Significance of Na+ current in the excitability of atrial and ventricular myocardium of the fish heart

The present study examines the importance of the Na+ current (I-Na) in the excitability of atrial and ventricular myocardium of the rainbow trout heart. Whole-cell patchclamp under reduced sarcolemmal Na+ gradient showed that the density of I-Na is similar in atrial and ventricular myocytes of the trout heart, and the same result was obtained when I-Na was elicited by chamber-specific action potentials (AP) in normal physiological saline solution. However, the maximum rate (V-max) of AP upstroke, measured with microelectrodes in intact trout heart, was 21% larger in atrium than ventricle, and thus in variance with the similar I-Na density of the two myocyte types. Furthermore, V-max calculated from the I-Na was 2.1 and 3.2 times larger for atrium and ventricle, respectively, than the values obtained from the APs. The discrepancy between I-Na of isolated myocytes and V-max, of intact muscle is only partly explained by the inward rectifier K+ current (I-K1), which overlaps I-Na and decreases the net depolarising current. Clear differences exist in the voltage dependence of steady-state activation and inactivation as well as in the inactivation kinetics of I-Na between atrial and ventricular myocytes. As a result of a more negative voltage dependence of I-Na activation, smaller I-K1 and higher input resistance of atrial myocytes, the voltage threshold for AP generation is more negative in atrium than ventricle of the trout heart. These findings suggest that atrial muscle is more readily excitable than ventricular muscle, and this difference is partly due to the properties of the atrial I-Na.