Role of sodium, bicarbonate, and plasma osmolality in biliary secretion

To examine the effect of changes in biliary sodium and bicarbonate secretion on bile formation, experiments were performed on fasted, pentobarbital-anesthetized pigs. During continuous intravenous secretin infusion (2.7 CU X kg body wt-1 X h-1) sodium secretion was altered by increasing or reducing plasma sodium concentration. Bicarbonate secretion was altered by varying arterial plasma pH. At increased biliary sodium secretion, bile formation was depressed, but changes in bicarbonate secretion were accompanied by parallel alterations in bile formation. Bile acid secretion was increased during elevated plasma sodium concentration, whereas reduced plasma sodium concentration depressed bile acid secretion. To distinguish between the effect of changes in plasma osmolality and sodium concentration, bile formation was also studied during intravenous sucrose infusion at normal plasma sodium concentration. About 50% of the effect on bile formation of changing plasma sodium concentration is solely caused by the changes in plasma osmolality. During secretin stimulation bile formation is mainly determined by bicarbonate. Changes in plasma osmolality affect bile secretion through alterations in the net osmotic force across the hepatocellular membrane. Sodium has an impact on the bile-acid-dependent fraction, whereas bicarbonate is the mediator of the bile-acid-independent fraction of bile secretion.