Expression and localization of aquaporin water channels in rat hepatocytes. Evidence for a role in canalicular bile secretion.
Although bile formation requires that large volumes of water be rapidly transported across liver epithelia, including hepatocytes, the molecular mechanisms by which water is secreted into bile are obscure. The aquaporins are a family of 10 channel-forming, integral membrane proteins of approximately 28 kDa numbered 0-9 that allow water to rapidly traverse epithelial barriers in several organs including kidney, eye, and brain. We found transcripts of three of 10 aquaporins in hepatocytes (aquaporin 8 aquaporin 9 > aquaporin 0) by reverse transcription-polymerase chain reaction and quantitative ribonuclease protection assays; immunohistochemistry confirmed the presence of these three proteins in liver. Immunoblots of subcellular fractions of hepatocytes showed enrichment of aquaporins 0 and 8 in microsomes and canalicular plasma membranes; aquaporin 9 was enriched only in basolateral plasma membranes. Immunofluorescence of hepatocyte couplets confirmed the intracellular/canalicular localization of aquaporins 0 and 8 and the basolateral localization of aquaporin 9. Upon exposure of couplets to a choleretic stimulus (i.e. dibutyryl cAMP), aquaporin 8 redistributed to the canalicular plasma membrane; the subcellular distributions of aquaporins 0 and 9 were unaffected. In addition, exposure of couplets to dibutyryl cAMP caused an increase in canalicular water transport in the presence and absence of an osmotic gradient, an effect that was blocked by aquaporin inhibitors. These results provide evidence that aquaporins are present in hepatocytes and that aquaporins are involved in agonist-stimulated canalicular bile secretion.
Huebert
Robert C
RC
Center for Basic Research in Digestive Diseases, Mayo Medical School, Clinic and Foundation, Rochester, Minnesota 55905, USA.
Splinter
Patrick L
PL
Garcia
Fabiana
F
Marinelli
Raul A
RA
LaRusso
Nicholas F
NF
D000818:Animals; D020346:Aquaporins / Q000737:chemistry* / Q000378:metabolism; D001646:Bile / Q000378:metabolism; D001648:Bile Canaliculi / Q000378:metabolism*; D003994:Bucladesine / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005136:Eye Proteins / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism*; D015151:Immunoblotting; D007150:Immunohistochemistry; D007473:Ion Channels; D008099:Liver / Q000378:metabolism; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D009995:Osmosis; D011485:Protein Binding; D011487:Protein Conformation; D012313:RNA / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D014867:Water / Q000737:chemistry*
C491637:Aqp9 protein, rat; D020346:Aquaporins; D005136:Eye Proteins; D007473:Ion Channels; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; C030328:aquaporin 0; C108330:aquaporin 8; D014867:Water; D012313:RNA; D003994:Bucladesine