Effect of noradrenaline on circulation in the genital tract of early and late pregnant guinea-pigs.

Noradrenaline infusions were given to anaesthetized guinea-pigs in early (19--23 days) and near-term (61--66 days) pregnancy. Radioactive microspheres (15 micrometers) were used to determine blood flow in various regions of the genital tract. In late gestation, a 10-min intravenous infusion of 1 micrograms noradrenaline/min/kg elicited moderate increases in vascular resistance that were counteracted by the pressor response. Uterine and placental blood flow remained unchanged, although there was a 27% decrease in perfusion of the uterine cervix and a fall of 19% in vaginal blood flow. In early pregnant guinea-pigs, blood pressure rose but vascular resistance was unaffected, resulting in an augmentation of placental blood flow. During infusion of 10 micrograms noradrenaline/min/kg, the vasonconstrictor response in late pregnancy was accentuated, and a strong decrease in tissue perfusion was noted in the placenta (-36%), uterine horns (-39%), cervix (-70%) and vagina (-56%). At the higher rate of noradrenaline infusion, vascular resistance also increased in the genital tract of early pregnant animals, although to a lesser extent than in late pregnancy. The results indicate an enhancement of the vasoconstrictor response to circulating catecholamines in near-term pregnancy.