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 H 2 O 2 production and lipid

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... eroxidation were measured for estimation of temperature-induced oxidative damage. Results: The increase of temperature in the febrile range (40 C) slightly activated mitochondrial function through stimulation of the respiratory module, without affecting the kinetics of the proton leak and phosphorylation modules. At 42 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 (D ) and diminished ATP synthesis (16% lower phosphorylation flux). Increase of temperature above 42 C caused dissipation of D and abolishment of ATP synthesis indicating complete uncoupling of oxidative phosphorylation. The changes in mitochondrial functions induced by incubation at 42 C were completely reversible in contrast to only partial recovery after incubation at higher temperature (45 C). Furthermore, hyperthermia stimulated the production of H 2 O 2 and membrane lipid peroxidation with maximal rates observed at 40 C. Conclusions: We demonstrated for the first time that febrile temperature (40 C) activates mitochondrial energy supplying functions, whereas further temperature increase by only a few degrees leads to severe impairment of mitochondrial ability to maintain DÉ and synthesise ATP. Int J Hyperthermia Downloaded from informahealthcare.com by Freie Universitaet Berlin For personal use only. 37 C 4 2 C J o , (nmol Á O min À1 Á mg protein À1 ) 266 AE 11 289 AE 8 J p , (nmol Á O min À1 Á mg protein À1 ) 254 AE 8 183 AE 20* J L , (nmol Á O min À1 Á mg protein À1 ) 2 8AE 1 9 2AE 11* DÉ, (mV) 149 AE 3 139 AE 4* 62 R. Zukiene et al. Int J Hyperthermia Downloaded from informahealthcare.com by Freie Universitaet Berlin For personal use only.