RELATIVE ROLES OF METABOLISM AND RENAL EXCRETORY MECHANISMS IN XENOBIOTIC ELIMINATION BY FISH

Renal clearance techniques were used to examine the relative contributions of metabolism and renal tubular transport in determining the rates of excretion of benzo(a)pyrene (BaP) and several of its phase I metabolites by southern flounder, Paralichthys lethostigma. Each compound (H-3-labeled) was injected at a dose of 2.5-mu-mole/kg, producing plasma concentrations of 1 to 5-mu-M. Despite extensive plasma binding (> 95%), the uncorrected renal clearance of BaP-7,8-dihydrodiol exceeded the glomerular filtration rate (GFR) by more than 20-fold. Phenolic BaP metabolites also showed net secretion (1.5- to 3-fold). At times prior to 3 hr, BaP itself showed an average clearance of only 0.2 times the GFR. After 3 hr, BaP clearance increased to three times the GFR. Decreasing the dose of BaP injected also dramatically increased its clearance. Clearances of all four compounds studied were reduced by probenecid and other organic anion, including the herbicide 2,4-dichlorophenoxyacetic acid. HPLC analysis demonstrated that the bulk of the material excreted in the urine was not the parent compound, but sulfate or glucuronide conjugates of its phenolic or dihydrodiol metabolites. Excretion of sulfate conjugates predominated over the first 24 hr, whereas the glucuronide conjugates were the primary excretory products in succeeding days. In vitro, isolated renal tubules transported both glucuronide and sulfate conjugates, but sulfates were the preferred substrates. Isolated tubules were shown to be capable of catalyzing conjugation reactions, producing predominantly glucuronide conjugates. Liver slices produced both types of conjugates. Thus, the rapid excretion of BaP-7,8-dihydrodiol reflected a combination of two processes. First, this metabolite was rapidly converted to its sulfate conjugate, primarily via extrarenal tissues. Second, the sulfate conjugate was preferentially transported to the urine via secretion on the organic anion transport system.