MCWHORTER, T.J.; KARASOV, W.H.; GREEN, A.K.; Univ. of Wisconsin, Madison: How the American robin absorbs glucose

American robins (Turdus migratorius) assimilated 92 � 2 % of D-glucose and 92 � 14% of L-glucose, the stereioisomer not actively transported. Does this finding, similar to that found in other birds with diverse diet and taxonomic associations, confirm that passive absorption is important for nutrient intake? Perhaps not, because L-glucose might be absorbed at a slower rate than D-glucose. An elegant approach to this issue has been to compare the extent and rate of absorption of L-glucose vs. D-glucose or its non-metabolized analogue 3-O-methyl D-glucose (3OMD-glucose) simultaneously. We applied a pharmacokinetic method, called deconvolution, to characterize the time course of absorption based on the time course of appearance of radiolabeled probes in blood of intact birds fed a meal by gavage. D-glucose was completely absorbed by 5-min post gavage, whereas complete absorption of the other two probes occurred between 20 and 45 min (3OMD-glucose and L-glucose, respectively). Thus, absorption of glucose analogues that are actively and passively absorbed (D- and 3OMD-) is faster than the glucose analogue only passively absorbed, and 3OMD-glucose is an imperfect proxy for D-glucose because it is absorbed relatively slower, probably due to its relatively lower affinity for glucose transporter(s). Assuming that absorption of L-glucose is a proxy for passive absorption, whereas the absorption of D-glucose or 3OMD-glucose represents the sum of passive plus mediated absorption, the ratio of the apparent absorption rates (L/D) indicates the proportion of glucose absorption that occurs via the passive pathway. Based on the ratios in our experiments, (0.5 for D-glucose, 0.7 for 3OMD-glucose), at least 50% of glucose absorption occurred by the passive pathway. Supported by NSF IBN-0216709.