Corticosteroid administration adversely affects fetal growth and consequently, unless metabolic rate increases (thereby increasing the need for fetal metabolic substrates), reduces fetal need for exogenous substrates. To find whether the uptake of all or only certain metabolic substrates was affected by fetal hypercortisolemia, we measured exogenous uptake of glucose, lactate, and alpha-amino nitrogen during fetal hypercortisolemia in seven late-gestation sheep fetuses. Hydrocortisone infusion increased fetal cortisol concentrations from normal to values usually associated with late gestation (4.3 +/- 0.9 vs. 72.5 +/- 5.0 ng/ml; P < 0.001). In association with increased fetal cortisol concentration, both fetal metabolic rate and the uptakes of glucose and lactate remained constant. However, the uptake of alpha-amino nitrogen-containing substances fell (39.6 +/- 3.1 vs. 28.5 +/- 3.4 mumol.kg-1.min-1; P < 0.002) as did the proportion of metabolic rate (0.45 +/- 0.04 vs. 0.32 +/- 0.04; P < 0.002) that could be supported by metabolism of these substances. The proportion of fetal metabolic rate that could be supported by complete metabolism of all substrates taken up across the placenta decreased (1.09 +/- 0.07 vs. 0.95 +/- 0.06; P < 0.02). In conclusion, hypercortisolemia alters the composition of oxidative substrates taken up by the sheep fetus and decreases total fetal substrate uptake. These changes occur within hours of exposure to increased cortisol concentration.
The Relation of Metabolic Rate to Body Weight and Organ Size
