Effect of external magnesium on intracellular free sodium: Na+ flux via Na+/Mg2+ antiport is masked by other Na+ transport systems in rat cardiac myocytes.
Mg2+ efflux from heart cells on a Na+/Mg2+ antiport has been postulated, but the Na+ flux component of the antiport has not been demonstrated. The study aimed to establish if the Na+ flux component could be measured by following changes in [Na+]i with SBFI during conditions known to reverse the antiport (5 mmol/L Mg2+(o), Na+(o)- & Ca2+(o)-free): and after minimising the activity of other Na+ transport pathways. Resting [Na+]i was 8 +/- 0.7 mmol/L (mean +/- S.E., n = 39 cells) in normal Tyrode's solution. [Na+]i decreased below the normal level in all cells (a decline of 4-5 mmol/L, n = 21) during perfusion with 5 mmol/L Mg2+(o) (Na+(o)- & Ca2+(o)-free). Controls using 1 mmol/L Mg2+(o) showed similar declines in [Na+]i, but the fall was greatest when Na+(o) was replaced by K+(o) (decline of 6 mmol/L) rather than the tetramethylammonium ion (TMA+). The rate of decrease in [Na+]i during perfusion with 5 mmol/L Mg2+(o) (Na+(o)- & Ca2+(o)-free) was slowed by 20 microM ouabain (n = 5) or by elevation of pHo to pH 9 (n = 7) so that [Na+]i remained close to the initial value. The decrease of [Na+]i was not affected by 10 microM imipramine (n = 15). These data suggest that the Na+ efflux component of the Na+/Mg2+ antiport is masked in Na+(o)- and Ca2+(o)-free conditions by other Na+(i) efflux pathways.
Odblom
M P
MP
Department of Biological Sciences, University of Plymouth, Devon.
Handy
R D
RD
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D006863:Hydrogen-Ion Concentration; D007099:Imipramine / Q000494:pharmacology; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology*; D009206:Myocardium / Q000166:cytology* / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013997:Time Factors
D002316:Cardiotonic Agents; D010042:Ouabain; D012964:Sodium; D008274:Magnesium; D007099:Imipramine; D002118:Calcium