Preserved high energy phosphate metabolic reserve in globally "stunned" hearts despite reduction of basal ATP content and contractility.

Impaired energy production has been proposed as one mechanism to explain the contractile abnormality in post-ischemic "stunned" myocardium. If energy production were impaired, administration of inotropic agents should result in a deterioration of cellular energy stores because of an inability of ATP synthesis to match the rate of increased utilization. In this study we correlated changes in myocardial high energy phosphates, measured by 31P-NMR spectroscopy, with changes in left ventricular function and energy requirement in buffer perfused rabbit hearts following ischemia and reperfusion, and during stimulation with isoproterenol. Hearts were stunned by 20 min of zero flow global ischemia at room temperature. After reperfusion, isovolumic developed pressure returned to 77.8 +/- 2.2% of baseline and ATP content was reduced to 80.9 +/- 4.1% of baseline. Isoproterenol (5 x 10(-8) M for 10 min) caused increases in developed pressure and rate-pressure product (to 134.1 +/- 12.6% and 195.0 +/- 21.4% of baseline, respectively) without a decrease in ATP or phosphocreatine (PCr) content (80.0 +/- 7.1% and 103.0 +/- 3.8% of preischemia, respectively), and without functional or metabolic deterioration of the hearts after discontinuation of the drug. Control hearts not subjected to ischemia showed similar functional and metabolic responses to isoproterenol. The phosphocreatine/inorganic phosphate (PCr/Pi) ratio, an index of the balance between energy production and utilization, was higher (not lower) than baseline in stunned hearts, thus confirming that energy production was not intrinsically impaired. Together these data indicate that despite reduced myocardial ATP content, mitochondrial function in stunned hearts is capable of sustaining a large increase in function and energetic requirements.