Inner mitochondrial membrane potential (ΔΨm), cytoplasmic ATP content and free Ca2+ levels in metaphase II mouse oocytes

Background: The relative magnitude of the inner mitochondrial membrane potential (DeltaPsim) has been suggested to indicate the competence of mammalian gametes and early embryos. This study examined the response of cultured somatic cells and mouse oocytes to inhibitors and conditions that affect DeltaPsim or metabolism, or both, and measured treatment-specific changes in ATP and cytoplasmic free Ca2+. Methods: During and after treatment, relative DeltaPsim, free Ca2+, and ATP levels and cortical granule density were determined. Results: Comparable responses of somatic cells and metaphase II mouse oocytes to experimental manipulations that affect DeltaPsim and metabolism were observed and reversible loss of DeltaPsim was associated with increased intracellular free Ca2+, which in certain instances resulted in parthenogenetic activation. Conclusion: The findings support a mitochondrial basis for pericortical J-aggregate fluorescence but not for a direct association between high DeltaPsim and metabolism. The results extend previous findings indicating that high-polarized (high DeltaPsim, JC-1 J-aggregate-forming) mitochondria occur in pericortical domains in mouse and human oocytes and early preimplantation stage embryos and support the notion that this spatial distribution may be related to localized ionic and metabolic regulation.