Characterization of verapamil binding sites in cardiac membrane vesicles

Specific, saturable, and reversible binding of verapamil has been demonstrated in crude cardiac sarcolemmal membranes. These receptors possess a Kd of approximately 50 nM for verapamil as determined by either equilibrium binding studies, competition binding analysis, or kinetic analysis of on and off rates and display an average density of 1.25 pmol/mg of protein. Specificity of binding is indicated by several criteria. Competition studies with the verapamil analog D-600 indicate that (-)D-600 is 200-fold more potent than the (+)-isomer in displacing bound verapamil. Likewise, several other aryl alkyl amine Ca2+ entry blockers effectively displace bound ligand. In addition, dihydropyridines and diltiazem promote partial (25-35%) displacement of bound verapamil with Ki values similar to the Kd values for their respective receptors. Characterization of nitrendipine binding in this preparation indicates an average density of 0.3 pmol of receptors/mg of protein suggesting that the verapamil:nitrendipine binding site ratio is approximately 4:1. Binding characteristics of verapamil and nitrendipine receptors in highly purified sarcolemmal vesicles are similar to those in the crude preparation except that the ratio of verapamil:nitrendipine sites approaches 1 and nitrendipine and diltiazem promote almost complete displacement of bound verapamil. Fractionation studies of crude sarcolemmal membranes indicate that excess verapamil receptors, insensitive to the action of dihydropyridines or diltiazem, are located in a high-density, nonmitochondrial, non-sarcolemmal membrane fraction. Thus, verapamil receptors exist in two locations in cardiac tissue but only in the sarcolemmal membrane are these receptors coupled to the dihydropyridine receptor.