We have previously reported that rat jejunal glucose absorption comprises an active component mediated by SGLT1 and a facilitated component mediated by GLUT2 (Kellett & Helliwell, 2000). At high glucose concentrations, similar to the effective concentrations likely to be present at the apical membrane after a meal, the facilitated component is as much as three to five times greater than the active. At 50 mM glucose the ratio of the passive to the active component was 2.1. These results were obtained by low stress jejunal perfusion in vivo with low flow rate (0.75 ml min^-1) and zero pressure head. Under these conditions, most of the facilitated component was dependent on the active component. Here we report the relationship of the two components in high stress perfusions with high flow rate and pressure head.

Jejunal loops of male Wistar rats (from humanely killed animals), anaesthetized I.P. with 1.0 ml Hypnorm plus 0.4 ml Hypnovel, were perfused in vivo with 50 mM D-glucose in Krebs-Henseleit buffer in the absence (control) or presence of inhibitor(s). The perfusate was recirculated at a rate of 7.0 ml min^-1 and segmented with gas (95 % O₂/5 % CO₂) at a rate of 2.0 ml min^-1: the pressure head of the perfusate reservoir was 18 cm, so that the jejunum became distended. Absorption was measured by the luminal disappearance of glucose; n = 4 for all perfusions, values given as means ± S.E.M. and statistical significance assessed by paired t test. The control rate was 42.0 ± 3.7 mmol min^-1 (g dry wt)^-1. Phloridzin (0.5 mM) inhibited absorption by 51 ± 4 % (P < 0.001), a value close to that reported by Debnam & Levin (1975) under similar high stress conditions. Cytochalasin B (0.2 mM) inhibited absorption by 49 ± 8 % (P < 0.001), demonstrating that the large phloridzin-insensitive component was mediated by a facilitative transporter, confirmed by Western blotting to be GLUT2. The ratio of the passive to active components was therefore close to 1. Phloridzin and cytochalasin B together inhibited absorption by 92 ± 7 % (P < 0.001).

The results provide additional support for our proposal that rat intestinal glucose absorption comprises a facilitated as well as an active component. They further show that in high stress perfusions the facilitated component is independent of the active component, in contrast to the situation in low stress perfusions. The sum of the two components determined by independent inhibition with cytochalasin B and phloridzin accounts within experimental error for all absorption. This finding was confirmed by the fact that simultaneous inhibition of both components abolishes all but a few per cent of absorption. Non-carrier-mediated absorption is therefore minimal.

This work was supported by The Wellcome Trust.

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