Fast and slow gating behaviour of single calcium channels in cardiac cells

Abstract

The Ca-channel gating behaviour during steady and stepwise depolarization was examined in recordings of single Ca-channel activity from cell-attached membrane patches of single ventricular cells isolated enzymatically from hearts of adult guinea pigs. The single-channel recordings were performed by means of the improved patch-clamp technique (Hamill et al. 1981) with 90 mM Ba in the pipettes. Upon step depolarization, two types of current records were regularly observed in the ensembles: (1) traces with Ca-channel activity (in the form of closely-spaced brief pulses of inward current with a unitary amplitude) of various length, and (2) blank sweeps without any detectable single-channel opening. The records with Ca-channel activity show a distinct tendency for openings to occur towards the beginning of the clamp pulse, followed by long periods of silence. The blank sweeps seem to reflect a condition or conditions where the Ca channel is unavailable for opening. The corresponding ensemble mean current I(t) displayed a rapid rising phase to its peak followed by a slow decay. During steady depolarization, kinetic analysis of the distributions of all open and shut lifetimes revealed a monoexponential probability density distribution function of all open times. By contrast, more than two exponential terms were required for an accurate description of the frequency distribution of all shut lifetimes. Corresponding to the two well-separated fast closed time components, individual Ca-channel openings were grouped into bursts of openings. The bursting behaviour reflected fast gating transitions and was related to the fluctuations of the Ca channel between two short-lived closed states and one open state. This fast gating was terminated by the entrance of the Ca channel into at least one long-lived closed state, exit from which was slow in comparison to the rapid cycling. As consequence, bursts of openings were further grouped together in clusters of bursts, the cluster behaviour being related to slow gating transitions in the kinetics of the Ca channel. The biphasic frequency distribution of the first latencies (resulting from the transit through the two short-lived shut states, before the open state is entered) superimposed on the first time derivative of the rising phase of the ensemble mean current, I(t), upon step depolarization. The time constant of the monoexponential distribution function of all cluster lifetimes matched the declining phase of I(t) during maintained depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)