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 (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^-4M) had no impact on FTR, yet decreased Js_amm 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 Js_amm, suggesting the absence of direct competition between Na⁺ and ammonia for transport. Finally, [¹⁴C]methylamine permeability (P_MA) measurements indicated the likely presence of an intestinal Rh-mediated ammonia transport system, as increasing NH₄Cl (0, 1, 5mmoll^-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.