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

Calcium and meiotic maturation of the mammalian oocyte

The role of calcium in the regulation of both the meiotic and mitotic cell cycles has been the subject of considerable investigation in the nonmammalian field. In contrast, the mechanisms for signalling meiotic maturation in the mammalian oocyte are not as well documented nor as clearly defined. In the mammalian oocyte, calcium is associated with both spontaneous and hormone-induced meiotic maturation. A transient release of endogenously stored calcium precedes germinal vesicle breakdown and can override cyclic AMP maintained meiotic arrest; it thus may signal the resumption of meiosis. Additionally, extracellular calcium is apparently required for meiotic progression past metaphase I. The time sequence for meiotic resumption and progression is very varied between species. The timing of cell cycle protein synthesis during meiosis suggests that cyclins may be expressed in oocytes of some species much earlier in their development than in others. A generic model is proposed for the mechanism for triggering meiotic resumption in the mammalian oocyte. In this model, the critical components of meiotic resumption involve the temporal relationship of cyclin synthesis and the subsequent activation of the MPF complex by the calcium signal generated, which accounts for differences among species.

The effects of gonadotropin on the phosphatidylinositol pathway in the primate corpus luteum

The current study was designed to investigate the effects of gonadotropin on basal and prostaglandin (PG) F2 alpha-induced activity of the phosphatidylinositol pathway in corpora lutea (CL) of rhesus monkeys. Luteal progesterone production in vitro was significantly stimulated (P < 0.05) by human chorionic gonadotropin (hCG). Neither basal nor PGF2 alpha-induced phosphatidylinositol 4,5-bisphosphate hydrolysis was significantly influenced by hCG in CL of various ages (P > 0.10). Gonadotropin did induce a slight, yet sustained, increase (P < 0.05) in [Ca2+]i in approximately 70% of luteal cells. The maximal increase in [Ca2+]i in response to hCG (approximately 100 nM) was about one-tenth that induced by PGF2 alpha (approximately 1000 nM). hCG treatment did not alter (P > 0.10) the increase in [Ca2+]i induced by PGF2 alpha Treatment-induced changes in [Ca2+]i did not differ between small (17-21 microns) and large (23-28 microns) luteal cells. Therefore, luteolytic agents are more potent activators of the phosphatidylinositol pathway than luteotropins. This is consistent with the hypothesis that the phosphatidylinositol pathway is involved in primate luteal regression. The inability of hCG to acutely alter the responsiveness of this pathway to PGF2 alpha suggests that CG may rescue the CL of early pregnancy via a mechanism other than direct inhibition of the luteolytic actions of PGF2 alpha.

Regulation of intracellular free Ca2+ by the LH/CG receptor in an established cell line 293 expressing transfected rat receptor

Luteinizing hormone (LH)/chorionic gonadotropin (CG) receptor (LHR) regulation of intracellular free Ca2+, [Ca2+]i, was studied by spectrofluorometric analysis and digital imaging of intracellularly trapped fura-2 fluorescence in a stable cell line 293 expressing transfected rat LHR. Exposure of the suspensions of 293-LHR cells to human CG (hCG) resulted in single, dose-dependent burst of elevated [Ca2+]i, the maximum being obtained at 0.1-1 microgram hCG/ml. The subconfluent individual 293-LHR cells responded to 1 microgram hCG/ml with either a single or oscillating [Ca2+]i bursts, the appearance of the oscillation being dependent upon the presence of extracellular Ca2+ ([Ca2+]e). 72% of the cells produced oscillation in response to hCG in the presence of [Ca2+]e and only 33% in the absence. Moreover, removal of [Ca2+]e from the incubation medium lowered the elevated basal [Ca2+]i level to or below the prestimulatory level and concomitantly damped out the oscillation, while its readdition without hCG was capable of re-elevating [Ca2+]i level and of gradually restoring the oscillation. The 293-LHR cells exposed to increasing doses of hCG also developed a dose-dependent desensitization of [Ca2+]i increase to renewed hormonal stimulation. The [Ca2+]i bursts within individual 293-LHR cells appeared in particular regions at the cell peripheries rather than distributed uniformly throughout the cytoplasm, pointing to compartmentation of the Ca2+ stores and to a local differences in receptor number in most cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic stimulation and cAMP mobilize Ca2+ from an IP3-insensitive pool in rat submandibular granular ducts

The beta-adrenergic agonist isoproterenol induced an increase in intracellular calcium concentration ([Ca2+]i) in rat submandibular granular ducts that was blocked by beta-adrenergic but not by alpha-adrenergic or muscarinic antagonists. This effect was only partially inhibited by the selective beta 1- and beta 2-adrenergic antagonists atenolol and ICI-118,551, but was completely blocked by the combination of the two, suggesting the involvement of multiple (or atypical) beta-adrenergic receptor subtypes. The response to isoproterenol was mimicked by forskolin, 3-isobutyl-1-methylxanthine, and dibutyryl adenosine 3',5'-cyclic monophosphate, but it was blocked by protein kinase inhibitors. The response of [Ca2+]i to isoproterenol was sustained in Ca(2+)-replete replete medium but transient in Ca(2+)-free medium, indicating the involvement of both Ca2+ entry and release from intracellular stores. However, isoproterenol stimulation produced no increase in ductal inositol phosphate levels. In addition, isoproterenol was still able to increase [Ca2+]i after the carbachol-induced depletion of inositol 1,4,5-trisphosphate (IP3)-sensitive calcium stores. We conclude that isoproterenol, acting through cAMP, releases Ca2+ from an IP3-insensitive intracellular store in salivary granular ducts.

Protein kinase C stimulatory activity in the pseudopregnant rat ovary

Ovarian cytosol from pseudopregnant rats was heated to 80-90 degrees C for 2 min and precipitated proteins removed by centrifugation. The supernatant of the heated ovarian cytosol contained no protein kinase C activity but when added to a control preparation containing protein kinase C, enzyme activity was increased to 200% of control. The stimulatory activity was stable to heating for 10 min, was retained on a centrifugal filtration device with a 100,000 M(r) cut-off, did not affect cAMP-dependent protein kinase, was not extractable in petroleum ether or chloroform/methanol (2:1), and enhanced the phosphorylation of protein kinase C-specific peptide substrates. The stimulatory factor was calcium-dependent and could substitute for phosphatidylserine and diacylglycerol in the protein kinase C assay. This stimulatory factor may provide a mechanism whereby the response of protein kinase C to hormonal activation could be regulated by the cell.

Biological actions of monoclonal antibodies to bovine lutropin receptor

Monoclonal antibodies (Mabs) were produced against bovine Lutropin receptor (LH-R). Antibodies were detected by an enzyme linked immunosorbant assay (ELISA). Hybridomas were subcloned to achieve monoclonality. Ascites were developed in Balb/c mice. Hybridoma supernatants were purified by ammonium sulfate precipitation and chromatography on hydroxylapatite columns. LH-R antibodies showed upto 50% inhibition of 125I-labeled hCG binding to bovine luteal cell membranes and up to 80% inhibition of testosterone production by hCG stimulated mouse Leydig cells. LH-R antibodies were predominantly IgM isotype. Purified antibodies showed a 78-kDa band, in SDS-PAGE, as the heavy chain of the immunoglobulin. LH-R antibodies were localized specifically in the thecal and luteal cells of the rat ovaries as well as in the Leydig cell of mouse testes. Injections of the LH-R antibody caused a constant estrus in normal rats. One month after the cessation of the injections the animals returned to normal estrus cycle and fertility. Pregnant mice injected with LH-R antibodies produced only 3 viable pregnancies and 10 pups, as compared to 8 pregnancies and 45 pups born to normal controls. LH-R antibodies also caused, approximately, a 50% reduction in testosterone production in normal male rats. These observations indicate a high degree of specificity of the Mab to LH-R and their potential in studies on gonadal function.

Effect of haloperidol on cyclic AMP and inositol trisphosphate in rat striatum in vivo

To investigate the effect of haloperidol (HAL) on second messengers in the brain striatum, the concentrations of cAMP and inositol trisphosphate (IP-3) were measured in the striatum of rats in vivo after intravenous administration of HAL, and their concentrations were compared with the severity of catalepsy and changes in dopamine (DA) metabolism in the striatum. Catalepsy developed both in the animals treated with 5 mg/kg and those with 0.5 mg/kg of HAL, but it appeared earlier, and the period of severe catalepsy was longer in the former than in the latter. In the animals treated with 5 mg/kg of HAL, DOPAC and HVA began to increase at 20 min after administration, and their percent increases were correlated with the severity of catalepsy. In the 5 mg/kg animals, both cAMP and IP-3 increased. The IP-3 showed a delayed peak but a greater increase as compared with the cAMP. In the 0.5 mg/kg animals, only IP-3 increased. These findings suggest that HAL might affect not only the adenylate cyclase system but also the phosphoinositide response in the striatum. Moreover, the changes in the phosphoinositide response might be secondarily induced by the blocking of D-2 receptors by HAL.

Inhibition of TSH bio-activity by synthetic beta TSH peptides

The in vitro bioactivity of the human beta TSH subunit was investigated utilizing eleven overlapping synthetic peptides representing the entire 112 residue sequence. The peptides were tested for both stimulatory and inhibitory activity in two sensitive bioassay systems: the first based on cAMP production in FRTL-5 rat thyroid cells, and the second based on stimulation of iodine trapping by the same continuous cell line. Peptides from three distinct regions of the beta-subunit showed concentration dependent inhibition of TSH bio-activity, including beta 1-15, beta 11-25, beta 31-45, beta 81-95, and beta 91-105 with IC50 values ranging from 150 to 304 microM. An additional peptide representing the entire sequence of the "intercysteine loop" region of beta TSH, beta 31-52, also inhibited TSH activity with somewhat higher potency than its fragment peptide beta 31-45 (IC50 of 87.5 +/- 14.7 microM for beta 31-52 versus 207 +/- 92.4 microM for beta 31-45). Three of these, beta 1-15, beta 31-45, and beta 31-52, also inhibited binding of TSH to the receptor in a radio-receptor assay, as previously reported (1), supporting their importance in receptor interaction. None of the synthetic peptides stimulated either cAMP production or iodine trapping. Two other overlapping peptides, beta 81-95 and beta 91-105, possessed bio-inhibitory activity but did not inhibit binding of labeled TSH. Computer analysis of this sequence predicted an extended turn structure for this region. This region has been referred to as the "determinant loop" as it is bounded by cysteine residues at positions 88 and 95 that many believe form a disulfide bond in the native subunit. The current data suggests the beta 88-95 region may play a role in receptor activation after initial binding of hormone to receptor.

Neomycin, an inhibitor of phosphoinositide hydrolysis, inhibits the resumption of bovine oocyte spontaneous meiotic maturation

The possibility that the intracellular signals generated upon phosphoinositide hydrolysis are involved in regulating bovine oocyte spontaneous meiotic resumption was investigated. Oocytes were mass-harvested and cultured in 2A-BMOC medium supplemented with 0.5% bovine serum albumin in the presence or absence of neomycin (an inhibitor of phosphoinositide hydrolysis) or phorbol myristate acetate (an activator of protein kinase C). The role of intracellular calcium was examined by preloading with BAPTA/AM (a calcium chelator) prior to culture. Meiotic maturation was scored cytogenetically. 1) Neomycin induces an irreversible inhibition of germinal vesicle breakdown which does not exceed 60% and is apparent at concentrations of 5 mM or above. Progression of meiosis past metaphase I is inhibited at concentrations of 2.5 mM or above. The full effect of neomycin is only apparent if it is presented to the oocytes within 3 h of follicular release, although germinal vesicle breakdown is not observed until 9 h culture under control conditions. 2) PMA alone has negligible effect on germinal vesicle breakdown, but it acts synergistically with 2 mM IBMX to inhibit this process. PMA has a dual effect on the progression of meiosis past metaphase I: 1 nM PMA has a stimulatory effect while 1 microM PMA blocks the ability of oocytes to reach anaphase I or beyond. These observations are not found with a non-tumor-promoting phorbol ester. 3) Spontaneous meiotic resumption is not significantly affected in the absence of added exogenous calcium. However, oocytes preloaded with BAPTA/AM exhibit a dose-dependent inhibition of germinal vesicle breakdown, even in the presence of extracellular calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective time-dependent effects of insulin on brain phosphoinositide metabolism

The effect of insulin on phosphoinositide metabolism in the cerebral cortex was examined using 32P as precursor. A maximal increase was detected as early as 15 s; phospholipid labeling declined after this initial peak but then increased to another maximum at 30 min. The levels of these phospholipids were unchanged at the earliest time examined, but at 30 min insulin caused an increase in the content of all phospholipids tested. In pulse-chase experiments, insulin stimulated depletion of 32P-labeled phosphoinositides only at 15 s. On the other hand, insulin treatment caused a biphasic diacyglycerol (DAG) production. We conclude that in cerebral cortex, insulin has a dual mechanism of action on phosphoinositide metabolism. First, insulin causes a rapid but transient hydrolysis of phosphoinositides by a phospholipase C-dependent mechanism, followed by subsequent resynthesis; thereafter, insulin increases de novo phospholipid synthesis.

Role of calcium in the control of steroidogenesis in preovulatory ovarian follicles of the goldfish

The possible involvement of calcium in the regulation of steroidogenesis in the goldfish was investigated using preovulatory ovarian follicles incubated in vitro. Incubation of follicles in media deficient in calcium impaired testosterone production in response to human chorionic gonadotropin (hCG) in both the presence and the absence of the phosphodiesterase inhibitor IBMX. Similarly, addition of calcium channel antagonists (verapamil, nifedipine, nicardipine, and CoCl2) caused a dose-dependent inhibition of hCG-stimulated testosterone production. TMB-8, an inhibitor of intracellular calcium mobilization, also suppressed hCG-stimulated testosterone production. Basal testosterone production was not affected by incubation in calcium-deficient media or with drugs which reduce intracellular calcium availability. In other studies, nifedipine blocked forskolin and dibutyryl cyclic AMP-stimulated testosterone production suggesting that one of the major sites of calcium action is distal to cyclic AMP generation. Two inhibitors of calmodulin, W5 and W7, significantly inhibited hCG-stimulated testosterone production. These findings suggest that calcium derived from intracellular and extracellular pools participate in the expression of gonadotropin effects on steroid production in goldfish ovarian follicles and that these effects are mediated intracellularly by interaction with calmodulin.

Rat oocyte maturation: role of calcium in hormone action

We studied the role of extracellular calcium (Ca0) in oocyte maturation and oocyte-cumulus cells interaction in rat follicles in vitro. Luteinizing hormone (LH) or a gonadotropin-releasing hormone analog (GnRHa) induced full maturation at [Ca0] = 1.3 mM. At [Ca0] = 0.6 mM, maturation induced by LH or GnRHa was inhibited by 65%. Chelatin of [Ca0] resulted in 45% maturation and neither hormone caused a further increase of maturation. [Ca0] = 20 mM enhanced the response to suboptimal concentrations of GnRHa but inhibited that to LH. Divalent cation ionophores caused [Ca0]-dependent maturation, which was fully inhibited by dibutyryl cAMP. Changes in [Ca0] also affected oocyte-cumulus interaction. At [Ca0] = 1.3 mM, either LH or GnRHa caused partial dispersion of the cumulus. Chelation of [Ca0] also resulted in an almost complete dispersion of the cumulus. The ionophores, however, caused maturation with the oocyte-cumulus complex preserved intact. Our data suggest that GnRHa may induce maturation via cAMP-sensitive calcium mobilization into the oocyte-cumulus-granulosa complex.

Two G protein oncogenes in human endocrine tumors

Somatic mutations in a subset of growth hormone (GH)-secreting pituitary tumors convert the gene for the alpha polypeptide chain (alpha s) of Gs into a putative oncogene, termed gsp. These mutations, which activate alpha s by inhibiting its guanosine triphosphatase (GTPase) activity, are found in codons for either of two amino acids, each of which is completely conserved in all known G protein alpha chains. The likelihood that similar mutations would activate other G proteins prompted a survey of human tumors for mutations that replace either of these two amino acids in other G protein alpha chain genes. The first gene so far tested, which encodes the alpha chain of Gi2, showed mutations that replaced arginine-179 with either cysteine or histidine in 3 of 11 tumors of the adrenal cortex and 3 of 10 endocrine tumors of the ovary. The mutant alpha i2 gene is a putative oncogene, referred to as gip2. In addition, gsp mutations were found in 18 of 42 GH-secreting pituitary tumors and in an autonomously functioning thyroid adenoma. These findings suggest that human tumors may harbor oncogenic mutations in various G protein alpha chain genes.

Phosphoinositides metabolism in primary culture of dog thyroid cells: effects of thyrotropin and carbachol

Thyrotropin (TSH) and carbachol stimulated in a dose-dependent manner the accumulation of 3H-glycerophosphoinositol (GPI), 3H-inositol monophosphate (IP1), 3H-inositol bisphosphate (IP2) and 3H-inositol trisphosphate (IP3) in primary cultures of dog thyroid cells prelabeled with myo-[2-3H]inositol. TSH, 250 mU/mL, stimulated 3H-IP3 level after a 10-minute incubation while 10 mU/mL TSH increased it during a 60-minute incubation. The effect of carbachol was more rapid and greater than that of TSH. Carbachol, 100 mumol/L, elevated 3H-IP3 after a 2-minute incubation and 3H-IP3 formation was increased by as little as 1 mumol/L carbachol. TSH stimulation was observed only if the cells were deprived of TSH for 5 days before being labeled with 3H-inositol. Prolongation of the labeling period or addition of TSH, (Bu)2cAMP or carbachol during the labeling increased 3H-inositol incorporation into polyphoinositides (PIPs). When the cells were labeled without any other addition, control and TSH-stimulated 3H-IP3 levels increased in parallel with 3H-PIP levels. However, TSH or carbachol-stimulated 3H-IP3 levels did not increase in proportion to 3H-PIPs level when the cells were labeled with TSH or (Bu)2cAMP. Thus, the ratio of 3H-IP3/3H-PIPs (both control and TSH or carbachol-stimulated) decreased in the cells labeled with TSH or (Bu)2cAMP, which might reflect TSH stimulation of 3H-inositol incorporation into PIPs pool(s) that do not participate in hormone-induced hydrolysis of PIPs.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Proopiomelanocortin-derived peptides, phosphoinositides, cAMP, and aldosterone secretion

Since the intracellular messengers of various proopiomelanocortin-derived peptides remain ambiguous at best, we have investigated the possible involvement of phosphoinositide metabolism in aldosterone secretion evoked by alpha-MSH, beta-LPH, as well as ACTH in rat and calf adrenal glomerulosa cells. We have also examined the cAMP responses in the adrenal glomerulosa cells to alpha-MSH comparing it with those of ACTH. Our results showed that neither alpha-MSH, beta-LPH, nor ACTH increased inositol triphosphate (IP3) or other inositol phosphates in adrenal glomerulosa cells while increasing aldosterone secretion from the same cells. Angiotensin II, known to cause hydrolysis of the phosphoinositides, increased IP3 in these adrenal cells in a dose-dependent manner. Both ACTH and alpha-MSH raised the cAMP levels in the calf adrenal glomerulosa cells, although the magnitude of the increase of cAMP in response to ACTH was greater. These findings suggest that IP3 as a mediator of alpha-MSH- and beta-LPH-induced aldosterone secretion is not likely and other mediator(s) may be involved.

Suppression of LH-stimulated prostaglandin and progesterone accumulation in rat granulosa cells by isoquinolinesulfonamide protein kinase inhibitors

Rat granulosa cells were incubated with isoquinolinesulfonamide inhibitors of protein kinases A and C and/or LH, dibutyryl cAMP (dbcAMP), tetradecanoylphorbol acetate (TPA), cholera toxin, or forskolin for 5 h. H7 (25 microM) was observed to inhibit LH, cholera toxin or dbcAMP stimulation of prostaglandin (PGE), and progesterone accumulation. H7 produced inhibition when added as little as 2 min before and as long as 1 h after LH. HA1004 was ineffective against LH or cholera toxin stimulation of PGE or progesterone at up to 100 microM. H9 blocked some LH and forskolin responses at 25 microM, but required a 50 microM concentration to minimally affect TPA stimulation. Cytotoxicity was not observed at the concentrations and times of isoquinolinesulfonamides tested. H7 and H9, therefore, suppress LH stimulation of granulosa cell functions in a dose- and time-dependent manner consistent with inhibition of protein kinases A and/or C, and consonant with a requirement for such kinases in LH action.

Effects of human chorionic gonadotropin, prostaglandin F2 alpha and protein kinase C activators on the cyclic AMP and inositol phosphate second messenger systems in cultured human granulosa-luteal cells

The effects of human chorionic gonadotropin (hCG) and prostaglandin F2 alpha (PGF2 alpha) on the adenylate cyclase-cAMP and inositol phospholipid-phospholipase C-inositol trisphosphate and diacylglycerol transmembrane signalling systems were evaluated in cultured human granulosa-luteal cells. Granulosa-luteal cells obtained from patients undergoing in vitro fertilization were cultured for 72 h prior to addition of hormones. During the last 24 h of culture granulosa-luteal cells were incubated with [3H]inositol. Neither hCG nor gonadotropin-releasing hormone (GnRH) stimulated the inositol phospholipid-phospholipase C signalling system. PGF2 alpha stimulated increases in inositol mono-, bis-, and trisphosphate accumulation in 30 min incubations. NaF (20 mM) mimicked the stimulatory effect of PGF2 alpha on inositol phosphate accumulation suggesting the involvement of a guanine nucleotide regulatory protein in the activation of phospholipase C. In contrast, hCG but not PGF2 alpha or NaF stimulated cAMP accumulation in 30 min incubations. Simultaneous treatment with hCG and PGF2 alpha did not alter the stimulatory effect of PGF2 alpha on inositol phosphate accumulation but reduced (37%) the stimulatory effect of hCG on cAMP accumulation. The protein kinase C activator, 12-O-tetradecanoylphorbol 13-acetate (TPA) inhibited the stimulatory effects of hCG (76%) and PGF2 alpha (62%) on cAMP and inositol phosphate accumulation, respectively. Thus, cultures of human granulosa-luteal cells possess multiple transmembrane signalling systems which may be modulated by the activation of protein kinase C.

Second messenger systems and progesterone secretion in the small cells of the bovine corpus luteum: effects of gonadotropins and prostaglandin F2a

The present studies were conducted to determine the effects of gonadotropins (LH and hCG) and prostaglandin F2a (PGF2a) on the production of "second messengers" and progesterone synthesis in purified preparations of bovine small luteal cells. Corpora lutea were removed from heifers during the luteal phase of the normal estrous cycle. Small luteal cells were isolated by unit-gravity sedimentation and were 95-99% pure. LH provoked rapid and sustained increases in the levels of [3H]inositol mono-, bis-, and trisphosphates (IP, IP2, IP3, respectively), cAMP and progesterone in small luteal cells. LiCl (10 mM) enhanced inositol phosphate accumulation in response to LH but had no effect on LH-stimulated cAMP or progesterone accumulation. Time course studies revealed that LH-induced increases in IP3 and cAMP occurred simultaneously and preceded the increases in progesterone secretion. Similar dose-response relationships were observed for inositol phosphate and cAMP accumulation with maximal increases observed with 1-10 micrograms/ml of LH. Progesterone accumulation was maximal at 1-10 ng/ml of LH. LH (1 microgram/ml) and hCG (20 IU/ml) provoked similar increases in inositol phosphate, cAMP and progesterone accumulation in small luteal cells. 8-Bromo-cAMP (2.5 mM) and forskolin (1 microM) increased progesterone synthesis but did not increase inositol phosphate accumulation in 30 min incubations. PGF2a (1 microM) was more effective than LH (1 microgram/ml) at stimulating increases in inositol phosphate accumulation (4.4-fold vs 2.2-fold increase for PGF2a and LH, respectively). The combined effects of LH and PGF2a on accumulation of inositol phosphates were slightly greater than the effects of PGF2a alone. In 30 min incubations, PGF2a had no effect on cAMP accumulation and provoked small increases in progesterone secretion. Additionally, PGF2a treatment had no significant effect on LH-induced cAMP or progesterone accumulation in 30 min incubations of small luteal cells. These findings provide the first evidence that gonadotropins stimulate the cAMP and IP3-diacylglycerol transmembrane signalling systems in bovine small luteal cells. PGF2a stimulated phospholipase C activity in small cells but did not reduce LH-stimulated cAMP or progesterone accumulation. These results also demonstrate that induction of functional luteolysis in vitro requires more than the activation of the phospholipase C-IP3/calcium and -diacylglycerol/protein kinase C transmembrane signalling system.

Sphingosine and psychosine, suggested inhibitors of protein kinase C, inhibit LH effects in rat luteal cells

The possible involvement of protein kinase C on luteinizing hormone (LH) effects in dispersed rat luteal cells was investigated using two substances that have been reported to be protein kinase C inhibitors, sphingosine and psychosine. Sphingosine efficiently inhibited protein kinase C activity both in brain and luteal cytosol fractions. Both substances inhibited LH-stimulated cyclic adenosine monophosphate (cAMP) accumulation in a dose-dependent fashion with an LD50 at 3-7 microM (sphingosine) and 40 microM (psychosine). LH-stimulated progesterone production was also inhibited with an ID50 at 6-10 microM (sphingosine) and 40-100 microM (psychosine). The inhibition was not due to an increased phosphodiesterase activity since IBMX (3-isobutyl-1-methylxanthine, 0.1 mM) and RO 20-1724 (4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone, 0.1 mM) did not abolish the inhibitory effect of sphingosine. To study the mode of action of sphingosine, forskolin and cAMP analogues were tested. The effect of these substances on steroidogenesis was inhibited, as well as the forskolin-induced cAMP accumulation, by sphingosine. This study demonstrates a clear inhibition of LH-stimulated effects by sphingosine and psychosine. LH action in rat luteal cells is discussed in relation to protein kinase C and the possible mode of sphingosine action.

Effects of quinacrine on vasopressin-induced changes in glycogen phosphorylase activity, Ca2+ transport and phosphoinositide metabolism in isolated hepatocytes

In isolated hepatocytes, quinacrine (150-250 microM) inhibited vasopressin-induced increases in glucose release, glycogen phosphorylase a activity and 45Ca2+ efflux; and glucagon-induced increases in glucose release and cyclic AMP formation. These results indicate that a phospholipase A2 enzyme sensitive to quinacrine is unlikely to be involved in the process by which vasopressin stimulates glycogen phosphorylase activity in the liver cell. In cells labelled with [3H]inositol, much lower concentrations of quinacrine (20-50 microM) inhibited the stimulation by vasopressin of the accumulation of [3H]inositol. The drug had little effect on vasopressin-induced accumulation of [3H]inositol mono-, bis- and tris-phosphates. In the absence of vasopressin, higher concentrations of quinacrine caused a small stimulation of glycogen phosphorylase activity, 45Ca2+ release and the formation of [3H]inositol polyphosphates. Quinacrine did not inhibit the degradation by liver homogenates of inositol 1-phosphate, inositol 4,5-bisphosphate or inositol 1,4,5-trisphosphate. It is concluded that concentrations of quinacrine comparable with those which inhibit phospholipase A2 [G.J. Blackwell, W.G. Duncombe, R.J. Flower, M.F. Parsons and J.R. Vane, Br. J. Pharmac. 59, 353-366 (1977)] inhibit the stimulation by vasopressin of inositol utilization without significantly affecting coupling between hormone receptors and adenyl cyclase or phosphoinositide-specific phosphodiesterase, the action of the phosphodiesterase, and the degradation of inositol triphosphate.

Thyroid-stimulating hormone stimulates increases in inositol phosphates as well as cyclic AMP in the FRTL-5 rat thyroid cell line

Studies were conducted to determine whether thyroid-stimulating hormone (TSH; thyrotropin), a hormone known to increase cytosol concentrations of cyclic AMP, also stimulates the formation of inositol phosphates in thyroid cells. TSH and noradrenaline both stimulated [3H]inositol phosphate formation in a concentration-dependent manner in the rat thyroid cell line, FRTL-5 cells, which had been prelabelled with [3H]inositol. The threshold concentration of TSH required to stimulate inositol phosphate formation was more than 20 munits/ml, which is approx. 10(3)-fold greater than that required for cyclic AMP accumulation and growth in these cells. We also demonstrate that membranes prepared from FRTL-5 cells possess a guanine nucleotide-activatable polyphosphoinositide phosphodiesterase, which suggests that activation of inositide metabolism in these cells may be coupled to receptors by the G-protein, Gp. Our findings suggest that two second-messenger systems exist to mediate the action of TSH in the thyroid.

Dual activation by thyrotropin of the phospholipase C and cyclic AMP cascades in human thyroid

In human thyroid slices prelabeled with myo-[2-3H]inositol, thyrotropin (TSH, 3-30 mU/ml) stimulated IP3, IP2 and IP1 generation over a prolonged time course. The cAMP response was much more sensitive to TSH, peaking between 1 and 5 mU/ml. Forskolin (10(-5) M) and isoproterenol had no effect on basal IP levels, while carbamylcholine (10(-5) M, 10(-4) M) also increased IP accumulation. These data suggest that in the human thyroid, TSH activates a phospholipase C generating IP3 and diacylglycerol independently of the well-known adenylate cyclase stimulation. They validate in the human model a dual mode of action of the hormone previously proposed on the basis of indirect observations.

An update on the role of phospholipid metabolism in the action of steroidogenic agents

Most steroidogenic agents which bind to cell surface receptors activate adenylate cyclase and/or phospholipase C. Activation of either signaling system may also be associated with rapid increases in de novo phospholipid synthesis, but it is at present uncertain whether this is a secondary or parallel event. Activation of phospholipase C leads to hydrolysis of phosphatidylinositol-4',5'-PO4 (PIP2) and generation of two second messengers, inositol-triphosphate and diacylglycerol (DAG), which mobilize Ca2+ and activate protein kinase C, respectively. Increases in de novo phospholipid synthesis lead to rapid increases in phosphatidic acid, DAG and C-kinase activity. The PIP2-phospholipase C system appears to initiate the steroidogenic response to certain agents, such as angiotensin-II, and this may be amplified by concomitant increases in phospholipid synthesis. With other agonists, the role of phospholipase C activation and de novo phospholipid (and DAG) synthesis is less certain. In some tissues, activation of protein kinase C by exogeneously added DAG analogues provokes an increase in steroidogenesis. However, this is not observed in other tissues, and it is uncertain whether this rules out involvement of the C-kinase system for steroidogenesis in these tissues, or whether endogenously produced DAG is a more effective activator of the relevant C-kinase system then exogenously added DAG analogues. The role of other potential intracellular signaling substances that may be derived from phospholipase C activation and de novo phospholipid synthesis is also at present uncertain, as are the interrelationships between these two phospholipid responses, cyclic nucleotides, and other steroidogenic factors.

Human chorionic gonadotropin activates the inositol 1,4,5-trisphosphate-Ca2+ intracellular signalling system in bovine luteal cells

Human chorionic gonadotropin, hCG, a hormone which increases intracellular cAMP, provoked rapid (30 s) and sustained (up to 30 min) increases in the levels of inositol mono-, bis- and trisphosphates (IP, IP2 and IP3, respectively) in bovine luteal cells. LiCl (10 mM) enhanced inositol phosphate accumulation in response to hCG. Concentration-dependent increases in inositol phosphates, cAMP and progesterone accumulation were observed in hCG-treated luteal cells. hCG also induced rapid and concentration-dependent increases in cytosolic free Ca2+ as measured by quin 2 fluorescence. These findings demonstrate that hCG stimulates the phospholipase C-IP3 and diacylglycerol 'second messenger' system in the bovine corpus luteum.