Effect of birth-related events on metabolism in fetal sheep

At birth, changes in utilization of metabolic substrates occur as the fetus, dependent upon carbohydrates and amino acids available continuously from the placenta, becomes a neonate, dependent primarily upon fat obtained intermittently by suckling. In addition to changes in substrates, at birth, metabolic rate increases dramatically as the activity of several organs and thermoregulatory mechanisms increase. To determine whether metabolic changes are related to certain events that occur at birth, we studied 15 fetal sheep instrumented chronically with vascular catheters and an endotracheal tube at 133 to 137 d gestational age. We measured blood flow with radionuclide-labeled microspheres and arteriovenous concentration differences for oxygen content, blood glucose, and lactate across the placental, cerebral, myocardial, and hindlimb circulations at rest, during in utero ventilation of the fetal lungs with 3% O2, during in utero ventilation with 100% O2, and during ventilation with 100% O2 and umbilical cord occlusion. Ventilation with 3% O2 decreased oxygen uptake by the fetus and by the cerebral circulation but produced no other significant changes. Ventilation with the low and high oxygen gas mixture reduced glucose uptake by the fetus from the placental circulation dramatically to zero. In contrast, blood glucose concentrations increased and glucose uptake by the fetal brain, heart, and hindlimb were not altered significantly. These data indicate that, for a short time at least, complete cessation of glucose supply to the fetus from the placenta is not associated with decreases in blood glucose concentrations or utilization of glucose by a major portion of the fetal body.