Mg²⁺ Buffering in Cultured Chick Ventricular Myocytes: Quantitation and Modulation by Ca²⁺: Quantitation and modulation by Ca²⁺

To characterize the Mg ²⁺ buffering of cultured chick ventricular myocytes, cytosolic Mg ²⁺ was increased by liberating Mg ²⁺ 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 Mg ²⁺ liberated (2.7 mM) and the 0.9 mM increase in cytosolic Mg ²⁺ activity measured fluorometrically with mag-fura-2 indicates a sizable Mg ²⁺ buffering. A new term, the Mg ²⁺ buffer coefficient (B _Mg ), was derived to quantify this buffering. We also determined that cytosolic Mg ²⁺ activity increased by only 0.6 mM in cells acutely exposed to zero external Ca ²⁺ during ATP depletion. In the absence of extracellular Ca ²⁺ , the basal cytosolic Ca ²⁺ activity (αCa _i ²⁺ ) was reduced by 72%, whereas the increase in αCa _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 Mg ²⁺ buffering is substantially greater in the absence of extracellular Ca ²⁺ (2.5) than when extracellular Ca ²⁺ is present (1.4), indicating that αCa _i ²⁺ affects cytosolic Mg ²⁺ activity in ventricular myocytes. Therefore the Mg ²⁺ 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.