Leukotrienes stimulate gonadotropin release in vitro

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).

The use of flufenamate and forskolin to evaluate the role of cAMP in gonadotropin-releasing hormone-stimulated luteinizing hormone secretion from pituitaries of ovariectomized rats

Flufenamate which inhibited gonadotropin-releasing hormone (GnRH)-stimulated cAMP production in pituitaries from ovariectomized (72 hr) rats, was used to determine whether ovariectomy induces a change in the role of cAMP as a mediator of luteinizing hormone (LH) secretion. Additionally, the study evaluated the practicability of utilizing forskolin to restore intracellular cAMP concentrations in the presence of flufenamate. Infusions of flufenamate to perifused pituitary tissue blocks did not affect the protein synthesis-independent component of GnRH-stimulated LH secretion, but completely inhibited the protein synthesis-dependent component. Dibutyryl cAMP (dbcAMP) and forskolin potentiated the GnRH-stimulated responses, and restored the LH secretion inhibited by flufenamate, even though these agents were ineffective secretagogues when administered singly. The LH responses affected by forskolin were dependent on protein synthesis, while dbcAMP affected both the protein synthesis-dependent and -independent components of GnRH-stimulated LH secretion. Since the effects of dbcAMP on the protein synthesis-independent component of LH secretion might be due to interactions with GnRH receptors, the results suggest that forskolin might be a better choice for restoring intracellular cAMP levels in the presence of flufenamate when assessing the role of cAMP in gonadotropes. The study also indicates that ovariectomy does not result in a change in the role of cAMP, which appears to be a pivotal, but indirect mediator of the protein synthesis-dependent component of GnRH-stimulated LH secretion.

Gonadotropin releasing hormone activates the lipoxygenase pathway in cultured pituitary cells: role in gonadotropin secretion and evidence for a novel autocrine/paracrine loop

The formation and role of arachidonic acid (AA) and its metabolites during gonadotropin releasing hormone- (GnRH-) induced gonadotropin secretion were investigated in primary cultures of rat pituitary cells. Prelabeled cells ([3H]AA) responded to GnRH challenge with increased formation (about 2-fold) of the leukotrienes LTC4, LTD4, and LTE4 as well as 5- and 15-eicosatetraenoic acids (5- and 15-HETE) as identified by HPLC. Formation of leukotrienes and 15-HETE was further verified by specific radioimmunoassays. No significant increase in the formation of 12-HETE or of the cyclooxygenase products prostaglandin E (PGE) and thromboxane A2 by GnRH was noticed. Addition of physiological concentrations of LTC4 enhanced basal LH release, while subphysiological concentrations of LTC4 (10(-15)-10(-12) M) inhibited GnRH-induced LH release by about 35% (p less than 0.02). Using specific lipoxygenase inhibitors L-656,224 and MK 886, we found inhibition of GnRH-induced LH release by about 40% at concentrations known to specifically inhibit the 5-lipoxygenase pathway. The peptidoleukotriene receptor antagonist ICI 198,615 inhibited LTC4- and LTE4-induced LH release and surprisingly also the effect of GnRH on LH release by 40%. The data strongly suggest a role for AA and its lipoxygenase metabolites in the on/off reactions of GnRH upon LH release. The data also present a novel amplification cycle in which newly formed leukotrienes become first messengers and establish an autocrine/paracrine loop.

Involvement of cyclic adenosine monophosphate in the stimulation of gonadotropin secretion from the pituitary of the teleost fish, tilapia

The present study examines the involvement of cAMP in the transduction of the short-term effect of gonadotropin-releasing hormone (GnRH) on gonadotropin release in the teleost fish, tilapia. A 5 min pulse of dibutyryl cyclic AMP (dbcAMP; 0.03-3 mM) or forskolin (0.1-10 microM) resulted in dose-dependent surges in tilapia gonadotropin (taGTH) secretion from the perifused pituitary. The initial increase in taGTH in response to dbcAMP (3 mM) occurred within 6 min. The concentration of cAMP in the effluent medium increased about 20-fold after a pulse of [D-Ala6,Pro9-NEt]-luteinizing hormone-releasing hormone (LHRH) (GnRHa; 100 nM). To rule out the possibility that the observed effects were due to stimulation by endogenous GnRH release from intact nerve terminals present in the fragments, further experiments were performed in primary cultures of dispersed pituitary cells. Exposure (30 min) of the cells to forskolin (0.01-1.0 microM) resulted in a dose-dependent increase in taGTH release similar to that achieved by GnRHa (1 pM to 10 nM). Also 8-bromo cAMP (0.01-1.0 mM) evoked a dose-related increase in taGTH release. A 3-fold increase in the release occurred in the presence of isobutylmethylxanthine (IBMX) (0.2 mM), similar to that obtained by GnRHa (1.0 nM) in the absence of IBMX. However, when combined, the increase in taGTH release was 16-fold. Moreover, exposure of the cultured cells to GnRHa (0.1 or 10 nM, 60 min) resulted in a dose-related elevation of intracellular cAMP levels and taGTH release.(ABSTRACT TRUNCATED AT 250 WORDS)

The lipoxygenase inhibitor nafazatrom inhibits stimulated prolactin secretion in cultured rat lactotrophs (GH4C1 cells)

Arachidonic acid metabolites are involved in the regulation of prolactin (PRL) secretion from rat lactotrophs (GH4C1 cells). Phospholipase A2 (PLA2) stimulated PRL secretion, while the PLA2 inhibitor quinacrine reduced both thyrotropin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulated PRL release. Hormonally stimulated PRL release was further increased in the presence of the cyclo-oxygenase inhibitor indomethacin. Nafazatrom, a lipoxygenase inhibitor, reduced stimulated PRL secretion regardless of the mechanism of stimulation [hormonally (VIP and TRH) or by increasing intracellular Ca2+ concentration by KCl or Bay-K and cAMP by forskolin]. None of the inhibitors used in this study had any effect either on the basal PRL secretion or on the production of cAMP. Lipoxygenase products seem to be involved in the regulation of PRL secretion, probably by affecting a late step in the signal transduction.

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.

Prostaglandin E2 and leukotriene C4-induced luteinizing hormone-releasing hormone release from immature and adult male rat median eminences in vitro: eicosanoid formation and binding parameters

The amounts of prostaglandin E2 formed in vitro by the median eminences of adult male rats were greater than those produced by the median eminences of immature, 22 day-old rats. However, the amount of leukotriene C4 produced by the adult rat median eminences was lower than that produced by the immature rat median eminences. Analysis of the prostaglandin E2 binding parameters of hypothalamic P2 membrane fractions indicates that there are two binding components, one high affinity (RH) and one low affinity (RL) in both adult and immature rats. The maximal binding capacity of RH from adult rat membranes was significantly lower than that of immature rat membranes, correlating with greater prostaglandin E2 production by the adult rat median eminence. Only one leukotriene C4 binding site was detected in both adult and immature rat membranes. Exogenous prostaglandin E2 and leukotriene C4 both stimulated, the release of luteinizing hormone-releasing hormone to the same extent from both the adult and immature median eminences.

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.

Differences in luteinizing hormone release stimulated in rat anterior pituitary cells by leukotriene C4 and by gonadotropin-releasing hormone in vitro

Continuous administration of leukotriene C4 (LTC4, 10(-10) M) to superfused rat anterior pituitary cells increased LH release for 40 min only, whereas in a parallel experiment gonadotropin-releasing hormone (GnRH, 10(-9) M) evoked a continuous increase in hormone secretion. In contrast to GnRH, LTC4 did not desensitize rat anterior pituitary cells. The secretory action resulting from the administration of LTC4 (10(-10) M) was abolished for 40 min after previous stimulation. The results documented the dual action of LTC4 on LH exocytosis.

Exogenous action of 5-lipoxygenase by its metabolites on luteinizing hormone release in rat pituitary cells

The stimulatory effect of exogenously administered potato 5-lipoxygenase (0.1-0.3 U/2 ml) on luteinizing hormone (LH) release was demonstrated in rat anterior pituitary cells in a superfusion system. Nordihydroguaiaretic acid (NDGA), an inhibitor of 5-lipoxygenase, abolished the effect of the enzyme on LH secretion. The secretory effect on LH after 5-lipoxygenase administration was biphasic and dependent on Ca2+ indicating that 5-lipoxygenase affects LH release through its oxygenation reaction. Another series of experiments demonstrated that activation of 5-lipoxygenase, expressed as production of leukotriene (LT) B4 and C4 (728 +/- 127 pg/10(6) cells and 178 +/- 23 pg/10(6) cells, respectively) occurs in rat pituitary cells after addition of Ca2+ ionophore A23187. However, LTB4 and LTC4 were not formed by pituitary cells that had previously been desensitized by gonadotropin-releasing hormone (GnRH), the physiological ligand of LH release. These results are consistent with a role of 5-lipoxygenase metabolites in the mechanism of GnRH-induced LH secretion.

Leukotriene C4-induced release of LHRH into the hypophyseal portal blood and of LH into the peripheral blood

Intracerebroventricular (i.c.v.) administration of leukotriene (LT) C4 at doses of 2, 0.5 and 0.2 micrograms/rat significantly stimulated (3-12 fold) the release of LH into the peripheral blood of male rats. Injection of anti-LHRH serum had no effect on LTC4-stimulated LH release, but did block PGE2- stimulated LH release. I.c.v.- infused LTC4 also stimulated the release of LHRH into the hypophyseal portal blood. This is the first report of an in vivo action of LTC4 on the release of a hypothalamic releasing factor (LHRH) and a pituitary hormone (LH). These observations, plus in vitro results, clearly show that LTC4 stimulates LH release by acting on both the hypothalamus, causing LHRH release, and on the pituitary. Then the action of LTC4 on LH release in vivo is quite different from the indirect action of PGE2.

Stimulation of progesterone and prostaglandin E2 production by lipoxygenase metabolites of arachidonic acid

The role of several lipoxygenase metabolites of arachidonic acid in the action of luteinizing hormone-releasing hormone (LHRH) on ovarian hormone production was investigated. Like LHRH, treatment of rat granulosa cells with 5-HETE, 5-HPETE, 12-HETE, 15-HETE or 15-HPETE stimulated progesterone (P) and prostaglandin E2 (PGE2) production. 12-HEPE was most potent and stimulated P and PGE2 equally well. By contrast, 5-HETE stimulated P better than PGE2, while 15-HETE was a potent stimulator of PGE2 but not of P. Stimulation of P and PGE2 by LHRH or 12-O-tetradecanoylphorbol 13-acetate (TPA) was further augmented by several HETEs and HPETEs. Like protein kinase C, arachidonic acid metabolites appear to mediate the multiple actions of LHRH in the ovary.

Epidermal growth factor stimulates luteinizing hormone and arachidonic acid release in rat pituitary cells

Epidermal growth factor (EGF) directly enhanced luteinizing hormone (LH) release from dispersed rat pituitary cells in monolayer cultures as well as in superfusion columns. This 2.3-fold stimulatory effect was dose and time dependent and was also reconfirmed in a superfusion system. Retinal, a protein kinase C inhibitor, counteracted the EGF effect only partially. Further experiments were therefore carried out to investigate alternate EGF mechanisms. Nordihydroguaiaretic acid and chloroquine suppressed the stimulatory effect of EGF in a dose-dependent manner. Moreover, EGF (10(-7) M) stimulated [3H]arachidonate release from pre-labelled rat pituitary cells. This indicates that phospholipase A2 and arachidonic acid may be involved in EGF action on LH release from rat pituicytes.