BATS ABSORB NUTRIENTS LIKE A BIRD

Abstract

If you've ever had to pay excess baggage on an aircraft, you've probably thought about the costs of flight when carrying additional loads. Another flying mammal, the bat, faces the same problem and seems to get round this problem by having less intestinal tissue than comparably sized but non-flying mammals. With this refinement bats manage to reduce the mass of material carried by the gut to improve their take-off and manoeuvrability during flapping flight. However, small bats have very high energy demands when flying and thus need to extract energy efficiently from food at high rates, despite their smaller intestines. So how do flying mammals like bats efficiently absorb energy rich carbohydrates, given their smaller intestinal surface area and high energy demands? Birds meet their high energy demands by absorbing nutrients by passive diffusion across the gut lumen into the blood(paracellular uptake). However, transporter proteins actively transport solutes across the gut lumen, via the transcellular uptake mechanism,in mammals. So is nutrient uptake in the bat's digestive system more bird like or mammal like?To answer this question, Enrique Caviedes-Vidal from Argentina and his international team of colleagues measured intestinal carbohydrate uptake in the great fruit-eating bat (Artibeus literatus). First they fed the bats, and then they injected the animals intraperitoneally with two metabolically inert, water soluble carbohydrates (l-rhamnose and cellobiose) and compared the carbohydrate concentrations in the blood plasma over 3 h after intake. As the molecular weight of cellobiose is twice that of l-rhamnose, l-rhamnose will pass into the bird's bloodstream faster than cellobiose if the uptake mechanism is paracellular,while the two carbohydrates will be absorbed at the same rate if uptake is mediated by transcellular uptake.The researchers found that intestinal uptake of l-rhamnose was much higher than the absorption of the large disaccharide cellobiose. And when the team repeated experiments with another carbohydrate,3-O-methyl-d-glucose, which is both actively and passively transported, they found out that even 3-O-methyl-d-glucose was predominantly absorbed by the paracellular pathway. The bats absorb nutrients via passive paracellular uptake. When comparing the bats'absorption rates with the data previously measured in other mammals, it turned out that passive absorption is significantly higher in great fruit-eating bats than in non-flying mammals and can amount to 70% of total glucose absorption. The observed uptake rates in the bats were more similar to those of birds,rather than mammals, and thus the authors conclude that paracellular absorption by passive diffusion may serve small bats as well as birds to compensate for their relatively small intestinal surface area.The team also propose several mechanisms that might account for high passive carbohydrate absorption including increased microvilli surface areas and more cell junctions, a higher pore radius to facilitate the sieving effect and an increase in water flux to elevated the carriage of solute across the junctions.Together, the study on intestinal absorption in the great fruit-eating bat provides evidence that small flying bats and birds compensate for their relatively small guts and reduce the surface area for absorption with paracellular carbohydrate absorption. However, the team point out that there are risks associated with paracellular absorption. High intestinal permeability might permit toxins to be absorbed in the intestinal lumen,leaving bats vulnerable to toxins that other mammals would not fall prey to.