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The differential regulation of cAMP levels within the oocyte and somatic (cumulus) cell compartments of the bovine follicle, and the subsequent regulation of oocyte meiotic maturation was examined through specific cell-type localisation of phosphodiesterases (PDEs). Selective PDE inhibitors were used to modulate cAMP levels in each of the two follicular compartments and to examine their effects on oocyte meiotic maturation. Ovaries were obtained from an abattoir and cumulus-oocyte complexes (COC) were aspirated from antral follicles into culture medium supplemented with 4 mg/ml BSA and 2mM 3-isobutyl-1-methylxanthine (IBMX). COC, denuded oocytes (DO), or mural granulosa cells (MGC) were cultured either with or without forskolin or FSH, in the presence of specific PDE inhibitors; either milrinone (PDE3 inhibitor), cilostamide (PDE3 inhibitor), or rolipram (PDE4 inhibitor). COC/DO cultures were assessed for meiotic progression and cAMP content, and MGC for cAMP production. The type 3 PDE inhibitor, but not the type 4, prevented spontaneous meiotic maturation and elevated intraoocyte cAMP in cultured denuded oocytes. In contrast, the type 4 PDE inhibitor had no effect on the oocyte, but elevated mural granulosa and cumulus cell cAMP production. The results of this study indicate that specific PDE subtypes are differentially localised within the two compartments of the bovine follicle--the type 3 PDE in the oocyte and the type 4 PDE in the granulosa cells. In addition, oocyte cAMP levels are primarily regulated in bovine oocytes by its degradation by PDE, whereas granulosa cell cAMP levels are controlled mainly by active adenylate cyclase, with both sources able to participate in oocyte meiotic regulation. Key Words: oocyte; meiotic maturation; meiotic inhibition; cAMP; phosphodiesterase; granulosa cell.

Oocytes secrete soluble factors that regulate the growth and differentiation of follicular cells, including maintenance of the distinctive cumulus cell phenotype. This study determines whether the mitogenic activity of oocytes is developmentally regulated and examines the responsiveness of follicular cells to oocytes at different stages of follicular development. Prepubertal SV129 mice of varying ages were primed with 5 IU equine chorionic gonadotropin (eCG) and oocytes/zygotes collected either 46 h post-eCG (immature oocytes), 12 h after administration of 5 IU human CG (hCG; ovulated ova), or 12 h post-hCG and mating (zygotes). Mural granulosa cells (MGC) from antral follicles and GC from preantral follicles were cultured [+ or -] denuded oocytes (DO) for 18 h, followed by a 6-h pulse of [[sup.3.H]]thymidine as an indicator of cellular DNA synthesis. Coculturing MGC with meiotically maturing oocytes led to a dose-dependent increase in [[sup.3.H]]thymidine incorporation (20-fold above control levels at 0.5 DO/ [micro] l). However, [3H] counts remained unchanged from control levels when cultured with meiotically incompetent DO from 11- to 15-day-old mice (3% germinal vesicle breakdown; GVB), irrespective of dose of DO or developmental status of GC (MGC or preantral GC). In some treatments, spontaneous meiotic resumption of competent oocytes was prevented by culturing with 5 [micro] M milrinone, a selective inhibitor of oocyte-specific cyclic nucleotide phosphodiesterase. The mitogenic capacity of oocytes was found to decline during and after oocyte maturation. [[sup.3.H]]Thymidine incorporation in MGC was highest (11-fold above controls) when cultured with meiotically inhibited (milrinone-treated) GV DO, stimulated 5.5-fold by culture with maturing oocytes, 3-fold with ovulated ova, and unstimulated by zygotes. [[sup.3.H]]Thymidine incorporation in MGC was not altered by the dose of milrinone, either in the presence or absence of DO. Metaphase I marked the beginning of the decline in the capacity of oocytes to promote MGC DNA synthesis. These results demonstrate that the capacity of oocytes to promote proliferation of granulosa cells follows a developmental program, closely linked to oocyte meiotic status, increasing with the acquisition of meiotic competence and declining during and after oocyte maturation. [c] 2001 Academic Press Key Words: oocyte mitogen; oocyte meiotic competence; folliculogenesis; granulosa cell; oocyte maturation.