Radioimmunoassay of vasoactive intestinal polypeptide (VIP) in plasma

A sensitive and specific radioimmunoassay for vasoactive intestinal polypeptide (VIP) has been developed, which can detect 3.3 pmol x L-1 of the peptide in plasma. Antisera to highly purified porcine VIP coupled to albumin were raised in eight rabbits. The final dilution, the avidity, and the specificity of each antiserum were determined. 125I-VIP served as label, and highly purified porcine VIP was used as standard. Separtation of antibody-bound and free VIP was achieved by plasma-coated charcoal. Nonspecific interference withe assay system was excluded by extraction of plasma samples with ethanol. The reliability of the assay was investigated by recovery experiments, by serial dilution of plasma samples with high concentration of endogenous VIP, and by immunosorption. The within-and between assay reproducibility at a concentration of 18.3 pmol x L-1 was 1.6 and 2.3 pmol x L-1 (1 S.D.), respectively. Median fasting concentration of VIP in plasma from 74 normal subjects was 7.3 pmol x L-1 (range: 0-20.0 pmol x L-1).