Abstract
The adaptations in the freshwater turtle that permit survival despite prolonged loss of mitochondrial ATP generation were investigated by comparing the bioenergetics of turtle brain slices with rat brain slices. Aerobic turtle brain shows no significant difference in basal levels of total ATP generation compared to rat brain; levels in turtle brain and rat brain were 18.4 +/- 2.8 (SD) and 19.4 +/- 2.2 mumol (100 mg of tissue)-1 hr-1, respectively. However, in turtle brain, a significantly greater fraction of ATP is derived from glycolysis both under aerobic and anaerobic conditions [aerobic turtle (24%) and rat (13%), P less than 0.02; anaerobic, turtle (28%) and rat (18%), P less than 0.05]. The increased glycolytic capacity is related to high levels of rate-limiting glycolytic enzymes, such as pyruvate kinase (EC 2.7.1.40). Turtle brain operates close to glycolytic capacity even under aerobic conditions, and no Pasteur effect can be demonstrated. Quantitatively, anaerobic glycolysis accounts for a maximum of 28% of basal aerobic ATP generation, suggesting that prolonged diving is also accompanied by a reduction in brain energy requirements. The adaptation subserving short-term (natural) diving is an increase in brain glycolytic capacity. The adaptation subserving prolonged diving (days to weeks) may be a reduction in the energy requirements of brain (and other cells).