Ionic regulation and Na(+)-K(+)-ATPase activity in gills and kidney of the freshwater stingray Paratrygon aiereba living in white and blackwaters in the Amazon Basin.

During low-water period, freshwater stingray Paratrygon aiereba collected in the whitewater (WW) of the River Amazon showed higher urea content, osmolality, Na(+) and Cl(-) concentrations in plasma and perivisceral fluid than those caught in blackwater (BW) of the River Negro. Gills and kidney Na(+)-K(+)-ATPase activities were significantly lower in WW than in BW fish. The high level of kidney Na(+)-K(+)-ATPase activity in P. aiereba may minimize ion loss and generate diluted solute-free urine in ion-poor BW environment.