Effects of extracellular potassium concentration on meiotic and cytoplasmic maturation in the porcine oocyte

Effects of extracellular potassium (K+) concentration in maturation media on the meiotic and cytoplasmic maturation of porcine oocytes were examined. Oocyte-cumulus cell complexes or cumulus cell denuded oocytes were cultured in Whittenʹs medium containing 0, 3, 6, 12 or 16 mM potassium. Absence of K+ in the media did not inhibit germinal vesicle breakdown (GVBD) in cumulus intact oocytes, but significantly decreased the frequency of meiotic maturation. In cumulus cell denuded oocytes, both GVBD and meiotic maturation were inhibited in K+-free medium. Millimole concentrations of K+ channel blockers, 4-aminopyridine or tetraethyl ammonium chloride inhibited GVBD and almost completely suppressed progression of meiotic maturation. The effect of varying the concentration of K+ on cytoplasmic maturation of pig oocytes was evaluated by the ability to form a male pronucleus after in vitro fertilisation. The percentage of sperm penetration or monospermic penetration was not different among treatments (P > 0.1). However, male pronuclear formation in oocytes in medium with 6 mM K+ was higher than in media with 12 and 16 mM K+. These results suggest that extracellular K+ is required for GVBD and meiotic maturation, and high concentrations (12 or 16 mM) of K+ in maturation media impair cytoplasmic maturation.