title: Pathways of Rb+ influx and their relation to intracellular [Na+] in the perfused rat heart. A 87Rb and 23Na NMR study.
abstract: The aims of this study were to characterize the routes of influx of the K+ congener, Rb+, into cardiac cells in the perfused rat heart and to evaluate their links to the intracellular Na+ concentration ([Na+]i) using 87Rb and 23Na nuclear magnetic resonance (NMR) spectroscopy. The rate constant for Rb+ equilibration in the extracellular space was 8.5 times higher than that for the intracellular space. The sensitivity of the rate of Rb+ accumulation in the intracellular space of the perfused rat heart to the inhibitors of the K+ and Na+ transport systems has been analyzed. The Rb+ influx rates were measured in both beating and arrested hearts: both procaine (5 mmol/L) and lidocaine (1 mmol/L) halved the Rb+ influx rate. In procaine-arrested hearts, the Na+,K(+)-ATPase inhibitor ouabain (0.6 mmol/L) decreased Rb+ influx by 76 +/- 24% relative to that observed in untreated but arrested hearts. Rb+ uptake was insensitive to the K+ channel blocker 4-aminopyridine (1 mmol/L). The inhibitor of Na+/K+/2 Cl- cotransport bumetanide (30 mumol/L) decreased Rb+ uptake only slightly (by 9 +/- 8%). Rb+ uptake was dependent on [Na+]i: it increased by 58 +/- 34% when [Na+]i was increased with the Na+ ionophore monensin (1 mumol/L) and decreased by 48 +/- 9% when [Na+]i was decreased by the Na+ channel blockers procaine and lidocaine. Dimethylamiloride (15 to 20 mumol/L), an inhibitor of the Na+/H+ exchanger, slightly reduced [Na+]i and Rb+ entry into the cardiomyocytes (by 15 +/- 5%). 31P NMR spectroscopy was used to monitor the energetic state and intracellular pH (pHi) in a parallel series of hearts. Treatment of the hearts with lidocaine, 4-aminopyridine, dimethylamiloride, or bumetanide for 15 to 20 minutes at the same concentrations as used for the Rb+ and Na+ experiments did not markedly affect the levels of the phosphate metabolites or pHi. These data show that under normal physiological conditions, Rb+ influx occurs mainly through Na+,K(+)-ATPase; the contribution of the Na+/K+/2 Cl- cotransporter and K+ channels to Rb+ influx is small. The correlation between Rb+ influx and [Na+bdi during infusion of drugs that affect [Na+]i indicates that, in rat hearts at 37 degrees C, Rb+ influx can serve as a measure of Na+ influx. We estimate that, at normothermia, at least 50% of the Na+ entry into beating cardiac cells is provided by the Na+ channels, with only minor contributions (< 15%) from the Na+/K+/2 Cl- cotransporter and the Na+/H+ exchanger.
authors.affiliation: Institute for Biodiagnostics, National Research Council, Winnipeg, Manitoba, Canada.
authors.forename: V V
authors.initials: VV
authors.lastname: Kupriyanov
authors.forename: L C
authors.initials: LC
authors.lastname: Stewart
authors.forename: B
authors.initials: B
authors.lastname: Xiang
authors.forename: J
authors.initials: J
authors.lastname: Kwak
authors.forename: R
authors.initials: R
authors.lastname: Deslauriers
mesh_terms: D000818:Animals; D004734:Energy Metabolism / Q000187:drug effects; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D010477:Perfusion; D011343:Procaine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012413:Rubidium / Q000378:metabolism*; D012964:Sodium / Q000032:analysis / Q000378:metabolism*
chemical_list: D011343:Procaine; D010042:Ouabain; D012964:Sodium; D012413:Rubidium