Ammonium ion trapping in mouse distal colon: Role of NHE3

Abstract

Despite its critical role in the development of hepatic encephalopathy, ammonium transport in the gastrointestinal tract is poorly understood. It is well known that overall in the gut net ammonium absorption occurs. However, we have shown that net secretion occurs in the distal colon of mice and thus limits overall net absorption. Ammonium secretion occurs in colonic epithelial cells via basolateral NH4+ uptake mediated by NKCC1 and Rhesus glycoprotein B (RhBG). Our data suggests apical ammonia exit occurs via Rhesus glycoprotein C (RhCG) combined with ion trapping via H+ transport on cH/K‐ATPase and/or NHE2/3. In the human colonic cell line T84, inhibition of NHE2/3 by amiloride produced a 50% decrease in secretory NH4+ flux, implicating NHE2/3 in ion trapping of ammonia. In NHE3−/− mice portal vein ammonium levels were 1.9±0.4 fold higher than in WT, while no significant change was observed in NHE2−/− or cH/K‐ATPase−/− mice. qRT‐PCR, Western blot and immunohistochemistry was used to define distal colon transporter expression in various transporter null mice to establish co‐operative function in ammonium secretion. In NHE2−/− mice NHE3−/− was up‐regulated, in cH/K‐ATPase−/− mice both RhBG and NKCC1 were upregulated but not NHE3. In the NHE3−/− mice, cH/K‐ATPase was up‐regulated. Taken together these data suggest that NHE3 plays a principle role in ammonia ion trapping. Supported by NIH DK079979.