Role of osmosis in biliary NaCl secretion and bile formation.

To challenge the osmotic hypothesis of biliary NaCl secretion and bile formation, experiments were performed in anaesthetized pigs. An increase in plasma osmolality of 7 +/- 1 mosm/kg H2O induced by intravenous sucrose infusion decreased NaCl secretion, NaHCO3 secretion, and bile flow by 36 +/- 3%, 34 +/- 2%, and 34 +/- 3%, respectively. There was no change in the biliary concentration of NaCl and NaHCO3. When bile acids were infused intravenously, the secretion of 1 mmol bile acids caused an osmotic flow of 12.0 ml bile containing 0.92 mmol NaCl and 0.30 mmol NaHCO3 in an isotonic solution. Bile acids are therefore much stronger choleretic substances than NaHCO3. When the plasma sodium concentration was increased to 200 mM, bile flow increased by 31 +/- 5% and the secretion of bile acids, NaHCO3, and NaCl was increased by 63 +/- 3%, 96 +/- 4%, and 93 +/- 4%, respectively. These data are consistent with osmotic transport as the main mode of bile formation, but diffusion could be responsible for a small fraction. A raised plasma sodium concentration stimulates osmotic formation of bile by increasing both the bile acid-dependent and -independent secretion through stimulation of biliary bile acid and NaHCO3 secretion.