Asymmetric proton block of inward rectifier K channels in skeletal muscle

Inward rectifier and delayed rectifier K currents were measured in frog skeletal muscle fibers with the vaseline gap voltage clamp technique as internal or external pH were lowered. Inward rectifier currents were only slightly reduced by low external pH but were completely and reversibly blocked when the internal pH was reduced to below 5.5 either by cutting the fiber ends in low pH solutions or by bathing the fiber exterior with permeant acetate buffers at low pH. The steepness of the pH dependence of this block suggests that more than one and perhaps three hydrogen ions are required to bind to the blocking site. The voltage dependence of inward rectifier gating was not shifted by low external pH. Either these channels are not located near the fixed negatively charged groups which apparently alter the voltage sensed by many other ionic channels or the membrane potential drop and the external [K+] are altered in a compensating manner such that the driving force on K+ (upon which inward rectification depends) remains unchanged. In contrast, delayed rectifier channels were blocked and their kinetics dramatically shifted by lowering external pH.