Mechanisms of gastrointestinal copper absorption in the African walking catfish: copper dose-effects and a novel anion-dependent pathway in the intestine.

In mammals, copper (Cu) absorption occurs mostly in the small intestine, and some of the Cu transporters involved in its uptake have been characterised. In fish, however, the regions of the gut involved in Cu absorption and the membrane transport mechanisms responsible for gastrointestinal Cu uptake are unknown. Everted gut sacs and isolated perfused intestine of Clarias gariepinus were used to explore Cu absorption (at 22 degrees C). Gut sacs exposed to 100 micromol l(-1) mucosal solution Cu ([Cu](m)) showed that Cu was mostly (70 %) absorbed in the middle and hind intestine. Most of the accumulated Cu was located in the mucosa. In perfused intestines, cumulative Cu absorption from the mucosal solution to the serosal perfusate was greatest at 10 micromol l(-1) [Cu](m) and decreased at higher values of [Cu](m), while tissue accumulation of Cu showed a dose-dependent elevation. Absorption efficiency therefore declined with increasing Cu dose, and basolateral transport was the limiting factor in Cu uptake. Serosal applications of the P-type ATPase inhibitor vanadate (100 micromol l(-1)) or the anion transport inhibitor DIDS (100 micromol l(-1)) caused threefold increases in net Cu uptake (at [Cu](m)=10 micromol l(-1)). The vanadate effect was explained by a reduction in transepithelial potential rather than inhibition of Cu-ATPase, but the DIDS effect was not. Transepithelial potential, water transport and tissue [Cu] were not affected by DIDS, but tissue [K(+)] was elevated. Removal of Cl(-) simultaneously from both the mucosal and serosal solutions caused a 10-fold reduction in the rate of Cu uptake, while removal of Cl(-) from the mucosal solution only completely abolished Cu absorption to the serosal perfusate. Transepithelial potential effects are discussed. We conclude that Cu absorption occurs mostly in the intestine and is normally driven by a basolateral Cu/anion symport that prefers Cl(-).