LAVIN, S.R.*; MCWHORTER, T.J.; KARASOV, W.H.; Univ. of Wisc.; Univ. of Wisc.; Univ. of Wisc.: Differences in paracellular absorption: a function of solvent drag or pore size?
Increasing evidence indicates that birds have more extensive intestinal non-mediated, paracellular absorption of hydrosoluble compounds than do mammals. The mechanism(s) underlying this difference has not been studied. More paracellular absorption may be due to increased water and dissolved solute flux across the tight junction between intestinal enterocytes (solvent drag) and/or a larger tight junction effective pore size. Using a recirculating duodenal perfusion technique and a range of perfusate osmolalities in pigeons and Sprague-Dawley rats, we measured the absorption of fluid and D-glucose, as well as a series of carbohydrate probes not known to be transported by mediated pathways and ranging in molecular weight. We chose these species because in our experiments on intact animals, pigeons had almost twice the absorption of smaller probes than rats. We hypothesized that if increased paracellular absorption is due to increased solvent drag, then pigeons would have greater water absorption concurrent with paracellular probe absorption than rats. We also hypothesized that a larger effective pore size in pigeons would lead to greater passage of larger molecules. We found that rats and pigeons absorb D-glucose at a comparable rate. Pigeons had greater absorption of inert carbohydrate probes, yet fluid absorption was not more extensive than rats, and pigeons absorbed carbohydrate probes when net water absorption was nil. These data are inconsistent with the notion that pigeons have greater solvent drag than rats. We also found an inverse relationship between probe molecular weight and probe absorption in both species. Our results suggest that greater paracellular nutrient absorption in pigeons is not due to increased solvent drag but instead may be a function of the tight junction effective pore size between intestinal enterocytes. Supported by IBN-0216709 to W.H.K.
