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First published online October 31, 2008
Journal of Experimental Biology 211, 3544-3553 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020644

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Mechanism and rate of glucose absorption differ between an Australian honeyeater (Meliphagidae) and a lorikeet (Loriidae)

Kathryn R. Napier^*, Todd J. McWhorter and Patricia A. Fleming

School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150, Australia

^* Author for correspondence (e-mail: K.Napier{at}murdoch.edu.au)

Accepted 22 September 2008

Efficient mechanisms of glucose absorption are necessary for volant animals as a means of reducing mass during flight: they speed up gut transit time and require smaller volume and mass of gut tissue. One mechanism that may be important is absorption via paracellular (non-mediated) pathways. This may be particularly true for nectarivorous species which encounter large quantities of sugar in their natural diet. We investigated the extent of mediated and non-mediated glucose absorption in red wattlebirds Anthochaera carunculata (Meliphagidae) and rainbow lorikeets Trichoglossus haematodus (Loriidae) to test the hypothesis that paracellular uptake accounts for a significant proportion of total glucose uptake in these species. We found that routes of glucose absorption are highly dynamic in both species. In lorikeets, absorption of L-glucose (non-mediated uptake) is slower than that of D-glucose (mediated and non-mediated uptake), with as little as 10% of total glucose absorbed by the paracellular pathway initially (contrasting previous indirect estimates of 0%). Over time, however, more glucose may be absorbed via the paracellular route. Glucose absorption by both mediated and non-mediated mechanisms in wattlebirds occurred at a faster rate than in lorikeets, and wattlebirds also rely substantially on paracellular uptake. In wattlebirds, we recorded higher bioavailability of L-glucose (96±3%) compared with D-glucose (57±2%), suggesting problems with the in vivo use of radiolabeled D-glucose. Further trials with 3-O-methyl-D-glucose revealed high bioavailability in wattlebirds (90±5%). This non-metabolisable glucose analogue remains the probe of choice for measuring uptake rates in vivo, especially in birds in which absorption and metabolism occur extremely rapidly.

Key words: paracellular absorption, glucose absorption, red wattlebird, Anthochaera carunculata, rainbow lorikeet, Trichoglossus haematodus, 3-O-methyl-D-glucose, L-glucose

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