Adrenergic and cholinergic responses in the uteroplacental vascular bed of the guinea pig.

The effects on uterine and maternal placental circulation of adrenergic and cholinergic drugs, injected selectively in the ovarian and uterine arteries of guinea pigs, were analysed by serial angiography. Noradrenaline, 0.5 nmol/kg, was found to cause a reduction in both ovarian and uterine blood flow, associated with arterial vasoconstriction and impairment of the placental circulation. This response could be prevented by alpha-adrenergic blockade with 25 nmol/kg phenoxybenzamine. At injection into the ovarian artery, phenoxybenzamine alone increased ovarian blood flow and elicited arterial vasodilatation. At injection into the uterine artery the response was more variable, but vasodilatation was observed in four animals of six. Acetylcholine, 0.5 to 5.0 nmol/kg, evoked an increase in both ovarian and uterine blood flow and arterial vasodilatation. When the dose was increased to 50 nmol/kg, dilatation of the extrinsic uterine arteries was maintained, but the placental circulation was reduced due to concomitant contraction of the myometrium. All the effects of acetylcholine could be blocked by prior administration of 10 nmol/kg atropine. This dose of atropine did not affect uterine or placental circulation when given alone.