Intracellular and extracellular concentrations of Na+ modulate Mg2+ transport in rat ventricular myocytes.
Apparent free cytoplasmic concentrations of Mg2+ ([Mg2+]i) and Na+ ([Na+]i) were estimated in rat ventricular myocytes using fluorescent indicators, furaptra (mag-fura-2) for Mg2+ and sodium-binding benzofuran isophthalate for Na+, at 25 degrees C in Ca2+-free conditions. Analysis included corrections for the influence of Na+ on furaptra fluorescence found in vitro and in vivo. The myocytes were loaded with Mg2+ in a solution containing 24 mM Mg2+ either in the presence of 106 mM Na+ plus 1 mM ouabain (Na+ loading) or in the presence of only 1.6 mM Na+ to deplete the cells of Na+ (Na+ depletion). The initial rate of decrease in [Mg2+]i from the Mg2+-loaded cells was estimated in the presence of 140 mM Na+ and 1 mM Mg2+ as an index of the rate of extracellular Na+-dependent Mg2+ efflux. Average [Na+]i, when estimated from sodium-binding benzofuran isophthalate fluorescence in separate experiments, increased from 12 to 31 mM and 47 mM after Na+ loading for 1 and 3 h, respectively, and decreased to approximately 0 mM after 3 h of Na+ depletion. The intracellular Na+ loading significantly reduced the initial rate of decrease in [Mg2+]i, on average, by 40% at 1 h and by 64% at 3 h, suggesting that the Mg2+ efflux was inhibited by intracellular Na+ with 50% inhibition at approximately 40 mM. A reduction of the rate of Mg2+ efflux was also observed when Na+ was introduced into the cells through the amphotericin B-perforated cell membrane (perforated patch-clamp technique) via a patch pipette that contained 130 mM Na+. When the cells were heavily loaded with Na+ with ouabain in combination with intracellular perfusion from the patch pipette containing 130 mM Na+, removal of extracellular Na+ caused an increase in [Mg2+]i, albeit at a very limited rate, which could be interpreted as reversal of the Mg2+ transport, i.e., Mg2+ influx driven by reversed Na+ gradient. Extracellular Na+ dependence of the rate of Mg2+ efflux revealed that the Mg2+ efflux was activated by extracellular Na+ with half-maximal activation at 55 mM. These results contribute to a quantitative characterization of the Na+-Mg2+ exchange in cardiac myocytes.
Tashiro
Michiko
M
Department of Physiology, Tokyo Medical University, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160-8402, Japan.
Tursun
Pulat
P
Konishi
Masato
M
D000666:Amphotericin B / Q000494:pharmacology; D000818:Animals; D001572:Benzofurans / Q000494:pharmacology; D001692:Biological Transport; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000737:chemistry; D002138:Calibration; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000494:pharmacology; D016257:Fura-2 / Q000031:analogs & derivatives / Q000494:pharmacology; D006352:Heart Ventricles / Q000166:cytology*; D017136:Ion Transport; D007477:Ions; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D008564:Membrane Potentials; D032342:Muscle Cells / Q000166:cytology*; D009206:Myocardium / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000737:chemistry; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction; D012964:Sodium / Q000737:chemistry / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013997:Time Factors
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D007477:Ions; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; C061665:sodium-binding benzofuran isophthalate; D010042:Ouabain; D000666:Amphotericin B; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium; D016257:Fura-2