Vasomotor responses of the umbilical circulation in fetal sheep.

The effects of vasoactive agents on the distribution of vascular resistance within the umbilical circulation of fetal sheep in utero has been assessed under general anesthesia. Under control conditions, 37% of the arterial to venous pressure drop occurred across the umbilical arteries and their major tributaries, 8% occurred across umbilical veins, and the remaining 55% occurred across the cotyledons. Isoproterenol had no significant effect on these resistances. Norepinephrine and angiotensin II both dramatically increased placental vascular resistance, but the distribution of resistances was altered in different ways. Angiotensin caused a 13-fold increase in the umbilical arterial resistance, whereas there were no changes in cotyledon or venous resistances. Norepinephrine also had no effect on cotyledon vascular resistance but it constricted downstream vessels (6-fold increase). These results suggest that norepinephrine and angiotensin II regulate fetal placental blood flow and that they can independently control pressure in placental exchange vessels. As a consequence, these agents may play an important role in controlling fetal-maternal fluid transport.