Effects of cadmium on the kinetics of calcium uptake in developing tilapia larvae, Oreochromis mossambicus

The toxic effects of Cd2+ on Ca2+ influx kinetics in developing tilapia (Oreochromis mossambicus) larvae were evaluated. Addition of 20 mu g l(-1) of Cd2+ to the environment of 0 and 3 day-old larvae competitively inhibited the Ca2+ uptake within 4h resulting in a great increase in K-m values for Ca2+ influx (19.3 and 17.4 fold, respectively) as compared with their respective controls. Consequently,the actual Ca2+ influx of larvae in solutions of 0.2 mM Ca2+ are suppressed by 32-45%. Also, 3 day-old larvae were more sensitive to internally accumulated Cd2+ than 0 day-old larvae. Although the Ca2+ influx in 0 and 3 day-old larvae may be restored to the levels of their respective controls with 24h of being transferred to a 20 mu g l(-1) Cd2+ solution, total body Ca2+ content was significantly reduced in 3 day-old larvae. Increased Ca2+ uptake efficiency ensures sufficient Ca2+ for normal growth. However, rapid increase in Ca2+ influx after hatching also leads to higher Cd2+ uptake. Exposure to Cd2+ will lead to a drop in body Ca2+ content resulting in retardation of larval growth. Therefore, we conclude that if Ca2+ uptake is interfered with at this critical stage of development, larvae will not be able to maintain normal levels of body Ca2+ and will show signs of Cd2+ poisoning.