The influence of cumulus cells on porcine oocyte maturation in the presence of L-carnitine

Cumulus-oocyte complexes (COCs) were recovered from 3–6 mm follicles of abattoir-derived ovaries, and either kept as intact COCs or denuded of their cumulus cells (CCs). Groups of COCs and denuded oocytes (DOs) were matured separately or co-cultured together to assess any indirect influence of CCs (DOs + CCs). Modified porcine oocyte medium (POM; Yoshioka et al. 2008) containing a low concentration (1.5 mM) of glucose and no pyruvate and lactate was supplemented with either 0 or 12 mM LC. Nuclear maturation was assessed at 44 h of IVM. The intra-oocyte concentration of ATP was also measured in oocytes in the presence (+PL) and absence (-PL) of pyruvate and lactate at 0, 22 and 44 h. Data were analysed using ANOVA and Fisher’s unprotected lease significant difference test (Genstat, 16th Edition; UK).

Supplementing LC had no significant effect on the proportion of oocytes that were mature at 44 hours (Fig. 1). As expected, significantly greater proportions of oocytes cultured in the presence of cumulus cells were mature when compared to DOs. An interaction was observed between time and treatment on the ATP concentrations observed per oocyte (Fig. 2). Mean ATP concentrations were increased (P < 0.05) in oocytes matured in the presence of pyruvate and lactate for 44 h compared to all other treatments across all time points. The results indicated that under low carbohydrate conditions LC does not enhance oocyte nuclear maturation, irrespective of the presence of CCs, nor does it significantly increase ATP production. Thus, the hypothesis that LC treatment would improve oocyte maturation by increasing cellular energy generation was rejected. Further studies are required to elucidate whether cumulus cells play a role in porcine oocyte lipid metabolism.

Fig. 1.  Effect of L-carnitine (LC) on oocyte maturation (mean ± SEM) at 44 hours of IVM. Oocytes were matured with 0 mM and 12 mM LC. Different letters indicate differences between treatments (P < 0.05). Denuded oocytes (DOs), cumulus-oocyte complexes (COCs), cumulus cells (CCs).

Fig. 2.  Effect of L-carnitine (LC) on intra-oocyte ATP concentrations (mean ± SEM) at 0, 22 and 44 hours IVM. Oocytes were matured with (+PL) or without (-PL) pyruvate and lactate, and treated with 0 (-LC) or 12 mM (+LC) L-carnitine (+LC/+PL, -LC/+PL, +LC/-PL, -LC/-PL). Values with an asterisk are significantly different (P < 0.05).