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 myocardia l ischemia in slow heart-rate species which include rabbits and all la rger mammals examined including humans, there is an IF1-mediated inhib ition 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 conductio n (Guerrieri, F., et al., FEES Lett. 213, 67-72, 1987). In the present study rabbit was used as the slow heart-rate species and rat as the f ast heart-rate species. Rat is a fast heart-rate species that contains too little IF1 to down regulate the ATPase activity present. Mitochon dria were prepared from control and ischemic hearts and ESMP were made from aliquots by sonication at pH 8.0 with 2 mM EDTA. Oligomycin-sens itive ATPase activity and IF1 content were measured in SMP prepared fr om the control and ischemic mitochondrial samples. After identical inc ubation procedures, oligomycin-sensitive ATPase activity, oligomycin-s ensitive proton conductivity, and IF1 content were also measured in ES MP samples. The study was undertaken to corroborate further what appea r to be fundamental differences in ATPase regulation between slow and fast heart-rate mammalian hearts evident during total myocardial ische mia. Thus, passive proton conductivity was used as an independent meas ure of these regulatory differences. The results show that, consistent with the low IF1 content of rat heart cardiac muscle mitochondria, co ntrol rat heart ESMP exhibit approximately twice as much passive proto n conductivity as control rabbit heart ESMP regardless of the pH of th e incubation and assay. Moreover, while total ischemia caused an incre ase in IF1 binding and a commensurate decrease in passive proton condu ctivity in rabbit heart ESMP regardless of pH, neither IF1 content nor proton conductivity changed significantly in rat heart ESMP as a resu lt of ischemia.