Regulatory volume decrease stimulates bile flow, bile acid excretion, and exocytosis in isolated perfused rat liver

To study the effect of volume regulation on bile secretory function, isolated perfused rat livers (IPRL) were exposed to hypotonic stress (45 mM NaCl) while bile flow and the biliary excretion of bile acids and horseradish peroxidase (HRP) were assessed. Hypotonic stress induced a biphasic increase in bile flow, which rose in the first minute from 1.1 ± 0.2 to 1.7 ± 0.1 μl · min^-1 · g liver^-1 (P < 0.01), an effect attributed to rapid osmotic equilibration of water, then increased further between 3 and 5 min to 1.6 ± 0.1 μl · min^-1 · g liver^-1 (P < 0.01, followed by a subsequent return to baseline. HRP excretion in bile increased during the second peak of bile flow from 0.9 ± 0.2 to 1.1 ± 0.2 ng · min^-1 · g liver^-1, P < 0.01. Pretreatment with colchicine but not lumicolchicine completely abolished the latter increase in bile flow and HRP excretion as did BaCl₂ (1 mM), an inhibitor of both K⁺ channels and regulatory volume decrease (RVD) in hepatocytes. When sodium taurocholate was infused (1 μmol/min), hypotonic stress induced an even larger increase in the second peak of bile flow (5.1 ± 0.7 μl/g liver, P < 0.01) and higher rates of bile acid excretion than in control perfusions with bile acid (126.2 ± 21.0 vs. 99.0 ± 17.1 nmol · min^-1 · g liver^-1, P < 0.05). These data suggest that both bile flow and bile acid excretion are stimulated during RVD by mechanisms that involve both K⁺ channels and microtubule-dependent exocytosis at the canalicular (apical) membrane domain.