Endocrine effects of ventilation, oxygenation and cord occlusion in near-term fetal sheep.

The transition from fetal to newborn life is associated with significant endocrine changes. Some of these changes may be triggered by the effects of spontaneous respiration, increased oxygenation and removal of the placental circulation at birth. We, therefore, studied endocrine function during in utero ventilation, oxygenation and cord occlusion, in nine chronically-instrumented fetal sheep aged 137 to 140 days of gestation. During the sequential effects of in utero ventilation and oxygenation, plasma renin activity (PRA) and angiotensin II (AII) remained at control levels of 6.3 +/- 1.9 ng/ml per h and 66 +/- 15 pg/ml. After cord occlusion, PRA increased to 10.3 +/- 2.2 ng/ml per hr and AII increased to 86 +/- 24 pg/ml, the latter being non-significant. Plasma cortisol levels also increased from control levels of 3.5 +/- 1.0 to 6.7 +/- 0.8 g/dl during cord occlusion (P = 0.02). During the sequential effects of ventilation, oxygenation and cord occlusion, plasma epinephrine increased from control levels of 87 +/- 17 to 492 +/- 142 pg/ml (P = 0.04), and plasma norepinephrine levels increased slightly from 620 +/- 112 to 850 +/- 214 pg/ml. These data provide evidence that many of the changes in endocrine function seen at birth result from factors other than spontaneous respiration, oxygenation and umbilical cord occlusion.