Placental Metabolic Activity

Although the barrier between the maternal and fetal circulation is effectively only one cell thick, the placenta is a substantial organ made up of many cell types. It is extremely active metabolically and has its own requirement for nutrients and this is consistent with the observations that the surface area of the maternal-facing membrane (MVM) is around 5 times greater than that of the fetal-facing membrane (BM), that the concentration of expression of GLUT1 is greater on the MVM, that the MVM contains additional fatty acid binding proteins that are not present on the BM, and that the amino acid transporters act to produce the maximum amino acid gradient across the MVM. The metabolic transformations within the placenta are intimately linked to fetal metabolism and represent another way in which the placenta can regulate nutrient transport availability within the fetal circulation.

In late pregnancy the overall contribution of fat to whole body oxidation is reduced and this is thought to result from the preferential utilization of carbohydrate and amino acids such as glutamate as an energy source in the feto-placental unit and the sparing of fatty acids to maximize fetal accretion of the critical LCPUFA in particular. The interrelationships between the placenta and fetus are particularly complex for the amino acids. The placenta is a net user of serine, glutamate, leucine, isoleucine, and valine and there is significant interconversion of alanine, pyruvate, and lactate between the placenta and fetal tissues. The concentration of lactate in the fetal circulation is considerably greater than that in the maternal circulation and a considerable proportion of the glucose taken up by the placenta is converted into lactate prior to export into the fetal circulation for use by the fetus. The placenta takes up serine from both the maternal and fetal circulation, converting this into glycine and exporting it into the fetal circulation for oxidation by the fetal liver and there is significant cycling of glutamate and glutamine between the placenta and fetal liver. This partition of the various segments of metabolic pathways between the placenta and fetal tissues is a general phenomenon and in many respects the feto-placental unit can be considered as a metabolic whole with the placenta acting as an extra fetal organ in addition to its role as a simple nutrient transporter. Metabolic activity in the feto-placental unit is also responsive to nutrient supply and fetal demand. For example, AA is an important precursor of the prostacyclins, pros-taglandins, thromboxanes, and leukotrienes, which play key roles in pregnancy. When the maternal circulation of AA is low there is net uptake from the fetal circulation to maintain placental synthesis of these compounds.

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