Effects of intracellular and extracellular concentrations of Ca 2+, K+, and Cl- on the Na+-dependent Mg2+ efflux in rat ventricular myocytes

Abstract

Intracellular Mg2+ concentration ([Mg2+]i) was measured in rat ventricular myocytes with the fluorescent indicator furaptra (25°C). After the myocytes were loaded with Mg2+, the initial rate of decrease in [Mg2+]i (initial Δ[Mg2+]i/Δt) was estimated upon introduction of extracellular Na+, as an index of the rate of Na +-dependent Mg2+ efflux. The initial Δ[Mg 2+]i/Δt values with 140 mM [Na+] o were essentially unchanged by the addition of extracellular Ca 2+ up to 1 mM (107.3 ± 8.7% of the control value measured at 0 mM [Ca2+]o in the presence of 0.1 mM EGTA, n = 5). Intracellular loading of a Ca2+ chelator, either BAPTA or dimethyl BAPTA, by incubation with its acetoxymethyl ester form (5 μM for 3.5 h) did not significantly change the initial Δ[Mg2+] i/Δt: 115.2 ± 7.5% (seven BAPTA-loaded cells) and 109.5 ± 10.9% (four dimethyl BAPTA loaded cells) of the control values measured in the absence of an intracellular chelator. Extracellular and/or intracellular concentrations of K+ and Cl- were modified under constant [Na+]o (70 mM), [Ca2+]o (0 mM with 0.1 mM EGTA), and membrane potential (-13 mV with the amphotericin-B-perforated patch-clamp technique). None of the following conditions significantly changed the initial Δ[Mg2+]i/Δt: 1), changes in [K+]o between 0 mM and 75 mM (65.6 ± 5.0% (n = 11) and 79.0 ± 6.0% (n = 8), respectively, of the control values measured at 140 mM [Na+]o without any modification of extracellular and intracellular K+ and Cl-); 2), intracellular perfusion with K+-free (Cs+-substituted) solution from the patch pipette in combination with removal of extracellular K+ (77.7 ± 8.2%, n = 8); and 3), extracellular and intracellular perfusion with K+-free and Cl--free solutions (71.6 ± 5.1%, n = 5). These results suggest that Mg2+ is transported in exchange with Na+, but not with Ca2+, K+, or Cl-, in cardiac myocytes. © 2006 by the Biophysical Society.