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 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.