Repeated short periods of regional myocardial ischemia: effect on local function and high energy phosphate levels.

The effect of recurrent periods of ischemia on the myocardium was investigated in 15 open-chest dogs. Ischemia was produced by 3 minutes of proximal occlusion of the left anterior descending coronary artery. Each occlusion was followed by reperfusion of 3 minutes duration. Forty occlusions with a total of 120 minutes of ischemia were performed, and regional function (sonomicrometry) as well as high energy phosphates (needle biopsies) were determined at the end of the 5th, 20th, and 40th period of ischemia and reperfusion. The first periods of ischemia had a cumulative effect both on regional postischemic function (44% and 59% respectively of preischemic control after 20 occlusions) and on the ATP content, but with increasing number of occlusions the additive effects became smaller (ATP reduction/mumol/g w w/per occlusion). The ATP breakdown per occlusion was diminished with increasing number of periods of ischemia, and no significant adenosine was measured in the ischemic myocardium. Higher than normal postischemic creatine phosphate levels (9.1 mumol/g w w at the 40th reperfusion vs. 6.7 mumol/g w w control) indicated a functioning oxidative phosphorylation in the presence of an ATP utilization problem at the sarcomere level, because indicators of the cellular energy level (energy charge, free energy change of ATP hydrolysis) quickly normalized during reperfusion. Stunned myocardium is therefore not a problem of energy supply but rather of energy utilization. Reduced ATP utilization and regional dysfunction are the expressions of the same cellular defect which resides either in the ATP-splitting contractile apparatus or in the electromechanical coupling. Contractile dysfunction during reperfusion protects the heart against subsequent periods of ischemia because ATP turnover is reduced.