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Am J Physiol Cell Physiol 279: C1955-C1962, 2000;
0363-6143/00 $5.00
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Vol. 279, Issue 6, C1955-C1962, December 2000

Sodium gradient-dependent transport of magnesium in rat ventricular myocytes

Michiko Tashiro1 and Masato Konishi1,2

1 Department of Physiology, The Jikei University School of Medicine, Tokyo 105 - 8461, and 2 Department of Physiology, Tokyo Medical University, Tokyo 160-8402, Japan

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.

Na+/Mg2+ exchange; cardiac muscle; antiport; magnesium; sodium


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