Effects of systemic norepinephrine (NE) on distribution of uterine blood flow (UBF) in pregnant ewes

Alterations in UBF may follow administration of pharmacologic agents employed in clinical obstetrics, adversely affecting fetal well being. Development of a chronic pregnant sheep preparation has permitted evaluation of responses in magnitude and distribution of the UBF to these agents. The authors studied the effects of a continuous systemic infusion of NE on the distribution of UBF in 7 pregnant ewes from 100 to 137 days gestation. Dose response curves were determined for each animal and a rate of infusion leading to a significant reduction in UBF without a significant change in systemic pressure was identified (mean 0.237 μg/min kg ± 0.035). Labeled microspheres were infused into the left ventricle prior to and during the constant infusion of NE at the previously identified rate. UBF decreased from a mean of 1233 ml/min ± 196 to 817.4 ml/min ± 131 (31.9% ± 5.24). Reductions in blood flow occurred in each uterine tissue: myometrium decreased from 41.2 ml/min ± 6.8 to 22.4 ml/min ± 4.1 (-45%), endometrium from 196 ml/min ± 24 to 72.6 ml/min ± 10.9 (-62%) and placental cotyledons from 996 ml/min ± 174 to 723 ml/min ± 125 (-25%). Decreases in blood flow also occurred in other reproductive tissues. Cardiac output decreased 17.8% ± 4.9. These observations demonstrate that NE is a potent vasoconstrictor in all tissues of the pregnant ovine uterus, that the response in each tissue is different, and that cardiac output and blood flow to other tissues may also be affected.