Characteristics of the phenylalkylamine binding site in canine cardiac sarcolemmal membranes.

We have investigated the phenylalkylamine binding site in canine cardiac sarcolemmal preparations using (-)-[3H]-desmethoxyverapamil as the labeled ligand. Radioligand binding experiments were carried out in 10 mM Hepes (Na+) buffer and 1 mM EGTA, at pH 7.4 and 20 degrees C. A single high affinity binding site for (-)-[3H]-desmethoxyverapamil was identified both by saturation and competition binding experiments. Several phenylalkylamine derivatives such as (-)-D600, (+)-D600, verapamil and (+)-desmethoxyverapamil completely inhibited (-)-[3H]-desmethoxyverapamil binding with the following order of potency: (-)-desmethoxyverapamil greater than (-)-D600 greater than verapamil greater than (+)-desmethoxyverapamil = (+)-D600. In contrast to this, ronipamil, a new long acting phenylalkylamine derivative, produced only a 70% inhibition. Diltiazem also completely inhibited (-)-[3H]-desmethoxyverapamil binding to canine cardiac sarcolemma while nifedipine displaced only 70% of binding. (-)-[3H]-desmethoxyverapamil binding was also inhibited by Ca++ and Mg++. These data suggest the presence of a saturable, reversible and stereoselective phenylalkylamine binding site in canine cardiac sarcolemmal preparations which may be a receptor for the phenylalkylamine Ca++ channel inhibitors.