Binding and movement of silver in the intestinal epithelium of a marine teleost fish, the European flounder (Platichthys flesus)

The intestine has been indicated as a site of waterborne silver toxicity in marine fish and chronic effects at the intestine have been observed at concentrations far below acutely toxic level. Thus, models of silver toxicity to marine fish need to consider the intestine as a biotic ligand. The present study characterises binding of silver to the intestine of the European flounder (Platichthys flesus). Everted intestinal sacks were prepared and submersed in a solution mimicking the intestinal fluid of the fish at the acclimation salinity (21‰). Silver was added as 110mAgNO3 or 110mAgNO3/AgNO3 mixtures at concentrations ranging from 1.6 to 950 nM total silver. Appearance of 110mAg was analysed in mucosal scrapings, muscle layers, and in the plasma saline on the serosal side of the intestine. The latter represented uptake into blood and other extra-intestinal compartments. Mucosal scrapings consisted of both epithelial cells and mucus and, thus contained adsorbed as well as absorbed silver. Most of the silver in mucosal scrapings was bound to mucus. There was no difference in silver binding between the anterior, mid, and posterior regions of the intestine. Concentration-dependency of silver binding was sigmoidal and saturated above 100 nM total silver. The saturable appearance of 110mAg in the plasma saline suggest that silver passage across the intestine is transcellular and carrier mediated. Mucus likely influences uptake of silver by altering its speciation from that in the lumen and by serving as physical barrier for silver binding to the brushborder membrane. A biotic ligand model for marine fish to silver may have to consider these interactions.