title: Rapid regulation of Na+ fluxes and ammonia excretion in response to acute environmental hypoxia in the Amazonian oscar, Astronotus ocellatus.
abstract: The Amazonian oscar is extremely resistant to hypoxia, and tolerance scales with size. Overall, ionoregulatory responses of small ( approximately 15 g) and large oscars ( approximately 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 (Po(2) approximately 10 mmHg), 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.
authors.affiliation: Dept of Biology, McMaster Univ, Hamilton, ON, Canada. woodcm@mcmaster.ca
authors.forename: Chris M
authors.initials: CM
authors.lastname: Wood
authors.forename: Makiko
authors.initials: M
authors.lastname: Kajimura
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Sloman
authors.forename: Graham R
authors.initials: GR
authors.lastname: Scott
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Walsh
authors.forename: Vera M F
authors.initials: VM
authors.lastname: Almeida-Val
authors.forename: Adalberto L
authors.initials: AL
authors.lastname: Val
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D049628:Body Size; D023681:Cichlids / Q000097:blood / Q000378:metabolism*; D004777:Environment; D006863:Hydrogen-Ion Concentration; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014508:Urea / Q000378:metabolism; D014867:Water / Q000737:chemistry; D014882:Water-Electrolyte Balance / Q000502:physiology
chemical_list: D014867:Water; D000641:Ammonia; D014508:Urea; D012964:Sodium; D010100:Oxygen