Pregnancy-induced alterations of uterine arteriolar reactivity in the rat: Observations with a new in vivo microcirculatory preparation

Objective: The aim of this study was to compare reactivity of uterine arterioles in vivo between virgin and gravid rats. Study Design: In anesthetized gravid and virgin rats circumferential arterioles were observed by videomicroscopy while vasoactive agonists were suffused over the uterus. Arteriolar diameter changes were compared. Results: Resting and maximum diameters of circumferential arterioles were significantly larger in pregnant animals. Sensitivity to acetylcholine was greater in pregnant animals than in virgin animals (50% effective concentration, 7.1 ± 0.17 negative log mol/L vs 6.1 ± 0.32 negative log mol/L), whereas that to phenylephrine was less (50% effective concentration, 5.3 ± 0.11 negative log mol/L vs 6.0 ± 0.17 negative log mol/L). Angiotensin II–induced constriction was significantly reduced in pregnant rats only at lower concentrations. Serotonin significantly dilated arterioles in virgin rats (76% ± 3% of maximum) but caused significant arteriolar constriction in pregnant animals (65% ± 4% of maximum). Conclusion: The in vivo uterine microcirculatory preparation allows measurement of resistance vessels in the intact animal. Although these results differ from some previous reports, differences may be attributable to vessel location or to in vivo versus in vitro conditions. (Am J Obstet Gynecol 2000;183:621-6.)