Effects of membrane potential on the capacitance of skeletal muscle fibers.

Abstract

A method for measuring frog muscle fiber capacitance using small test pulses applied with the 3-microelectrode voltage clamp is presented. Using this method, 3 membrane potential-dependent changes in capacitance were observed: capacitance of polarized fibers increased by 5-15% with depolarization from V < -100 mV to voltages slightly below the contraction threshold; capacitance of fibers depolarized to -30 mV by 100 mM Rb solution decreased by roughly 8% with further depolarization to about +50 mV and increased with repolarization, exhibiting a maximum increase of about 10% at -80 to -90 mV; and capacitance of fibers depolarized to -15 mV by 100 mM K solution increased by about 19% with further depolarization to +43 mV and decreased by about 23% with repolarization to -62 mV. The first 2 effects are attributed to changes in specific membrane capacitance due to voltage-dependent redistribution of mobile charged groups within surface or transverse(T)-tubule membranes. The 3rd effect is caused by changes in the T-system space constant .lambda.T due to the voltage dependence of K conductance (inward rectification). Analysis of the latter effect showed that in 100 mM K solution .lambda.T .apprxeq. .mu.m when inward rectification was fully activated by hyperpolarization and that the density of inward rectifier channels is about the same in surface and tubular membranes. Fiber internal resistance was independent of voltage, a necessary condition for the interpretation of the capacitance measurements.