Responses of the pelvic vascular bed to intra-arterial stimulation of beta-adrenergic and cholinergic receptors in pregnant and nonpregnant sheep.

The responses of different parts of the pelvic vascular bed to beta-adrenergic and cholinergic receptor stimulation were studied in chronically instrumented, unanesthetized pregnant and nonpregnant sheep. Isoproterenol and acetylcholine were administered intra-arterially in progressively increasing bolus injections, and dose-response curves were constructed for changes in blood flows, arterial pressure, and heart rate. Response to beta-adrenergic and cholinergic stimulation was measured before and after the agents reached the central circulation. In addition, data were obtained from the same animals before and after pharmacologic cardiac denervation. Results show: (1) Stimulation of beta-adrenergic and cholinergic receptors produced active vasodilatation in extrauterine vascular beds with minor changes in uterine blood flow of the pregnant horn; (2) the changes in uterine flow observed during beta-adrenergic and cholinergic stimulation were probably secondary and related to a shift of blood from the uterus to other vascular beds that were actively dilated; (3) the response of the pregnant animal to any given dose of the autonomic agonist was strikingly smaller to that of the nonpregnant animal. A number of physiologic factors peculiar to pregnancy may contribute to this difference; among these are: (1) dilution factor related to the greater volume flow; (2) near-maximal dilatation of some blood vessels and the presence of the two resistance system of placental circulation, and (3) the effect of progesterone on the vascular smooth muscle.