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Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235.
1. Properties of Cl- channels from surface membranes of acutely dissociated rat cerebral cortical neurons were studied with the patch clamp technique. These channels were present in the majority of excised inside-out membrane patches. 2. Cl- channels were rarely observed in cell-attached membrane patches, and usually several minutes elapsed following excision of the patch before Cl- channels became active. 3. Under asymmetric ionic conditions (1000 mM-KCli, 140 mM-KClo), neuronal Cl- channels are fairly selective for Cl- over K+ and Na+, with permeability ratios, determined by reversal potential shifts of 4.8 for both PCl/PK and PCl/PNa. 4. Neuronal Cl- channel kinetic activity remained stable over periods of time long enough to collect up to 500,000 open and closed intervals. Occasionally, the channels entered altered modes of activity. In the 'buzz mode' the open and closed interval durations became much shorter than normal for several hundreds of intervals. In the 'subconductance mode' the channel opened to a current level about two-thirds of the normal level. 5. Using the method of maximum likelihood, sums of exponentials were fitted to the distributions of open and closed interval durations. Open interval distributions required at least two exponential components with time constants of less than 1 ms. At least six or seven exponential components were required to fit the closed interval distributions with time constants ranging from 30 microseconds to several hundreds of milliseconds. This suggests that neuronal Cl- channels enter at least two open and six or seven closed kinetic states during normal activity. 6. Cl- channels often entered long-duration closed states of several minutes which could not be accounted for by the sums of exponentials fitted to the distribution of closed interval durations. 7. Neuronal Cl- channels exhibit a marked voltage dependence with the percentage of time the channels are open increasing with depolarization. Most of the observed voltage dependence can be accounted for by a decrease in the mean closed interval duration with depolarization. The mean open interval was relatively independent of voltage. 8. These results suggest a high degree of similarity in kinetic behaviour and conductance properties between the fast Cl- channels of tissue-cultured rat skeletal muscle and fast Cl- channels in acutely dissociated rat cerebral cortical neurons.
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