The stability of synthetic substance P in blood.

Experiments with synthetic substance P incubated in whole blood show that apart from a moderate loss of activity immediately on exposure of the peptide to whole blood, it is inactivated slowly in this tissue, approximately 25% of control activity remaining after 30 min incubation. Incubation with plasma did not result in the degradation of substance P. The attenuation of substance P activity in blood may be due to enzymatic destruction within erythrocytes.