IF1 FUNCTION IN-SITU IN UNCOUPLER-CHALLENGED ISCHEMIC RABBIT, RAT, AND PIGEON HEARTS

Rabbit, rat, and pigeon are species representative of three cardiac mu scle mitochondrial ATPase regulatory classes, a, b and c, respectively , Class a species contain a full complement of higher affinity ATPase inhibitor subunit, IF1, in their cardiac muscle mitochondria and show marked IF1-mediated mitochondrial ATPase inhibition during myocardial ischemia. Class b species contain low levels of higher affinity IF1 an d show very little IF1-mediated ATPase inhibition during ischemia. Cla ss c species contain a full complement of a lower affinity form of IF1 and show a low-to-moderate level of IF1-mediated ATPase inhibition du ring ischemia, In the present study we perfused hearts of a member of each regulatory class through the coronary arteries with the uncoupler , carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), before ma king them ischemic, We then compared net rates of cell ATP depletion d uring ischemia in the FCCP-treated hearts to identically treated FCCP- free hearts, Thus, we tested the relative capacities of cardiac muscle mitochondria of the three species to avert a potentially greatly incr eased net rate of cell ATP depletion due to ATP hydrolysis by the full y uncoupled mitochondrial ATPase, We found that FCCP-uncoupling in sit u had a relatively small effect on ATP depletion during ischemia in ra bbit hearts, that it dramatically accelerated ATP depletion in ischemi c rat hearts, and that it had an intermediate effect on GTP depletion in ischemic pigeon hearts. These results demonstrate for the first tim e the relative extents to which IF1-mediated mitochondrial ATPase inhi bition can slow cell ATP depletion due to the fully uncoupled mitochon drial ATPase in these three classes of hearts. They show that, in cont rast to the situation in rabbit hearts, the low level of higher affini ty IF1 present in the cardiac muscle mitochondria of the rat is, under these conditions, essentially nonfunctional, while the full complemen t of the lower affinity form of IF1 present in the cardiac muscle mito chondria of the pigeon is partially functional in that it appeared to provide an intermediate level of protection against rapid cell ATP dep letion.