Glucose absorption by a nectarivorous bird: the passive pathway is paramount.

We tested the hypothesis that most sugar absorption across the small intestine's brush border is normally by a mediated pathway, i.e., the Na(+)-glucose cotransporter. In nectar-eating rainbow lorikeets (Trichoglossus haematodus, 120 g), we measured mediated D-glucose uptake in vitro using the everted-sleeve technique. The apparent Michaelis constant (7.7 mM) was similar to that observed in hummingbirds and other birds in general. Maximal mediated D-glucose uptake summed along the entire length of intestine (48 cm) was not notably high (5.34 mumol/min) when compared with other avian species and was an order of magnitude too low to explain observed rates of glucose absorption in vivo (54.5 mumol/min). This implied that nonmediated glucose absorption predominated, and independent verification of that was sought. We applied a pharmacokinetic technique to measure in vivo absorption of L-glucose, the stereoisomer that does not interact with the Na(+)-glucose cotransporter. Eighty percent of L-glucose that was ingested was absorbed, confirming that nonmediated absorption can be substantial. We discuss how equating L-glucose absorption with passive D-glucose absorption depends on certain assumptions regarding the relative importance of diffusive and convective passive flux. In either case, the conclusion about the relative importance of passive absorption should still hold.