Late gestation alterations in fetal pulmonary lactate metabolism in vivo

We have previously shown that lactate is produced by the ovine fetal lung. Inasmuch as factors that might affect lactate production, such as pulmonary glucose and oxygen uptake, change late in gestation we investigated whether pulmonary lactate metabolism also changes. Eleven chronically catheterized fetal lambs were studied over 119-141 d gestation. Lactate, glucose, and oxygen concentrations were measured in the pulmonary artery (PA) and vein while lung blood flow was determined using labeled microspheres. Between early studies (less than or equal to 127 d) and studies near term (greater than or equal to 134 d) PA lactate levels did not change, but due to increasing pulmonary blood flow, lung lactate delivery rose 51% (p less than 0.05). Because of a decline in PA glucose, lactate also made up a larger fraction of the major nonnitrogenous substrate in PA blood near term (p less than 0.001). Despite this, no net pulmonary uptake of lactate occurred. Lactate production continued, but decreased by 80% between early and late studies (p less than 0.05) and the maximum fraction of glucose uptake that could be accounted for by lactate production dropped from 0.78 to 0.20 (p less than 0.025). Correlations were found between lung lactate production and glucose uptake also correlated with PA glucose (p less than 0.05). No relationships were observed between lactate production and PA oxygen content, oxygen delivery, lactate concentration, or lactate delivery. The decreasing fraction of glucose uptake explained by lactate production suggests that metabolism of pulmonary glucose is altered near term. The correlation between decreasing glucose delivery and declining lactate production also suggests that glucose itself influences this change.