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Single-channel studies of the action of (+)-tubocurarine on locust muscle glutamate receptors

The effects of (+)-tubocurarine (TC) on single glutamate-activated channels in voltage-clamped locust muscle fibres have been examined using the patch-clamp technique. Glutamate alone produced a concentration-dependent increase in the probability of the channel being in the open state (p o ), but an increase in the concentration of glutamate (5×10 −5 -5×10 −3 moll −1 ) in the presence of 5×10 −4 moll −1 TC left p o essentially unchanged. TC (5×10 −6 -5×10 −4 moll −1 ) caused a concentration- dependent decrease in the mean open time and in p o for channels opened by 10 −4 moll −4 glutamate. Correlations between successive openings and successive closings, which are characteristic of the kinetics of the muscle glutamate-receptorgated channel of locust muscle, were weakened in the presence of TC. There was little evidence of voltage sensitivity of TC action over the limited membrane potential (V m ) range −70 to −120 mV. The results are consistent with the idea that TC blocks the cation-selective channel gated by glutamate receptors in insect muscle and that the unblocking rate is low. They suggest also that block is at the level of the open channel, a conclusion supported by the wholly activation-induced depression of the neurally evoked twitch contraction of locust muscle by TC. Based upon a simple model for open channel block, TC is estimated to have a dissociation constant of 1.57 μmoll −1 (V m = −100mV). The rate of association of blocker with channel is estimated to be 8.74×10 −3 ms −1 (moll −1 ) −1 (V m =−100mV). The rate of dissociation, estimated indirectly from the single-channel data, is 1.53×10 −2 ms −1 , which gives a mean channel block time of 65.4 ms.