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First published online May 8, 2007
Journal of Experimental Biology 210, 1726-1734 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02766

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Absorption of sugars in the Egyptian fruit bat (Rousettus aegyptiacus): a paradox explained

Christopher R. Tracy^1,2,*, Todd J. McWhorter^3,4, Carmi Korine¹, Michal S. Wojciechowski^1,5, Berry Pinshow¹ and William H. Karasov³

¹ Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel
² School of Science, Charles Darwin University, Darwin, NT 0909, Australia
³ Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706, USA
⁴ Department of Veterinary Biology & Biomedical Science, Murdoch University, Murdoch, WA 6150, Australia
⁵ Department of Animal Physiology, Institute of General and Molecular Biology, Nicolas Copernicus University, Torun, Poland

^* Author for correspondence (e-mail: chris.tracy{at}cdu.edu.au)

Accepted 1 March 2007

Two decades ago D. J. Keegan reported results on Egyptian fruit bats (Rousettus aegyptiacus, Megachiroptera) that were strangely at odds with the prevailing understanding of how glucose is absorbed in the mammalian intestine. Keegan's in vitro tests for glucose transport against a concentration gradient and with phloridzin inhibition in fruit bat intestine were all negative, although he used several different tissue preparations and had positive control results with laboratory rats. Because glucose absorption by fruit bats is nonetheless efficient, Keegan postulated that the rapid glucose absorption from the fruit bat intestine is not through the enterocytes, but must occur via spaces between the cells. Thus, we hypothesized that absorption of water-soluble compounds that are not actively transported would be extensive in these bats, and would decline with increasing molecular mass in accord with sieve-like paracellular absorption. We did not presume from Keegan's studies that there is no Na⁺-coupled, mediated sugar transport in these bats, and our study was not designed to rule it out, but rather to quantify the level of possible non-mediated absorption. Using a standard pharmacokinetic technique, we fed, or injected intraperitonealy, the metabolically inert carbohydrates L-rhamnose (molecular mass=164 Da) and cellobiose (molecular mass=342 Da), which are absorbed by paracellular uptake, and 3-O-methyl-D-glucose (3OMD-glucose), a D-glucose analog that is absorbed via both mediated (active) and paracellular uptake. As predicted, the bioavailability of paracellular probes declined with increasing molecular mass (rhamnose, 62±4%; cellobiose, 22±4%) and was significantly higher in bats than has been reported for rats and other mammals. In addition, fractional absorption of 3OMD-glucose was high (91±2%). We estimated that Egyptian fruit bats rely on passive, paracellular absorption for the majority of their glucose absorption (at least 55% of 3OMD-glucose absorption), much more than in non-flying mammals.

Key words: paracellular nutrient uptake, carbohydrate absorption, Chiroptera, Egyptian fruit bat, Rousettus aegyptiacus

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