Inhibition of HCG-induced cyclic AMP accumulation and steroidogenesis in rat luteal cells by an LHRH agonist

GnRH receptors and GnRH endocrine effects on luteoma cells

An ovary implanted into the spleen of an ovariectomized rat develops into a luteinized tumor, growing in response to gonadotrophins. Previously, it was shown that in vivo Buserelin, a gonadotrophin-releasing hormone (GnRH) analog, inhibited tumor growth. To determine if GnRH had a direct effect on tumor cells, the presence of GnRH receptors as well as the endocrine effects of buserelin were studied on tumoral tissue. GnRH receptors were present in luteoma in similar concentrations and dissociation constant (Kd) to control estrous ovaries. In vivo treatment with buserelin did not modify luteoma GnRH receptors. In organ incubations, luteoma secreted significantly higher estradiol and lower progesterone than estrous ovaries; addition of buserelin did not modify steroid secretion. The same difference in basal steroid secretion between luteoma cells and luteal cells superovulated prepubertal ovaries was observed in cell cultures. Although luteinizing-hormone (LH)-stimulated progesterone in both kinds of cells, buserelin significantly inhibited LH-stimulated progesterone only in luteoma cells. These results describe clear differences in basal steroid secretion between tumoral and normal tissue. Furthermore, they show that luteoma possess GnRH receptors similar to those in normal ovarian tissue, and that GnRH analogs have endocrine effects on these cells. Therefore, a direct effect of buserelin on luteoma cells can be postulated.

Modulation of cyclic AMP formation and progesterone secretion by human chorionic gonadotropin, epinephrine, buserelin and prostaglandins in normal or human chorionic gonadotropin desensitized rat immature luteal cells in monolayer culture

It is now well recognized that hCG-induced luteolysis is associated with hCG-induced desensitization, but the physiological significance of luteal cell GnRH, PGS and beta-receptors is still undefined. Therefore, we intend in this study to observe the effects of prostaglandin F2 alpha and prostaglandin E2 and the interactions between epinephrine, a potent LHRH agonist [(D-Ser-(TBu)6, des-Gly-NH2(10)) LHRH ethylamide: Buserelin] and hCG in normal and in vitro hCG-desensitized rat immature luteal cells in monolayer culture, on basal, hCG or cholera toxin stimulated intracellular and extracellular cyclic AMP and progesterone secretion. The present report shows that incubation of immature rat luteal cells in monolayer culture with Buserelin, led to 25-50% inhibition of the epinephrine--as well as PGE2--induced cyclic AMP and progesterone responses. The LHRH agonist can also reverse the stimulatory effects of cholera toxin in the presence of hCG and led with PGF2 alpha, to additive inhibitory effects on extracellular cyclic AMP accumulation induced by cholera toxin. Both Buserelin and PGF2 alpha can reverse the hCG-induced cyclic AMP and progesterone release but no effect could be observed when the incubation was carried out with either substance in the absence of hCG. Prostaglandin E2, in acute conditions of incubation, seems to share agonist properties with hCG when both were incubated with luteal cells. Buserelin reversed the stimulatory effects of PGE2, hCG, epinephrine, and cholera toxin on cyclic AMP and progesterone responses to these substances. These results suggest that Buserelin and PGF2 alpha have luteolytic-like effects and that there may be a complementary action for the two substances. Preincubation of rat luteal cells in monolayer culture with 1 nM hCG for a 24 h period led to the inhibition of cyclic AMP and progesterone responses after a subsequent exposure to hCG and epinephrine. Luteal cells were no longer responsive to hCG while the presence of epinephrine in hCG-desensitized cells led to a 40% stimulation of cAMP and progesterone production. These observations suggest that there occurred a partial alteration of the N component activity of the adenylyl cyclase system.

Modulation of cyclic AMP formation and progesterone secretion by human chorionic gonadotropin, epinephrine, buserelin and prostaglandins in normal or human chorionic gonadotropin desensitized rat immature luteal cells in monolayer culture

It is now well recognized that hCG-induced luteolysis is associated with hCG-induced desensitization, but the physiological significance of luteal cell GnRH, PGs and beta-receptors is still undefined. Therefore, we intend in this study to observe the effects of prostaglandin F2 alpha and prostaglandin E2 and the interactions between epinephrine, a potent LHRH agonist [(D-Ser-(TBu)6, des-Gly-NH10(2) LHRH ethylamide: Buserelin] and hCG in normal and in vitro hCG-desensitized rat immature luteal cells in monolayer culture, on basal, hCG or cholera toxin stimulated intracellular and extracellular cAMP and progesterone secretion. The present report shows that incubation of immature rat luteal cells in monolayer culture with Buserelin, led to 25-50% inhibition of the epinephrine-as well as PGE2-induced cAMP and progesterone responses. The LHRH agonist can also reverse the stimulatory effects of cholera toxin in the presence of hCG and led with PGF2 alpha, to additive inhibitory effects on extracellular cAMP accumulation induced by cholera toxin. Both Buserelin and PGF2 alpha can reverse the hCG-induced cAMP and progesterone release but no effect could be observed when the incubation was carried out with either substance in the absence of hCG. Prostaglandin E2, in acute conditions of incubation, seems to share agonist properties with hCG when both were incubated with luteal cells. Buserelin reversed the stimulatory effects of PGE2, hCG, epinephrine and cholera toxin on cAMP and progesterone responses to these substances. These results suggest that Buserelin and PGF2 alpha have luteolytic-like effects and that there may be a complementary action for the two substances. Preincubation of rat luteal cells in monolayer culture with 1 nM hCG for a 24 h period led to the inhibition of cAMP and progesterone responses after a subsequent exposure to hCG and epinephrine. Luteal cells were no longer responsive to hCG while the presence of epinephrine in hCG-desensitized cells led to a 40% stimulation of cAMP and progesterone production. These observations suggest that occurred a partial alteration of the N component activity of the adenylyl cyclase system.

LHRH rapidly stimulates phosphatidylinositol metabolism in enriched gonadotrophs

The effects of luteinizing hormone-releasing hormone (LHRH) and human pancreatic growth hormone-releasing factor (hpGRF(1-40)-NH2) on phospholipid metabolism were studied in rat anterior pituitary cells in primary culture. In a 4-fold enriched population of gonadotrophs, 30 nM LHRH increased 32Pi incorporation into phosphatidic acid (PA) as early as 1 min after its addition. Phosphatidylinositol (PI) labeling was increased 1 min later. The stimulatory action of LHRH was observed in both phospholipids up to 100 min, the last time interval studied. The decapeptide did not affect 32Pi labeling of phosphatidylcholine (PC), lysoPC, phosphatidylethanolamine or phosphatidylserine. Dose-response studies performed after 25 min of incubation showed an ED50 value of LHRH action at approximately 1 nM for PI labeling. In contrast, the addition of 0.1 microM GRF to anterior pituitary cells enhanced 32Pi incorporation only into PC after a 60 min incubation period. The present data suggest that stimulation of acidic phospholipid metabolism, particularly an increase in PA-PI turnover, may represent an early event in the mechanism of action of LHRH but not GRF in the anterior pituitary gland.

GnRH agonist-induced inhibitory and stimulatory effects during ovarian follicular maturation

The in vivo regulation of ovarian gonadotropin and prolactin receptors and adenylate cyclase activity by FSH, and the potent GnRH agonist [D-Ala6]des-Gly10-GnRH N-ethylamide (GnRHa), was studied in immature hypophysectomized diethylstilbestrol-implanted rats. During FSH treatment over a 48 h period, FSH receptors increased 2-fold with the maximum response during the first 12 h, whereas LH and prolactin receptors increased by 10-fold and 6-fold with the maximum response from 12 to 48 h. Administration of GnRHa at any time during the 48 h period of FSH treatment inhibited the subsequent development of gonadotropin and PRL receptors. In contrast, administration of a single dose of 10 micrograms GnRHa after 48 h of FSH treatment stimulated follicular luteinization and caused increases in basal adenylate cyclase activity, ovarian weight and PRL receptor content, and concomitant decreases in gonadotropin receptors and adenylate cyclase responses. In the immature follicles of animals not primed with FSH, GnRHa caused progressive inhibition of FSH-sensitive adenylate cyclase activity, with a decrease in FSH receptors, but increased both basal and GMP-P(NH)P-stimulated adenylate cyclase activities. These results demonstrate that GnRHa causes marked inhibition of gonadotropin receptor expression in the basal and FSH-stimulated ovary. This decrease in gonadotropin receptors is an important component of the mechanism by which GnRH agonists inhibit ovarian gonadotropin-sensitive adenylate cyclase activity. In addition, these peptides exert stimulatory effects upon ovarian weight and basal adenylate cyclase activity, and cause an increase in PRL receptors and luteinization of mature ovarian follicles.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of calcium in luteinizing hormone-releasing hormone agonist (ICI 118630)-stimulated steroidogenesis in rat Leydig cells

The luteinizing hormone-releasing hormone (LHRH) agonist ICI 118630 was found to increase testosterone production in purified rat testis Leydig cells in a concentration- and time-dependent manner, but no consistent changes in cyclic AMP levels were detectable. The stimulation of steroidogenesis by LHRH agonist was found to be dependent on the concentration of Ca2+ in the incubation medium; at least 1 mM was required. The calcium ionophore A23187 mimicked the effects of the LHRH agonist on steroidogenesis, and addition of both compounds together did not further increase testosterone production. The calcium ionophore caused a small increase in cyclic AMP which was independent of the concentration of the ionophore and of the calcium concentrations. The evidence obtained in this study indicates that LHRH agonist-stimulated steroidogenesis in rat testis Leydig cells is primarily mediated by calcium and not cyclic AMP.

The effect of calcium on the potentiation of LH-stimulated steroidogenesis and inhibition of LH-stimulated cyclic AMP production by LHRH agonist (ICI 118630) in rat Leydig cells

A luteinizing hormone-releasing hormone (LHRH) agonist (ICI 118630) potentiated the effects of luteinizing hormone (LH) and dibutyryl cyclic AMP on steroidogenesis during 4 h incubations with rat Leydig cells. LH-stimulated cyclic AMP levels were decreased by the addition of the LHRH agonist. The potentiation of the LH-increased steroidogenesis was dependent on Ca2+; maximum effects required at least 2.5 mM Ca2+ in the incubation medium. The calcium ionophore A23187 negated the potentiation in a dose-dependent manner (ED50 = 0.2-0.3 microM), but had no effect on LH-induced steroidogenesis, despite a 90% decrease in cyclic AMP production. The latter decrease was found to be dependent on the Ca2+ concentration. In the presence of the phosphodiesterase inhibitor methylisobutylxanthine (MIX), the ionophore A23187 induced a dose-dependent decrease in both LH and LH plus LHRH agonist-stimulated steroidogenesis and cyclic AMP production. The results obtained indicate that calcium, rather than cyclic AMP, is the mediator of the potentiating effects of LHRH agonist on LH-increased steroidogenesis in rat Leydig cells. The marked inhibition of the synergism in the presence of calcium ionophore A23187 suggests that Leydig cell calcium homeostasis must be intact for LHRH agonist action to occur. LHRH agonist causes a Ca2+-dependent decrease in LH-stimulated cyclic AMP production.

Stimulation by prostaglandin F2 alpha of phosphatidic acid-phosphatidylinositol turnover in rat luteal cells

Prostaglandin F2 alpha (PGF 2 alpha) causes a rapid and marked increase of [32P]-orthophosphate incorporation into phosphatidylinositol (PI) and phosphatidic acid (PA) in rat luteal cells in culture. The incorporation of radioactivity is increased as early as 2 and 5 min after PGF 2 alpha addition into PA and PI, respectively, and by 10 min has reached a 2-fold stimulation over control in both lipid moieties. The labeling of other phospholipids is not affected. PGF 2 alpha exerts its stimulatory effect at an ED50 value of approximately 200 and 60 nM on PI and PA labeling, respectively. By contrast, human chorionic gonadotropin has no effect alone and does not interfere with the PGF 2 alpha-induced stimulation of PA-PI labeling. The striking similarity between the effects of PGF 2 alpha and LHRH on PA-PI labeling suggests that the two agents may exert their direct action on the corpus luteum via a common intracellular mechanism involving acidic phospholipid metabolism.

Distribution of luteinizing hormone-releasing hormone receptors in the rat ovary

The distribution of luteinizing hormone-releasing hormone (LHRH) receptors was studied in the adult rat ovary using autoradiography after injection of the stable LHRH agonist 125I-labelled [D-Ser(TBU)6,des-Gly-NH2(10)]LHRH ethylamide (Buserelin) and by radioreceptor assay using the same tracer. In intact cycling female rats, no differences in ovarian LHRH receptor levels could be observed between day diestrus I and day proestrus. Moreover, similar levels are observed in total ovarian homogenate, corpora lutea and the remaining ovarian tissue in adult animals treated with PMSG (pregnant mare's serum gonadotropins) and hCG (human chorionic gonadotropin). Radioautographic data show a comparable distribution of grains over theca interna and externa, granulosa and luteal cells. The present findings indicate the presence of LHRH receptors in both the interstitial and follicular cells throughout all stages of cellular differentiation.

Mechanism by which GnRH inhibits androgen synthesis directly in ovarian interstitial cells

The mechanism by which GnRH acts on ovarian interstitial cells to inhibit androgen synthesis was studied in primary cultures of ovarian cells from hypophysectomized immature rats. Interstitial cells cultured in defined medium with LH showed a 200-fold increase in steroid production, of which androsterone was the principal metabolite (88% of the total steroid content). Treatment with GnRH (10(-8) M) inhibited LH-stimulated androsterone production by 92%. This inhibitory effect of GnRH was not due to changes in cell number, cell viability, or 125-I-hCG binding capacity. Prostaglandin E2, cholera toxin and 8-Br-cyclic AMP mimicked the LH effect on androsterone synthesis and these increases were also inhibited by GnRH. Metabolic studies of GnRH-treated cultures revealed that LH-stimulated androsterone and 5 alpha-androstane-3 alpha, 17 beta-diol were decreased by 90%; androstenedione, testosterone and DHEA were decreased by 70%; 17 alpha-hydroxypregnenolone and 17 alpha-hydroxyprogesterone were decreased by 50%; pregnenolone was unchanged; and progesterone was increased 40%. Collectively, these results suggest that GnRH directly inhibits androgen synthesis in ovarian interstitial cells by selectively inhibiting the 17 alpha-hydroxylase and C17-20 desmolase activities.