Voltage‐clamp experiments in normal and denervated mammalian skeletal muscle fibres.

Abstract

The vaseline-gap voltage-clamp method was applied to the study of ionic currents in cut pieces from innervated and 5-7 day denervated rat skeletal muscle fibers. Kinetic analysis of Na currents in innervated rat muscle showed them to be similar to those in frog muscle, except that rat Na channels are activated at slightly more negative potentials. Peak Na conductances of 40-50 m-mho/cm2 were measured, corresponding to values of .hivin.GNa of 100-120 m-mho/cm2. The permeability sequence of the Na channel to several organic and inorganic cations is Li+ > Na+ > hydroxylammonium > hydrazinium > guanidinium .apprxeq. ammonium > K+. TMA+ and Ca2+ were not measurably permeant. Denervation appears to shift activation and inactivation parameters of Na currents by .apprx. 10 mV to more negative rotentials but does not appreciably affect the maximum peak Na conductance or the time constants for activation and inactivation. Dose-response curves for block by tetrodotoxin in innervated fibers are fitted well by assuming binding of toxin to a single population of channels with a dissociation constant of about 5 nM. In denervated fibers, there appears in addition a 2nd population of channels with a dissociation constant in the micromolar range. These relatively toxin-insensitive channels respond less rapidly to potential changes and can contribute up to 25-30% of the total Na conductance. The K currents of innervated rat muscle were similar to those of frog muscle in their voltage dependence of activation. The time constant for inactivation of the Na current, .tau.h, at -13 mV showed a temperature dependence measured between 10-20.degree. C equivalent to an average Q10 of 2.3. The Q10 for the time constant of activation of the K current, .tau.n, averaged 2.5 between -40 and +40 mV, measured over the same temperature range.