Characterization of alpha-1 adrenergic receptors in the heart using [sup 3H]WB4101: Effect of 6-hydroxydopamine treatment

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 B(max) of 2.57 fmol/mg of tissue (27.7 fmol/mg of protein). Adrenergic agonists competed for specific [³H]WB4101 binding in the order: (-)-epinephrine > (-)-norepinephrine >> (-)-isoproterenol. Stereospecificity of the [³H]WB4101 binding sites was also demonstrated with (-)-epinephrine, (Ki = 90) nM being 270 times as potent as (+)-epinephrine, (K₁ = 24 μM). Adrenergic antagonists competed for the binding in the order: WB4101 = prazosin >> 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 five 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 week 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. We conclude: 1) [³H]WB4101 selectively labels postsynaptic alpha-1 adrenoceptors in the rat heart; 2) there is a definite regional variation for cardiac alpha-1 adrenoceptors; and 3) 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.