Adaptive response of equine intestinal Na+/glucose co-transporter (SGLT1) to an increase in dietary soluble carbohydrate

Abstract

Experimental and epidemiological evidence suggests that consumption of hydrolyzable carbohydrate, hCHO (grain), by horses is an important risk factor for colic, a common cause of equine mortality. It is unknown whether the small intestinal capacity to digest hCHO and/or to absorb monosaccharides is limiting, or even if horses can adapt to increased carbohydrate load. We investigated changes in the brush-border membrane carbohydrate digestive enzymes and glucose absorptive capacity of horse small intestine in response to increased hCHO. Expression of the Na(+)/glucose co-transporter, SGLT1, was assessed by Western blotting, immunohistochemistry, Northern blotting, QPCR, and Na(+)-dependent D-glucose transport. Glucose transport rates, SGLT1 protein, and mRNA expression were all 2-fold higher in the jejunum and 3- to 5-fold higher in the ileum of horses maintained on a hCHO-enriched diet compared to pasture forage. Activity of the disaccharidases was unaltered by diet. In a well-controlled study, we determined SGLT1 expression in the duodenal and ileal biopsies of horses switched, gradually over a 2-month period, from low (<1.0 g/kg bwt/day) to high hCHO (6.0 g/kg bwt/day) diets of known composition. We show that SGLT1 expression is enhanced, with time, 2-fold in the duodenum and 3.3-fold in the ileum. The study has important implications for dietary management of the horse.

Glucose sensing and signalling; regulation of intestinal glucose transport

“…These monosaccharides, when accumulated in the enterocytes, exit the cell across the basolateral membrane into the systemic system by another Na + -independent monosaccharide transporter, GLUT2; a bidirectional transporter that can move glucose out or into the cell depending on its concentration gradient (6) . SGLT1 has been shown to be expressed on the brush-border membrane of villus enterocytes in most mammalian species studied (20)(21)(22)(23)(24)(25) . In general, expression levels are higher in jejunum >duodenum > ileum.…”

Sodium/glucose cotransporter-1, sweet receptor, and disaccharidase expression in the intestine of the domestic dog and cat: two species of different dietary habit

“…PVDF membranes were incubated for 1 h at room temperature in PBS-TM [PBS containing 0.5% (wt/vol) nonfat dried milk and 0.05% (vol/vol) Tween-20] and then incubated for 1 h with a 1:1,000 dilution of anti-SGLT1 antibody in PBS-TM. The antibody to SGLT1 was raised in rabbits to a synthetic peptide corresponding to amino acids 402-420 (STLFTMDIYTKIRKKASEK), an intracellular loop region of SGLT1 that is conserved among various species (13,18). Immunoreactive bands were detected by incubation for 1 h with affinity-purified horseradish peroxidase-linked anti-rabbit secondary antibody (DAKO) diluted 1:2,000 in PBS-TM, and visualized using an enhanced chemiluminescence system (GE Healthcare).…”

“…Immunohistochemistry was performed as previously described (13). Frozen fixed tissue was cut into 10-m sections, followed by a 1-h incubation in blocking solution [10% (vol/vol) donkey serum in PBS], at 25°C in a humidifier chamber.…”

“…The other antibody to GLUT2 (ab1342; Millipore, UK) was raised against a peptide (SHYRHVLGVPLDDRRA) corresponding to residues 40 -55 of rat GLUT2. These sequences are conserved in GLUT2 in various species (13). The antibodies to T1R2 (H-90) and T1R3 (H-145) were raised in rabbits and obtained from Santa Cruz Biotechnology.…”

Reply to Zhao and Zhang: Loss of taste receptor function in mammals is directly related to feeding specializations

“…However, numerous studies, including direct sequencing (5,6), have reported that both the pig and horse possess the sweet taste receptor and these animals are sweet-preferring. Therefore, there is no convincing evidence that any omnivorous or herbivorous mammal has lost the Tas1r2 gene function.…”