title: Uncoupling of oxidative phosphorylation and ATP synthase reversal within the hyperthermic heart.
abstract: Heart failure is a common cause of death with hyperthermia, and the exact cause of hyperthermic heart failure appears elusive. We hypothesize that the energy supply (ATP) of the heart may become impaired due to increased inner-mitochondrial membrane permeability and inefficient oxidative phosphorylation (OXPHOS). Therefore, we assessed isolated working heart and mitochondrial function. Ex vivo working rat hearts were perfused between 37 and 43.5°C and showed break points in all functional parameters at ~40.5°C. Mitochondrial high-resolution respirometry coupled to fluorometry was employed to determine the effects of hyperthermia on OXPHOS and mitochondrial membrane potential (ΔΨ) in vitro using a comprehensive metabolic substrate complement with isolated mitochondria. Relative to 37 and 40°C, 43°C elevated Leak O2 flux and depressed OXPHOS O2 flux and ∆Ψ. Measurement of steady-state ATP production from mitochondria revealed decreased ATP synthesis capacity, and a negative steady-state P:O ratio at 43°C. This approach offers a more powerful analysis of the effects of temperature on OXPHOS that cannot be measured using simple measures such as the traditional respiratory control ratio (RCR) or P:O ratio, which, respectively, can only approach 1 or 0 with inner-membrane failure. At 40°C there was only a slight enhancement of the Leak O2 flux and this did not significantly affect ATP production rate. Therefore, during mild hyperthermia (40°C) there is no enhancement of ATP supply by mitochondria, to accompany increasing cardiac energy demands, while between this and critical hyperthermia (43°C), mitochondria become net consumers of ATP. This consumption may contribute to cardiac failure or permanent damage during severe hyperthermia.
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Amelia
authors.initials: A
authors.lastname: Power
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Pearson
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand.
authors.forename: Toan
authors.initials: T
authors.lastname: Pham
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand.
authors.forename: Carlos
authors.initials: C
authors.lastname: Cheung
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand Department of Surgery, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand Maurice Wilkins Center, University of Auckland, Auckland, New Zealand.
authors.forename: Anthony
authors.initials: A
authors.lastname: Phillips
authors.affiliation: School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand Maurice Wilkins Center, University of Auckland, Auckland, New Zealand.
authors.forename: Anthony
authors.initials: A
authors.lastname: Hickey
keywords: ATP synthesis; hyperthermia; mitochondrial membrane potential; mitochondrial respiration