Natural sugars and artificial sweeteners are sensed by receptors in taste buds. T2R bitter and T1R sweet taste receptors are coupled through G-proteins, α-gustducin and transducin, to activate phospholipase C β2 and increase intracellular calcium concentration. Intestinal brush cells or solitary chemosensory cells (SCCs) have a structure similar to lingual taste cells and strongly express α-gustducin. It has therefore been suggested over the last decade that brush cells may participate in sugar sensing by a mechanism analogous to that in taste buds. We provide here functional evidence for an intestinal sensing system based on lingual taste receptors. Western blotting and immunocytochemistry revealed that all T1R members are expressed in rat jejunum at strategic locations including Paneth cells, SCCs or the apical membrane of enterocytes; T1Rs are colocalized with each other and with α-gustducin, transducin or phospholipase C β2 to different extents. Intestinal glucose absorption consists of two components: one is classical active Na + -glucose cotransport, the other is the diffusive apical GLUT2 pathway. Artificial sweeteners increase glucose absorption in the order acesulfame potassium ∼ sucralose > saccharin, in parallel with their ability to increase intracellular calcium concentration. Stimulation occurs within minutes by an increase in apical GLUT2, which correlates with reciprocal regulation of T1R2, T1R3 and α-gustducin versus T1R1, transducin and phospholipase C β2. Our observation that artificial sweeteners are nutritionally active, because they can signal to a functional taste reception system to increase sugar absorption during a meal, has wide implications for nutrient sensing and nutrition in the treatment of obesity and diabetes.

Perfusion of rat jejunum in vitro with PMA increased fructose transport by 70% compared with control values and was blocked by the protein kinase C (PKC) inhibitor chelerythrine. The brush-border membrane contained both the fructose transporters GLUT5 and GLUT2; the presence of the latter was confirmed by luminal biotinylation. PMA increased the GLUT2 level 4-fold within minutes, so that the level was comparable with that of the basolateral membrane, but had no effect on GLUT5 level. GLUT2 was functional, accessible to luminal fructose and could be inhibited selectively by phloretin to permit determination of GLUT2- and GLUT5-mediated transport components. The 4-fold increase in GLUT2 level induced by PMA was matched by a 4-fold increase in GLUT2-mediated transport: there was a compensatory fall in the GLUT5-mediated rate. The pattern of dynamic trafficking was seen only for GLUT2, not GLUT5 or SGLT1, implying that GLUT2 trafficks to the brush-border membrane by a different pathway. Trafficking of GLUT2 to the brush-border membrane correlated with activation of PKC betaII, implying that this isoenzyme is likely to control trafficking. Since PKC is activated by endogenous hormones, GLUT2 levels in vivo are 3-4-fold those in vitro; moreover, because PKC is inactivated as soon as intestine is excised, GLUT2 is lost from the brush-border within minutes in vitro. It is therefore difficult to detect GLUT2 in most in vitro preparations and its role in intestinal sugar absorption across the brush-border membrane has accordingly been overlooked.

The adaptation of d-fructose transport in rat jejunum to experimental diabetes has been studied. In vivo and in vitro perfusions of intact jejunum with d-fructose revealed the appearance of a phloretin-sensitive transporter in the brush-border membrane of streptozotocin-diabetic rats which was not detectable in normal rats. The nature of the transporters involved was investigated by Western blotting and by d-fructose transport studies using highly purified brush-border and basolateral membrane vesicles. GLUT5, the major transporter in the brush-border membrane of normal rats, was not inhibited by d-glucose or phloretin. In contrast, GLUT2, the major transporter in the basolateral membrane of normal rats, was strongly inhibited by both D-glucose and phloretin. In brush-border membrane vesicles from diabetic rats, GLUT5 levels were significantly enhanced; moreover the presence of GLUT2 was readily detectable and increased markedly as diabetes progressed. The differences in stereospecificity between GLUT2 and GLUT5 were used to show that both transporters contributed to the overall enhancement of d-fructose transport measured in brush-border membrane vesicles and in vitro isolated loops from diabetic rats. However, overall d-fructose uptake in vivo was diminished. The underlying mechanisms and functional consequences are discussed.

T1R taste receptors are present throughout the gastrointestinal tract. Glucose absorption comprises active absorption via SGLT1 and facilitated absorption via GLUT2 in the apical membrane. Trafficking of apical GLUT2 is rapidly up-regulated by glucose and artificial sweeteners, which act through T1R2 + T1R3/α-gustducin to activate PLC β2 and PKC βII. We therefore investigated whether non-sugar nutrients are regulated by taste receptors using perfused rat jejunum in vivo. Under different conditions, we observed a Ca 2+ -dependent reciprocal relationship between the H + /oligopeptide transporter PepT1 and apical GLUT2, reflecting the fact that trafficking of PepT1 and GLUT2 to the apical membrane is inhibited and activated by PKC βII, respectively. Addition of l-glutamate or sucralose to a perfusate containing low glucose (20 mm) each activated PKC βII and decreased apical PepT1 levels and absorption of the hydrolysis-resistant dipeptide l-Phe( S)-l-Ala (1 mm), while increasing apical GLUT2 and glucose absorption within minutes. Switching perfusion from mannitol to glucose (75 mm) exerted similar effects. l-Glutamate induced rapid GPCR internalization of T1R1, T1R3 and transducin, whereas sucralose internalized T1R2, T1R3 and α-gustducin. We conclude that l-glutamate acts via amino acid and glucose via sweet taste receptors to coordinate regulation of PepT1 and apical GLUT2 reciprocally through a common enterocytic pool of PKC βII. These data suggest the existence of a wider Ca 2+ and taste receptor-coordinated transport network incorporating other nutrients and/or other stimuli capable of activating PKC βII and additional transporters, such as the aspartate/glutamate transporter, EAAC1, whose level was doubled by l-glutamate. The network may control energy supply.

We have proposed a model of intestinal glucose absorption in which transport by SGLT1 induces rapid insertion and activation of GLUT2 in the apical membrane by a PKC βII-dependent mechanism. 2+ absorption represents a complex nutrient-sensing system, which allows both absorptive pathways to be regulated rapidly and precisely to match dietary intake.