Hepatic extraction of bile salts in conscious dogs.
The enterohepatic circulation of cholic acid conjugates (CAC) was studied in three conscious dogs by comparing the relationship of the concentration of CAC in portal, hepatic, and peripheral venous plasma samples collected simultaneously. The pool of CAC in each dog was labeled with 14C. Catheters were surgically placed in the jugular, left hepatic, and portal veins. Each dog was studied on 2 consecutive days, and each study consisted of a series of samples withdrawn from each catheter at 15-min intervals before and after gallbladder contraction with cholecystokinin. The concentration of CAC in the portal vein ranged from 3 micron (fasting) to 235 micron (after gallbladder contraction). In individual studies, the concentration of CAC increased four to sixfold. A linear relationship exists between the concentration of CAC in the portal vein to that in the hepatic and jugular veins. Thus, the fractional hepatic extraction of CAC is constant over the physiological range of the concentration of CAC in portal venous plasma. Mean extraction varied among the six studies from 0.618 +/- 0.072 (+/- 1 SD) to 0.983 +/- 0.010.