EFFECT OF NOREPINEPHRINE ON FETAL BREATHING IN SHEEP

We tested the hypothesis that the surge of norepinephrine at birth is associated with the establishment of continuous breathing. Therefore, we studied whether the administration of norepinephrine could enhance fetal breathing during administration of oxygen, or 100% O-2 plus cord occlusion, and if phenoxybenzamine would reverse these changes. Fetal sheep were instrumented in late gestation to measure electrocortical activity and diaphragmatic electromyography. These parameters and bloo d gases were measured before and during in utero administration of nit rogen. 100% O-2, 100% O-2 plus umbilical cord occlusion, and subsequen tly during umbilical reperfusion and recovery. Nine fetuses (14 experi ments) received continuous norepinephrine (0.13 mu g/kg/min) throughou t the experiment while 9 other fetuses (18 experiments) underwent the same treatment without the hormonal infusion. We found that norepineph rine inhibited the breathing induced by 100% O-2 plus cord occlusion, despite a significant increase in the duration of low-voltage electroc ortical activity; phenoxybenzamine reverted these changes. The finding s suggest that the surge of norepinephrine at birth is probably not th e primary mechanism for establishment of continuous breathing.