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Volume 271, Number 39, Issue of September 27, 1996 pp. 23638-23641
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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COMMUNICATION:
IF₁ Function in Situ in Uncoupler-challenged Ischemic Rabbit, Rat, and Pigeon Hearts

(Received for publication, July 1, 1996, and in revised form, July 25, 1996)

From the Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0575

Rabbit, rat, and pigeon are species representative of three cardiac muscle 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 and show very little IF₁-mediated ATPase inhibition during ischemia. Class 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 during 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 making them ischemic. We then compared net rates of cell ATP depletion during 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 increased net rate of cell ATP depletion due to ATP hydrolysis by the fully uncoupled mitochondrial ATPase. We found that FCCP-uncoupling in situ had a relatively small effect on ATP depletion during ischemia in rabbit hearts, that it dramatically accelerated ATP depletion in ischemic rat hearts, and that it had an intermediate effect on ATP depletion in ischemic pigeon hearts. These results demonstrate for the first time the relative extents to which IF₁-mediated mitochondrial ATPase inhibition can slow cell ATP depletion due to the fully uncoupled mitochondrial ATPase in these three classes of hearts. They show that, in contrast to the situation in rabbit hearts, the low level of higher affinity IF₁ present in the cardiac muscle mitochondria of the rat is, under these conditions, essentially nonfunctional, while the full complement of the lower affinity form of IF₁ present in the cardiac muscle mitochondria of the pigeon is partially functional in that it appeared to provide an intermediate level of protection against rapid cell ATP depletion.

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