The binding of the calcium channel blocker, bepridil, to calmodulin.

Abstract Bepridil had the highest relative potency for inhibition of myosin light chain kinase (MLCK) activated by Ca 2+ -calmodulin of all the calcium channel blockers we examined. Kinetic analysis indicated that the primary effect of bepridil was mediated through a competitive inhibition of the enzyme activation by interaction with calmodulin and the apparent K i value of this agent was 2.2 μM. We then examined the binding of bepridil to calmodulin, using the equilibrium column binding technique. [ 3 H]bepridil bound to the calcium-calmodulin complex, but not to calmodulin in the presence of 2 mM EGTA. Scatchard analysis of the binding of bepridil to calmodulin demonstrated that the dissociation constant was 6.2 μM and the calculated number of specific binding sites was about 5 sites per molecule of calmodulin. The concentrations of unlabeled bepridil, W-7, prenylamine, verapamil and diltiazem producing 50% inhibition ( ic 50 ) of the binding of [ 3 H]bepridil to calmodulin were 4 μM, 28 μM, 45 μM, 130 μM and 700 μM, respectively. However, nifedipine and nicardipine did not displace [ 3 H]labeled bepridil from calmodulin. There was a good correlation between the displacement of [ 3 H]bepridil from calmodulin and the inhibitory effect on MLCK by these calcium channel blockers and W-7. These results suggest that bepridil binds to calmodulin in the presence of calcium and potently inhibits the phosphorylation of myosin light chain.