Abstract:
The loss of hemoglobin in Antarctic icefishes is correlated with high mitochondrial volume densities and altered mitochondrial morphology in their oxidative muscle compared to red-blooded Antarctic notothenioid fish species. We hypothesized that differences in mitochondrial morphology between icefishes and red-blooded species might be correlated with differences in mitochondrial properties at their habitat temperature, near 0�C. We further hypothesized that differences in function might become more pronounced as temperature increases and might contribute to the lower thermal tolerance of icefishes compared to red-blooded species. Proton leak, rates of reactive oxygen species (ROS) production, membrane susceptibility to peroxidation and levels of antioxidants were measured in mitochondria isolated from hearts of the icefishes Chaenocephalus aceratus and Chionodraco rastrospinosus, and the red-blooded species Gobionotothen gibberifrons and Notothenia coriiceps. In addition, levels of oxidized proteins and lipids, and antioxidant levels were quantified in oxidative muscles of icefishes and red-blooded species exposed to their critical thermal maximum (CTmax) - an acute, short-term heat stress, and in animals exposed to 4�C for one week. Rates of ROS production increased as temperature increased in mitochondria isolated from both white- and red-blooded fishes. Yet, isolated mitochondria of icefishes are less protected against ROS. Antioxidant levels normalized to phospholipid content were lower in icefishes compared to red-blooded species, suggesting that icefishes might be more likely to experience oxidative stress as temperature increases. These findings were supported by measurements made in animals exposed to their CTmax. Levels of oxidized lipids increased in hearts of both icefishes, while levels of oxidized proteins increased only in C. aceratus in response to exposure to CTmax. In contrast, neither levels of oxidized lipids nor proteins increased in red-blooded fishes in response to exposure to CTmax. Similarly, levels of oxidized proteins did not increase in C. rastrospinosus or N. coriiceps in response to exposure to 4�C. Antioxidant levels tended to be lower in icefishes compared to red-blooded species and did not increase in any species in response to exposure to CTmax or 4�C. In summary, our findings suggest that icefishes are more vulnerable to heat-induced oxidative stress compared to red-blooded fishes.
