Sodium gradient-dependent transport of magnesium in rat ventricular myocytes.
Cytoplasmic concentration of Mg(2+) ([Mg(2+)](i)) was measured with a fluorescent indicator furaptra in ventricular myocytes enzymatically dissociated from rat hearts (25 degrees C). To study Mg(2+) transport across the cell membrane, cells were treated with ionomycin in Ca(2+)-free (0.1 mM EGTA) and high-Mg(2+) (10 mM) conditions to facilitate passive Mg(2+) influx. Rate of rise of [Mg(2+)](i) due to the net Mg(2+) 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 Mg(2+) efflux from cells loaded with Mg(2+). After [Mg(2+)](i) was raised by ionomycin and high Mg(2+) to the level 0.5-0.6 mM above the basal value ( approximately 0.7 mM), washout of ionomycin and lowering extracellular [Mg(2+)] to 1.2 mM caused rapid decline of [Mg(2+)](i) in the presence of 140 mM Na(+). This net efflux of Mg(2+) was completely inhibited by withdrawal of extracellular Na(+) and was largely attenuated by imipramine, a known inhibitor of Na(+)/Mg(2+) exchange, with 50% inhibition at 79 microM. The relation between the rate of net Mg(2+) 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 Mg(2+) transport, which is consistent with Na(+)/Mg(2+) exchange, in cardiac myocytes.
Tashiro
M
M
Department of Physiology, The Jikei University School of Medicine, Tokyo 105 - 8461, Japan.
Konishi
M
M
D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002614:Chelating Agents / Q000494:pharmacology; D004232:Diuretics / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D005110:Extracellular Space; D007099:Imipramine / Q000494:pharmacology; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000493:pharmacokinetics; D008094:Lithium / Q000494:pharmacology; D008274:Magnesium / Q000493:pharmacokinetics*; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000493:pharmacokinetics*
D018759:Adrenergic Uptake Inhibitors; D002614:Chelating Agents; D004232:Diuretics; D007476:Ionophores; D004533:Egtazic Acid; D015759:Ionomycin; D000584:Amiloride; D008094:Lithium; D012964:Sodium; D008274:Magnesium; D007099:Imipramine; D011188:Potassium