VASODILATION INDUCED BY SUBSTANCE-P IN GUINEA-PIG CAROTID ARTERIES

In the guinea pig carotid artery with an intact endothelium, substance P (SP, 10(-10)-10(-7) M) relaxed the norepinephrine- (NE) contracted smooth muscles transiently, in a concentration-dependent manner. Acetylcholine (ACh, 10(-6) M) produced a sustained relaxation. SP and ACh also relaxed muscles contracted with high-K (29.6 mM) solution, with a similar form but with a reduced amplitude compared with findings in NE-contracted muscles. In the presence of nitroarginine (10(-5) M) and NE, the ACh-induced relaxation was transient, with a reduced amplitude, whereas the SP-induced relaxation was not significantly changed. In muscles contracted with high-K solution containing nitroarginine, neither SP nor ACh produced relaxation. SP (>10(-11) M) transiently hyperpolarized the membrane, but only when this peptide was applied from the intimal side of the intact vessel, and the peak amplitude reached similar to 20 mV from the resting potential at 10(-8) M. ACh transiently hyperpolarized the membrane (the peak amplitude being similar to 10 mV), in both the adventitial and intimal applications. In high-K solution, neither SP nor ACh produced hyperpolarization. The amplitude of hyperpolarizations produced by SP did not significantly change in the presence of nitroarginine, oxyhemoglobin, or indomethacin. Thus, SP-induced relaxation seems to be produced mainly by endothelium-derived hyperpolarizing factor-induced hyperpolarization.