Relaxation kinetics of rat aorta during pregnancy

OBJECTIVE: To examine the hypothesis that kinetics of vasorelaxation are altered during pregnancy. METHODS: Rings of aorta from rats at different stages of pregnancy (early, late, and term) and from nonpregnant female rats were precontracted with high-K⁺ and then allowed to relax in normal-K⁺ physiologic saline solution. The time to reach 50% and 80% relaxation were determined in the absence or presence of a cyclooxygenase inhibitor (indomethacin) and a nitric oxide synthase inhibitor (N(Ω)-nitro-L-arginine methyl ester) or after removal of the endothelium. RESULTS: The aortic relaxation was progressively faster in later stages of gestation. N(Ω)- nitro-L-arginine methyl ester and indomethacin had no significant effect whereas removal of the endothelium caused a slowing of relaxation in all the groups. Even in the presence of N(Ω)-nitro-L-arginine methyl ester and indomethacin or after de-endothelization, the relaxation remained faster at term as compared with the other groups. CONCLUSION: Aortic relaxation is faster in the presence of endothelium. The effect of endothelium on relaxation is independent of nitric oxide synthase or cyclooxygenase systems. Progression of gestation is associated with acceleration of aortic relaxation, which cannot be totally ascribed to an endothelial factor and may involve a change intrinsic to the vascular smooth muscle. Faster relaxation kinetics of the vasculature during pregnancy may be a mechanism to decrease peripheral vascular resistance.