific [3H]WB4101 binding (Bmax) significantly greater in the ventricles and intraventricular septae than in atria. At 1 wk after 6-hydroxydopamine treatment, there was a significant increase (40%) in the Bmax for [3H]WB4101 binding to ventricles and intraventricular septae without a change in Kd. Apparently [3H]WB4101 selectively labels postsynaptic .alpha. adrenoceptors in the rat heart; there is a definite regional variation for cardiac .alpha.-1 adrenoceptors; and 6-hydroxydopamine treatment caused a significant increase in the density of .alpha.-1 adrenoceptors in ventricles and intraventricular septae, compatible with a postsynaptic localization of the [3H]WB4101 binding site.