Glucose metabolism in separated embryos and investing membranes during organogenesis in the rat

Glucose metabolism provides an essential energy source in the mammalian embryo. We used the rat embryo culture system to investigate the activity of several critical pathways for glucose metabolism during early organogenesis, before (day 10 of gestation) and during (day 11) the establishment of the chorioallantoic circulation and closure of the neural tube. We studied glucose metabolism in the intact conceptus, the separated embryo, and its investing membranes, including the visceral yolk sac, allantois, and amnion. Short-term incubations were performed for 4 hours in culture media containing U-¹⁴C-, ¹⁴C-1-, or ¹⁴C-6-labeled d-glucose on day 10 and day 11 of gestation, and the rates of glucose utilization by glycolysis and oxidative metabolism, including the pentose phosphate pathway (PPP), were measured. Glycolytic metabolism, estimated by the accumulation of lactate, was high on day 10 in the intact conceptus, embryo, and membranes (92 ± 6, 63 ± 5, and 99 ± 8 nmol/μg protein/4 h, respectively) and decreased by two thirds by day 11 in each tissue. Of note, the rate of glycolysis was greater in membranes than in the embryo on both days (P < .01). On the other hand, oxidative metabolism, reflected in the production of ¹⁴CO₂, was relatively low on day 10 (0.78 ± 0.04, 0.73 ± 0.05, and 0.6 ± 0.06 nmol/μg protein/4 h for intact conceptus, embryo, and membranes, respectively) and increased significantly in each tissue by day 11 (P < .01 for each). Rates of glucose metabolism via the PPP in the embryo and membranes were 0.42 ± 0.02 and 0.45 ± 0.02 nmol/μg protein/4 h, respectively, on day 10; this rate was significantly higher on day 11 in the embryo (0.55 ± 0.03 nmol,/μg protein/4 h, P < .05 v day 10) and also tended to increase in the membranes on day 11. The production of CO₂ by the mitochondrial pathway (calculated by the difference of total and PPP metabolism) increased 3.2- and 2.4-fold from day 10 to day 11 in the embryo and membranes, respectively. Our results indicate that the energy production system in the intact conceptus, separated embryo, and investing membranes is dependent on anaerobic glycolysis before the establishment of the yolk sac circulation. With the establishment of the chorioallantoic circulation and continued growth of the conceptus, mitochondrial oxidative glucose metabolism matures in the intact conceptus and in both the embryo and its investing membranes. The PPP remains fully active in each tissue during this period of major metabolic adaptation, providing a source of necessary synthetic precursors for continued growth and development during the process of organogenesis.