Substance P induces biphasic endothelium-dependent relaxations in pig and rabbit carotid arteries.

Careful handling and preparation of freshly harvested vessels from 22 pigs and 12 rabbits revealed a two-phase vasorelaxation response to cumulative doses of substance P (SP). A rapid, transient relaxation was observed during the cumulative dose-response and a new plateau of equilibrium was seen following an increase in developed force after the last dose of SP. The phase 2 response is also produced by submaximal doses of SP and is not altered by pretreatment of the rings with Indomethacin. Acetylcholine (ACh) caused an endothelium-dependent relaxation but without evidence of a phase 2 plateau. N omega-Nitro-L-Arginine (L-NNA) and N omega-Nitro-L-Arginine Methylester (L-NAME) pretreatment resulted in a shift to the right in the phase 1 response to SP and a complete blockade of phase 2. Methylene blue caused nearly complete block of both phases. Nitroglycerin caused a dose-dependent and prolonged vasorelaxation with no phase 2.