Suppressing actions of butyrate on growth hormone (GH) secretion induced by GH-releasing hormone in rat anterior pituitary cells

We assessed the inhibitory effects of butyrate on the growth hormone (GH) secretion in order to investigate the cellular mechanisms in rat somatotrophs. Isolated anterior pituitary cells were cultured in DMEM for several hours, either in the presence (1, 3, or 10mM) or absence of butyrate, and then stimulated with 10(-7)M GHRH for 30 min, in the presence of butyrate at the concentrations used for the previous culture. The increase in GHRH-induced GH release was significantly reduced in a time-dependent and concentration-dependent manner in the cells previously cultured with butyrate. GH content (the sum of GH released into the medium induced by GHRH stimulation and the GH remaining in the cells after stimulation) was reduced by the culture of cells in the presence of butyrate, which was also inversely dependent on the concentrations used for the culture. Simultaneous addition of an L-type Ca(2+) channel blocker, nifedipine (10 pM), to the medium during 10(-9)M GHRH stimulation significantly reduced the stimulated GH release, which was further significantly decreased by a simultaneous addition of 10 mM butyrate. Butyrate blunted the GHRH (10(-9)M)-induced increase in cellular cyclic AMP and calcium ion concentrations, the activity of protein kinases (A and C), and GHmRNA expression. The expression of mRNA for GPR 41 and 43, known as receptors for short-chain fatty acids, was confirmed in the anterior pituitary cells. These findings suggest that butyrate inhibits GHRH-induced GH release as well as GH production, and the cellular inhibitory actions of butyrate occur in diverse cellular signaling pathways of rat somatotrophs.     

Effect of continuous somatostatin and growth hormone-releasing hormone (GHRH) infusions on the subsequent growth hormone (GH) response to GHRH. Evidence for somatotroph desensitization independent of GH pool depletion

Continuous infusions of growth hormone-releasing hormone (GHRH) attenuate the subsequent growth hormone (GH) response to GHRH. To test whether this phenomenon can occur in the absence of GH pool depletion, we examined the effects of continuous infusions of 10 nM GHRH and of 10 nM somatostatin (SRIH), separately or in combination, on dispersed, perifused rat anterior pituitary cells. Columns of these cells were given either GHRH alone for 5 h, GHRH and SRIH together for 3 h followed by GHRH alone, or SRIH alone for 3 h followed by GHRH or medium. SRIH blunted both basal GH release and the GH response to GHRH, without affecting the subsequent GH responses to GHRH. The GHRH infusions attenuated the subsequent GH response to GHRH, even when GH release was initially prevented by the concurrent infusion of SRIH. Furthermore, the degree of attenuation was similar in the presence or absence of SRIH, suggesting that pool depletion plays little role in the desensitization process under these experimental conditions. The results are consistent with the hypothesis that a short-term infusion of GHRH leads to attenuation of the GH response in rat anterior pituitary cells primarily through receptor effects rather than through GH pool depletion.     

Direct modification of somatotrope function by long-term leptin treatment of primary cultured ovine pituitary cells.

Leptin is produced primarily in adipocytes and regulates body energy balance. A close link between leptin and pituitary hormones, including GH, has been reported. The mechanisms employed by leptin to influence somatotropes are not clear, however. Here we report a direct action of recombinant ovine leptin on primary cultured ovine somatotropes by analyzing the levels of mRNA encoding for GH or the receptors for GHRH (GHRH-R) and GH-releasing peptides (GHRP). Treatment of ovine somatotropes with leptin (10(-7)-10(-9) M) for 1-3 d reduced the mRNA levels encoding GH and GHRH-R, but increased GHRP receptor mRNA levels in a time- and dose-dependent manner. Three-day treatment of cells with leptin decreased the GH response to GHRH stimulation, but the GH response to GHRP-2 stimulation was increased. The combined effect of GHRH and GHRP-2 on GH secretion was not altered by treatment of the cells with leptin. These results demonstrated a direct action of leptin on ovine pituitary cells, leading to a reduced sensitivity of somatotropes to GHRH. It is also suggested that GHRP may be useful to correct the decrease in GHRH-induced GH secretion by leptin.     

Mechanisms involved in the effects of TRH on GHRH-stimulated growth hormone release from ovine and bovine pituitary cells

Incubation of cultured ovine pituitary cells with growth hormone-releasing hormone (GHRH) (10(-12)-10(-7) M) stimulated growth hormone secretion up to 3-fold. At a maximal stimulatory concentration of GHRH (10(-10) M), thyrotropin-releasing hormone (TRH) (10(-7) M) caused an inhibition of growth hormone release to approx. 50% of the response obtained with GHRH alone (during a 15 min incubation period). TRH also caused a small inhibition of the GHRH-stimulated cellular cyclic AMP level but this effect was only significant at a relatively high concentration of GHRH (10(-9) M). Incubation of cultured bovine pituitary cells with GHRH (10(-11)-10(-8) M) plus TRH (10(-7) M) caused a significant stimulation of growth hormone release by up to 40%, compared with the response obtained with GHRH alone (at all concentrations of GHRH). TRH (10(-7) M) had no effect on GHRH (10(-8) M)-stimulated cellular cyclic AMP levels in a partially purified bovine pituitary cell preparation. The effects of varying extracellular [Ca2+] (0.1-10 mM) on intracellular [Ca2+] and on the responsiveness to releasing hormones were also determined using ovine pituitary cells. GHRH (10(-10) M)-stimulated growth hormone release was inhibited when cells were incubated at both high (10 mM) and low (0.1 mM) [Ca2+] (compared with 1 mM or 3 mM Ca2+) with or without TRH (10(-7) M). At 1 mM Ca2+, TRH produced a synergistic effect with GHRH to stimulate growth hormone release. However, at 3 mM Ca2+ TRH inhibited GHRH-stimulated growth hormone release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone regulation in primary fetal and neonatal rat pituitary cell cultures: the role of thyroid hormone

GH is first detectable in the fetal rat pituitary between gestational days 18 and 19. The reasons for the GH surge soon after birth and subsequent postnatal decline to adult levels remain unclear. We therefore determined whether GH gene regulation in the developing pituitary could be distinguished from adult rat somatotroph function. In primary cultures of fetal and neonatal rat pituitary cells, GH secretion was detected by the 20th gestational day. These cells were stimulated by GH-releasing hormone (GHRH), but not by T3 or the morphogen retinoic acid. The stimulatory effect of T3 (0.25 mM) on GH secretion was detected only on the 2nd neonatal day and was similar to that seen in mature rat pituitary cell cultures. GHRH (10 nM) treatment for 24 h caused a 5-fold induction of GH secretion in pituitary cells derived from 2-, 5-, and 12-day-old neonatal rats. The presence or absence of T3 in the culture medium did not alter the response to GHRH. In contrast, only 2-fold induction of GH was observed in adult male pituitary cells during the same time course. Insulin-like growth factor-I (IGF-I; 6.5 nM), the peripheral target hormone for GH, resulted in a modest (20%) attenuation of GH secretion from pituitary cells derived from 20-day-old fetuses. IGF-I, however, produced a 70% reduction in GH levels in adult male pituitary cells grown under similar conditions. The effects of IGF-I on adult pituitary cells grown in T3-depleted medium were blunted. Addition of T3 partially restored the responsiveness of these cells to IGF-I. The results suggest that the high circulating GH levels in the fetal and neonatal rat may be secondary to relative insensitivity of the immature somatotroph to the inhibitory actions of IGF-I in addition to enhanced responsiveness to GHRH compared with the adult rat pituitary. Relative thyroid hormone deficiency in the immature rat may be contributory to this early transient state of pituitary IGF-I resistance.     

Pituitary vasoactive intestinal peptide regulates prolactin secretion in the hypothyroid rat

In this study, we demonstrated that the cell content and basal secretion of vasoactive intestinal peptide (VIP) in primary rat pituitary cell cultures were increased in hypothyroidism. VIP release from hypothyroid pituitary cells in vitro was stimulated by thyrotropin releasing hormone (TRH 10(-8) to 10(-6) M) and growth hormone (GH)-releasing hormone (GHRH 10(-9) to 10(-8) M) but not by corticotropin-releasing hormone or luteinizing hormone-releasing hormone in concentrations up to 10(-6) M. In the presence of anti-VIP antisera, there was a significant decrease in basal prolactin secretion from cultured hypothyroid pituitary cells (p less than 0.005) indicating that VIP exerts a tonic stimulatory effect on prolactin (PRL) secretion. The increment in PRL secretion following TRH was not affected by exposure to anti-VIP indicating that PRL release after TRH is not mediated by VIP at the pituitary level. In contrast to changes in PRL, exposure to anti-VIP had no effect on basal GH secretion, indicating that the PRL changes are hormone specific. Similarly, GHRH-induced GH release was unaffected by VIP immunoneutralization.     

Effects of acute and prolonged glucose excess on growth hormone release by cultured rat anterior pituitary cells.

The aim of this study was to verify whether prolonged exposure of cultured rat anterior pituitary cells to high glucose can alter growth hormone (GH) release and responsiveness to secretagogues. Therefore, we cultured anterior pituitary cells obtained from normal male Sprague-Dawley rats in presence of normal (6 mM) or high (22 mM) glucose concentrations. After 3 days, the acute effects of glucose, growth hormone-releasing factor (GRF), dibutyryl cyclic AMP(db-cAMP) and somatostatin were studied during 2-hour incubations. High glucose did not alter basal GH release from cells cultured in 6 mM glucose. However, basal GH release from cells cultured in 22 mM glucose was moderately higher in the 2-hour incubation (by 46%) than in cells cultured in 6 mM glucose. In contrast, GH stimulation by GRF or db-cAMP was significantly reduced in cells cultured in 22 mM as compared to cells cultured in 6 mM glucose. This inhibitory effect of high glucose on GRF-stimulated GH release was completely reversible after 24 h of exposure of the cultured cells to 6 mM glucose and testing on the 4th day of culture. Finally, GH inhibition by somatostatin was also attenuated in cells cultured with high glucose. We conclude that prolonged exposure to high glucose could act directly at the pituitary level to modulate GH release and responsiveness.     

Effects of hypophysiotropic factors on growth hormone and prolactin secretion from somatotroph adenomas in culture

In an attempt to characterize GH and PRL secretion in acromegaly, the effects of various stimuli on GH and PRL release by cultured pituitary adenoma cells derived from acromegalic patients were studied. In addition, the PRL responses of somatotroph adenoma cells were compared to those of prolactinoma cells. GH-releasing hormone-(1-44) (GHRH) consistently stimulated GH secretion in all 14 somatotroph adenomas studied in a dose-dependent manner. The sensitivity as well as the magnitude of the GH responses to GHRH were highly variable in individual tissues. Somatotroph adenomas that did not respond to dopamine were more sensitive and had greater GH responses to GHRH. In 8 of 9 somatotroph adenomas that concomitantly secreted PRL, the addition of GHRH likewise increased PRL release. Omission of extracellular Ca2+ blocked the stimulatory effect of GHRH on GH and PRL secretion. When cells were coincubated with 0.1 nM somatostatin, GH and PRL secretion induced by 10 nM GHRH were completely blocked in most adenomas. Similarly, coincubation of dopamine resulted in inhibition of GHRH-induced hormone secretion in some adenomas. Addition of TRH to the incubation medium, on the other hand, significantly stimulated GH secretion in 8 of 14 adenomas, while TRH stimulated PRL release in all of the adenomas. Vasoactive intestinal peptide (VIP) and corticotropin-releasing hormone (CRH) produced an increase in GH and PRL secretion in other adenomas. In prolactinoma cells, somatostatin and dopamine unequivocally suppressed PRL secretion; however, other stimuli including GHRH, VIP, and CRF were ineffective. TRH induced a significant increase in PRL secretion in only one prolactinoma. These results suggest that responsiveness to GHRH and somatostatin is preserved in somatotroph adenomas; the responsiveness to GHRH is inversely correlated to that to dopamine; and PRL cells associated with somatotroph adenomas possess characteristics similar to those of GH cells. Further, the GH stimulatory actions of TRH and VIP are different.     

GH-releasing peptide-6 overcomes refractoriness of somatotropes to GHRH after feeding.

After a meal, somatotropes are temporarily refractory to growth hormone-releasing hormone (GHRH), the principal hormone that stimulates secretion of growth hormone (GH). Refractoriness is particularly evident when free access to feed is restricted to a 2-h period each day. GH-releasing peptide-6 (GHRP-6), a synthetic peptide, also stimulates secretion of GH from somatotropes. Because GHRH and GHRP-6 act via different receptors, we hypothesized that GHRP-6 would increase GHRH-induced secretion of GH after feeding. Initially, we determined that intravenous injection of GHRP-6 at 1, 3 and 10 microg/kg body weight (BW) stimulated secretion of GH in a dose-dependent manner. Next, we determined that GHRP-6- and GHRH-induced secretion of GH was lower 1 h after feeding (22.5 and 20 ng/ml respectively) than 1 h before feeding (53.5 and 64.5 ng/ml respectively; pooleds.e.m.=8.5). However, a combination of GHRP-6 at 3 microg/kg BW and GHRH at 0.2 microg/kg BW synergistically induced an equal and massive release of GH before and after feeding that was fivefold greater than GHRH-induced release of GH after feeding. Furthermore, the combination of GHRP-6 and GHRH synergistically increased release of GH from somatotropes cultured in vitro. However, it was not clear if GHRP-6 acted only on somatotropes or also acted at the hypothalamus. Therefore, we wanted to determine if GHRP-6 stimulated secretion of GHRH or inhibited secretion of somatostatin, or both. GHRP-6 stimulated secretion of GHRH from bovine hypothalamic slices, but did not alter secretion of somatostatin. We conclude that GHRP-6 acts at the hypothalamus to stimulate secretion of GHRH, and at somatotropes to restore and enhance the responsiveness of somatotropes to GHRH.     

Sex-related naloxone influence on growth hormone-releasing hormone-induced growth hormone secretion in normal subjects

The effect of opiate-receptor antagonist naloxone on growth hormone (GH) release after growth hormone-releasing hormone (GHRH) 1-44 administration was investigated in ten normal men and 18 normal women during different phases of their menstrual cycle. Naloxone was infused at a rate of 1.6 mg/h in women and 1.6- and 3.2 mg/h in men, starting one hour before GHRH administration (50 micrograms iv as a bolus). On different day sessions, naloxone, GHRH, or saline were administered as controls. Naloxone infusion reduced the GHRH-induced GH release in normal women. The mean % inhibition of peak GH response was 83% during follicular phase, 46.5% during periovulatory phase, and 77.6% during luteal phase. On the contrary, in normal men, both doses of naloxone infusion were ineffective in blunting the GH response to GHRH. Our studies indicate that naloxone infusion was capable of inhibiting GH release induced by direct stimulation with GHRH in normal women, suggesting an opiate-antagonist action at the anterior pituitary level. The absence of such an effect in normal men strongly indicates a sex dependence of naloxone effects and suggests a role of the sexual steroid environment in opioid modulation of pituitary hormone secretion.     

Effects of ghrelin on growth hormone secretion from cultured adenohypophysial cells in cattle

To clarify the direct effects of ghrelin on growth hormone (GH) release from anterior pituitary (AP) cells in cattle, GH-releasing effects of human ghrelin (hGhrelin) and rat ghrelin (rGhrelin) on bovine AP cells were compared with those of GH-releasing hormone (GHRH) in vitro. The AP cells were obtained from Holstein steers and were incubated for 2 h with the peptides after incubating in DMEM for 3 days. hGhrelin and rGhrelin significantly stimulated GH release from the cultured cells at doses from 10(-10) to 10(-7) M and from 10(-9) to 10(-7) M, respectively (P<0.05). The rates of increase in GH at 10(-10), 10(-9), 10(-8) and 10(-7) M hGhrelin were 26, 26, 59 and 100% compared with controls, respectively, and those of increase in GH at 10(-9), 10(-8) and 10(-7) M rGhrelin were 58, 74 and 106%, respectively. GHRH significantly increased GH concentrations in cultured media at a dose as low as 10(-13) M compared with the control (P<0.05). When hGhrelin (10(-8) M) and GHRH (10(-8) M) were added together, the release of GH induced by both peptides was significantly greater than that by hGhrelin alone (P<0.05), and tended to be greater than that by GHRH alone. Somatostatin (SS, 10(-7) M) significantly blunted GH release induced by hGhrelin (10(-8) M) and GHRH (10(-8) M) (P<0.05). In the presence of SS, the percent increase in GH released with hGhrelin plus GHRH was 42% and 14% greater than that by either hGhrelin or GHRH alone, respectively (P<0.05). These results show that ghrelin directly stimulates the release of GH from anterior pituitary cells, and that SS modifies ghrelin-stimulated GH release in cattle.     

Atropine blockade of growth hormone (GH)-releasing hormone-induced GH secretion in man is not exerted at pituitary level

The role of acetylcholine (Ach) in the regulation of human GH secretion was assessed using atropine, which selectively blocks cholinergic muscarinic receptors. Paired tests were performed in seven normal subjects using GH-releasing hormone (GHRH) 1-44 (1 microgram/kg iv), with and without atropine pretreatment (1 mg im). The GHRH 1-44-induced GH secretory peak [20.7 +/- 4.5 (SEM) ng/ml] was completely blocked by atropine administration (2.3 +/- 0.6 ng/ml) (P less than 0.01). To determine whether this atropine blockade was at the pituitary level, a series of in vitro studies were conducted using monolayer cultures of cells from bovine anterior pituitary glands. GHRH 1-44 (10(-8) M) stimulated bovine GH release (11.1 +/- 1.5 micrograms/ml) as compared to control values (5.1 +/- 0.4 microgram/ml) (P less than 0.01). This response was not altered by 10(-6) M atropine (14.9 +/- 0.9 microgram/ml). Similar results were obtained with GHRH, 10(-9) M, with or without atropine, 10(-7) M. Addition of 10(-6) M Ach to the incubation medium significantly increased bovine GH release (12.7 +/- 1.2 microgram/ml) and the effect of 10(-6) M Ach and 10(-8) M GHRH was additive (20.9 +/- 2.1 micrograms/ml) (P less than 0.01). Similar results were obtained with Ach, 10(-5) M, and GHRH, 10(-9) M. Atropine or eserine alone did not alter basal GH secretion, and atropine blocked Ach-stimulating activity. In conclusion, atropine blockade of GHRH-induced GH secretion appears to be exerted at a site other than pituitary.     

Differential ontogenetic patterns of in vitro desensitization to GHRH in fetal and neonatal anterior pituitary

The aim of this study was to assess the ontogenetic changes in vitro in both the responsiveness of anterior pituitary tissue to growth hormone-releasing hormone (GHRH) and the critical role of GHRH in the long-term regulation of pulsatile GH secretion during perinatal porcine life. A superfusion system was used to apply three consecutive 10-min pulses of GHRH (the first of 1 nM and the other two of 10 nM) for 3 consecutive days in pituitary glands isolated from fetal (95- and 110-day) and neonatal (12-day) male pigs. In fetuses, total GHRH-induced GH release decreased progressively over the 3 days. However, in neonates, GH did not decrease until day 3, but remained higher than in fetuses. When each GH pulse was assessed individually, fetuses showed a similar pattern. GH secretion induced by the first GHRH pulse on days 1 and 2 was lower than that induced by the second and third pulses. By day 3, GH release lowered dramatically after all pulses. In contrast, in neonates no differences were observed among the GH levels induced by the three GHRH pulses at any day, although day 3 showed lower GH rates. In conclusion, during perinatal development, a desensitizing effect to long-term repetitive GHRH pulses was observed in both fetuses and neonates, but this effect was delayed in neonates. Thus, the capacity of somatotrope cells to maintain GH response to GHRH seems to be developmentally regulated during perinatal stages. Furthermore, the frequency of GHRH pulses, rather than the concentrations, might be a key factor to elicit desensitization.     

The role of pituitary ghrelin in growth hormone (GH) secretion: GH-releasing hormone-dependent regulation of pituitary ghrelin gene expression and peptide content

Ghrelin is a GH-releasing peptide originally purified from the rat stomach. It has been demonstrated that ghrelin expression, within the gastroenteric system, is regulated by both the metabolic and GH milieu. Our laboratory and others have previously reported that ghrelin is also produced in the pituitary. Given that the receptor for ghrelin [GH secretagogue receptor (GHS-R)] is also expressed by the pituitary, the possibility exists that locally produced ghrelin plays an autocrine/paracrine role in regulating GH release. Because we have previously reported that GHRH infusion increases pituitary levels of ghrelin mRNA, we hypothesized that GHRH could be a key regulator of pituitary ghrelin expression. In this report, we demonstrate that 4-h GHRH infusion increased pituitary ghrelin peptide content. Interestingly, under experimental conditions in which hypothalamic GHRH expression is increased, e.g. GH deficiency due to GH gene mutation, glucocorticoid deficiency, and hypothyroidism, we observed that pituitary ghrelin expression (mRNA levels and peptide content) was also increased. Consistent with this positive correlation between GHRH and ghrelin, pituitary ghrelin expression (mRNA levels and peptide content) was found to be decreased in conditions in which hypothalamic GHRH expression is decreased, e.g. GH treatment, glucocorticoid excess, hyperthyroid state, and food deprivation. Collectively, these results suggest that pituitary ghrelin expression is GHRH dependent. We also conducted functional studies to examine whether the pituitary ghrelin/GHS-R system contributes to GH release after GHRH stimulation, by challenging pituitary cell cultures with GHRH in the presence of a GHS-R-specific inhibitor ([d-Lys-3]-GHRP-6). The GHS-R inhibitor did not affect GH release in the absence of GHRH, but significantly reduced GHRH-mediated GH release. This is the first report demonstrating that endogenous pituitary ghrelin can play a physiological role in GH release, by optimizing somatotroph responsiveness to GHRH.     

Acute hyperglycemia and activation of the beta-adrenergic system exhibit synergistic inhibitory actions on growth hormone (GH) releasing hormone-induced GH release

OBJECTIVE: Acute hyperglycemia stimulates somatostatin (SRIH) release by the hypothalamus which, in turn, suppresses growth hormone (GH) secretion from the anterior pituitary gland. Although it has been suggested that the cholinergic pathway mediates glucose-induced SRIH release, other regulatory systems have not been examined. Therefore, we investigated whether blocking or activating the beta-adrenergic pathway alters glucose-mediated inhibition of GH release. DESIGN AND METHODS: One set of experiments was performed with a beta-adrenergic antagonist, propranolol, and the other set with a beta-adrenergic agonist, isoproterenol. Each set of experiments was performed in ten healthy subjects and consisted of four tests. Test 1, a 100 microg GHRH bolus i.v. at 0 min; test 2, 100 g glucose orally at -30 min, followed by a 100 microg GHRH bolus at 0 min; test 3, after a 100 microg GHRH bolus i.v. at 0 min, a continuous infusion of propranolol (0.2 mg/kg) or isoproterenol (0.012 microg/kg) was administered between 0 and 120 min; test 4, after a 100 g glucose oral load at -30 min, and a 100 microg GHRH bolus i.v. at 0 min, a continuous infusion of propranolol (0.2 mg/kg) or isoproterenol (0.012 microg/kg) was administered between 0 and 120 min. Blood was drawn every 10 min from -30 min to 120 min to measure GH and glucose concentrations. RESULTS: Pretreatment with glucose significantly suppressed GHRH-induced GH secretion. Propranolol infusion significantly increased the GHRH-induced GH secretion, but it did not block glucose-induced suppression of GH secretion. Isoproterenol infusion alone significantly suppressed GHRH-induced GH secretion and augmented the inhibitory action of glucose on GH release. CONCLUSION: This study demonstrates that glucose-induced suppression of GHRH-stimulated GH release is independent of beta-adrenergic tone. Since previous data supports a role for SRIH in both glucose and beta-adrenergic suppression of GH release, the current results suggest that subsets of SRIH neurons are differentially responsive to these external cues. Therefore, a combined glucose and isoproterenol test may provide a useful assessment of hypothalamic somatostatinergic activity.     

GLYCOPROTEIN HORMONE ALPHA-SUBUNIT AND PROLACTIN RELEASE BY CULTURED PITUITARY ADENOMA CELLS FROM ACROMEGALIC PATIENTS: CORRELATION WITH GH RELEASE

In-vitro data of pituitary adenoma cells from 28 acromegalic patients were evaluated. In addition to GH, PRL was produced by 16 adenomas (57%) and alpha-subunit by 15 adenomas (54%) while there was a significantly higher incidence of tumours producing PRL and alpha-subunit simultaneously. From 26 pituitary adenomas enough cells were obtained in order to perform secretion studies. Percentage basal hormone release (medium: (medium + intracellular hormone)) x 100% of GH and alpha-subunit by 11 adenomas showed a close correlation while such a correlation for GH and PRL was present only in a subgroup of 10 of 13 adenomas. The responses of GH and alpha-subunit release to 10nM SMS201-995, 10nM bromocriptine, 100 nM TRH and 10nM GHRH were closely related in that a response or an absent response of GH release to the four secretagogues was virtually always attended with a response or an absent response respectively of alpha-subunit release. Such a relationship was less evident with respect to the effects of SMS201-995, bromocriptine. TRH and GHRH on GH and PRL release. We conclude that basal and secretagogue-induced alpha-subunit release by cultured pituitary adenoma cells from acromegalic patients closely follows the pattern of GH release while such a relationship for GH and PRL is present only in a subgroup of the adenomas secreting GH and PRL simultaneously.     

Growth hormone-releasing hormone and gonadotropin-releasing hormone stimulate nitric oxide production in 17beta-estradiol-primed rat anterior pituitary cells

It was reported that neuronal nitric oxide synthase (nNOS) was expressed only in gonadotrophs and folliculo-stellate cells in the anterior lobe of the pituitary gland. However, recent studies have demonstrated the occurrence of nNOS in the somatotrophs and lactotrophs. In the present study, we investigated effects of growth hormone-releasing hormone (GHRH), gonadotropin-releasing hormone (GnRH), and 17β-estradiol on nitric oxide (NO) release in cultured rat anterior pituitary cells in vitro. The NO ₂ ⁻ level in the incubation medium of the rat anterior pituitary cells was dependent on the cell density. Pretreatment with 10 μM 17β-estradiol resulted in an increase in medium NO ₂ ⁻ level. GHRH and GnRH failed to change medium NO ₂ ⁻ levels, but they elicited increases in medium NO ₂ ⁻ levels in estrogen-treated cells. The GHRH-induced increase in NO ₂ ⁻ level was inhibited by Nχ-nitro-l-arginine methyl ester, a NOS inhibitor. These findings suggest that GnRH and GHRH could activate nNOS in the gonadotrophs and the somatotrophs, respectively.     

Normal and growth hormone (GH)-secreting adenomatous human pituitaries release somatostatin and GH-releasing hormone

Neuropeptides such as vasoactive intestinal peptide, LHRH, or TRH have been found in rat pituitary tissue and could act via paracrine or autocrine actions in this tissue. In this study we investigated whether normal human pituitary tissue and GH-secreting human pituitary adenomas could release somatostatin (SRIH) and GHRH. Fragments from three human pituitaries and dispersed cells from six GH-secreting adenomas (four adenomas were studied for GHRH release and five for SRIH release) were perifused using a Krebs-Ringer culture medium, and the perifusion medium was collected every 2 min (1 mL/fraction for 5 h). GH, GHRH, and SRIH were measured by RIA under basal conditions and in the presence of 10(-6) mol/L TRH or SRIH. Both normal pituitaries and GH-secreting pituitary adenomas released SRIH and GHRH. SRIH release commenced 90-180 min after initiation of the perifusion, at which time GH secretion had decreased significantly. TRH stimulated SRIH release from normal pituitary tissue and inhibited SRIH release from adenoma tissue. GHRH was present at the start of the perifusion, but rapidly disappeared. However, SRIH stimulated GHRH release from normal pituitary tissue, but not from adenoma tissue. Significant amounts of GHRH and SRIH were released during the experiments, suggesting their local synthesis. These results indicate that pituitary cells can release hypothalamic peptides. The liberation of these neuropeptides is regulated, and moreover, their regulation differs between normal and adenomatous pituitaries.