Selectivity of sodium channels in denervated tonic muscle fibre membrane of the frog.

Ionic selectivity of sodium channels was examined under voltage clamp conditions in normal and denervated twitch fibres and denervated tonic fibres isolated from m. ileofibularis of the frog (R. temporaria). Membrane currents were recorded by means of the Hille-Campbell vaseline-gap voltage clamp method from muscle fibre segments exposed to a potassium-free artificial internal solution. Permeability ratio (PS/PNa) were determined from changes in the reversal potential after replacing all Na ions in the solution bathing the voltage clamped external membrane area with sodium substituting ions (S). The permeability sequence was: Na+ greater than Li+ greater than NH4+ greater than K+. No inward currents were observed for Ca2+. The permeability ratios were as follows. Denervated tonic fibres: 1:0.88:0.23:0.012; control twitch fibres: 1:0.94:0.22:0.076; denervated twitch fibres: 1:0.91:0.14:0.082. The permeability to Li+ ions deviates from independence to a greater extent in tonic than in phasic fibres. Our results are consistent with the Hille model of sodium channel selectivity, and they support the hypothesis that sodium channels formed in denervated tonic muscle fibres of the frog are of the same genetic origin as Na channels expressed under physiological conditions.