Potassium Contracture and Its Modification by Cations in the Heart Muscle of the Marine Mollusc, Dolabella Auricula

Inotropism and Contracture of Aplysiid Ventricles as Related to the Action of Neurohumors on Resting and Action Potentials of Molluscan Hearts

“…[K + ]o produces a reversible long-lasting depolarization, giving rise to transient contracting force (Nomura, 1965). In zero LK + ]o the membrane depolarized by more than 10 mV, as would be expected if there were an electrogenic sodium pump.…”

“…The resting potentials of molluscan ventricular myocardia appear to be balanced between the predominance of transmembrane potassium ion distribution and several other factors which together result in a rather low value of resting potential (Irisawa et al, 1967a;Irisawa et al, 1968;Wilkens, 1972a;Kiss and S.-R6zsa, 1973;Nomura, 1965). Primarily, there appears to be a leakiness to sodium ion which reduces the value of the resting potential, but this is counteracted by a chloride contribution and by an electrogenic sodium-potassium pump which may function to keep the membrane potential value at the level where pacemaker activity appears.…”

“…Wilkens, 1972a. Wilkens, 1972a 1 O7î y/j Nomura, 1963Nomura, 1965Kuwasawa, 1967Kuwasawa, 1967 This paper tuating electrogenic sodium pump in pacemaker activity. The membrane potential of the heart of Modiolus depends primarily on the distribution of potassium ion across the membrane, as in Mytilus, but replacement of chloride with sulphate or propionate ions shifts the K + depolarization curve, indicating a contribution of chloride ions to the resting membrane potential.…”

“…In vertebrate myocardia, increased [K + ]o leads to a dramatic shortening of the duration of the cardiac action potential (reviewed by Noble, 1975). Similarly, 10 mV depolarization of ventricular fibers of Dolabella in 50 mM KC1 causes a reduction in amplitude and duration of action potential, which is correlated with a decrease in force of contractions (Nomura, 1965). It should be illuminating to ascertain whether or not inward-going rectification, attributed to some K + channels of vertebrate cardiac muscle, can be demonstrated in cardiac muscle oi Dolabella.…”