Sodium gradient-dependent transport of magnesium in rat ventricular myocytes

Abstract

Cytoplasmic concentration of Mg2+([Mg2+]i) was measured with a fluorescent indicator furaptra in ventricular myocytes enzymatically dissociated from rat hearts (25°C). To study Mg2+ transport across the cell membrane, cells were treated with ionomycin in Ca2+-free (0.1 mM EGTA) and high-Mg2+ (10 mM) conditions to facilitate passive Mg2+ influx. Rate of rise of [Mg2+]i due to the net Mg2+influx was significantly smaller in the presence of 130 mM extracellular Na+ than in its absence. We also tested the extracellular Na+ dependence of the net Mg2+efflux from cells loaded with Mg2+. After [Mg2+]i was raised by ionomycin and high Mg2+ to the level 0.5–0.6 mM above the basal value (∼0.7 mM), washout of ionomycin and lowering extracellular [Mg2+] to 1.2 mM caused rapid decline of [Mg2+]i in the presence of 140 mM Na+. This net efflux of Mg2+ was completely inhibited by withdrawal of extracellular Na+ and was largely attenuated by imipramine, a known inhibitor of Na+/Mg2+ exchange, with 50% inhibition at 79 μM. The relation between the rate of net Mg2+ efflux and extracellular Na+ concentration ([Na+]o) had a Hill coefficient of 2 and [Na+]o at half-maximal rate of 82 mM. These results demonstrate the presence of Na+ gradient-dependent Mg2+ transport, which is consistent with Na+/Mg2+ exchange, in cardiac myocytes.