Luminal fructose modulates fructose transport and GLUT-5 expression in small intestine of weaning rats.

In neonatal rats, precocious introduction of dietary fructose significantly enhances brush-border fructose transport rates and GLUT-5 mRNA levels during early weaning. In this study, these rates and levels were more than two times higher in the anastomosed intestine compared with those in the bypassed loop of weaning pups that underwent Thiry-Vella surgery and consumed high-fructose (HF) diets. In Thiry-Vella pups fed fructose-free (NF) diets, uptake rates and mRNA levels in the anastomosed intestine were very low and similar to those in the bypassed loop. In sham-operated littermates, transport rates and mRNA levels were similar between intestinal regions that corresponded to anastomosed and bypassed loops in Thiry-Vella pups and were two to three times greater in pups fed HF than in those fed NF diet. In contrast, rates of brush-border glucose transport and levels of SGLT-1 and of GLUT-2 mRNA were independent of diet and were similar between bypassed and anastomosed regions. Changes in GLUT-5 expression did not follow a distinct diurnal rhythm. When pups were fed HF diet after 12 h of starvation to empty the intestinal lumen, fructose transport rates increased with feeding duration and reached a plateau 12-24 h after feeding; in contrast, GLUT-5 mRNA levels were highest within 4 h after arrival of chyme in the jejunum and then decreased gradually and returned to baseline levels 24 h later. In littermates fed NF diet, mRNA levels and uptake rates were each independent of feeding duration. Luminal, and not endocrine, signals regulate GLUT-5 expression in weaning pups.