Structure‐activity relationships for some substance P‐related peptides that cause wheal and flare reactions in human skin.

Substance P (6.25‐25 p‐mole) produced dose‐dependent flare and wheal responses when injected intradermally into the volar surface of the human forearm. The maximum flare response was obtained within the first 3 min of injection and declined thereafter. The wheal response reached a maximum after 12 min following the injection. Only those peptides having one or more basic residues in the N‐terminal region were effective in producing a flare reaction. Eledoisin‐related peptide and SP1‐9 were 17 and 7 times less active than substance P respectively, whilst [D‐pro2, D‐phe7, D‐trp9]SP1‐11 was twice as active. The N‐terminal tetrapeptide, SP1‐4 and eledoisin were inactive in the dose range tested. Wheal‐producing activity was not dependent on the presence of basic residues and the rank order of relative potencies was: physalaemin (2.0): [D‐pro2, D‐phe7, D‐trp9]SP1‐11 (1.1): SP1‐11 (1.0): SP4‐11 (0.4): SP1‐9 (0.15): eledoisin‐related peptide (0.08): eledoisin (0.06). The N‐terminal tetrapeptide failed to produce a wheal response in the dose range tested. Substance P was approximately equi‐active with poly‐L‐arginine in the production of wheal and flare and both of these agents were about 10 times more potent than histamine. Adenosine triphosphate (25‐400 n‐mole) produced dose‐dependent wheal and flare responses and was 10,000 times less potent than substance P. Pre‐treatment of the subjects with the H1 histamine antagonist, chlorpheniramine, (20 mg I.V.) reduced the wheal and flare responses to substance P. Local anaesthetic injection into the skin reduced the spread of the flare response but did not affect the development of the wheal response. Pre‐treatment of the skin with capsaicin reduced the flare but not the wheal response to intradermal injection of histamine. The results are discussed in relation to the mechanism of the ‘axon reflex’ vasodilatation in skin. This is thought to involve mast cells in addition to substance P‐containing primary afferent neurones.