Comparison of different dietary sugars as inducers of intestinal sugar transporters.

Intestinal sugar transport increases with dietary carbohydrate levels, but the specific regulatory signals involved have been little studied. Hence we compared rations containing one of five sugars [D-glucose, D-galactose, 3-O-methyl-D-glucose (3-O-MG), D-fructose, and maltose] in their effects on brush-border uptake of five transported solutes (D-glucose, D-galactose, 3-O-MG, D-fructose, and L-proline) by everted sleeves of mouse small intestine. As confirmed by transepithelial potential difference (PD) measurements, there is a distinct fructose transporter that does not evoke a PD, along with one or more aldohexose transporters that do evoke a PD. Galactose and 3-O-MG rations cause a twofold increase in feeding rates, mucosal hyperplasia, and hence nonspecific increases in uptake per unit length of intestine for all transported solutes. Dietary fructose is by far the best specific inducer of the fructose transporter. The five dietary sugars are of fairly similar potency as specific inducers of aldohexose transport, but dietary galactose and fructose may be slightly more potent than glucose. Regulatory signals need not be transported substrates, or vice versa, and need not be metabolizable. Variation in uptake ratios of pairs of aldohexoses with ration and intestinal position suggest multiple aldohexose transporters of overlapping specificity, with different relative activities at different positions and with different susceptibilities to induction by different dietary sugars.