AMMONIA EFFECT ON CALCIUM-ACTIVATED CHLORIDE SECRETION IN T84 INTESTINAL EPITHELIAL MONOLAYERS

We recently showed that ammonia profoundly inhibits cyclic nucleotide- regulated Cl- secretion in model human T84 intestinal epithelia but do es not impair the secretory response to the Ca2+ agonist carbachol. Us ing transepithelial transport, fura 2 fluorescence, and radioisotopic efflux techniques, we further explored this dichotomy and arrived at a preliminary explanation for the inhibitory action of ammonia. The sec retory response to the Ca2+-adenosinetriphosphatase inhibitor thapsiga rgin is unaffected by ammonia, which suggests that an increase in intr acellular Ca2+ stimulates secretory pathways that are insensitive to a mmonia. Surprisingly, Cl- Secretion elicited by the Ca2+ ionophores io nomycin and A23187 is markedly blunted in monolayers pretreated with a mmonia. However, ammonia posttreatment does not inhibit the secretory response to ionophore, which suggests that ammonia may interfere with the ability of these ionophores to increase intracellular [Ca2+]. This hypothesis is directly supported by fura 2 experiments. The inhibitor y action of ammonia parallels the behavior of the K+ channel blocker B a2+, and ammonia reduces the basolateral Rb-86(+) efflux rate constant in forskolin- but not in carbachol-treated monolayers. Ammonia, which is present in high concentrations in the normal gastrointestinal trac t, may serve as a novel endogenous regulator of epithelial electrolyte transport by interfering with a Ba2+ sensitive basolateral K+ conduct ance distinct from the Ca2+ activated basolateral K+ conductance.