Abstract
Previous studies have shown two components of glucose absorption in the small intestine: a secondary active transport through SGLT1, and unsaturated component, recently attributed mainly to the facilitated diffusion through GLUT2, but the relationship between these two components under physiological conditions remains controversial. In chronic experiments on nonanesthetized rats we investigated for the first time the kinetics of maltose hydrolysis and glucose absorption in the isolated loop of the small intestine in a wide range of maltose and glucose concentrations (25 ÷ 200 mmol/l glucose). The processes were simulated on mathematical models which took into account the current views about mechanisms of hydrolysis and transport of nutrients and geometric characteristics of the intestinal surface. The results of chronic experiments and mathematical simulation have shown that under the close to physiological conditions the glucose transport mediated by SGLT1 is the main mechanism of its absorption in comparison with the unsaturated component. This was demonstrated not only at low, but also at high substrate concentrations. We conclude that correct evaluation of the relative contribution of different mechanisms in glucose transport through the intestinal epithelium requires taking into account the geometric specificities of its surface.
Highlights
In accordance with the classic view, glucose is transported across the brush border membrane by Na+/glucose co-transporter SGLT1, while the facilitated diffusion mediated by GLUT2 provides the output of glucose from enterocytes across the basolateral membrane into the extracellular space [1,2,3,4,5,6,7]
To evaluate the relative roles of “active” and “passive” components of glucose absorption in the small intestine, we investigated for the first time the kinetics of hydrolysis of maltose and glucose absorption in the isolated loop of the small intestine in chronic experiments on nonanaesthetizes rats in a wide range of the substrate
Curves are the results of modeling. 1—maltose hydrolysis; 2— released glucose absorption; 2a—the “active” transport component; 2b—the unsaturated transport component
Summary
In accordance with the classic view, glucose is transported across the brush border membrane by Na+/glucose co-transporter SGLT1, while the facilitated diffusion mediated by GLUT2 provides the output of glucose from enterocytes across the basolateral membrane into the extracellular space [1,2,3,4,5,6,7].The results of numerous experiments in vivo have revealed the existence of unsaturated (“diffusion”) component of glucose absorption, which may exceed greatly the secondary active transport through SGLT1 at high substrate concentrations [6,8,9].To explain an importance of the unsaturated component J.R. In accordance with the classic view, glucose is transported across the brush border membrane by Na+/glucose co-transporter SGLT1, while the facilitated diffusion mediated by GLUT2 provides the output of glucose from enterocytes across the basolateral membrane into the extracellular space [1,2,3,4,5,6,7]. Along with SGLT1, it participates in glucose transfer across this membrane [9,12]. It is assumed that at high carbohydrate loads, due to saturation of SGLT1 transporters, the facilitated diffusion via GLUT2 behaves as the unsaturated component and becomes the predominant mechanism of glucose absorption in the small intestine [9,13]
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