| Summary: | Antarctic fishes of the suborder Notothenioidei are noted for their narrow temperature tolerance, yet the physiological factors underlying their limited capacity to endure elevated temperatures are unknown. Previous work by our group indicates that cardiac function may, in fact, delineate thermal tolerance. For example, the hemoglobinless (i.e., icefish) Chaenocephalus aceratus experiences persistent ventricular asystole at 14.1°C while the red‐blooded Notothenia coriiceps experiences asystole at 16.7°C, corresponding with established CT max (associated with loss of right response) temperatures of 13.9°C and 17.1°C, respectively. In the current study, we tested the hypothesis that failure of membrane integrity is associated with collapse of cardiac function. Plasma membranes were isolated from ventricle tissue in both fish species and liposomes were prepared by rehydrating extracted lipids with the self‐quenching fluorophore 5(6)‐carboxyfluorescein (CF). Membrane structural integrity was assessed by quantifying percent of CF leakage from liposomes warmed in 1°C increments over a range of 0°C to 20°C, compared with CF leakage upon complete liposome dissolution. At physiological temperature (0°C), liposomes prepared from membranes of C. aceratus were significantly more permeable with initial leakage of 31% extending to 42% at 20°C, while membrane leakage was 1.5‐times less in N. coriiceps , beginning at 21% and increasing to only 27% over the same temperature range ( P = 0.015). Liposomes prepared from membranes of the icefish also demonstrated increased thermal sensitivity compared with liposomes prepared from the red‐blooded species. Membrane leak increased most significantly between 0°C and 5°C in the icefish ( P < 0.001), while liposomes from the red‐blooded species experienced the greatest change between 0°C and 9°C ( P < 0.001), reflecting a reduced resilience to thermal change for the membrane integrity of C. aceratus compared to N. coriiceps ( P < 0.001). Our data indicate that physical properties ...
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