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Studies in temperate fishes provide evidence that cardiac mitochondrial function and the capacity to fuel cardiac work contributes to thermal tolerance. Here we tested the hypothesis that decreased cardiac aerobic metabolic capacity contributes to the lower thermal tolerance of the haemoglobinless Antarctic icefish, Chaenocephalus aceratus, compared to the red-blooded Antarctic species, Notothenia coriiceps. Maximal activities of citrate synthase (CS) and lactate dehydrogenase (LDH), respiration rates of isolated mitochondria, adenylate levels, and changes in mitochondrial protein expression were quantified from hearts of animals held at ambient temperature or exposed to their critical thermal maximum (CTMAX). Compared to C. aceratus, activity of CS, ATP concentration, and energy charge were higher in hearts of N. coriiceps at ambient temperature and CTMAX. While state 3 mitochondrial respiration rates were not impaired by exposure to CTMAX in either species, state 4 rates, indicative of proton leakage, increased following exposure to CTMAX in C. aceratus but not N. coriiceps. The interactive effect of temperature and species resulted in an increase in antioxidants and aerobic metabolic enzymes in N. coriiceps, but not C. aceratus. Together, our results support the hypothesis that the lower aerobic metabolic capacity of C. aceratus hearts contributes to its low thermal tolerance.

Cardiac mitochondrial metabolism may contribute to differences in thermal tolerance of red- and white-blooded Antarctic notothenioid fishes