Induction and inhibition of in vitro oocyte maturation and production of steroids in fish follicles by forskolin

Characterization of G protein-coupled estrogen receptors in Japanese medaka, Oryzias latipes

The G protein-coupled estrogen receptor 1 (Gper1) is a membrane-bound estrogen receptor that mediates non-genomic action of estrogens. A Gper1-mediating pathway has been implicated in reproductive activities in fish, including oocyte growth, but Gper1 has been characterized in only a very limited number of fish species. In this study, we cloned and characterized two genes encoding medaka (Oryzias latipes) Gper1s, namely, Gper1a and Gper1b, and phylogenic and synteny analyses suggest that these genes originate through a teleost-specific whole genome duplication event. We found that Gper1a induced phosphorylation of mitogen-activated protein kinase (MAPK) in 293T cells transfected with medaka Gper1s on exposure to the natural estrogen, 17β-estradiol (E2) and a synthetic Gper1 agonist (G-1), and treatment with both E2 and G-1 also decreased the rate of spontaneous maturation in medaka oocytes. These findings show that the processes for oocyte growth and maturation are sensitive to estrogens and are possibly mediated through Gper1a in medaka. We also show that 17α-ethinylestradiol (EE2), one of the most potent estrogenic endocrine-disrupting chemicals, and bisphenol A (BPA, a weak environmental estrogen) augmented phosphorylation of MAPK through medaka Gper1s in 293T cells. Interestingly, however, treatment with EE2 or BPA did not attenuate maturation of medaka oocytes. Our findings support that Gper1-mediated effects on oocytes are conserved among fish species, but effects of estrogenic endocrine-disrupting chemicals on oocytes acting through Gper1 may be divergent among fish species.

A trial to cryopreserve immature medaka (Oryzias latipes) oocytes after enhancing their permeability by exogenous expression of aquaporin 3

Fish oocytes have not been cryopreserved successfully, probably because it is difficult to prevent intracellular ice from forming. Previously, we have shown in medaka that immature oocytes are more suitable for cryopreservation than mature oocytes or embryos, in terms of permeability. We have also shown in immature medaka oocytes that the exogenous expression of aquaporin 3 (AQP3), a water/cryoprotectant channel, promotes the movement of water and cryoprotectants through the plasma membrane. In the present study, we attempted to cryopreserve immature medaka oocytes expressing AQP3. We first examined effects of hypertonic stress and the chemical toxicity of cryoprotectants on the survival of the AQP3-expressing oocytes. Exposure to hypertonic solutions containing sucrose decreased the survival of oocytes, but the expression of AQP3 did not affect sensitivity to hypertonic stress. Also, AQP3 expression did not markedly increase sensitivity to the toxicity of cryoprotectants. Of the four cryoprotectants tested, propylene glycol was the least toxic. Using a propylene glycol-based solution, therefore, we tried to cryopreserve immature oocytes by vitrification. During cooling with liquid nitrogen, all intact oocytes became opaque, but many AQP3-expressing oocytes remained transparent. This indicates that the expression of AQP3 is effective in preventing intracellular ice from forming during cooling. During warming, however, all the AQP3-expressing oocytes became opaque, indicating that intracellular ice formed. Therefore, the dehydration and permeation by propylene glycol were still insufficient. Further studies are necessary to realize the cryopreservation of fish oocytes.

Regulation and actions of insulin-like growth factors in the ovary of zebrafish (Danio rerio)

Insulin-like growth factors (Igf) are known paracrine/autocrine regulators of ovarian development in teleosts. Initial studies investigated the hormonal and intracellular signalling cascades involved in regulating the expression of ovarian-derived Igfs in zebrafish (Danio rerio). Quantitative real-time PCR was used to quantify the expression of igf3, igf2a, and igf2b in full grown immature (FG; 0.57-0.65 mm) and mid-vitellogenic (MV; 0.45-0.56 mm) follicles. Addition of the gonadotropin analogue human chorionic gonadotropin (hCG) and the adenylate cyclase activator forskolin increased igf3 expression in FG and MV follicles, but had no effect on igf2a or igf2b expression. The effects of hCG on igf3 expression were blocked by the addition of the protein kinase A inhibitor H-89. Pituitary adenylate cyclase activating peptide also stimulated a small increase in igf3 expression in FG follicles, while growth hormone and salmon gonadotropin releasing hormone had no effect on igf3, igf2a, or igf2b expression. Secondary studies investigated the involvement of ovarian-derived Igfs in mediating the ovarian actions of gonadotropins on cell survival and steroidogenesis. Treatment of FG follicles with recombinant human IGF1, hCG, or forskolin inhibited the induction of caspase-3/7 activity, which was used as a measure of apoptosis. The effects of hCG and forskolin on caspase-3/7 were attenuated by co-treatment with NVP-AEW54, an IGF1 receptor antagonist. In other studies, hCG was shown to increase the production of the maturation-inducing steroid 17,20β-dihydroxy-4-pregnen-3-one, but this action was not affected by co-treatment with NVP-AEW54. These results suggest there is a high degree of hormonal specificity in regulating Igfs in the zebrafish ovary and the ovarian-derived Igfs, presumably Igf3, are downstream mediators of gonadotropin-dependent cell survival, but are not involved in gonadotropin-induced steroidogenesis.

Characteristics of ovarian follicle steroidogenesis during vitellogenesis in an asynchronously ovulating stock of rainbow trout Oncorhynchus mykiss

This study explored several physiological criteria that could be used to assess the steroidogenic condition of the ovarian follicles of individual fish of an asynchronously ovulating captive rainbow trout Oncorhynchus mykiss stock. In these fish, the date of sampling, morphological variables such as gonado-somatic index or ovarian follicle mass and visual assessment of the ovary provided accurate indications of the maturational condition of an individual. The physiological variables measured included the in vitro basal and cyclic adenosine monophosphate (cAMP)-stimulated synthesis by ovarian follicles of 17β-oestradiol (E(2)) and testosterone (T); in addition, quantitative reverse-transcription (RT)-PCR was used to measure the relative expression of star and p450scc genes by ovarian follicles. The ratios of cAMP-stimulated E(2) and T synthesis to basal E(2) and T synthesis provided a reliable indication of differences in the steroidogenic status of the follicles of individual animals. On the basis of these criteria, together with the use of gene expression profiles, it was possible to classify individual fish as being at an early, mid or late-vitellogenic stage.

Development of a reliable in vitro maturation system for zebrafish oocytes

In zebrafish oocytes, it has been reported that a 60 or 75% Leibovitz L-15 medium or simple balanced saline solution containing 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP) is effective for nuclear maturation. However, most of the oocytes that matured under these conditions were not fertilized and did not hatch. Thus, these in vitro maturation methods could not support the cytoplasmic maturation of zebrafish oocytes. Therefore, we tried to develop a reliable in vitro maturation method for zebrafish oocytes, which supports their ability to be fertilized and to develop till hatching. When zebrafish oocytes at stage III were cultured in 50-100% Leibovitz L-15 medium supplemented with DHP, the highest rates of cleavage (24%) and hatching (12%) were obtained from oocytes matured in 90% Leibovitz L-15 medium. When we examined the suitable pH (7.5-9.5) of the 90% medium, higher rates of cleavage (45%) and hatching (33%) were obtained in oocytes matured at pH 9.0 than at pH 7.5, 8.5, or 9.5 (cleavage rate, 16-29%; hatching rate, 8-21%). In oocytes matured in 90% Leibovitz L-15 medium at pH 9.0, high rates of cleavage (70%) and hatching (63%) were obtained when oocytes were cultured for 270 min with 0.5 mg/ml BSA. Thus, 90% Leibovitz L-15 medium at pH 9.0 containing 0.5 mg/ml BSA was effective for normal maturation of zebrafish oocytes. This method will become a powerful tool for understanding the mechanism of in vitro maturation in zebrafish oocytes and for the practical use of immature oocytes.

Expression of aquaporin-3 improves the permeability to water and cryoprotectants of immature oocytes in the medaka (Oryzias latipes)

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes and embryos. Several efforts have been made to facilitate the movement of water and cryoprotectants across the plasma membrane of fish oocytes/embryos because of their large size. Aquaporin-3 is a water/solute channel that can also transport various cryoprotectants. In this study, we tried to improve the permeability of immature medaka (Oryzias latipes) oocytes to water and cryoprotectants by artificially expressing aquaporin-3. The oocytes were injected with aquaporin-3 cRNA and cultured for 6-7 h. Then, hydraulic conductivity (L(P)) and cryoprotectant permeability (P(S)) were determined from volume changes in a hypertonic sucrose solution and various cryoprotectant solutions, respectively, at 25 degrees C. The L(P) value of the cRNA-injected oocytes was 0.22+/-0.04 microm/min/atm, nearly twice larger than that of intact or water-injected oocytes (0.14+/-0.02 and 0.14+/-0.03 microm/min/atm, respectively). P(S) values of intact oocytes for ethylene glycol, propylene glycol, and DMSO were 1.36+/-0.34, 1.97+/-0.20, and 1.17+/-0.52 x 10(-3) cm/min, respectively. The permeability to glycerol could not be calculated because oocytes remained shrunken in the glycerol solution. On the other hand, cRNA-injected oocytes had significantly higher P(S) values (glycerol, 2.20+/-1.29; ethylene glycol, 2.98+/-0.36; propylene glycol, 3.93+/-1.70; DMSO, 3.11+/-0.74 x 10(-3) cm/min) than intact oocytes. When cRNA-injected oocytes were cultured for 12-14 h, 51% matured to the metaphase II stage, and 43% of the matured oocytes were fertilized and hatched following in vitro fertilization and 14 days of culture. Thus, the permeability of medaka oocytes to water and cryoprotectants was improved by the artificial expression of aquaporin-3, and the oocytes retained the ability to develop to term.

Water- and cryoprotectant-permeability of mature and immature oocytes in the medaka (Oryzias latipes)

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes/embryos. To identify a stage feasible for the cryopreservation of teleost oocytes, we investigated the permeability to water and various cryoprotectants of medaka (Oryzias latipes) oocytes at the germinal vesicle (GV) and metaphase II (MII) stages. In sucrose solutions, the volume changes were greater in GV oocytes than MII oocytes. Estimated values for osmotically inactive volume were 0.41 for GV oocytes and 0.74 for MII oocytes. Water-permeability (microm/min/atm) at 25 degrees C was higher in GV oocytes (0.13+/-0.01) than MII oocytes (0.06+/-0.01). The permeability of MII oocytes to various cryoprotectants (glycerol, propylene glycol, ethylene glycol, and DMSO) was quite low because the oocytes remained shrunken during 2 h of exposure in the cryoprotectant solutions at 25 degrees C. When the chorion of MII oocytes was removed, the volume change was not affected, except in DMSO solution, where dechorionated oocytes shrunk and then regained their volume slowly; the P(DMSO) value was estimated to be 0.14+/-0.01x10(-3) cm/min. On the other hand, the permeability of GV oocytes to cryoprotectants were markedly high, the P(s) values (x10(-3) cm/min) for propylene glycol, ethylene glycol, and DMSO being 2.21+/-0.29, 1.36+/-0.18, and 1.19+/-0.01, respectively. However, the permeability to glycerol was too low to be estimated, because GV oocytes remained shrunken after 2 h of exposure in glycerol solution. These results suggest that, during maturation, medaka oocytes become less permeable to water and to small neutral solutes, probably by acquiring resistance to hypotonic conditions before being spawned in fresh water. Since such changes would make it difficult to cryopreserve mature oocytes, immature oocytes would be more suitable for the cryopreservation of teleosts.

5-HT causes an increase in cAMP that stimulates, rather than inhibits, oocyte maturation in marine nemertean worms

In the nemertean worms Cerebratulus lacteus and Micrura alaskensis, 5-HT (=5-hydroxytryptamine, or serotonin) causes prophase-arrested oocytes to mature and complete germinal vesicle breakdown (GVBD). To identify the intracellular pathway that mediates 5-HT stimulation, follicle-free oocytes of nemerteans were assessed for GVBD rates in the presence or absence of 5-HT after being treated with various modulators of cAMP, a well known transducer of 5-HT signaling and an important regulator of hormone-induced maturation in general. Unlike in many animals where high levels of intra-oocytic cAMP block maturation, treatment of follicle-free nemertean oocytes with agents that elevate cAMP (8-bromo-cAMP, forskolin or inhibitors of phosphodiesterases) triggered GVBD in the absence of added 5-HT. Similarly, 5-HT caused a substantial cAMP increase prior to GVBD in nemertean oocytes that had been pre-injected with a cAMP fluorosensor. Such a rise in cAMP seemed to involve G-protein-mediated signaling and protein kinase A (PKA) stimulation, based on the inhibition of 5-HT-induced GVBD by specific antagonists of these transduction steps. Although the downstream targets of activated PKA remain unknown, neither the synthesis of new proteins nor the activation of MAPKs (mitogen-activated protein kinases) appeared to be required for GVBD after 5-HT stimulation. Alternatively, pre-incubation in roscovitine, an inhibitor of maturation-promoting factor (MPF), prevented GVBD, indicating that maturing oocytes eventually need to elevate their MPF levels, as has been documented for other animals. Collectively, this study demonstrates for the first time that 5-HT can cause immature oocytes to undergo an increase in cAMP that stimulates, rather than inhibits, meiotic maturation. The possible relationship between such a form of oocyte maturation and that observed in other animals is discussed.

Serotonin inhibition of steroid-induced meiotic maturation in the teleost Fundulus heteroclitus: role of cyclic AMP and protein kinases

The transduction of the serotonin (5-HT) signal in Fundulus heteroclitus ovarian follicles leading to the inhibition of oocyte meiosis reinitiation (oocyte maturation) in vitro induced by the naturally occurring maturation-inducing steroid 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P) was investigated. Steroid-induced oocyte maturation was inhibited by 5-HT in a dose-dependent manner; maximum inhibition (90%) was observed with 10(-4) M 5-HT. Groups of follicle-enclosed oocytes were cultured in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and treated with increasing doses of 5-HT. Serotonin was found to slightly increase the levels of follicular 3',5'-cyclic adenosine monophosphate (cAMP) in a dose-dependent manner; 10(-4) M 5-HT induced approximately a 3-fold increase in cAMP with respect to the controls. The changes in cAMP were then evaluated in follicles treated with 17,20 beta P in IBMX-free culture media in the presence or absence of 10(-4) M 5-HT. The exposure of follicles to 17,20 beta P alone produced a small and transient reduction in cAMP (40%) within 1-3 hr of steroid stimulation, and these early changes in cAMP appeared associated with a high incidence of germinal vesicle breakdown (80% GVBD) by 24 hr of incubation. Under these conditions, treatment of follicles with 5-HT also increased significantly the production of cAMP, and when 5-HT was combined with 17,20 beta P, the steroid-mediated reduction in cAMP was prevented and the levels of GVBD inhibited by 95%. Meiosis also was reinitiated with either the protein kinase A (PKA) inhibitor H8 or the protein kinase C (PKC) activator PMA, and the 5-HT inhibitory action on GVBD was found to be 100-fold reduced or completely ineffective, respectively. Preincubation of follicles with the PKC inhibitor GF109203x abolished PMA-induced GVBD in a dose-dependent manner, whereas this inhibitor had no effect on 17,20 beta P-triggered meiotic maturation, indicating that activation of PKC is apparently sufficient but not necessary to reinitiate meiosis. Taken together, these findings suggest that 5-HT may inhibit 17,20 beta P-induced meiotic reinitiation through the activation of a cAMP-PKA transduction pathway and that PKC possibly induces oocyte maturation by a different pathway than the steroid and thus is not affected by 5-HT.

Inhibitory guanine-nucleotide-binding-regulatory protein alpha subunits in medaka (Oryzias latipes) oocytes--cDNA cloning and decreased expression of proteins during oocyte maturation

We have previously shown that pertussis-toxin-sensitive inhibitory guanine-nucleotide-binding-regulatory proteins (G proteins) are involved in the signal transduction of steroidal maturation-inducing hormone (MIH) of rainbow trout (Oncorhynchus mykiss) oocytes, 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP) [Yoshikuni, M. & Nagahama, Y. (1994) Dev. Biol. 166, 615-622]. In this study, we obtained five different cDNA fragments of G protein alpha subunits from medaka (Oryzias latipes) intact ovarian follicles (three subtypes of G(i alpha), G(i alpha a), G(i alpha b) and G(i alpha c); two subtypes of G(s alpha), G(s alpha d), and G(s alpha e)). Using a newly developed extraction method for medaka oocyte RNA, we demonstrated that oocytes expressed both G(i alpha a) and G(i alpha c), but not G(i alpha b). Full-length cDNA clones for G(i alpha a) and G(i alpha c) were then isolated from a medaka ovarian follicle cDNA library. The predicted amino acid sequences of G(i alpha a) and G(i alpha c) exhibited significant similarity with G(i alpha1) and G(i alpha2) of other species, respectively. Both G(i alpha a) and G(i alpha c) possessed a specific Cys residue in the C-terminal region that was the site for ADP-ribosylation by pertussis toxin. G(o alpha), another G protein that is ADP-ribosylated by pertussis toxin, was not detected in oocytes, although it was expressed in brain tissue. Western blot analyses using a specific antibody against G(i alpha1) and G(i alpha2) subunit proteins revealed that in both medaka and rainbow trout G(i alpha) subunit protein (40 kDa) contents were abundant in plasma membranes of postvitellogenic immature oocytes, decreased in mature oocytes, and were absent in ovulated eggs. Furthermore, specific 17alpha,20beta-DP binding to plasma membranes was higher in postvitellogenic immature oocytes than in ovulated eggs. Taken together, these results suggest that G(i alpha a) and/or G(i alpha c) may be involved in the transduction of the signal from 17alpha,20beta-DP receptors during oocyte maturation of fish oocytes.

Effects of isoquinolinesulfonamide H-8 on Fundulus heteroclitus ovarian follicles: role of cyclic nucleotide-dependent protein kinases on steroidogenesis and oocyte maturation in vitro

The possible role of cyclic nucleotide-dependent protein kinases in mediating the stimulatory actions of Fundulus heteroclitus pituitary extract (FPE) during ovarian steroidogenesis and oocyte maturation in vitro was investigated. Follicle-enclosed oocytes were cultured in the presence of FPE and/or N-[2-Methylamino)ethyl]-5-isoquinolinesulfonamide (H-8), a compound that inhibits protein kinase A (PKA) and cGMP-dependent protein kinase. H-8 alone (0.1-1 mM) promoted oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner. However, the process of GVBD initiated by H-8 was much slower that that triggered by 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 betaP), the natural inducer of oocyte maturation in F. heteroclitus. Treatment with H-8 also increased 17,20 betaP production by the follicles and the accumulation of this steroid in the media was much slower than that initiated by FPE. However, in contrast to the FPE action on the oocyte, which is mediated by 17,20 betaP, the stimulatory action of H-8 on GVBD appears to be independent of follicular steroid production, since aminoglutethimide (AGI), an inhibitor of steroidogenesis, did not-block H-8-induced GVBD while inhibiting H-8 induced 17, 20 betaP production. Moreover, addition of H-8 to FPE-treated follicles significantly reduced 17,20 betaP secretion and the percentage of GVBD. These results provide further support for the involvement of PKA in the mechanism by which FPE stimulates ovarian steroidogenesis in F. heteroclitus. Furthermore, the fact that H-8 alone increased 17,20 betaP levels may imply that basal follicular production of this steroid could be induced by inactivation of cyclic nucleotide-dependent protein kinases. Data also indicate that inhibition of PKA and/or c-GMP-dependent protein kinase in the oocyte may be involved in the mechanism leading to resumption of meiosis in this species.

The in vitro effects of forskolin, IBMX and cyanoketone on meiotic maturation in follicle-enclosed catfish (Clarias batrachus) oocytes

The effects of an adenylate cyclase activator (forskolin, FK), phosphodiesterase inhibitor (3-isobutyl-l-methyl-xanthine, IBMX) and an inhibitor of steroidogenesis (cyanoketone, CK) on germinal vesicle breakdown (GVBD) in the catfish (Clarias batrachus) were investigated in vitro. In most of the experiments GVBD was induced by using 1 microgram/ml 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-DP), which is the maturation-inducing steroid (MIS) for this species. Adenosine 3':5'-cyclic monophosphate (cAMP) levels were also measured in the control, MIS-induced and/or FK- and IBMX-treated follicle-enclosed oocytes. MIS-induced GVBD was inhibited by FK (> or = 0.5 microM) or IBMX (> or = 1.0 mM), but oocyte exposed to 0.1 microM FK or 0.5 mM IBMX, after MIS stimulation, underwent GVBD. However, an inhibition of GVBD was recorded when the MIS-induced folliculated oocytes were preincubated with CK (1 microgram/ml) and subsequently treated with 0.1 microM FK. In the time course study, when the oocytes were stimulated by MIS for various time intervals and then treated with 1.0 microM FK or 1.0 mM IBMX, both the substances blocked maturation if they were added up to 12 hr after MIS. The extent of inhibition was gradually decreased and was completely removed after 30 hr of post-MIS stimulation. The stimulatory dose of 17 alpha,20 beta-DP (1 microgram/ml) not only induced GVBD (83.2 +/- 1.50%) in vitro but also reduced oocyte cAMP level (65.3 +/- 2.85 pmol/100 micrograms protein) significantly after 6 hr of incubation. However, FK (10.0 microM) or IBMX (1.0 mM) countered these effects and promoted the accumulation of cAMP in the oocytes; FK being more potent. On the other hand, when unstimulated full-grown but immature oocytes were cultured in vitro in the presence of different concentrations of FK, an induction of oocyte maturation was recorded in dose- and time-dependent manner. These results strongly suggest the involvement of cAMP in the regulation of catfish oocyte maturation.

Steroidogenesis in Fundulus heteroclitus. IV. Dichotomous effects of a phorbol ester on ovarian steroid production and oocyte maturation

The possible role of protein kinase C (PKC) activation in mediating the stimulatory actions of a Fundulus pituitary extract (FPE) on ovarian steroidogenesis and oocyte maturation was investigated. The phorbol ester, phorbol 12-myristate 13-acetate (PMA), alone slightly increased basal 17 alpha-hydroxy,20 beta-dihydroprogesterone (DHP) and 17 beta-estradiol (E2) synthesis and significantly stimulated germinal vesicle breakdown (GVBD). Addition of FPE promoted synthesis of DHP, testosterone (T), and E2, and initiated GVBD. Phorbol ester inhibited FPE-induced steroidogenesis but increased the number of oocytes that underwent GVBD. Phorbol ester also markedly impeded induction of steroidogenesis by dibutyryl cAMP and differentially affected the conversion of 25-hydroxycholesterol, pregnenolone, or progesterone to DHP, T, and E2: DHP production was not affected; T production diminished; and E2 synthesis increased (T aromatization also increased). These results suggest an inhibitory role for the PKC pathway on FPE-induced ovarian steroid production, with PMA appearing to affect various steroidogenic steps. The stimulatory action of PMA on oocyte maturation seems to be independent of follicular steroid production since aminoglutethimide, an inhibitor of steroidogenesis, did not block PMA-induced GVBD. Moreover, PMA had a marked stimulatory effect on GVBD in denuded oocytes. Thus, in contrast to the inhibitory role found for the PKC pathway on ovarian follicular steroidogenesis, activation of PKC in the oocyte may serve as a signal-transducing mechanism leading to GVBD.

Steroid production by ovarian follicles of the viviparous guppy (Poecilia reticulata) and its regulation by precursor substrates, dibutyryl cAMP and forskolin

Production in vitro of estradiol-17 beta, testosterone, 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), 17 alpha-hydroxyprogesterone, and progesterone by follicles of the guppy at various stages of oocyte growth and gestation was investigated. Basal production of estradiol-17 beta was highest in 0.8- and 1.2-mm follicles, whereas that of testosterone and 17 alpha,20 beta-P was highest in 1.6-mm (postvitellogenic) follicles. Levels of these steroids declined after fertilization and were undetectable in late gestation and postpartum follicles. 17 alpha-Hydroxyprogesterone and progesterone levels were low at all stages. Thus, none of these steroids appears to be involved in maintaining gestation. Regulation of estradiol-17 beta and 17 alpha,20 beta-P secretion by vitellogenic (1.0 mm) and postvitellogenic follicles by precursor substrates, dbcAMP (0.1 to 10 mM) and forskolin (1 to 100 microM), was also investigated. Vitellogenic follicles synthesized increased quantities of estradiol-17 beta in the presence of exogenous testosterone, whereas estradiol-17 beta production by postvitellogenic follicles was not altered by testosterone. These results suggest decreased aromatase activity in the postvitellogenic follicles. Dibutyryl cAMP and/or forskolin stimulated testosterone and estradiol-17 beta production by vitellogenic follicles but did not stimulate conversion of testosterone to estradiol-17 beta, suggesting that the adenylate cyclase system stimulates estradiol-17 beta production by stimulating testosterone production but does not mediate conversion of testosterone to estradiol-17 beta. Postvitellogenic follicles synthesized increased quantities of 17 alpha,20 beta-P in response to 17 alpha-hydroxyprogesterone in a dose-dependent manner. Although 1 microM of forskolin stimulated 17 alpha,20 beta-P production by postvitellogenic follicles in the absence of exogenous 17 alpha-hydroxyprogesterone, 100 microM of forskolin inhibited 17 alpha,20 beta-P production. Dibutyryl cAMP, however, did not affect 17 alpha,20 beta-P production. In the presence of 50 ng of 17 alpha-hydroxyprogesterone, dbcAMP (10 mM) and forskolin (1 to 100 microM) suppressed 17 alpha,20 beta-P production. It is suggested that cAMP mediates 17 alpha,20 beta-P production up to a certain threshold level, beyond which it inhibits 17 alpha,20 beta-P production.

Induction and inhibition of meiotic maturation of amphibian (Rana dybowskii) follicular oocytes by forskolin and cAMP in vitro

Previous studies have demonstrated that direct or indirect elevation of cAMP levels in cultured amphibian ovarian follicles simultaneously stimulated production of oocyte maturation-inducing steroid (progesterone) by the follicles and inhibited oocyte maturation induced by endogenous or exogenous hormone. The duration of cAMP stimulation influenced arrest and reinitiation of oocyte meiotic maturation in ovarian follicles of Rana dybowskii. Addition of forskolin (adenylate cyclase stimulator) to cultured follicles inhibited both progesterone- and frog pituitary homogenate (FPH)-induced oocyte maturation. Similar inhibitory results were obtained when hormone-treated follicles were cultured in the continual presence of cAMP. Oocyte maturation increasingly occurred in follicular oocytes when cAMP or forskolin addition was delayed following treatment with FPH or progesterone. Transient exposure (6-8 hr) of ovarian follicles to forskolin or cAMP markedly stimulated oocyte maturation as well as accumulation of progesterone as measured by radioimmunoassay within the ovarian follicles. Forskolin was more effective than cAMP, at the dose tested, in stimulating progesterone production and accumulation by the follicles. The data demonstrate that transient manipulation (elevation) of cAMP levels in cultured follicles, without added FPH or steroid, was sufficient to initiate oocyte maturation. Results suggest that, with transient exposure to forskolin or exogenous cAMP, there is a sequential increase and decrease in endogenous cAMP levels in the somatic cells and germ cell components of the ovarian follicle. These changes appear to mediate production of maturation-inducing steroid and secondarily allow its effects on the oocyte to be expressed.

Steroidogenesis in Fundulus heteroclitus. I. Production of 17 alpha-hydroxy,20 beta-dihydroprogesterone, testosterone, and 17 beta-estradiol by prematurational follicles in vitro

In order to understand better the mechanism of gonadotropin action on steroidogenesis in prematurational follicles of Fundulus heteroclitus, follicle synthesis of 17 alpha-hydroxy, 20 beta-dihydroprogesterone (17 alpha-OH,20 beta-DHP), testosterone (T), and 17 beta-estradiol (E2) from a variety of precursors and the maturational response of oocytes were simultaneously followed in vitro. The addition of 25-hydroxycholesterol, pregnenolone, or progesterone to unstimulated follicles increased media 17 alpha-OH,20 beta-DHP, T, and E2, as well as oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner. Inhibition of cholesterol side-chain cleavage by aminoglutethimide blocked 25-hydroxycholesterol-promoted steroid accumulation and GVBD, indicating that 25-hydroxycholesterol does not directly induce GVBD, but rather is metabolized in the follicle to an active steroid (presumably 17 alpha-OH,20 beta-DHP). Likewise, trilostane, an inhibitor of delta 5-3 beta-hydroxysteroid dehydrogenase, blocked pregnenolone action. Both inhibitors also completely abolished steroid accumulation and GVBD promoted by a F. heteroclitus pituitary extract (FPE), but not GVBD induced by exogenous 17 alpha-OH,20 beta-DHP. FPE also significantly depressed T but enhanced E2 production from exogenous precursors. We have concluded from these observations that (1) cholesterol side-chain cleavage and pregnenolone conversion to progesterone are essential for gonadotropin-promoted follicle steroid production and the resulting reinitiation of meiosis by the oocyte, (2) the enzymes necessary for the conversion of cholesterol to 17 alpha-OH,20 beta-DHP, T, and E2 are present in the unstimulated, prematurational follicle, and (3) gonadotropin initiates steroidogenesis by acting at a step prior to the conversion of cholesterol to pregnenolone; it also appears to enhance aromatase activity.

Adenylate cyclase of catfish hepatocyte membranes: basal properties and sensitivity to catecholamines and glucagon

Some characteristics of adenylate cyclase of catfish (Ictalurus melas) liver membranes were studied, and the effects of catecholamines and of glucagon were tested. The enzyme has an optimum temperature of 40 degrees C, and a Km for ATP of 0.16 mM at 30 degrees C, and requires Mg2+ for its activity. The enzyme activity is inhibited with a Ca2+ concentration higher than 5 X 10(-5) M, and enhanced with F- higher than 10(-4) M. The response of adenylate cyclase to GTP is biphasic, with a maximum of activity at 10(-5) M GTP. Catecholamines (epinephrine, norepinephrine, isoproterenol, phenylephrine) enhance cyclase activity. Propranolol inhibits the increase in enzyme activity induced by catecholamines, whereas phentolamine is ineffective. This indicates that catecholamines (phenylephrine included) activate adenylate cyclase through a beta-adrenergic mechanism. Glucagon (mammalian) has a smaller effect than epinephrine in increasing the enzyme activity of catfish hepatocyte membranes. This fact is the opposite of that observed for the cyclase activity of rat liver membranes.