Modulation of vascular tone by nitric oxide and endothelin 1 in myometrial resistance arteries from pregnant women at term, ,

Objective: We evaluated the role of endothelium-derived nitric oxide and endothelin 1 in the modulation of myogenic tone, norepinephrine-induced tone, and flow-mediated responses in resistance arteries from pregnant women at term. Study Design: Arteries (approximately 200 μm at 50 mm Hg; n = 27) were dissected from myometrial biopsies obtained from women undergoing elective cesarean delivery at term and mounted in a pressure arteriograph. Responses to intraluminal flow, pressure, and norepinephrine were studied in the absence and presence of the nitric oxide synthase inhibitor Nω-nitro-L-arginine and the endothelin-converting enzyme inhibitor phosphoramidon. Results: Pressure-induced (80 mm Hg) myogenic tone was significantly enhanced after incubation with Nω-nitro-L-arginine (33% ± 8% vs 24% ± 4%; P< .05), whereas phosphoramidon significantly reduced myogenic tone (24% ± 5% vs 33% ± 5%; P< .05). A combination of Nω-nitro-L -arginine and phosphoramidon did not affect myogenic tone. Norepinephrine-induced tone was significantly enhanced after nitric oxide synthase inhibition (49% ± 6% vs 41% ± 5%; P< .05) but was not affected by phosphoramidon. Flow-mediated dilatation was increased in the presence of phosphoramidon compared with flow-induced dilatation in physiologic salt solution (maximum dilatation, 57% ± 12% vs 30% ± 5%; analysis of variance, P< .05), and all flow-induced dilatation was abolished by Nω-nitro-L -arginine. Conclusions: Nitric oxide and endothelin 1 may play a significant role in modulation of myogenic tone and flow-mediated responses in the resistance vasculature of the uterine circulation in normal pregnancy. (Am J Obstet Gynecol 2000;182:87-93.)