Interaction of diltiazem with single L-type calcium channels in guinea-pig ventricular myocytes

The effects of diltiazem on cardiac L-type calcium channels were studied at the single channel level, using Ba2+ ions as the charge carrier. Patch clamp experiments were performed on enzymatically isolated guinea-pig ventricular myocytes. It was shown in cell-attached configuration on multichannel patches that diltiazem, when applied to the bath, can approach the calcium channel under the pipette after diffusion through the membrane phase. The time constant of the onset of the effect was 60 s. The rate of recovery seemed to be of the same order. Diltiazem had most prominent effect on calcium channel open state probability by reducing the frequency of openings, and by increasing the frequency of records without channel opening (nulls). The effect on mean open time was found to be insignificant at 1 kHz resolution. Diltiazem had no effect on the amplitude of unitary currents. These data are consistent with the assumption that diltiazem interacts mainly with the inactivated state (although interaction with the closed states was not ruled out), and does not bind to the open state of the calcium channel.