Noise analysis of drug induced voltage clamp currents in denervated frog muscle fibres.

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Abstract
Voltage clamp currents were recorded during iontophoretic application of steady doses of acetylcholine (ACh), carbachol or suberyldicholine to hypersensitive extrasynaptic regions of chronically denervated frog [Rana esculenta and R. temporaria] muscle fibers. Autocorrelation functions of drug induced current fluctuations were calculated and estimates of conductance .gamma. and average open time .tau. of the extrasynaptic ion channels were derived. The average open time of an extrajunctional channel induced by acetylcholine was .tau.ACh = 11 .+-. 1.6 ms (.+-.SE) at -80 mV and 8.degree. C. Carbachol and suberyldicholine open channels of .tau.Carb = 3.9 .+-. 0.4 ms and .tau.SubCh = 19 .+-. 2.5 ms (.+-.SE) duration under the same conditions. The average open time of the extrasynaptic channel produced by each drug was 3-5 times longer than the value found for junctional channels in normal fibers. The average open time of the extrajunctional channel was dependent on temperature and membrane potential. Lowering the temperature or increasing the membrane potential increased the average open time of channels induced by any one of the drugs. The conductance of a single extrajunctional channel opened by the action of acetylcholine was estimated to be .gamma.extra = 15 .+-. 1.8 pmho (.+-.SE). This was somewhat lower than the value of .gamma.ep = 23 .+-. 2 pmho (.+-.SE) found for the conductance of a single open channel in the junctional membrane of normal fibers. The extrasynaptic channels opened by the action of carbachol and suberyldicholine had similar conductances to those produced by ACh. The autocorrelation function of drug-induced current fluctuations, recorded at the former end-plate region of chronically denervated fibers often showed a fast and a slow time constant. They corresponded in value to the time constant of the autocorrelation function obtained from end-plate currents in normal fibers and from extrasynaptic currents in denervated fibers, respectively. Two populations of channels may exist at the former end-plate region of denervated muscle fibers.