Effects of Cd injection on osmoregulation and stress indicators in freshwater Nile tilapia

Freshwater tilapia (Oreochromis niloticus) were intraperitoneally injected with sublethal doses of cadmium (1.25 or 2.5 mg Cd kg(-1) body mass) and sampled after 1, 4 and 7 days in order to evaluate the mechanisms of Cd toxicity at physiological and biochemical levels. Cd levels were significantly elevated in the gill and kidney following injection however levels in the kidney continued to accumulate while levels in the gill either did not change or decreased with time. Cd caused a generalized stress condition as indicated by an increase in blood glucose, lactate and cortisol levels as well as an oxidative stress indicated by increases in lipid peroxidation and protein carbonyl content. Furthermore, tilapia exhibited impairment in their osmoregulatory status based on the fall in plasma sodium levels. Concerning ion regulatory disruption, the kidney was the most affected organ since there was a generalized increase in renal Na(+)/K(+)-ATPase activity after 1 day of exposure to Cd followed by a significant decrease in day 7. This study provides some insights into the mechanisms of Cd toxicity at physiological and biochemical levels and complements previously reported findings on O. niloticus. The disruption of ion homeostasis, alterations in Na(+)/K(+)-ATPase activity and oxidative damage are the effects of Cd exposure that can be integrated in a comprehensive model for Cd impacts.