The long term goal of this proposal is to understand the pathogenesis of the human disease, reflux esophagitis (RE), and this approached mechanistically and from the relatively uncommon vantage point of the target tissue, the esophageal epithelium (EE). In this grant, we focus on the barrier properties of the EE, and especially that of the intercellular junctions (ICJs). The ICJs have importance as an essential defense against H+ entry into EE and as the initial target for H+ attack and damage that culminates in esophagitis. Therefore, one major goal is to characterize the barrier properties to select ions and molecules of the lumen-facing, apical cell membranes (ACMs) and ICJs using in vitro rabbit EE in Ussing chambers, and to determine how exposure to luminal acid alters these properties. Also, since many patients with RE have normal acid contact time on pH monitoring, experiments in Ussing chambered-EE are designed to identify (non-acidic) factors within the refluxate, meals and the esophageal inflammatory reaction that can contibute to breaking the barriers to H+ entry into EE, and for those identified, to localize the site of action to ACMs and/or ICJs and characterize, by pH-microelectrodes, the ability to lower pHi. Further, there is evidence to suggest that patients with RE have defects in (epithelial) barrier function, and that those with nonerosive esophagitis have damage localized to the ICJs. Since localization to the ICJs is key to validating our proposed pathogenesis of heartburn and esophagitis, another goal is to confirm, using in vivo PD measurements during salt superfusions, that this defect is localized to the ICJs. Moreover, there is a high rate of relapse following medical therapy and preliminary data suggest that this is attributable to persistent defects within the EE. Therefore, we hypothesize that the defects are due to persistent esophageal inflammation and studies proposed to correlate histologic esophagitis on endoscopic biopsy with relapse frequency after cessation of medical therapy. Also, defects in epithelial defense are sought by PD measurements during acid perfusion in asymptomatic elderly Caucasian males, a group at high risk for RE, and if found, to determine if the defect is in the barrier, and age or genetics related. Another goal is to use morphology (epifluorescence/confocal microscopy, freeze fracture, TEM, immunocytochemistry), Ussing chambers and SDS-PAGE technology, to study the structure/function of the ICJs in healthy EE and to assess the mechanisms of H+ damage. This effort coupled with the studies above should provide new insights into the pathogenesis of RE.