Glucagon induces the gene expression of aquaporin-8 but not that of aquaporin-9 water channels in the rat hepatocyte

Am J Physiol Regul Integr Comp Physiol. 2009 Apr;296(4):R1274-81. doi: 10.1152/ajpregu.90783.2008. Epub 2009 Feb 4.


Glucagon stimulates the vesicle trafficking of aquaporin-8 (AQP8) water channels to the rat hepatocyte canalicular membranes, a process thought to be relevant to glucagon-induced bile secretion. In this study, we investigated whether glucagon is able to modulate the gene expression of hepatocyte AQP8. Glucagon was administered to rats at 0.2 mg/100 g body wt ip in 2, 3, or 6 equally spaced doses for 8, 16, and 36 h, respectively. Immunoblotting analysis showed that hepatic 34-kDa AQP8 was significantly increased by 79 and 107% at 16 and 36 h, respectively. Hepatic AQP9 protein expression remained unaltered. AQP8 mRNA expression, assessed by real-time PCR, was not modified over time, suggesting a posttranscriptional mechanism of AQP8 protein increase. Glucagon effects on AQP8 were directly studied in primary cultured rat hepatocytes. Immunoblotting and confocal immunofluorescence microscopy confirmed the specific glucagon-induced AQP8 upregulation. The RNA polymerase II inhibitor actinomycin D was unable to prevent glucagon effect, providing additional support to the nontranscriptional upregulation of AQP8. Cycloheximide also showed no effect, suggesting that glucagon-induced AQP8 expression does not depend on protein synthesis but rather on protein degradation. Inhibitory experiments suggest that a reduced calpain-mediated AQP8 proteolysis could be involved. The action of glucagon on hepatocyte AQP8 was mimicked by dibutyryl cAMP and suppressed by PKA or phosphatidylinositol-3-kinase (PI3K) inhibitors. In conclusion, our data suggest that glucagon induces the gene expression of rat hepatocyte AQP8 by reducing its degradation, a process that involves cAMP-PKA and PI3K signal pathways.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Aquaporins / genetics
  • Aquaporins / metabolism*
  • Cells, Cultured
  • Cyclic AMP / metabolism
  • Cyclic AMP-Dependent Protein Kinases / antagonists & inhibitors
  • Cyclic AMP-Dependent Protein Kinases / metabolism
  • Glucagon / administration & dosage
  • Glucagon / metabolism*
  • Hepatocytes / drug effects
  • Hepatocytes / enzymology
  • Hepatocytes / metabolism*
  • Injections, Intraperitoneal
  • Male
  • Nucleic Acid Synthesis Inhibitors / pharmacology
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoinositide-3 Kinase Inhibitors
  • Protease Inhibitors / pharmacology
  • Protein Kinase Inhibitors / pharmacology
  • Protein Processing, Post-Translational
  • Protein Synthesis Inhibitors / pharmacology
  • RNA, Messenger / metabolism
  • Rats
  • Rats, Wistar
  • Time Factors
  • Up-Regulation


  • Aqp9 protein, rat
  • Aquaporins
  • Nucleic Acid Synthesis Inhibitors
  • Phosphoinositide-3 Kinase Inhibitors
  • Protease Inhibitors
  • Protein Kinase Inhibitors
  • Protein Synthesis Inhibitors
  • RNA, Messenger
  • aquaporin 8
  • Glucagon
  • Cyclic AMP
  • Cyclic AMP-Dependent Protein Kinases