Regulation of glycolysis in the mouse blastocyst during delayed implantation

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Abstract
The rate of oxidation of glucose is reduced in mouse embryos in the prolonged free living phase associated with delayed implantation and increases when the embryos are reactivated by estrogen. To determine how these changes in metabolism are regulated, several aspects of glucose metabolism were evaluated in dormant and reactivated blastocysts: 1) Embryos were exposed to 14C‐pyruvate in vitro and evolved 14CO2 was measured. It was found that the rate of production of CO2 was equal in the two types of blastocysts, suggesting that aerobic pathways are fully functional during delayed implantation. 2) Production of lactate in the presence of O2 was measured and a decrease of 30% was found in delayed implanting embryos, suggesting that the overall regulatory mechanism for glucose metabolism resides in the glycolytic portion of the pathway. 3) Capacity for uptake and phosphorylation of glucose was evaluated using 3H‐2‐deoxyglucose and was found to be equal in the two types of embryos. 4) Total amounts of the rate‐controlling enzymes for glycolysis (i.e., hexokinase and phosphofructokinase) in lysates of delayed and reactivated embryos were found to be equal, indicating that amounts of these enzymes are not limiting in delayed implantation. 5) Lactate production, measured under anaerobic conditions, was found to be equal, demonstrating that it is not the capacity for glycolysis but a difference in the degree of allosteric inhibition that is responsible for reduced glucose oxidation in delayed implantation. 6) Levels of ATP, ADP, and hexose‐6‐phosphates were found to be consistent with allosteric inhibition of the glycolytic pathway at phosphofructokinase during delay and a release of this inhibition with reactivation. In addition, finding a high ATP/ADP ratio in delayed implanting blastocysts indicates that the associated depression of macromolecular synthesis is not due to limited energy supply. Rather it suggests that the reduction in glucose metabolism is probably due to a decrease in energy demand.