Radioimmunoassay measurement of substance P release following a meat meal.

We report the development of a sensitive and specific radioimmunoassay for substance P based on a new extraction technique for this peptide. In this technique, substance P-like immunoreactivity (SPLI) was extracted from midly acidified plasma successively with 0.05M ammonium sulfate and ethanol; recovery was almost quantitative (89.5 +/- 3.2%) and was independent of concentration. Basal substance P levels averaged 22 +/- 3 pg/ml in 33 dogs. Following initiation of a high-protein meal, mean SPLI levels in three dogs (in a total of 12 experiments) increased significantly from 26 +/- 3 to a peak of 37 +/- 4 pg/ml at 15 minutes, after which they slowly returned to the initial levels.