Interactions of Calcium with Sodium and Potassium in Membrane Potentials of the Lobster Giant Axon

Experiments were performed on the lobster giant axon to determine the relation between intracellular spike amplitude and external calcium ion concentration. Action potential decline in low external calcium is greatly accelerated by simultaneous removal of external sodium ion. Correlation of the time course of spike decline in low calcium-low sodium solution with the time courses of spike decline in low calcium alone and in low sodium alone indicates that the effect of simultaneous removal of both ions is significantly greater than the sum of the individual effects. For a given time of treatment, spike amplitude was a function of external calcium concentration. While spike height is proportional to the log of the external calcium concentration over the range 2.5 to 50 millimolar, the proportionality constant is dependent upon the sodium concentration. Under the conditions of low external sodium (50 per cent reduction) the slope of the linear relationship between the spike height and the log of the external calcium concentration is about 5 times greater than in normal external sodium. Decreasing external calcium concentration and simultaneously increasing external potassium concentration produce a greater spike reduction than the arithmetic sum of spike reductions in low calcium alone and in high potassium alone. It is suggested that calcium interacts strongly with sodium and potassium in the spike-generating mechanism. A theoretical basis for these results is discussed.