Excretion of phenol red and its glucuronide in the dogfish shark

1. The biliary and urinary excretion of phenol red and its glucuronide was measured in dogfish sharks, Squalus acanthias, in both intact animals and animals with biliary fistula. 2. Phenol red is extensively metabolized to its glucuronide by the dogfish shark and both forms of the compound are actively transported into bile and urine. 3. Both compounds are transferred from plasma to urine and bile against a large concentration gradient; the transfer process is saturable most easily in the renal compartment but also in the hepatic compartment; and both excretion routes for the free drug and the glucuronide are inhibited by probenecid. 4. There were no significant differences in the 48 h biliary excretion of total (free + glucuronide) phenol red, but the urine or intact fish contained two-fold more total drug than did animals with fistulae.