Coordinate actions of arachidonic acid and protein kinase C in gonadotropin-releasing hormone-stimulated secretion of luteinizing hormone

Activation of the p38 mitogen-activated protein kinase pathway by gonadotropin-releasing hormone

Previous studies have shown that interaction of GnRH with its serpentine, G protein-coupled receptor results in activation of the extracellular signal regulated protein kinase (ERK) and the Jun N-terminal protein kinase (JNK) pathways in pituitary gonadotropes. In the present study, we examined GnRH-stimulated activation of an additional member of the mitogen-activated protein kinase (MAPK) superfamily, p38 MAPK GnRH treatment of alphaT3-1 cells resulted in tyrosine phosphorylation of several intracellular proteins. Separation of phosphorylated proteins by ion exchange chromatography suggested that GnRH receptor stimulation can activate the p38 MAPK pathway. Immunoprecipitation studies using a phospho-tyrosine antibody resulted in increased amounts of immunoprecipitable p38 MAPK from alphaT3-1 cells treated with GnRH. Immunoblot analysis of whole cell lysates using a phospho-specific antibody directed against dual phosphorylated p38 kinase revealed that GnRH-induced phosphorylation of p38 kinase was dose and time dependent and was correlated with increased p38 kinase activity in vitro. Activation of p38 kinase was blocked by chronic phorbol ester treatment, which depletes protein kinase C isozymes alpha and epsilon. Overexpression of p38 MAPK and an activated form of MAPK kinase 6 resulted in activation of c-jun and c-fos reporter genes, but did not alter the expression of the glycoprotein hormone alpha-subunit reporter. Inhibition of p38 activity with SB203580 resulted in attenuation of GnRH-induced c-fos reporter gene expression, but was not sufficient to reduce GnRH-induced c-jun or glycoprotein hormone alpha-subunit promoter activity. These studies provide evidence that the GnRH signaling pathway in alphaT3-1 cells includes protein kinase C-dependent activation of the p38 MAPK pathway. GnRH integration of c-fos promoter activity may include regulation by p38 MAPK.

Protein kinase C as a signal for exocytosis

We have studied signaling mechanisms that stimulate exocytosis and luteinizing hormone secretion in isolated male rat pituitary gonadotropes. As judged by reverse hemolytic plaque assays, phorbol-12-myristate-13-acetate (PMA) stimulates as many gonadotropes to secrete as does gonadotropin-releasing hormone (GnRH). However, PMA and GnRH use different signaling pathways. The secretagogue action of GnRH is not very sensitive to bisindolylmaleimide I, an inhibitor of protein kinase C, but is blocked by loading cells with a calcium chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. The secretagogue action of PMA is blocked by bisindolylmaleimide I and is not very sensitive to the intracellular calcium chelator. GnRH induces intracellular calcium elevations, whereas PMA does not. As judged by amperometric measurements of quantal catecholamine secretion from dopamine- or serotonin-loaded gonadotropes, the secretagogue action of PMA develops more slowly (in several minutes) than that of GnRH. We conclude that exocytosis of secretory vesicles can be stimulated independently either by calcium elevations or by activation of protein kinase C.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Exocytosis in chromaffin cells: evidence for a MgATP-independent step that requires a pertussis toxin-sensitive GTP-binding protein

We have previously described that mastoparan, an amphiphilic tetradecapeptide that activates heterotrimeric G-proteins, inhibits Ca(2+)-induced MgATP-dependent secretion from streptolysin-O-permeabilized chromaffin cells [Vitale, Mukai, Rouot, Thiersé, Aunis and Bader (1993) J. Biol. Chem. 268, 14715-14723]. Our observations suggest the involvement of an inhibitory G(o)-protein, possibly located on the membrane of secretory granules, in the final stages of the exocytotic pathway in chromaffin cells. Here, we demonstrate that mastoparan is also able to stimulate the Ca(2+)-dependent secretion of catecholamines in the absence of MgATP in the medium. This MgATP-independent secretion is totally blocked by tetanus toxin, a potent inhibitor of exocytosis in all neurosecretory cells so far investigated, suggesting that the mastoparan target is a component of the exocytotic machinery. Mas17, a mastoparan analogue inactive on G-proteins, had no effect on catecholamine secretion whereas both Mas7, a highly active analogue of mastoparan, and AlF4-, which selectively activates trimeric G-proteins, triggered MgATP-independent secretion. Non-hydrolysable GTP analogues (GTP[S] and p[NH]ppG) mimicked the dual effects of mastoparan on secretion: they inhibited exocytosis in the presence of MgATP and stimulated MgATP-independent secretion. The different potencies displayed by these two analogues suggest the involvement of two distinct G-proteins. Accordingly, the mastoparan-induced MgATP-independent secretion is highly sensitive to pertussis toxin (PTX) whereas the inhibition by mastoparan of secretion in the presence of MgATP is resistant to PTX treatment. When permeabilized cells were incubated with mastoparan, the release of arachidonic acid increased in a PTX-sensitive manner. 7,7-Dimethyl-5,8-eicosadienoic acid, a potent inhibitor of intracellular phospholipase A2, inhibited both the arachidonate release and the MgATP-independent catecholamine secretion evoked by mastoparan. In contrast, neomycin, an inhibitor of phospholipase C, had no significant effect on either the release of arachidonic acid or the secretion of catecholamines provoked by mastoparan. We conclude that two distinct heterotrimeric G-proteins act in series in the exocytotic pathway in chromaffin cells: one controls an ATP-dependent priming step through an effector pathway that remains to be determined, and the second is involved in a late Ca(2+)-dependent step which does not require MgATP but possibly involves the generation of arachidonic acid.

Prostaglandin F2 alpha and gonadotropin-releasing hormone increase intracellular free calcium in rat granulosa cells

Changes in cytosolic free calcium concentration ([Ca2+]i) in response to prostaglandin F2 alpha (PGF2 alpha) and gonadotropin-releasing hormone (GnRH) were measured in single rat granulosa cells, using the calcium-sensitive fluorescent dye, fura-2AM. In 90 out of 135 granulosa cells (67%), there was a 3- to 4-fold increase in resting [Ca2+]i within 30 s of administration of PGF2 alpha (10(-6) M). The resting [Ca2+]i returned to pre-stimulation levels in approximately 80 s. Granulosa cells were responsive to PGF2 alpha at concentrations ranging from 10(-7) M to 10(-4) M (n = 7). Within this range of concentrations, the magnitude of the calcium response did not differ. In another series of experiments, the majority (93%, n = 57) of the granulosa cells which responded to PGF2 alpha also responded to GnRH. Neither the magnitude of the [Ca2+]i response nor the time to response differed between PGF2 alpha and GnRH. Furthermore, simultaneous treatment of granulosa cells with both hormones did not generate a larger response than with either hormone alone. During perifusion with low calcium media, the characteristic [Ca2+]i response to PGF2 alpha decreased, and was eliminated within 10 min (n = 9). Similar observations were made in response to GnRH under these conditions. These data confirm that PGF2 alpha and GnRH stimulate a transient increase in [Ca2+]i in rat granulosa cells, the source of which may be shared intracellular stores.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Relationship between arachidonic acid release and Ca2(+)-dependent exocytosis in digitonin-permeabilized bovine adrenal chromaffin cells

The relationship between Ca2(+)-dependent arachidonic acid release and exocytosis from digitonin-permeabilized bovine adrenal chromaffin cells was investigated. The phospholipase A2 inhibitors mepacrine, nordihydroguaiaretic acid and indomethacin had no effect on either arachidonic acid release or secretion. The phospholipase A2 activator melittin had no effect on secretion. The specific diacylglycerol lipase inhibitor RG80267 had no effect on secretion, but decreased basal arachidonic acid release to such an extent that the level of arachidonic acid in treated cells in response to 10 microM-Ca2+ was equivalent to that of control cells in the absence of Ca2+. Staurosporine, a protein kinase C inhibitor, was found to abolish Ca2(+)-dependent arachidonic acid release completely, but had only a slight inhibitory effect on Ca2(+)-dependent secretion. It is concluded that arachidonic acid is not essential for Ca2(+)-dependent exocytosis in adrenal chromaffin cells.

Direct inhibition of smooth muscle myosin light chain kinase by arachidonic acid in a purified system

The direct effect of arachidonic acid (AA) on the phosphorylation of smooth muscle myosin light chain (SMLC) by smooth muscle myosin light chain kinase (SMLCK) was assessed in a purified system. AA inhibited the phosphorylation of SMLC by SMLCK in a dose dependent manner. Increasing the amount of calmodulin (59 nM and 590 nM) did not reverse this inhibition. Linoleic acid and oleic acid also inhibited the phosphorylation. The inhibitory potency of these unsaturated fatty acids paralleled the number of cis double bonds. These results show that SMLCK is directly inhibited by unsaturated fatty acids including AA.

Constant turnover of arachidonic acid and inhibition of a potassium current in Aplysia giant neurons

Steady-state currents at hyperpolarized membrane potentials were studied in the homologous giant neurons, LP1 and R2, of Aplysia using two-electrode voltage clamp. Nearly half of the steady-state current at voltages more hyperpolarized than -70 mV had characteristics similar to the inwardly rectifying potassium current (IR) described previously in Aplysia neurons. The pharmacological agents 4-bromophenacylbromide, indomethacin, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate were found to modulate IR. IR was stimulated with BPB and indomethacin and inhibited with TPA. These agents altered IR by a mechanism independent of cAMP, which can also modulate IR. The effects of these modulators are consistent with their actions on arachidonic acid (AA) metabolism in Aplysia nervous system, suggesting AA may constitutively inhibit IR. When ganglia were perfused for 12 hr with medium containing BSA to absorb extracellular fatty acids, IR was increased nearly twofold. This increase was partially inhibited by addition of AA to the perfusion medium, and completely inhibited by pretreatment of ganglia with BPB. Although no direct effect of short-term exposure to exogenous AA was observed, long term exposure to exogenous AA and several other unsaturated fatty acids was accompanied by a decrease in IR.

Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. III. A23187, cAMP, phorbol ester and DiC8-stimulated luteinizing hormone release

Dispersed estradiol-treated rat pituitary cells were used to characterize progesterone (P) modulation of luteinizing hormone (LH) secretion in response to a variety of pharmacologic secretagogues which influence cell biochemistry. Acute (less than 3 h) and chronic (24 h) exposures to P prior to secretagogue challenge respectively enhanced and inhibited Ca2+ ionophore (A23187)-stimulated and gonadotropin-releasing hormone (GnRH)-stimulated LH release in similar quantitative fashion without any effect on concurrent prolactin release. Similar responses were also noted with cholera toxin-stimulated secretion. However, when protein kinase C activators such as phorbol esters and dioctanoylglycerol were used to trigger LH release, chronic exposure to P did not inhibit, but rather enhanced, LH release. Again, P had no effect on prolactin release. 'Washout' studies indicated that chronic treatments with P would suppress LH secretion stimulated by these compounds, but only when the steroid was cleared from the cells 4 h beforehand. These studies provide further evidence that P specifically modulates gonadotroph secretory function via mechanisms which bypass GnRH receptors. Moreover, they suggest that P exerts many different actions within the gonadotroph and question the role of protein kinase C in GnRH action.

Arachidonic acid stimulates steroidogenesis in goldfish preovulatory ovarian follicles

The possibility that arachidonic acid (AA) plays a role in the regulation of steroidogenesis in goldfish was investigated using preovulatory ovarian follicles incubated in vitro. AA was shown to act in a time- and dose-dependent manner to stimulate testosterone production. AA in the range of 10(-5) to 10(-4) M increased testosterone production within 2 hr and had a maximal effect by 9 hr. The magnitude of the testosterone response to AA was similar to that observed when ovarian follicles were incubated with human chorionic gonadotropin (hCG). Ovarian follicles incubated with AA and either hCG or forskolin (adenylate cyclase activator) produced more testosterone than follicles incubated with either of these compounds alone. The actions of AA on testosterone production were completely blocked by cyclooxygenase inhibitors (indomethacin or ibuprofen) and were reduced by 50% by the lipoxygenase inhibitor nordihydroguaiaretic acid. Phospholipase C was far more effective than phospholipase A2 in the stimulation of testosterone production. Taken together, these results suggest that AA formed subsequent to the action of phospholipase C on membrane phospholipids has a role in the regulation of steroidogenesis in preovulatory goldfish ovarian follicles.

Mechanism of action of GnRH: the participation of calcium mobilization and activation of protein kinase C in gonadotropin secretion

The role of protein kinase C in luteinizing hormone (LH) release was analyzed in studies on the secretory responses to gonadotropin releasing hormone (GnRH) and phorbol esters in pituitary cell cultures. 12-O-tetradecanoyl-phorbol 13-acetate (TPA), 4 beta-phorbol 12,13-bibenzoate, and 4 beta-phorbol 12,13-diacetate stimulated LH release with ED50s of 5, 10 and 1000 nM, respectively, and with about 70% of the efficacy of GnRH. Phorbol ester-stimulated LH secretion was decreased but not abolished by progressive reduction of [Ca2+] in the incubation medium, and the residual response was identical with that of GnRH in Ca2+-deficient medium. TPA increased [Ca2+]i to a peak after 30 s in normal medium but not in the absence of extracellular Ca2+, indicating that protein kinase C promotes calcium entry but can also mediate secretory responses without changes in calcium influx and [Ca2+]i. The extracellular Ca2+-dependent action of TPA on LH release was blocked by CoCl2 but not by nifedipine. The secretory actions of TPA and GnRH were additive at low doses and converged to a common maximum LH response at high concentrations of the agonists. TPA caused rapid translocation of cytosolic protein kinase C to the particulate fraction, followed by a progressive decrease in total enzyme activity to less than 10% after 6 h. Partial recovery of the cytosolic enzyme (to 20%) occurred after washing and reincubation for 15 h. Such kinase C-depleted cells showed prominent dose-dependent reductions in the actions of both GnRH and TPA on LH release in normal and Ca2+-deficient media. These observations show that the actions of kinase C on LH release include extracellular Ca2+-dependent and independent components, and support the hypothesis that protein kinase C participates in the LH secretory response to GnRH in pituitary gonadotrophs.

Participation of arachidonic acid metabolism in gonadotropin-releasing hormone stimulation of goldfish gonadotropin release

Two intraperitoneal injections of a mammalian gonadotropin-releasing hormone (GnRH) analog, [D-Ala6, Pro9-N-ethylamide]-GnRH (mGnRHa; 0.1 micrograms/g), at 12-hr intervals increased serum gonadotropin (GTH) levels in sexually mature and sexually regressed female goldfish 2 and 6 hr after the second injection. This serum GTH response was decreased by the coinjection of a lipoxygenase enzyme inhibitor, nordihydroguaiaretic acid (NDGA: 0.1 to 10 micrograms/g) at the time of the second mGnRHa application. In static cultures of dispersed goldfish pituitary cells, 1-100 microM arachidonic acid (AA) and 0.1-1000 nM [Trp7, Leu8]-GnRH (salmon GnRH, sGnRH) and [D-Arg6, Pro9-N-ethylamide]-sGnRH (sGnRHa) caused dose-dependent increases in GTH release. Additions of 1-40 microM NDGA reduced the sGnRH-stimulated GTH release in a dose-dependent manner, and completely inhibited the GTH response to increasing concentrations of AA. NDGA 40 microM also decreased the elevated GTH levels induced by sGnRHa treatment. Exposure to 10 microM 5,8,11,14-eicosatetraynoic acid, an inhibitor with mixed action on lipoxygenase and cyclooxygenase enzymes, reduced the dose-dependent GTH response to sGnRH and AA. In contrast, coincubation with another cyclooxygenase blocker, indomethacin, at 10 microM, did not alter AA and sGnRH-induced GTH release. These results provide in vivo and in vitro evidence for the participation of AA metabolism in mediating GnRH-stimulated GTH release in the goldfish. The importance of AA metabolism through the lipoxygenase pathway is also indicated.

Calmodulin and protein kinase C activation duplicates the biphasic secretion of luteinizing hormone

Ionomycin, which activates the Ca2+-calmodulin system, and phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), were used to investigate potential roles of these systems as mediators of the biphasic secretion of luteinizing hormone. Quartered pituitaries from diestrous II female rats were perifused at 37 degrees C, and sequential effluent fractions collected every 10 min. Gonadotropin-releasing hormone administration resulted in a biphasic response: an initial, protein synthesis-independent secretion, followed 60 min later by a secondary, augmented, protein synthesis-dependent component. Ionomycin-stimulated gonadotropin secretion was immediate and partially independent of protein synthesis, whereas the PMA-induced secretion was delayed (approximately 70 min), and was completely dependent on protein synthesis. Simultaneous infusions of ionomycin and PMA resulted in an initial, protein synthesis-independent response followed by the secondary, augmented, protein synthesis-dependent component, which exhibited synergistic interactions between calmodulin and PKC. These results suggest that calmodulin mediates the initial, protein synthesis-independent secretion, PKC mediates part of the secondary, augmented response, while calmodulin and PKC synergize to mediate the remaining component of the secondary response.

Differential role of protein kinase C in the action of luteinizing hormone-releasing hormone on hormone production in rat ovarian cells

This study was undertaken to determine the involvement of arachidonic acid and protein kinase C in the actions of luteinizing hormone-releasing hormone on steroid and prostaglandin formation in the ovary. In primary culture of rat granulosa cells, treatment with 3 x 10(-7) mol/L melittin stimulates progesterone and prostaglandin E2 accumulation after a 5-hour culture period. Concomitant treatment of the cells with melittin and luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate further enhances the stimulatory action of either luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate by itself on prostaglandin E2 production. In contrast, no synergistic effects are observed on progesterone production by the same treatments. Treatment with luteinizing hormone-releasing hormone for 24 hours significantly decreases follicle-stimulating hormone-induced progesterone production by approximately 50%. Treatment of the cells with either follicle-stimulating hormone or luteinizing hormone-releasing hormone stimulates prostaglandin E2 production at least tenfold in the same cultures. When follicle-stimulating hormone and luteinizing hormone-releasing hormone are present concomitantly, they synergistically enhance prostaglandin E2 formation (p less than 0.01). Similar effects are observed with the phorbol ester, 12-0-tetradecanoylphorbol 13-acetate, which causes a dose-dependent inhibition of progesterone production by follicle-stimulating hormone whereas follicle-stimulating hormone-stimulated prostaglandin E2 formation is enhanced. Thus luteinizing hormone-releasing hormone-induced activation of protein kinase C may play multiple roles (stimulatory or inhibitory) in hormone production in the ovary.

Gonadal steroid effects on LH response to arachidonic acid and protein kinase C

Cultured rat pituitary cells were used to examine, first, the effects of luteinizing hormone-releasing hormone, Ca2+ mobilization, protein kinase C (PKC) activation, and arachidonic acid (AA) on luteinizing hormone (LH) secretion and AA release, and, second, gonadal steroid modulation of these effects. A23187, 12-O-tetradecanoylphorbol 13-acetate (TPA), and AA stimulated LH secretion by both perifused and static cultures; TPA facilitated the responses to both A23187 and AA. LHRH, A23187, and TPA stimulated AA release. Inhibition of AA metabolism reduced the LH responses to LHRH, A23187, TPA, and melittin. Pretreatment with testosterone inhibited the LH response to LHRH but not the responses to TPA or AA. Pretreatment with 17 beta-estradiol stimulated the LH responses to LHRH, TPA, and low concentrations of AA. These results suggest that LHRH action involves a cascade of events, in which the effects of Ca2+ mobilization and PKC activation are mediated at least in part by AA release. They further suggest that both testosterone and 17 beta-estradiol modulate LH secretion by affecting AA release; 17 beta-estradiol may also affect some process subsequent to AA release.

Desensitization of pituitary gonadotropes by mediators of LH release

Desensitization of pituitary gonadotropes by exposure to 10 nM gonadotropin-releasing hormone (GnRH) for 6 h severely impaired the luteinizing hormone (LH) response to a second 3-h treatment with GnRH, and reduced the secretory responses to 50 microM arachidonic acid (AA), 100 nM tetradecanoyl phorbol-13-acetate (TPA), and AA + TPA. Pretreatment with AA blocked subsequent responses to AA but not to other secretagogues. Pretreatment with TPA attenuated the LH response to TPA, but not to GnRH, AA, and AA + TPA. After exposure to AA + TPA, all subsequent responses were abolished. Each of the secretagogues reduced GnRH receptor binding, but only GnRH-induced receptor loss and desensitization were reversed by simultaneous incubation with a GnRH antagonist. Similar results were obtained when 16-h pretreatment periods were used, or when the data were normalized for the concomitant reduction of cellular LH content. These findings indicate that GnRH-receptor loss and depletion of LH content are not the sole causes of GnRH-induced desensitization. Receptor uncoupling and impairment of AA- and protein kinase C-dependent pathways may also be involved in this process.

Inhibition of follicle-stimulating hormone- and adenosine-3',5'-cyclic monophosphate-induced progesterone production by calcium and protein kinase C in the rat ovary

In this study we examined the effects of A23187 (a calcium ionophore) and 12-O-tetradecanoyl phorbol-13-acetate, a known activator of protein kinase C, on progesterone production. Granulosa cells obtained from pregnant mare serum gonadotropin-primed rats were maintained in primary culture. Treatment with follicle-stimulating hormone (0.5 microgram/ml), 8-bromo-adenosine-3',5'-cyclic monophosphate (2 mmol/L), or cholera toxin (0.1 microgram/ml) for 5 hours or 24 hours markedly stimulated progesterone production. The concomitant presence of A23187 attenuated the elevated levels of progesterone induced by follicle-stimulating hormone, 8-bromo-adenosine-3',5'-cyclic monophosphate, or cholera toxin, with or without the presence of a phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (0.2 mmol/L). Likewise, treatment of the cells with 12-O-tetradecanoyl phorbol-13-acetate suppressed follicle-stimulating hormone-induced progesterone production, whether or not 1-methyl-3-isobutylxanthine was present in the cultures. The effect of 12-O-tetradecanoyl phorbol-13-acetate was not mimicked by phorbol-13-monoacetate or 4 alpha-phorbol-12, 13-didecanoate. These results indicate that both A23187 and 12-O-tetradecanoyl phorbol-13-acetate inhibit follicle-stimulating hormone-induced progesterone production, in part at a step or steps beyond adenosine-3',5'-cyclic monophosphate generation and degradation. They further support a role of calcium and protein kinase C in the intraovarian action of luteinizing hormone-releasing hormone.

Luteinizing hormone-releasing hormone (LHRH)-induced arachidonic acid release in rat granulosa cells. Role of calcium and protein kinase C

In rat granulosa cells prelabeled with [3H]arachidonic acid, addition of LHRH, A23187 or 12-0-tetradecanoyl phorbol-13-acetate (TPA) enhanced the release of [3H]arachidonic acid into the culture medium. The effect of A23187 was significant as early as 5 min and the lowest effective dose was 5 X 10(-8)/M. On the other hand, TPA was effective only at dosages greater than 10(-6)M. These results suggest that the stimulatory effect of LHRH on arachidonic acid release is coupled more tightly to a Ca2+-dependent rather than a protein kinase C-mediated pathway.