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

Area of Science:

• Physiology
• Animal Nutrition
• Gastrointestinal Physiology

Background:

• Sugar absorption in the small intestine is crucial for energy metabolism.
• The Na(+)-glucose cotransporter is widely considered the primary mediated pathway for glucose uptake.
• Understanding avian glucose absorption is important given diverse dietary adaptations.

Purpose of the Study:

• To test the hypothesis that mediated transport via the Na(+)-glucose cotransporter dominates sugar absorption in rainbow lorikeets.
• To quantify the contribution of nonmediated glucose absorption in this species.
• To compare glucose absorption mechanisms across avian species.

Main Methods:

• In vitro everted-sleeve technique to measure mediated D-glucose uptake.
• Pharmacokinetic analysis of in vivo L-glucose absorption.
• Comparison of in vitro and in vivo absorption rates with avian species data.

Main Results:

• Apparent Michaelis constant for D-glucose uptake was 7.7 mM, similar to other birds.
• Maximal mediated D-glucose uptake was insufficient to explain in vivo absorption rates.
• Substantial absorption (80%) of L-glucose, a non-cotransporter interacting isomer, confirmed significant nonmediated absorption.

Conclusions:

• Nonmediated glucose absorption plays a predominant role in rainbow lorikeets.
• The Na(+)-glucose cotransporter's contribution to overall glucose absorption is less significant than previously assumed.
• Passive glucose absorption mechanisms are critical for energy uptake in nectar-feeding birds.