Modulation of fetal cardiovascular responsiveness to norepinephrine by autonomic nervous system

To determine the extent to which the autonomic nervous system modifies the fetal cardiovascular responses to exogenous norepinephrine (NE), NE was infused intravenously (0.39-39 micrograms/min) for 30 min into chronically catheterized sheep fetuses averaging 132 days gestation. The resulting changes in arterial pressure, venous pressure, heart rate, and blood volume were compared between fetuses with and without ganglionic blockade. Autonomic blockade did not alter the relationship between the rise in NE concentration and NE infusion rate. In fetuses with a blocked autonomic nervous system, the arterial pressure response to exogenous NE was shifted 0.8 log units to the left when compared with normal fetuses. The venous pressure response to NE infusion was not altered in the blocked fetuses when compared with normal fetuses. Heart rate in the autonomically blocked fetuses increased with plasma NE concentration, which was opposite to the initial suppression of heart rate during NE infusion in control fetuses. Fetal blood volume decreased progressively with increasing NE infusion rate in the blocked fetuses, which was similar in autonomically intact fetuses, except for an increase in blood volume at low NE infusion rates. Thus it appears that the autonomic nervous system modifies the fetal arterial pressure, heart rate, and blood volume responses to exogenous NE but not the response of venous pressure.