Insulin-like growth factor-I in the plasma of two reptiles: assay development and validations

Radioimmunoassays (RIAs) of insulin-like growth factor-I (IGF-I) in nonmammals are complicated by two problems: decreased specificity due to heterologous assays and assay interference from IGF binding proteins. We have developed and validated an RIA for the detection of IGF-I in reptilian plasma that circumvents these problems. Biochemical validations show parallelism between standard curves, plasma dilutions, and internal standards. Further, biological validations indicate that (1) plasma IGF-I levels are significantly reduced in nonreproductive, female American alligators (Alligator mississippiensis) with high plasma estradiol-17 beta concentrations compared to individuals with low plasma estradiol-17 beta concentrations and (2) plasma IGF-I concentrations are elevated in juvenile Trachemys scripta elegans turtles fed higher quality diets (defined by protein and energy content). These results provide impetus for future studies of IGF-I in reptiles.