Diffusion of calcium, cadmium and mercury in a mucous solution from rainbow trout

Diffusion of calcium, cadmium and mercury was studied in a solution of mucus from rainbow trout. Diffusion rates for Cd and Hg were reduced by at least 50% when compared with that in a borate buffer solution. Calcium diffusion rate was unaffected by the presence of mucus. The effect of mucus on the diffusion rates is most likely the consequence of binding of the metals to the mucous proteins. Equilibrium dialysis studies of equimolar concentrations of the metals in a mucous solution demonstrated that about 50% of the Ca, 95% of the Cd and 99% of the Hg remained bound. The dissociation constants (Kd) for mucus and the metals are: 15 microM for Ca2+, 0.95 microM for Cd2+ and 0.33 microM for Hg2+. The significance of the results for uptake of these metals via the gills is discussed.