Physiologic Gating Properties of Unitary Cardiac L-type Ca2+ Channels

The contraction of adult mammalian ventricular cardiomyocytes is triggered by the influx of Ca(2+) ions through sarcolemmal L-type Ca(2+) channels (LCCs). However, the gating properties of unitary LCCs under physiologic conditions have remained elusive. Towards this end, we investigated the voltage-dependence of the gating kinetics of unitary LCCs, with a physiologic concentration of Ca(2+) ions permeating the channel. Unitary LCC currents were recorded with 2mM external Ca(2+) ions (in the absence of LCC agonists), using cell-attached patches on K-depolarized adult rat ventricular myocytes. The voltage-dependence of the peak probability of channel opening (Po vs. Vm) displayed a maximum value of 0.3, a midpoint of -12 mV, and a slope factor of 8.5. The maximum value for Po of the unitary LCC was significantly higher than previously assumed, under physiologic conditions. We also found that the mean open dwell time of the unitary LCC increased twofold with depolarization, ranging from 0.53+/-0.02 ms at -30 mV to 1.08+/-0.03 ms at 0 mV. The increase in mean LCC open time with depolarization counterbalanced the decrease in the single LCC current amplitude; the latter due to the decrease in driving force for Ca(2+) ion entry. Thus, the average amount of Ca(2+) ions entering through an individual LCC opening ( approximately 300-400 ions) remained relatively constant over this range of potentials. These novel results establish the voltage-dependence of unitary LCC gating kinetics using a physiologic Ca(2+) ion concentration. Moreover, they provide insight into local Ca(2+)-induced Ca(2+) release and a more accurate basis for mathematical modeling of excitation-contraction coupling in cardiac myocytes.