BACKGROUND/AIMS: Active absorption of glucose stimulates passive absorption of small solutes. Part of this effect may be caused by glucose-induced water absorption. Increased water absorption can enhance passive solute absorption by solvent drag and by passive diffusion if the luminal solute concentration increases as water is removed from the lumen. The purpose of this research was to quantitate the contribution of these two forces when glucose enhances the absorption of L-xylose. METHODS: The effect of solvent drag on L-xylose absorption was determined by measuring the effect of water absorption stimulated by hypotonicity on L-xylose absorption when the L-xylose concentration was constant. The effect of diffusion on L-xylose absorption was determined by measuring the effect of L-xylose concentration on L-xylose absorption when water absorption was nil. RESULTS: Glucose increased L-xylose absorption by 1.8 mmol.h-1 x 30 cm-1 (from 1.4 to 3.2 mmol.h-1 x 30 cm-1). The increase attributable to solvent drag was 1.03 mmol.h-1 x 30 cm-1; the increase attributable to passive diffusion was 0.75 mmol.h-1 x 30 cm-1. CONCLUSIONS: When glucose stimulates the passive absorption of L-xylose, 57% of the increase can be attributed to solvent drag and 42% to passive diffusion. Because the combined effect of these two forces can account for 99% of the observed effect, virtually all of the glucose effect on L-xylose absorption can be explained by glucose-induced water absorption.
BACKGROUND/AIMS: Active absorption of glucose stimulates passive absorption of small solutes. Part of this effect may be caused by glucose-induced water absorption. Increased water absorption can enhance passive solute absorption by solvent drag and by passive diffusion if the luminal solute concentration increases as water is removed from the lumen. The purpose of this research was to quantitate the contribution of these two forces when glucose enhances the absorption of L-xylose. METHODS: The effect of solvent drag on L-xylose absorption was determined by measuring the effect of water absorption stimulated by hypotonicity on L-xylose absorption when the L-xylose concentration was constant. The effect of diffusion on L-xylose absorption was determined by measuring the effect of L-xylose concentration on L-xylose absorption when water absorption was nil. RESULTS:Glucose increased L-xylose absorption by 1.8 mmol.h-1 x 30 cm-1 (from 1.4 to 3.2 mmol.h-1 x 30 cm-1). The increase attributable to solvent drag was 1.03 mmol.h-1 x 30 cm-1; the increase attributable to passive diffusion was 0.75 mmol.h-1 x 30 cm-1. CONCLUSIONS: When glucose stimulates the passive absorption of L-xylose, 57% of the increase can be attributed to solvent drag and 42% to passive diffusion. Because the combined effect of these two forces can account for 99% of the observed effect, virtually all of the glucose effect on L-xylose absorption can be explained by glucose-induced water absorption.