Regulation of intestinal sugar transport.

The recent surge in knowledge of cellular and molecular mechanisms of intestinal sugar transport has fueled an enormous interest in adaptive mechanisms regulating sugar transport. We first review several functional considerations that help us interpret the different patterns of adaptation for different nutrients. We then distinguish nonspecific adaptive mechanisms leading to parallel changes in transport of different nutrients from specific adaptive mechanisms only affecting the transport of a single nutrient. Nonspecific adaptive mechanisms include changes in mucosal surface area and in the ratio of transporting to nontransporting cells; specific mechanisms include changes in site density of transporters and in affinity constants. We also enumerate the patterns of regulation and describe how sugar transport is affected by changes in diet, energy budgets, and environmental salinity as well as by intestinal resection, starvation, stress, and age. We relate the various signals linking these stimuli to adaptive mechanisms and make predictions about the nature of these signals. Finally, we describe the significance of the interactions among sugar, fluid, and electrolyte transport mechanisms and of the paracellular pathway to transepithelial transport of sugars. We close by drawing attention to promising directions for future research.