title: Mechanism and rate of glucose absorption differ between an Australian honeyeater (Meliphagidae) and a lorikeet (Loriidae).
abstract: 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 approximately 80%). 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.
authors.affiliation: School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150, Australia. K.Napier@murdoch.edu.au
authors.forename: Kathryn R
authors.initials: KR
authors.lastname: Napier
authors.forename: Todd J
authors.initials: TJ
authors.lastname: McWhorter
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Fleming
mesh_terms: D019325:3-O-Methylglucose / Q000378:metabolism; D000042:Absorption; D000818:Animals; D001315:Australia; D005947:Glucose / Q000378:metabolism* / Q000493:pharmacokinetics; D011563:Psittaciformes / Q000378:metabolism*; D020308:Songbirds / Q000378:metabolism*
chemical_list: D019325:3-O-Methylglucose; D005947:Glucose