Inward rectification in the transverse tubular system of frog skeletal muscle studied with potentiometric dyes.

Abstract

The non-penetrating potentiometric dyes NK2367 and WW375 were used to investigate the effect of inward rectification on the weighted-average tubular membrane potential in single frog muscle fibers voltage clamped using a 3 Vaseline-gap method. In 100 mM-K solution, when inward rectification was activated by hyperpolarization the steady-state amplitude of the transverse tubular system (T-system) optical signal was reduced and its rise time was faster than that recorded for an equivalent depolarization. The voltage dependence of the optical attenuation followed that of inward rectification, increasing with increasing hyperpolarization. For a voltage-clamp step of -140 mV the optical atenuation was 0.72 which correspondes to a weighted-average T-system potential change of 100 mV. When inward rectification was blocked in a Cs, TEA [tetra ethyl-ammoinum] solution the optical attenuation was also abolished. The voltage dependence of the block of the inward currents in solutions containing low concentrations of Cs was also reflected in the T-system optical signals. A radial cable model of the T-system, assuming the same specific inward rectifier conductance in surface and tubular membranes predicted that the measured optical attenuation corresponds to a decrease in the tubular space constant, .lambda.T, from 120 .mu.m under passive conditions to .apprx. 40 .mu.m when inward rectification is fully activated. The voltage dependence of inward rectification measured at the surface membrane was reasonably well predicted by assuming that the specific conductance obeyed a Boltzmann type of voltage dependence; that major effect of tubular decrements was to reduce the steepness of the total (surface + T-system) conductance-voltage relation.