A radioimmunoassay of gastric inhibitory polypeptide in human plasma.

A sensitive radioimmunoassay for the measurement of human gastric inhibitory polypeptide (GIP), using pure porcine GIP, has been developed. Cross-reactivity of the antiserum with all available mammalian gut peptide preparations was negligible with the exception of glucagon when it was approximately 1%. Two major molecular forms of GIP were detectable in plasma and tissue extracts, one of large molecular size and the other corresponding to the elution coefficient of pure porcine standard. Concentrations of GIP in plasma from 50 normal subjects after overnight fasting were 9 +/- 1.0 (S.E.M.) pmol/l rising to a peak of 34 +/- 2.8 pmol/l following the ingestion of a small mixed test meal. Ingestion of glucose or fat resulted in a similar rise of plasma GIP, whereas no change was observed after the ingestion of protein.