Aquaporin water channels in gastrointestinal physiology

Fluid transport is a major function of the gastrointestinal (GI) tract with more than 9 litres of fluid being absorbed or secreted across epithelia in human salivary gland, stomach, the hepatobiliary tract, pancreas, small in testine and colon. This review evaluates the evidence that aquaporin-type w ater channels are involved in G-I fluid transport. The aquaporins are a fam ily of small (similar to 30 kDa) integral membrane proteins that function a s water channels. At least seven aquaporins are expressed in various tissue s in the GI tract: AQP1 in intrahepatic cholangiocytes, AQP4 in gastric par ietal cells, AQP3 and AQP4 in colonic surface epithelium, AQP5 in salivary gland, AQP7 in small intestine, AQP8 in liver, pancreas and colon, and AQP9 in liver. There are functional data suggesting that some GI cell types exp ressing aquaporins have high or regulated water permeability; however, ther e has been no direct evidence for a role of aquaporins in GI physiology. Re cently, transgenic mice have been generated with selective deletions of var ious aquaporins. Preliminary evaluation of GI function suggests a role for AQP1 in dietary fat processing and AQP4 in colonic fluid absorption. Furthe r study of aquaporin function in the GI tract should provide new insights i nto normal GI physiology and disease mechanisms, and may yield novel therap ies to regulate fluid movement in GI diseases.