Tolerance of isolated rat hearts to low-flow ischemia and hypoxia of increasing duration: Protective role of down-regulation and ATP during ischemia

We tested the hypothesis that down-regulated hearts, as observed during low-flow ischemia, adapt better to low O₂ supply than non-down-regulated, or hypoxic, hearts. To address the link between down-regulation and endogenous ischemic protection, we compared myocardial tolerance to ischemia and hypoxia of increasing duration. To that end, we exposed buffer-perfused rat hearts to either low-flow ischemia or hypoxia (same O₂ shortage) for 20, 40 or 60 min (n = 8/group), followed by reperfusion or reoxygenation (20 min, full O₂ supply). At the end of the O₂ shortage, the rate pressure product was less in ischemic than hypoxic hearts (p <0.0001). The recovery of the rate pressure product after reperfusion or reoxygenation was not different for t = 20 min, but was better in ischemic than hypoxic hearts for t = 40 and 60 min (p <0.02 and p <0.0002, respectively). The end-diastolic pressure remained unchanged during low-flow ischemia (0.024 ± 0.013 mmHg·min^-1), but increased significantly during hypoxia (0.334 ± 0.079 mmHg·min^-1). We conclude that, while the duration of hypoxia progressively impaired the rate pressure product and the end-diastolic pressure, hearts were insensitive of the duration of low-flow ischemia, thereby providing evidence that myocardial down-regulation protects hearts from injury. Excessive ATP catabolism during ischemia in non-down-regulated hearts impaired myocardial recovery regardless of vascular, blood-related and neuro-hormonal factors. These observations support the view that protection is mediated by the maintenance of the ATP pool.