Principal substrates of fetal metabolism: fuel and growth requirements of the ovine fetus.

In those fetuses studied, the glucose uptake by the fetus has been a major source of calories, although in all instances it has been insufficient to account for the total fuel requirements of the fetus. The glucose/oxygen quotients in different mammalian fetuses vary from about 0.5 to 0.8. Glucose transport across the placenta has been altered by fetal hyperinsulinaemia and by maternal fasting. Fetal hypoglycaemia is common to both conditions. However, umbilical glucose uptake increases with fetal hyperinsulinaemia and decreases with maternal fasting. During fasting, concomitant with a decrease in fetal glucose supply, there is an increase in amino acid catabolism. A high rate of placental production of both ammonia and lactate has been demonstrated in several mammalian species. Umbilical lactate uptake is sufficient to account for about 25% of the oxygen consumption in the ovine fetus. Ammonia production by the sheep placenta is reflected in increased ammonia concentrations in both the maternal and fetal circulations. In the ovine fetus, transport of umbilical amino acid has exceeded that required for new tissue growth, supporting the observations of a high urea production rate during fetal life. Neutral and basic amino acids represent the bulk of the amino acids transported across the placenta. In contrast there is a net uptake of glutamate from the fetal circulation into the placenta and very little umbilical uptake of aspartic acid. The umbilical uptake of free fatty acids varies markedly among species. In some species, such as the sheep and the cow, no umbilical veno-arterial differences for free fatty acids can be demonstrated.