Transcellular water transport in hepatobiliary secretion and role of aquaporins in liver.

In the context of the osmotic model of bile formation, we used isolated rat hepatocyte couplets and performed volume measurements by video image analysis to analyze the transport of water between the bile canalicular lumen, liver cells and the surrounding bathing medium. Increasing bath osmolarity by the addition of sucrose led to shrinkage of cells that preceded shrinkage of the canalicular lumen by approx. 1 sec. Thermodynamic modeling of water transport across the basolateral and apical cell membranes and across a paracellular pathway (tight junctions) revealed high hydraulic water permeabilities of both cell membranes of approx. 310(-4) cmsec(-1)*(osmol/kg)(-1) indicating transcellular water flux between bathing medium and bile. Tight junctions exhibited low water permeability but allowed for electrolyte permeation that enables canalicular spaces to shrink below van't Hoff equilibrium during the osmotic maneuver. The results are discussed with respect to the role of different types of membrane aquaporins being expressed in hepatocytes.