The recent identification of taste-receptor signaling elements in the gastrointestinal tract has fueled tremendous excitement with regards to the potential role of the taste system in nutrient sensing and nutrient metabolism. The G-protein coupled receptors, T1R2 and T1R3 which form the sweet taste receptor (STR) and gustducin, the G protein involved in taste signal transduction are all expressed in the small intestine of humans and other animals. Although the functional significance of these receptors in humans has yet to be demonstrated, recent animal model studies provide intriguing indications. Stimulation of the STR upregulates the intestinal sodium-glucose transporter (SGLT1), thereby, promoting glucose absorption from the intestinal lumen. In addition, because the taste signaling elements are co-expressed in the intestinal L-cells which secrete the hormone, glucagon-like peptide (GLP);during food ingestion, stimulation of the STR also elicits the release of GLP. Blockade of the STR with the inhibitor lactisole or genetic knock out of gustducin or T1R3 inhibits upregulation of SGLT1 and the GLP response to glucose. These data provide evidence for a role of the STR in regulating glucose absorption and intestinal hormone release. The possibility that the STR could increase or decrease GLP levels is of significant clinical interest as GLP plays a critical role in glucose homeostasis by stimulating insulin release, delaying gastric emptying and inhibiting hepatic glucose production. Patients with type 2 diabetes mellitus (T2DM) exhibit blunted GLP levels and enhancing GLP is the target of new diabetes therapeutics. In the proposed studies, we will bring our expertise in glucose metabolism and apply it to this exciting new area by using stable isotope methodologies to monitor post-prandial hormonal release as well as endogenous glucose production (EGP),the rate of appearance of ingested glucose and glucose disposal. To date, the effect of the STR on these indices of glucose metabolism have not been examined in humans. We are proposing to investigate the effects of stimulating (Specific Aim 1) and inhibiting (Specific Aim 2) the sweet taste receptor on GLP and post-prandial hormone release as well as indices of glucose metabolism in healthy subjects and patients with Type 2 diabetes mellitus (T2DM). Findings from the proposed studies will have important clinical implications with respect to the impact of dietary sugars and non- nutritive sweeteners on glucose homeostasis and the etiology of T2DM. Our overall hypothesis is that in healthy subjects, the STR receptor is involved in the regulation of post-prandial glucose metabolism and GLP levels but in T2DM, the STR receptor is unresponsive to activation or inhibition
Exciting new findings demonstrate that sweet taste receptors, originally thought to be located specifically on the tongue, are also present in the lumen of the small intestine of humans and other animals. Animal's studies suggest that these intestinal sweet taste receptors may play a role in glucose absorption from the intestine and regulation of intestinal hormone secretion. The proposed studies will explore the functional significance of these intestinal taste cell receptors in healthy control subjects and type 2 diabetes mellitus patients.