Demonstrates that the cardiac action potential overshoot in frog ventricles depends on a competition between sodium and calcium ions, challenging the pure sodium-hypothesis of excitation.
Findings in frog ventricle should not inform human care; challenges sodium hypothesis and leaves open calcium's role in mammalian action potentials.
The height of the overshoot of the action potential recorded from frog ventricles is markedly less sensitive to lowering the external sodium concentration than predicted by the sodium-hypothesis of excitation, and it is surprisingly sensitive to changes in the external calcium concentration. These observations are explained by a mechanism in which there is competition between sodium and calcium ions, in the excitable membrane, for anionic sites which control the inward current of sodium ions at the crest of the action potential.
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Orkand et al. (1964) studied this question.
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