Gastrointestinal uptake and distribution of copper in rainbow trout

A single dose of radioactive copper (Cu-64 or new Cu) was infused into the stomach of rainbow trout (Oncorhynchus mykiss) to model dietary copper (Cu) uptake under conditions of a normal nutritional dose and optimum environme ntal temperature (16 degrees C, 0.117 mu g Cu g(-1) body mass). The distrib ution of new Cu to the gut and internal organs occurred in two phases: rapi d uptake by the gut tissues (almost complete by 24 h postinfusion) followed by slower uptake by the internal organs. By 72 h, 60% of the dose had been excreted, 19% was still retained in the gut tissue, 10% remained in the lu men and 12% had been absorbed across the gut and partitioned amongst the in ternal organs. A reduction in water temperature of 10 degrees C (to 6 degre es C) significantly retarded components of new Cu distribution (movement of the bolus along the gut and excretion); nonetheless, by 72 h, the fraction absorbed by all the internal organs was similar to that at 16 degrees C. A n increase in water temperature of 3 degrees C (to 19 degrees C) caused a p ronounced increase in internal organ uptake by 24 h to approximately double the uptake occurring at 16 degrees C. The uptake of new Cu by the gut tiss ue had a low temperature coefficient (Q(10)<1) consistent with simple diffu sion, while the temperature coefficient for transfer of new Cu from gut tis sue to the internal organs was high (Q(10)>2), consistent with facilitated transport. Internally, the liver and gall bladder (including bile) were the target org ans for dietary Cu partitioning since they were the only organs that concen trated new Cu from the plasma. Individual tissues differed in terms of the exchange of their background Cu pools with new Cu. The background Cu in the walls of the gastrointestinal tract (excluding stomach) exchanged 45-94% w ith new Cu from the gut lumen, while tissues such as the stomach, gills, ki dney, carcass and fat had 5-7% exchangeable background Cu. The liver, heart , spleen, ovary, bile and plasma had only 0.2-0.8% exchangeable background Cu. The gastrointestinal tissues appear to act as a homeostatic organ, regu lating the absorption of nutritional (non-toxic) doses of Cu (0.117 mu g g( -1) body mass day(-1)) by the internal organs. Within the dose range we use d and at optimal temperature (16 degrees C), the new Cu content of the gut tissues fluctuated, but absorption of new Cu by the internal organs remaine d relatively constant. For example, predosing the fish with non-radioactive Cu caused new Cu absorption by the gut tissues to double and decreased new Cu excretion from 38 to 1.5%, but had no effect on new Cu uptake by the in ternal organs. Feeding fish after application of the normal liquid dose of new Cu also had no effect on new Cu uptake by the internal organs, even tho ugh the presence of food in the digestive tract reduced the binding of new Cu to the gut tissues and assisted with the excretion of new Cu. The gut wa s therefore able to regulate new Cu internalization at this dosage. Higher new Cu doses (10, 100 and 1000 times the normal dose), however, evok ed regurgitation and increased new Cu excretion within 4 h of application b ut did not elevate new Cu levels in gut tissue beyond a threshold of approx imately 40 mu g of new Cu. Only at the highest dose (1000 times the normal dose, 192 mu g g(-1) body mass), equivalent to toxic concentrations in the daily diet (7000 mu g Cu g(-1) dry mass food), was the buffering capacity o f the gut overwhelmed, resulting in an increase in internal new Cu uptake.