POWERS, D.R.*; MCWHORTER, T.J.; KIMBERLY, D.J.; BAKER, T.L.; THORNBURG, K.L.: Characterization and Distribution of Aquaporin Water Channels in the Hummingbird Gastrointestinal Tract

Because hummingbirds have a high-energy demand, and their diet a high-water content, they can consume in excess of twice their body weight in water each day. It has been thought that much of the water consumed simply passes through the gut. However, recent work suggests that nearly 80% of all water consumed is absorbed. While absorbing so much water poses physiological challenges, it makes sense when you consider the high evaporation rates of hummingbirds. Rapid water absorption might protect against dehydration We looked at endothelial cell morphology of hummingbird gastrointestinal tracts to understand how water absorption occurs so quickly. Of interest was distribution of aquaporin (AQP) water channels along the luminal surfaces of the gut. AQPs are an important component in all tissues through which bulk water flow occurs, so it is likely they are involved in water absorption by hummingbirds guts. We determined where in the gut AQPs were located using immunohistochemistry, then used standard molecular protocols to locate, sequence, and characterize the AQP protein. Our immunohistochemistry showed that AQPs were abundant on the surfaces of the stomach and small intestine suggesting that this is where most water is absorbed. Few AQPs were found in the esophagus, crop, or large intestine indicating that these areas do not participate substantially in water absorption. At this point the amino acid structure of the hummingbird gut AQP most closely resembles that of a mammalian AQP 1, which is insensitive to antidiuretic hormone (a primary regulator of tissue water permeability).