Diltiazem facilitates inactivation of single L-type calcium channels in guinea pig ventricular myocytes

Diltiazem is a benzothiazepine Ca2+ channel blocker used clinically for its antihypertensive and antiarrhythmic effects. We studied the mechanism of diltiazem blockade by recording L-type Ca2+ channel currents from cell-attached patches in isolated guinea pig ventricular myocytes using Ba2+ as the charge carrier. With diltiazem (200 microM) in the superfusate, multichannel currents showed a use-dependent decline in amplitude reflecting reductions in the numbers of superpositions of channel openings. Analysis of single-channel currents revealed that both open and closed times were little affected by diltiazem (50 and 100 microM). However, the rate of decay of the averaged current during 150-ms depolarization steps was significantly accelerated and the open state probability in current containing-sweeps was significantly decreased by diltiazem, suggesting that the drug accelerates transition from the activated state to the inactivated state. The effect of diltiazem on the slow gating process was studied by repetitively applying 500-1000 step pulses at selected holding potentials. Decreased channel availability by diltiazem was reflected by the increasing number of blank sweeps per run at depolarized holding potentials. These results suggest that diltiazem reduces Ca2+ influx by accelerating inactivation during action potentials, and that the use-dependent blockade is due to increases in the number of channels in a sustained closed state.