How the house sparrow Passer domesticus absorbs glucose

According to the hypothesis that most glucose absorption occurs passively across intestinal tight junctions (paracellular absorption), one would predict fairly similar rates of in vivo absorption of L-glucose, the stereoisomer of D-glucose that is absorbed only passively and is not catabolized, and of 3-O-methyl-D-glucose (3OMD-glucose), the D-glucose analogue that is actively and passively transported and not catabolized. In house sparrows Passer domesticus, we applied a pharmacokinetic method to measure simultaneous in vivo absorption of [¹⁴C] L-glucose and [³H]3OMD-glucose in a situation in which intestinal glucose transporters were relatively saturated (gavage solution contained 200 mmol l^-1 3OMD-glucose). Fractional absorptions (F) were not significantly different between [³H]3OMD- and [ ¹⁴C]L-glucose (0.80 vs 0.79), and the apparent rates of absorption did not differ significantly. When we performed the same experiment on other sparrows in a situation in which intestinal glucose transporters were relatively unsaturated (200 mmol l^-1 mannitol replaced 3OMD-glucose in the gavage solution), the apparent rate of absorption was significantly reduced for [¹⁴C]L-glucose by 39% and for [³H]3OMD-glucose by 26%. A simulation model showed that a reduction is not predicted if most of the [ ³H]3OMD-glucose is actively absorbed, because the absorption rate of the tracer should increase when competitive inhibitor (unlabeled 3OMD-glucose) is removed. The similar extent and rates of absorption of [³H]3OMD- and [¹⁴C]L-glucose, and the acceleration of their rates of absorption in the presence of luminal 3OMD-glucose, are most consistent with Pappenheimer's hypothesis that the majority of dietary D-glucose is absorbed passively.