The effect of temperature on mitochondrial respiration in permeabilized cardiac fibres from the freshwater turtle, Trachemys scripta

Ectothermic animals regularly experience fluctuations in body temperature, which have profound effects on biochemical and metabolic processes. To cope with cold environments, the freshwater turtle, Trachemys scripta, exhibits inverse thermal compensation, where physiological processes exhibit exaggerated Q10 effects and are actively downregulated to limit energy requirements. The present study aimed to identify potential sites of temperature sensitivity in mitochondria from the turtle heart. The effect of acute temperature change on ADP sensitivity and respiratory flux through different components of the electron transport chain (Complexes I, II and IV) was analysed in permeabilized cardiac fibres from the turtle ventricle. An acute decrease in temperature significantly reduced the acceptor control ratio and maximum respiration rate (ṀO2) through all complexes of the electron transport chain. Calculated Q10 values for ṀO2across the three experimental temperatures tested (5, 13 and 21 °C) were in the range of 1.19–3.65, and although there was a tendency for exaggerated Q10 values in the lower temperature range (5–13 °C), there were no significant differences in Q10 between any temperatures or complexes examined. These results suggest the large-scale reductions in turtle cardiac function and high Q10 values at acutely low temperatures are likely due to a reduction in energy demand (contractile function), rather than supply (mitochondrial respiration).