Influence of ammonia on sodium absorption in rat proximal colon

Abstract

The influence of ammonia on sodium and chloride fluxes in rat proximal colon was studied in Ussing chamber experiments. Under short-circuit conditions, the proximal colon absorbed sodium and secreted chloride. The presence of ammonia (30 mmol 1(-1) mucosal) diminished Na+ absorption, but had hardly any influence on Cl- fluxes. Blocking the apical Na+/H+ exchanger isoform NHE2 by amiloride or HOE642 diminished absorptive Na+ and Cl- fluxes. In contrast, the NHE3-specific antagonist S3226 was ineffective. Amiloride and HOE642 also inhibited the effect of ammonia on net sodium absorption. In bicarbonate-free buffer solution, ammonia failed to alter the absorptive fluxes of sodium and chloride, but increased the secretory fluxes of Na+ and Cl-. The latter effect was blocked by HOE642. These results suggest that basal NaCl absorption in rat proximal colon depends to a large extent on NHE2. Mucosal ammonium decreases Na+ absorption and this effect can be antagonized by blocking NHE2. This observation suggests that ammonium interacts with the apical Na+/H+ exchanger, thereby diminishing sodium absorption.

Apical ammonium inhibition of cAMP-stimulated secretion in T84 cells is bicarbonate dependent

“…However, the extent to which K ϩ secretion may have contributed to the measured current was unclear. In contrast, Cermak et al (6) found little effect of luminal NH 4 ϩ on Cl Ϫ flux in rat colon. It is unlikely that an apical K ϩ conductance is a contributing factor in this study, (15).…”

“…Previous studies in rat and human colon as well as in cultured epithelial cell lines have demonstrated that NH 4 ϩ can inhibit Na ϩ absorption (6, 7) as well as Cl Ϫ secretion (6,16,31,32,37,48). The T84 secretory colonic cell line was used to show that basolateral NH 4 ϩ can affect cAMP-and cGMPdependent Cl Ϫ secretion but not carbachol-induced Ca 2ϩ -dependent secretion (31).…”

“…However, the extent to which K ϩ secretion may have contributed to the measured current is unclear. In contrast, using Cl Ϫ flux measurements, Cermak et al (6) found little effect of luminal NH 4 ϩ on Cl Ϫ secretion in rat colon.…”

“…Previous studies in rat and human colon demonstrated that luminal NH 4 ϩ can inhibit Na ϩ and Cl Ϫ absorption, an effect that involves interaction of NH 4 ϩ with an apical Na ϩ /H ϩ exchanger (6,7). Luminal NH 4 ϩ inhibition of forskolin-activated short-circuit current (I sc ) has been reported in rat and human colon by Mayol et al (32), but little effect was observed with basolateral NH 4 ϩ .…”

Uroguanylin Regulates Net Fluid Secretion via the NHE2 Isoform of the Na⁺/H⁺ Exchanger in an Intestinal Cellular Model

“…Even more, our results indicate that while NHE1 and NHE4 would have a housekeeping role in steady state pH i NHE2 would be mainly involved in Na + uptake. In fact, NHE2 has shown to be a major pathway for NaCl absorption in rat proximal colon [34] and to participate in Na + absorption in mouse colonic crypts [35]. Additionally, the capacity of T84 cells for absorbing Na + has been reported and informed as HOE-694 sensitive, indicating the participation of the Na + /H + exchanger [19].…”