Spin label oximetry to assess extracellular oxygen during myocardial ischemia

We describe real-time measurement of myocardial oxygen consumption during ischemia in the intact heart. Measurement of extracellular oxygen concentration during myocardial ischemia by spin label oximetry has been limited by ischemia-induced reduction of the neutral, water-soluble nitroxide TEMPONE. We have overcome this problem by encapsulating the nitroxides. Isolated immature (7-10 d old) rabbit hearts (n=8) were perfused aerobically within the cavity of a loop gap resonator with bicarbonate buffer containing an oxygen-sensitive, lipid-soluble nitroxide (N-14-TEMPO laurate in FC-43 perfluorocarbon micelles) and a much less oxygen-sensitive and positively charged nitroxide (N-15-TEMPO choline in multilamellar vesicles) as an internal standard. The ratio of the ESR signal amplitudes of these nitroxides was used as a sensitive index of oxygen concentration. Sequestration of the nitroxides decreased their reduction rate by ascorbate in comparison with nonsequestered nitroxides. Hearts were subjected to 60 min of global no-flow ischemia at 20 degrees C. Extracellular oxygen content (mean+/-SD) during aerobic perfusion was 1195+/-55 mu mol/liter. The electron spin resonance signal from TEMPO laurate increased with the onset and progression of ischemia, consistent with a decrease in extracellular oxygen, while the signal for TEMPO choline was relatively unchanged. Extracellular oxygen content after 40 and 60 min of ischemia was reduced to 393+/-27 mu mol/liter (p <.05) and 61+/-5 mu mol/liter (p <.05), respectively. We conclude that spin-label oximetry can directly and precisely measure myocardial oxygen consumption at constant temperature during ischemia in the intact heart. Copyright (C) 1996 Elsevier Science Inc.