Inhibitory action of hypoxanthine on meiotic resumption of denuded pig follicular oocytes in vitro

Abstract

The present study was conducted to investigate the direct action of hypoxanthine, which has been reported to be present in pig follicular fluid, on spontaneous meiotic resumption of pig denuded oocytes in vitro. No oocytes, which were surrounded by a cumulus and some membrana granulosa cells directly adjacent to it from healthy antral follicles, underwent germinal vesicle breakdown (GVBD) when they were cultured in serum-supplemented 199 with or without 4 mM hypoxanthine. On the other hand, 81% of denuded oocytes prepared from such cumulus-oocyte-granulosa cell complexes showed GVBD after 24 h of culture. Hypoxanthine inhibited significantly this spontaneous resumption of meiosis in denuded oocytes. The inhibitory action of hypoxanthine was dose dependent at concentrations of 1-6 mM and was reversible following subsequent culture of the oocytes in hypoxanthine-free medium. However, premature chromosome condensation was observed in oocytes cultured in hypoxanthine-supplemented medium, and the oocytes released from the influence of hypoxanthine completed GVBD and reached the second metaphase slightly faster than those freshly isolated from the follicle. These results suggest that hypoxanthine inhibits spontaneous meiotic resumption of pig oocytes in vitro. However, the inhibitory action of hypoxanthine does not prevent all events involved in GVBD from occurring.

Phosphatidylinositol 3-kinase in cumulus cells is responsible for both suppression of spontaneous maturation and induction of gonadotropin-stimulated maturation of porcine oocytes

“…Hypoxanthine was present in ovarian follicular fluid from pigs, and suppressed spontaneous maturation of cumulusenclosed oocytes (Downs et al 1985, Miyano et al 1995. It has been reported that hypoxanthine maintains meiosisarresting levels of cAMP by inhibiting cAMP phosphodiesterase activity in mouse oocytes (Downs et al 1989).…”

Hypoxanthine Promotes the Acquisition of Meiotic Competence in Pig Oocytes from Early Antral Follicles during Growth Culture

“…Since h y p o x a n t h i n e i s a n a t u r a l l y o c c u r r i n g c A M Pphosphodiesterase inhibitor, hypoxanthine in our culture medium was thought to maintain the cAMP level in granulosa cells as well as in oocytes. If so, it may be a case similar to that of cultures supplemented with dibutyryl cAMP or forskoline, an adenylate cyclase activator, that are shown to promote the acquisition of meiotic competence in naked growing mouse oocytes [25]. But it has not yet been made clear how the increase in cAMP in oocytes promotes the acquisition of the competence.…”

“…The follicular fluid eventually contains hypoxanthine at a high concentration of 1.4 mM [14]. It is well known that this high level of hypoxanthine is one of the meiosisinhibiting factors contained in follicular fluid, and is cr ucia l i n pre v ent ing t he spon tan eous mei oti c resumption of oocytes in their follicles [25]. In addition to this role of maintaining meiotic arrest in meioticcompetent oocytes, as suggested in this study, hypoxanthine may participate in the acquisition of meiotic competence by pig oocytes during their final growth stage in the ovary.…”

<i>In Vitro</i> Growth of Mammalian Oocytes

“…In the ovarian follicles, oocytes are arrested by the meiosisarresting substances. Hypoxanthine in follicular fluid has been identified as one of such meiosisarresting substances [37][38][39]. The oocytes at the mid-growth phase themselves have no ability to resume meiosis.…”