The ability of nutrients in the intestinal lumen to initiate changes in secretory and motor function in the gastrointestinal tract is well established. The precise nature of the """"""""sensors"""""""" is not well characterized. Dietary carbohydrate in the intestinal lumen is well established to inhibit gastric emptying and food intake. The hypothesis to be tested is that dietary carbohydrate-induced inhibition of gastric emptying is dependent on binding of glucose to the sodium-glucose co-transporter SGLT on enterochromaffin (EC) cells, leading to release of 5-hydroxytryptamine (5-HT) that activates 5-HT3 receptors on extrinsic afferent nerve terminals in the intestinal mucosa. To test this hypothesis, studies are proposed to determine that (1) inhibition of gastric emptying by glucose is dependent on binding of glucose to SGLT; (2) glucose-induced release of 5-HT from EC cells is dependent on binding of glucose to SGLT; (3) activation of vagal and spinal afferents in response to glucose is dependent on SGLT and 5-HT3 receptor activation and (4) to demonstrate the expression of 5-HT3 receptors on vagal and spinal afferent terminals innervating the duodenum and their association with EC cells. The role of SGLT-1 and 5-HT release in glucose-induced inhibition of gastric emptying will be determined in awake rats by intestinal perfusion of analogues of glucose that are or are not substrates for SGLT, and the role of binding to SGLT will be determined by infusion of phloridzin that binds but is not internalized. Finally, the role of sodium ions or protons will be determined by alteration of their concentrations in intestinal glucose perfusates. 5-HT secretion will be measured in vitro from BON cells, a model for native EC cells, to determine that glucose acts directly on EC cells. The expression of SGLT by BON cells and native EC cells will be determined by measurement of expression of SGLT protein by Western blot and by immunocytochemistry. Duodenal vagal and spinal afferent discharge will be measured to determine whether glucose activates extrinsic afferent fiber activity and whether this is mediated by SGLT and a 5-HT3-receptor pathway. The cellular sites of expression of 5-HT3 receptors in the intestinal wall and their origins will be determined in immunohistochemical studies using antisera raised to the 5-HT3 receptor in tissue from intact, capsaicin-treated and extrinsically denervated rats. The relationship between primary afferent nerve terminals expressing 5-HT3 receptors and EC cells expressing 5-HT will be determined in double labeling experiments. These studies address sensory transduction in the gastrointestinal tract, which is important in the regulation of normal digestive function. There is evidence that sensory function of the intestine is altered in pathological conditions such as inflammatory bowel disease, functional bowel disease and obesity. A greater understanding of these sensory mechanisms will help in understanding the pathophysiology of these diseases.
