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

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 (Jsamm) in the anterior intestine, however it did not alter Jsamm in the mid- or posterior intestine suggesting similar mechanisms of ammonia handling in freshwater and seawater fish. Both fluid transport rate (FTR) and Jsamm 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 Jsamm caused by low Na+ treatment indicated a Na+ linked transport mechanism. Mucosal bumetanide (10− 4 M) had no impact on FTR, yet decreased Jsamm in the anterior and mid-intestine, suggesting NH4+ 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 Jsamm, suggesting the absence of direct competition between Na+ and ammonia for transport. Finally, [14C]methylamine permeability (PMA) measurements indicated the likely presence of an intestinal Rh-mediated ammonia transport system, as increasing NH4Cl (0, 1, 5 mmol l− 1) concentrations reduced PMA, 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.