137 ROLE OF GLUCOSE AND FATTY ACID METABOLISM IN PORCINE EARLY EMBRYO DEVELOPMENT

Abstract

Glucose metabolism plays an important role in the preimplantation development of porcine embryos in vitro. As in mammalian species generally, a proportion of glucose consumed is converted to lactate by aerobic glycolysis generating small amounts of ATP, with the remainder oxidized by the TCA cycle. However, a striking feature of the porcine early embryo is the large amount of lipid present as triglyceride (TG), which represents an alternative energy source. The TG is metabolized via β-oxidation, producing acetyl Co A, which in turn is oxidized by the TCA cycle. This sequence of reactions requires a constant supply of carbohydrate to provide oxaloacetate (OA) to prime the TCA cycle. The provision of OA from pyruvate arising from glycolysis may represent an alternative role for glucose in early pig embryo development. We have therefore sought to determine the importance of interplay between glucose and TG metabolism in porcine embryos in vitro. Porcine embryos were generated in vitro by fertilization of in vitro-matured oocytes collected from abattoir-derived ovaries. Oocytes were matured in defined maturation medium and embryos cultured in NCSU23. Glucose consumption, lactate production, and TG content of single porcine blastocysts cultured throughout development in the presence of methyl palmoxirate (MP), an inhibitor of TG metabolism, were measured as described by Sturmey RG and Leese HJ 2003 (Reprod. 126, 197–204). The capacity of zygotes to form blastocysts when cultured with OA in place of glucose in the presence or absence of MP and the amount of TG in blastocysts grown in either glucose or OA-containing medium were then determined (6 replicates). When TG metabolism was inhibited, porcine blastocysts consumed significantly more glucose (32 ± 9 pmol/embryo/h v. 11 ± 1 pmol/embryo/h; P < 0.05; n = 34) and produced higher amounts of lactate (35 ± 4 pmol/embryo/h v. 10 ± 0.8 pmol/embryo/h; P < 0.01; n = 34). Blastocyst rates did not differ significantly between embryos grown in the presence of glucose or OA, and in blastocysts grown in OA-NCSU the TG content was significantly reduced (155 ± 8 ng v. 240 ± 12 ng; P = 0.015; n = 41). All embryos cultured in OA-containing medium in the presence of MP failed to develop beyond the zygote stage. The data support the notion that porcine embryos can use endogenous TG as a metabolic energy source. When this is prevented by chemical inhibition, the embryo upregulates glycolysis and glucose oxidation as an alternate means of generating ATP. When cultured in medium containing OA, a compound that cannot generate ATP per se, embryo development was similar to controls, again suggesting the ability to use endogenous energy stores, a proposition reinforced by a significant fall in the levels of TG in the presence of OA. However, by inhibiting β-oxidation in the absence of glucose, porcine embryos were unable to develop. The relationship between TG and glucose metabolism by porcine embryos is analogous to the glucose/fatty acid cycle in whole animals where glucose and TG can be used as energy sources, but in a reciprocal manner. The data also demonstrate the plasticity of energy metabolism by porcine early embryos.