Ammonia blockade of intestinal epithelial K+ conductance

Ammonia profoundly inhibits cAMP-dependent Cl- secretion in model T84 human intestinal crypt epithelia. Because colonic lumen concentrations of ammoni a are high (10-70 mM), ammonia may be a novel regulator of secretory diarrh eal responsiveness. We defined the target of ammonia action by structure-fu nction analysis with a series of primary amines (ammonia, methylamine, ethy lamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, and octylamine) that vary principally in size and lipid solubilities. The amine concentrations required for 50% inhibition of Cl- secretion in intact mono layers and 50% inhibition of outward K+ current (I-K) in apically permeabil ized monolayers vs. the logs of the respective amine partition coefficients give two plots that are strikingly similar in character. Half-maximal inhi bition of short-circuit current (I-sc) by ammonia was seen at 6 mM and for I-K at 4 mM; half-maximal inhibition for octylamine was 0.24 mM and 0.19 mM for I-sc and I-K, respectively. The preferentially water-soluble hydrophil ic amines (ammonia, methylamine, ethylamine) increase in blocking ability w ith decreasing size and Lipophilicity. Conversely, the preferentially lipid -soluble hydrophobic (propylamine, butylamine, pentylamine, hexylamine, hep tylamine, octylamine) amines increase in blocking ability with increasing s ize and lipophilicity. Ammonia does not affect isolated apical Cl- conducta nce; amine-induced changes in cytosolic and endosomal pH do not correlate w ith secretory inhibition. We propose that ammonia in its protonated ammoniu m form (NH4+) inhibits cAMP-dependent Cl- secretion in T84 monolayers by bl ocking basolateral K+ channels.