Effect of sodium concentration and plasma sugar concentration on hexose absorption by the rat jejunum in vivo. Further evidence of two transport mechanisms

Differential responses of intestinal glucose transporter mRNA transcripts to levels of dietary sugars

Dietary sugars are known to stimulate intestinal glucose transport activity, but the specific signals involved are unknown. The Na(+)-dependent glucose co-transporter (SGLT1), the liver-type facilitative glucose transporter (GLUT2) and the intestinal-type facilitative glucose transporter (GLUT5) are all expressed in rat jejunum [Miyamoto, Hase, Taketani, Minami, Oka, Nakabou and Hagihira (1991) Biochem. Biophys. Res. Commun. 181, 1110-1117]. In the present study we have investigated the effects of dietary sugars on these glucose transporter genes. A high-glucose diet stimulated glucose transport activity and increased the levels of SGLT1 and GLUT2 mRNAs in rat jejunum. 3-O-Methylglucose, D-galactose, D-fructose, D-mannose and D-xylose can mimic the regulatory effect of glucose on the SGLT1 mRNA level in rat jejunum. However, only D-galactose and D-fructose increased the levels of GLUT2 mRNA. The GLUT5 mRNA level was increased significantly only by D-fructose. Our results suggest that the increase in intestinal transport activity in rats caused by dietary glucose is due to an increase in the levels of SGLT1 and GLUT2 mRNAs, and that these increases in mRNA may be caused by an enhancement of the transcriptional rate. Furthermore, for expression of the SGLT1 gene, the signal need not be a metabolizable or transportable substrate whereas, for expression of the GLUT2 gene, metabolism of the substrate in the liver may be necessary for signalling. Only D-fructose is an effective signal for expression of the GLUT5 gene.

Effect of chronic ethanol intake on lactase activity and active galactose absorption in rat small intestine

The effects of feeding a nutritionally adequate liquid diet containing 5% ethanol to rats over a four week period on intestinal lactase activity and the kinetics of jejunal galactose absorption in vivo have been determined. Both lactase activity and the maximum capacity for active, saturable galactose absorption (Jmax) were increased significantly after chronic ethanol ingestion. In contrast, uptake of the sugar via the phlorhizin-insensitive (passive) route was unaffected by ethanol. Our results imply the presence of an increased maturity of the enterocyte population on the villus surface in response to ethanol. The relevance of this work to uptake studies in alcoholics is briefly discussed.

Effects of neomycin on galactose absorption across rat jejunum

The effects of neomycin sulphate on galactose absorption have been studied using in vivo and in vitro preparations of rat small intestine. Neomycin (10(-3)M) produced an increase in the maximum transport capacity (Jmax) for the active component of absorption in vivo. The apparent Kt for absorption was unaffected. The antibiotic caused a dose-dependent increase in the potential difference across the mucosal membrane (Vm) measured in vitro, a maximal effect being seen at a concentration of 10(-4)M. Furthermore, the magnitude of the depolarization induced by the addition of galactose (4 mM) to the mucosal fluid was enhanced by neomycin (10(-4)M). Phlorhizin (10(-4)M) abolished the galactose-induced depolarization in both the absence and presence of the antibiotic. It is concluded that neomycin increases the electrical driving force for Na+ during Na+-coupled galactose entry into the enterocyte.