title: Water transporting properties of hepatocyte basolateral and canalicular plasma membrane domains.
abstract: Previous work from our laboratory supports an important role for aquaporins (AQPs), a family of water channel proteins, in bile secretion by hepatocytes. To further define the pathways and molecular mechanisms for water movement across hepatocytes, we directly assessed osmotic water permeability (Pf) and activation energy (Ea) in highly purified, rat hepatocytes basolateral membrane vesicles (BLMV) and canalicular membrane (CMV) vesicles by measuring scattered light intensity using stopped-flow spectrophotometry. The time course of scattered light for BLMV and CMV fit well to a single-exponential function. In BLMV, Pf was 108 +/- 4 mum.s-1 (25 degrees C) with an Ea of 7.7 kcal/mol; in CMV, Pf was 86 +/- 5 mum.s-1 (25 degrees C) with an Ea of 8.0 kcal/mol. The AQP blocker, dimethyl sulfoxide, significantly inhibited the Pf of both basolateral (81 +/- 4 mum.s-1; -25%) and canalicular (59 +/- 4 mum.s-1; -30%) membrane vesicles. When CMV were isolated from hepatocytes treated with dibutyryl cAMP, a double-exponential fit was needed, implying two functionally different vesicle populations; one population had Pf and Ea values similar to those of CMV from untreated hepatocytes, but the other population had a very high Pf (655 +/- 135 mum.s-1, 25 degrees C) and very low Ea (2.8 kcal/mol). Dimethyl sulfoxide completely inhibited the high Pf value in this second vesicle population. In contrast, Pf and Ea of BLMV were unaltered by cAMP treatment of hepatocytes. Our results are consistent with the presence of both lipid- and AQP-mediated pathways for basolateral and canalicular water movement across the hepatocyte plasma membrane barrier. Our data also suggest that the hepatocyte canalicular membrane domain is rate-limiting for transcellular water transport and that this domain becomes more permeable to water when hepatocytes are exposed to a choleretic agonist, presumably by insertion of AQP molecules. These data suggest a molecular mechanism for the efficient coupling of osmotically active solutes and water transport during canalicular bile formation.
authors.affiliation: Instituto de Fisiología Experimental, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, 2000 Rosario-Santa Fe, Argentina.
authors.forename: Raul A
authors.initials: RA
authors.lastname: Marinelli
authors.forename: Pamela S
authors.initials: PS
authors.lastname: Tietz
authors.forename: Ariel J
authors.initials: AJ
authors.lastname: Caride
authors.forename: Bing Q
authors.initials: BQ
authors.lastname: Huang
authors.forename: Nicholas F
authors.initials: NF
authors.lastname: LaRusso
mesh_terms: D000818:Animals; D001648:Bile Canaliculi / Q000378:metabolism; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D022781:Hepatocytes / Q000378:metabolism* / Q000502:physiology*; D007700:Kinetics; D008027:Light; D008297:Male; D008854:Microscopy, Electron; D009995:Osmosis; D017434:Protein Structure, Tertiary; D051381:Rats; D011916:Rats, Inbred F344; D012542:Scattering, Radiation; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D004121:Dimethyl Sulfoxide