High [Na⁺]_iin cardiomyocytes from rainbow trout

Abstract

Intracellular Na(+)-concentration, [Na(+)](i) 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(+)](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 degrees 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 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.

Intracellular diffusion restrictions in isolated cardiomyocytes from rainbow trout

“…In addition, trout and neonatal rabbit cardiomyocytes have similar morphology (compare Fig. 1 with [59,60]) and possibly also excitation-contraction coupling [61,62]. Thus, it seems that cardiomyocytes from low-performance hearts are very similar independent of species.…”

The cellular force-frequency response in ventricular myocytes from the varanid lizard, Varanus exanthematicus

“…As frequency increases, Na ϩ entry through voltage-gated Na ϩ channels increases and can accumulate in the cytosol, thereby reducing Ca 2ϩ extrusion via forward-mode NCX. Intracellular Na ϩ activity (aNa ϩ ) increases with stimulation frequency in trout (5) but not in the rat, even though both show increases in diastolic Ca 2ϩ (17). Canine heart also increased aNa ϩ at high frequencies (8).…”

Comparison of sarcoplasmic reticulum calcium content in atrial and ventricular myocytes of three fish species

“…The composition of the pipette solution was as follows (in mmol/l): 140 CsCl, 3.4 NaCl, 5 Na2ATP, 1 MgCl2, 0.025 EGTA, 0.01 Na2GTP, 10 HEPES (pH 7.2 with CsOH). The total Na ϩ concentration of the pipette solution (13.4 mM) is as determined for rainbow trout ventricular myocytes (7). When a giga seal was obtained, the myocyte was lifted up from the bottom of the chamber and placed in front of two glass capillaries (0.5 mm in diameter) of the solution changer (RCS-200; Biologic, Claix, France) for a rapid exchange of the control external saline solution to the one containing 10 mM caffeine and back again.…”