Activation of the primary kinetic modes of large‐ and small‐conductance cholinergic ion channels in Xenopus myocytes.

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Abstract
1. The kinetic properties of single acetylcholine (ACh)-activated ion channels in tissue-cultured Xenopus myocytes have been examined in cell-attached patches. The rates of agonist binding and channel gating were inferred from the durations of open and closed intervals from channels exposed to 40 nM-200 .mu.M-ACh. The predominant kinetic forms of large- (.gamma.60) and small-conductance (.gamma.40) cholinergic channels were compared. 2. At high [ACh], bursts were defined so that they primarily reflect sojourns in activatable states. The probability that a channel is open without a burst (Po) increases between 2 and 200 .mu.M-ACh. Po is half-maximal at .apprx. 5 .mu.M for .gamma.40 channels and at .apprx. 25 .mu.M for .gamma.60 channels. 3. Open interval durations for .gamma.40 channels are distributed as the sum of two exponentials, with the slow component (.tau. .apprx. 2.8 ms) accounting for > 80% of the total. Open interval durations for .gamma.60 channels are often distributed as a single exponential with an apparent time constant of .apprx. 0.8 ms. For both conductance forms of channel, open interval durations show no significnat dependence on [ACh] in the range 0.04-10 .mu.M, but decrease at higher [ACh] in a manner consistent with channel block by agonist molecules. 4. Closed interval durations within bursts (2-100 .mu.M-ACh) for .gamma.60 or .gamma.40 channels are described by the sum of two or three exponentials. For both conductance forms of channel the apparent time constant of the fastest component is .apprx. 40 .mu.s and does not change significantly with [ACh], and the time constant of the predominant, slowest component (.tau.slow) decreases with increasing [ACh]. 5. For .gamma.40 channels, at high [ACh].tau.slow saturates at .apprx. 0.17 ms, while no saturation is apparent for .gamma.60 channel .tau.slow values up to 200 .mu.M-ACh. Below 50 .mu.M-ACh, .gamma.40 .tau.slow values are .apprx. 1.5 times shorter than .gamma.60 values. 6. Estimates of rate constants for agonist binding and channel gating were obtained by fitting closed interval durations in the range 2-100 .mu.M-ACh. .gamma.60 channels have a > 5-fold faster opening rate, .apprx. 10-fold faster closing rate, and .apprx. 3-fold lower affinity than do .gamma.40 channels. There is some indication of positive co-operativity of ACh binding to .gamma.40 channels.