The resting and action potentials of sartorius muscles of the toad, Bufo marinus, have been measured under varying conditions of external environment. At the same time, analyses for Na+ and K+ content were carried out.

There was a slight elevation of 2 mv. when the measurements were made in phosphate-Ringer instead of in bicarbonate-Ringer. The R.P. was independent of the hydrogen ion concentration between pH 6.5 and 8.5, although at these pH's there was marked alteration in the level of Na+ and K+ in the muscle.

Alteration of the external K+ level between 0 and 50 m.eq./liter has little influence on the internal K+ concentration. When the log of the external K+ concentration is plotted against the R.P. there is not a linear relationship until the external K+ is raised above 12 m.eq./liter, at which point the cell is unexcitable. Above this value a straight line with a slope of 58 mv. per ten-fold change in concentration is obtained, but the absolute values at any point are about 35 per cent higher than those which would be given by the Nernst equation.

Alteration of the external Na+ level within a range of 45 to 650 m.eq./liter resulted in marked changes in the internal Na+ content, without, however, having any effect on the ratio Na+out/Na+in. This ratio has remained at about 3 in spite of marked fluctuations in the absolute value of the internal and external Na+ levels. When the Na+ level is lowered there is a decrease in the height of the action potential although there is no alteration in the ratio Na+out/Na+in. As the Na+ level is raised the height of the action potential is not affected even in the presence of a fivefold increase in Na+ in the Ringer.

The results do not support the conclusion that the bioelectric potentials can be calculated from the ionic ratios by means of simple physical chemical hypotheses such as the Nernst or Goldman equations.

The maintenance of the normal K+ content of the cell cannot be accounted for by a Donnan mechanism. No definite evidence has been produced to explain the mechanism of a Na+ "pump." In other words, the concept of a Na+ pump requires that there shall be a physico- or organochemical mechanism which will distinguish between Na+ and K+ (or other) ions. There is evidence that Na+ can be extruded against a concentration gradient. On the other hand the cell is able to maintain a constant ratio of external to internal Na+ even when the cell has been severely damaged by very high external Na+ levels.

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