8 Do Mitochondria Limit Hot Fish Hearts decreases with increasing environmental temperatures. However, oxygen diffusion rates increase in warm water, and we found no immediate decline in blood saturation that can be associated with hypoxia. In general, glycogen and free glucose become the main metabolic fuels in stressed and hard-working vertebrate muscles including the heart. Increased plasma lactate and succinate, and decreased malate of thermally challenged N. celidotus indicate enhanced anaerobic metabolism, interrupted ETS, or mitochondrial disruption in this species. This conclusion is supported by an increase in LDH activities in heart tissues from fish exposed to acute heat stress. Elevated glycolytic capacities may represent attempts to offset increased ATP demands under heat stress, and compensatory energetic 9 Do Mitochondria Limit Hot Fish Hearts 26669264 responses to loss of ATP production have been observed in other studies on heat stressed ectotherms. Furthermore, the accumulation of EAAs in experimental plasma could be attributable to increased protein catabolism to fuel metabolism in the presence of heat stress. However, the accumulation of plasma glutamate, a NEAA that is a TCA cycle intermediate and neurotransmitter, may also be associated with a global decrease in mitochondrial oxidation with acute heat stress. Thus the altered patterns of metabolites in this study imply the onset of mitochondrial dysfunction for this species. a decreased OXP efficiency. The depression in RCR and relative increase in Leak-I may indicate a response to ROS as production trended 26646986 higher in heat stressed A-83-01 cost fibres in OXP states and was significantly higher in the chemically uncoupled ETS state. Mitochondrial uncoupling at high temperatures is thought to result from increased superoxide up-regulating uncoupling proteins. However, the significantly higher production of ROS in the experimentally exposed heart fibres could also be attributed to damage at either CI or CIII as these are the most commonly accepted sites of mitochondrial ROS production. Impacts of increasing in situ temperature on heart mitochondria. Overall, substantial changes to both the inner Thermal Sensitivity of Cardiac Mitochondria Mitochondrial function following acute heat stress in intact fish. Following acute heat stress exposure, N. celidotus hearts most likely face decreased OXP efficiencies supported by lower Leak-I and OXP-I fluxes in experimental animals, compared to controls. These changes were reflected by decreased RCRs in acutely temperature exposed fish. This can be caused by irreversible changes in mitochondrial inner membrane integrity with extreme heat stress which has been observed in mammalian heart fibres, 
and this increases the permeability to ions, decreases membrane potential, and results in and outer mitochondrial membranes were evident at 25uC before THF. Leak-I rates increased by 75% at 25uC relative to those at 15uC indicating increased inner membrane permeability. Whereas OXP states were less temperature sensitive and this resulted in a measurable depression of the RCR suggesting increased oxygen flux rates are required to maintain mitochondrial membrane potentials. An initial dose-dependent analysis indicated that RCR values were inhibited by 20uC when increasing temperature was considered as an inhibitory dose. The Leak-I, II/ETS provides a measure of ETS capacity relative to the leak respiration state when phosphorylation is inhibited by atractyloside
