Rapid upregulation of sodium-glucose transporter SGLT1 in response to intestinal sweet taste stimulation

Abstract

Objective: We set out to examine the short-term regulation of the intestinal sodium/glucose cotransporter SGLT1 by its substrate glucose and sweet taste analogs.

Summary background data: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

FIGURE 1

Infusion with 250 mM D-glucose leads to a 2.9-fold up-regulation in jejunal SGLT1 protein expression (A, *P < 0.05 compared with water infusion), but no change in SGLT1 mRNA. Expression is shown relative to water infusion. These effects are specific to SGLT1, with no effect when looking at GLUT2, either at transcriptional level or on Western blotting (B). Similarly, no effect is seen when infusing 250 mM D-mannitol compared with water: protein densitometry results are shown in (C).

FIGURE 2

SGLT1 Western blot results are shown for whole bowel infusions of 250 mM D-fructose and 0.3% saccharin compared with infusions with water. Results for each show relative SGLT1 expression (as measured by densitometry) compared with water infusion (**P < 0.01 compared with water). Results from infusion of D-glucose are presented for comparison.

FIGURE 3

Expression of taste receptors (α-gustducin, T1R2, T1R3) were measured in proximal duodenum, proximal jejunum, mid small bowel, and terminal ileum, using qPCR. Both α-gustducin and T1R3 showed topographical variation, peaking in proximal to midsmall bowel. While T1R2 trended towards a regional variation, this did not meet significance.

FIGURE 4

A–C, are schematic representations of isolated regional infusions. In (A), a clip placed at the ligament of Trietz, and a drainage catheter in situ, allow infusion of just the stomach and duodenum (labeled D in [Fig. D, E]). In (B), a clip is placed at both ligament of Trietz and at 15 cm distal to this, infusing only proximal jejunum (PJ). Lastly, in (C), infusion is restricted to distal jejunum and ileum (DJ). D, shows densitometry results for Western blots of protein extracts taken from proximal jejunum in each case, compared with whole bowel water infusion. Whole bowel glucose infusion is also shown for comparison. E, shows a representative SGLT1 immunoblot from proximal jejunal protein extracts obtained after regional infusion. WI, Whole intestine; D, Duodenum; PJ, proximal jejunum; DJ, Distal jejunum. *P < 0.05 compared with water; **P < 0.01 compared with water.

FIGURE 5

A, shows the results of intestinal D-glucose infusion on jejunal SGLT1 expression, compared with water, in animals pretreated with capsaicin to de-afferent the vagus nerve. There is no significant difference in SGLT1 expression, either in terms of mRNA or lysate protein. B, examines the effects of pretreatment with ondansetron. The figure shows SGLT1 protein expression after infusion with D-glucose or water, in animals injected 30 minutes previously with ondansetron (1 mg/kg intraperitoneally; labeled “Ondansetron +”). This shows a blunted response compared with previous experiments without ondansetron pretreatment (labeled “Ondansetron −”; see Fig. 1A), reproduced alongside for comparison.