Effect of beta-adrenergic stimulation on oxygen metabolism in the fetal lamb

Catecholamines, which are released into the circulation during stress, increase fetal metabolism. This effect appears to be related to beta-adrenoreceptor stimulation. We examined the effect of isoproterenol infusion on umbilical blood flow, oxygen delivery and consumption, and glucose and lactate uptake in late-gestation fetal lambs. Isoproterenol increased umbilical blood flow, but oxygen delivery to the fetus did not increase. Umbilical venous oxygen content fell linearly with the increase in umbilical blood flow. It is proposed that oxygen delivery to the sheep fetus is at or near a maximum and that oxygen delivery cannot be raised by increasing umbilical blood flow because oxygen diffusion at the placental site is limited. Fetal oxygen consumption increased initially but returned to control levels with an increase in infusion rate. Blood glucose concentration increased during isoproterenol infusion; this was due to release of glucose and not because of increased placental uptake. Fetal blood pH values fell in association with elevated lactate levels. It is proposed that elevated glucose concentrations resulted in increased metabolism of glucose, and because oxygen delivery could not be enhanced, increased anaerobic glycolysis caused lactate concentration to rise.