Cyclic Adenosine Monophosphate and Ovarian Follicular Fluid Act Synergistically to Inhibit Mouse Oocyte Maturation*

Abstract

The relationship between cAMP and the capacity of two partially-purified fractions of porcine follicular fluid (PFF) to suppress mouse oocyte maturation was investigated. The fractions used were: a low MW filtrate (< 10,000) derived from Amicon PM10 filtration of PFF (PM10), and a fraction from Bio-Gel P2 chromatography of the PM10 filtrate (Bio-Gel) that has meiotic inhibitory activity in porcine oocytes. PM10 alon produced a transient inhibition of the maturation of both cumulus cell-enclosed and denuded mouse oocytes, as manifested by germinal vesicle breakdown. The addition of FSH or (Bu)2cAMP to medium containing either of the PFF fractions resulted in dramatic synergism of the inhibitory effect of PFF alone in cumulus cell-enclosed oocytes. Forskolin or (Bu)2cAMP promoted a similar synergistic response with either PFF fraction in denuded oocytes. The degree of inhibition was consistently greater in cumulus cell-enclosed than in denuded oocytes. The putative inhibitor was dialyzable through tubing having a nominal MW cutoff of 1,000. Proteolysis, acid hydrolysis or ether extraction of PM10 did not reduce its inhibitory synergism with (Bu)2cAMP. Inhibition was completely abolished by charcoal extraction. Steroid hormones did not mimic the PM10-induced synergism when added to (Bu)2cAMP-containing medium in a concentration at least 14-fold greater than that present in PM10-supplemented medium. The presence of a factor(s) in PFF that acts synergistically with a cAMP-dependent process to inhibit oocyte maturation in vitro was demonstrated. Furthermore, although PFF fractions suppressed the maturation of both denuded and cumulus cell-enclosed oocytes, cumulus cells appear to mediate the inhibitory activity of PFF. Apparently the PFF inhibitor is a small (molMW < 1,000) hydrophobic molecule but not a peptide or nonpolar lipid.