EFFECTS OF AMMONIUM ION AND AMMONIA ON FUNCTION AND MORPHOLOGY OF IN-VITRO FROG GASTRIC-MUCOSA

The effects of ammonium ion (NH+) and ammonia (NH3) on function and mo rphology of gastric epithelial cells were studied in intact sheets of in vitro frog (Rana catesbeiana) gastric mucosa. Luminal 115 mM NH4Cl at luminal pH 8.0 (calculated [NH3] 2.7 mM), but not at 5.0 (calculate d [NH3] 3 muM) induced 1) an increase in intracellular pH (pH(i)) in o xynticopeptic cells (OPC) and decreases in transmucosal potential diff erence (PD) and electrical resistance (R) in resting tissues, 2) a dec rease in histamine-stimulated H+ secretion and an increase in H+ back- diffusion after removal of luminal NH4Cl, and 3) augmented acidificati on of OPC during luminal acidification. Serosal 30 mM NH4Cl at serosal pH 7.2 (calculated [NH3] 0.47 mM) induced 1) an increase in pH(i) in OPC and inhibition of the alkalinization of OPC after removal of ambie nt Cl-, 2) a decrease in PD associated with the increase in R and decr ease in short-circuit current, effects attenuated by serosal 15 mM K+, accentuated by 0.2 mM Ba2+, and abolished by removal of ambient Cl-, 3) a sudden drop of PD in resting. but not in stimulated tissues, effe cts prevented by high serosal pH (7.8), serosal HCO3-, or removal of l uminal Cl-, 4) a decrease in histamine-stimulated H+ secretion and an increase in H+ backdiffusion after removal of NH4Cl, and 5) augmented acidification of OPC during luminal acidification. These results sugge st that 1) luminal NH3, but not NH4+, increases backdiffusion of H+ fr om the lumen to the mucosa, 2) serosal NH3 and/or NH4+ induces depolar ization of OPC and decreases electrogenic Cl- transport, thereby atten uating the activity of the basolateral Cl-HCO3-exchanger in OPC, and 3 ) both of these effects contribute to th augmented acidification of OP C during exposure to high luminal [H+].