Contribution of fructose and lactate produced in placenta to calculation of fetal glucose oxidation rate

Abstract

We examined the rate of production of [14C]fructose and [14C]lactate from [U-14C]glucose by the placenta and the contribution of 14CO2 from fetal oxidation of these metabolic products to the calculation of glucose oxidation rate in fetal sheep. During fetal tracer infusions (n = 16), oxidation of fructose contributed 16 +/- 3% of total fetal CO2 production; oxidation of lactate accounted for 3.3 +/- 0.1%. Thus 80% of total fetal CO2 production resulted from direct oxidation of carbon atoms in glucose; the "direct" glucose oxidation fraction was 0.46 +/- 0.04. During maternal tracer infusion (n = 15), CO2 production from fructose was 21 +/- 3, 20 +/- 3, and 30 +/- 4% and from lactate was 16 +/- 3, 13 +/- 3, and 11 +/- 4% in hypo-, normo-, and hyperglycemic animals, respectively; the direct glucose oxidation fraction was 0.40 +/- 0.04, not different from the fraction obtained with the fetal tracer infusion. Fetal oxidation of substrates derived from glucose metabolism in the placenta contributes significantly to fetal CO2 production. Fetal oxidation of placental products of a metabolic substrate tracer should be considered in studies of fetal oxidative metabolism.

Evidence of increased substrate availability to in vitro-derived bovine foetuses and association with accelerated conceptus growth

“…Placental fructose production may be a byproduct of the foetal need on occasion to supply glucose to the placenta, preventing glucose loss to the mother, as fructose does not cross the foetomaternal barrier, and limiting foetal glucose uptake when placental needs are high (Gu et al 1987). Fructose and lactate, another by-product of glucose metabolism by the placental tissue, are of significance to the total conceptus and placental oxidative metabolism (McGowan et al 1995). Aldoretta and Hay (1999) demonstrated in sheep that the rate at which the placental tissue synthesizes fructose seems to depend directly on the placental glucose supply.…”

Endocrine And Metabolic Adaptations Of Calves To Extra-Uterine Life

“…During the transition from intra-uterine to extra-uterine life tremendous metabolic changes occur [91]. Those changes are strongly related to the hormonal pattern as well as to the developing rate of mechanisms involved in metabolic pathways of certain metabolites.…”

Comparative aspects of fetal carbohydrate metabolism

“…Fetal plasma concentrations of fructose range from 3.0 to 8.0 mmol/l in these species with virtually no detectable fructose in the maternal plasma (Silver 1984). Fructose is made from glucose in the placenta and its concentration in the fetus rises with increases in the fetal glucose level in the pig, sheep and horse (Silver 1984;McGowan et al 1995). Tracer studies in the sheep fetus show that fructose is utilised at a very low rate and accounts for only 6% of the fetal CO, production (Meznarich et al 1987).…”

“…Since some of the products (e.g. lactate, fructose) are released back into the umbilical circulation, not all the glucose carbon taken up by the uteroplacental tissues is lost to the fetus (McGowan et al 1995). However, uteroplacental glucose consumption does reduce the amount of glucose directly reaching the fetus.…”