Mg2+ buffering in cultured chick ventricular myocytes: quantitation and modulation by Ca2+.
To characterize the Mg2+ buffering of cultured chick ventricular myocytes, cytosolic Mg2+ was increased by liberating Mg2+ normally chelated by ATP upon total depletion of ATP content. Because the total Mg content and cell volume remained constant during this time, the difference between the amount of Mg2+ liberated (2.7 mM) and the 0.9 mM increase in cytosolic Mg2+ activity measured fluorometrically with mag-fura-2 indicates a sizable Mg2+ buffering. A new term, the Mg2+ buffer coefficient (BMg), was derived to quantify this buffering. We also determined that cytosolic Mg2+ activity increased by only 0.6 mM in cells acutely exposed to zero external Ca2+ during ATP depletion. In the absence of extracellular Ca2+, the basal cytosolic Ca2+ activity (alpha Ca2+i) was reduced by 72%, whereas the increase in alpha Ca2+i induced by ATP depletion was substantially blunted; no difference in either the time course of adenine nucleotide changes or the Ca and Mg content was observed. The BMg value calculated for these cells indicates that Mg2+ buffering is substantially greater in the absence of extracellular Ca2+ (2.5) than when extracellular Ca2+ is present (1.4), indicating that alpha Ca2+i affects cytosolic Mg2+ activity in ventricular myocytes. Therefore the Mg2+ buffering of ventricular myocytes appears to be comprised of at least two components: 1) a Ca(2+)-insensitive adenine nucleotide pool and 2) a Ca(2+)-sensitive nonadenine nucleotide pool.
Department of Pharmacology and Toxicology, School of Medicine, Wright State University, Dayton, Ohio 45435.
K L
KL
Koss
R W
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Putnam
R D
RD
Grubbs
D000227:Adenine Nucleotides / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002478:Cells, Cultured; D002642:Chick Embryo; D003600:Cytosol / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D006321:Heart / Q000187:drug effects*; D006352:Heart Ventricles; D007461:Iodoacetates / Q000494:pharmacology; D019807:Iodoacetic Acid; D007700:Kinetics; D008274:Magnesium / Q000032:analysis / Q000378:metabolism*; D008433:Mathematics; D008856:Microscopy, Fluorescence; D008962:Models, Theoretical; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D012966:Sodium Cyanide / Q000494:pharmacology; D013997:Time Factors
D000227:Adenine Nucleotides; D005456:Fluorescent Dyes; D007461:Iodoacetates; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D008274:Magnesium; D012966:Sodium Cyanide; D002118:Calcium; D016257:Fura-2; D019807:Iodoacetic Acid