Studies of Sulfobromophthalein Sodium (BSP) Metabolism in Man. III. Demonstration of a Transport Maximum (Tm) for Biliary Excretion of BSP*

Convincing direct evidence that in dogs the se-cretion of BSP into bile is limited by a maximal rate, similar to the transport maximum (Tm) for certain substances secreted by the renal tubules, has been provided by Wheeler and co-workers (1, 2). They also showed that the Tm for BSP may be estimated from observations on peripheral plasma concentrations during different rates of constant infusion without resort to bile collections, hepatic venous sampling, or measurement of he-patic blood flow. The colorimetrically determined maximal output of BSP in dog bile was equal, on the average, to 87.57o % of the estimated Tm. The existence of a rate-limited BSP transport mecha-nism in man was inferred from the achievement of a state in which the hepatic removal rate was independent of the absolute plasma concentration and related only to the rate of change of plasma concentration. Utilizing the constant-infusion technic, estimations of Tm in man, analogous to those in the dog, have been reported by Wheeler and his group and by workers in other laborator-ies as well (3, 4). The experiments reported herein were per-formed on a person with complete obstruction of the distal common bile duct due to cancer of the pancreas; a T-tube had been placed in his bile duct without an attempt to remove the obstructing lesion. This circumstance provided an opportunity to obtain evidence supporting the concept of a transport maximum for biliary excretion of BSP in man, even though there was mild (presumably secondary) hepatic damage. * Submitted for publication December 23, 1963; ac-