Intestinal Adaptation Symposium

Abstract

The absorptive function of the intestinal tract depends on the intact villous and crypt structures which are the functional units of the small intestine. The villi increase the absorptive surface area of the intestine 7to 14-fold. The epithelial cells covering the surface of the villus are mature cells that are covered with a microvillous membrane, which increases the apical surface area 14to 4O-fold. Digestive enzymes such as disaccharides and peptidases, transport proteins such as glucose Na+ carriers, and receptors and channels for electrolyte transport are primarily associated with the microvillous membrane. The intestinal cells at the tip of the villus are renewed every 48 -72 h by migrating cells from the crypts. Like other organs the intestinal tract is subject to adaptational responses when subjected to various hormonal, neuroendocrine, nutritional, and surgical stimuli. The occurrence and importance of intestinal adaptation is well recognized for d l ages. The regulation and specific features s f the adaptation process are now only beginning to be characterized. The papers presented in this symposium are each unique in their own right and present specific aspects of small intestinal adaptation. Current evidence suggests that glucose transport across the brush border membranes occurs by a carriermediated process. This process is electrogenic and is driven by the Na+ gradient across the cell. Dr. Richard Fedorak emphasized the fact that surface messengers of transport are involved in intestinal adaptation. He used the diabetic rat model to explore regulation of the glucose transporter and Na+ -K+ ATPase. Enhanced glucose transport in the diabetic model appears to result from recruitment of glucose transporters. which in turn may be regulated by brush border membrane fluidity. It is clear that differences in chain length and double bond arrangement of fatty acids may alter membrane physical properties and affect certain membrane properties such as receptor-dependent transport and enzyme activity. The small intestinal cell, the enterocyte, has multiple membrane transpor? functions and enzyme activities that may be subject to regulation by enterocyte membrane composition. Dr. Manohar Garg presented evidence that enterocyte microsomd desaturases may regulate the enterocyte membrane composition. A9 and A6 desaturase activity, present in the microsomd fraction of enterocytes, influences the polyunsaturated fatty acid content of the enterocyte membrane. The activity of enterocytes is modified by nutrient supply and psition along the intestinal tract. Dr. Monika Keelan gave evidence for critical period programming sf intestinal transport hnction by diet. When the polyunsaturated to saturated fatty acid content of the diet was changed, jejunal microcellular membrane lipid composition was likewise accordingly modified, and this in turn influenced hexose intestinal uptake. Dr. Keelan also presented evidence that a high cholesterol diet will prevent the age-related decline in glucose transport. After resection, the remaining small intestind tract undergoes hyperplasia and hypertrophy. Dr. Vijay Grey gave evidence of the effect of a novel growth-stimulating activity derived fkom the proximal smdl intestine on the adaptive response of the small intestine following resection. The partially purified growth-stimulating protein was only present in the jejunum for 96 h post-resection and required oral feeding for its induction. Dr. Henry Koopmans gave evidence that gut signals from the lower small intestine have the potential to regulate food intake. Surgical transposition of the ileum to the upper small intestine decreased food intake. An optimally hwctioning gastrointestinal tract is a prerequisite for nutrient absorption and hence optimum health. The small intestinal tract undergoes adaptive responses to wide-ranging stimuli. This symposium presents specific aspects of intestinal adaptation that should be of interest to neonatologists, pediatric gastroenterologists, nutritionists, physiologists, and cellular biologists. We wish to thank the sponsors of this symposium, Mead Johnson Canada and Ross Laboratories.