Fig. 1. The time course for absorption of radiolabeled 3OMD-glucose and L-glucose differs according to whether most glucose absorption is mediated by the Na⁺-glucose cotransporter or is passive. The predicted patterns shown were generated using a chemical reactor model of intestinal absorption of radiolabeled probes, with a starting substrate concentration of 100 mmol l^-1 (see Discussion). (A,B) Mediated uptake is assumed to dominate and so the maximal rate of 3OMD-glucose transport (V_max) was taken to be 5x that which was measured in house sparrow small intestine in vitro (2.083 nmol min^-1 µl^-1; Caviedes-Vidal and Karasov, 1996) whereas the passive permeability coefficient (K_a) was taken to be the measured value (0.05 nmol µl^-1). In this situation, percent cumulative absorption of the 3OMD-glucose (A), whose uptake is both mediated and passive (broken line), increases faster and is overall greater than the percent cumulative absorption of L-glucose, whose uptake is only passive (solid line). The apparent absorption rate (i.e. instantaneous slope; B) of 3OMD-glucose exceeds that of L-glucose. (C,D) Passive uptake is assumed to dominate and so the V_max was taken to be the measured value and K_a was taken to be 5x the measured value. In this situation, the percent cumulative absorption of the 3OMD-glucose is only marginally faster and higher than L-glucose (C), and the apparent absorption rate of the 3OMD-glucose is only marginally higher than that of L-glucose (D). The Michaelis constant for mediated uptake (apparent K_m) was fixed at 16.5 mmol l^-1, 3x the measured value. These figures are shown because these types of data can be empirically generated using the pharmacokinetic methods described in Materials and methods and Results.