268 LACTATE UPTAKE BY THE FETAL AND NEONATAL SHEEP LIVER:

In the fetal sheep, lactate is an important metabolic substrate and previous studies have shown placental lactate production and lactate uptake by the fetal liver. In the neonatal sheep, there is no placental lactate source and blood lactate concentrations are lower than in the fetus. To compare the importance of lactate as a substrate for fetal and neonatal liver metabolism, we measured lactate flux across the liver in 16 fetal sheep (mean gestational age 129 days) which had catheters chronically placed in the right or left hepatic vein, portal vein, descending aorta and umbilical vein and in 5 neonatal sheep (mean age 9 days) with similar catheters except for the umbilical vein. In addition, we measured umbilical lactate uptake in the fetal sheep. In the fetus, net hepatic lactate uptake was 5.0 mg/min/100 g. There was a linear correlation between hepatic and umbilical lactate uptake (r = 0.74, p < .005); hepatic uptake could account for nearly 90% of umbilical uptake, but hepatic lactate extraction was only 7.7% of hepatic lactate delivery (94.2 mg/min/100 g). In the neonate, net hepatic lactate uptake was more than twice the fetal rate (11.0 mg/min/100 g) and hepatic lactate extraction was 32.9% of hepatic lactate delivery (31.0 mg/min/100 g). Lactate is therefore an important substrate for both fetal and neonatal liver metabolism. Although lactate delivery to the neonatal liver decreases, increased hepatic extraction compensates so that hepatic lactate uptake actually increases. (Supported by HL24056.)