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

Relation to activation and inactivation of calcium-channel current

Abstract

  1. (1)

    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.

  2. (2)

    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 currentI(t) displayed a rapid rising phase to its peak followed by a slow decay.

  3. (3)

    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.

  4. (4)

    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 ofI(t) during maintained depolarization. Thus, the decrease of the probability of channel opening and the resulting decline ofI(t) seemed to be due to a transition of the Ca channel into the long-lived third class of shut state(s).

  5. (5)

    The responsiveness of the Ca channel in a series of trials decreased at positive holding potentials with a sigmoidal dependence on the potential of the conditioning depolarization due to an increasing number of blank single-channel current records within the ensembles. Traces with channel openings as well as blank sweeps tended to form sequences of consecutive single-channel current records upon conditioning depolarization. The Ca channel was only activatable, if a command pulse was applied during the occupancy of either the open state or the two short-lived shut states. If the long-lived shut state(s) was already occupied at the conditioning potential preceding the step depolarization, the Ca channel was unavailable for opening and a blank sweep was observed upon the voltage pulse.

  6. (6)

    A quantitative patch-to-patch variability in Ca-channel gating behaviour was detected. It was interpreted as the statistical deviation from the average kinetic behaviour of a single population of Ca channels.

  7. (7)

    The total time course of the ensemble mean currentI(t) was reconstructed by a convolution of the first latency and cluster lifetime distribution functions. The peak amplitude ofI(t) was mainly determined by the steady-state occupancy of the activatable states of the Ca channel.

  8. (8)

    Under comparable experimental conditions (90 mM external Ba), the pooled average behaviour of individual Ca channels in different membrane patches was the same as the bulk behaviour of all the Ca channels in the cardiac cell.

  9. (9)

    The fast and slow Ca-channel gating transitions are discussed in terms of a channel-state model where, according to lifetimes and transition rates, the respective channel states are divided into two subsets.

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Author information

Correspondence to Dieter Pelzer.

Additional information

Supported by the Deutsche Forschungsgemeinschaft (DFG), SFB 38 (Membranforschung), project G1

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Cavalié, A., Pelzer, D. & Trautwein, W. Fast and slow gating behaviour of single calcium channels in cardiac cells. Pflugers Arch. 406, 241–258 (1986). https://doi.org/10.1007/BF00640910

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Key words

  • Single cardiac ventricular cells
  • Single Ca-channel gating behaviour
  • Reconstruction of ensemble average Ca-channel currents
  • Relation to whole-cell Ca-channel current