Electrophysiological Studies on the Heart of the Bivalve Mollusc,Modiolus Demissus

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Abstract
1. The membrane potential in the heart of Modiolus demissus is dependent primarily on the unequal distribution of potassium ions across the cell. 2. The membrane potential is low, however, and the reduced slope of the potassium equilibrium potential is not predictable by the familiar Nernst equation. 3. The membrane undergoes a sustained depolarization in Cl-free SW which reduces the magnitude of subsequent potassium depolarization. Chloride ions contribute, perhaps independently, up to 15-30% of the total potential. 4. The high degree of variability in membrane potential is due, in part, to a sodium leakage current. Membrane hyperpolarization in Na-free SW is greatest in hearts with a low initial potential. 5. An electrogenic sodium pump is described which contributes to the membrane potential in the form of a rhythmical after-hyperpolarization (positive after-potential) following each spike. 6. Inhibition of the pump (reduction of the after-potential) can be achieved using ouabain and cyanide, by substituting lithium for sodium, by removing extracellular potassium and by cooling the heart. 7. The ionic basis of the membrane potential is discussed in relation to the diffuse pacemaker properties of the heart.