Intracellular Na+ and Na+ pump rate in rat and rabbit ventricular myocytes

Intracellular Na+ (Na+i) is centrally involved in regulation of cardiac Ca2+ and contractility via Na+-Ca2+ exchange (NCX) and Na+-H+ exchange (NHX). Previous work has indicated that Na+i is higher in rat than rabbit ventricular myocytes. This has major functional consequences, but the reason for the higher Na+i in rat is unknown. Here, resting Na+i was measured using the fluorescent indicator SBFI, with both traditional calibration and a novel null-point method (which circumvents many limitations of prior methods). In rabbit, resting Na+i was 4.5 +/- 0.4 mM (traditional calibration) and 4.4 mM (null-point). Resting Na+i in rat was significantly higher using both the traditional calibration (11.1 +/- 0.7 mM) and the null-point approach (11.2 mM). The rate of Na+ transport by the Na+ pump was measured as a function of Na+i in intact cells. Rat cells exhibited a higher V(max) than rabbit (7.7 +/- 1.1 vs. 4.0 +/- 0.5 mM x min(-1)) and a higher K(m) (10.2 +/- 1.2 vs. 7.5 +/- 1.1 mM). This results in little difference in pump activity for a given Na+i below 10 mM, but at measured resting Na+i levels the pump-mediated Na+ efflux is much higher in rat. Thus, Na+ pump rate cannot explain the higher Na+i in rat. Resting Na+ influx rate was two to four times higher in rat, and this accounts for the higher resting Na+i. Using tetrodotoxin, HOE-642 and Ni2+ to block Na+ channels, NHX and NCX, respectively, we found that all three pathways may contribute to the higher resting Na+ influx in rat (albeit differentially). We conclude that resting Na+i is higher in rat than in rabbit, that this is caused by higher resting Na+ influx in rat and that a higher Na+,K+-ATPase pumping rate in rat is a consequence of the higher Na+i.