Uptake of exogenous substrates during hypoxia in fetal lambs

Fetal uptakes of oxygen, glucose, lactate, and alpha-amino-nitrogen containing substrates were measured in eight near-term fetal lambs by simultaneously drawing blood samples from the umbilical vein and descending aorta, then measuring umbilical blood flow using the microsphere method. These procedures were repeated after 3 h of hypoxia induced by lowering maternal inspired oxygen concentration. On the next day the experiment was repeated, except the ewes were first made hypoxic then allowed to breathe room air. These conditions decreased the delivery of oxygen, but not the delivery of other metabolic substrates to the fetus. During hypoxia, fetal oxygen uptake was 82% of normal (mean of both days); fetal glucose and amino-nitrogen uptakes were 74 and 23% of normal, respectively, and fetal lactate uptake became insignificant. These data indicate that endogenous rather than exogenous substrates are used to support fetal oxidative metabolism during hypoxia. Also, because exogenous uptake of amino-nitrogen is less than normal nitrogen accretion rates, fetal growth must be reduced as a consequence of 3-4 h of hypoxia.