Inhibitory effects of cadmium on carbonic anhydrase activity and ionic regulation of the estuarine crab Chasmagnathus granulata (Decapoda, Grapsidae).

This work was aimed at evaluating the gill carbonic anhydrase (CA) activity of the estuarine crab Chasmagnathus granulata exposed in vivo to cadmium, at different salinities. The in vivo effect of the specific inhibitor acetazolamide (AZ) was also assayed. Besides, the inhibition of CA activity by different heavy metals (cadmium, copper, zinc) and AZ were evaluated under in vitro conditions. For the in vivo assays, adult males were acclimated to salinities of 2.5 or 30/1000. The corresponding 96-h LC50 of cadmium was 2.69 mg l-1 at 2.5/1000, and > 50 mg l-1 at 30/1000. Cadmium only caused a significant lower CA activity than control at 2.5/1000. EC50 for CA inhibition was estimated to be 1.59 mg l-1 at 2.5/1000. Statistical differences in Na+ hemolymphatic levels (P < 0.05) were only detected at 2.5/1000, between 0 and 1.25 mg l-1 of cadmium, but no statistical differences were observed for Cl- levels at any assayed salinity. As CA inhibition registered at 2.5/1000 was followed by only changes in Na+ concentration, it is likely that cadmium exposure could differentially affect ions permeability, among others factors. The concentrations that inhibited in vitro 50% of enzymatic activity (IC50) were 2.15 x 10(-5), 1.62 x 10(-5), 3.75 x 10(-6) and 4.4 x 10(-10) M for cadmium, copper, zinc and AZ, respectively. The comparison with IC50 values of other aquatic species, indicates a higher CA sensitivity for C. granulata to pollutants.