APICAL ACIDIFICATION INDUCES PARACELLULAR INJURY IN CANINE GASTRIC-MUCOSAL MONOLAYERS

We used primary monolayer cultures of enzyme-dispersed canine oxyntic mucosal cells mounted in Ussing chambers to characterize the apical ba rrier to H+, [H-3]mannitol Aux (MF) and [C-14]inulin Aux (IF) were e u sed as size probes for tight junctions. Apical H+ produced a three-pha se effect. In phase 1, as the apical pH was decreased from 7 to about 2.5, resistance (R) increased, but Short-circuit current (I-sc) did no t change. In phase 2, an increased paracellular permeability developed at pH below 2.5-1.7, evidenced by decreased R and increased MF but no t IF. Size sieving and monolayer integrity were preserved, and this pa racellular leak was either fully reversed dr stabilized by epical neut ralization, depending on the duration of the paracellular leak: In pha se 3, after sustained exposure to an apical pH below similar to 2, tra nsepithelial integrity was lost; R decreased to fluid R, and both MF a nd IF increased. Basolateral acidification below pH 5.5 produced rapid monolayer disruption. Low concentrations of cytochalasin D (CD) decre ased R and increased MF but not IF; apical acidification to pH 4 after CD increased R and decreased the MF, indicating reduced paracellular permeability by apical H+. Apical amiloride did not alter I-sc; howeve r, after 48 h of treatment with hydrocortisone and insulin, an amilori de-sensitive I-sc component became evident, Our data indicate that the increase in R observed with apical acidification reflects decreased p aracellular permeability and that the earliest injury with apical acid ification is a selective paracellular leak.