Norepinephrine regulation of fetal heart rate: multiple mechanisms of action

Norepinephrine was infused intravenously for 30 minutes into chronically catheterized sheep fetuses averaging 133 days' gestation. At infusion rates of 3.9, 12, and 39 micrograms/min, heart rate initially decreased as much as 40 bpm and then gradually increased to 50 bpm above the control value by the end of the infusion. Thereafter, heart rate increased further, and by the end of a 30-minute recovery period, heart rate was still 66 bpm above the control value. In fetuses pretreated with either a ganglionic blocker or a parasympathetic blocker, norepinephrine caused a large and sustained rise in heart rate by 124 bpm, which declined rapidly when the infusion was terminated. These results suggest that circulating norepinephrine affects fetal heart rate by several mechanisms: a baroreceptor-mediated suppression, a direct stimulation by norepinephrine, a gradual weakening of the vagal and an increase in cardiac sensitivity to sympathetic stimulation. In addition, there appears to be a long-term positive correlation between fetal heart rate and circulating norepinephrine levels.