An atypical distribution of lactate dehydrogenase isoenzymes in the hooded seal (Cystophora cristata) brain may reflect a biochemical adaptation to diving
The brains of some diving mammals can withstand periods of severe hypoxia without signs of deleteriouseffects. This may in part be due to an enhanced cerebralcapacity for anaerobic energy production. Here, wehave tested this hypothesis by comparing various parametersof the lactate dehydrogenase (LDH) in the brain of thehooded seal (Cystophora cristata) with those in the brainsof the ferret (Mustela putorius furo) and mouse (Mus musculus).We found that mRNA and protein expression of lactatedehydrogenase a (LDHA) and lactate dehydrogenaseb (LDHB), and also the LDH activity were significantlyhigher in the ferret brain than in brains of the hooded sealand the mouse (p < 0.0001). No conspicuous differences inthe LDHA and LDHB sequences were observed. There wasalso no difference in the buffering capacities of the brains.Thus, an enhanced capacity for anaerobic energy productionlikely does not explain the higher hypoxia tolerance ofthe seal brain. However, the brain of the hooded seal hadhigher relative levels of LDHB isoenzymes (LDH1 andLDH2) compared to the non-diving mammals. Moreover,immunofluorescence studies showed more pronounced co-localization of LDHB and glial fibrillary acidic proteinin the cortex of the hooded seal. Since LDHB isoenzymesprimarily catalyze the conversion of lactate to pyruvate,this finding suggests that the contribution of astrocytes tothe brain aerobic metabolism is higher in the hooded sealthan in non-diving species. The cerebral tolerance of thehooded seal to hypoxia may therefore partly rely on differentLDH isoenzymes distribution.