Use of Low-Salt Culture Medium for in Vitro Maturation of Porcine Oocytes is Associated with Elevated Oocyte Glutathione Levels and Enhanced Male Pronuclear Formation after in Vitro Fertilization1

The effects of sodium chloride (NaCl) in Whitten's medium on intracellular glutathione concentration and on cytoplasmic maturation, as determined by monospermic penetration and male pronuclear formation of porcine oocytes, were examined. Porcine cumulus-oocyte complexes were cultured for 20 h in BSA-free Whitten's medium containing different NaCl concentrations (44.50, 68.49, 92.40, 116.40, or 140.35 mM) and supplemented with 10% porcine follicular fluid and hormonal supplements; the complexes were then cultured without hormonal supplements for an additional 20-h period. The mean width of the perivitelline space of oocytes was increased with decreased concentration of NaCl in the culture medium. Intracellular glutathione concentration was elevated in oocytes cultured in medium with lower NaCl concentrations. After co-culture with spermatozoa for 6 h and culture in modified Whitten's medium for an additional 6 h, there were no differences in maturation and penetration rates among experimental groups. However, the rate of male pronuclear formation was higher in oocytes matured in media with the lower NaCl concentrations. In addition, the rates of monospermic penetration and male pronuclear formation were higher in oocytes matured in medium containing 44.50 mM NaCl (59.3 +/- 8.1 and 70.9 +/- 2.0%, respectively) than in medium containing 68.49 mM NaCl (39.4 +/- 5.5 and 57.1 +/- 4.5%, respectively). These data indicated that decreasing NaCl concentration in maturation medium for porcine oocytes below the customary level improved the quality of the matured oocytes as reflected in higher intracellular glutathione content, wider perivitelline space, higher monospermic penetration rate, and increased frequency of male pronuclear formation.