Abstracts
1998 Digestive Disease Week

#3355

AMMONIA INHIBITS REGULATED CHLORIDE SECRETION IN IN VITRO HUMAN COLON. TC O'Brien, M Riegler*, EC Mun, M Prasad, C Pothoulakis*, JB Matthews. Department. of Surgery and *Division of Gastroenterology, Beth Israel Deaconess Medical Center, Boston, MA.

The colonic lumen normally contains up to 70 mM ammonia (mostly as NH4+), due to bacterial metabolism of dietary protein. We have shown (J. Clin. Invest. 1995;96:2142-51) that NH4Cl profoundly inhibits cAMP-but not Ca2+-regulated Cl- secretion in cultured T84 intestinal epithelia independent of changes in intracellular pH. This suggested that ammonia may be a novel endogenous inhibitor of intestinal secretion. We now examine whether our findings can be extended to native human tissue.

METHODS: Segments of freshly harvested surgical specimens of normal human colonic mucosa were stripped, mounted in Ussing chamber, bathed in standard HCO3- buffer, and gassed with 95%O2/5%CO2. Agonist-evoked changes in short-circuit current (_Isc, which reflects electrogenic Cl- secretion) were measured by voltage-current clamp. Potential difference (PD) and electrical resistance (R) were also measured. Tissues were bathed in control buffer or buffer containing 25 mM or 50 mM NH4Cl (pH 7.4) in either the mucosal or serosal chamber for 90 minutes prior to stimulation with the receptor-dependent Ca2+ -agonist carbachol (100 µM), the non-receptor Ca2+-agonist thapsigargin (5 µM), or the cAMP-agonist forskolin (10 µM). Mean ± SEM, stats by ANOVA. RESULTS: Baseline values for PD, Isc, and R were not significantly different pre- and post NH4Cl treatment. Mucosal 50 mM NH4Cl significantly inhibited _Isc elicited by carbachol (_Isc = 25.9 ± 7.3 vs 122.4 ± 18.6 µA/cm2 for NH4Cl vs. control, n=7, p < 0.05). Serosal 50 mM NH4Cl also inhibited carbachol-elicited secretion (_Isc = 5.7 ± 7.9 vs 125.7 ± 26.0 µA/cm2, n=7, p<0.05). Mucosal NH4Cl dose-dependently inhibited the response to thapsigargin (41.8 ± 8.1 vs 12.2  ± 13.3 vs 5.1 ± 5.7 µA/cm2 for control, 25 mM, and 50 mM NH4Cl, respectively, n=6, p < 0.05). Mucosal exposure to 25 or 50 mM NH4Cl also inhibited the response to forskolin (129.5 ± 4.5 vs 0.5 ± 7.5 vs 5.0 ± 14 µA/cm2 for control, 25 mM, and 50 mM NH4Cl, respectively, n=2, p < 0.05).

CONCLUSIONS: Ammonia inhibits chloride secretion in human colon. However, in contrast to findings in the T84 model, ammonia inhibits not only cAMP- but also Ca2+ -regulated secretion, and it appears to be equally effective whether exposed mucosally or serosally. The data support the contention that ammonia may be an endogenous negative regulator of intestinal transport, perhaps serving to dampen secretory responsiveness to toxins or other potentially diarrheagenic substances.

Copyright 1996 - 1998, SSAT, Inc. Revised 29 June 1998.