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.