Origin and Metabolic Fate of Plasma Glycerol in the Rat and Rabbit Fetus

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Abstract
Summary: On day 21.5 a pregnant rat received a single injection of [1-14C]glycerol. The purpose was to study the transfer of glycerol through the placenta from the maternal to fetal plasma. From 3–20 min after injection the specific activity of glycerol in maternal and fetal plasma was measured. The results indicate that the mother can provide this molecule to the fetus. Similar results were obtained with the rabbit on day 28 of pregnancy. The possibility of the conversion of plasma glycerol to glucose has been investigated in the rat and rabbit fetus. This molecule was chosen chiefly to see whether the gluconeogenic pathway was functioning in the fetus above the triose phosphate step. At two stages of fetal development the capacity of the fetus to incorporate [1-14C]glycerol into glucose plus glycogen has been shown in the two species. In the rat fetus the conversion of [1-14C]glycerol to [14C)glucose increases from 19.5 to 21.5 days of gestation. For the rabbit this parameter increases from 25 to 28 days of gestation. On day 25 in the rabbit and day 19.5 in the rat the liver glycogen was labeled, but it did not accumulate the [14C]glucose from [1-14C]glycerol during the time that we have studied. In contrast, on day 28 in the rabbit and day 21.5 in the rat the incorporation of radioactivity increased as function of the time. However, the relative importance of glycerol as precursor of the glucose plus glycogen in the fetus remains to be elucidated. Speculation: The available evidence suggests that in the fetal rat liver the gluconeogenesis does not appear until after birth. Quantitatively, the most important source of glucose for the fetus must be the mother. The existence of glycerol in the plasma of the rat and rabbit fetus may contribute to produce glucose and increase liver glycogen reserves near term.

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