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A. M. CARTER and T. OLIN

Summary.

Responses of the myometrial and maternal placental circulations to adrenergic stimulation and blockade have been studied in the rabbit by serial angiography after selective catheterization of the urogenital artery. Changes in myometrial activity and arterial blood pressure have been registered and taken into account. Myometrial and placental blood flow was reduced by a few nanograms of noradrenaline or adrenaline, but increased after blockade of αadrenoceptors with phenoxybenzamine or thymoxamine. The main effect of phentolamine was to evoke a long series of myometrial contractions, which counteracted the tendency of this drug to improve placental perfusion. The β-blocking agent, propranolol, had no effect at all on the parameters studied. Isoprenaline did not clearly affect myometrial and placental blood flow. Isoxsuprine, however, caused a slight increase by dilating the uterine arteries, possibly due to its papaverine-like properties. Both isoprenaline and isoxsuprine tended to depress myometrial activity. None of the drugs tested markedly affected distribution of uterine blood flow between the uterine wall and the placentae.

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A. M. CARTER and T. OLIN

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.

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A. M. CARTER, J. GÖTHLIN and T. OLIN

Summary.

The uterine and maternal placental vasculature of the rabbit has been studied by serial angiography after selective catheterization of the urogenital artery or the corresponding vein and in specimens injected with barium sulphate suspension and gelatin. The uterus is supplied with blood through long, spiral arteries. Some of these perfuse the myometrium whilst others enter the placentae, opening into large arterial sinuses which supply the labyrinth through wide efferent vessels. It is suggested that blood pressure is reduced and pulse pressure damped during passage of the blood through the spiral arteries and that the function of the sinuses is to raise the lateral pressure and reduce the linear velocity, so that a slow even perfusion of the placental labyrinth can be achieved. Cross connections between the spiral arteries and a plexus of finer arterial vessels beneath each placenta enable redistribution of blood flow between the myometrium and placentae. In addition, there are probably arteriovenous anastomoses in pregnant as well as non-pregnant uteri. A sphincter mechanism in the placental feeding arteries is postulated to prevent intrapartum haemorrhage after delivery of the placentae and possibly to regulate maternal placental blood flow during pregnancy. Total uterine blood flow was estimated to be about 30 ml/min. Circulation time through the myometrium was much less than that through the placenta.