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First published online January 19, 2006
Journal of Experimental Biology 209, 549-557 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02044

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

Jaakko Haverinen and Matti Vornanen^*

University of Joensuu, Department of Biology, PO Box 111, 80101 Joensuu, Finland

^* Author for correspondence (e-mail: matti.vornanen{at}joensuu.fi)

Accepted 13 December 2005

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 patch-clamp 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.

Key words: trout heart, sodium current, action potential upstroke, impulse conduction, Oncorhynchus mykiss.

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