Radioimmunoassay of vasoactive intestinal polypeptide in plasma

A sensitive, precise, and specific radioimmunoassay for vasoactive intestinal polypeptide (VIP) is described, with a detection limit of 0.8 pmol/l, within assay precision of 7.8%, and between assay precision of 13.1%. The final dilution of the antiserum to bind 50% of 0.9 fmol 125I-labeled VIP was 1:250000. The antiserum showed an effective equilibrium constant (Keff) according to Scatchard of 7.4 x 1011 l/mol, an average equilibrium constant (Ko) according to Sips of 7.3 x 1011 l/mol, an index of heterogeneity (alpha) according to Sips of 0.99, and a negligible cross-reactivity with secretin. 125I-labeled VIP was prepared by a modified Chloramine-T method, and the label purified on a Sephadex G-15 column followed by a SP Sephadex C-25 column had a specific radioactivity of 1700 muCi/nmol. The present assay allows measurements of fasting plasma VIP in the very low pmol/l range and the increase and gradual fall in plasma VIP subsequent to a brief period of duodenal acidification.