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