MG(2+) BUFFERING IN CULTURED CHICK VENTRICULAR MYOCYTES - QUANTITATION AND MODULATION BY CA(2+)

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 cytosilic Mg2+ activity measured fluorometrically with mag-fura-2 indicates a sizable Mg2+ buffering. A new term, the Mg2+ buffer coefficient (B(Mg)), 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 (alphaCa(i)2+) was reduced by 72%, whereas the increase in alphaCa(i)2+ 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 B(Mg) 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 alphaCa(i)2+ 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 Ca2+-insensitive adenine nucleotide pool and 2) a Ca2+-sensitive nonadenine nucleotide pool.