Metabolism of Exogenous Steroids by Anuran Larvae

Uptake and metabolism of exogenous steroids [testosterone (T), estradiol (E 2), 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 E 2 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 E 2 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