A Comparison of Zinc and Cadmium Uptake Via the Intestinal Tract of Rainbow Trout

Abstract

The absorption and distribution of metals via the gut of fish is not well known. Consequently, the aim of the present study was to describe the movement of metals along the gut, their absorption and binding to gut tissues, and their distribution to the internal tissues following model dietary exposure. Two different approaches were employed, an in vivo gastric dosing procedure, and an in vitro gut bag protocol and two different metals were studied: an essential metal, zinc, and a non-essential (and more toxic) metal, cadmium. The dietary uptake and distribution of zinc and cadmium to 0.3 kg rainbow trout (Oncorhynchus mykiss) was examined at l5°C at 1, 2, 3, or 7 days following a single bolus dose to the stomach of 0.5 mM of radio labelled metaL After exposure, all internal organs and the remaining carcass were individually counted for radioactivity. Uptake, distribution and excretion of both zinc and cadmium was rapid, occurring largely within the first 24 h of exposure. By 24 h, fish exposed to Zn had absorbed 20.0% of the dose, 21.0% was bound in the gastrointestinal tissues and the remainder was either excreted (38.1 %) or was present in the gut lumen (20.9%). Cadmium showed a much different pattern of uptake, with only 2.9% of the dose absorbed after 24 h, and the remainder found either in the gut tissue (30.2%) and the lumen (19.0%) or excreted (47.9%). Over the following six days, very little uptake and internal metal redistribution occurred.When exposed to higher doses of metal in vivo (0.5 - 50 mM), there were distinct differences in the handling of the two metals. Zinc concentrations in the gut tissues continued to rise at higher doses until apparent saturation. In contrast, gut tissues were saturated with cadmium at the lowest dose employed (0.5 mM). Both metals bound most avidly to the distal intestine but all gut tissues had a higher binding capacity for zinc, as compared to cadmium. Target tissues (liver, gills, kidney) all saturated with zinc at high doses. In contrast, cadmium concentrations in these tissues continued to rise in a linear fashion with increasing dose. In vitro studies revealed that the most important region of the gut for metal uptake in rainbow trout was the mid-intestine. Studies using the metabolic uncoupler, 2,4-DNP, suggested that the transfer of both zinc and cadmium across intestinal cells was passive at the brush border membrane, but was at least partly dependent on A TP for movement across the basolateral membrane. Furthermore, this transport mechanism was not shared by calcium, as the presence of calcium had no inhibitory effect on the transport of either metal. Mucus within the intestinal lumen appears to have a higher binding affinity but lower capacity for cadmium than zinc. Calcium did not displace cadmium from the mucus layer. In contrast, zinc was displaced by an equimolar exposure to calcium in the medium. Gut mucus apparently impedes the movement of metals along the intestine with the extent of the delay likely being related to the binding affinity of the metal. The impediment was greatest for cadmium, as 10% of the metal remained in the lumen of fish exposed in vivo, after a period of7 days. In contrast, only 2% ofthe original dose of zinc remained in the gut lumen after only 3 days.