Role of Sodium, Bicarbonate, and Plasma Osmolality in Biliary Secretion

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Abstract
To examine the effect of changes in biliary Na and bicarbonate secretion on bile formation, experiments were performed on fasted, pentobarbital-anesthetized pigs. During continuous i.v. secretin infusion (2.7 CU [clinical unit] .cntdot. kg body wt-1 .cntdot. h-1) Na secretion was altered by increasing or reducing plasma Na concentration. Bicarbonate secretion was altered by varying arterial plasma pH. At increased biliary Na 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 Na concentration, whereas reduced plasma Na concentration depressed bile acid secretion. To distinguish between the effect of changes in plasma osmolality and Na concentration, bile formation was also studied during i.v. sucrose infusion at normal plasma Na concentration. About 50% of the effect on bile formation of changing plasma Na 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. Na has an impact on the bile-acid-dependent fraction, whereas bicarbonate is the mediator of the bile-acid-independent fraction of bile secretion.

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