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 barrier to H+. [3H]mannitol flux (MF) and [14C]inulin flux (IF) were used as size probes for tight junctions. Apical H+ produced a three-phase effect. In phase 1, as the apical pH was decreased from 7 to about 2.5, resistance (R) increased, but short-circuit current (Isc) did not change. In phase 2, an increased paracellular permeability developed at pH below 2.5-1.7, evidenced by decreased R and increased MF but not IF. Size sieving and monolayer integrity were preserved, and this paracellular leak was either fully reversed or stabilized by apical neutralization, depending on the duration of the paracellular leak. In phase 3, after sustained exposure to an apical pH below approximately 2, transepithelial integrity was lost; R decreased to fluid R, and both MF and IF increased. Basolateral acidification below pH 5.5 produced rapid monolayer disruption. Low concentrations of cytochalasin D (CD) decreased 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 Isc; however, after 48 h of treatment with hydrocortisone and insulin, an amiloride-sensitive Isc component became evident. Our data indicate that the increase in R observed with apical acidification reflects decreased paracellular permeability and that the earliest injury with apical acidification is a selective paracellular leak.