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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

High [Na⁺]_i in cardiomyocytes from rainbow trout

Faculty of Life Sciences, The University of Manchester, United Kingdom

Submitted 21 March 2007 ; accepted in final form 15 May 2007

Intracellular Na⁺-concentration, [Na⁺]_i modulates excitation-contraction coupling of cardiac myocytes via the Na⁺/Ca²⁺ exchanger (NCX). In cardiomyocytes from rainbow trout (Oncorhyncus mykiss), whole cell patch-clamp studies have shown that Ca²⁺ influx via reverse-mode NCX contributes significantly to contraction when [Na⁺]_i is 16 mM but not 10 mM. However, physiological [Na⁺]_i has never been measured. We recorded [Na⁺]_i using the fluorescent indicator sodium-binding benzofuran isophthalate in freshly isolated atrial and ventricular myocytes from rainbow trout. We examined [Na⁺]_i at rest and during increases in contraction frequency across three temperatures that span those trout experience in nature (7, 14, and 21°C). Surprisingly, we found that [Na⁺]_i was not different between atrial and ventricular cells. Furthermore, acute temperature changes did not affect [Na⁺]_i in resting cells. Thus, we report a resting in vivo [Na⁺]_i of 13.4 mM for rainbow trout cardiomyocytes. [Na⁺]_i increased from rest with increases in contraction frequency by 3.2, 4.7, and 6.5% at 0.2, 0.5, and 0.8 Hz, respectively. This corresponds to an increase of 0.4, 0.6, and 0.9 mM at 0.2, 0.5, and 0.8 Hz, respectively. Acute temperature change did not significantly affect the contraction-induced increase in [Na⁺]_i. Our results provide the first measurement of [Na⁺]_i in rainbow trout cardiomyocytes. This surprisingly high [Na⁺]_i is likely to result in physiologically significant Ca²⁺ influx via reverse-mode NCX during excitation-contraction coupling. We calculate that this Ca²⁺-source will decrease with the action potential duration as temperature and contraction frequency increases.

atrial and ventricular myocytes; excitation-contraction coupling; intracellular sodium; reverse-mode sodium-calcium exchange; temperature