Acute temperature resistance threshold in heart mitochondria: Febrile temperature activates function but exceeding it collapses the membrane barrier

Purpose: Molecular mechanisms underlying hyperthermia-induced cellular injury are not fully understood. The aim of this study was to identify the components of mitochondrial oxidative phosphorylation affected by mild hyperthermia and to quantify the contribution of each component to changes in system behaviour. Methods: Temperature effects on the oxidative phosphorylation in isolated rat-heart mitochondria were assessed using modular kinetic analysis. Mitochondrial H2O2 production and lipid peroxidation were measured for estimation of temperature-induced oxidative damage. Results: The increase of temperature in the febrile range (40 degrees C) slightly activated mitochondrial function through stimulation of the respiratory module, without affecting the kinetics of the proton leak and phosphorylation modules. At 42 degrees C, state 3 respiration rate remained unchanged, the proton leak across the inner mitochondrial membrane was substantially increased, the respiratory module slightly inhibited, leading to decreased membrane potential (Delta psi) and diminished ATP synthesis (16% lower phosphorylation flux). Increase of temperature above 42 degrees C caused dissipation of Delta psi and abolishment of ATP synthesis indicating complete uncoupling of oxidative phosphorylation. The changes in mitochondrial functions induced by incubation at 42 degrees C were completely reversible in contrast to only partial recovery after incubation at higher temperature (45 degrees C). Furthermore, hyperthermia stimulated the production of H2O2 and membrane lipid peroxidation with maximal rates observed at 40 degrees C. Conclusions: We demonstrated for the first time that febrile temperature (40 degrees C) activates mitochondrial energy supplying functions, whereas further temperature increase by only a few degrees leads to severe impairment of mitochondrial ability to maintain Delta psi and synthesise ATP.