Na/K selectivity, ion conductances and net fluxes of K+ and Na'n metabolically exhausted muscle fibres.

The high resting potassium conductance, induced by stimulating skeletal muscle fibres of the frog to complete exhaustion of contractile activity, was investigated in more detail. In exhausted fibres the Na/K-selectivity (alpha = PNa/PK) calculated by applying the equation for zero-current potentials (Goldman, Hodgkin, Katz) remained as high or became even larger than in normal fibres (alpha less than 0.009). Net fluxes of Na and K derived from a flame photometric estimation of the internal concentrations of these ions rose for Na to 24-49 and for K to 50-100 pmole/(cm2.s). The measured fluxes were by up to more than two orders of magnitude smaller than those calculated from the Goldman-Hodgkin-Katz flux equation based upon the high potassium conductance measured with electrophysiological methods. This discrepancy may be explained by assuming that these channels do not obey the 'independence principle'. The results can probably better be interpreted in terms of multi ion channels with single file characteristics, although accumulation of potassium ions in the transverse tubular system or in the vicinity of the surface membrane may partly account for the observed deviation.