Ontogenetic development of intestinal nutrient transporters

Abstract

New information about intestinal transporter development is accumulating rapidly. Most transporters are first expressed prenatally, but the time varies with species and transporter. Striking postnatal changes in expression include the declines of galactose and taurine transport, the postweaning turn-on of mammalian fructose, bile acid, and (in rats) lysine transporters, surges in chicken transporters when yolk reserves are exhausted and when feather molt peaks, and the steeper postnatal decline in transport for essential AAs than nonessential AAs. There are major shifts in transporter distribution along the axes from duodenum to colon and from crypt to villus. Mechanisms of these developmental changes may include changes in transporter number and membrane fluidity, plus sequential replacement of related transporters by each other. In the few well-studied cases to date, genetically hard-wired signals prove more important than external dietary signals in controlling transporter development. Growth of intestinal absorptive capacity is closely matched to the growing animal's needs. We are just acquiring the background facts needed to address the most interesting questions in this area. Obvious unsolved problems include to resolve developmental patterns for discrete hexose and amino acid transporters, to understand the functional significance of why transporters appear either prenatally and postnatally, to identify the underlying molecular mechanisms and signals, and to test whether species differ in excess absorptive capacity in ecologically meaningful ways.