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.