Role of intestinal basolateral membrane in absorption of nutrients.

Organic solutes leave the intestinal epithelium and enter the circulation via specific facilitated carriers located in the basolateral membrane. In the case of glucose it is a low-affinity, high-capacity transport system that can adapt to the carbohydrate content of the diet. Chronic diabetes also promotes the exit of glucose, and in both cases the effect results from an increased density of carriers in the basolateral membrane. In contrast, a rapid upregulation of this system that can be induced within 30 min by hyperglycemia does not involve large changes in the amount of transporter protein. Similarly, the absorptive capacity of the small intestine from some amino acids can be influenced by events occurring at the basolateral membrane. In the case of dibasic amino acid absorption, exit from the epithelium is the rate-limiting step. The activity of the basolateral carrier can be increased almost 10-fold within 60 s by the addition of micromolar concentrations of the neutral amino acid leucine to either the lumen or the plasma. This response does not involve the second messenger adenosine 3',5'-cyclic monophosphate and may represent an allosteric modulation of the carrier. These observations are discussed in relation to the role of the basolateral membrane as a locus for controlling intestinal absorption of organic nutrients.