Aquaporin‐9 is Downregulated in the Liver of Rat with Extrahepatic Cholestasis

Abstract

Hepatocytes express aquaporin‐9 (AQP9), a basolateral channel permeable to water, glycerol and other neutral solutes. While liver AQP9 is known for mediating the uptake of glycerol its relevance in bile secretion remains elusive. Here, we evaluated whether defective expression of AQP9 is associated to secretory dysfunction of rat hepatocytes following bile duct ligation (BDL). By immunoblotting, 1‐day BDL resulted in a slight decrease of AQP9 protein in basolateral membranes and a simultaneous increase of AQP9 in intracellular membranes. This pattern was steadily accentuated in the subsequent days of BDL as at 7‐days BDL basolateral membrane AQP9 decreased by 85% whereas intracellular AQP9 increased by 115%. However, the total amount of AQP9 in the livers from day 7 of BDL rats was reduced by 49% compared to the sham counterpart. Results were confirmed by immunocytochemistry and consistent with biophysical studies showing considerable decrease of the basolateral membrane water and glycerol permeabilities of cholestatic hepatocytes. The AQP9 mRNA was slightly reduced only at day 7 of BDL indicating that the dysregulation was mainly occurring at a posttranslational level. The altered expression of liver AQP9 during BDL was not dependent on insulin, a hormone known to downregulate AQP9, since insulinemia was unchanged in 7‐days BDL rats. These results suggest that extrahepatic cholestasis leads to downregulation of liver AQP9 and dysregulated AQP channels contribute to bile flow dysfunction of cholestatic hepatocyte.