Metabolism of exogenous steroids by anuran larvae

Uptake and metabolism of exogenous steroids [testosterone (T), estradiol (E2), and corticosterone (B)] were examined in anuran larvae. Radioimmunoassay (RIA) of steroids dissolved in water revealed that as much as 0.03 mg of T or E2 were cleared from water by a single Xenopus laevis tadpole in 24 hr. Studies using RIA in combination with counting of total [3H]steroids suggested that clearance was mainly due to metabolism and not to uptake and storage by the tadpole. Thin-layer chromatography and HPLC analysis confirmed metabolism and revealed that T was metabolized to at least three ether-extractable compounds as well as to water soluble compounds (nonextractable in ether) by nine anuran species, representing eight families. Most species metabolized E2 to nonextractable compounds, but some produced organic metabolites. In contrast, B was not taken up, cleared, or metabolized by any of the nine species. Studies in X. laevis using [3H]T and radioinert T in the same container confirmed that the decrease in radioimmunoassayable T was due to metabolism as determined by TLC. In a separate study, [3H]T was metabolized more extensively when injected directly into the tadpole than when dissolved in water. Additional studies with X. laevis revealed that steroids were also metabolized in tadpole-conditioned water (water exposed to tadpoles), but the metabolites produced represented < 10% of those produced when tadpoles were present. This metabolism was confined to particulate matter from the gut of tadpoles. Metabolites extracted from tadpoles were similar to metabolites found in the water.