The effects of rat corticotrophin‐releasing factor‐41 peptide fragments on bioassay and immunoassay determination of corticotrophin‐R-releasing factor‐41 Levels

Peptide fragments of rat corticotrophin‐releasing factor‐41 (CRF‐41) containing amino‐acid residues 21–33 antagonized the 5 nmol/l CRF‐41‐stimulated adrenocorticotrophin secretion from the adult rat pituitary gland in vitro. The CRF 6‐33 sequence had antagonistic effects at equimolar (5 nmol/l) concentrations which were not observed at high (50 nmol/l) concentrations whilst the CRF 21–41 sequence had effects only at high (50 nmol/l) concentrations. Similar effects were observed with CRF 6‐33 on basal release of adrenocorticotrophin. Peptide fragments elevated radioimmunoassay measurement of CRF‐41 whilst inhibiting measurement of CRF‐41 in a two‐site enzyme amplified immunometric assay. The inhibitory effects of peptide fragments in the enzyme amplified immunometric assay could be removed by dilution to within the lower end of the standard curve or by increasing the concentration of antibody bound to the solid phase. These inhibitory effects mimic those of peptide fragments on basal adrenocorticotrophin release seen in a rat pituitary gland in vitro bioassay indicating that such two‐site immunoassay determinations bear closer relation to bioactivity than those obtained using radioimmunoassay.