Kinetics and mechanisms of glucose absorption in the rat small intestine under physiological conditions

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.

Mechanisms of Glucose Absorption in the Small Intestine in Health and Metabolic Diseases and Their Role in Appetite Regulation

“…From a methodological point of view, it should be noted that the relative contribution of various mechanisms (active transport mediated by SGLT1 and passive diffusion mediated by GLUT2) to the total glucose absorption in the small intestine may significantly depend on the experimental in vivo approaches. For example, surgery and narcosis accompanying acute and chronic in vivo experiments significantly influence the levels and kinetics of nutrient hydrolysis and absorption in the small intestine [ 57 , 58 , 59 ]. In this regard, it is possible that different mechanisms of glucose absorption in the small intestine may be differently sensitive to anesthesia.…”

“…Moreover, a quantitative evaluation of the relative contribution of different mechanisms of glucose absorption to the total glucose absorption may also depend on the method of calculation of the corresponding kinetic constants; in particular, whether the effects of the pre-epithelial diffusion layer and the peculiarities of the intestinal absorptive surface (presence of villi) are included in the calculations [ 28 , 59 , 60 ]. For example, according to the theoretical estimates, the saturation of the SGLT1 starts on the top of the villi and then, with a further increase of the luminal substrate concentration, it continues along the lateral surface of the villi to the direction of the crypt [ 60 , 61 ].…”

The role of fructose transporters in diseases linked to excessive fructose intake

“…Due to limited space, this review will emphasize papers published after our 2008 review, and will not discuss the functional characteristics of these GLUTs as well as the large number of studies linking fructose to metabolic syndrome. It will also not cover the controversial but relevant hypothesis related to the translocation of GLUT2 to the apical membrane where it becomes the main sugar transporter (Au et al 2002; Le Gall et al 2007; Kellett et al 2008; Chaudhry et al 2012) but that has not been observed by others (Moran et al 2010; Scow et al 2011; Gorboulev et al 2012; Gruzdkov et al 2012) for reasons outlined in the review of Wright et al (2011) on sodium glucose cotransport. Finally, we are not reviewing the extensive literature regarding GLUT2 and diabetes.…”

In vitro gastrointestinal digestibility of native, hydroxypropylated and cross-linked wheat starches

“…On the other hand, under SIC, the changes in the "a" exponent of the two-term exponential model showed the amounts of glucose release in the plateau state, which were 4.99-9.48, 4.46-9.73 and 4.19-7.71 (mg/ 100 ml) for NWS, CLWS and HPWS, respectively. There were no specific investigations on modeling of gastrointestinal glucose release from the gelatinized native or modified starches, whether in vitro or in vivo experiments, but several studies were found to be involved in the modeling of intestinal glucose absorption, 32,33 disintegration kinetics of solid foods during gastric digestion 34 and elution profile of sodium caseinate. 35 It is important to note that the observed digestion behaviors and the obtained glucose release extent were in a simple digestive system included only in each starch sample separately, so the calculated amounts of glucose release both under SGC and SIG cannot be interpreted as the in vivo results for starchy foods, because they are much more complex.…”