A variation in acetylcholine-induced relaxation of rat aorta in pregnancy

The tension of isolated ring preparations of the thoracic aortae from pregnant rats on Days 7, 14, and 21 was measured isometrically to study the effect of advancing pregnant stage on endothelium-dependent relaxing factor activity. The endothelium-dependent relaxation caused by acetylcholine (ACh) (10(-8) - 3x10(-5) M) in the aortae of pregnant rats on Days 7, 14, and 21 were greater than that in those of diestrus (D-2) rats. In pregnant rats, the endothelium-dependent relaxation caused by acetylcholine was greatest in the aortae from pregnant rats on Day 21. In contrast with ACh, the relaxation caused by sodium nitroprusside was decreased in the aortae from pregnant rats on Day 21, compared with metestrus (D-1) and estrus (E) rats. The peaks of serum estradiol and progesterone levels in pregnant rats were observed on Days 21 and 14, respectively. These results suggest that there is a fluctuation of endothelium-dependent relaxing factor activity in the aortae during pregnancy of rats and ACh-induced relaxation is greatest on Day 21 in pregnancy, and that it may be due to the enhancement of NO synthesis in endothelium mainly by estradiol and partly by progesterone.