Glucose transporter gene expression in lactating bovine gastrointestinal tract.

Absorption of glucose from the intestine and probably from various portions of the gastrointestinal tract (GIT) in lactating dairy cows can have profound implications because the availability of glucose, or glucose precursors, seems to be a limiting factor for milk synthesis. This article reports on the expression of the Na+/glucose cotransporter gene (SGLT1) in mucosal tissues from rumen, omasum, abomasum, duodenum, jejunum, ileum, and cecum of lactating cows. Sodium/glucose cotransporter mRNA transcripts were detected in polyadenylated RNA of all the GIT tissues examined and in total RNA from rumen, omasum, duodenum, jejunum, ileum, and cecum. The pattern of SGLT1 mRNA distribution in these tissues was different from that of the facilitative glucose transporter GLUT5. Sodium/glucose cotransporter protein was also detected in small intestinal membranes of lactating dairy cows. Sodium-dependent glucose uptake across small intestinal brush border membrane vesicles could be resolved into linear nonsaturable (diffusion) and saturable, carrier-mediated components. The diffusion constant of glucose from jejunal membrane vesicles was 18.3+/-2.1 pmol x mM(-1) x mg protein(-1) x s(-1). The saturable component of glucose uptake had a Vmax of 27.1+/-5.8 pmol x mg protein(-1) x s(-1) and a Km of .12+/-.05 mM. Similar transport kinetics were found in the duodenum, and they were less in the ileum. These data suggest that the GIT of lactating cows is capable of active transport of glucose from the lumen across the brush border membrane of the epithelial cells. In addition, the relatively high abundance of SGLT1 mRNA in the rumen, omasum, and cecum may imply new functional and nutritional roles for these tissues in the active transport of glucose.