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First published online July 20, 2007
Journal of Experimental Biology 210, 2754-2764 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006114

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Mechanistic bases for differences in passive absorption

Shana R. Lavin¹, Todd J. McWhorter^1,2 and William H. Karasov^1,*

¹ Department of Wildlife Ecology, University of Wisconsin-Madison, Madison, WI 53706, USA
² School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150, Australia

^* Author for correspondence (e-mail: wkarasov{at}wisc.edu)

Accepted 15 May 2007

Increasing evidence indicates that small birds have more extensive non-mediated, paracellular intestinal absorption of hydrosoluble compounds than do mammals, although studies have not employed uniform methodologies or demonstrated differences at the tissue level. The mechanistic bases behind apparent species differences are poorly understood. We show using uniform methodology at the whole-animal level that intact, unanesthetized pigeons had significantly higher absorption of L-arabinose and L-rhamnose, two water-soluble compounds used to measure paracellular absorption, than similarly sized laboratory rats. The species differences were also evident using perfused isolated loops of duodenum, showing that the difference in paracellular absorption occurred at the tissue level, even when D-glucose absorption rates (transcellular+paracellular) were similar between the two species. The greater absorption of these probes in pigeons could not be explained by mediated uptake of the putative paracellular probes, or by increased nominal surface area, increased villus area or increased number of tight junctions. Rats and pigeons had comparable absorption of larger probes, which is consistent with similar effective pore size of the tight junction between enterocytes. The elimination of these mechanistic explanations might suggest that pigeon intestine has relatively higher paracellular solvent drag, but pigeon duodenal segments did not have higher net water absorption than rat duodenal segments. Whatever the exact mechanism(s), the paracellular pathway of both species limits substantial (>5%) fractional absorption to molecules smaller than about 4.8 Å (M_r ca. 350), and permeability to smaller molecules at the tissue level is higher in pigeons than in rats.

Key words: paracellular nutrient absorption, tight junction, solvent drag, gut morphology

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