The capacity for paracellular absorption in the insectivorous bat Tadarida brasiliensis
Water-soluble nutrients are absorbed by the small intestine via transcellular and paracellular processes. The capacity for paracellular absorption seems greater in fliers than in nonfliers, although that conclusion rests mainly on a comparison of flying birds and nonflying mammals because only two frugivorous bat species have been studied. Furthermore, the bats studied so far were relatively large (>85 g, compared with most bat species which are <20 g) and were not insectivores (like about 70 % of bat species). We studied the small (11 g) insectivorous bat Tadarida brasiliensis and tested the prediction that the capacity for paracellular absorption would be as high as in the other bat and avian species studied so far, well above that in terrestrial, nonflying mammals. Using standard pharmacokinetic technique, we measured the extent of absorption (fractional absorption = f) of inert carbohydrate probes: L-arabinose (MM = 150.13) absorbed exclusively by paracellular route and 3OMD-glucose (MM = 194) absorbed both paracellularly and transcellularly. As predicted, the capacity of paracellular absorption in this insectivorous bat was high (L-arabinose f = 1.03 ± 0.14) as in other frugivorous bats and small birds. Absorption of 3OMD-glucose was also complete (f = 1.09 ± 0.17), but >80 % was accounted for by paracellular absorption. We conclude that passive paracellular absorption of molecules of the size of amino acids and glucose is extensive in this bat and, generally in bats, significantly higher than that in nonflying mammals, although the exact extent can be somewhat lower or higher depending on molecule size, polarity and charge.