Pentose phosphate pathway activity: effect on in vitro maturation and oxidative status of bovine oocytes

Abstract

The relationship between pentose phosphate pathway (PPP) activity in cumulus–oocyte complexes (COCs) and oxidative and mitochondrial activity in bovine oocytes was evaluated with the aim of analysing the impact of two inhibitors (NADPH and 6-aminonicotinamide (6-AN)) and a stimulator (NADP) of the key enzymes of the PPP on the maturation rate, oxidative and mitochondrial activity and the mitochondrial distribution in oocytes. The proportion of COCs with measurable PPP activity (assessed using brilliant cresyl blue staining), glucose uptake, lactate production and meiotic maturation rate diminished when 6-AN (0.1, 1, 5 and 10 mM for 22 h) was added to the maturation medium (P < 0.05). The addition of NADPH did not modify glucose uptake or lactate production, but reduced PPP activity in COCs and meiotic maturation rates (P < 0.05). The presence of NADP (0.0125, 0.125, 1.25 and 12.5 mM for 22 h of culture) in the maturation medium had no effect on PPP activity in COCs, glucose uptake, lactate production and meiotic maturation rate. However, in the absence of gonadotropin supplementation, NADP stimulated both glucose uptake and lactate production at 12.5 mM (the highest concentration tested; P < 0.05). NADP did not modify cleavage rate, but decreased blastocyst production (P < 0.05). During IVM, oocyte oxidative and mitochondrial activity was observed to increase at 15 and 22 h maturation, which was also related to progressive mitochondrial migration. Inhibiting the PPP with 6-AN or NADPH led to reduced oxidative and mitochondrial activity compared with the respective control groups and inhibition of mitochondrial migration (P < 0.05). Stimulation of the PPP with NADP increased oxidative and mitochondrial activity at 9 h maturation (P < 0.05) and delayed mitochondrial migration. The present study shows the significance of altering PPP activity during bovine oocyte IVM, revealing that there is a link between the activity of the PPP and the oxidative status of the oocyte.