Insulin-like immunoreactivity in serum and pancreas of metamorphosing tadpoles.

Insulin-like immunoreactivity (ILI) in pancreatic extracts and serum of Rana catesbeiana tadpoles of various developmental stages was determined by radioimmunoassay. Serum ILI levels increased from 2.4 +/- 0.4 microU/ml in limb bud stage tadpoles (I-V) to 13.9 +/- 2.2 microU/ml in foot stage animals (XI-XVII). During climax, serum ILI levels declined reaching 5.0 microU/ml in postmetamorphic juveniles. Embryonic pancreases (stage 25) contained sizeable amounts of ILI (115 +/- 14.4 microU). A fourfold increase in total pancreatic ILI occurred during the limb bud stages, and a twelvefold increase occurred by the end of the foot development stages. Total pancreatic ILI declined throughout climax stages to 410 microU in postmetamorphic juveniles. Serum glucose increased slightly over the course of larval development; the peak concentration occurred in foot stage tadpoles.