Activation and inactivation of single calcium channels in snail neurons.

Activation and inactivation properties of Ca currents were investigated by studying the behavior of single Ca channels in snail neurons. The methods described in the previous paper were used. In addition, a zero-phase digital filter has been incorporated to improve the analysis of latencies to first opening, or waiting times. It was found that a decrease in the probability of single channel opening occurred with time. This was especially marked at 29 degrees C and paralleled the inactivation observed in macroscopic currents. The fact that a single channel was observed means that there is a significant amount of reopening from the "inactivated" state. Small depolarizations at 18 degrees C showed little inactivation. From these measurements, histograms of single channel open, closed, and waiting times were analyzed to estimate the rate constants of a three-state model of activation. Two serious discrepancies with the model were found. First, waiting time distributions at -20 mV were slower than those predicted by parameters obtained from an analysis of the single channel closed times. Second, it was shown that the time and the magnitude of the peak of the waiting time histogram were inconsistent with a three-state model. It is concluded that a minimum of four states are involved in activation. Some four-state models may be eliminated from further consideration. However, a comprehensive model of Ca channel kinetics must await further measurements.