DIFFERENCES IN UPTAKE OF INORGANIC MERCURY AND CADMIUM IN THE GILLS OF THE ZEBRAFISH, BRACHYDANIO-RERIO

The influence of calcium and calcium channel blockers on the apical uptake of cadmium and inorganic mercury was studied in the branchial epithelium of the zebrafish (Brachydanio rerio). Autoradiography was used to determine the distribution of Cd and Hg after uptake in the epithelium. Zebrafish were exposed to 20 nM Cd-109 or Hg-203 for 15-30 min. Cd uptake was considerably lower at 2 mM Ca2+ than at 0.2 mM Ca2+, whereas uptake of Hg was not influenced. The calcium-channel blocker verapamil caused a concentration-dependent decrease in uptake of Cd. Uptake of Hg increased when fish were exposed to 150 mu M verapamil, but was not affected by 250 mu M verapamil. Fish exposed to Cd or Hg at 0.2 mM Ca2+ and in the presence of 1 mu M lanthanum (La3+) showed a lower branchial uptake of the metals than the controls. In another experiment fish were exposed to Cd-109 or Hg-203 (10 nM) for 24 h. Autoradiograms of the gills indicated a high Cd uptake in some epithelial cells of the primary filament. Identifiable Cd-109-positive cells had the appearance of chloride cells. Hg was relatively evenly distributed between the primary filament and the secondary lamella. No specific type of cell with a high uptake of Hg was detected. The results show that there are differences in apical uptake of Cd and Hg, probably due to speciation differences between Cd and Hg in fresh water.