NH4+ secretion in the avian colon. An actively regulated barrier to ammonium permeation of the colon mucosa

Experiments were designed to characterize an active, electrogenic transport of NH4+ ions across the colonic epithelium of the domestic fowl (Gallus gallus). Colonic segments were isolated and stripped of underlying muscle. The mucosal epithelia were mounted in Ussing chambers and voltage-clamped to measure the short-circuit currents (ISC) associated with transport. Bilateral addition of NH4+ caused a dose-dependent outward current (negative ISC), with a Km of 34 ± 8 mM and a maximal current response of 311 ± 47 µA cm− 2 (12 ± 2 µEq cm− 2 h− 1). A similar effect was seen with unilateral addition of NH4+ to the serosal (s) side, but not with mucosal (m) addition. Pre-treatment with 10− 4 M amiloride exposed a net outward (negative) ISC, and serosal NH4+ addition further increased this outward current with a Km of 53 ± 24 mM. Decreasing the bath pH from 7.3 to 6.0 did not affect the ISC response to NH4+. Unidirectional NH4+ flux measurements revealed a net secretory flux (8.8 ± 3.1 µmol cm− 2 h− 1 s-m, versus 2.6 ± 1.4 µmol cm− 2 h− 1 m-s). Furthermore, the secretory flux closely matched the resulting change in ISC with serosal NH4+, showing that the transepithelial flux of NH4+ could account for the outward current response. Addition of 50 nM bafilomycin A to the mucosal solution completely eliminated serosal to mucosal NH4+ transport, implicating an apical V-type H+-ATPase in this transport process. The ISC response to NH4+ was partially inhibited by ouabain, a blocker of the Na+/K+-ATPase, but only minimally affected by bumetanide, an inhibitor of the serosal Na+-K+-2Cl− cotransporter. Active NH4+ extrusion across the mucosal membrane, combined with low permeability to NH3 in this tissue, allow for maintenance of steep ammonia gradients across the colonic epithelium and protection from ammonia toxicity. Furthermore, these studies indicate that the hen colon may be a useful new model system for the study of NH4+ transport.