The Contribution of CLC-1 and Kir Channels to the Resting Electrical Properties of Skeletal Muscle Fibers from Normal and Myotonic Mice

Abstract

The membrane resistance (Rm) of mammalian skeletal muscle fibers at rest has been typically assumed to be dominated by the chloride conductance (gCl). We have demonstrated, in fibers internally equilibrated with 70 mM Cl, bathed in TEA-Cl (145mM), and voltage-clamped at −20mV, that the majority of the Cl currents (ICl; ∼1mA/cm2) are contributed by 9-ACA-sensitive chloride (ClC-1) channels expressed in the transverse tubular system (TTS) membranes. Likewise, in fibers internally equilibrated with 150mM K, bathed with 150mM KMeSO4, and clamped at 0mV, the majority of the K-currents arise from inward rectifier K (KIR; ∼1mA/cm2) channels in the TTS. Here we address the question: what are the relative contributions of gCl and gKIR to Rm in intact fibers bathed in Tyrode (4mM K and 155mM Cl; RP∼-90mV)? Short fibers isolated from flexor digitorum brevis (FDB) muscles were impaled with 2 microelectrodes and stimulated with step current pulses (ΔI<30nA; 400ms) eliciting small voltage responses (ΔV<15mV). Rm and Cm were calculated by fitting ΔV traces to the solution of equations for a short cylindrical cable (Hodgkin & Nakajima, JPhysiol. 221:105-120.1972). For fibers of adult control mice, Rm in Tyrode is 721±52 cm2 (n=7); this value increased to 1,672±116 cm2 (n=5) by blocking gKIR with 10mM Rb (or 1mM Ba), and to 1,804±190 cm2 (n=5) by blocking gCl with 1mM 9-ACA. The results demonstrate that, unlike commonly assumed, gCl and gKIR represent approximately 58% and 42% (respectively) of the blockable resting conductances in fibers of normal mice. In contrast, similar analysis in fibers of young HSALR mice, which have ∼80% reduction in ClC-1 expression, gCl and gKIR contributions represent approximately 23% and 77%, respectively. Supported by NIH grants AR047664, AR041802, and AR054816.