Biliary secretion in elasmobranchs. II. Hepatic uptake and biliary excretion of organic anions

Abstract

[35S]Bromosulfophthalein ([35C]BSP), [14C]sodium taurocholate ([14C]NaTC), AND 10 MG OF UNLABELED BSP.and of phenol-3,6-dibromophthalein disulfonate (DBSP) per kilogram body weight were injected in the caudal artery of free-swimming dogfish sharks (Squalus acanthias) and small skates (Raja erinacea). Twenty-four hours later, 85.8 +/- 15.7% of [35S]BSP was recovered in bile and liver in dogfish and 78.4 +/- 9.9% in skates. Similar results were obtained for [14C]NaTC. Unlabeled BSP or DBSP (10 mg/kg body wt) were also selectively excreted in bile over a 4-day period and at comparable rates in both species. More than 85% of [35S]BSP, BSP, and DBSP in bile was in unconjugated form. Selective hepatic clearance of BSP occurred despite nonselective binding to liver homogenates and very low concentrations of binding proteins in liver cytosol. Analysis of the organic anion plasma disappearance curves suggest that the clearance of anions into bile in elasmobranchs is delayed disproportionately relative to hepatic uptake. Albumin-BSP infusions did not prevent selective hepatic uptake of [35S]BSP, although biliary excretion was delayed further. These studies demonstrate that transport systems for biliary excretion of organic anions evolved prior to migration of marine life from the sea and relatively independently of intrahepatic conjugation and organic anion-binding proteins.