Flare and itch induced by substance P in human skin.

Intradermal injection of synthetic substance P (10(-7)--10(-5) M in humans produced flare, wheal and itching. These responses were inhibited by oral pretreatment of the subjects with an antihistaminic drug (chlorcyclizine) or by local pretreatment with Compound 48/80 administered to deplete the local stores of mast-cell bound histamine. The findings indicate that the responses induced by substance P were mainly mediated by histamine released from the dermal mast cells. In contrast to previously studied histamine liberators, substance P was less potent when acting on rat mast cells in vitro than on human skin mast cells in vivo. When incubated with rat peritoneal mast cells, about 100 times higher concentrations (10(-5) M) were required to induce histamine release than in the in vivo studies on humans. It was concluded that substance P is a potent histamine liberator in human skin.