Rapid adaptation of intestinal glucose transport: a brush-border or basolateral phenomenon?

Regulation of intestinal nutrient absorption can occur on a very short time scale (i.e., 3-5 h). In this study we set out to determine whether this phenomenon of very rapid adaptation is mediated by changes at the level of the brush-border or basolateral membrane of enterocyte. We employed two experimental manipulations that had been reported to cause increased glucose transport in jejunum within as short a time as 3 h: 1) hyperglycemia caused by jugular vein infusion of glucose, and 2) perfusion of the ileal lumen with glucose. We measured glucose absorption by two methods. 1) An in vitro method that tested directly for an effect at the brush border of enterocytes, and 2) an in vivo method whose measurement is a composite phenomenon including events at both boundaries of enterocytes. Carrier-mediated D-glucose absorption in jejunum in vivo was significantly enhanced by approximately 60% in both experimental situations, but there was no significant difference in in vitro unidirectional uptake at the brush border of tissues removed from the same region of the jejunum. The experimental manipulations also had no effect on glucose uptake by brush-border membranes in the other regions of the gut, nor on the passive uptake of L-glucose or the total uptake of L-proline. These results, plus a comparison of the Vmax values for glucose absorption in vitro and in vivo suggest the following: 1) in the "nonadapted" (i.e., control) rats the rate-limiting step in glucose absorption in vivo is efflux at the basolateral membrane, and 2) the observed rapid increase in glucose absorption in vivo appears to be mediated by change at the level of the basolateral membrane.