Effects of Membrane Potential on Na〔+〕-Dependent Mg〔2+〕 Extrusion from Rat Ventricular Myocytes

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  • Effects of Membrane Potential on Na+ -Dependent Mg2+ Extrusion from Rat Ventricular Myocytes.

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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°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 Mg2+, 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, ∼40%. Hyperpolarization from −80 to −120 mV slightly but significantly slowed the decrease in [Mg2+]i by ∼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.<br>

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