VARIATION IN THE EFFECT OF ACETYLCHOLINE ON MYOMETRIAL ACTIVITY AND MATERNAL PLACENTAL BLOOD FLOW IN THE RABBIT

Summary. The effect of acetylcholine upon uteroplacental blood flow was studied by X-ray angiography following selective catheterization of the urogenital artery of the rabbit. Injection of acetylcholine into the artery usually increased the total uterine blood flow. In some cases, this entailed an increase in maternal placental blood flow but, in others, the blood supply to the placenta was cut off and the increased flow passed through the superficial myometrial vessels and, possibly, arteriovenous short circuits. The dichotomy in vascular response depended upon a variation in the myometrial response. In some animals, acetylcholine evoked a large contraction, which curtailed placental flow by compressing blood vessels. In others, the myometrium reacted slightly or not at all to acetylcholine and placental blood flow was able to increase. The vascular and myometrial responses to acetylcholine could be inhibited by previous administration of atropine or butylscopolamine.