Effects of intracellular and extracellular concentrations of Ca2+, K+, and Cl- on the Na+-dependent Mg2+ efflux in rat ventricular myocytes

Intracellular Mg2+ concentration ([Mg2+](i)) was measured in rat ventricular myocytes with the fluorescent indicator furaptra (25 degrees C). After the myocytes were loaded with Mg2+, the initial rate of decrease in [Mg2+](i) (initial Delta[Mg2+](i)/Delta t) was estimated upon introduction of extracellular Na+, as an index of the rate of Na+-dependent Mg2+ efflux. The initial D[Mg2+](i)/Delta t values with 140 mM [Na+](o) were essentially unchanged by the addition of extracellular Ca2+ 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 mu M for 3.5 h) did not significantly change the initial D[Mg2+](i)/Delta t: 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 1 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 Delta[Mg2+](i)/Delta t: 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 +/- 6 8.2%, n=8); and 3), extracellular and intracellular perfusion with K+-free and Cl--free solutions (71.6 +/- 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.