Cytosolic free magnesium levels in ischemic rat heart.

Changes in cytosolic free magnesium ion concentration (Mgi) during myocardial ischemia were measured by 19F NMR in perfused rat hearts loaded with fluorine-labeled derivatives of the magnesium chelator o-aminophenol-N,N,O-triacetate. The perfused rat hearts were loaded intracellularly with the appropriate magnesium indicator by perfusion with 200-400 ml of Krebs-Henseleit buffer containing 5 microM acetoxymethyl ester of the indicator. Basal Mgi concentrations measured by three different indicators averaged 0.85 +/- 0.10 mM (n = 9) and showed no correlation with the KD of the indicator used. 31P NMR measurements of the magnesium-dependent shift between alpha- and beta-phosphates of ATP demonstrate that there is no measurable lowering of Mgi during loading with fluorinated o-aminophenol-N,N,O-triacetate. Between 10 and 15 min of ischemia, Mgi rose nearly 3-fold to 2.1 +/- 0.4 mM. This increase in Mgi occurred over the same time course as the decrease in ATP. After 20 min of reperfusion with Krebs-Henseleit buffer, Mgi declined to 1.5 +/- 0.5 mM. This sustained elevation of Mgi above basal levels may inhibit calcium release from sarcoplasmic reticulum, thereby contributing to the well documented impairment of mechanical function that occurs after a reversible period of ischemia.