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

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 microM-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 within a burst (Po) increases between 2 and 200 microM-ACh. Po is half-maximal at approximately 5 microM for gamma 40 channels and at approximately 25 microM 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 approximately 2.8 ms) accounting for greater than 80% of the total. Open interval durations for gamma 60 channels are often distributed as a single exponential with an apparent time constant of approximately 0.8 ms. For both conductance forms of channel, open interval durations show no significant dependence on [ACh] in the range 0.04-10 microM, but decrease at higher [ACh] in a manner consistent with channel block by agonist molecules. 4. Closed interval durations within bursts (2-100 microM-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 approximately 40 microseconds 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 approximately 0.17 ms, while no saturation is apparent for gamma 60 channel tau slow values up to 200 microM-ACh. Below 50 microM-ACh, gamma 40 tau slow values are approximately 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 microM-ACh. Gamma 60 channels have a greater than 5-fold faster opening rate, approximately 10-fold faster closing rate, and approximately 3-fold lower affinity than do gamma 40 channels. There is some indication of positive co-operativity of ACh binding to gamma 40 channels.