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In vivo substrate utilization has not been described for the maturing fetal lung. We, therefore, studied pulmonary delivery and use of major fetal substrates in six chronically catheterized fetal lambs over 119-141 days gestation. Oxygen, glucose, lactate, and alpha-amino nitrogen concentrations were measured in the pulmonary artery and pulmonary vein whereas lung blood flow was determined using labeled microspheres. We found that lung oxygen availability and use increased near term. Oxygen delivery averaged 4960 +/- 480 (SEM) microliters/min and rose with fetal age (p less than 0.05); uptake averaged 708 +/- 111 microliters/min and increased 93% near term (p less than 0.05). In contrast, glucose availability and use fell with gestation. Pulmonary artery glucose decreased with time (p less than 0.001), with a mean drop of 5.05 +/- 1.71 mg/dl in individual animals (p less than 0.05). Average glucose uptake was 844 +/- 225 micrograms/min and fell near term (p less than 0.05). Lactate was produced by the lung at a mean rate of 534 +/- 176 micrograms/min; this did not change with gestation. Lung amino nitrogen availability increased with fetal age. Pulmonary artery amino nitrogen rose by 1.35 mg/dl, or 43% (p less than 0.001) and lung delivery of amino nitrogen increased (p less than 0.05). The mean pulmonary glucose/O2 ratio was 1.48 +/- 0.26 and decreased with gestation (p less than 0.05), being less than 1.0 near term. The glucose-lactate/O2 ratio was 0.67 +/- 0.26, implying that 30% of lung oxidative metabolism is still unaccounted for. The alterations in delivery of substrates to the lung which were identified may serve as signals for pulmonary maturation, inducing the changes in lung metabolism that were found near term.

Substrate Utilization by the Fetal Sheep Lung during the Last Trimester