The small intestinal fructose transporters: site of dietary perception and evidence for diurnal and fructose sensitive control elements.

To obtain an insight into the mechanisms responsible for GLUT5 diurnality and fructose responsiveness, rats were gavaged at 9:00 AM or 6:00 PM with 1 g of fructose in the presence or absence of cycloheximide. After 4 h of fructose exposure, GLUT5 mRNA and protein levels increased 2-3.5-fold above the natural diurnal levels of expression. In situ hybridization and immunochemical analysis of GLUT5 mRNA and protein demonstrated that both diurnality and fructose responsiveness was confined to mature enterocytes. The protein synthesis inhibitor, cycloheximide, blunted the diurnal and fructose driven increase in GLUT5 mRNA expression in the morning, but had minimal effect on the pattern of expression in the evening. This differential sensitivity of intestinal GLUT5 mRNA to de novo protein synthesis may reflect the increasing presence of diurnal and fructose sensitive control factors during the day. Following vehicle gavage, Cycloheximide was more effective in reducing GLUT5 protein expression levels in the morning when compared to the evening. These data suggest that the turnover of GLUT5 protein may be diurnally influenced.