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

Abstract

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.

Oxidant and redox signaling in vascular oxygen sensing mechanisms: basic concepts, current controversies, and potential importance of cytosolic NADPH

“…In contrast, studies supporting the Nox oxidases in hypoxic responses have generally been conducted in bovine pulmonary arteries where mitochondrial inhibitors seem to have minimal effects on force generation or the response to hypoxia (58,60). One possible explanation for these differences is the potential absence in rats of the mitochondrial ATPase inhibitor subunit IF 1 that functions to prevent the consumption of cytosolic ATP for maintenance of mitochondrial membrane potential under conditions such as severe hypoxia (92). Our studies also indicate that force generation in endothelium-removed bovine coronary arteries is essentially not altered by inhibition of mitochondrial electron transport by antimycin A (33), suggesting that the inhibition of mitochondrial function does not mimic hypoxia in these arteries.…”

Ischemia/reperfusion-induced death of cardiac myocytes: possible involvement of nitric oxide in the coordination of ATP supply and demand during ischemia

“…This means that mitochondrial F1,F0-ATPase is a ATP consumer, not a ATP producer, in the absence of oxygen [35]. Since rat cardiac myocytes contain small amounts of higher affinity F1-ATPase inhibitory subunit (IF1) [36], there is a possibility that the most effective strategy of cardiac myocytes reducing ATP utilization during ischemia is a down-regulation of proton leak in the mitochondria. Further studies will be needed to clarify this.…”

Interleukin-3 Withdrawal Induces an Early Increase in Mitochondrial Membrane Potential Unrelated to the Bcl-2 Family

“…a small inhibitory protein, IF1, which modulates the ATPase activity and proton conductance (17,18), is thought to confer additional regulation. IF1 is pH-sensitive, increasing its inhibitory activity by binding the F 0 F 1 -ATPase in an acidic environment (19,20).…”