The stomach must survive HCl secreted by the gastric epithelium. This challenge is heightened by the frequent presence of compounds (e.g. non-steroidal anti-inflammatory drugs) or organisms (Helicobacter pylori) -which compromise the gastric barrier to acid, leading to ulcers when the barrier is breached for sustained intervals. An alkaline juxtamucosal gel layer is the first line of gastric defense: acting to protect the gastric epithelium from back-diffusion of acid from the lumen. We developed an in vivo confocal microscopy approach to non-invasively measure pH in the pre-epithelial compartment, and have extended this methodology to include use of two-photon light absorption to fatally damage selected surface epithelial cells in a few seconds. In response, the tissue manifests a large but transient surge of alkali in the juxtamucosal layer, creates a mucoid cap to cover the "micro-lesion" of exfoliated cells, and undergoes epithelial restitution in a matter of minutes. This is the first model of the microscopic epithelial disruptions that must occur daily due to epithelial renewal, or physical injury of the epithelium by ingested food stuffs. Our objective is to define the elements limiting progression of gastric damage and favoring restoration of epithelial and juxtamucosal defenses after microscopic lesions, using the micro-lesion model with confocal microscopy to follow epithelial and pre-epitheila repair in real-time. Our first aim asks if surface pH regulation is important to the progression, of acute gastric damage and repair. Using wild-type mice, we will test if the transient alkali surge is protective to the epithelium, and localize the site of the restored epithelial barrier during repair. Using COX-knockout mice and COX inhibitors, we will define which cycloxygenase isoforms regulate tissue repair from acute damage. The second aim examines the roles of motogens in pre-epithelial and epithelial defense. Using wild-type and motogen null mice (TFF1, TFF2 and TGF-beta1 knockouts) we will ask if motogens regulate surface pH in healthy tissue, or stimulate epithelial restitution after micro-lesions. Fixed specimens will be used to ask if motogens additionally act on cells adjacent to the micro-lesion to inhibit proliferation or inhibit apoptosis. In the third aim, we will ask if more extensive damage caused by H. pylori can be initiated on micro-lesions. We will test if micro-lesions are an attractant guiding H. pylori to the gastric surface during initial colonization, and if H. pylori-induced gastritis affects the response to focal epithelial lesions. Results will extend our knowledge about conditions that strengthen or weaken gastric defense, in response to a superficial lesion of a size that is encountered daily during normal epithelial renewal.