BIOCHEMICAL ADJUSTMENTS TO HYPOXIA BY AMAZON CICHLIDS

The isozyme distribution of cichlid lactate dehydrogenase (LDH) is rel ated to species environmental preferences. Cichlasoma amazonarum occur s in different environments and presents LDH tissue distribution patte rns that correlate with oxygen tension at the capture location. Cichla soma amazonarum was exposed to longterm severe hypoxia (51 days at 36. 4 +/- 5.9 mmHg), tissue LDH isozyme distribution was analyzed by elect rophoresis and enzyme activities were measured by monitoring the oxida tion of NADH as pyruvate was reduced to lactate. The exposure of Cichl asoma amazonarum to long-term severe hypoxia resulted in changes in th e tissue distribution of LDH isozymes. The major changes in response t o hypoxia occurred in heart, liver and brain: isozyme A(4) was activat ed in heart and brain, whereas isozyme B-4 was activated in liver. The most significant quantitative change occurred in brain LDH of hypoxia -exposed animals which adopted muscle type kinetics, reflecting a new LDH isozyme distribution, LDH activity was significantly reduced (P<0. 05) in animals exposed to hypoxia (N = 8): suggesting an overall LDH s uppression. Pyruvate inhibition decreased in all hypoxia-exposed tissu es, Thus, the ability of Cichlasoma amazonarum to regulate LDH tissue expression according to oxygen availability allows the animal to survi ve chronic hypoxic environments. This phenotypic plasticity may occur in other hypoxia-tolerant fish species.