Differential mechanisms of ammonia transport in rat duodenum and colon

Abstract

The major source of serum ammonia is colonic luminal bacteria, which contribute to ammonia production from amino acid metabolism. The ammonia transporters, rhesus (Rh) glycoprotein B (RhBG) and C (RhCG), expressed in the GI tract, are implicated in ammonia absorption. Since little is known about ammonia transport in the GI tract, including segmental differences, and since ammonia absorption is a major risk factor for hepatic encephalopathy, we investigated ammonia transport in the duodenum and colon.We measured NH3/NH4+ transport in Ussing chambered mucosa‐submucosa preparation of rat duodenum, and proximal (PC) and distal colon (DC). Isc changes and serosal ammonia concentrations were measured before and after luminal application of NH4Cl (1 – 30 mM). We also perfused a proximal duodenal or distal colonic loop in vivo with an NH4Cl solution while measuring portal venous (PV) ammonia concentrations.Luminal application of NH4Cl rapidly and concentration‐dependently increased Isc in the duodenum, whereas Isc gradually increased in response to NH4Cl in the colon. The peak response of NH4Cl‐induced Isc was duodenum = DC >> PC, while the time to peak was duodenum << PC = DC. NH4Cl‐induced Isc was greater when luminal pH was alkaline (pH 7.4 > 7.0 >> 6.4) in the colon, but pH‐independent in the duodenum. Serosal ammonia appearance was consistent with Isc changes in the duodenum, but inconsistent with Isc changes, and constantly increasing, in the colon. NH4Cl‐induced Isc in the duodenum was sensitive to ouabain, but insensitive to luminal Gd3+ and serosal bumetanide, most likely due to cation absorption. NH4Cl‐induced Isc in the colon was sensitive to luminal Gd3+ and serosal bumetanide, but insensitive to ouabain, suggesting that absorbed ammonia may induce Cl− secretion. Luminal perfusion of the duodenum and DC with NH4Cl similarly increased PV ammonia concentration. Quantitative PCR showed that RhBG was ubiquitously expressed in the GI mucosa, whereas RhCG was expressed in the duodenal mucosa, but not in the PC or DC mucosa.Electrogenic, pH‐independent, ammonia absorption in the duodenum may be explained by apical RhCG‐mediated NH4+ transport, whereas luminal pH‐dependent ammonia absorption in the colon characterized by a gradual increase of Isc may be related to simple NH3 diffusion, followed by stimulated secretion. These results may explain why oral NH4Cl induces acute metabolic acidosis, whereas slow NH3 absorption in the distal colon may be implicated in hepatic encephalopathy.Support or Funding InformationVA Merit Review, NIH R01 DK54221