Mechanisms of ischemic myocardial cell damage assessed by phosphorus-31 nuclear magnetic resonance

Phosphorus-31 nuclear magnetic resonance (³¹P NMR) can estimate tissue intracellular pH as well as the content of high-energy phosphate metabolites in isolated perfused hearts. We used ³¹P NMR to examine mechanisms associated with the recovery of ventricular function in hearts subjected to global ischemia and reperfusion, with special emphasis on intracellular pH, a previously unreported variable. Single-dose and multiple-dose administration of a hyperkalemic cardioplegic solution were compared with hypothermia alone in 18 isolated perfused rabbit hearts. Hearts in group 1 were subjected to 24°C hypothermia during 60 minutes of global ischemia; group 2 hearts received a single injection of 37-mM KCL cardioplegic solution at 10°C at the onset of ischemia; and group 3 hearts received a similar initial cardioplegic injection followed by two subsequent 24°C injections at 20-minute intervals during the ischemic period. Using an intraventricular balloon, maximal dP/dt provided a quantitative index of left ventricular performance before and after ischemia. Return of ventricular function expressed as a percentage of control was 54 ± 11% for group 1, 84 ± 6% for group 2, and 101 ± 18% for group 3. Differences in the rate of development of intracellular acidosis were noted during the 60-minute ischemic period. Intracellular pH fell to 6.09 ± 0.12 in group 1, 6.31 ± 0.09 in group 2, and 6.79 ± 0.03 in group 3. In all three groups intracellular pH returned to control (pH 7.20) within 10 minutes of reflow. The metabolic correlates of functional recovery appeared to be the tissue content of ATP at the end of ischemia and after reflow. ATP content at the end of ischemia was 22 ± 2% of control in group 1 hearts, 31 ± 4% in group 2 and 64 ± 2% in group 3. After 45 minutes of reperfusion, ATP levels recovered to 33 ± 9% of control in group 1, to 71 ± 9% in group 2 and to 86 ± 6% in group 3. Although there were no differences between groups in the content of creatine phosphate after 60 minutes of ischemia, the rates of creatine phosphate decline were dissimilar. Further, during the early reflow period, a marked overshoot in tissue creatine phosphate was detected, especially 6% in groups 1 and 2. Histologic damage assessed by light microscopy correlated with the metabolic data, confirming that multidose cardioplegia provided the best preservation of cellular morphology. These results demonstrate that the magnitude of intracellular acidosis and the associated increase in inorganic phosphate correlate inversely with recovery of postischemic ventricular structure and function. ATP, but not creatine phosphate, content correlates with return of contractile performance after reperfusion. The overshoot in creatine phosphate during early reperfusion might impede optimal restoration of ATP content and, as a result, optimal recovery of cell functions.