Gonadotropin-releasing hormone rapidly alters polyphosphoinositide metabolism in rat granulosa cells

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.

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.

Calcium-dependent actions of gonadotropin-releasing hormone agonist and luteinizing hormone upon cyclic AMP and progesterone production in rat ovarian granulosa cells

The Ca2+ dependency of the direct stimulatory effect of the gonadotropin-releasing hormone (GnRH) agonist analog [D-Ser(t-Bu)6]des-Gly10-GnRH-N-ethylamide (GnRHa) on progesterone production was investigated and compared to that of luteinizing hormone (LH) in rat granulosa cells from preovulatory follicles. Removal of extracellular Ca2+ by EGTA, or the use of the Ca2+ channel blockers verapamil and La3+, resulted in complete inhibition of GnRHa-induced progesterone production and a partial inhibition of LH-stimulated progesterone production (80, 80 and 50% inhibition respectively for EGTA, verapamil and La3+). Removal of extracellular Ca2+ increased the ED50 for LH-induced cAMP production by four-fold (from 80 to 330 ng/ml) and decreased maximal nucleotide formation by 44%. LH-induced cAMP production was also inhibited partially by verapamil (35%) at 10(-4) M drug concentration. GnRHa had no effect on cAMP production in the presence or absence of Ca2+. GnRHa and LH were found to have maximal effects on progesterone production at about 0.5 mM of Ca2+ in the incubation medium. On the other hand the stimulatory effect of dibutyryl cAMP [Bu)2cAMP) on progesterone production showed little dependency on extracellular Ca2+. The calmodulin antagonist trifluoperazine (TFP) caused concentration-dependent inhibition of the stimulatory action of GnRHa and LH on progesterone production with IC50 values of 3 and 8 microM, respectively. The stimulatory effect of (Bu)2cAMP on progesterone synthesis was attenuated by verapamil and TFP. These results indicate that the direct stimulatory effect of GnRH on ovarian progesterone production is absolutely dependent on Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Luteinizing hormone stimulates the formation of inositol trisphosphate and cyclic AMP in rat granulosa cells. Evidence for phospholipase C generated second messengers in the action of luteinizing hormone

The following studies were conducted to determine whether luteinizing hormone (LH), a hormone which increases cellular levels of cyclic AMP, also provokes increases in 'second messengers' derived from inositol lipid metabolism (i.e. inositol phosphates and diacylglycerol). Rat granulosa cells isolated from mature Graafian follicles were prelabelled for 3 h with myo-[2-3H]inositol. LH provoked rapid (5 min) and sustained (up to 60 min) increases in the levels of inositol mono-, bis, and trisphosphates (IP, IP2 and IP3, respectively). Time course studies revealed that IP3 was formed more rapidly than IP2 and IP following LH treatment. The response to LH was concentration-dependent with maximal increases at LH concentrations of 1 microgram/ml. LiCl (2-40 mM) enhanced the LH-provoked accumulation of all [3H]inositol phosphates, presumably by inhibiting the action of inositol phosphate phosphatases. The effectiveness of LH, however, was dependent on the concentration of lithium employed; maximal increases in IP were observed at 10 mM-LiCl, whereas maximal increases in IP2 and IP3 were observed at 20 mM- and 40 mM-LiCl, respectively. The stimulatory effects of LH on inositol phosphate and progesterone accumulation were also compared with changes in cyclic nucleotide levels. LH rapidly increased levels of inositol phosphates, progesterone and cyclic AMP, but transiently reduced levels of cyclic GMP. These results demonstrate that LH increases both cyclic AMP and inositol trisphosphate (and presumably diacylglycerol) in rat granulosa cells. Our findings suggest that two messenger systems exist to mediate the action of LH in granulosa cells.

Mechanisms of action of gonadotropin releasing hormone on rat granulosa cells

To elucidate the mechanisms of stimulatory actions of GnRH on rat granulosa cells (GC), we have compared the actions of a GnRH agonist with those of a tumor-promoting phorbol ester, 12-0-tetradecanoylphorbol 13-acetate (TPA) and Ca+2 ionophore, A23187. GC were obtained from immature (28-29 days old) rats 48 h after injection of 20 IU PMSG. Following prelabeling with 3[H]arachidonic acid (AA), the cells were incubated with the test substances for 10 min and AA release determined. A GnRH agonist, [D-Ala6, des-Gly-NH2(10)] GnRH ethylamide (GnRHa; 10 ng/ml) increased AA release 175% compared to the control value. AA release in the presence of GnRHa was larger than that due to 1 microM A23187 or 40 nM TPA alone. A23187 or TPA increased GnRHa-stimulated AA release further. GC were incubated with the test substances for longer time periods, i.e., up to 5 h. GnRHa caused a 4-fold increase in prostaglandin (PG) synthase activity at 5 h. GnRHa increased PGE accumulation to the same extent as TPA, but only increased PG synthase activity about half as much. In combination with TPA, GnRHa had no influence on TPA-stimulated PG synthase activity, but increased PGE accumulation to levels comparable to those with A23187 plus TPA. GnRHa caused a 2.5 fold increase in progesterone (P) accumulation, which was the same as TPA. P accumulation in the presence of GnRHa was affected by neither A23187 nor TPA. These data indicate that the combination of TPA and A23187 can substitute for GnRH action on PGE and P accumulation in rat GC.

Gonadotropin-releasing hormone as a modulator of ovarian function

GnRH and its agonist analogs exert direct inhibitory and stimulatory effects on the ovaries of animals from several species. In the immature follicle, GnRH inhibits the actions of FSH on an integrated array of biochemical responses that lead to follicular development and corpus luteum formation. GnRH also suppresses gonadotropin action in mature follicles, and stimulates certain ovarian processes such as steroidogenesis and oocyte maturation. The inhibitory and stimulatory effects of GnRH are mediated through the binding of the peptide to high-affinity receptors in granulosa and thecal cells. Recent studies have shown that GnRH action in the ovary is dependent upon calcium mobilization and probably operates through stimulation of phospholipid turnover and activation of protein kinase C.

Luteinizing hormone-releasing hormone enhances polyphosphoinositide breakdown in rat granulosa cells

A 2-min addition of LHRH to [3H]inositol-prelabeled rat granulosa cells in primary culture evoked significant increases in the accumulation of [3H]inositol phosphates, i.e. radiolabeled inositol monophosphate (IP), inositol diphosphate (IP2), and inositol triphosphate (IP3) levels increased to 210, 590 and 520%, respectively, when compared to control cultures. By contrast, addition of FSH failed to elicit such a response. The effect of LHRH was completely blocked by the concomitant presence of a specific LHRH antagonist. LHRH evoked increase in [3H]IP3 and [3H]IP2 accumulation as early as 30 sec, while the increase in [3H]IP became significant at 2 min. These data support the hypothesis that polyphosphoinositide breakdown may be an early step in the intracellular signal mechanism which mediates the action of LHRH.

Inhibition of granulosa cell differentiation by dioctanoylglycerol--a novel activator of protein kinase C

The actions of dioctanoylglycerol, a novel and specific activator of protein kinase C, on granulosa cell steroidogenesis and LH/hCG receptor induction were examined in vitro. Granulosa cells were cultured for 2 days in media containing 100 nM androstenedione. FSH treatment stimulated progesterone, 20 alpha-hydroxy-pregn-4-en-3-one (20 alpha-OH-P) and estrogen production by 60- to 80-fold over basal levels. Treatment with dioctanoylglycerol increased progesterone production 3-fold over basal levels but failed to affect 20 alpha-OH-P or estrogen production. Concomitant treatment of cells with FSH and increasing amounts of dioctanoylglycerol produced dose-dependent inhibition of FSH-stimulated progesterone, 20 alpha-OH-P and estrogen production with IC50 values of 75, 40 and 50 microM, respectively. Dioctanoylglycerol also inhibited production of all three steroids induced by forskolin, an activator of adenylate cyclase, and dibutyryl cAMP. The effects of dioctanoylglycerol on cAMP responses to FSH were also examined. The increase in both intracellular and extracellular cAMP at 1 h following treatment with FSH was suppressed in the presence of dioctanoylglycerol as was the extracellular accumulation of cAMP at 48 h of culture. FSH induction of LH/hCG receptors, known to be a cAMP-mediated event, was also inhibited in the presence of dioctanoylglycerol. In contrast, a thio-derivative of dioctanoylglycerol (3-thio-1,2-dioctanoylglycerol), which does not activate protein kinase C, failed to inhibit both steroidogenesis and LH/hCG receptor induction in response to FSH.(ABSTRACT TRUNCATED AT 250 WORDS)