Granulosa cells regulate oocyte intracellular pH against acidosis in preantral follicles by multiple mechanisms

Mammalian oocytes grow within ovarian follicles in which the oocyte is coupled to surrounding granulosa cells by gap junctions. We report here that growing oocytes isolated from mouse preantral follicles are incapable of recovering from an experimentally induced acidosis, and that oocytes acquire the ability to manage acid loads by activating Na⁺/H⁺ exchange during growth. By contrast, granulosa cells from similar preantral follicles possess substantial Na⁺/H⁺ exchange capacity, which is attributable to the simultaneous action of two Na⁺/H⁺ exchanger isoforms: NHE1 and NHE3. Granulosa cells were also found to possess a V-type H⁺-ATPase that drives partial acidosis recovery when Na⁺/H⁺ exchange is inactivated. By monitoring intracellular pH (pH_i) in small follicle-enclosed oocytes, we found that the oocyte has access to each of these acidosis-correcting activities, such that small follicle-enclosed oocytes readily recover from acidosis in a manner resembling granulosa cells. However, follicle-enclosed oocytes are unable to access these activities if gap-junction communication within the follicle is inhibited. Together, these experiments identify the NHE isoforms involved in regulating oocyte pH_i, indicate that gap junctions allow granulosa cells to exogenously regulate oocyte pH_i against acidosis until the oocyte has acquired endogenous pH_i regulation, and reveal that granulosa cells possess multiple mechanisms for carrying out this function.