Tissue levels of oxolinic acid after oral or intravascular administration to freshwater and seawater rainbow trout

Abstract

The concentrations of oxolinic acid in tissues of seawater-acclimated rainbow trout were compared with those of freshwater rainbow trout after a single oral administration (40 mg/kg). Up to 24 h after administration, little difference in the tissue concentrations was observed between the two groups of trout. However, tissue concentrations of the drugs in seawater trout decreased to undetectable levels by 72 h, whereas the concentrations in the freshwater trout peaked at 48 h and were detectable for at least 244 h. The kinetics of oxolinic acid uptake and excretion in the seawater trout was similar to that reported for seawater teleosts. Both groups of trout metabolized oxolinic acid by the same pathway, because the same oxolinic acid metabolites were observed in the bile of both groups of trout after a single oral administration. When oxolinic acid was injected (20 mg/kg) into the caudal vessels of each group of trout, serum levels of oxolinic acid decreased immediately after injection in both groups. Serum concentrations were 8.69 ± 2.14 μg/ml in the freshwater trout and 8.03 ± 2.27 μg/ml in seawater trout by 3 h after injection, respectively. Subsequently, the serum levels of oxolinic acid in the seawater trout decreased to barely detectable levels by 72 h. In contrast, oxolinic acid levels in freshwater trout were higher and persisted for longer, up to 244 h. These results confirm that oxolinic acid is excreted more rapidly in seawater trout than in freshwater trout.