Internal bioavailability of zinc in rainbow trout (Oncorhynchus mykiss) following chronic waterborne and dietary exposure

The principal objective of this thesis was to illuminate the internal bioavailability and the nature of interactions between the branchial and gastrointestinal routes of Zn uptake in rainbow trout (Oncorhynchus mykiss). A comprehensive analysis of tissue Zn distribution following waterborne and dietary Zn exposure, singly and in combination, was carried out. We demonstrate differences in Zn accumulation, at the tissue and subcellular levels, and distribution patterns, with the majority of the accumulated Zn being metabolically active (i.e. bioavailable). At the tissue level the carcass acted as the main sink for Zn containing 84-90% of whole-body Zn, with the intestine (3-7%) and gill (4-6%) holding the majority of the remainder. The estimated metabolically active pools were: gill 81-90%; liver 65-78%, and intestine 59-75%. Interestingly, in the two primary tissues of Zn uptake (gill and intestine) the nuclei-cellular debris fraction bound the highest amounts of Zn while in the liver heat-stable proteins were dominant. A shift in Zn distribution between the metabolically active and detoxified pools with Zn exposure was most prominent in the intestine in which the proportion of detoxified Zn increased from 25 to 41%. Interactions between the branchial and gastrointestinal uptake pathways were complex and appeared to be tissue-specific. In plasma, blood cells, and gill, uptake from water drove the Zn accumulation, while uptake from food drove the accumulation of Zn in the intestine. Both pathways appeared to contribute equally to Zn accumulation in the liver, carcass, and kidney. At the subcellular level there was minimal evidence of cross-talk between waterborne and gastrointestinal pathways. Only the branchial heat-stable, hepatic heat-denaturable, and intestinal microsomes-lysosomes fractions showed some evidence of cross-talk. We also found that the intestine was the main site for dietary Zn detoxification, while the gill had a similar role for waterborne Zn. Finally, the mitochondrial fraction was identified as a potential common target site of Zn accumulation independent of tissue and route of exposure. Overall the Zn exposure regimes employed in this research caused minimal toxicity manifested by a transient inhibition of protein synthesis in the waterborne exposure. We concluded that Zn bioaccumulation is very highly regulated in rainbow trout.