A Radioimmunoassay for Gonadotropin-Releasing Hormone (Gn-RH) in Serum1

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Abstract
Double antibody radioimmunoassays have been developed to measure gonadotropinreleasing hormone (Gn-RH). Thyrotropin-releasing hormone, melanocyte-stimulating hormone inhibiting factor, various Gn-RH analogs and the components of serum per se did not appear to influence the assay. Serum containing endogenous or endogenous plus exogenous Gn-RH, crude hypothalamic extracts, and various Gn-RH analogues all produced inhibition curves which were parallel to those obtained with the synthetic Gn-RH standard. Exogenous Gn-RH could be quantitatively recovered from 200 μl of serum obtained from sheep in different reproductive states. Sensitivity of the radioimmunoassay ranged from 1 to 3.8 pg per assay tube with the more sensitive antibody. Using this same radioimmunoassay system the coefficient of variation for 12 samples in the same assay was 5.9% while the coefficient of variation for 2 samples in 6 different radioimmunoassays was 14.7%. The level of Gn-RH in serum of normal rams was 71 ± 1.4 (se) pg/ml as compared to 128 ± 6.1 (se) pg/ml in the serum of long-term castrate ewes and could be easily measured in 200 μl of serum. The time required for a 50% reduction in serum levels following intracarotid administration of Gn-RH to 4 sheep was 6.7 ± 1.1 (se) min.

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