Comparative brain oxygenation and mitochondrial redox activity in turtles and rats

Comparisons were made between brain oxygenation and cytochrome c oxidase (cytochrome aa3) redox status in turtles (Pseudymys scripta) and rats. Although average brain O2 tension (PtO2) during normoxia was similar in turtle telencephalon and rat parietal cortex, a greater frequency of high and low values of PtO2 occurred in rats, which may indicate a steeper O2 gradient between capillaries and cells. When the fractional inspired O2 concentration (FIO2) was increased, PtO2 was elevated and cytochrome aa3 became more oxidized, whereas decreased FIo2 produced opposite changes in both species. In turtle brain, however, the relationship between PtO2 and cytochrome aa3 redox state was nearly linear over a wide range of PtO2 values. In rats, this relationship was steeper at PtO2 below normoxia and approached a plateau at Pto2 values above normoxia. Turtle brain cytochrome aa3 appeared more reduced in normoxia, which may be due either to lower affinity for O2 or to enhanced substrate supply. These results indicate that differences in resistance to hypoxia/anoxia observed between rats and turtles are probably not due to differences in O2 availability. Rather, it is likely that differences in redox activities of cytochrome aa3 and/or in substrate use play a role in the relative insensitivity of turtle brain to O2 deprivation.