Rapid upregulation of sodium-glucose transporter SGLT1 in response to intestinal sweet taste stimulation.
ABSTRACT We set out to examine the short-term regulation of the intestinal sodium/glucose cotransporter SGLT1 by its substrate glucose and sweet taste analogs.
Intestinal SGLT1 is a putative target for antidiabetic therapy; however, its physiological regulation is incompletely understood, limiting its application as a pharmacological target. While it is clearly regulated by dietary composition over a period of days, its short-term regulation by nutrients is unknown.
Sprague-Dawley rats were anesthetized, and the duodenum cannulated. D-glucose, D-fructose, saccharin, D-mannitol, and water were infused for 3 hours, before harvest of proximal jejunum for SGLT1 analysis with Western blotting and quantitative polymerase chain reaction. In further experiments, the receptor region was identified by D-glucose infusion of isolated regions. Lastly, the vagus was de-afferented with capsaicin, and 5HT3-receptor activation of vagal afferents inhibited using ondansetron, before repeating experiments using water or D-glucose infusion.
Infusion of D-glucose led to 2.9-fold up-regulation in SGLT1 compared with water or iso-osmotic D-mannitol; this effect was replicated by D-fructose or saccharin. This response was strongest following isolated infusions of duodenum and proximal jejunum, with a blunted effect distally; topography matched the expression profile of sweet taste receptor T1R2/T1R3. The reflex was abolished by capsaicin pretreatment, and blunted by ondansetron.
The agonist response implicates the luminal-based sweet-taste receptor T1R2/T1R3, with the reflex apparently involving vagal afferents. The proximal nature of the sensor coincides with the excluded biliopancreatic limb in Roux-en-Y gastric bypass, and this may provide a novel explanation for the antidiabetic effect of this procedure.
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ABSTRACT: Active uptake of D-glucose and L-proline at 50 mM was measured in everted intestinal sleeves of mice whose dietary carbohydrate and protein levels were being varied experimentally. Compared to a nearly carbohydrate-free meat diet, a 50% carbohydrate laboratory chow diet stimulated active glucose uptake in the proximal intestine without affecting proline uptake, passive glucose permeability, or several measures of mucosal mass. Switching from a low-protein high-carbohydrate to a high-protein no-carbohydrate diet reversibly stimulated proline uptake while inhibiting glucose uptake. For each solute and diet switch, the stimulation of transport was complete within 1 day, while the inhibition required several days. The results imply induction and repression of intestinal glucose and proline transport by dietary substrate levels. This mechanism, in conjunction with the normal gradient of nutrient concentrations along the intestine, is probably largely responsible for the gradient in nutrient transport along the intestine.Proceedings of the National Academy of Sciences 01/1984; 80(24):7674-7. · 9.74 Impact Factor
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ABSTRACT: Over the last decade, a debate has developed about the mechanism of the passive or 'diffusive' component of intestinal glucose absorption and, indeed, whether it even exists. Pappenheimer and colleagues have proposed that paracellular solvent drag contributes a passive component, which, at high concentrations of sugars similar to those in the jejunal lumen immediately after a meal, is severalfold greater than the active component mediated by the Na+-glucose cotransporter SGLT1. On the other hand, Ferraris & Diamond maintain that the kinetics of glucose absorption can be explained solely in terms of SGLT1 and that a passive or paracellular component plays little, if any, part. Recently, we have provided new evidence that the passive component of glucose absorption exists, but is in fact facilitated since it is mediated by the rapid, glucose-dependent activation and recruitment of the facilitative glucose transporter GLUT2 to the brush-border membrane; regulation involves a protein kinase C (PKC)-dependent pathway activated by glucose transport through SGLT1 and also involves mitogen-activated protein kinase (MAP kinase) signalling pathways. This topical review seeks to highlight the significant points of the debate, to show how our proposals on GLUT2 impact on different aspects of the debate and to look at the regulatory events that are likely to be involved in the short-term regulation of sugar absorption during the assimilation of a meal.The Journal of Physiology 04/2001; 531(Pt 3):585-95. · 4.38 Impact Factor
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ABSTRACT: Nutrient feedback from the small intestine modulates upper gastrointestinal function and energy intake; however, the molecular mechanism of nutrient detection is unknown. In the tongue, sugars are detected via taste T1R2 and T1R3 receptors and signalled via the taste G-protein alpha-gustducin (G alpha(gust)) and the transient receptor potential ion channel, TRPM5. These taste molecules are also present in the rodent small intestine, and may regulate gastrointestinal function. Absolute transcript levels for T1R2, T1R3, G alpha(gust) and TRPM5 were quantified in gastrointestinal mucosal biopsies from subjects with and without type 2 diabetes; immunohistochemistry was used to locate G alpha(gust). Effects of luminal glucose on jejunal expression of taste molecules were also quantified in mice. T1R2, T1R3, G alpha(gust) and TRPM5 were preferentially expressed in the proximal small intestine in humans, with immunolabelling for G alpha(gust) localised to solitary cells dispersed throughout the duodenal villous epithelium. Expression of T1R2, T1R3, TRPM5 (all p<0.05) and G alpha(gust) (p<0.001) inversely correlated with blood glucose concentration in type 2 diabetes subjects but, as a group, did not differ from control subjects. Transcript levels of T1R2 were reduced by 84% following jejunal glucose perfusion in mice (p<0.05). Taste molecules are expressed in nutrient detection regions of the proximal small intestine in humans, consistent with a role in "tasting". This taste molecule expression is decreased in diabetic subjects with elevated blood glucose concentration, and decreased by luminal glucose in mice, indicating that intestinal "taste" signalling is under dynamic metabolic and luminal control.Gut 12/2008; 58(3):337-46. · 10.73 Impact Factor