High-affinity binding sites for [3H] verapamil in cardiac membranes.

The calcium channel blocker [3H]verapamil showed specific binding of high affinity (equilibrium dissociation constant, KD, of 4.25 nM) and low density (maximal number of binding sites, Bmax, 50 fmol/mg protein) in frog heart membranes. Nitrendipine, a new calcium channel blocker, had a potency comparable to verapamil in the inhibition of [3H]verapamil binding. Concentrations of Ca2+ in the medium exceeding 3 mM decreased [3H]verapamil binding. These findings are consistent with the concept that high-affinity binding sites for calcium channel blockers are associated with sarcolemmal calcium channels.