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Electrophysiological studies on the heart of the bivalve mollusc, Modiolus demissus. I. Ionic basis of the membrane potential.

Intracellular electrical recordings from the isolated ventricles of three bivalves (Irisawa, Shigeto & Otani, 1967; Irisawa, Noma & Ueda, 1968; Ebara, 1969; Wilkens & Greenberg, 1970) have been reported in the literature. The bivalve heart features electrical waveforms similar to those of the vertebrate myocardial pacemaker tissues (Trautwein & Kassebaum, 1961), and behaves functionally as a unitary smooth muscle where impulses are transmitted from cell to cell by way of tight junctions or nexuses. These properties are ideal for extracellular recording by the sucrose-gap technique (StMrnpfli, 1954), the method which has been used predominately in the present study. Recordings from Modiolus heart suggest that the pacemaker mechanism is a diffuse property of the entire ventricle. The myogenic pacemaker of bivalves and other molluscs has been discussed in the reviews of Krijgsman & Divans (1955) and of Hill & Welsh (1966). This paper deals with the ionic basis of the membrane potential in Modiolus heart in terms of its characteristic spontaneous nature. Of special significance to this preparation is the low membrane potential due, in part, to chloride and sodium conductances, and the rhythmical contributions of an electrogenic sodium pump. A preliminary account of some of these data has been given previously (Wilkens & Greenberg, 1970).