Aim: 

Liver expresses AQP9, a basolateral channel permeable to water, glycerol and other small neutral solutes. Although AQP9 is known for mediating the uptake of sinusoidal blood glycerol its relevance in bile secretion physiology and pathobiology remains elusive. Here, we evaluate whether defective expression of AQP9 underlies secretory dysfunction of rat hepatocytes following bile duct ligation (BDL).

Methods: 

BDL rats were used as a surgical model of obstructive extrahepatic cholestasis. The mRNA and protein levels of AQP9 were defined by RT-PCR and immunoblotting. The osmotic permeability of the basolateral membrane was assessed by stopped flow light scattering.

Results: 

One-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 AQP9 immunoreactivity of the total liver membranes from day 7 of BDL rats was reduced by 49% compared to the sham counterpart. Results were confirmed by immunofluorescence and immunogold electron microscopy and consistent with biophysical studies showing considerable decrease of the basolateral membrane osmotic water permeability of cholestatic hepatocytes. The AQP9 mRNA was slightly reduced only at day 7 of BDL indicating that the AQP9 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 regulate negatively AQP9 at a mRNA level, since insulinemia was unchanged in 7-days BDL rats.

Conclusions: 

The results suggest that extrahepatic cholestasis leads to downregulation of AQP9 in the basolateral membrane and dysregulated AQP9 contributes to bile flow dysfunction of cholestatic hepatocyte.