Late Gestation Alterations in Fetal Pulmonary Lactate Metabolism in Vivo

Abstract

ABSTRACT. 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 (5127 d) and studies near term (2134 d) PA lactate levels did not change, but due to increasing pulmonary blood flow, lung lactate delivery rose 51% ( p < 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 < 0.001). Despite this, no net pulmonary uptake of lactate occurred. Lactate production continued, but decreased by 80% between early and late studies ( p < 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 < 0.025). Correlations were found between lung lactate production and both glucose delivery ( p < 0.005) and PA glucose concentration ( p < 0.05). The ratio between lactate production and glucose uptake also correlated with PA glucose ( p < 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. (Pediatr Res 27: 274-277,1990) Abbreviations PA, pulmonary artery PV, pulmonary vein In previous studies in chronically catheterized fetal sheep, we found that glucose is taken up by the fetal lung and lactate is produced (1). During late gestation, the average quantity of lactate produced is equivalent to about 50% of pulmonary glucose uptake. However, metabolic factors that may influence lung lactate metabolism are not static during gestation. Near term, Received March 20, 1989; accepted October 19, 1989 pulmonary arterial glucose concentration and pulmonary glucose uptake decline, whereas pulmonary delivery of oxygen increases (1). We, therefore, investigated whether lactate metabolism in the fetal lung changes with gestation or with either pulmonary delivery of glucose or oxygen. Further, because lactate levels themselves might affect lung metabolism, we investigated the relationship between pulmonary lactate concentration and both lung lactate production and glucose use. MATERIALS AND METHODS Surgical preparation and recovery. Eleven mixed Western,breed-dated, pregnant sheep of 115 to 125 d gestation were surgically prepared by methods described previously (1-3). With the ewe under spinal anesthesia (induced by 1 % tetracaine) and ...