Previous studies based upon competition between different organic anions for biliary excretion in vivo have suggested that all organic anions share a common hepatic secretory mechanism. Corriedale sheep with an inherited defect in organic anion excretion by the liver were used to study this problem directly without the need for competition studies, the results of which are difficult to analyze. Maximal biliary excretion of sulfobromphthalein (BSP) in mutant Corriedale sheep was less than 7% of that observed in normal sheep whereas maximal biliary excretion of taurocholate, the major organic anion in sheep bile, was not different in mutant and normal sheep. Taurocholate infusion enhanced maximal hepatic excretion of BSP in normal but not in mutant sheep. These studies of an inheritable disorder which appears to be identical to the Dubin-Johnson syndrome in man, demonstrate that taurocholate excretion requires at least one step in biliary excretion which is not required by other organic anions such as bile pigment, porphyrins, drugs, and dyes.

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