OOCYTE MATURATION INVOLVES COMPARTMENTALIZATION AND OPPOSING CHANGES OF CAMP LEVELS IN FOLLICULAR SOMATIC AND GERM-CELLS - STUDIES USING SELECTIVE PHOSPHODIESTERASE INHIBITORS

The second messenger cAMP has been implicated in the regulation of mam malian and amphibian oocyte maturation. Although a a decrease in intra oocyte levels of cAMP precedes germinal vesicle breakdown (GVBD), the gonadotropin induction of ovulation and oocyte maturation is associate d with major increases of cAMP in ovarian follicles. In the mammalian system, isolated oocytes undergo spontaneous maturation in vitro but t his process is blocked by treatment with a phosphodiesterase (PDE) inh ibitor, IBMX, which increases intraoocyte cAMP levels. In contrast, th e same inhibitor, when added to cultured follicles for a brief time, i ncreases follicle cAMP levels, followed by the induction of GVBD. To r esolve the paradoxical actions of this PDE inhibitor on the maturation of isolated and follicle-enclosed oocytes, we hypothesized that meiot ic maturation requires opposing fluctuations of cAMP levels in the som atic granulosa and germ cells. Such opposing fluctuations may result f rom selective expression and regulation of PDEs in the somatic and ger m cell compartments of the follicle. To test this hypothesis, PDE acti vity was manipulated in different follicular cells using type-specific inhibitors. The impact of the ensuing changes in cAMP levels in the t wo compartments was monitored by the induction of GVBD. In isolated oo cytes, spontaneous GVBD was blocked by two inhibitors of type 3 PDE (c GMP-inhibited: CGI-PDE), milrinone and cilostamide. In contrast, treat ment with an inhibitor for type 4 PDE (cAMP-specific), rolipram, was i neffective. These findings suggest that the oocyte expresses type 3 bu t not type 4 PDE and that increases in intraoocyte cAMP suppress GVBD. This hypothesis was confirmed by in situ hybridization studies with P DE3 and PDE4 probes. PDE3B mRNA was concentrated in oocytes while PDE4 D was mainly expressed in granulosa cells. In cultured follicles, LH t reatment induced oocyte maturation but the gonadotropin action was blo cked by inhibitors of type 3 but not the type 4 PDE inhibitors. Furthe rmore, treatment with the type 4, but not the type 3, PDE inhibitor mi mics the action of LH and induces oocyte maturation, presumably by inc reasing cAMP levels in granulosa cells. Our findings indicate that PDE subtypes 4 and 3 are located in follicle somatic and germ cells, resp ectively. Preferential inhibition of PDE 3 in the oocyte may lead to a delay in oocyte maturation without affecting the cAMP-induced ovulato ry process in the somatic cells. Conversely, selective suppression of granulosa cell cAMP-PDE may enhance the gonadotropin induction of ovul ation and oocyte maturation. Thus, in addition to the well-recognized differential expression and regulation of adenylate cyclase in the som atic and germ cell compartments of the follicle, we suggest that selec tive regulation and expression of PDEs may be involved in the regulati on of cAMP levels and control of oocyte maturation in the preovulatory mammalian follicle. (C) 1996 Academic Press, Inc.