EPR monitoring of ischemia-induced myocardial oxygen depletion and acidosis in isolated rat hearts using soluble paramagnetic probes

A new low-field EPR approach for noninvasive measurements of myocardial oxygen tension and tissue acidity was developed. The approach was applied to monitor myocardial pO2 and pH in a model of global no-flow ischemia (30 min) and reperfusion (I/R) in isolated perfused rat hearts. The myocardial oxygen measurements were performed using deuterated Finland trityl radical probe. A rapid decrease of myocardial pO2 from 160 mmHg to about 2 ± 1 mmHg was observed within the first minute of ischemia followed by incomplete restoration of pO2 to 50 mmHg during 30 minutes of reperfusion. The lower oxygen concentration after ischemia was attributed to the 50% reduction in coronary flow after ischemia as a consequence of myocardial I/R damage. Myocardial pH measurements using a specially designed imidazoline pH-sensitive nitroxide showed severe myocardial acidification to pH 6.25 during 30 minutes of ischemia. Preconditioning of the hearts with two 5 minute periods of ischemia significantly reduced the acidification of myocardial tissue during sustained ischemia. Noninvasive EPR monitoring of myocardial oxygenation and pH may provide important insights into the mechanisms of I/R injury and a background for development of new therapeutic approaches.