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Voltage-dependent decrease in the availability of single calcium channels by nitrendipine in guinea-pig ventricular
1Department of Physiology, School of Medicine, Juntendo University, Tokyo, Japan.
Abstract:
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.
Insights
Nitrendipine blocks calcium channels by prolonging unavailable states. This drug binding mechanism affects channel availability and alters state transition rates in guinea-pig ventricular cells.
Area of Science:
- Cardiovascular Pharmacology
- Ion Channel Physiology
Background:
- Calcium (Ca2+) channels are crucial for cardiac function.
- Nitrendipine is a known calcium channel blocker.
Purpose of the Study:
- To elucidate the mechanism of calcium channel block by nitrendipine.
- To investigate the effects of nitrendipine on channel gating kinetics.
Main Methods:
- Single-channel patch-clamp recordings from guinea-pig ventricular cells.
- Utilized barium (Ba2+) as the charge carrier.
- Applied voltage-clamp protocols with depolarization pulses.
Main Results:
- Nitrendipine shifted the availability-voltage relationship hyperpolarizing.
- The drug shortened the mean duration of the available channel state.
- Nitrendipine prolonged the slow component of the unavailable state duration.
Conclusions:
- The mechanism of nitrendipine-induced calcium channel block involves prolonged unavailable states.
- Drug binding during depolarization and maintained during holding potentials underlies the blockade.
- Rate constants for state transitions were quantified.