LAVIN, S.R.; MCWHORTER, T.J.; KARASOV, W.H.; Univ of WI : Do birds exhibit greater paracellular absorption than mammals?
Water-soluble compounds can be absorbed paracellularly (in between epithelial cells) in the small intestine, and this absorption is enhanced by transport of luminal nutrients possibly due to solvent drag. Differential paracellular uptake may make certain taxa more vulnerable to toxicant absorption, but also may offer a physiologically inexpensive means of nutrient absorption. Previous studies suggest that birds have more extensive paracellular absorption than mammals, but a formal comparison using uniform methodology has not been done. Using Sprague-Dawley rats and pigeons, we measured the absorption of a series of carbohydrate probes not known to be transported by mediated pathways and ranging in molecular weight. We hypothesized that fractional absorption of carbohydrate probes would be greater in pigeons than in rats and that absorption would be inversely related to molecular weight and greater in the presence of D-glucose. Using a standard pharmacokinetic technique, we gavaged or injected the carbohydrates and measured the appearance of probes in urine and/or blood over time to calculate the bioavailability of these probes when fed in the presence and absence of D-glucose. As predicted,1)carbohydrate absorption was more extensive in pigeons than rats,2)absorption by rats was inversely related to probe molecular weight, and 3)absorption was greater in the presence of D-glucose. These data provide a direct comparison between similar-sized avian and mammalian species and are consistent with previous studies suggesting that birds have more extensive paracellular absorption than mammals. This study confirms that two common laboratory species can be used as models for future experiments examining the mechanistic basis for species differences in paracellular transport. The methodology used in this research can be applied to additional avian and mammalian species for more extensive taxonomic comparisons. Supported by IBN-0216709 to W.H.K.
