Effects of Intracellular and Extracellular Concentrations of Ca 2+, K +, and Cl − on the Na +-Dependent Mg 2+ Efflux in Rat Ventricular Myocytes

Abstract

Intracellular Mg 2+ concentration ([Mg 2+] i) was measured in rat ventricular myocytes with the fluorescent indicator furaptra (25°C). After the myocytes were loaded with Mg 2+, the initial rate of decrease in [Mg 2+] i (initial Δ[Mg 2+] i/Δ t) was estimated upon introduction of extracellular Na +, as an index of the rate of Na +-dependent Mg 2+ efflux. The initial Δ[Mg 2+] i/Δ t 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 [Ca 2+] o in the presence of 0.1 mM EGTA, n = 5). Intracellular loading of a Ca 2+ 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[Mg 2+] i/Δ 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 + and Cl − were modified under constant [Na +] o (70 mM), [Ca 2+] 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[Mg 2+] i/Δ 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 ± 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 Mg 2+ is transported in exchange with Na +, but not with Ca 2+, K +, or Cl −, in cardiac myocytes.