Changes in the activity of defense mechanisms against induced acidosis during meiotic maturation in mouse oocytes

Previous work has indicated that although activity of the HCO₃⁻/Cl⁻exchanger (AE), which regulates intracellular alkalosis, is high in the germinal vesicle (GV) stage, the oocyte is inhibited as it progresses through meiotic maturation. In this study, we aimed to investigate the defense mechanisms against acidosis during the meiotic maturation stages. Intracellular pH (pH(i)) was recorded using a microspectrofluorometric technique, and Na(+)/H(+) (NHE) and Na(+)-dependent HCO₃⁻/Cl⁻ exchanger (NDCBE) activity were determined by measuring the recovery rate from induced acidosis. Additionally, SLC9A1 (for NHE) and SLC4A8 (for NDCBE) gene transcription levels were determined by real-time PCR. The recovery rate of first meiotic prophase (GV) oocytes was high, but it decreased during the meiotic metaphase II (MII) stage in HCO₃⁻-free medium; it became high again at the pronuclear zygote (PN) stage. Recovery rate was significantly inhibited by 5-(N-ethyl-N-isopropyl) amiloride and cariporide or in the absence of extracellular Na(+), implicating NHE, specifically NHE1 activity. Moreover, the level of SLC9A1 transcription correlated with the observed changes in NHE activity. The changes in NHE activity during meiotic maturation displayed a similar pattern to that of AE. The recovery rate from acidosis was significantly higher in MII stage oocytes and PN zygotes in HCO₃⁻-containing medium; however, the increase was significantly inhibited in Na(+)-free medium or 4,4'-diisocyanatostilbene-2,2'-disulfonic acid. Furthermore, changes in the transcription of SLC4A8 during meiotic maturation were concordant with the level of exchanger activity. These results indicate that NDCBE activity is present in mouse oocytes and zygotes, and that this activity exhibits a different pattern than that of AE and NHE during meiotic maturation.