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.
Despa
Sanda
S
Department of Physiology and Cardiovascular Institute, Loyola University Chicago, Maywood, IL 60153, USA.
Islam
Mohammed A
MA
Pogwizd
Steven M
SM
Bers
Donald M
DM
D000818:Animals; D001572:Benzofurans; D002138:Calibration; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D006352:Heart Ventricles; D007425:Intracellular Membranes / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; C061665:sodium-binding benzofuran isophthalate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase