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The oyster heart muscle can continue to develop spike potential rhythmically in a solution where practically all Na ions were removed and replaced by isosmolar Tris. Tetrodotoxin did not show any appreciable effect on both contraction and excitation at the concentration of 5×10-7g/ml. The application of 10mM manganese ion caused the frequency to increase, but after 30 minutes the rhythmical contraction stopped at a diastole with a concomitant cessation in electrical activity. The amplitude and maximum rate of rise of spike potential which developed in Na+ free solution increased as the concentration of Ca++ in the solution was elevated and decreased when it was reduced. A tenfold increase in [Ca++] 0 elevated the amplitude of the spike potential by approximately 19mV. This indicates that Ca++ can also play a dominant role in the generation of spikes in the bivalve heart muscle.
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Irisawa et al. (1968) studied this question.
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