Ammonia inhibits sodium and chloride absorption in rat distal colon.

It was recently demonstrated that ammonia inhibits sodium absorption in the proximal colon of rats. In order to investigate the effect of luminal ammonia in the distal colon, sodium and chloride transport were measured in Ussing chambers. Under short-circuit conditions, distal colon absorbed sodium and chloride. When luminal ammonia (30 mmol l(-1)) was present, sodium and chloride absorption was diminished. Inhibition of the two Na(+)-H(+) exchanger isoforms NHE2 and NHE3, which are known to be located in the apical membrane of the distal colon epithelium, failed to influence the effect of ammonia on transepithelial sodium and chloride fluxes. The inhibitory effect of ammonia was eliminated under the following conditions: after block of carbonic anhydrases with acetazolamide, in the presence of an unspecific blocker of Na(+)-H(+) exchangers, and under chloride-free conditions. Ammonia did not alter electrogenic sodium absorption. These results demonstrate that luminal ammonia inhibits sodium and chloride absorption in rat distal colon. We suggest that ammonia inhibits NaCl absorption by interfering with a Na(+)-H(+) exchanger that is not NHE2 or NHE3

Influence of ammonia on sodium absorption in rat proximal colon.

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.

K(+) secretion in rat distal jejunum.

The Ussing chamber technique was used to measure unidirectional Rb(+) fluxes under short-circuit conditions across tissue sheets from proximal, central, and distal jejunum of rats. Whereas the proximal and central parts of the jejunum did not show any net transport of Rb(+), there was a net secretion of around 0.2 micromol hr(-1) cm(-2) in the distal segment. This secretion could not be influenced significantly by mucosal application of K(+) channel blockers such as Ba(2+) (5 mm), tetraethylammonium (20 mm) or quinine (1 mm). Serosal ouabain (1 mm) blocked net secretion by increasing mucoserosal flux. Blockers of H(+)/K(+) ATPases could not alter net fluxes of Rb(+). Stimulation of Cl(-) secretion by forskolin (10 microm) or of Na(+) absorption by serine (10 mm) failed to influence the observed secretion of Rb(+). Adrenaline (10 microm) also had no effect on Rb(+) fluxes. Blocking Na(+)/H(+) exchange by 5-(N-Ethyl-N-isopropyl)-amilorid (100 microm) blocked net secretion by increasing mucoserosal flux, as did the addition of Na(+) acetate (30 mm) to the mucosal solution. We conclude that the distal jejunum of the rat secretes K(+) under short-circuit conditions. This secretion does not seem to occur via K(+) channels, but through a pH dependent mechanism.