Summary. The guinea-pig uterine artery responded to acetylcholine (ACh) with vasodilatation only during pregnancy or after oestrogen treatment. Even with high doses (1 mg/day) oestradiol-17β esters had to be administered for several days to effect sensitization to ACh, but oestradiol-17β itself was active within a few hours. Oestriol was equipotent with oestradiol. Sensitization was prevented when protein synthesis was inhibited over the period of oestrogen administration, but was not dependent on the integrity of the cholinergic vasodilator nerve supply to the artery.
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