Perturbation of potassium homeostasis impairs oocyte maturation

Movement of ions such as calcium and zinc are well known regulators of oocyte maturation and fertilization. The specific contribution of potassium ions is less well understood. We tested whether acute disruption of K+ homeostasis perturbs oocyte maturation. A preliminary screen identified PA-6 (IK1/KIR2.x inhibitor) as a robust inhibitor of oocyte maturation. We compared effects of PA-6 to Valinomycin (K+ ionophore) and tetraethylammonium chloride (TEAC; broad voltage-gated K+ channel blocker). PA-6 and Valinomycin completely prevented germinal vesicle breakdown (GVBD) and polar body extrusion. The nuclear envelope remained intact but with the chromatin stretching across the nucleus (PA-6) or remained semi condensed in the nucleus (Valinomycin). TEAC permitted GVBD but 66% were arrested at MI and both MI and MII spindles were abnormal. Thallium-based flux assays showed higher K+ transport in GV than MII oocytes with PA-6 increasing transport in GV only. Valinomycin reduced transport in MII, but not GV oocytes. TEAC enhanced transport in both stages with delayed onset at GV. PA-6 did not elevate cAMP, while Valinomycin and TEAC modestly increased cAMP. Only PA-6 increased mitochondrial membrane potential, while only Valinomycin increased plasma membrane potential. Both PA-6 and Valinomycin blunted depolarization-induced Zn2+ accumulation. Surprisingly, KCNJ2/4/12 transcripts were detected in cumulus oocyte complexes but not in denuded oocytes. However, KCNJX immunoreactivity was present in oocytes indicating uncertainty about the PA-6 targets in oocytes. Collectively, these data identify K+ flux as a central regulator of oocyte maturation that integrates plasma membrane excitability, organelle function, chromatin remodeling, spindle integrity, and zinc dynamics.