Ammonia blockade of intestinal epithelial K1 conductance

Hrnjez, Bruce J., Jaekyung C. Song, Madhu Prasad, Julio M. Mayol, and Jeffrey B. Matthews. Ammonia blockade of intestinal epithelial K1 conductance. Am. J. Physiol. 277 (Gastrointest. Liver Physiol. 40): G521–G532, 1999.—Ammonia profoundly inhibits cAMP-dependent Cl2 secretion in model T84 human intestinal crypt epithelia. Because colonic lumen concentrations of ammonia are high (10–70 mM), ammonia may be a novel regulator of secretory diarrheal responsiveness. We defined the target of ammonia action by structure-function analysis with a series of primary amines (ammonia, methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, and octylamine) that vary principally in size and lipid solubilities. The amine concentrations required for 50% inhibition of Cl2 secretion in intact monolayers and 50% inhibition of outward K1 current (IK) in apically permeabilized monolayers vs. the logs of the respective amine partition coefficients give two plots that are strikingly similar in character. Half-maximal inhibition of short-circuit current (Isc) by ammonia was seen at 6 mM and for IK at 4 mM; half-maximal inhibition for octylamine was 0.24 mM and 0.19 mM for Isc and IK, respectively. The preferentially water-soluble hydrophilic amines (ammonia, methylamine, ethylamine) increase in blocking ability with decreasing size and lipophilicity. Conversely, the preferentially lipid-soluble hydrophobic (propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine) amines increase in blocking ability with increasing size and lipophilicity. Ammonia does not affect isolated apical Cl2 conductance; amine-induced changes in cytosolic and endosomal pH do not correlate with secretory inhibition. We propose that ammonia in its protonated ammonium form (NH4 ) inhibits cAMP-dependent Cl2 secretion in T84 monolayers by blocking basolateral K1 channels.