During bile formation by the liver, large volumes of water are transported across two epithelial barriers consisting of hepatocytes and cholangiocytes (i.e. intrahepatic bile duct epithelial cells). We recently reported that a water channel, aquaporin-channel-forming integral protein of 28 kDa, is present in cholangiocytes and suggested that it plays a major role in water transport by these cells. Since the mechanisms of water transport across hepatocytes remain obscure, we performed physiological, molecular, and biochemical studies on hepatocytes to determine if they also contain water channels. Water permeability was studied by exposing isolated rat hepatocytes to buffers of different osmolarity and measuring cell volume by quantitative phase contrast, fluorescence and laser scanning confocal microscopy. Using this method, hepatocytes exposed to hypotonic buffers at 23 °C increased their cell volume in a time and osmolarity-dependent manner with an osmotic water permeability coefficient of 66.4 10 cm/s. In studies done at 10 °C, the osmotic water permeability coefficient decreased by 55% (p < 0.001, at 23 °C; t test). The derived activation energy from these studies was 12.8 kcal/mol. After incubation of hepatocytes with amphotericin B at 10 °C, the osmotic water permeability coefficient increased by 198% (p < 0.001) and the activation energy value decreased to 3.6 kcal/mol, consistent with the insertion of artificial water channels into the hepatocyte plasma membrane. Reverse transcriptase polymerase chain reaction with hepatocyte RNA as template did not produce cDNAs for three of the known water channels. Both the cholesterol content and the cholesterol/phospholipid ratio of hepatocyte plasma membranes were significantly (p < 0.005) less than those of cholangiocytes; membrane fluidity of hepatocytes estimated by measuring steady-state anisotropy was higher than that of cholangiocytes. Our data suggests that the osmotic flow of water across hepatocyte membranes occurs mainly by diffusion via the lipid bilayer (not by permeation through water channels as in cholangiocytes).

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

This article has been cited by other articles:

				X. Guo, Y. Lanir, and G. S. Kassab Effect of osmolarity on the zero-stress state and mechanical properties of aorta Am J Physiol Heart Circ Physiol, October 1, 2007; 293(4): H2328 - H2334. [Abstract] [Full Text] [PDF]

				B. Yang, Y. Song, D. Zhao, and A. S. Verkman Phenotype analysis of aquaporin-8 null mice Am J Physiol Cell Physiol, May 1, 2005; 288(5): C1161 - C1170. [Abstract] [Full Text] [PDF]

				R. A. Marinelli, P. S. Tietz, A. J. Caride, B. Q. Huang, and N. F. LaRusso Water Transporting Properties of Hepatocyte Basolateral and Canalicular Plasma Membrane Domains J. Biol. Chem., October 31, 2003; 278(44): 43157 - 43162. [Abstract] [Full Text] [PDF]

				A. Mennone, A. S. Verkman, and J. L. Boyer Unimpaired osmotic water permeability and fluid secretion in bile duct epithelia of AQP1 null mice Am J Physiol Gastrointest Liver Physiol, September 1, 2002; 283(3): G739 - G746. [Abstract] [Full Text] [PDF]

				R. C. Huebert, P. L. Splinter, F. Garcia, R. A. Marinelli, and N. F. LaRusso Expression and Localization of Aquaporin Water Channels in Rat Hepatocytes. EVIDENCE FOR A ROLE IN CANALICULAR BILE SECRETION J. Biol. Chem., June 14, 2002; 277(25): 22710 - 22717. [Abstract] [Full Text] [PDF]

				M. L. Garcia-Martin, M. A. Garcia-Espinosa, P. Ballesteros, M. Bruix, and S. Cerdan Hydrogen Turnover and Subcellular Compartmentation of Hepatic [2-13C]Glutamate and [3-13C]Aspartate as Detected by 13C NMR J. Biol. Chem., March 1, 2002; 277(10): 7799 - 7807. [Abstract] [Full Text] [PDF]

				G. P. Nicchia, A. Frigeri, B. Nico, D. Ribatti, and M. Svelto Tissue Distribution and Membrane Localization of Aquaporin-9 Water Channel: Evidence for Sex-linked Differences in Liver J. Histochem. Cytochem., December 1, 2001; 49(12): 1547 - 1556. [Abstract] [Full Text] [PDF]

				T. Ma and A S Verkman Aquaporin water channels in gastrointestinal physiology J. Physiol., June 1, 1999; 517(2): 317 - 326. [Abstract] [Full Text] [PDF]

				R. A. Marinelli, P. S. Tietz, L. D. Pham, L. Rueckert, P. Agre, and N. F. LaRusso Secretin induces the apical insertion of aquaporin-1 water channels in rat cholangiocytes Am J Physiol Gastrointest Liver Physiol, January 1, 1999; 276(1): G280 - G286. [Abstract] [Full Text] [PDF]

				Y. Koyama, T. Yamamoto, D. Kondo, H. Funaki, E. Yaoita, K. Kawasaki, N. Sato, K. Hatakeyama, and I. Kihara Molecular Cloning of a New Aquaporin from Rat Pancreas and Liver J. Biol. Chem., November 28, 1997; 272(48): 30329 - 30333. [Abstract] [Full Text] [PDF]

				R. A. Marinelli, L. Pham, P. Agre, and N. F. LaRusso Secretin Promotes Osmotic Water Transport in Rat Cholangiocytes by Increasing Aquaporin-1 Water Channels in Plasma Membrane. EVIDENCE FOR A SECRETIN-INDUCED VESICULAR TRANSLOCATION OF AQUAPORIN-1 J. Biol. Chem., May 16, 1997; 272(20): 12984 - 12988. [Abstract] [Full Text] [PDF]

				F. Garcia, A. Kierbel, M. C. Larocca, S. A. Gradilone, P. Splinter, N. F. LaRusso, and R. A. Marinelli The Water Channel Aquaporin-8 Is Mainly Intracellular in Rat Hepatocytes, and Its Plasma Membrane Insertion Is Stimulated by Cyclic AMP J. Biol. Chem., April 6, 2001; 276(15): 12147 - 12152. [Abstract] [Full Text] [PDF]

				M. W. Roe, A. L. Moore, and S. D. Lidofsky Purinergic-independent Calcium Signaling Mediates Recovery from Hepatocellular Swelling. IMPLICATIONS FOR VOLUME REGULATION J. Biol. Chem., August 10, 2001; 276(33): 30871 - 30877. [Abstract] [Full Text] [PDF]