Physiological processes, such as secretion, digestion,

absorption, and motility, occur in response to these luminal substances, implying the presence of mucosal chemosensors, which evoke protective mucosal defense mechanisms.1 The duodenal mucosa rapidly responds to luminal chemical stimuli, not only by enhancing local defense factors, such as mucosal blood flow and HCO3- and mucus secretion, but also by inhibiting gastric emptying and secretion, in addition to producing symptoms, such as bloating, nausea, and fullness. Gastric inhibition in response to duodenal luminal substances is termed “duodenal feedback” or “duodenal brake”, originally described by Andersson in 1960.2 Intraduodenal acid inhibits gastric acid secretion and delays gastric emptying via neuronal reflexes and the release of gastric inhibitory peptide/glucose-dependent insulinotropic BTK pathway inhibitors peptide (GIP), glucagon-like peptide-1 (GLP-1), glucagon, cholecystokinin (CCK),

secretin, and somatostatin. Our laboratory has provided data supporting the hypothesis that luminal acid is sensed via submucosal cation channels expressed on afferent nerves. Luminal acid is converted to this website CO2 at the surface of the epithelial cells through mixture with secreted HCO3- and membrane-bound carbonic anhydrase activity.3 CO2 enters the cell and is hydrated to HCO3- and H+ by cytosolic carbonic anhydrase, with the H+ exiting across the basolateral membrane via the Na+/H+ exchanger NHE1, with HCO3- secreted across the apical cell membrane.4 In this fashion, large quantities of gastric acid are absorbed as the non-toxic acid equivalent CO2, which will not injure the epithelial cells. As a consequence, luminal acid is rapidly sensed by submucosal chemosensors, such as transient receptor potential vanilloid-1, which transduce the luminal chemical signal into neural

Ureohydrolase afferent responses and can then trigger efferent neurohormonal responses.5 Disruption or dysregulation of these duodenal physiological responses to postprandial luminal acid could be related to the pathogenesis of mucosal injury and nociception. Functional dyspepsia (FD) is a heterogeneous symptom complex including upper abdominal discomfort or pain, postprandial fullness, early satiety, nausea, vomiting, and bloating in the absence of organic disease, as defined by the Rome III criteria.6 Although the pathogenesis of FD is unknown, dysmotility, gastric relaxation disorders, or sensory disorders have been hypothesized. Recently, FD symptoms were correlated with duodenal acidity in basic and clinical studies.7 Although proton pump inhibitors relieve dyspepsia, gastric acid secretion or gastric mucosal acid sensitivity is normal in most FD patients.