Does apical membrane GLUT2 have a role in intestinal glucose uptake?

Abstract

It has been proposed that the non-saturable component of intestinal glucose absorption, apparent following prolonged exposure to high intraluminal glucose concentrations, is mediated via the low affinity glucose and fructose transporter, GLUT2, upregulated within the small intestinal apical border. The evidence that the non-saturable transport component is mediated via an apical membrane sugar transporter is that it is inhibited by phloretin, after exposure to phloridzin. Since the other apical membrane sugar transporter, GLUT5, is insensitive to inhibition by either cytochalasin B, or phloretin, GLUT2 was deduced to be the low affinity sugar transport route. As in its uninhibited state, polarized intestinal glucose absorption depends both on coupled entry of glucose and sodium across the brush border membrane and on the enterocyte cytosolic glucose concentration exceeding that in both luminal and submucosal interstitial fluids, upregulation of GLUT2 within the intestinal brush border will usually stimulate downhill glucose reflux to the intestinal lumen from the enterocytes; thereby reducing, rather than enhancing net glucose absorption across the luminal surface. These states are simulated with a computer model generating solutions to the differential equations for glucose, Na and water flows between luminal, cell, interstitial and capillary compartments. The model demonstrates that uphill glucose transport via SGLT1 into enterocytes, when short-circuited by any passive glucose carrier in the apical membrane, such as GLUT2, will reduce transcellular glucose absorption and thereby lead to increased paracellular flow. The model also illustrates that apical GLUT2 may usefully act as an osmoregulator to prevent excessive enterocyte volume change with altered luminal glucose concentrations.

Conflict of interest statement

Competing interests: No competing interests were disclosed.

Figures

Figure 1.. These diagrams show snapshots of the simulated glucose flows from intestinal lumen loaded with 50 mM in 150 mM NaCl to capillaries perfusing the submucosal spaces, whose afferent arterial concentration contains 150 mM NaCl and 5 mM D glucose.

The tissue in panelA has low apical GLUT2 and GLUT5 activity and low capillary permeability and perfusion rates (clearance). In panelB the tissue apical membrane GLUT2 activity is increased by 4-fold above that in panelA, capillary perfusion is unchanged. In panelC, the apical GLUT2 activity is the same as in panelA, but capillary clearance is increased by 10-fold. In panelD, the apical GLUT2 is raised, as in panelB and the capillary clearance raised, as in panelC. The rates of glucose uptake are normalized relative to the rate of SGLT1 glucose uptake (panelA). Altering either GLUT2, or capillary clearance have negligible effects on glucose inflow via SGLT1. However, after raising the apical GLUT2 activity, the steady state glucose concentration within the cytosol decreases from 68 to 52 mM (c.f. PanelsA andC). On raising capillary clearance, the steady state of cytosolic glucose concentration also decreases (c.f. PanelA versus PanelC and PanelB versus PanelD). Raising capillary glucose clearance increases the rate of glucose inflow from the interstitial to capillary fluid by fourteen fold (c.f. PanelA andC). These changes are accompanied by decreased interstitial fluid glucose from 52 to 40 mM and reductions in the mean capillary glucose from 23 to 18 mM. Reduced interstitial glucose concentrations reverse the direction of the glucose gradient across the paracellular pathway from -2 to + 10 mM. Thus raising the capillary clearance of glucose, reverses the direction of paracellular glucose flow from (-0.38) to (+2.46) and increases the net glucose inflow across the luminal surface from (0.22 to 3.23). Although raising apical membrane GLUT2 activity by fourfold reduces net glucose influx across the apical border from 0.63 to 0.15, it also indirectly leads to an increase in paracellular glucose flux and thereby causes a slight increase in net glucose flux across the luminal border. When capillary clearance is raised, either by enhanced perfusion rates, or increased endothelial permeability, increasing apical membrane GLUT2 enhances apical membrane glucose reflux from -0.14 to -0.31. This has no significant effect on glucose flow from the interstitial to capillary fluid. (c.f. panelC andD).

Figure 2.. Simulation of altered apical membrane GLUT2 activity on intestinal glucose fluxes from zero (Panel A) to an arbitrary of maximal flux value of 2 (Panel B).

The simulations show the glucose fluxes via apical SGLT1 (blue); apical GLUT2 (red); paracellular pathway (green); the total transluminal membrane, (SGLT1 + GLUT2 + paracellular fluxes), (black) and interstitial to capillary flow (pink crosses) inset on the black square. The main effect of increasing GLUT2 is to cause a negative glucose flux (backflux) via GLUT2 (PanelB). This is accompanied by a increased paracellular flux without any significant change in net transluminal or transepithelial glucose flux. The point at which paracellular glucose flux and SGLT1 flux are equal lies between 20 and 30 mM as has been previously observed – . This value is used as one of the key registration points for the model. The cytosolic (red) interstitial (blue) and mean capillary glucose concentrations (black) and enterocyte volume per unit weight of tissue (green) are shown in panelC with zero apical GLUT2 and in PanelD with GLUT2 V max = 2. Increased apical GLUT2 activity decreases cytosolic glucose concentration (panelD). With rising luminal glucose concentration raised GLUT2 activity prevents the non-linear increase in enterocyte volume seen with zero GLUT2 (PanelC).

Figure 3.. Simulation of effects of varying paracellular glucose permeability on intestinal glucose fluxes and accumulation.

The effects of varying paracellular glucose permeability P glc from 0 to 0.02 cm s -1 are shown in Figure 3A increasing P glc on paracellular glucose flux (Blue) .As P glc is increased from zero the point of equality of paracellular glucose flux Jglpc with glucose flux via SGLT1 decreases from infinity at P glc = 0 to around 20–30 mM luminal glucose when P glc = 0.01–0.02 cm s -1. Increases P glc raises interstitial glucose concentrations 3C (blue) and in parallel, cysosolic concentrations Figure 3C (red). The reduction in glucose gradient across the basolateral membrane with raised P glc reduces and then reverses glucose flux via GLUT2 ( Figure 3B).

Figure 4.. Simulation of sequential additions of phloridzin then phloretin to the luminal fluid on intestinal glucose fluxes.

GLUT2 is present in the apical membrane V max 2 and the capillary perfusion rate = 10 is similar to that shown in Figure 2 panelsB andD. The luminal glucose is 30 mM and afferent capillary glucose is 5 mM to simulate the conditions used by Kellett & Helliwell 2000 . In Figure 4, panelsA andB inhibition of SGLT1 activity at 0.5h to zero reduces glucose flux via SGLT1 to zero. Simultaneously glucose flux via GLUT2 increases thereby reversing the backflux from -1.1. to 0.05 and also paracellular glucose flux increases from 0.55 to 0.95. In panelsC andD the glucose concentration changes in the cytosol (red) interstitial fluid (blue) capillary fluid (black) and cytosolic volume (green). Following phloridzin addition and inhibition of SGLT1, cytosolic glucose falls from 33 to 30 mM; cytosolic volume falls from 0.62 to 0.56 ml at 1 hour without significant changes in capillary or interstitial fluid glucose concentration. This explains both the fall in glucose reflux via GLUT2 and the decrease in basolateral membrane flux is compensated by the rise in paracellular flux thereby nullifying interstitial glucose concentration changes. Phloretin addition at 1h is simulated by blocking apical GLUT2 (panelA) and by blocking both apical GLUT2 and paracellular glucose and Na permeability (panelB). GLUT2 fluxes fall to zero in both panelsA andB and in panelA there is a small increase in paracellular glucose flow but the total transluminal glucose flux is unaffected by addition of phloretin after phloridzin. There is a small decrease in cytosolic volume from 0.56 to ≈ 0.55. the paracellular flux falls to zero as does the transluminal glucose flux in panelB, simulating the effect observed by Kellett and Helliwell 2000 . This is accompanied by a large decrease in cell volume from 0.56 to 0.51 ml. Since no net glucose transport now occurs from the luminal fluid, capillary glucose concentration also decreases to 5 mM.

Figure 5.. Diagram showing the predicted effects from the simulation model of loading enterocytes with luminal fluid containing 50 mM glucose and 5 mM glucose in the capillary perfusion fluid in A normal enterocytes expressing apical membrane GLUT2 and in B GLUT2 KO enterocytes.

The KO cells have higher enterocyte glucose concentrations in the proximal intestine, but higher paracellular flow and larger cell volumes. The normal enterocytes have lower cytosolic glucose concentrations and SGLT1 is more widely dispersed along the intestinal length with higher rates of glucose permeation in distal regions of the small intestine. Long term exposure may lead to higher maximal glucose absorption rates in normal intestine than with GLUT2 KO.