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.
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