Rapid regulation of Na+ fluxes and ammonia excretion in response to acute environmental hypoxia in the Amazonian oscar, Astronotus ocellatus

The Amazonian Oscar is extremely resistant to hypoxia, and tolerance scales with size. Overall, ionoregulatory responses of small (similar to 15 g) and large Oscars (similar to 200 g) to hypoxia were qualitatively similar, but the latter were more effective. Large Oscars exhibited a rapid reduction in unidirectional Na+ uptake rate at the gills during acute hypoxia (PO2 similar to 10 rnmHg), which intensified with time (7 or 8 h); Na-= efflux rates were also reduced, so net balance was little affected. The inhibitions were virtually immediate (1st h) and preceded a later 60% reduction (at 3 h) in gill Na+-K+-ATPase activity, reflected in a 60% reduction in maximum Na+ uptake capacity without change in affinity (Km) for Na+. Upon acute restoration of normoxia, recovery of Na+ uptake was delayed for 1 h. These data suggest that dual mechanisms may be involved (e.g., immediate effects of O-2 availability on transporters, channels, or permeability, slower effects of Na+-K+-ATPase regulation). Ammonia excretion appeared to be linked indirectly to Na+ uptake, exhibiting a Michaelis-Menten relationship with external [Na+], but the Km was less than for Na-= uptake. During hypoxia, ammonia excretion fell in a similar manner to Na+ fluxes, with a delayed recovery upon normoxia restoration, but the relationship with [Na+] was blocked. Reductions in ammonia excretion were greater than in urea excretion. Plasma ammonia rose moderately over 3 h hypoxia, suggesting that inhibition of excretion was greater than inhibition of ammonia production. Overall, the Oscar maintains excellent homeostasis of ionoregulation and N-balance during severe hypoxia.