ATPase activity,IF1 content,and proton conductivity of ESMP from control and ischemic slow and fast heart-rate hearts

Earlier studies by Rouslin and coworkers showed that, during myocardial ischemia in slow heart-rate species which include rabbits and all larger mammals examined including humans, there is an IF₁-mediated inhibition of the mitochondrial ATPase due to an increase in the amount of IF₁, bound to the ATPase (Rouslin, W., and Pullman, M.E.,J. Mol. Cell. Cardiol. 19, 661–668, 1987). Earlier work by Guerrieri and colleagues demonstrated that IF₁ binding to bovine heart ESMP was accompanied by parallel decreases in ATPase activity and in passive proton conduction (Guerrieri, F.,et al., FEBS Lett. 213, 67–72, 1987). In the present study rabbit was used as the slow heart-rate species and rat as the fast heart-rate species. Rat is a fast heart-rate species that contains too little IF₁ to down regulate the ATPase activity present. Mitochondria were prepared from control and ischemic hearts and ESMP were made from aliquots by sonication at pH 8.0 with 2 mM EDTA. Oligomycin-sensitive ATPase activity and IF₁ content were measured in SMP prepared from the control and ischemic mitochondrial samples. After identical incubation procedures, oligomycin-sensitive ATPase activity, oligomycin-sensitive proton conductivity, and IF₁ content were also measured in ESMP samples. The study was undertaken to corroborate further what appear to be fundamental differences in ATPase regulation between slow and fast heart-rate mammalian hearts evident during total myocardial ischemia. Thus, passive proton conductivity was used as an independent measure of these regulatory differences. The results show that, consistent with the low IF₁ content of rat heart cardiac muscle mitochondria, control rat heart ESMP exhibit approximately twice as much passive proton conductivity as control rabbit heart ESMP regardless of the pH of the incubation and assay. Moreover, while total ischemia caused an increase in IF₁ binding and a commensurate decrease in passive proton conductivity in rabbit heart ESMP regardless of pH, neither IF₁ content nor proton conductivity changed significantly in rat heart ESMP as a result of ischemia.This paper is dedicated to the memory of Dr. G. Capozza who died in 1994.