Circulating thyroglobulin in tadpoles and adult frogs of Rana catesbeiana.

Using purified thyroglobulin (TG) of Rana catesbeiana and its antisera, a specific double-antibody radioimmunoassay (RIA) was developed. Serum TG content was measured in tadpoles, froglets, and adult frogs by RIA. Serum TG increased during pre- and prometamorphosis and reached a peak of 476.9 +/- 29.1 ng/ml at the climax. Thereafter the amount of circulating TG decreased slowly toward the end of metamorphosis, and it was at the lowest level (154.1 +/- 18.4 ng/ml) in froglets. Serum TG in adult frogs was 267.3 +/- 31.7 ng/ml. The serum TG in tadpoles and adult frogs was identified by immunoblotting as only one component corresponding to the purified TG. The circulating TG level was much higher than that in mammalian species and chickens reported so far. These results seem to show that the release of TG is a preferential function in the bullfrog thyroid gland.