AbstractThe effects of the putative maturation inhibitor in porcine follicular fluid on gonadotropinstimulated reversal of cyclic adenosine monophosphate (cAMP)‐maintained meiotic arrest in mouse oocytes in vitro were assessed in this study. When cumulus cell‐enclosed oocytes were cultured in a suboptimal inhibitory concentration of dibutyryl cAMP (dbcAMP), the effect of follicle‐stimulating hormone (FSH) on oocyte maturation was initially inhibitory at 3 hr, but stimulatory at 6 hr. Supplementation of the medium with an ultrafiltrate of porcine follicuiar fluid (PM10‐filtrate) completely suppressed FSH‐promoted reversal of inhibition at 6 hr. Charcoal extraction eliminated this effect of the PM10‐filtrate. FSH reversed the inhibition of maturation of cumulus cell‐enclosed oocytes maintained by a high concentration of dbcAMP and suboptimal concentrations of the phosphodiesterase inhibitor, 3‐isobutyl‐1‐methyl xanthine (IBMX), during a 21–22‐hr culture period. However, the effect of a completely inhibitory concentration of IBMX was not reversed by gonadotropin. A component of serum was also found to inhibit FSH reversal of dbcAMP‐maintained meiotic arrest, and this activity was removed by charcoal extraction. In addition, when oocytes were cultured in medium containing a suboptimal concentration of dbcAMP plus a low molecular weight fraction (<1,000) of porcine follicular fluid, porcine serum, or fetal bovine serum, a synergistic inhibition of maturation was observed. Experiments with highly purified gonadotropins revealed that reversal of dbcAMP‐maintained meiotic arrest occurred only in response to FSH; neither highly purified luteinizing hormone nor human chorionic gonadotropin could mimic this action of FSH. Also, this effect was mediated by the cumulus cells, since FSH could not reverse dbcAMP‐maintained meiotic arrest in denuded oocytes. Furthermore, elevating cAMP levels in denuded oocytes augmented, rather than reversed, the inhibitory action of dbcAMP on oocyte maturation. These data therefore suggest that dbcAMP‐ or IBMX‐maintained meiotic arrest in vitro is reversed by an FSH‐stimulated, cAMP‐dependent process mediated by the cumulus cells and demonstrate that a factor present both in follicular fluid and serum pre