The ionic fluxes in frog muscle

Abstract Methods are described for determining the absolute sizes of the inward and outward fluxes of radioactive sodium and potassium in frog muscle fibres. Most of the work was concerned with a small muscle from the frog’s foot, the M. extensor longus dig. IV, but the fluxes in sartorius and abdominal muscles were also measured. In normal Ringer’s solution (containing 2·5 mM-K) the mean potassium flux in the toe muscle was estimated as 4·5 pmole/cm2s, the influx being slightly smaller than the efflux. In 5 mM-K the fluxes were nearly doubled. A simplified theoretical treatment of the error in the potassium flux likely to arise from the slowness of diffusion in the extracellular space of the muscle, showed that in this small cylindrical muscle it was only about 10 %. Similar calculations for sartorius muscles suggested that the true flux was about 12 pmole/cm2s in normal Ringer, but this result depended on a rather large correction for the diffusion effect. It proved difficult to make really satisfactory measurements of the sodium fluxes, because, as has been noted by other authors, the time course of the exchange of 24Na in a whole muscle did not conform exactly to that for a simple two-stage process. However, the sodium fluxes in the toe muscle were estimated to be of the order of 10 pmole/cm2s. There were several possible causes for the observed deviations from ideal behaviour, but there was insufficient evidence to decide between them. The self-diffusion coefficient of sodium ions in the extracellular space of the muscle was found to be 3·1 x 10-6cm2/s. Sartorius muscles gave fluxes and diffusion coefficients of the same order. It was shown that the sodium efflux in frog muscle was reduced in potassium-free Ringer, and increased in a potassium-rich medium. This suggests that there may be, as in cephalopod axons, some form of coupling between sodium efflux and potassium influx.