HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) All Versions of this Article: 296/4/R1274    most recent 90783.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Soria, L. R. Articles by Marinelli, R. A. Search for Related Content PubMed PubMed Citation Articles by Soria, L. R. Articles by Marinelli, R. A.

WATER AND ELECTROLYTE HOMEOSTASIS

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

Instituto de Fisiología Experimental, Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Santa Fe, Argentina

Submitted 18 September 2008 ; accepted in final form 28 January 2009

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.

aquaporins; protein kinase A; phosphatidylinositol-3-kinase; calpains; liver