1. A voltage-dependent gating mechanism for the fast Cl- channel was developed from the analysis of single-channel current records obtained with the patch clamp technique from primary cultures of rat skeletal muscle. Up to 106 open and shut intervals were analysed from each of five different excised patches of membrane containing a single fast Cl- channel. 2. Rate constants for a kinetic scheme with six closed and two open states (scheme I) were estimated at a given voltage by maximum likelihood fitting of open and closed dwell-time distributions obtained at that voltage. This procedure was then repeated for data obtained at each of three to eight different membrane potentials for each channel. 3. Plots of the estimated rate constants against membrane potential typically appeared linear on semilogarithmic co-ordinates, consistent with rate constants that are exponentially dependent on voltage. 4. Regression analysis of these plots yielded two parameters for each rate constant: the value of the rate constant at -50 mV (B) and its voltage sensitivity (A). The dwell-time distributions predicted with these parameters and scheme I gave a good description of the experimental dwell-time distributions at all the studied voltages, lending further support for an exponential dependence of rate constants on membrane potential. 5. Estimates of A and B were also obtained by simultaneously fitting dwell-time distributions obtained at three to eight different voltages, in order to better define these parameters. Predicted dwell-time distributions obtained with these estimates and scheme I could approach the theoretical best description of the data for discrete-state Markov models. 6. Eight to twelve of the fourteen rate constants in scheme I appeared voltage sensitive, with effective gating charges ranging from about -1.5 to + 1.0 units of electronic charge. 7. The estimated rate constants and their voltage sensitivities for the five analysed channels were generally similar, but showed some heterogeneity. 8. Gating mechanisms which had fewer kinetic states than scheme I, or equal and opposite effective gating charges over each transition barrier, or four or five identical and independent voltage-dependent subunits, all gave poorer descriptions of the data than scheme I. These simpler mechanisms were also ranked below scheme I by the Schwarz criterion, which applies a heavy penalty for additional free parameters. 9. These findings indicate that the voltage dependence of the fast Cl- channel is consistent with a kinetic scheme with six closed and two open states, in which a majority of transitions among the states are voltage dependent. If functional subunits are involved in the gating, then they are either not identical or interact co-operatively.
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