Normothermic ischemic cardiac arrest of the isolated working rat heart. Effects of time and reperfusion on myocardial ultrastructure, mitochondrial oxidative function, and mechanical recovery.

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Abstract
The ischemic state of the myocardium of the isolated working rat heart after induction of normothermic ischemic cardiac arrest was assessed by the interrelationship among changes in myocardial ultrastructure, mitochondrial oxidative phosphorylation, and tissue high energy phosphate contents. At all time intervals (10-40 minutes) studied, the ultrastructural changes were more severe in the subendocardium than in the subepicardium. After 25-40 minutes of normothermic ischemic cardiac arrest, the mitochondrial oxygen uptake (state 3) became increasingly depressed, particularly in mitochondria isolated from the subendocardium. Mitochondrial oxidative function, as measured in vitro, did not correlate well with mitochondrial ultrastructural damage. In addition, the effects of coronary reperfusion on the ability of the ischemic heart to recover in terms of ultrastructure, mechanical, and metabolic function were evaluated. Hearts subjected to 10-40 minutes of normothermic ischemic cardiac arrest showed almost complete ultrastructural recovery of the subepicardium upon reperfusion; regression of ultrastructural changes occurred to a lesser extent in the subendocardium. Reperfusion for 30 minutes did not alleviate the depression in mitochondrial oxidative function, while tissue ATP levels did not return to control, preischemic levels. After 20 minutes of normothermic ischemic cardiac arrest, the mechanical performance of the working heart during reperfusion was significantly depressed, compared with pre-ischemic control values. Normal ultrastructure of the subendocardium always accompanied mechanical recovery, while improvement of mitochondrial oxidative function was not essential.