Kinetic properties of the cardiac T‐type calcium channel in the guinea‐pig.

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Abstract
The kinetic properties of T-type Ca2+ channels were examined in single ventricular cells from guinea-pig hearts using the cell-attached configuration of the patch-clamp technique. T-type Ca2+ channel activity has been observed in 44 out of 139 patches. The density of these channels was estimated at 0.1-0.3 .mu.m-2. The T-type Ca2+ channel responds to a depolarizing voltage step either with a burst of openings which appears with a distinct delay or with no openings at all. The mean number of bursts per record for the records showing channel activity is 1.1. The probability of observing a blank sweep is high and amounts to 0.65 .+-. 0.02 (n = 26). With 110 mM-Ca2+ in the pipette solution, the slope conductance calculated from the current-voltage relationship of the single-channel current in the range between -50 and +10 mV is 6.8 pS. Openings to a subconductance level of about 50% of the main level could be resolved. All possible transitions between the subconductance and the main level were observed, indicating that the cardiac T-type Ca2+ channel possesses a substrate. The macroscopic steady-state activation and inactivation, as determined from ensemble-averaged currents, could be described by Boltzmann functions. Half-maximal activation and inactivation occur at -14 and -60.7 mV, the slope parameters of these curves are 10.8 nd 5.6 mV respectively. The maximum (peak) open probability is 0.15. The ensemble-averaged current decays monoexponentially. The time constant is strongly voltage dependent and decreases at less negative potentials. The open times are monoexponentially distributed. The mean open time of the channel does not depend on either the holding or the test potential, and has a mean value of 1.4 ms. The distribution of the closed times is biexponential. The fast mean closed time is also voltage independent with a mean value of 0.48 ms. The slow mean closed time increases with voltage from 1.9 ms at -40 mV to 8.8 ms to 0 mV. The mean burst duration also increases with voltage from a value of 4.9 ms at -40 mV to 13.9 ms at -10 mV. The convolution of the first-latency distribution with that of the burst duration closely fits the open probability calculated from the ensemble-averaged current. The mean first latency is also closely correlated with the macroscopic time constant of inactivation. It was strongly voltage dependent, and decreased from a value 41 ms at -50 mV to 9.1 ms to 0 mV. The microscopic kinetics of the T-type Ca2+ channel is characterized by a high and voltage-independent probability of reopening p = 0.8 .+-. 0.01 (n = 26). The probability f that the channel bypasses the open state is 0.84 at -40 mV, reaches a minimum value of 0.52 at -30 mV and increases again at more positive values (0.74at 0 mV). The number of openings per non-empty sweep is only weakly voltage dependent with a mean value of 4.7 .+-. 0.3 openings per sweep. The high probabilities with which the channel reopens and bypasses the open state can be described by a model in which inactivation occurs from a closed state. The strong correlation between first latency and time constant of macroscopic inactivation on the other hand indicate that inactivation might occur from the open state. Our analysis does not, however, discriminate between kinetic schemes with inactivation coupled to or independent of activation.