In vivo characterisation of intestinal zinc uptake in freshwater rainbow trout

Knowledge of the uptake mechanisms and metabolism of metals is essential for understanding the factors governing metal toxicity, discerning means by which acclimation and homeostasis may be achieved and characterising interactions between the metal of interest and other environmental moieties. Zinc is both an important aquatic contaminant and a vital micronutrient. The physiological characterisation of dietary zinc absorption in fish has, therefore, important implications for environmental protection and aquaculture. The present study aimed to elucidate the mechanism of intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss), using an in vivo cannulation technique. Only a saturable component of zinc uptake, with a concentration giving half-maximal rate of accumulation (K_0.5) of 309 μmoll^-1, and a maximal rate of accumulation (J_max) of 933 nmol kg^-1 h^-1, was described. This characterised the intestine as a low-affinity, high-capacity zinc absorption pathway. Physiological mechanisms appear to regulate zinc uptake. Intestinal mucus was one important regulatory locus, promoting zinc uptake at low concentrations yet buffering the animal against high luminal zinc loads. Regulatory mechanisms also seemed to limit subepithelial zinc accumulation. Experiments using ethylene glycol tetraacetic acid (EGTA) to wash the intestinal lumen following zinc perfusion exhibited a higher proportion of loosely associated zinc at higher perfused concentrations. This was attributed to saturation of the uptake process or efflux from the subepithelium. Two distinct pathways for passage of zinc across the epithelium were discerned, with post-intestinal transfer possibly mediated by sulphydryl groups, as illustrated by N-ethylmaleimide perfusion experiments. Putative roles of zinc transporters and/or intracellular-binding proteins are discussed.