Effects of K⁺ and Cl⁻ on Na⁺-dependent Mg²⁺ efflux from rat ventricular myocytes

Abstract

We measured intracellular free Mg²⁺ concentration ([Mg²⁺]_i) with fluorescent Mg²⁺ indicator furaptra (mag-fura-2) in Ca²⁺-free condition (0.1 mM EGTA) at 25°C. After the cells loaded with Mg²⁺ in 24 mM-Mg²⁺ solution for 3 h, reduction of [Mg²⁺]_o to 1 mM caused a decrease in [Mg²⁺]_i in the presence of extracellular Na⁺ (Na⁺-dependent Mg²⁺ efflux). To study the effects of K⁺ on Na⁺-dependent Mg²⁺ efflux, the initial rate of decrease in [Mg²⁺]_i (initial Δ[Mg²⁺]_i) /Δt) was compared at high extracellular [K⁺] (75 mM) and K⁺-free (replaced by N-methyl-D-glucamine) conditions. [Na⁺]_o was kept constant at 70 mM, and membrane potential was set at -13 mV with amphotericin-B-perforated patch clamp technique. With the K⁺-based pipette solution, the initial Δ[Mg²⁺]_i/Δt values obtained in the presence of 75 mM K⁺ and 0 mM K⁺ in the perfusate were not significantly different (79.0±6.0% and 65.6±5.0%, respectively, of the control values measured at 140 mM [Na⁺]_o without any modification of extracellular and intracellular K⁺ and Cl⁻). Intracellular perfusion with K⁺-free (Cs⁺-substituted) solution from the patch pipette in combination with removal of extracellular K⁺ did not significantly change the initial Δ[Mg²⁺]_i) /Δt (77.7±8.2% of the control). Finally, the initial Δ[Mg²⁺]_i/Δt was unchanged by extracellular and intracellular perfusion with K⁺-free and Cl⁻-free solutions (71.6±5.1% of the control). These results suggest that K⁺ and Cl⁻ are not involved in the Na⁺-dependent Mg²⁺ efflux. [J Physiol Sci. 2006;56 Suppl:S113]