Intracellular-free magnesium in the smooth muscle of guinea pig taenia caeci: a concomitant analysis for magnesium and pH upon sodium removal.
This study is concerned with the regulation of intracellular-free Mg2+ concentration ([Mg2+]i) in the smooth muscle of guinea pig taenia caeci. To assess an interaction of Ca2+ on the Na(+)-dependent Mg(2+)-extrusion mechanism (Na(+)-Mg2+ exchange), effects of Na+ removal (N-methyl-D-glucamine substitution) were examined in Ca(2+)-containing solutions. As changes in pHi in Na(+)-free solutions perturb estimation of [Mg2+]i using the single chemical shift only of the beta-ATP peak in 31P NMR (nuclear magnetic resonance) spectra, [Mg2+]i and pHi were concomitantly estimated from the chemical shifts of the gamma- and beta-peaks. When extracellular Na+ was substituted with N-methyl-D-glucamine, [Mg2+]i was reversibly increased. This increase in [Mg2+]i was eliminated in Mg(2+)-free solutions and enhanced in excess Mg2+ solutions. ATP content fluctuated little during removal and readmission of Na+, indicating that [Mg2+]i changes were not induced by Mg2+ release from ATP, and that Mg(2+)-extruding system would not be inhibited by fuel restriction. A slow acidification in Na(+)-free solutions and transient alkalosis by a readmission of Na+ were observed regardless of the extracellular Mg2+ concentration. When the extracellular Ca2+ concentration was increased from normal (2.4 mM) to 12 mM, only a marginal increase in [Mg2+]i was caused by Na+ removal, whereas a similar slow acidosis was observed, indicating that extracellular Ca2+ inhibits Mg2+ entry, and that the increase in [Mg2+]i is negligible through competition between Mg2+ and Ca2+ in intracellular sites. These results imply that Na(+)-Mg2+ exchange is the main mechanism to maintain low [Mg2+]i even under physiological conditions.
University Department of Pharmacology, Oxford, UK.
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Nakayama
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Nomura
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Tomita
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002432:Cecum; D005260:Female; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D007424:Intracellular Fluid / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D009130:Muscle, Smooth / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D012964:Sodium; D008274:Magnesium; D002118:Calcium