Radioimmunoassay of plasma gastric inhibitory polypeptide (GIP), release of GIP after a test meal and duodenal infusion of bile, and immunoreactive plasma GIP components in man.

A sensitive, precise and specific radioimmunoassay method for measuring plasma gastric inhibitory polypeptide (GIP) is described. The present assay system, using a label purified on a Sephadex G-15 and a SP Sephadex C-25 column as well as careful corrections for nonspecific plasma effects, allows measurements of fasting plasma GIP in the lower picomole per liter range, and the significant rise in plasma GIP subsequent to a test meal and duodenal infusion of cattle bile. Apparent immunoreactive fasting plasma GIP was eluted from a Sephadex G-150 Fine column in one peak probably representing plasma GIP bound to plasma proteins and nonspecific plasma effects. Apparent immunoreactive meal-stimulated plasma GIP was eluted from a Sephadex G-50 Fine column in two large and one diminutive peak. The first peak probably represents plasma GIP bound to plasma proteins and nonspecific plasma effects, the almost negligible second peak probably represents big GIP with a molecular weight of some 9,000, and the third and major peak, little GIP with a molecular weight of about 5,000.