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Department of Physiology, School of Medicine, Juntendo University, Tokyo, Japan.

1. The mechanism of Ca2+ channel block by nitrendipine was studied by recording single-channel activity from cell-attached patches on guinea-pig ventricular cells using patch pipettes containing 50 mM-Ba2+. Test depolarization pulses to around 10 mV with a duration of 100 ms were applied repetitively at 2 Hz. 2. The percentage of non-blank sweeps was maximal (about 40%) at a holding potential between -65 and -130 mV and decreased sigmoidally with its depolarization. Nitrendipine shifted the availability-voltage relationship in a hyperpolarizing direction. 3. From the number of consecutive non-blank sweeps and that of blank sweeps, the duration of the available state and that of the unavailable state were estimated. 4. The histogram of the duration of the available state showed a single-exponential distribution. Its mean duration was about 1.5 s and was shortened by nitrendipine. Correspondingly, the decay of the mean current during the depolarization step was accelerated by nitrendipine. 5. In the presence of 100 nM-nitrendipine the histogram of the duration of the unavailable state at large negative holding potentials was simulated as the sum of two exponential components, one with a time constant similar to that in the control and the other with a time constant of 6-7 s. 6. The histogram of the duration of the unavailable state at depolarized holding potentials was simulated by a double-exponential curve also in the control. The duration of the slow component was prolonged by nitrendipine. 7. The prolongation of the unavailable states initiated by drug binding during depolarization steps and maintained during depolarized holding potentials is the mechanism of the blockade. The rate constants of the state transitions between an available state and two unavailable states were estimated.

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