Ion conductances of the surface and transverse tubular membranes of skeletal muscle.

A combination voltage clamp and admittance analysis of single skeletal muscle fibers showed that moderate depolarizations activated a steady-state negative sodium conductance in both the surface and transverse tubular membranes. The density of the voltage-dependent channels was similar for the surface and tubular conductances. The relaxation times associated with the negative conductance were in the millisecond range and markedly potential dependent. The negative tubular conductance has the consequence of increasing the apparent steady-state radial space constant to large values. This occurs because the positive conductance is counterbalanced by the maintained inward-going sodium current. The enhancement of the space constant by a negative conductance provides a means for the nearly simultaneous activation of excitation-contraction coupling.