Dexamethasone is a potent stimulator of growth hormone-releasing factor-induced cyclic AMP accumulation in the adenohypophysis

The stimulatory effect of maximal concentrations of synthetic human pancreatic growth hormone (GH)-releasing factor (GRF)(1-40)NH on cyclic AMP accumulation in rat anterior pituitary cells in culture is 4.5-fold increased following a 48-h preincubation with the potent glucocorticoid dexamethasone while the sensitivity of GRF action is increased by approximately 4-fold. Dexamethasone pretreatment, on the other hand, has no effect on basal cyclic AMP levels but approximately doubles both basal and GRF-induced GH release. The present data suggest that the potent stimulatory effect of glucocorticoids on GH secretion is exerted on the adenylate cyclase system at a step preceding cyclic AMP formation.     

Muscarinic acetylcholine receptor activation causes inhibition of cyclic AMP accumulation, prolactin and growth hormone secretion in GH3 rat anterior pituitary tumour cells

Acetylcholine, oxotremorine and carbachol, compounds that exhibit muscarinic agonist activity, maximally inhibited basal prolactin secretion from GH3 cells by approx. 50% and intracellular cyclic AMP levels by approx. 20%. Both parameters were inhibited with similar potencies by each agonist. These inhibitory effects were blocked by a muscarinic but not by a nicotinic receptor antagonist. In the presence of VIP or IBMX, which raise intracellular cyclic AMP levels and stimulate hormone release, the degree of muscarinic inhibition was increased, but the potency remained unchanged. Similar changes in the secretory rate of prolactin and growth hormone occurred in these and in cell perifusion experiments. These results suggest that the inhibition of hormone secretion from GH3 cells by muscarinic agonists is mediated by a decrease in intracellular cyclic AMP levels.     

Ontogeny of the response to growth hormone-releasing factor

Primary cell cultures were prepared from fetal, neonatal and adult rat pituitaries and evaluated for their ability to secrete growth hormone (GH) in response to growth hormone-releasing factor (GRF). Pituitary cells prepared from fetuses at days 19 and 21 of gestation, neonatal animals at the day of birth (day 0) or the following day (day 1) and peripubertal male rats showed full dose response curves to GRF with maximal GH release when stimulated with 1 X 10(-10) M rat GRF. At this concentration of GRF, the amount of GH released was not different from that elicited by activation of adenylate cyclase with 1 X 10(-5) M forskolin. In contradistinction, a preparation of cells from fetuses at day 18 of gestation did not show the same release of GH when challenged with 1 X 10(-10) M GRF and forskolin (0.057 +/- 0.001, compared to 0.076 +/- 0.003 micrograms/10(5) cells per 4.5 h), although the cells clearly responded to both secretagogues (basal levels of GH, 0.029 +/- 0.002 micrograms/10(5) cells per 4.5 h). While cells prepared from fetuses at day 21 of gestation or from animals after birth released 5-10% of their total cellular GH content, those prepared from 18- and 19-day fetuses released as much as 40% of their total GH suggesting there is a maturation of intracellular GH processing that occurs late in gestation. The results show that, in late pregnancy, the rat fetal pituitary is highly responsive to growth hormone-releasing factor and suggest that this peptide participates in regulating GH levels during the perinatal period.     

Inhibition of prolactin and growth hormone secretion by nickel

Nickel (Ni++) is a potent inhibitor of prolactin (PRL) secretion from isolated rat pituitary quarters in vitro, suppressing both basal PRL release and the stimulation of PRL secretion due to theophylline and dibutyryl cyclic AMP. Stimulation of growth hormone (GH) secretion by synthetic GHRH is also blunted by Ni++, although basal GH release and stimulated GH release due to theophylline or dibutyryl cyclic AMP are not suppressed. Ni++ antagonizes the stimulation of both PRL and GH secretion by barium (Ba++) ion, suggesting that the inhibitory effects of Ni++ on hormone release are due to an antagonism of calcium uptake or redistribution.     

Somatostatin prevents the desensitizing action of growth hormone-releasing factor on growth hormone release

We have investigated the effect of prior exposure to somatostatin (SRIF) alone or in combination with growth hormone-releasing factor (GRF) on the subsequent cyclic AMP and GH responses to GRF in rat anterior pituitary cells in primary culture. The maximal 4.5-fold stimulation of GH release induced by a 3-hr incubation with GRF is reduced by 60% following a prior 3-hr exposure to 30 nM GRF. A 3-hr preincubation with GRF in the presence of 30 nM SRIF doubles spontaneous GH release while the maximal amount of GH released during a subsequent 3-hr exposure to GRF is similar to that measured in cells pretreated with control medium, thus completely preventing the loss of GH responsiveness induced by prior exposure to GRF. The prevention by SRIF of the desensitizing action of GRF on GH release is not observed on the cyclic AMP response which remains almost completely inhibited in GRF-pretreated cells. Similar protective effects are obtained when SRIF is incubated with prostaglandin E2 (PGE2), thus completely preventing the desensitizing action of PGE2 on GH release. Prior treatment with pertussis toxin completely prevents the protective action of SRIF on GH responsiveness. Pretreatment with GRF + SRIF increases by 85 and 60% the maximal amount of GH release induced by cholera toxin and 8-bromoadenosine 3',5'-monophosphate, respectively. The post-SRIF rebound effect on GH release occurs mainly during the first 30 min following withdrawal of the tetradecapeptide. The present data demonstrate that simultaneous preincubation with SRIF and GRF prevents the marked inhibition of GH release during subsequent exposure to GRF.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dopamine on prolactin secretion and cyclic AMP accumulation in the rat anterior pituitary gland

The effects of dopamine on pituitary prolactin secretion and pituitary cyclic AMP accumulation were studied by using anterior pituitary glands from adult female rats, incubated in vitro. During 2h incubations, significant inhibition of prolactin secretion was achieved at concentrations between 1 and 10nm-dopamine. However, 0.1–1μm-dopamine was required before a significant decrease in pituitary cyclic AMP content was observed. In the presence of 1μm-dopamine, pituitary cyclic AMP content decreased rapidly to reach about 75% of the control value within 20min and there was no further decrease for at least 2h. Incubation with the phosphodiesterase inhibitors theophylline (8mm) or isobutylmethylxanthine (2mm) increased pituitary cyclic AMP concentrations 3- and 6-fold respectively. Dopamine (1μm) had no effect on the cyclic AMP accumulation measured in the presence of theophylline, but inhibited the isobutylmethylxanthine-induced increase by 50%. The dopamine inhibition of prolactin secretion was not affected by either inhibitor. Two derivatives of cyclic AMP (dibutyryl cyclic AMP and 8-bromo cyclic AMP) were unable to block the dopamine (1μm) inhibition of prolactin secretion, although 8-bromo cyclic AMP (2mm) significantly stimulated prolactin secretion and both compounds increased somatotropin (growth hormone) release. Cholera toxin (3μg/ml for 4h) increased pituitary cyclic AMP concentrations 4–5-fold, but had no effect on prolactin secretion. The inhibition of prolactin secretion by dopamine was unaffected by cholera toxin, despite the fact that dopamine had no effect on the raised pituitary cyclic AMP concentration caused by this factor. Dopamine had no significant effect on either basal or stimulated somatotropin secretion under any of the conditions tested. We conclude that the inhibitory effects of dopamine on prolactin secretion are probably not mediated by lowering of cyclic AMP concentration, although modulation of the concentration of this nucleotide in some other circumstances may alter the secretion of the hormone.     

Effectors of ATP-sensitive K+ channels inhibit the regulatory effects of somatostatin and GH-releasing factor on growth hormone secretion.

Somatostatin inhibition of growth hormone (GH) secretion from adenohypophysis cells in culture was antagonized by the antidiabetic sulfonylurea glipizide (K0.5 = 10 +/- 5 nM). Although all cells that hyperpolarize with somatostatin have ATP-sensitive K+ channels, the antagonistic actions of the hormone and of the antidiabetic drug are due to effects on different types of K+ channels. Diazoxide, an opener of ATP-sensitive K+ channels, abolished the increase of intracellular Ca2+ provoked by growth hormone releasing factor (GRF) and induced inhibition of GRF stimulated GH secretion (K0.5 = 138 microM). This inhibition by diazoxide was largely suppressed by glipizide which blocked the ATP-sensitive K+ channels opened by diazoxide. In summary, hormonal activation of GH secretion is inhibited by openers of ATP-sensitive K+ channels, while hormonal inhibition of GH secretion is suppressed by blockers of ATP-sensitive K+ channels.     

Dynamics of secretory granules in somatotrophs of rats after stimulation with growth hormone-releasing factor: a stereological analysis

The anterior pituitary tissue of male rats injected with growth hormone-releasing factor (GRF) was either processed for stereology at the light-and electron-microscopic levels, or homogenized for growth hormone (GH) assay 2–60 min after GRF injection. Secretory granules of somatotrophs became smaller but increased in numerical density 2 min after GRF injection. Their volume density began to increase at 5 min. The frequency of exocytosis of the granules was most prominent as early as 2 min after GRF injection and reduced thereafter. GH levels in the tissue were lowest at 2–5 min, and returned to the control value by 60 min. Serum GH levels were highest at 15 min; even at 60 min, this value was higher than in the controls. These findings suggest that secretory granules in somatotrophs are stimulated to divide by GRF, resulting in a decrease in size and an increase in number. The discrepancy between the earlier formation of new secretory granules and the later restoration of intracellular GH levels implies that GRF first stimulates the synthesis of constituents of granules other than GH, and only later the synthesis of GH, and that newly formed small secretory granules contain less GH. From the clearance rate of serum GH and the frequency of granule exocytosis, it can be estimated that about a half million granules are released to maintain 1 ng/ml of serum GH in rats.     

Prolactin and growth hormone regulate adiponectin secretion and receptor expression in adipose tissue

Adiponectin is a hormone secreted from adipose tissue, and serum levels are decreased with obesity and insulin resistance. Because prolactin (PRL) and growth hormone (GH) can affect insulin sensitivity, we investigated the effects of these hormones on the regulation of adiponectin in human adipose tissue in vitro and in rodents in vivo. Adiponectin secretion was significantly suppressed by PRL and GH in in vitro cultured human adipose tissue. Furthermore, PRL increased adiponectin receptor 1 (AdipoR1) mRNA expression and GH decreased AdipoR2 expression in the cultured human adipose tissue. In transgenic mice expressing GH, and female mice expressing PRL, serum levels of adiponectin were decreased. In contrast, GH receptor deficient mice had elevated adiponectin levels, while PRL receptor deficient mice were unaffected. In conclusion, we demonstrate gene expression of AdipoR1 and AdipoR2 in human adipose tissue for the first time, and show that these are differentially regulated by PRL and GH. Both PRL and GH reduced adiponectin secretion in human adipose tissue in vitro and in mice in vivo. Decreased serum adiponectin levels have been associated with insulin resistance, and our data in human tissue and in transgenic mice suggest a role for adiponectin in PRL and GH induced insulin resistance.     

Cycloheximide blocks insulin-like growth factor I but not somatostatin inhibition of growth hormone secretion

Growth hormone secretion is controlled by the two hypothalamic hormones, growth hormone releasing factor (GRF) and somatostatin. In addition, the insulin-like growth factors (IGF or somatomedins) which are themselves growth hormone dependent, inhibit growth hormone release in vitro, therefore acting to close the negative feedback loop. The studies reported here examine some of the differences between inhibition of growth hormone secretion by somatostatin and IGF-I in vitro. The major finding is that cycloheximide, a protein synthesis inhibitor, blocks inhibition of GRF-stimulated growth hormone release caused by IGF-I, without changing the inhibition caused by somatostatin. The experiments were done by exposing mixed rat adenohypophysial cells to secretagogues with or without cycloheximide for 24 h in a short term culture. Somatostatin (0.6 nM) totally blocked rat GRF (1 nM) stimulated growth hormone release to values 48% of control (nonstimulated values), while IGF-I (27 nM) only reduced the GRF-stimulated growth hormone release by 27 +/- 3% (N = 5). Cycloheximide (15 micrograms/mL) totally blocked the effect of IGF-I but not somatostatin. A low concentration (0.12 nM) of somatostatin, which only partly inhibited growth hormone release, was also unaffected by cycloheximide. In purified rat somatotrophs, somatostatin (0.1 nM) inhibited GRF-stimulated cAMP levels slightly and reduced growth hormone release while IGF-I (40 nM) had no effect. We suggest that IGF-I inhibits only the secretion of newly synthesized growth hormone, while somatostatin inhibits both stored and newly synthesized growth hormone pools.     

Effects of growth hormone-releasing factor and somatostatin on growth hormone secretion and cellular cyclic AMP levels. Cultured ovine and rat anterior pituitary cells show markedly different responses

Human pancreatic growth hormone-releasing factor (GRF-44-NH2) stimulated growth hormone (GH) secretion and intracellular cyclic AMP levels in cultured pituitary cells from both sheep and rat. Somatostatin (SRIF), over a wide range of doses and time, showed no significant effect on the elevated cyclic AMP levels in sheep cells, but did block the GH release in a dose-dependent manner. In rat cells, however, SRIF inhibited GRF-stimulated cyclic AMP levels by 75% maximum (still 8-fold greater than the basal levels) and GH release to almost half the basal value. We conclude that somatostatin inhibits GRF-elevated cyclic AMP levels in rat pituitary cells but not in sheep cells.     

Actions of L-363,586, a cyclic hexapeptide analogue of somatostatin, on GH secretion by human somatotrophinoma cells in vitro.

L-363,586 is a cyclic, hexapeptide analogue of somatostatin-14 with potent inhibitory actions on rat growth hormone (GH) release in vitro. The studies reported here investigate the direct effects of L-363,586 on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from 3 human somatotrophinomas in dispersed cell culture. 1nM and 10nM L-363,586 inhibited both basal and GRF-stimulated GH release from cells of all 3 somatotrophinomas during a 2h treatment period, whilst 100nM L-363,586 had a prolonged inhibitory action on basal GH secretion from cells of 2 of the tumours throughout treatment and recovery periods. Rebound release of GH was observed with cells of 1 tumour following treatment with L-363,586 plus GRF. The actions of L-363,586 were similar to those of somatostatin-14. These data suggest that L-363,586 may have a role in the treatment of acromegaly.     

Effects of pretreatment with tetradecanoyl phorbol acetate on regulation of growth hormone and prolactin secretion from ovine anterior pituitary cells

Tetradecanoyl phorbol acetate (TPA) stimulates growth hormone (GH) and prolactin secretion from ovine anterior pituitary cells. Pretreatment of the cells with TPA abolishes this effect, presumably due to down-regulation of protein kinase C. Such pretreatment did not alter effects of thyrotropin-releasing hormone or dopamine on prolactin secretion, suggesting no involvement of protein kinase C. Pretreatment with TPA attenuated actions of GH-releasing hormone on GH release (but not actions on cyclic AMP levels), possibly due to depletion of cellular stores of GH. Such pretreatment also attenuated inhibition of GH release by somatostatin, possibly due to phosphorylation of receptors or associated proteins by protein kinase C.     

Dominant dwarfism in transgenic rats by targeting human growth hormone (GH) expression to hypothalamic GH-releasing factor neurons.

Expression of human growth hormone (hGH) was targeted to growth hormone-releasing (GRF) neurons in the hypothalamus of transgenic rats. This induced dominant dwarfism by local feedback inhibition of GRF. One line, bearing a single copy of a GRF-hGH transgene, has been characterized in detail, and has been termed Tgr (for Transgenic growth-retarded). hGH was detected by immunocytochemistry in the brain, restricted to the median eminence of the hypothalamus. Low levels were also detected in the anterior pituitary gland by radioimmunoassay. Transgene expression in these sites was confirmed by RT-PCR. Tgr rats had reduced hypothalamic GRF and mRNA, in contrast to the increased GRF expression which accompanies GH deficiency in other dwarf rats. Endogenous GH mRNA, GH content, pituitary size and somatotroph cell number were also reduced significantly in Tgr rats. Pituitary adrenocorticotrophic hormone (ACTH) and thyroid-stimulating hormone (TSH) levels were normal, but prolactin content, mRNA levels and lactotroph cell numbers were also slightly reduced, probably due to feedback inhibition of prolactin by the lactogenic properties of the hGH transgene. This is the first dominant dwarf rat strain to be reported and will provide a valuable model for evaluating the effects of transgene expression on endogenous GH secretion, as well as the use of GH secretagogues for the treatment of dwarfism.     

Protein kinase C enhances growth hormone releasing factor (1-40)-stimulated cyclic AMP levels in anterior pituitary. Actions of somatostatin and pertussis toxin

The brain peptide human growth hormone releasing factor (1-40) (GRF), which stimulates adenylate cyclase activity in the anterior pituitary, is the predominant hormone signal for pituitary growth hormone (GH) release. Activators of protein kinase C such as teleocidin and 4 beta-phorbol 12-myristate 13-acetate (PMA) double the cyclic AMP accumulation induced by GRF, with no apparent effect on GRF potency; an inactive 4-alpha-PMA has no such action in cultured anterior pituitary cells. This PMA potentiation can be measured as early as 60 s, is maximal by 15 min, and wanes such that by 3-4 h there is no such amplifying effect of PMA. PMA, phorbol 12,13-dibutyrate, and teleocidin ED50 values for potentiating GRF activity are similar to those obtained for direct protein kinase C activation. The major inhibitory peptide somatostatin reduced both GRF- and GRF + PMA-stimulated cyclic AMP accumulation. Pertussis toxin totally blocked this somatostatin action without affecting the degree of maximal GRF potentiation achieved with PMA. Thus, the pertussis toxin target(s) are required for somatostatin inhibition of the cyclic AMP generating system, but may not be involved in the PMA potentiation of GRF-stimulated cyclic AMP accumulation.     

Antagonism of prostaglandin-promoted pituitary cyclic amp accumulation and growth hormone secretion in vitro by 7-oxa-13-prostynoic acid

The studies reported here confirm the previously observed potent stimulus to growth hormone (GH) secretion by prostaglandin E₁ (PGE₁). Proportional increments in GH secretion were observed following $\text{in vitro}$ addition of PGE₁ over a concentration range of 10^?7 to 10^?5 M. Growth hormone secretion could not be further stimulated by higher concentrations of prostaglandin. Prostaglandin E₁ also increased cyclic AMP concentration in the pituitary explants in a proportional fashion, which correlated closely with its potency as a growth hormone secretogogue. In order to define more precisely the mechanism by which prostaglandin acts, the effects of prostaglandin antagonist, 7-oxa-13-prostynoic acid, on GH secretion and cyclic AMP accumulation were investigated. Addition of the antagonist alone had no consistent effects on GH secretion or cyclic AMP levels in the pituitary. However, the antagonist significantly reduced the stimulation of hormone release and cyclic AMP accumulation found following addition of PGE₁. Increasing the concentration of antagonist further diminished prostaglandin stimulated hormone release and nucleotide accumulation. The antagonist failed to block the stimulatory effects of theophylline and dibutyryl cyclic AMP on GH release, indicating that the inhibition observed occurred prior to intracellular accumulation of the cyclic nucleotide. These results are consistent with the hypothesis that a prostaglandin receptor on the pituitary somatotrope is linked to the adenyl cyclase-cyclic AMP system.     

The effect of thyroid hormone and glucocorticoids on carp growth hormone-releasing factor (GRF)-induced growth hormone (GH) release in rainbow trout (Oncorhynchus mykiss).

1. The effect of thyroid hormone and glucocorticoids on carp growth hormone-releasing factor (GRF)-induced growth hormone (GH) secretion was studied on rainbow trout using a dispersed pituitary cell culture system. 2. A combined administration of lower doses (0.01 microM) of 3,5,3'-triiodo-L-thyronine (T3) and dexamethasone (Dex) significantly increased spontaneous as well as carp GRF-induced GH release. 3. Lower doses of Dex alone had no effect, and T3 had a marginal effect on GH release. Higher doses of either Dex or T3 potentially reduced GH release. 4. This study indicates an important role of thyroid hormone and/or glucocorticoids in the hypothalamic regulation of GH secretion in fish.     

Peptide T bolus normalizes the growth hormone secretion pattern in two children with AIDS

In humans, HIV infection reduces growth hormone (GH) secretion contributing to AIDS wasting. In rats, the HIV envelope protein gp120 alone blocks GH secretion both in vitro and in vivo through GH-releasing hormone receptors. Peptide T, a modified octapeptide derived from gp120, normalizes GH secretion. We now report that an intravenous bolus of peptide T normalizes nocturnal GH secretion in two out of three children with AIDS. These results, coupled with the lack of toxicity of this experimental AIDS therapeutic, justify clinical trials for AIDS wasting and pediatric AIDS. A clinical and basic science update on peptide T appears in Current HIV Research.     

Regulation of GH3-cell function via adenosine A1 receptors. Inhibition of prolactin release, cyclic AMP production and inositol phosphate generation.

We examined the mechanism by which adenosine inhibits prolactin secretion from GH3 cells, a rat pituitary tumour line. Prolactin release is enhanced by vasoactive intestinal peptide (VIP), which increases cyclic AMP, and by thyrotropin-releasing hormone (TRH), which increases inositol phosphates (IPx). Analogues of adenosine decreased prolactin release, VIP-stimulated cyclic AMP accumulation and TRH-stimulated inositol phospholipid hydrolysis and IPx generation. Inhibition of InsP3 production by R-N6-phenylisopropyladenosine (R-PIA) was rapid (15 s) and was not affected by the addition of forskolin or the removal of external Ca2+. Addition of adenosine deaminase or the potent adenosine-receptor antagonist, BW-A1433U, enhanced the accumulation of cyclic AMP by VIP, indicating that endogenously produced adenosine tonically inhibits adenylate cyclase. The potency order of adenosine analogues for inhibition of cyclic AMP and IPx responses (measured in the presence of adenosine deaminase) was N6-cyclopentyladenosine greater than R-PIA greater than 5'-N-ethylcarboxamidoadenosine. This rank order indicates that inhibitions of both cyclic AMP and InsP3 production are mediated by adenosine A1 receptors. Responses to R-PIA were blocked by BW-A1433U (1 microM) or by pretreatment of cells with pertussis toxin. A greater amount of toxin was required to eliminate the effect of R-PIA on inositol phosphate than on cyclic AMP accumulation. These data indicate that adenosine, in addition to inhibiting cyclic AMP accumulation, decreases IPx production in GH3 cells, possibly by directly inhibiting phosphoinositide hydrolysis.     

生长激素/胰岛素样生长因子1与阿尔茨海默病

生长激素／胰岛素样生长因子-1（GH／IGF-1）轴的合成、分泌、调节及生物学活性与阿尔茨海默病（AD）有密切关系。生长激素（GH）的合成和分泌受生长激素释放激素（GHRH）正向调节。GH／IGF-1轴活性下降导致一系列生理功能变化。GH／IGF-1缺乏可引起衰老及神经退行性变（AD）而导致认知功能的下降,相应激素的补给可以抑制或逆转这种认知障碍。越来越多的证据表明：GH／IGF-1参与AD型痴呆病理过程,对AD有很好的治疗应用前景。本文就生长激素／胰岛素样生长因子1在AD发病中的机理和药理学研究做一综述。