Abstract
Extract: Whole blood lactate, pyruvate, and oxygen concentrations were measured simultaneously in the umbilical vein, fetal femoral artery, maternal artery, and uterine vein in 14 chronically catheterized pregnant ewes and their fetuses. Lactate was found to be taken up in significant amounts across the placental circulation by the fetuses, whereas pyruvate was not.
The lactate concentration of fetal blood was higher than that of maternal blood; however, fetal lactate levels correlated with maternal arterial levels (P ≤ 0.01). The mean lactate concentrations in all samples were: common umbilical vein, 2.105 mM; fetal femoral artery, 1.986 mM; and maternal artery, 0.823 mM. Where uterine venous lactate concentrations were measured, the lactate content of the uterine vein exceeded that of the maternal artery by a mean of 0.088 mmol/liter (P < 0.005). The mean fetal gain in lactate across the placental circulation was 0.118 mmol/liter (P < 0.005). This is equivalent to a gain of 1.2 g carbon/kg/24 hr by the growing lamb fetus. The mean fraction of fetal oxygen consumption that could be accounted for by oxidation of lactate was 0.32.
The pyruvate concentration of fetal blood was higher than that of maternal blood; however, fetal pyruvate levels correlated with maternal arterial levels (P < 0.05). The mean pyruvate concentrations in all samples were: common umbilical vein, 0.084 mM, fetal femoral artery, 0.094 mM; and maternal artery, 0.053 mM. Where uterine venous pyruvate concentrations were measured, they exceeded the maternal arterial concentrations by a mean of 0.005 mmol/liter (P = 0.001). Pyruvate appeared to be lost by the fetus across the placental circulation by a mean of 0.010 mmol/liter. This loss of pyruvate correlated with the placental fetal to maternal pyruvate concentration gradient (P < 0.05).
Correlations between maternal arterial and fetal lactate concentrations imply that fetal lactate levels are influenced by maternal levels. The increase in lactate concentration of both fetal and maternal blood during circulation through the placenta indicates placental production of lactate. The pyruvate concentrations observed, however, are consistent with either fetal to maternal flow of pyruvate or placental production
Speculation: Lactate, after glucose and amino acids, is the third most important fetal substrate identified in the fetal lamb. Its role as a fetal fuel now needs to be evaluated in other animal species.
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