Activation and inactivation of single calcium channels in snail neurons.

Abstract

Activation and inactivation properties of Ca currents were investigated by studying the behavior of single Ca channels in snail [Helix pomatia] neurons. Methods described in a previous paper were used. A zero-phase digital filter was incorporated to improve the analysis of latencies to 1st 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.degree. 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.degree. 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 3-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 3-state model. A minimum of 4 states are evidently involved in activation. Some 4-state models may be eliminated from further consideration. A comprehensive model of Ca channel kinetics must await further measurements.