Stimulatory role of cyclic adenosine monophosphate as a mediator of meiotic resumption in rabbit oocytes.

The present study was undertaken to assess the role of alterations of intraoocyte cAMP concentrations in the meiotic maturation of isolated and follicle-enclosed oocytes. In isolated oocyte culture, the intracellular cAMP content of denuded oocytes declined within 15 min of incubation, whereas the cAMP content of cumulus-enclosed oocytes did not change substantially for 1.5 h of incubation, and then declined abruptly. Commitment to meiotic maturation was preceded by reduced concentrations of intraoocyte cAMP. Forskolin inhibited the spontaneous maturation of cumulus-enclosed oocytes in a dose-dependent manner. However, this inhibition was attenuated as the duration of incubation increased. Forskolin significantly stimulated the intracellular cAMP content of denuded and cumulus-enclosed oocytes, but intraoocyte cAMP returned to pretreatment values within 4 h. The decline in intraoocyte cAMP was followed by the meiotic maturation of isolated oocytes. In in vitro perfused rabbit ovaries, exposure to forskolin at 10(-4) M, as well as to 50 IU human CG, accelerated the meiotic maturation of follicle-enclosed oocytes. The intraoocyte cAMP content increased significantly within 30 min and reached its maximum 2 h following exposure to forskolin. Thereafter, cAMP decreased abruptly and returned to pretreatment levels by 6 h. These alterations of intraoocyte cAMP contents following exposure to forskolin paralleled those observed in human CG-treated ovaries. The decline in cAMP content of follicle-enclosed oocytes was followed by their meiotic maturation. In conclusion, the sustained elevation of intraoocyte cAMP levels inhibits the initiation of meiotic maturation in isolated and follicle-enclosed oocytes. Within the follicle, resumption of meiosis is triggered via a transient increase in intraoocyte cAMP, but commitment to meiosis must await the decline of intraoocyte cAMP.