Validation of a heterologous radioimmunoassay for insulin-like growth factor-I in bovine serum.

A quantitative, repeatable, heterologous radioimmunoassay (RIA) for insulin-like growth factor-I (IGF-I) was developed for bovine serum. Untreated serum could not be assayed due to interference by IGF-I-binding protein. Serum acidified to pH 3.6 in glycine-HCl (0.1 mol/l) for 24 h at 37 degrees C and neutralized with NaOH produced inhibition curves non-parallel to the [Thr59]-IGF-I standard. Neutralization by 40-fold dilution of acidified serum samples with assay buffer produced inhibition curves nearly parallel to the IGF-I standard. Complete parallelism was achieved by utilizing preprecipitated normal rabbit serum-sheep anti-rabbit gamma-globulin to separate antibody-bound 125I-labelled IGF-I from free 125I-labelled IGF-I. Recovery of IGF-I (1.3-52.3 fmol) added to serum was quantitative. The sensitivity of the RIA (n = 6) was 8.25 +/- 0.17 (S.E.M.) fmol. Intra- and interassay coefficients of variation were 3.03 and 4.95% respectively. Serum IGF-I levels measured in beef calves at weaning were positively correlated with weaning weight, total weight gain from birth to weaning and average daily weight gain. In conclusion, a heterologous RIA for IGF-I in beef serum which is sensitive, accurate, precise and repeatable has been developed.