The Role Of The AQP1 And AQP8 In Hepatobiliary System

Members of the AQP family are distributed in many cell types in different organ systems where they play important roles in various physiological functions and pathological conditions including urinary concentrating function, exocrine glandular fluid secretion, brain edema formation, regulation of intracranial and intraocular pressure, skin hydration, fat metabolism, tumor angiogenesis,cell migration and apoptosis.The major function of hepatobiliary system includes bile formation and modification by secretory and absorptive processes in the epithelial cells of intrahepatic bile ducts and gallbladder. Aquaporin (AQP)-mediated transepithelial water transport mechanism may play an important role in bile formation. To date, 13 AQPs have been found in mammals, with many numbers (AQP0, AQP1, AQP4, AQP5, AQP8, AQP9, AQP11) being localized to the hepatobiliary system in the plasma membranes of hepatocytes, the epithelial cell of bile ducts , the epithelial cells of gallbladder and intrahepatic vascular endothelial cells. In the present study, we focus on the expression and potential physiological roles of AQP1 and AQP8 in bile secretion and concentration.We collected bile from wild type and AQP8 null mice. Compare with wild type mice, bile flow was significantly decreased in AQP8 null mice under both basal and bile salt-stimulated conditions. Bile composition analysis showed no difference in the total bile acid and glutathione concemtration under physiological condition between wild type and AQP8 null mice. But after bile salt stimulation, total bile acid and glutathione concentration in bile were significantly increased in AQP8 null mice compared with wild type mice. Those data indicated that AQP8-facilitated canalicular membrane water transport is involved in hepatic bile secretion.To further confirm the reduced hepatic bile secretion in AQP8-/- liver, we isolated hepatocyte couplets from wild type and AQP8 null mice. Although the couplets number and morphology are similar in both genotypes, fluorescence assay showed significantly decreased bile secretion into canalicular space of hepatocyte couplets in AQP8 null mice compare with wild type mice. These results provide direct evidence that AQP8-mediated water transport playa an important role in hepatic bile secretion.Water transport across gallbladder epithelium is driven by osmotic gradients generated from active salt absorption and secretion. Aquaporin (AQP) water channels have been proposed to facilitate transepithelial water transport in gallbladder and to modulate bile composition. We found strong AQP1 immunofluorescence at the apical membrane of mouse gallbladder epithelium.Transepithelial osmotic water permeability (Pf) was measured in freshly isolated gallbladder sacs from the kinetics of luminal calcein self-quenching in response to an osmotic gradient. Pf was very high (0.12 cm/s) in gallbladders from wildtype mice, cAMP-independent, and independent of osmotic gradient size and direction. Although gallbladders from AQP1 knockout mice had similar size and morphology to those from wildtype mice, their Pf was reduced by ~10-fold. Apical plasma membrane water permeability was greatly reduced in AQP1-deficient gallbladders, as measured by cytoplasmic calcein quenching in perfluorocarbon-filled, inverted gallbladder sacs. However, neither bile osmolality nor bile salt concentration differed in gallbladders from wildtype vs. AQP1 knockout mice. Our data indicate constitutively high water permeability in mouse gallbladder epithelium involving transcellular water transport through AQP1. The similar bile salt concentration in gallbladders from AQP1 knockout mice argues against a physiologically important role for AQP1 in mouse gallbladder.In conclusion, our results provide evidence for AQP8-mediated water transport across hepatocyte canalicular membrane plays an important role in hepatic bile secretion. AQP1 creates high osmotic water permeability in mouse gallbladder epithelium, but does not affect bile concentration.