Postjunctional localization of substance P receptors on the rat portal vein.

A dose-dependent contractile effect of substance P (SP) on the isolated, everted rat portal vein was competitively inhibited by two selective SP antagonists (pro2, phe7, trp9)-SP and (pro4, trp7,9)-SP 4-11. Phentolamine, atropine, methysergide, mepyramine, cimetidine, Sar1, Ile8-angiotensin II, Leu8, des-Arg9-bradykinin and indomethacin did not block the action of SP. However, some of these antagonists differentially reduced SP responses, but such inhibitory effects were shown to be nonspecific. The results suggest that the SP-induced contractions of the rat portal vein were directly mediated by specific receptors localized on the smooth muscle cells. In addition, the response to SP appeared to be independent of prostaglandin biosynthesis.