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NH 4 + secretion in the avian colon. An actively regulated barrier to ammonium permeation of the colon mucosa
Author(s) -
Laverty Gary,
Holtug Klavs,
Árnason Sighvatur S.,
Skadhauge Erik
Publication year - 2008
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.22.1_supplement.1239.11
Subject(s) - amiloride , bafilomycin , chemistry , ion transporter , ussing chamber , permeation , ammonium , ammonia , biophysics , transepithelial potential difference , analytical chemistry (journal) , ion , secretion , sodium , biochemistry , chromatography , membrane , biology , apoptosis , organic chemistry , autophagy
Experiments were done to characterize active, electrogenic transport of NH 4 + ions across the colonic epithelium of the domestic fowl ( Gallus gallus ). Stripped colonic mucosal segments were mounted in Ussing chambers and voltage‐clamped to measure the short‐circuit currents (I SC ). Bilateral addition of NH 4 + caused a dose‐dependent outward current (negative I SC ), 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 NH 4 + to the serosal (s) side, but not with mucosal (m) addition. Pre‐treatment with 10 −4 M amiloride exposed a net outward (negative) I SC , and NH 4 + addition (s) further increased this outward current (Km of 53 + 24 mM). Unidirectional NH 4 + 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), which closely matched the change in I SC with NH 4 + (s), showing that the transepithelial flux of NH 4 + could account for the outward current response. Addition of 50 nM bafilomycin A to the mucosal solution completely eliminated s‐m NH 4 + transport, implicating an apical V‐type H + ‐ATPase in this transport process. Active NH 4 + extrusion, combined with low permeability to NH 3 in this tissue, allow for maintenance of steep ammonia gradients across the colonic epithelium and protection from ammonia toxicity. The hen colon may be a useful system for the study of NH 4 + transport.