Metallothionein induction in early larval stages of tilapia (Oreochromis mossambicus)

Amounts of whole-body metallothionein (MT) in tilapia (Oreochromis mossambicus) larvae increased to a peak (1,500 ng mg^-1 protein) 1 d after hatching (H1), decreased rapidly thereafter, and was maintained at a constant level (700 ng mg^-1) 3 d after hatching (H3). Waterborne Cd²⁺ could stimulate MT expression in newly hatched (H0) larvae in dose-dependent and time-dependent patterns. H0 larvae, which were treated with 35 μg L^-1 Cd²⁺ for 24 h, showed a 1.7-fold increase in the MT amount (174.0 ± 64.7) and a 6.5-fold increase in accumulated Cd²⁺ but no significant change in Ca²⁺ content, compared with the H0 control (MT, 102.6 ± 48.1). H3 larvae with the same treatment revealed about a 10-fold increase in accumulated Cd²⁺, a 10% decrease in Ca²⁺ content, but no change in MT (261.2 ± 120.0), compared with the H3 control (MT, 330 ± 74.0). H0 larvae could synthesize more MT to bind Cd²⁺ for detoxification in 35 μg L^-1 Cd²⁺, a dose that would not affect normal physiology or survival of H0 larvae. On the other hand, 35 μg L^-1 Cd²⁺ caused H3 larvae to experience hypocalcemia, an abnormal physiological condition, in which H3 larvae could not synthesize sufficient MT, thus causing greater than 25% mortality. These results indicate for the first time that the inducibility of MT by waterborne Cd²⁺ is development dependent, being correlated with inconsistent sensitivities to Cd²⁺ during larval development.