INTERCONVERSION BETWEEN DISTINCT GATING PATHWAYS OF THE HIGH THRESHOLD CALCIUM-CHANNEL IN RAT VENTRICULAR MYOCYTES

1. High-voltage-activated Ca2+ current (I(Ca)) waveforms in adult rat ventricular myocytes comprise two components, referred to here as I(Ca(fc)) and I(Ca(sc)) to denote the fast and slow components, respectively, of I(Ca) decay. At all test potentials, the two time constants of I(Ca) decay, tau(fc) and tau(sc), differ by approximately an order of magnitude. Neither tau(fc) nor tau(sc) varies appreciably with test potential, however, suggesting that current inactivation is not markedly voltage dependent. 2. Current activation at all test potentials follows a sigmoidal time course and is best described by a power function with n = 4. Deactivation of the currents, examined following variable length depolarizations to various test potentials, however, follows a single exponential time course. In addition, the kinetics of activation and deactivation Of I(Ca(fc)) and I(Ca(sc)) are indistinguishable. 3. Although both begin to activate at approximately - 30 mV, the voltage dependences of I(Ca(fc)) and I(Ca(sc)) are distinct: I(Ca(fc)) peaks at -10 mV and I(Ca(sc)) peaks at + 10 mV. 4. The relative amplitudes Of I(Ca(fc)) and I(Ca(sc)) vary with the holding potential from which the currents are evoked and with the frequency of current activation: hyperpolarized holding potentials and low stimulation frequencies reveal preferential activation Of I(Ca(fc)), whereas depolarized holding potentials and high stimulation frequencies potentiate I(Ca(sc)). In addition, the observed voltage- and frequency-dependent changes in I(Ca(fc)) and I(Ca(sc)) amplitudes are reciprocal. 5. The apparent voltage dependences of steady-state inactivation of I(Cs(fc)) and I(Ca(sc)) are also distinct. I(Ca(fc)) is reduced to approximately 50 % of its maximal amplitude at - 45 mV, whereas I(Ca(sc)) is approximately 50 % inactivated at -30 mV. 6. Recovery Of I(Ca(peak)) from steady-state inactivation follows a complex time course. Following inactivation at -10 mV, I(Ca(peak)) recovers at -90 mV to its maximal value over a biexponential time course; I(Ca(peak)) then decreases over the next several seconds to a steady-state level. 7. The time course of recovery from steady-state inactivation of I(Ca(fc)) at - 90 mV is best described by the sum of two exponentials; the two time constants of recovery differ by approximately a factor of 25. I(Ca(sc)), in contrast, recovers rapidly and over a single exponential time course to its maximal value. When the recovery time at -90 mV is increased, however, I(Ca(sc)) amplitude decreases slowly and over a single exponential time course to a steady-state level. The time courses of the slow increase in I(Ca(fc)) and the slow decrease in I(Ca(sc)) are similar, suggesting that the changes in the two current components are reciprocal. 8. From the experimental data, a model involving two parallel pathways for gating of a single type of high-voltage-activated Ca2+ channel was developed which was capable of generating currents with time- and voltage-dependent properties similar to those of I(Ca) measured in typical adult rat ventricular myocytes. 9. Experimental testing of the predictions of the parallel pathway gating model suggests that the experimentally observed time- and voltage-dependent interconversions between the two gating pathways only occur via the closed states of the channel.