PLATEAU POTENTIAL OF OYSTER MYOCARDIUM

Transmembrane potential was recorded by the use of a microelectrode inserted into the myocardium in the connected half-ventricle, or in the whole ventricle and a fragment ventricle of oyster respectively and the formation and property of the plateau were studied. 1. The plateau becomes less prominent in consequence of a decrease in size of the material. 2. Size and shape of the plateau frequently change according to the length of delay between action potential spikes recorded simultaneously from two half-ventricles. When the plateau is manifested, the beat interval is prolonged. 3. The preceding action potential obtained from one of the two half-ventricles has a remarkable plateau, more frequently than in the succeeding action potential recorded from the other half-ventricle. 4. Frequently the plateau becomes prominent by electric stimulation applied during the repolarization phase, often followed by hyperpolarization. 5. The small potential induced by the action potential of adjacent fibers sometimes causes a local response of the recorded fiber. Occasionally this induces a noticeable plateau in the action potential of the other half-ventricle. 6. The small potential sometimes elicits the plateau potential on the recorded fiber itself. 7. Frequently plateaus disappear after treatments, such as addition of acetylcholine to the physiological saline. 8. The plateau is considered to be formed not only because of the effect of adequately delayed potential deflection caused by adjacent fibers, but also because of the character of the membrane associated probably with a change in membrane ion permeability.