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During each reproductive cycle, a preovulatory surge of gonadotropins induces meiotic maturation of the oocyte in the preovulatory follicle followed by ovulation. Although gonadotropins stimulate cAMP production in somatic cells of the follicle, a decrease in intra-oocyte cAMP levels is required for resumption of meiosis in oocytes. Based on the observed compartmentalization of the cAMP-degrading enzyme, phosphodiesterase, in follicular somatic and germ cells, inhibitors of phosphodiesterase 3 were used to block meiosis in ovulating oocytes in rodents. By this strategy, we demonstrated that fertilization and pregnancy could be prevented without disturbing follicle rupture and normal estrous cyclicity. In contrast to conventional contraceptive pills that disrupt ovarian steroidogenesis and reproductive cycles, the present strategy achieves effective contraception by selective blockage of oocyte maturation and development without alterations in ovulation and reproductive cyclicity. ( J. Clin. Invest. 1998. 102:532-537.)

In the preovulatory follicle, oocyte meiotic resumption occurs soon after the LH surge and is associated with a decrease in cAMP. Inhibition of cAMP degradation blocks germinal vesicle breakdown as well as activation of meiotic promoting factor, both hallmarks of reentry into the cell cycle. In situ and pharmacological analysis of rodent ovaries suggested the presence of a phosphodiesterase 3 (PDE3) in the germ cell but not the somatic cell compartment. Here we have investigated the structure and properties of the PDE form expressed in mouse oocytes. Polymerase chain reactions using a mouse oocyte cDNA library as a template, and primers based on the conserved sequence of rat and human PDE3As, yielded partial fragments corresponding to mouse PDE3A. Further screening of the mouse oocyte cDNA library and subsequent ligation of individual cDNA clones yielded PDE3A cDNA containing the entire coding region of mouse PDE3A. To determine the kinetic properties of this PDE, the cDNAs encoding the full-length PDE3A and NH(2)-truncation forms Delta 1 (Delta346aa) and Delta 2 (Delta608aa) were expressed in mouse Leydig tumor cells. Whereas the full-length recombinant protein was always found in the particulate fraction, the Delta 1 and Delta 2 truncated PDE3As were recovered mostly in the soluble fraction. The Michaelis constant values for hydrolysis of cAMP of PDE3A Delta 1 and PDE3A Delta 2 were similar to those of intact full-length PDE3A or oocyte PDE (0.2-0.5 microM). More importantly, there was good correlation between the rank of potency of selective and nonselective compounds in inhibiting recombinant PDE3A or PDE activity derived from cumulus-oocyte complexes and in blocking resumption of meiosis. These data provide evidence that the PDE expressed in the oocyte is a soluble form of PDE3A and that activity of this enzyme is involved in the control of resumption of meiosis.

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