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

To characterize the Mg2+ buffering of cultured chick ventricular myocy tes, cytosolic Mg2+ was increased by liberating Mg2+ normally chelated by ATP upon total depletion of ATP content. Because the total Mg cont ent 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 coeffi cient (B(Mg)), was derived to quantify this buffering. We also determi ned that cytosolic Mg2+ activity increased by only 0.6 mM in cells acu tely exposed to zero external Ca2+ during ATP depletion. In the absenc e of extracellular Ca2+, the basal cytosolic Ca2+ activity (alphaCa(i) 2+) was reduced by 72%, whereas the increase in alphaCa(i)2+ induced b y ATP depletion was substantially blunted; no difference in either the time course of adenine nucleotide changes or the Ca and Mg content wa s observed. The B(Mg) value calculated for these cells indicates that Mg2+ buffering is substantially greater in the absence of extracellula r Ca2+ (2.5) than when extracellular Ca2+ is present (1.4), indicating that alphaCa(i)2+ affects cytosolic Mg2+ activity in ventricular myoc ytes. 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.