Cadmium tolerance in the Nile tilapia (Oreochromis niloticus) following acute exposure: assessment of some ionoregulatory parameters.

The Nile tilapia (Oreochromis niloticus) can tolerate very high levels of waterborne cadmium. It has one of the highest 96 h LC50 recorded for a freshwater teleost fish (14.8 mg/L Cd; hardness 50 mg/L CaCO(3)). Cadmium is known to perturb ion balance in teleost fishes. However, in an acute time course experiment, plasma Na(+) concentrations were unaffected, and plasma Ca(2+) values only decreased after 96 h exposure in a dose-independent manner. Branchial Na(+)/K(+)-ATPase activity and alpha-subunit protein level expression in crude gill homogenates were not affected by Cd exposure during this period. Branchial chloride cell numbers, identified as Na(+)/K(+)-ATPase immunoreactive cells using immunohistochemistry, decreased 24 h after exposure but recovered thereafter. Histopathological changes did not follow a consistent pattern of variation with exposure time, and the alterations noted in gill epithelium were basically nonspecific to cadmium. Because of its tolerance, it can be concluded that the tilapia O. niloticus would not be a suitable test organism to evaluate sublethal toxicity of cadmium and the realistic impact of this pollutant in the environment. However, it certainly could contribute significantly to our understanding of the toxic mechanism of cadmium exposure in aquatic organisms. This is the first work to investigate the effect of waterborne pollutants on Na(+)/K(+)-ATPase alpha-subunit protein expression in fish gills.