In vitro inhibition of rat oocyte meiosis by human follicular fluid fractions

We studied the effect of low molecular weight fractions of human follicular fluid (FF1) on rat oocyte meiosis and progesterone secretion by the granulosa cells. Steroid-free FF1 extracts were filtered through Sephadex G50 gel. One highly retarded fraction was obtained (GF-3), which was either used for testing or further purified by filtration on Sephadex G10, the G10-3 fraction of which was used for experiments. The GF-3 and G10-3 fractions of FF1 inhibited (in a dose-dependent and reversible manner) meiosis of isolated rat oocytes during a 4-h culture. Similarly treated fractions of serum had no effect. The inhibition was not abolished by ether extraction, trypsin treatment, heating to 56 degrees C for 1 h or boiling for 5 min, whereas heating to 105 degrees C for 18 h decreased the effect. Gonadotropin-stimulated progesterone secretion by cumulus and mural granulosa cells was also dose-dependently and reversibly inhibited by the GF-3 fractions. Our results confirm those obtained in earlier studies on porcine oocyte maturation inhibitor (OMI).

Oxygen consumption by rat oocytes and cumulus cells during induced atresia

Oxygen consumption was measured in denuded oocytes and oocyte-cumulus complexes isolated from atretic rat follicles. Adult cyclic rats or immature PMSG-treated rats were used, and follicular atresia was induced by hypophysectomy on the morning of pro-oestrus or by repeated pentobarbitone injections beginning on the day of pro-oestrus. The later stages of atresia were accompanied by meiosis-like changes in the oocytes. Oxygen consumption by oocytes that had resumed meiosis (germinal vesicle breakdown, GVB) was higher than in oocytes with an intact germinal vesicle, a change similar to that observed in oocytes maturing in healthy follicles. This may indicate that the meiotic process in the atretic follicles is similar to that in normal ones. Oxygen consumption by the cumulus cells was not altered during pentobarbitone-induced atresia. Hypophysectomy led to a rapid and marked increase in cumulus oxygen consumption in cyclic rats but there was no change in PMSG-treated young animals. Since both pentobarbitone-treatment and hypophysectomy result in follicular atresia, but changes in cumulus respiration occurred only in hypophysectomized adult rats, it is concluded that an increase in cumulus respiration is not inherent to the atretic process.

Biphasic effect of gonadotropin releasing hormone on progestin secretion by rat granulosa cells

The effect of an agonistic gonadotropin releasing hormone (GnRH)-analog (D-Ala6, des-Gly10-NH2-GnRH-ethylamide, GnRHa) on granulosa cell steroidogenesis in the presence or absence of follicle-stimulating hormone (FSH) or luteinizing hormone (LH) was studied. Granulosa cells, isolated from preovulatory follicles of pregnant mare's serum gonadotropin (PMSG)-treated immature rats or from the less mature follicles of untreated immature rats, were cultured for a period of 72 h with daily changes of medium, and progesterone and its metabolite, 20 alpha-dihydro-progesterone (20 alpha-OHP), were assayed in the medium. In granulosa cells from preovulatory follicles, LH and FSH caused a much greater stimulation of steroidogenesis than did GnRHa. There appeared to be no interaction between GnRHa and FSH during the first 10 h, but at 24 h and later the presence of GnRHa clearly inhibited the steroidogenic response to LH and FSH. Steroidogenesis in granulosa cells from immature rats was considerably lower and the effects of GnRHa and FSH alone less pronounced. In these cells, FSH-stimulated progesterone secretion was inhibited by GnRHa only at 72 h. In contrast, 20 alpha-OHP secretion in the same cultures was potentiated by the combined presence of FSH and GnRHa. In conclusion, it seems as though the effects of GnRHa on granulosa cell steroidogenesis varies with exposure time, the initial response being stimulatory and the later inhibitory. Furthermore, the response is also to some extent determined by the maturational stage of the granulosa cells.

Aspects concerning the role of prostaglandins for ovarian function

The present review briefly summarizes the current view of the mechanisms whereby prostaglandins (PGs) may act to modulate ovarian function. Particular concern is devoted to the aspects of ovarian function which are under current investigation by the authors, namely follicular maturation and rupture, granulosa cell and oocyte maturation, and the formation and maintenance as well as the regression of the corpus luteum (CL). PGs are formed in the granulosa cells of the preovulatory follicle in response to gonadotropin action and are at their highest levels around the time of ovulation. PGs are believed to be obligatory for follicular rupture, but their mechanism of action is still unknown. In the process of oocyte maturation PGs can mimic the stimulatory effects induced by LH or hCG, but are probably not obligate mediators of these effects. The importance of PGs for CL formation and maintenance is so far unsettled, and PGs may be regarded more as modulators than as inducers for these processes. The most well-documented effect of PGs is the induction of CL regression. Evidence is presented that PGs may have this effect also in the human.

Comparison between the progestin secretion responsiveness to gonadotrophins of rat cumulus and mural granulosa cells in vitro

Several studies have shown differences in gonadotrophin receptor content between different granulosa cell (gc) populations within the ovarian follicle, but little is known about the gonadotrophin sensitivity in gc from different parts of the follicle. This study is an investigation of progestin synthesis and responsiveness to highly purified human and partly purified rat gonadotrophins. It involves short-term culture of rat cumulus and mural gc obtained from preovulatory follicles of PMSG-treated immature rats. The responsiveness to FSH in terms of progestin accumulation was similar in the two gc populations, whereas the responsiveness to LH was greater in the mural gc than in the cumulus. The morphological response in the cumulus cells (mucification) correlated with the steroidogenic response. The pattern of progestin synthesis differed between the two gc populations. In the cumulus gc progesterone was dominant, whereas 20 alpha-dihydroprogesterone was the main progestin secreted in the mural gc. The difference in responsiveness to gonadotrophins correlates well with the reported differences in receptor content in the two gc populations.

A gonadotrophin-releasing hormone (GnRH) antagonist inhibits GnRH- but not LH-induced meiosis in follicle-enclosed rat oocytes in vitro

Previous studies have shown that gonadotrophin-releasing hormone (GnRH) can induce resumption of meiosis in follicle-enclosed rat oocytes. In the present study a GnRH antagonistic analogue ([D-pGlul,D-Phe2,-D-Trp3,6]LRF) was found to effectively abolish the stimulatory effect of a GnRH agonist upon resumption of meiosis and lactate accumulation in isolated pre-ovulatory rat follicles but the have no effect on LH stimulation of these parameters. It is concluded that although LH and GnRH can evoke a similar response they act through separate receptor sites and that it is unlikely that GnRH mediates the effect of LH on meiosis or glycolysis.

Ovulation induced by a gonadotropin releasing hormone analog in hypophysectomized rats involves prostaglandins

A potent analogue of gonadotropin releasing hormone [D-Ala6- Des-Gly10-NH2]-GnRH ethylamide (GnRHa) caused oocyte maturation and ovulation when injected in the afternoon of proestrus in immature PMSG-treated female rats, hypophysectomized on the morning of proestrus. This action of GnRHa was accompanied by a marked increase in ovarian PGE levels. Furthermore, the pretreatment of the animals with a prostaglandin synthetase inhibitor (indomethacin) completely inhibited this PGE increase and ovulation. These data suggest a role for prostaglandins in GnRHa induced ovulation.

Further studies on the gonadotrophin-induced inhibition of respiration in the preovulatory rat cumulus oophorus

It has been shown earlier that LH decreases the oxygen consumption of the rat cumulus oophorus cells, i.e. the granulosa cells immediately surrounding the follicular oocyte. The decrease occurs in the preovulatory period at the time when the oocyte resumes and completes meiosis. This study examines in more detail the time-course of LH-induced inhibition of cumulus respiration. The results show that when isolated cumuli were exposed to LH during in vitro culture a significant inhibition of respiration occurred within one hour. When follicle-enclosed cumuli were exposed to LH, in vivo or in vitro, the decrease in respiration was not evident until 4 hours after the hormone exposure. By this time oocyte maturation was already in progress. The results suggest that the decrease in cumulus respiration may not be directly linked to the onset of oocyte meiosis.

A gonadotrophin-releasing hormone agonist stimulates oxygen consumption and maturation of follicle-enclosed rat oocytes in vitro

Rat oocytes were induced to resume meiosis by incubating isolated preovulatory follicles with LH or a gonadotrophin-releasing hormone (GnRH) agonist. Oocyte maturation was accompanied by a 32% increase in oxygen consumption by the denuded oocyte after incubation with LH and a 67% increase after incubation with the GnRH agonist.

Stimulation by hCG in vivo of oxygen consumption by rabbit oocytes in vitro

Rabbits were injected with 100 i.u. hCG to induce oocyte maturation. Oocytes removed from the ovaries at various times and denuded showed increased oxygen consumption which began before the start of maturation.

Lack of specific prostaglandin antagonistic effects of 7-oxa-13-prostynoic acid on ovarian metabolism in vitro

Prostaglandins (PGs) have been proposed to function as obligatory intermediates in the action of luteinizing hormone (LH) (Kuehl et al. 1970). This based on experiments on isolated mouse ovaries where the substance 7-oxa-13-prostynoic acid (7-oxa-13-PA) was found to block the effect of both PGE1 and LH on ovarian cyclic AMP formation and progesterone synthesis. We have studied whether 7-oxa-13-PA acts as a PG antagonist also on the isolated rat ovary, where PGs of the E type have many effects. Prepubertal rat ovaries were incubated up to 4 h in modified Krebs bicarbonate buffer containing glucose with PGE1 and 7-oxa-13-PA, alone and in combination. PGE1 stimulated the uptake of the non-utilizable amino acids alpha-aminoisobutyric acid (AIB) and cycloleucine in an equally wide dose-range as has earlier been found for the stimulation of lactic acid and cyclic AMP production as well as protein synthesis. 7-oxa-13-PA alone, in concentrations exceeding 25 microgram/ml, stimulated lactic acid production, but inhibited the incorporation of labelled leucine and the uptake of AIB. The uptake of cycloleucine was slightly stimulated. 7-oxa-13-PA did not antagonize the PGE1 effect on lactic acid production. 7-oxa-13-PA diminished the PGE1 effect on the uptake of amino acids and the incorporation into protein, but only in concentrations where it in itself inhibited the protein synthesis. These results show that 7-oxa-13-PA does not act as a PG-antagonist in the prepubertal rat ovary, since it cannot block the effects of PGs without having marked inherent effects.

Inhibition of maturation and metabolism in rat oocytes by cyclic amp

It is known that dibutyryl cyclic AMP (dbcAMP) and theophylline inhibit the spontaneous maturation of isolated mouse oocytes. The present study demonstrates that dbcAMP (0.01-1.0 mM) as well as cyclic AMP (cAMP, 10 mM) and a phosphodiesterase inhibitor (IBMX, 0.01-1.0 mM) prevent spontaneous maturation of isolated rat oocytes. As reported earlier an increase in oxygen consumption by the oocyte was found following maturation. When the oocytes were cultured in the presence of dbcAMP or cAMP no change in respiration occurred during culture. These results argue against the theory that cAMP acts as a direct mediator of the action of luteinizing hormone (LH) on oocyte maturation. Furthermore they suggest that changes in oocyte energy metabolism are closely related to the maturation process.