High [Na+]i in cardiomyocytes from rainbow trout.

Intracellular Na(+)-concentration, Na(+) modulates excitation-contraction coupling of cardiac myocytes via the Na(+)/Ca(2+) exchanger (NCX). In cardiomyocytes from rainbow trout (Oncorhyncus mykiss), whole cell patch-clamp studies have shown that Ca(2+) influx via reverse-mode NCX contributes significantly to contraction when Na(+) is 16 mM but not 10 mM. However, physiological Na(+) has never been measured. We recorded Na(+) using the fluorescent indicator sodium-binding benzofuran isophthalate in freshly isolated atrial and ventricular myocytes from rainbow trout. We examined Na(+) at rest and during increases in contraction frequency across three temperatures that span those trout experience in nature (7, 14, and 21 degrees C). Surprisingly, we found that Na(+) was not different between atrial and ventricular cells. Furthermore, acute temperature changes did not affect Na(+) in resting cells. Thus, we report a resting in vivo Na(+) of 13.4 mM for rainbow trout cardiomyocytes. Na(+) 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(+). Our results provide the first measurement of Na(+) in rainbow trout cardiomyocytes. This surprisingly high Na(+) is likely to result in physiologically significant Ca(2+) influx via reverse-mode NCX during excitation-contraction coupling. We calculate that this Ca(2+)-source will decrease with the action potential duration as temperature and contraction frequency increases.