Biomedical subjects

R Rasolonjanahary

Publications and source records attributed to R Rasolonjanahary.

17 recordsLinked to original sources

PubMed2000

Vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptides stimulate mitogen-activated protein kinase in the pituitary cell line GH4C1 by a 3',5'-cyclic adenosine monophosphate pathway.

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP38) regulate anterior pituitary cell secretion and proliferation. In the somatolactotrope GH4C1 cell line, these effects are mediated through the type-II-like PACAP receptor (VPAC2) coupled to the cAMP pathway. In this study, the control of the extracellularly responsive kinases (ERKs) by VIP and PACAP38 was investigated in GH4C1 cells. VIP and PACAP38 increased ERK1 and ERK2 phosphorylation and were equipotent stimulators of both kinases. ERK activation was mimicked by cholera toxin, forskolin and 8bromo-cAMP. VIP and PACAP38 activation of ERK2 was blocked by the protein kinase A inhibitor H89, whereas the protein kinase C inhibitor GF109203X, or prior PMA-induced depletion of the protein kinases C, failed to inhibit VIP and PACAP38 activation of ERK2. In contrast, thyrotropin-releasing hormone (TRH) elicited ERK activation by a PKC-dependent process. ERK activation by VIP or PACAP38 and TRH were additive and both sensitive to the MEK inhibitors PD98059 and U0126. In parallel, U0126 reduced prolactin (PRL) mRNA levels induced by VIP. These results demonstrate for the first time that VIP and PACAP38 activate ERK in GH4C1 cells. Cyclic AMP increase is sufficient to elicit ERK activation in these cells and thus likely to represent the transduction pathway underlying VIP- and PACAP38-dependent ERK activation. This mechanism seems to be involved in VIP-induced PRL gene regulation.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1995

Psycholeine, a natural alkaloid extracted from Psychotria oleoides, acts as a weak antagonist of somatostatin.

The effects of psycholeine, a plant alkaloid, were investigated on binding of radiolabelled somatostatin ([125I]N-Tyr-SRIF) and on somatostatin (SRIF)-induced inhibition of adenylate cyclase activity and growth hormone (GH) secretion by rat anterior pituitary cells. Psycholeine was shown to displace specific binding of [125I]N-Tyr-SRIF to pituitary membrane preparations, with an IC50 of 10(-5) M. At this concentration, psycholeine was also effective in significantly reducing the SRIF-induced inhibition of adenylate cyclase activity previously stimulated by growth hormone releasing factor (GRF). In parallel, it reduced the SRIF-induced inhibition of GH release stimulated by GRF in primary pituitary cell cultures in a dose-dependent manner. At a moderate concentration, the alkaloid affected neither adenylate cyclase activity nor GH release when applied in the absence of SRIF. These data suggest that psycholeine has antagonistic properties at the SRIF receptor. Quadrigemine C, a precursor of psycholeine, has a similar action.

Adenylyl Cyclases↗

PubMed1994

Angiotensin II effects on second messengers involved in prolactin secretion are mediated by AT1 receptor in anterior pituitary cells.

The two forms of angiotensin II (Ang II) receptors, AT1 and AT2 subtypes, have been demonstrated in many other cells beside the anterior pituitary cells. Attempting to investigate the subtype(s) of Ang II receptors implicated in the multiple transduction mechanisms involved in Ang II stimulation of prolactin (PRL) release by lactotropes, we studied the effect of selective nonpeptidergic Ang II antagonists on the PRL release, adenylate cyclase (AC), and phospholipase C activities. In intact cells, the AT1 antagonist DuP753 blocked Ang II-induced PRL release, reversed in a dose dependent manner Ang II-evoked inositol phosphates production, and inhibited completely the PLC and protein kinase C (PKC) dependent cAMP accumulation induced by Ang II. In membrane preparations, the Ang II receptors were negatively coupled to AC. The AT1 antagonist blocked in a dose dependent manner the inhibitory effect of Ang II on cAMP production. In intact cells, the negative coupling of Ang II receptor with AC was observed only when PKC was down regulated by long term 12-O-tetradecanolylphorbol-13-acetate pretreatment. Ang II was able to inhibit vasoactive intestinal peptide-induced cAMP accumulation, a response which was also prevented by DuP753. The different coupling of Ang II receptor described above implicated only the AT1 type receptor since the AT2 antagonists (PD123177 and PD123319) were ineffective at any doses tested (10(-8) to 10(-5) M). The obtained results indicate that the regulation of PRL secretion involves the AT1 receptor subtype and that this receptor might be coupled to multiple effectors.

Angiotensin I↗

PubMed1994

Neuropeptide Y enhances LHRH binding to rat gonadotrophs in primary culture.

Incubation of dispersed adenohypophyseal cells from intact male rats with Neuropeptide Y (NPY) or Peptide YY (YY) at 21 degrees C increased maximal 125I LHRHa binding (Bmax) by about 50%. In presence of 10(-7) M NPY, Bmax calculated from saturation isotherm curves was 15.3 +/- 1.9 fmoles x mg-1 proteins, as compared to 10 +/- 1 fmoles x mg-1 in control incubates. The increase was dose dependent with an EC50 of 6.3 +/- 1.8 10(-10) M NPY. Preincubation of the cells with pertussis toxin (PT, 15 ng/ml) for 24 h abolished the effect, suggesting coupling of NPY receptors to G alpha o or G alpha i proteins. NPY 10(-7) M inhibited basal and Forskolin 10(-5) M stimulated intracellular cyclic AMP formation by 31.9 +/- 3.4% and 30.6 +/- 2.3% respectively. Desensitization of protein kinase C by overnight preincubation of the cells with 10(-6) M phorbol ester (PMA) did not interfere with the effect of NPY. In contrast, W7, a calmodulin inhibitor, as well as H7, a protein kinase C inhibitor with a relatively wide spectrum, suppressed the effect of NPY with IC50 of 1.4 +/- 0.6 10(-6) M and 2.2 +/- 0.5 10(-5) M, respectively. Taken together, these results suggest that NPY is able to control unmasking of a cryptic LHRH receptor pool in pituitary cells by a process dependent upon both GTP binding proteins and calmodulin dependent protein kinase.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1994

Epidermal growth factor treatment induces D2 dopamine receptors functionally coupled to delayed outward potassium current (IK) in GH4C1 clonal anterior pituitary cells.

GH4C1 cells, a clonal cell line from a rat pituitary tumor, have been widely used as a model to study the regulation of prolactin secretion. These cells, however, do not express dopamine D2 receptors and are therefore not suitable for exploring mechanisms involved in dopamine inhibition of prolactin secretion. The recent demonstration that epidermal growth factor (EGF) is able to induce functional expression of D2 receptors in GH3 cells, a parental clonal cell line, overcomes this difficulty. We have thus undertaken an electrophysiological study in order to check whether coupling of D2 receptors to K+ channels could be restored in that model. Effects of dopamine on the non-inactivating voltage-dependent outward K+ current (IK) were investigated both in control and in EGF-treated GH4C1 cells. The K+ current was not modified by EGF treatment alone. In control cells, IK measured before and during dopamine application was unchanged. In contrast, dopamine application markedly enhanced the K+ current in cells that had previously been exposed to EGF. The effect was mimicked by the specific D2 receptor agonist bromocriptine and blocked by sulpiride, a D2 receptor antagonist, thus indicating that the effect of dopamine was effectively due to the activation of D2 receptors. These results bring further evidence that EGF-induced D2 receptors in clonal strains from rat pituitary tumors are functional and are coupled to the delayed outward K+ current IK.

Animals↗

PubMed1994

Selective effect of a diet-induced decrease in the arachidonic acid membrane-phospholipid content on in vitro phospholipase C and adenylate cyclase-mediated pituitary response to angiotensin II.

Young rats were fed on an essential fatty acid (EFA)-deprived diet for 6 weeks after weaning. Their pituitary was removed and adenohypophyseal cells dispersed and maintained in culture. Membrane lipids were analyzed and basal and stimulated levels of hormone secretion were measured after 4-day incubation in a culture medium containing or not 160 microM arachidonic acid 20:4n-6 (AA) in order to obtain EFA-deficient or EFA-restored pituitary cells, respectively. In EFA-deficient cells membrane phosphoglycerides (PGL) were depleted in AA and adrenic acid 22:4n-6; the deficit was overcome by incubation in the presence of AA. Depletion diversely affected PGL classes. AA was highly depleted in choline phosphoglycerides (ChoPG), only moderately depleted in serine and ethanolamine phosphoglycerides (SerPG and EtnPG) and not depleted at all in inositol phosphoglycerides, suggesting preferential preservation of AA in that class of PGL. Restoration of AA by addition of the fatty acid to the culture medium was complete for ChoPG and EtnPG and only partial for SerPG. Depressed levels of AA and adrenic acid in PGL were compensated for by a concomitant increase in 20:3n-9 and 22:3n-9. Growth hormone and prolactin (PRL) secretion was assessed by radioimmunoassay and possible effects of a membrane AA deficit on hormone regulation were tested in cells challenged by either growth hormone-releasing hormone, thyrotropin-releasing hormone, angiotensin II (AII), vasoactive intestinal peptide (VIP) or dopamine. Neither basal nor stimulated growth hormone secretion was different from controls in EFA-deficient cells. PRL modulation by VIP or dopamine was not affected either in EFA-deficient cells. In contrast, the capacity of AII, but not of thyrotropin-releasing hormone, to release PRL was markedly decreased in EFA-deprived cells. It was restored by addition of AA to the incubation medium. Parallel depression of AII-induced inositol phosphates and cAMP accumulation was also observed after EFA deficiency. When tested on membranes, the paradoxical inhibition of adenylate cyclase by AII documented by previous observations was reinforced in EFA-deficient membranes. In contrast, binding of AII was not affected by EFA deficiency. It is concluded that under our experimental conditions EFA deficiency affects selectively coupling of the AII receptor to its effectors without alteration of binding. The effect could involve changes in receptor interactions with coupling proteins.

Adenylyl Cyclases↗

PubMed1992

A second endogenous molecular form of mammalian hypothalamic luteinizing hormone-releasing hormone (LHRH), (hydroxyproline9)LHRH, releases luteinizing hormone and follicle-stimulating hormone in vitro and in vivo.

In vitro and in vivo release of pituitary hormones were studied in the presence of (hydroxyproline9)LHRH ((Hyp)LHRH), a newly characterized endogenous molecular form of LHRH. Results were compared to those obtained with LHRH itself. (Hyp)LHRH, as LHRH, stimulated both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a homothetic manner. The hydroxylated compound was, however, 24 times (in vitro) and 5 times (in vivo) less potent than LHRH. The lower activity of (Hyp)LHRH than of LHRH in the in vitro assay correlated well with a 28-fold lesser potency in a binding test using pituitary membrane preparations. The higher relative potency and the prolonged effect of (Hyp)LHRH in the in vivo test were related to a lesser susceptibility of the hydroxylated form to proteolytic degradation. Effects of LHRH and of (Hyp)LHRH were not additive, both peptides were equally able to desensitize gonadotrophs to a subsequent challenge by the other. Taken together, these observations suggest that both forms of LHRH act at the same receptor site. The lesser affinity of the hydroxylated compound is compensated to a certain extent by its higher resistance to enzymatic degradation. It is concluded that in spite of its lesser potency, (Hyp)LHRH may participate in the regulation of gonadotropins.

Animals↗

PubMed1992

Alkaloids from Psychotria oleoides with activity on growth hormone release.

Bioactivity-guided purification of a crude alkaloid extract of Psychotria oleoides has afforded a new alkaloid, psycholeine [1], together with quadrigemine C [2], a tetrameric pyrrolidinoindoline compound of unknown stereochemistry. A comparison study of nmr and cd spectra of quadrigemine C and hodgkinsine [3], a trimeric pyrrolidinoindoline substance, led us to suggest the stereochemistry of quadrigemine C. The structure and configuration of psycholeine was determined by spectroscopic means and chemical correlation with quadrigemine C. Psycholeine interacts with somatostatin receptors and exhibits a somatostatin antagonistic activity on GH secretion by pituitary cells in primary culture.

Alkaloids↗

PubMed1992

Somatostatin receptors on thyrotropin-secreting pituitary adenomas: comparison with the inhibitory effects of octreotide upon in vivo and in vitro hormonal secretions.

The in vivo and in vitro inhibitory effects of a somatostatin (SRIH) analog, octreotide, upon TSH, alpha-subunit, GH, and PRL have been studied, as well as SRIH receptors and their coupling to adenylate cyclase, in nine TSH-secreting pituitary adenomas. From in vivo and cell culture studies, the TSH- and alpha-subunit-secreting adenomas appeared heterogeneous, with four out of the nine tumors cosecreting GH and/or PRL. A single sc injection of octreotide (100 micrograms) lowered plasma concentration of TSH by 40 +/- 5% (mean +/- SE of 5), of alpha-subunit by 27 +/- 9% (n = 5), of GH by 60 +/- 5% (n = 4), and of PRL by 27 +/- 9% (n = 4). In cells cultures, octreotide (10(-8) mol/L) inhibited equally TSH, alpha-subunit, and GH release. 125I-Tyr0-DTrp8-SRIH binding sites were measurable in the nine TSH-secreting adenomas either on membrane preparations (n = 6; Bmax: 152 +/- 73 fmol/mg protein) or on frozen sections by radioautography (n = 3). Their density was variable among TSH adenomas and was lower than that measured in GH-secreting adenomas but higher than in nonfunctioning tumors. Two out of three TSH-secreting adenoma displayed an heterogeneous distribution of 125I-Tyr0-DTrp8-SRIH binding sites. 125I-Tyr0-DTrp8-SRIH specific binding was inhibited by guanosine triphosphate (GTP: 10(-4) mol/L). SRIH inhibited adenylate cyclase in 5/5 TSH-secreting adenomas and a good correlation (r = 0.92, P less than 0.02) was found between 125I-Tyr0-DTrp8-SRIH binding capacity (Bmax) and maximal adenylate cyclase inhibition by SRIH. These results demonstrate in vivo and in vitro inhibition of TSH, alpha-subunit, PRL, and GH secretion by octreotide in TSH-secreting pituitary adenomas. Functional SRIH receptors are present on these tumors and the effect of SRIH on hormonal secretion could be mediated, at least in part, by inhibition of adenylate cyclase. These findings support the medical treatment of this rare type of tumors by SRIH analogs.

Adenoma↗

PubMed1991

Involvement of protein kinase-C in the effect of angiotensin-II on adenosine 3',5'-monophosphate production in lactotroph cells.

When applied to rat anterior pituitary cells, angiotensin-II (AII) exerted two opposite effects on adenylate cyclase (AC) activity: a pertussis toxin (PTX)-sensitive inhibition of the enzyme with a maximal effect of -42 +/- 2% in crude cell membrane preparations, and, in contrast, a non-PTX-sensitive stimulation of cAMP production (maximal effect = 38 +/- 3%) in intact cells. The apparent affinity of both effects was equal to 1.8 nM. The stimulation of cAMP formation parallels the stimulation of PRL release. Under the same conditions, dopamine (DA) inhibited both membrane AC activity and cAMP formation in intact cells by a PTX-sensitive mechanism. After separation of pituitary cell types by sedimentation at unit gravity, the effects of AII and DA on intracellular cAMP and membrane AC activity coincided in the same fractions (those enriched in PRL cells). The stimulatory effect of AII on cAMP formation was about 5 times weaker than that of peptides positively coupled to AC as vasoactive intestinal peptide in total as well as in PRL-enriched cells. Since the AII receptor is also coupled to phospholipase-C (PLC) in a non-PTX-sensitive manner, we investigated whether protein kinase-C (PKC) could indirectly account for the positive effect of AII on cAMP formation. 12-O-Tetradecanolylphorbol 13-acetate (TPA), a stimulator of PKC was indeed able to increase intracellular cAMP; this effect was not additive with that of AII. conversely, application of the PKC inhibitors H7 [1-(5-isoquinolylsulfonyl)2-methyl-piperazine] and staurosporine or desensitization of PKC by long exposure of the cells to TPA abolished the cAMP response to TPA as well as that to AII. In addition, thyreoliberin, another activator of the PLC pathway, was able to stimulate cAMP formation in a PKC-dependent manner. DA inhibition of intracellular cAMP was not affected by any PKC inhibition. We conclude that in lactotroph cells, 1) the AII inhibitory coupling to AC observed in membrane preparations does not exist in intact cells, at least under basal conditions; and 2) the AII intracellular cAMP stimulation observed is not accounted for by a direct coupling with AC; it is due to a cross-talk of the PLC pathway mediated by PKC, an effect that might be shared by other PLC-stimulating mediators and may participate in the regulation of PRL release.

Adenylyl Cyclases↗

PubMed1990

Dual mechanisms of inhibition by dopamine of basal and thyrotropin-releasing hormone-stimulated inositol phosphate production in anterior pituitary cells. Evidence for an inhibition not mediated by voltage-dependent Ca2+ channels.

In primary cultures of anterior pituitary cells, dopamine inhibited basal and thyrotropin-releasing hormone (TRH)-stimulated inositol monophosphate, bisphosphate, and trisphosphate production. This inhibition by dopamine can be resolved into two distinct components. One of the components was rapid and already present after 10 s. The other was slower, starting after 1 min, and was mimicked by nimodipine, a dihydropyridine calcium channel antagonist. The effects of dopamine and nimodipine were not additive on both basal and TRH-stimulated inositol phosphate production. Furthermore, the dopamine inhibition in the presence of TRH was much higher than the inhibition induced by nimodipine. It is thus likely that calcium entry through voltage-dependent calcium channels triggers a positive feedback on TRH stimulation of phospholipase C. However, depolarizing concentrations of K+ or BAY-K-8644, a voltage-dependent calcium channel agonist, had no effect on inositol monophosphate and bisphosphate accumulation. Ionomycin, even at a very high concentration (10 microM), had only a slight and transient effect on inositol phosphate formation. In addition, these agents did not affect the TRH dose-dependent stimulation of inositol phosphate production. These results suggest that the intracellular calcium concentrations that we measured under basal and TRH-stimulated conditions are sufficient to allow the maximal activity of phospholipase C which can be obtained under these two experimental conditions. In contrast, any decrease in the intracellular calcium concentration by a dihydropyridine antagonist, suppression of extracellular calcium, or inactivation of a voltage-dependent calcium channel by long term depolarization with K+ decreased the phospholipase C activities measured under basal and TRH-stimulated conditions. From these data it can be concluded that dopamine inhibits inositol phosphate production by two distinct mechanisms. The slow dopamine-induced inhibition of TRH-stimulated inositol phosphate production which is mimicked by nimodipine is likely because of an inhibition of a voltage-dependent calcium channel. This is substantiated further by the fact that ionomycin (10 microM) was able to reverse the nimodipine inhibitions as well as this slow component of dopamine inhibition. The nature of the rapid inhibition of TRH-stimulated inositol phosphate production induced by dopamine, but not by nimodipine, remains to be determined. It is suppressed in the absence of extracellular Ca2+. This may suggest that this inhibition is related to blockade of non-dihydropyridine-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1989

Differential modulation of D1 and D2 dopamine-sensitive adenylate cyclases by 17 beta-estradiol in cultured striatal neurons and anterior pituitary cells.

Primary cultures of anterior pituitary cells from female rats and of mouse embryonic striatal neurons were used to study the effects of 17 beta-estradiol on D1- and D2-dopamine (DA)-sensitive adenylate cyclase. 17 beta-Estradiol pretreatment (10(-9) M, 72 h) suppressed the D2-DA-induced inhibition of adenylate cyclase activity in anterior pituitary cells. The steroid (10(-9) M, 24 h) also blocked the D2-DA-evoked response in striatal neurons whereas it enhanced by twofold the D1-DA-induced stimulation of the enzyme activity in these neurons. All these effects of the steroid were dose dependent and specific, as neither 17 alpha-estradiol, dexamethasone, nor progesterone used at the same concentration (10(-9) M) was effective. Furthermore, the modulation of DA-sensitive adenylate cyclases by the steroid required long-term exposure of living cells to 17 beta-estradiol since neither 17 beta-estradiol pretreatment for 4 h nor its addition to broken cells directly into the adenylate cyclase assay induced any alteration in the DA-sensitive adenylate cyclase activity. These results are in agreement with a genomic effect of the steroid. Using both anterior pituitary cells and striatal neurons in culture, 17 beta-estradiol affected neither the total number of DA (D1 and D2) receptors nor the estimated number of adenylate cyclase catalytic units. Therefore, it is suggested that the steroid modifies the coupling process by a mechanism that still has to be elucidated. These results demonstrate an effect of 17 beta-estradiol on DA target cells in both systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenylyl Cyclases↗

PubMed1989

Involvement of central somatostatin in the alteration of GH secretion in starved rats.

In order to determine the central or peripheral origin of the starvation-induced modifications of growth hormone (GH) and thyroid-stimulating hormone (TSH) secretions, the effects of starvation were studied in freely moving male rats with hypothalamo-hypophyseal disconnection. Five days after the disconnection GH secretion exhibited lower maximal values and higher trough levels and ultradian pulsatile secretion was lost as compared to controls. TSH levels were also decreased. The lesion did not modify pituitary somatostatin (SRIF) receptors as assessed by 125I-Tyr-O-D-Trp-8-SRIF binding or inhibition of adenylate cyclase activity. On the other hand, the growth hormone releasing factor (GRF) capacity to stimulate adenylate cyclase was strongly reduced by the lesion without modification of the affinity. Exposure to 72 h food deprivation decreased GH pulses and TSH levels in control rats but did not modify GH secretory profiles or TSH levels of lesioned rats. Plasma glucose and insulin levels were equally decreased after fasting in control and lesioned rats. Altogether, our results demonstrate that starvation-induced modifications of GH and TSH secretions are of central origin while glucose and insulin changes are peripherally triggered. They suggest that the hypothalamus is the only source of SRIF implicated in this effect.

Adenylyl Cyclase Inhibitors↗

PubMed1989

Resistance to bromocriptine in prolactinomas.

Bromocriptine therapy normalizes PRL secretion in most, but not all, patients with prolactinomas. This study was undertaken to determine the mechanism(s) responsible for bromocriptine resistance in patients with a PRL-secreting macroadenomas (n = 5) or microadenomas (n = 3). Their mean basal plasma PRL value was 807 +/- 220 (+/- SE) micrograms/L before treatment, and their nadir mean value was 354 +/- 129 micrograms/L during chronic therapy with 15-30 mg bromocriptine daily; four of the eight patients had an increase in tumor size during therapy. In cultures of prolactinoma cells from patients normally responsive to bromocriptine therapy (n = 10), considered as controls, 10(-9) mol/L bromocriptine inhibited PRL release by 71 +/- 6% (+/- SE), and the half-inhibitory dose was 7 x 10(-11) mol/L. In contrast, in cultures of prolactinoma cells from five patients resistant to bromocriptine, PRL release was inhibited by only 3-42% at 10(-9) mol/L bromocriptine. This partial inhibition was reversed by a 100-fold excess of haloperidol. In contrast, the effects of other inhibitors of PRL release (10(-8) mol/L T3 and 10(-8) mol/L somatostatin) or of a stimulator (10(-8) mol/L angiotensin-II) on cells from resistant and normally responsive patients were similar. In cell membranes from five bromocriptine-responsive adenomas the density of dopaminergic binding sites, labeled by [3H] spiroperidol was 243 +/- 65 (+/- SE) fmol/mg protein. In adenomas from the eight patients resistant to bromocriptine therapy the density of [3H]spiroperidol-binding sites lower (145 +/- 31 fmol/mg protein). In adenomas from five resistant patients whose tumor had grown during therapy the density of binding sites was 25 +/- 3 fmol/mg protein, 10% of that in normally responsive patients. The effects of dopamine on adenylate cyclase activity also were different in the three groups of adenomas. Dopamine inhibited adenylate cyclase activity by 28.8 +/- 5.6% in five bromocriptine-responsive tumors and by 16.5 +/- 4.3% in adenomas from eight resistant patients. In contrast, in the five patients whose tumors grew during therapy dopamine paradoxically stimulated adenylate cyclase activity (+26.4 +/- 9.8%). There was a very good correlation between the density of dopaminergic binding sites and maximal inhibition of adenylate cyclase activity in bromocriptine-responsive prolactinoma patients (r = 0.90) and resistant patients who had no tumor growth during therapy (r = 0.94).(ABSTRACT TRUNCATED AT 400 WORDS)

17-Hydroxycorticosteroids↗

PubMed1988

Dihydropyridine-sensitive calcium channel activity related to prolactin, growth hormone, and luteinizing hormone release from anterior pituitary cells in culture: interactions with somatostatin, dopamine, and estrogens.

In the present work, we determined the activity of voltage-dependent dihydropyridine (DHP)-sensitive Ca2+ channels related to PRL, GH, and LH secretion in primary cultures of pituitary cells from male or female rats. We investigated their modulation by 17 beta-estradiol (E2) and their involvement in dopamine (DA) and somatostatin (SRIF) inhibition of PRL and GH release. BAY-K-8644 (BAYK), a DHP agonist which increases the opening time of already activated channels, stimulated PRL and GH secretion in a dose-dependent manner. The effect was more pronounced on PRL than on GH release. BAYK-evoked hormone secretion was further amplified by simultaneous application of K+ (30 or 56 mM) to the cell cultures; in parallel, BAYK-induced 45Ca uptake by the cells was potentiated in the presence of depolarizing stimuli. In contrast, BAYK was unable to stimulate LH secretion from male pituitary cells, but it potentiated LHRH- as well as K+-induced LH release; it had only a weak effect on LH secretion from female cell cultures. Basal and BAYK-induced pituitary hormone release were blocked by the Ca2+ channel antagonist nitrendipine. Under no condition did BAYK affect the hydrolysis of phosphoinositides or cAMP formation. Pretreatment of female pituitary cell cultures with E2 (10(-9) M) for 72 h enhanced LH and PRL responses to BAYK, but was ineffective on GH secretion. DA (10(-7) M) inhibited basal and BAYK-induced PRL release from male or female pituitary cells treated or not treated with E2 (10(-9) M). SRIF (10(-9) and 10(-8) M) reversed BAYK-evoked GH release to the same extent in cell cultures derived from male or female animals. It was ineffective on BAYK-induced PRL secretion in the absence of E2, but antagonized it after E2 pretreatment. The effect was dependent upon the time of steroid treatment and was specific, since 17 alpha-estradiol was inactive. In addition, DA and SRIF decreased the 45Ca uptake induced by the calcium agonist. These data demonstrate that DHP-sensitive voltage-dependent calcium channels of the L type present on different pituitary cells are not equally susceptible to BAYK activation under steady state basal conditions, indicating that their spontaneous activity and/or distribution vary according to the cell type; their activity is modulated by sex steroids. In addition, these data suggest that Ca2+ channels represent a possible site of DA and SRIF inhibition of PRL and GH release, respectively, by gating calcium entry into the corresponding cells.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1987

Somatostatin receptors on pituitary somatotrophs, thyrotrophs, and lactotrophs: pharmacological evidence for loose coupling to adenylate cyclase.

Pharmacological characterization of somatostatin (SRIF) receptors located on somatotrophs, thyrotrophs, and lactotrophs was attempted by measuring the effects of 14 structural agonists of somatostatin (SRIF) on the inhibition of basal and GRF-stimulated GH and basal and TRH-stimulated PRL and TSH secretion. We also checked the abilities of the analogs to displace [125I]N-Tyr-SRIF binding to pituitary cell membranes and their potency to inhibit adenylate cyclase activity. There was a very good correlation (r = 0.975) between the displacement of [125I]N-Tyr-SRIF and the inhibition of adenylate cyclase activity by the analogs. The effects of the analogs on secretion of the three hormones followed the same rank order of potency. However, the active analogs displayed 2-6 times lower affinities in inhibiting PRL than GH or TSH secretions. The shift in affinity was even more pronounced in the case of the lower potency of the analogs as inhibitors of adenylate cyclase activity compared to hormone secretions. Pretreatment of the cells with pertussis toxin (100 ng/ml; 24 h) blocked SRIF inhibition of basal and GRF-stimulated adenylate cyclase activity and decreased by 83% [125I]N-Tyr-SRIF binding. It also blocked the ability of SRIF to inhibit GRF-induced GH and TRH-induced PRL and TSH secretion. However, pertussis toxin also increased GRF stimulation of GH secretion and decreased TRH stimulation of both TSH and PRL secretion. We conclude from our data that SRIF-binding sites located on the three target cells of the adenohypophysis are of a single class. These binding sites are negatively coupled to adenylate cyclase, but the inhibition of hormone secretions by SRIF cannot be explained solely through adenylate cyclase inhibition. Another mechanism of transduction must be involved in the actions of SRIF on its three pituitary target cells.

Adenylyl Cyclases↗

PubMed1983

Guanine nucleotide sensitivity of [125I]-Iodo NTyr somatostatin binding in rat adenohypophysis and cerebral cortex.

Specific [125I]-Iodo-NTyr somatostatin binding sites are present in adenohypophyseal and cerebral cortical membranes. Guanine nucleotides reduce the maximal binding capacity of adenohypophyseal binding sites without significantly affecting their apparent affinity. In pituitary as well as in cortex, GTP is the most potent nucleotide followed by GDP and guanylyl imidodiphosphate (GMP-PNP). The effect appears specific of guanine nucleotides since ATP, ADP and AMP are inactive on [125I]-Iodo-NTyr somatostatin binding. These results, showing the nucleotide sensitivity of [125I]-Iodo-NTyr somatostatin binding in pituitary and cerebral cortex, are compatible with a coupling of somatostatin receptors with adenylate cyclase.

Animals↗