Tachykinins (Substance P, Neurokinin A, Neuropeptide K, and Neurokinin B) in the Cerebral Circulation: Vasomotor Responses in vitro and in situ

The vasomotor responses of tachykinins have been studied in the cerebral vasculature of human, pig, cat, and guinea pig. Substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and neuropeptide K (NPK) induced concentration-dependent relaxations of precontracted cerebral arteries in all species when examined by a sensitive in vitro technique. In addition, the relaxant responses to SP, NKA, and NKB were studied in cat pial arterioles by peptide microapplication in situ. In human pial vessels, the order of relaxant potency was SP > NKB > NKA > NPK; in the pig middle cerebral artery, there was no difference in potency between the tachykinins; in the cat middle cerebral artery, SP = NKB > NKA = NPK; and in the guinea pig basilar artery, SP » NPK = NKA > NKB. Responses induced by SP, NKA, and NKB in the cat were comparable in vitro and in situ. Removal of the endothelium abolished relaxation induced by all four tachykinins. The relaxant responses of guinea pig basilar arteries to SP, NKA, and NPK were competitively antagonized by the SP antagonist Spantide. However, Spantide lowered the I max of the NKB concentration–response curve without any rightward shift, suggesting action at a different site than the other tachykinins. In the guinea pig basilar artery, the relaxation seems to be exerted via a NK-1 receptor subtype while the receptor subtype is more unclear in cerebral arteries from human, cat, and pig. It is concluded that relaxations induced by SP, NKA, NKB, and NPK are dependent on the endothelium, and are antagonized either competitively or non-competitively by the SP antagonist Spantide. The origin of tachykinins acting through the endothelium is discussed.