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Abstract

This study examined the role of hepatocyte calcium and cytoskeleton in activation of hyposmotic stress-induced increases in hepatocyte transmembrane potential and control of cell volume. Hepatocyte transmembrane potential was measured by glass microelectrodes in mouse liver slices before and after exposure to hyposmotic medium. Hepatocytes were loaded with tetramethylammonium by briefly exposing liver slices to nystatin, a cation poreforming antibiotic. Changes in hepatocyte steady-state water volume were determined by changes in intracellular tetramethylammonium activity measured with tetramethylammonium-sensitive, double-barrel microelectrodes 4 min after exposure to hyposmotic medium. Hyposmotic stress of 74% of the control osmolality (∼280 mOsm) hyperpolarized hepatocyte transmembrane potential by 1.83 times the control hepatocyte transmembrane potential, and cell water volume increased by a factor of 1.19. The Ca2+ channel blocker verapamil (100 μmol/L) completely inhibited hyposmotic stress-induced hyperpolarization of hepatocyte transmembrane potential. This inhibitory effect diminished at doses of 37.5 or 50 μmol/L, but even these hyperpolarizations were decreased significantly compared with control. Hyposmotic stress during added verapamil dosage (50 μmol/L) also resulted in 23% greater cell swelling compared with control. Ca2+ free medium plus ethylene glycol-bis (β-aminoethylether)-N,N'-tetraacetic acid (5 mmol/L) inhibited hyposmotic stress-induced increases in hepatocyte transmembrane potential and resulted in 16% greater cell swelling compared with control. Calmodulin inhibitors trifluoperazine (100 μmol/L) and promethazine (100 μmol/L) inhibited the hyperpolarization of hepatocyte transmembrane potential caused by hyposmolality, as did 3,4,5-trimethoxybenzoate 8-(N,N-diethylamino)octyl ester) (50 μmol/L), which inhibits mobilization of Ca2+ from intracellular stores. Cytochalasin B (50 μmol/L), which disrupts microfilaments, also inhibited hyperpolarization of hepatocyte transmembrane potential with osmotic stress. In contrast, colchicine (10 μmol/L) under identical conditions had no effect. These results show that extracellular and intracellular calcium, the calmodulin system and microfilaments are involved in control and activation of hyposmotic stress-induced increases in hepatocyte hepatocyte transmembrane potential. These changes in hepatocyte transmembrane potential play a role in cell volume regulation. (HEPATOLOGY 1991;13:962–969.)