Substance P: immunohistochemical localization and effect upon cat pial arteries in vitro and in situ.

1. Nerve fibres containing substance P immunoreactivity were present in the adventitia and the adventitia-media border of all cat cerebral arteries which were examined. Substance P immunoreactivity was most abundant in cerebral arteries from the rostral portion of the circle of Willis. 2. Substance P effected a dose-dependent relaxation of feline middle cerebral arteries which had been contracted with prostaglandin F2 alpha. The maximum relaxation (16 +/- 0.3 mN) was achieved with substance P at a concentration of 10-6 M. 3. In cats anaesthetized with alpha-chloralose, the perivascular microinjection of substance P effected dose-dependent increases in arteriolar calibre. The maximum increase in calibre (19 +/- 3%) was observed following the injection of 10(-6) M-substance P.