Intestinal ammonia transport in freshwater and seawater acclimated rainbow trout (Oncorhynchus mykiss): evidence for a Na+ coupled uptake mechanism.

Abstract In vitro gut sac experiments were performed on freshwater and 60% seawater acclimated trout ( Oncorhynchus mykiss ) under treatments designed to discern possible mechanisms of intestinal ammonia transport. Seawater acclimation increased ammonia flux rate into the serosal saline (Js amm ) in the anterior intestine, however it did not alter Js amm in the mid- or posterior intestine suggesting similar mechanisms of ammonia handling in freshwater and seawater fish. Both fluid transport rate (FTR) and Js amm were inhibited in response to basolateral ouabain treatment, suggesting a linkage of ammonia uptake to active transport, possibly coupled to fluid transport processes via solvent drag. Furthermore, decreases in FTR and Js amm caused by low Na + treatment indicated a Na + linked transport mechanism. Mucosal bumetanide (10 − 4  M) had no impact on FTR, yet decreased Js amm in the anterior and mid-intestine, suggesting NH 4 + substitution for K + on an apical NKCC, and at least a partial uncoupling of ammonia transport from fluid transport. Additional treatments (amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), phenamil, bafilomycin, 4′,6-diamidino-2-phenylindole (DAPI), high sodium) intended to disrupt alternative routes of Na + uptake yielded no change in FTR or Js amm , suggesting the absence of direct competition between Na + and ammonia for transport. Finally, [ 14 C]methylamine permeability (P MA ) measurements indicated the likely presence of an intestinal Rh-mediated ammonia transport system, as increasing NH 4 Cl (0, 1, 5 mmol l − 1 ) concentrations reduced P MA , suggesting competition for transport through Rh proteins. Overall, the data presented in this paper provide some of the first insights into mechanisms of teleost intestinal ammonia transport.