Gonadotropin releasing hormone enhances polyphosphoinositide hydrolysis in rat pituitary cells

The multifunctional protein PACS-1 is required for HDAC2- and HDAC3-dependent chromatin maturation and genomic stability

Phosphofurin acidic cluster sorting protein-1 (PACS-1) is a multifunctional membrane traffic regulator that plays important roles in organ homeostasis and disease. In this study, we elucidate a novel nuclear function for PACS-1 in maintaining chromosomal integrity. PACS-1 progressively accumulates in the nucleus during cell cycle progression, where it interacts with class I histone deacetylases 2 and 3 (HDAC2 and HDAC3) to regulate chromatin dynamics by maintaining the acetylation status of histones. PACS-1 knockdown results in the proteasome-mediated degradation of HDAC2 and HDAC3, compromised chromatin maturation, as indicated by elevated levels of histones H3K9 and H4K16 acetylation, and, consequently, increased replication stress-induced DNA damage and genomic instability.

In vitro nitric oxide effects on basal and gonadotropin-releasing hormone-induced gonadotropin secretion by pituitary gland of male crested newt (Triturus carnifex) during the annual reproductive cycle

The objective of this study was to test the possible nitric oxide (NO) involvement in pituitary gonadotropin secretion in the male crested newt, Triturus carnifex. Pituitaries were incubated in vitro with medium alone, GnRH, NO donor (NOd, sodium nitroprusside), NO synthase inhibitor (NOSi, Nomega-nitro-L-arginine methyl ester), cGMP analogue (cGMPa, 8-bromo-cGMP), soluble guanylate cyclase inhibitor (sGCi, cystamine), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi during the annual reproductive cycle: pre-reproduction, reproduction (noncourtship and courtship), and the refractory, recovery, and estivation periods. To determine pituitary gonadotropin secretion indirectly, newt testes were superfused in vitro with preincubated pituitaries, and androgen release was determined. NO synthase (NOS) activity and cGMP levels were assessed in the preincubated pituitaries. Medium alone- and GnRH-preincubated pituitary increased androgen secretion during pre-reproduction, noncourtship, courtship, and recovery; the GnRH-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOd and cGMPa increased androgens in all reproductive phases considered except courtship; the NOd- and cGMP-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOS activity was highest during courtship and lowest during the refractory and estivation periods. GnRH increased NOS activity during pre-reproduction, noncourtship, and recovery. Cyclic GMP levels were highest during courtship and lowest during the refractory period and estivation. GnRH increased cGMP levels during pre-reproduction, noncourtship, and recovery, while NOd did so during all reproductive phases considered. These results suggest that basal and GnRH-induced gonadotropin secretion are up-regulated by NO in the pituitary gland of the male Triturus carnifex.

Delayed luteolysis after intra-uterine infusions of nordihydroguaiaretic acid in the ewe

Intrauterine administration of nordihydroguaiaretic acid (5 mg, bid. NDGA), an inhibitor of the enzyme 5-lipoxygenase, on days 10-14 of the oestrous cycle, maintained luteal function and delayed oestrus in the ewe. The duration (mean +/- SD) of the oestrous cycle in the treatment group (n = 4) was 24 +/- 1 days, which was significantly (P < 0.001) longer than that of 16 +/- 1 days in vehicle-treated controls (n = 4); plasma progesterone concentrations were also significantly (P < 0.01) higher in the treatment group. On days 13 and 14 of the cycle (oestrus = Day 0) in the control group large pulses of 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) were evident, with mean (+/- SD) maximum concentrations of 232.5 +/- 66 and 415 +/- 309 pg ml(-1), respectively. In the treatment group, however, concentrations of PGFM were below detection level (< 50 pg ml(-1)). Similarly, in the control group, oxytocin release was highly pulsatile, with mean (+/- SD) peak concentrations of 21.8 +/- 5 and 18.5 +/- 6 pg ml(-1) on days 13 and 14, respectively; these were significantly (P < 0.01) higher than values of 7.6 +/- 3 and 6.1 +/- 3 pg ml(-1) in NDGA-treated ewes, where pulses were of relatively low amplitude. These results suggest that 5-lipoxygenase products of arachidonic acid metabolism may be involved in the positive feedback mechanism between luteal oxytocin and uterine PGF2alpha during luteolysis in the ewe.

Nitric oxide mediates gonadotropin-releasing hormone effects on frog pituitary

We studied the possible role of nitric oxide (NO) in GnRH-induced gonadotropin secretion in the female water frog, Rana esculenta. During pre-reproduction, pre-ovulation, ovulation, post-ovulation, refractory, recovery and hibernation, pituitaries were incubated with medium-alone, GnRH, NO donor (NOd), NO synthase inhibitor (NOSi), cyclic GMP analogue (cGMPa), soluble guanylate cyclase inhibitor (sGCi), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi. Because antisera raised against gonadotropins are not available for this species, we measured these hormones indirectly through their effects on ovarian progesterone secretion. The ovaries were superfused with the pituitaries pre-incubated as reported above. In addition, NOS activity and cGMP levels were determined in the pre-incubated pituitaries. Those pre-incubated with medium-alone and with GnRH increased progesterone secretion during pre-reproduction, pre-ovulation, ovulation and recovery; the increase induced by GnRH was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOd and cGMPa increased progesterone in all considered reproductive phases except ovulation; the increase induced by NOd and cGMP was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOS activity was highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased NOS activity during pre-reproduction, pre-ovulation and recovery. Cyclic GMP levels were highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased cGMP levels during pre-reproduction, pre-ovulation and recovery, NOd during all considered reproductive phases. These results suggest that NO mediates basal and GnRH-induced gonadotropin secretion in female Rana esculenta.

Inhibitory effect of leukotrienes on luteinizing hormone release

The aim of this work was to study the effect of high concentrations of leukotrienes on luteinizing hormone (LH) secretion in rat anterior pituitary cells. We also investigated the effect of leukotrienes in parallel with gonadotropin-releasing hormone (GnRH) action. Experiments were on cells gained from trypsinized pituitaries of female rats. Tests were performed by superfusion of the cells attached to cytodex-1 carrier beads. The LH content in samples of perfusate was estimated by radioimmunoassay. This work reports 48% inhibition of basic LH release by action of leukotriene C4 in superfused cells when applied continuously at a concentration of 100 nmol/l. Moreover, we have shown that leukotrienes suppressed GnRH-induced LH secretion in rat pituitary cells when applied in parallel to GnRH (1 nmol/l) as a 4-min pulse at a concentration of 0.1 nmol/l. GnRH-induced LH release was reduced to 66% of its value by leukotriene (LT) B4 (0.1 nmol/l) action; also to 54% by LTC4, 66% by LTD4 and 74% by LTE4 action. In contrast, arachidonic acid (50 pmol/l) and its other 5-lipoxygenase metabolites: 5-hydroperoxyeicosatetraenoic acid (5-HPETE) (50 pmol/l), or 5-hydroxyeicosatetraenoic acid (5-HETE) (50 pmol/l), had no inhibitory effect on GnRH-induced LH release. Arachidonic acid and 5-HETE potentiated GnRH-induced LH release up to 249% and 429%, respectively, when applied in parallel with GnRH (1 nmol/l) as a 4-min pulse at a concentration of 10 pmol/l. In our earlier work we have shown that several leukotrienes are potent stimulants of LH release. The present report documents the finding that the 5-lipoxygenase pathway is also involved in the inhibitory regulation of hormone release in anterior pituitary cells.

Role of leukotriene C4 in follicle-stimulating hormone (FSH) secretion in female rat pituitary

Leukotriene C4, at doses of 0.01 and 0.1 nmol/l added to superfused cells in pulse of 4-min duration, evoked follicle-stimulating hormone (FSH) release up to 12- to 26-fold of basal secretion. Higher and lower concentrations of leukotriene C4 were not able to induce FSH secretion. Gonadotropin-releasing hormone (GnRH)-induced FSH release was reduced by 38-57% by the leukotriene receptor antagonist FPL 55712 (10 mumol/l). Moreover, we have shown that FSH release occurs parallel to leukotriene C4 synthesis in rat anterior pituitary cells. Mellitin (100 nmol/l), an activator of phospholipase A2, induced FSH and radioactivity secretion in rat anterior pituitary cells previously preincubated for 24 h with [3H]arachidonic acid (AA).

A novel neuropeptide cellular mechanism in amphibian interrenal steroidogenesis

Interrenals of female Rana esculenta were incubated with gonadotropin-releasing hormone (GnRH), 9-ketoreductase inhibitor (palmitic acid), acetyl salicyclic acid, prostaglandin F2 alpha (PGF2 alpha), forskolin, isobutylmethyl xanthine (IBMX), dibutyril cyclic adenosine monophosphate (dbcAMP). Prostaglandin E2 (PGE2), PGF2 alpha, testosterone and 17 beta-estradiol were assessed on the incubation media. In addition, in the same interrenals, 9-ketoreductase and aromatase activities were evaluated. GnRH increased PGF2 alpha, 17 beta-estradiol, 9-ketoreductase and aromatase, and decreased PGE2 and testosterone. PGF2 alpha increased 17 beta-estradiol and aromatase, and decreased testosterone. Palmitic acid counteracted GnRH effects, while forskolin, IBMX and dbcAMP showed the same PGF2 alpha effects. These results suggest that GnRH stimulates 9-ketoreductase enhancing PGF2 alpha which in turn activates aromatase through cAMP mediation in the interrenal of Rana esculenta.

Mammalian gonadotropin-releasing hormone increases PGF2 alpha production activating diacylglycerol lipase in Rana esculenta interrenal

The aim of the present paper was to clarify if the prostaglandin F2 alpha (PGF2 alpha) production stimulated by mammalian gonadotropin-releasing hormone (mGnRH) comes from arachidonic acid (AA) freed by diacylglycerol (DAG) and/or membrane phospholipids in the interrenal of Rana esculenta. Interrenals of Rana esculenta were incubated with inhibitors of phospholipase A1 (PLA1), phospholipase A2 (PLA2), phospholipase C (PLC), protein kinase C (PKC) and diacylglycerol lipase (DAGlipase) in the presence or absence of mGnRH. In parallel, the same experiments were carried out using [3H]AA-labelled interrenals. The results of the experiments with non-labelled and [3H]AA-labelled interrenals were in agreement. PLA1, PLA2, PLC, PKC and DAGlipase inhibitors induced a decrease in PGF2 alpha production in interrenals without mGnRH, and PLA2 inhibitor was more effective than other inhibitors. PLC and DAGlipase inhibitors decreased the PGF2 alpha production by interrenals incubated with mGnRH, and PLC inhibitor was more effective than DAGlipase inhibitor. These findings suggest that the main source of AA used for mGnRH-induced PGF2 alpha synthesis is DAG; probably this decapeptide increases PGF2 alpha production enhancing the DAGlipase activity.

Mammalian GnRH involvement in prostaglandin F2 alpha and sex steroid hormones testicular release in two amphibian species: the anuran water frog, Rana esculenta, and the urodele crested newt, Triturus carnifex

The present work was carried out to study the in vitro effects of mammalian gonadotropin-releasing hormone (mGnRH) on Rana esculenta and Triturus carnifex testis production of prostaglandin F2 alpha (PGF2 alpha) and sex steroid hormones during the prereproduction, reproduction, and postreproduction periods. In R. esculenta, testicular PGF2 alpha release was lower during postreproduction, and mGnRH increased PGF2 alpha in prereproduction and reproduction. Androgens were higher during prereproduction, and mGnRH induced an androgens increase in prereproduction and reproduction. In T. carnifex testicular PGF2 alpha release was lower during reproduction, and mGnRH increased PGF2 alpha in prereproduction and reproduction. Androgens were higher in reproduction and lower in postreproduction, and mGnRH induced an androgens increase in reproduction. Estradiol-17 beta release was higher in postreproduction, and mGnRH induced an estradiol decrease in reproduction and an increase in postreproduction. These results seem to indicate the involvement of PGF2 alpha in the testicular reproductive activity, and a similar mGnRH mechanism of action, both in R. esculenta and in T. carnifex. In addition, taken together with previous studies, they seem to suggest that the relationship found between mGnRH and PGF2 alpha or sex steroids could be widespread in amphibians.

Effects of diacylglycerol and gonadotropin-releasing hormone on human chorionic gonadotropin release by cultured trophoblast cells

The effect of activation of calcium- and phospholipid-dependent protein kinase (protein kinase C) on human chorionic gonadotropin (hCG) release by cultured trophoblast cells was studied and a role of protein kinase C in the GnRH-mediated hCG release was also evaluated. Both GnRH and 1-oleoyl-2-acetylglycerol (OAG), a protein kinase C activator, stimulated hCG release after 3 h incubation in a dose-dependent manner with ED50 of 55 nmol/l and 4.0 nmol/l, respectively. A tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) also stimulated hCG release while two non-tumor-promoting compounds, phorbol and 4 alpha-phorbol, failed to stimulate hCG release. hCG release by maximal effective dose of GnRH (10 mumol/l) or OAG (1 mumol/l) was further stimulated when cells were incubated with same concentrations of GnRH and OAG. OAG-stimulated hCG release was completely inhibited by a protein kinase C inhibitor, H-7, with ID50 of 23 nmol/l while H-7 did not affect GnRH-mediated hCG release. These results indicate that GnRH-stimulated hCG release is not mediated by protein kinase C pathway, however, the secretion of hCG is also regulated by the mechanism that involves protein kinase C activation.

Production of leukotrienes in gonadotropin-releasing hormone-stimulated pituitary cells: potential role in luteinizing hormone release

Gonadotropin-releasing hormone (GnRH) stimulated the formation of two major metabolites of the 5-lipoxygenase pathway, leukotriene (LT) B4 and LTC4, as well as luteinizing hormone (LH) release in primary cultures of rat anterior pituitary cells. Several lines of evidence suggested the presence of a GnRH-dependent pituitary endocrine system in which LTs act as second messengers for LH release: (i) GnRH-dependent LT formation was observed within 1 min and immediately preceded GnRH-induced LH release, whereas exogenous LTs stimulated LH release at low concentrations; (ii) the dose responses of GnRH-induced LT production and LH release were similar and both effects required the presence of extracellular Ca2+ ions; (iii) GnRH-induced LH release was blocked by up to 45% following the administration of several LT receptor antagonists; (iv) LTE4 action on LH secretion was entirely abolished by LT receptor antagonists; and (v) an activator of protein kinase C acted synergistically with LTE4 to induce LH release. The major source of LT formation in the pituitary cell cultures appeared to be the gonadotrophs, as shown by GnRH receptor desensitization experiments. The results demonstrate the presence of a GnRH-activatable 5-lipoxygenase pathway in anterior pituitary cells and provide strong support for the hypothesis that LTs play a role in LH release in the GnRH signaling pathway.

Effects of mammalian gonadotropin-releasing hormone on plasma level of prostaglandin F2 alpha in the water frog, Rana esculenta

The present investigation was performed to evaluate the effects of mammalian gonadotropin-releasing hormone (mGnRH) on prostaglandin F2 alpha (PGF2 alpha) plasma level in adult male and female water frog, Rana esculenta, during three different periods of the reproductive cycle: recovery period (October), breeding period (May), and postreproductive period (June). Intact, hypophysectomized (HYP), gonadectomized (GON), and hypophysectomized-and-gonadectomized (HYP/GON) animals were injected with 0.6 micrograms of mGnRH and sacrificed 1 hr and 5 hr after peptide administration. Some of each of the groups were sacrificed without having received mGnRH. PGF2 alpha plasma levels were assessed by radioimmunoassay. Hypophysectomy induced a significant increase of PGF2 alpha levels in October and June males. mGnRH induced a significant increase of PGF2 alpha plasma levels only in HYP and HYP/GON frogs. The tissue target of this GnRH action is, at present, unknown, although interrenals could be putative responsive tissues. At present, it is also difficult to assign any physiological role to observed phenomena unless to suppose that the pituitary inhibition is not constant throughout the year. It cannot be excluded that the prostaglandin induction depends on a local paracrine action of GnRH, which could be performed outside any pituitary control.

Characterization of leukotriene C4 binding in anterior pituitary membrane preparations

Pituitary cells produce leukotrienes (LTs) and respond to exogenous administration of LTs by releasing gonadotropins. Specific high affinity leukotriene C4 (LTC4) binding has been found in membrane preparations of bovine anterior pituitaries. Unlabelled LTC4 displaced specific [3H]LTC4 binding. Other leukotrienes (LTB4, LTD4, LTE4, LTF4) did not compete with [3H]LTC4 for binding sites when administered at increasing concentrations together with a constant amount of radioligand indicating that the binding is highly specific for LTC4. Scatchard analysis of binding data obtained from saturation studies revealed a single binding site for [3H]LTC4 with a Kd of 8.95 +/- 5.53 nM and a B max of 15.44 +/- 6.93 pmol per mg of membrane protein. Glutathione S-transferase, a possible LTC4 binding site, did not display activity in the membrane fraction although the two glutathione derivates S-octylglutathione and S-decylglutathione competed with LTC4 in binding experiments. As leukotrienes are potent stimulators of gonadotropin secretion and modulators of gonadotropin-releasing hormone (GnRH)-induced gonadotropin release it is concluded that leukotrienes may be involved in the signal transduction pathway of GnRH and that they may act via a specific and high affinity receptor.

Mechanism of action of gonadotropin releasing hormone upon gonadotropin secretion: involvement of protein kinase C as revealed by staurosporine inhibition and enzyme depletion

The role of protein kinase C (PKC) in the mechanism of action of gonadotropin-releasing hormone (GnRH) upon gonadotropin secretion is controversial and therefore was investigated in primary cultures of rat anterior pituitary cells. A relatively selective PKC inhibitor, staurosporine, inhibited both GnRH- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced luteinizing hormone (LH) release with half-maximal inhibition (IC50) of about 80 nM. Inhibition of GnRH action was not complete suggesting also a PKC-insensitive component in GnRH-induced gonadotropin release. Staurosporine had no effect on basal LH release, or on cellular LH content, neither did the drug interfere with the binding of [125I]iodo-[D-Ser(t-Bu)6]des-Gly10-GnRH N-ethylamide to its receptor in pituitary cells. When cultured pituitary cells were incubated with TPA (1 microM) for 24-48 h no measurable cellular PKC activity could be detected. The decrease in total PKC activity was accompanied by an increase in Ca2+, phosphatidylserine (PS), diacylglycerol (DG)-insensitive activity suggesting the release of a portion of the catalytic domain of PKC (M-kinase) by the phorbol ester treatment. TPA-induced LH release was nearly abolished in PKC-depleted cells and the response to GnRH was markedly reduced (40%). The stimulatory effect of the Ca2+ ionophore, ionomycin, was not impaired in PKC-depleted cells. Impaired responses to GnRH in PKC-depleted cells were only noticed at a later phase (2-4 h) of the exocytotic response of the neurohormone. The data strongly suggest a role for PKC during the second phase of GnRH-induced gonadotropin secretion.

Calcium mobilization and influx during the biphasic cytosolic calcium and secretory responses in agonist-stimulated pituitary gonadotrophs

Stimulation of enriched pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) elicits dose-dependent biphasic elevations of cytosolic calcium ([Ca2+]i) and luteinizing hormone (LH) release, with rapid initial peaks followed by sustained plateaus during continued exposure to the agonist. A potent GnRH-antagonist, [N-acetyl-D-p-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10]GnRH, prevented the biphasic [Ca2+]i and LH responses when added before GnRH, and rapidly abolished both responses to GnRH when added during the plateau phase. In low Ca2+ medium the LH peak responses to GnRH were reduced and the subsequent sustained responses were almost completely abolished; reduction of extracellular Ca2+ during exposure to GnRH caused a prompt decline of LH release. The initial [Ca2+]i peak is derived largely from intracellular calcium mobilization with a partial contribution from calcium influx, while the sustained phase is dependent on the entry of extracellular Ca2+ through both L-type and dihydropyridine-insensitive channels. The presence of L-type voltage-sensitive calcium channels (VSCC) in pituitary gonadotrophs was indicated by the ability of elevated extracellular [K+] to stimulate calcium influx and LH release, and the sensitivity of these responses to dihydropyridine agonist and antagonist analogs. In cells pretreated with high [K+], the peak [Ca2+]i response to GnRH was enhanced but the subsequent plateau phase was markedly attenuated. This divergent effect of sustained membrane depolarization on the biphasic [Ca2+]i response suggests that calcium entry through VSCC initially potentiates agonist-induced mobilization of Ca2+ from intracellular storage sites. However, established Ca2+ entry through depolarization-activated VSCC cannot be further increased by agonist stimulation because both processes operate through the same channels, probably by changes in their activation-inactivation kinetics. Finally, the reciprocal potentiation by the dihydropyridine agonist, BK 8644, and GnRH of [Ca2+]i and LH responses confirms that both compounds act on the same type of channels, i.e., L-type VSCC, that participate in agonist-mediated calcium influx and gonadotropin secretion.

[Treatment of endometriosis]

In a randomized study, the effect of gestrinone (2 x 2.5 mg/week) was compared with the effect of danazol (3 x 200 mg/day) in treating 30 patients with laparoscopically proven endometriotic implants for 6 months. Therapy was effective in 80%-90% of cases and pregnancy rates were similar, but the incidence of side effects was different in the groups. In addition, buserelin (900 micrograms/day intranasally) was investigated in a multinational, multicenter trial in 275 patients. In 80%, GnRH agonist treatment induced the disappearance or reduction of endometriotic implants. The main side effects were due to estrogenic suppression. Presently, various methods of hormonal treatment are under investigation, especially to determine recurrency rates of endometriosis and long-term side effects.

Inositol 1,4,5-trisphosphate binds to a specific receptor and releases microsomal calcium in the anterior pituitary gland

The properties of inositol 1,4,5-trisphosphate (InsP3) receptor sites in the anterior pituitary were evaluated by binding studies with InsP3 labeled with 32P to high specific radioactivity. Specific binding of Ins[32P]P3 was demonstrable in pituitary membrane preparations and was linearly proportional to the amount of membrane added over the range 0.5-2 mg of protein. Kinetic studies showed that specific InsP3 binding was half-maximal in about 40 sec and reached a plateau after 15 min at 0 degree C. Addition of 1 microM unlabeled InsP3 was followed by rapid dissociation of the bound ligand, according to a single exponential function with half-time of about 1 min. Scatchard analysis of the binding data was consistent with a single set of high-affinity sites with Kd of 1.1 +/- 0.4 nM and maximal binding capacity of 28 +/- 15 fmol/mg of protein (n = 6). The specificity of Ins[32P]P3 binding to these sites was illustrated by the much weaker affinity for structural analogs such as inositol 1-phosphate, phytic acid, 2,3-bisphosphoglycerate, and fructose 1,6-bisphosphate. To assess the functional relevance of the InsP3 binding sites, the Ca2+-releasing activity of InsP3 was measured in pituitary membrane preparations. In the presence of oligomycin (2.5 micrograms/ml), Ca2+ movements were monitored with the fluorescent indicator fura-2 (free acid). Under these conditions, 1 mM ATP caused rapid uptake of Ca2+ by a vesicular component of the membrane fraction. Addition of InsP3 (50-2000 nM) caused a dose-dependent release of Ca2+ with a half-maximal effect at 240 nM. In the presence of 3% polyethylene glycol, GTP also stimulated calcium release. However, a nonhydrolyzable GTP analog, guanosine 5'-[gamma-thio]triphosphate, did not release calcium and completely blocked the effect of GTP. Under physiological conditions within the cytosol, the high-affinity InsP3 binding sites characterized in pituitary membranes could serve as the putative receptors through which InsP3 triggers Ca2+ mobilization in the anterior pituitary gland.

Effect of phorbol ester on stimulus-secretion coupling mechanisms in gonadotropin releasing hormone-stimulated pituitary gonadotrophs

The feedback regulatory control mechanism exerted by activated Ca2+/phospholipid-dependent protein C kinase upon gonadotropin releasing hormone (GnRH) binding, stimulation of phosphoinositide turnover and gonadotropin secretion was investigated in cultured pituitary cells. Addition of the tumor promoter phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), at concentrations which activate pituitary protein C kinase, to cultured pituitary cells resulted in up-regulation of GnRH receptors (155% at 4 h). The stimulatory effect of GnRH on [3H]inositol phosphates (Ins-P) production in myo-[2-3H]inositol prelabeled pituitary cells was not inhibited by prior treatment of the cells with TPA (10(-9)-10(-7) M). Higher concentrations of TPA (10(-6)-10(-5) M) inhibited the effect of GnRH on [3H]Ins-P production. Increasing concentrations of TPA or the permeable analog of diacylglycerol 1-oleoyl-2-acetylglycerol (OAG) stimulated luteinizing hormone (LH) release from cultured pituitary cells with ED50 values of 5 x 10(-9) M and 10 micrograms/ml, respectively. No consistent inhibition or additivity of LH release was observed when increasing doses of TPA or OAG were added with a submaximal dose of GnRH. These results suggest that protein C kinase might mediate the known homologous up-regulation of GnRH receptors during the reproductive cycle. Protein C kinase is positively involved in mediating the process of gonadotropin secretion. Unlike many other systems, activation of protein C kinase in pituitary gonadotrophs is not involved in negative feed-back regulation of stimulus-secretion-coupling mechanisms in GnRH-stimulated gonadotrophs.

Effect of inositol 1,4,5-trisphosphate and GTP on calcium release from pituitary microsomes

Microsomal vesicles from bovine anterior pituitary accumulate Ca2+ and maintain a steady-state ambient Ca2+ level of 200 nM. IP3 and GTP both induce calcium release from the microsomal vesicles. The effect of IP3 is inhibited by polyethylene glycol (PEG), and the effect of GTP is absolutely dependent on PEG. Half-maximal effect of IP3 (without PEG) is 0.26 micron, the maximal calcium release attaining 7% of the A23187-releasable pool. The same values for GTP (in the presence of PEG) are 80 microM and 10%, respectively. GTP potentiates the effect of IP3. This potentiation is not mediated by protein phosphorylation.

Binding of inositol phosphates and induction of Ca2+ release from pituitary microsomal fractions

Bovine anterior-pituitary microsomal fractions exhibit high-affinity, saturable and reversible binding of inositol 1,4,5-[32P]trisphosphate; 50% of the labelled ligand is displaced by 3.5 nM-inositol 1,4,5-trisphosphate. 0.5 microM-inositol 1,4-bisphosphate and 10 microM-ATP. Inositol 1,4,5-trisphosphate induces the release of Ca2+ from the microsomal vesicles (half-maximal effect at 290 nM), and its action is potentiated by inositol tetrakisphosphate (half-maximal effect at 4 microM).

Stimulation of luteinizing hormone release by melittin and phospholipase A2 in rat pituitary cells

Gonadotropin release in rat pituitary monolayer cultures was stimulated by phospholipase A2, as well as by its activator melittin. A dose-dependent stimulation of luteinizing hormone secretion by melittin was observed in a dose range of 10(-8) to 10(-4) M. A higher dose (1 mM) melittin had a sub-optimal effect. The stimulatory action of melittin was calcium-dependent and blocked by phospholipase A2 inhibitors, chloroquine and quinacrine. Similar to melittin, phospholipase A2 enhanced the effect of LH release in a dose range of 0.1-100 units/ml. The effect of this enzyme was also calcium-dependent with optimal calcium concentrations at 1.5 mM, as obtained also for melittin. In superfusion experiments, the stimulatory action of melittin and phospholipase A2 was reproducible in their effects on LH release in gonadotrophs. In addition, melittin (10(-7) M) stimulated LH and 3H-arachidonic acid efflux in superfused pituicytes following prelabelling with radiolabelled arachidonate. These data suggest that phospholipase A2, which releases arachidonic acid from phospholipids, may participate in controlling gonadotropin secretion in gonadotrophs, since arachidonic acid and its metabolites have previously been found to enhance gonadotropin release.

Stimulation of gonadotropin release by arachidonic acid and its lipoxygenase metabolites in superfused pituitary cells

Luteinizing hormone and follicle stimulating hormone secretion was stimulated by 4 min pulses of arachidonic acid (3 X 10(-5) to 10(-4)M) in superfused rat pituitary cells. The effect of its lipoxygenase metabolites, 5-hydroxy-6,8,11,14-eicosatetranoic acid (5-HETE) and 15-hydroxy-5,8,10,14-eicosatetranoic acid (15-HETE) was more potent on hormone release when added in the same dose. Using 3 X 10(-5)M 5-HETE, its releasing activity on gonadotropins was comparable to that of GnRH (10(-9)M). 15-HETE (3 X 10(-5)M) was even more potent on LH and FSH secretion than 5-HETE. The secretory profile induced by 5-HETE and 15-HETE was also similar to that shown for GnRH, resulting in a rapid increase and a more prolonged decline of the hormone release. The addition of these fatty acids to superfused pituitary cells did not alter the response of the cells to their physiological ligand. These findings give further support to the proposal that metabolites of arachidonic acid may be involved in receptor-mediated mechanisms of gonadotropin release in pituitary cells.

Leukotrienes stimulate gonadotropin release in vitro

Monolayer cell cultures of female rat anterior pituitaries were used to investigate the effect of leukotrienes (LT) LTA4, LTB4, LTC4, LTD4, LTE4 and other lipoxygenase metabolites of arachidonic acid (5-HETE, 5-HPETE, and 15-HETE) in vitro. 3H-arachidonic acid was rapidly incorporated into pituicytes and its release was enhanced by gonadotropin releasing hormone (GnRH) in superfused pituitary cells. Leukotrienes were found to be very potent stimulators of the release of luteinizing hormone (LH) when added as pulses to superfused pituicytes. At equimolar concentrations, LTA4, LTB4, LTC4 and LTE4 were found to be more potent than the physiological stimulus GnRH. LTD4 did not affect gonadotropin secretion. Other lipoxygenase metabolites of arachidonic acid, such as 5-HETE, 5-HPETE and 15-HETE were less effective on the exocytosis of LH. These results suggest that leukotrienes are potential mediators of GnRH action on gonadotropin secretion and are possible sites of regulation of pituitary function.

Contraceptive progestins and gonadotropin secretion in vitro

In an in vitro bioassay using rat pituitary cell cultures the effect of contraceptive progestins was tested on basal and gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion in vitro. Progestins diminished gonadotropin release in pituitary cells stimulated with GnRH, but did not alter basal values. This inhibitory effect was dose dependent in a range of 10(-10)-10(-5) M tested and the inhibitory action of most of the progestins examined was more potent than that of progesterone. The maximal reduction of LH and FSH values was by 60% of GnRH-induced control levels. Progestins also caused a shift in sensitivity of cells to GnRh (10(-12)-10(-6) M). When time dependence was investigated, some progestins potentiated GnRH effect on gonadotropins in pituitary cell cultures pre-incubated for a short time (4 h) with steroids. More prolonged pre-incubations from 23 to 71 h resulted in a progressive suppression of LH and FSH response to GnRH (10(-7) M). In order to examine intracellular effects, cells were pretreated with progestins and inositol phosphate metabolism was investigated. The data obtained in pituitary cells give evidence that polyphosphoinositide breakdown is potentially an early step in the action of GnRH on gonadotropin secretion by providing diacylglycerol and inositol phosphates. Addition of gonadotropin-releasing hormone to myo-2[3H]inositol-prelabeled rat pituitary cells in primary culture evoked a dose-dependent increase of the accumulation of [3H]inositol phosphates with a rise of inositol triphosphate, inositol diphosphate and inositol monophosphate within 1 min. Using one contraceptive progestin, gestoden, inositol phosphate production was inhibited by 80% compared to controls of GnRH-treated cells without the addition of steroids. The data obtained in this study suggest that this in vitro bioassay using rat pituitary cells is a useful tool in testing progestational compounds regarding their potency on gonadotropin release. In addition, these results show that one possible site of interference of progestins with GnRH-induced gonadotropin secretion may involve polyphosphoinositide breakdown.