Effects of membrane potential on Na+ -dependent Mg2+ extrusion from rat ventricular myocytes

To study Mg2+ transport across the cell membrane, the cytoplasmic concentration of Mg2+ (Mg2+](i)) in rat ventricular myocytes was measured with the fluorescent indicator furaptra (mag-fura-2) under Ca2+ -free conditions (0.1 mM EGTA) at 25 degrees C. The fluorescence ratio signal of furaptra was converted to Mg2+](i) using calibration parameters previously estimated in myocytes (Watanabe and Konishi, Pflügers Arch 442: 35-40, 2001). After Mg2+](i) was raised by loading the cells with Mg2+ in a solution containing 93 mM Mg(2+), the cells were voltage-clamped at a holding potential of -80 mV using the perforated patch-clamp technique with amphotericin B. At the holding potential of -80 mV, the reduction of extracellular Mg2+ to 1.0 mM caused a rapid decrease in Mg2+](i) only in the presence of extracellular Na(+). The rate of the net Mg2+ efflux appeared to be dependent on the initial level of Mg2+](i); the decrease in Mg2+](i) was significantly faster in the myocytes markedly loaded with Mg2+. The rate of decrease in Mg2+](i) was influenced little by membrane depolarization from -80 to -40 mV, but the Mg2+](i) decrease accelerated significantly at 0 mV by, on average, approximately 40%. Hyperpolarization from -80 to -120 mV slightly but significantly slowed the decrease in Mg2+](i) by approximately 20%. The results clearly demonstrate an extracellular Na(+)- and intracellular Mg2+ -dependent Mg2+ efflux activity, which is consistent with the Na(+)-Mg2+ exchange, in rat ventricular myocytes. We found that the apparent rate of Mg2+ transport depends slightly on the membrane potential: facilitation by depolarization and inhibition by hyperpolarization with no sign of reversal between -120 and 0 mV.