The active and passive components of glucose absorption in rat jejunum under low and high perfusion stress

In order to determine how perfusion design affects the relationship of the apparent 'active' and 'passive' components of glucose absorption, rat jejunum was perfused with 50 mM glucose under conditions of low and high mechanical stress. Phloretin or cytochalasin B was used to inhibit GLUT2 and phloridzin to inhibit SGLT1. In low stress perfusions, the ratios of the 'passive' to the 'active' components determined using phloretin and phloridzin were 2.2 and 0.43, respectively. This discrepancy was explained by the fact that phloridzin inhibits not only SGLT1 but also indirectly that part of the GLUT2-mediated component controlled by SGLT1 through the glucose-induced activation and recruitment of GLUT2 to the brush-border membrane. In high stress perfusions, the ratios of the 'passive' to the 'active' components determined using phloretin and phloridzin were 0.94 and 0.95, respectively; cytochalasin B gave 0.95. The identity of these results was explained by the observation that the passive component is not dependent on the active component, because glucose-induced activation and recruitment of GLUT2 does not occur in high stress perfusions. Simultaneous inhibition of SGLT1 and GLUT2 in high stress perfusions with phloridzin and cytochalasin B inhibited absorption by 92+/-7%; non-carrier-mediated transport is therefore minimal. Our data provide support for the view that the term 'facilitated' should be used to replace the term 'passive' in describing the component now known to be mediated by GLUT2. The study of the mechanism and regulation of this facilitated component depends crucially on the design of the perfusion system.