The importance of nitric oxide (NO) and dilator prostaglandins in uterine resistance arterioles was investigated. In pentobarbital anaesthetized rats at dioestrus-2, the uterine microcirculation in vivo was transilluminated by a fibreoptic probe and microvessels (circumferential arterioles) viewed by video microscopy. Arteriolar diameters were measured while increasing concentrations of acetylcholine (ACh), serotonin (5-HT), phenylephrine (PE), or angiotensin II (AII) were applied topically (suffused) over the uterus. Agonists were applied alone or with ibuprofen (IBU; cyclooxygenase inhibitor), N\g=w\-nitro-L-arginine (L-NA; nitric oxide synthase inhibitor) or both. Circumferential arterioles were dilated by ACh and 5-HT (10\m=-\8\p=n-\10\m=-\4mol l \ m=-\ 1) and constricted by PE (10\ m=-\ 8\ p=n-\ 10\ m=-\ 5 mol l \m=-\1) and AII (10\ m=-\ 11\ p=n-\ 10\ m=-\ 7 mol l \ m=-\ 1).
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