GHRH-induced Growth Hormone Release Research Articles

Effect of Melatonin Injected into the Third Ventricle on Growth Hormone Secretion in Holstein Steers

The effects of melatonin (MEL) injection into the third ventricle (3V) on growth hormone (GH) secretion were investigated in conscious Holstein steers. A stainless steel cannula was stereotaxically implanted in the 3V based on the ventriculogram. In Exp. 1, three doses of MEL (100, 300 or 600 microg) were injected into the 3V through the cannula and the GH concentration after the injection was determined. In Exp. 2, intracerebroventricular (icv) and intravenous (iv) injections of MEL (100 microg) and GH-releasing hormone (GHRH; 0.25 microg/kg body weight), respectively, were performed simultaneously to examine the effect of MEL on GHRH-induced GH release. The icv injection of MEL significantly stimulated GH release at 100 microg. The increase in GH concentrations by 100 microg of MEL was persistent. Intravenous injection of GHRH dramatically increased GH release. The injection of MEL did not alter GHRH-induced GH release. These results suggest that MEL stimulates GH secretion possibly through the hypothalamus in cattle.

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 10 mM) 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.

Rapid suppressing action of insulin-like growth factor-I (IGF-I) on GH release from anterior pituitary cells of goats

Goat anterior pituitary cells were cultured to investigate the effects of insulin-like growth factor-I (IGF-I), insulin, and growth hormone (GH) on basal and GH-releasing hormone (GHRH)-induced GH release. Changes in cellular Ca 2+ concentrations were also assessed to enable discussion of the cellular mechanisms of IGF-I. The cells were cultured for 48 h, and then stimulated with GHRH (10 nmol/l) for 30 min, with or without each test substance. In the control cells, IGF-I (10 and 100 ng/ml) significantly raised the basal, but did not change GHRH-induced GH release, resulting in the abolishment of GH release induced by GHRH in the presence of 100 ng/ml IGF-I. However, there was no significant effect of insulin (10, 100, and 1000 μU/ml) on basal and GHRH-induced GH release. In the cells cultured for 48 h with each test substance but stimulated for 30 min without the test substance, no significant change in the basal and GHRH-stimulated GH release was observed. Regardless of treatment, there was no significant effect on intra-cellular GH content. Analysis with a confocal laser microscope revealed that IGF-I (100 ng/ml) significantly increased the basal, but significantly reduced GHRH (10 nmol/l)-induced increase in cellular Ca 2+ concentrations. From these findings we conclude that IGF-I exerts an acute suppressing action on the GHRH-induced GH release, which partly involves changes in cellular Ca 2+ metabolism in goat somatotrophs.

Effect of withdrawal of somatostatin plus growth hormone (GH)-releasing hormone as a stimulus of GH secretion in Cushing's syndrome.

Somatostatin (SS) may not merely be inhibitory to GH secretion but, under appropriate temporal conditions, may act in a paradoxically positive manner to sensitize somatotroph responsiveness to GHRH. SS infusion withdrawal (SSIW) produces a rebound GH rise in humans and increases GHRH-induced GH release. Theoretically SSIW leaves the somatotroph cell in a situation of low endogenous SS. In Cushing's syndrome, GH secretion appears blunted to all stimuli. The mechanisms by which glucocorticoids impair GH secretion in Cushing's syndrome are unknown. There are no data evaluating GH responsiveness to SSIW plus GHRH in Cushing's syndrome patients. The aim of the present study was to evaluate the GH response to SSIW plus GHRH in a group of Cushing's syndrome patients, in order to further understand the deranged GH secretory mechanisms in Cushing's syndrome. Eight female patients with Cushing's syndrome were studied. As a control group, eight normal subjects of similar age and sex were studied. Three tests were done. On one day, SS intravenous (i.v.) infusion (500 micro g for 0-90 min) was performed followed by placebo i.v. bolus at min 90 after SS withdrawal (SSIW). On another day, SS i.v. infusion (500 micro g for 0-90 min) was performed followed by GHRH (100 micro g) i.v. bolus at min 90 after SS withdrawal. On a third day, slow infusion of 150 mmol/l NaCl administration was performed followed by GHRH (100 micro g) i.v. bolus at min 90 after the start of the infusion. Blood samples were taken at appropriate intervals for determination of GH. GHRH-induced GH secretion in normal subjects showed a mean peak of 15.4 +/- 2.1 micro g/l (conversion factor: 1 micro g/l = 1.2 mUI/l). Normal control subjects had a mean peak of 3.3 +/- 1.6 micro g/l after SSIW-induced GH secretion. When GHRH was administered after SSIW there was increased GH secretion with a mean peak of 23.7 +/- 4.2 micro g/l significantly greater than the response after SSIW alone (P < 0.05) and GHRH alone (P < 0.05). The patients with Cushing's syndrome had a blunted GH response after GHRH administration with a mean peak of 1.4 +/- 0.4. After SSIW, Cushing's syndrome patients had a mean peak of 1.0 +/- 0.5 micro g/l. When GHRH was administered after SSIW there was a similar GH response with a mean peak of 1.7 +/- 0.6 micro g/l, not statistically different than the response after SSIW alone (P = ns) and GHRH alone (P = ns). When we compare the response of normal subjects and Cushing's syndrome patients, after SSIW plus GHRH, it was decreased in Cushing's syndrome patients (P < 0.05), with a mean GH peak of 23.7 +/- 4.2 micro g/l and 1.7 +/- 0.6 micro g/l for normal subjects and Cushing's syndrome patients, respectively. This study has demonstrated a significantly blunted peak GH response to somatostatin infusion withdrawal plus GHRH in Cushing's syndrome patients. In this theoretical situation of decreased somatostatinergic tone there is persistence of GH hyposecretion in Cushing's syndrome, suggesting the existence of a pituitary defect responsible for the decreased GH secretion in Cushing's syndrome.

Short-Chain Fatty Acids Inhibit the Release and Content of Growth Hormone in Anterior Pituitary Cells of the Goat

The effects of short-chain fatty acids (SCFA: acetate, propionate, and butyrate) on growth hormone (GH)-releasing hormone (GHRH)-induced GH secretion from pituitary somatotrophs were assessed on isolated anterior pituitary cells of goats. Cells were cultured in Dulbecco's modified Eagle's medium for 3 days, either in the presence (1, 3, or 10 mM) or in the absence of each SCFA, and then stimulated with GHRH (10−12 to 10−7 M) for 30 min, again in the presence of and at the concentration of SCFA used over the previous 3 days. In the cells cultured in the absence of SCFA, the addition of SCFA to the medium during the GHRH stimulation period did not significantly change GHRH-induced GH release. However, in cells cultured in the presence of either propionate (3 or 10 mM) or butyrate (1, 3, or 10 mM), the addition of SCFA to the medium during GHRH stimulation significantly reduced the GHRH-induced GH release. The inhibitory effects of SCFA were dependent on the concentrations of SCFA and were greater for butyrate than for propionate. In the cells cultured in the presence of butyrate, but not in the absence, the total GH production (the sum of the released GH and the remaining GH after stimulation) was also significantly reduced. The GHmRNA expression was reduced in the cells cultured with 10 mM butyrate, whereas it was enhanced by the stimulation with 10−7 M GHRH. These findings suggest that propionate and butyrate may inhibit GHRH-induced GH release and GH production by caprine anterior pituitary cells.

Suppressing effects of short-chain fatty acids on growth hormone (GH)-releasing hormone-induced GH release in isolated anterior pituitary cells of goats☆

The involvement of tetrodotoxin-sensitive Na + channels and receptor-operated nonspecific Ca 2+ channels, and the effects of short-chain fatty acids, on growth hormone (GH) release induced by GH-releasing hormone (GHRH) were investigated in cultured and freshly isolated caprine anterior pituitary cells. In 3-d cultured cells in Dulbecco’s modified Eagle’s medium, an increase in GH release induced by GHRH (10 nmol/l) was moderately, but significantly, reduced by a voltage-sensitive Na + channel antagonist tetrodotoxin (1 μmol/l). The GHRH-induced GH increase, which was not affected by a simultaneous addition of a receptor-operated nonspecific Ca 2+ channel antagonist tetramethrine (0.1 mmol/l), was significantly reduced by a voltage-sensitive L-type Ca 2+ channel antagonist nifedipine (1 μmol/l). Propionate and butyrate at 10 mmol/l, however, not only suppressed basal GH release but also significantly reduced the GH increase induced by 10 nmol/l of GHRH. The inhibitory action of these acids was also reproduced by an addition of β-hydroxy butyrate (10 mmol/l) and octanoate (10 mmol/l). In freshly isolated and perifused cells, butyrate (10 mmol/l) as well as somatostatin (100 nmol/l) significantly reduced the GH increase induced by GHRH. From these findings we conclude that tetrodotoxin-sensitive Na + channels and voltage-dependent L-type Ca 2+ channels are involved in the cellular mechanism for GHRH-induced GH release, and that short-chain fatty acids such as propionate and butyrate have a direct action on somatotrophs to reduce basal and GHRH-induced GH release, in caprine somatotrophs.

Effects of beta-adrenergic agonists and antagonists on the growth hormone response to growth hormone-releasing hormone in anorexia nervosa

In anorexia nervosa (AN), growth hormone (GH) hypersecretion and low insulin-like growth factor I (IGF-I) levels are present. It is unclear whether this is due to a peripheral GH resistance and a reduced IGF-I negative feedback on GH secretion or to a primary hypothalamic dysfunction. In AN, in contrast to normal subjects, cholinergic antagonists and agonists, whose action is somatostatin (SS)-mediated, have reduced and absent effects on the GH response to growth hormone-releasing hormone (GHRH). Since arginine, another substance acting via inhibition of SS, maintains its potentiating effect on GH secretion in AN, it has been hypothesized that somewhat specific alteration of the SS-mediated cholinergic influence may be present in this condition. To further clarify the neural control of AH secretion in AN, we evaluated the effects of beta-adrenergic agonists and antagonists, which are known to inhibit and increase, respectively, the GHRH-induced GH secretion in normal subjects. We studied the effect of atenolol (ATE), a beta 1-adrenergic antagonist, and salbutamol (SALB), a beta 2-adrenergic agonist, on the GHRH-induced GH release in 10 patients with AN and in 10 normal age-matched women (NW). Basal GH levels were higher, whereas IGF-I were lower in AN than in NW. The GHRH-induced GH rise in AN was higher than that in NW. ATE significantly enhanced the GH response to GHRH in NW, but not in AN. The GH responses to GHRH after ATE pretreatment were similar in NW and in AN. The GH response to GHRH was inhibited by SALB in both NW and AN. The GH responses to GHRH after SALB pretreatment were similar in NW and AN. These data reveal an exaggerated somatotrope responsiveness to GHRH in AN that is not further increased by beta-adrenergic blockade, while is abolished by beta-adrenergic activation. This suggests that an impairment of beta-adrenergic influence on GH secretion is present in anorexia nervosa.

Enhanced growth hormone (GH) responsiveness to GH-releasing hormone after dietary restriction in patients with Cushing's syndrome.

In patients with Cushing's syndrome, decreased growth hormone (GH) secretion is observed although the basic mechanism is not yet understood. A short-term hypocaloric diet is known to increase both spontaneous and GHRH-stimulated GH secretion in normal subjects. In order to gain further insight into the altered GH secretion in patients with Cushing's syndrome, we assessed the effect of a short-term hypocaloric diet on GH responses to GHRH in these patients. Two GHRH tests (1 microgram/kg i.v.) were performed, the first under basal conditions (normocaloric diet) and the second after a 3-day hypocaloric diet (650 cal/day). Six female patients with untreated Cushing's disease. Plasma GH levels were measured by immunoradiometric assay. GHRH-induced GH release was impaired in patients with Cushing's disease on a normal diet (mean peak 12.4 +/- 6.4 mU/l, area under the curve (AUC) 744 +/- 332 mU/l/120 min). Following a hypocaloric diet, GH responses to GHRH were markedly enhanced in the same group of patients (mean peak 46.2 +/- 14.8 mU/l, AUC 3142 +/- 1032 mU/l/120 min, P < 0.05). This study demonstrates that in patients with Cushing's disease the somatotroph hyporesponsiveness to growth hormone releasing home is improved after a short-term hypocaloric diet. Therefore, blunted growth-hormone secretion in chronic hypercortisolism is a potentially reversible state and the secretory capacity of the somatotroph appears not to be severely compromised in patients with Cushing's disease.

Differential desensitization response of the neonatal and adult rat somatotroph to growth hormone-releasing hormone and phorbol ester

Elevated levels of circulating growth hormone (GH) in the perinatal animal may be caused in part by relative resistance to the desensitizing effects of GH secretagogues. We compared the effects of 4-day exposure of primary pituitary cell cultures from adult male and 2-day-old rats to GH-releasing hormone (GHRH; 10 nM) or 12- O-tetradecanoyl-phorbol-13-acetate (TPA; 1 μM) on subsequent acute GH response to these secretagogues. Prolonged exposure to GHRH reduced subsequent GHRH-induced GH release from pituitary cells of both age groups, but the reduction in GH response was significantly less in neonates than adults. In addition, GH secretion from neonatal pituitaries rose progressively during each day of GHRH exposure, to reach levels almost 7 times basal; by contrast, GH secretion from adult pituitaries increased only transiently and then declined. Prolonged exposure to TPA reduced the subsequent GH response to TPA equally in neonates and adults, but differentially affected the GH response to GHRH; TPA exposure reduced the GH response to GHRH in neonates, but not in adults. These data suggest a fundamental difference between the GH regulatory processes of neonatal and adult pituitaries. The ability of the somatotroph to exhibit attenuated GH response on exposure to secretagogue is developmentally regulated, and relative resistance of the immature somatotroph to homologous desensitization by GHRH may contribute to elevated serum GH levels during the perinatal period.

Regulation of hypothalamic somatostatin by glucocorticoids.

Glucocorticoids (GCs) play a key role in the physiology of the hypothalamic-somatotroph axis, since these steroids enhance growth hormone (GH) gene transcription and increase GHRH receptor synthesis. However, GC excess inhibits normal growth in all species studied. This is mainly due to the impaired GH secretion observed during hypercortisolism, a situation in which GH responses to a number of stimuli, including GHRH, are blunted. The inhibitory effect of GCs on GH secretion seems to be dependent on enhanced hypothalamic SS secretion. Since SS release is stimulated by beta-adrenergic agonism we tested the possibility that GC inhibition of GH secretion would depend on increased beta-adrenoceptor activity in SS-producing neurons. The experimental design consisted in evaluating the GH response to GHRH in normal subjects after having induced hypercortisolism, with DEX, and blocking beta-adrenoceptors with propranolol (PRO). Moreover, to investigate the specificity of this mechanism, GHRH-induced GH release was tested after inducing hypercortisolism and enhancing alpha 2-adrenergic or muscarinic cholinergic tone, by giving clonidine (CLO) or pyridostigmine (PD), respectively. As expected, nocturnal DEX administration inhibited the GH response to GHRH. In this situation of hypercortisolism, both PRO and CLO, but not PD, were able to reverse the inhibitory effect of DEX on GHRH-elicited release. However, the potentiating effect of these drugs on the GHRH-induced GH secretion was only observed for PRO. These data confirm that GC excess inhibits GH release by increasing hypothalamic SS secretion, and that the mechanism is mediated by GC-induced enhanced beta-adrenergic responsiveness. Therefore, the defective GHRH secretion observed in chronic hypercortisolism must be a consequence of the continuous blockade that SS excess exerts on GHRH-producing neurons. Our postulate agrees with other data in the literature showing that GCs modulate the secretion of some hypothalamic peptides by changing the responsiveness of the producing neurons from alpha 2-adrenoceptors to that of beta-adrenoceptors.

Inhibitory effect of amylin on growth hormone responsiveness to growth-hormone-releasing hormone in the rat.

The effects of intracerebroventricular (i.c.v.) or intracarotid (i.a.) administration of amylin (AMY) on growth hormone (GH) release induced by GH-releasing hormone (GHRH) were examined in conscious male rats. Amylin (25 ng-5 micrograms/rat, i.c.v. or 10 micrograms/rat, i.a.) was administered 10 min before GHRH (2 micrograms/kg, i.a.). I.c.v. administration of AMY dose-dependently inhibited GH secretion induced by GHRH but when given peripherally, AMY did not modify the GH response to GHRH. Amylin (10(-8)-10(-6) M) had no direct effect on the rat anterior pituitary in vitro either alone or when incubated with GHRH. To characterize the mechanism(s) involved in vivo in the inhibition of GH by AMY, we examined, at first, the effects of AMY on GHRH-induced GH release in rats depleted of somatostatin by pretreatment (4 h before) with cysteamine (300 mg/kg s.c.). The inhibitory activity of AMY on GH secretion elicited by GHRH seems to be independent of hypothalamic somatostatin; in fact, AMY was still active in rats treated with cysteamine. In addition, we examined the effects of i.c.v. AMY administration on clonidine (CLO)-induced GH secretion and on dopamine and noradrenaline content in the brain, since it is known that GHRH is a stimulus sensitive to changes in central catecholaminergic activity. The failure of AMY to affect GH secretion induced by activation of postsynaptic alpha 2-receptors by CLO and the finding that the peptide decreased noradrenaline content in the hypothalamus and striatum, indicates that AMY may inhibit GH release by interfering with the facilitatory influence of the catecholaminergic system on GH secretion.

Blunted GH Response to Growth Hormone-Releasing Hormone (GHRH) Alone or Combined with Arginine in Non-Insulin-Dependent Diabetes Mellitus

An increased spontaneous and stimulated growth hormone (GH) secretion is well documented in insulin-dependent diabetes mellitus. On the contrary, in non-insulin-dependent diabetes mellitus (NIDDM) conflicting results arise from literature. In 14 patients with NIDDM, 7 normal weight (NWD) and 7 obese (OD), we investigated the somatotrope responsiveness to GHRH (1 microgram/kg) alone or combined with arginine (ARG, 0.5 g/kg), which is able to enhance the GH response to GHRH, probably by inhibiting somatostatin release from hypothalamus. Baseline IGF-I, IRI FFA and glucose levels were also determined. Twelve healthy normal subjects (NS) and 12 obese patients (OP) were evaluated as control groups. GH but not IGF-I levels were higher (p < 0.05) in NS than in OP (1.5 +/- 0.5 vs 0.5 +/- 0.2 microgram/l). Insulin levels were higher (p < 0.05) in OP than in NS, NWD and OD (18.7 +/- 1.8 vs 8.7 +/- 0.5, 6.4 +/- 1.9 and 11.8 +/- 1.2 microU/l). FFA were higher (p < 0.05) in NWD. OD and OP than in NS (0.69 +/- 0.04, 0.70 +/- 0.04 and 0.65 +/- 0.06 vs 0.39 +/- 0.03 mmol/l). Plasma glucose was higher (p < 0.05) in diabetic patients than in normal and obese subjects. GH responses to GHRH in NWD, OD and OP were similar (AUC: 221.6 +/- 33.3, 206.0 +/- 35.9 and 177.2 +/- 57.3 micrograms/l/min, respectively) and all lower (p < 0.05) than that in NS (776.7 +/- 206.5 micrograms/l/min). ARG determined a significant increase of GHRH-induced GH release in all groups (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and biological activities of highly potent antagonists of growth hormone-releasing hormone.

In the search for antagonists of human growth hormone-releasing hormone (hGHRH) with high activity, 22 analogs were synthesized by solid-phase methods, purified, and tested biologically. Within the N-terminal sequence of 28 or 29 amino acids of hGHRH, all the analogs contained D-Arg2, Phe(4-Cl)6 (para-chlorophenylalanine), Abu15 (alpha-aminobutyric acid), and Nle27 and most of them had Agm29 (agmatine) substituents. All the peptides, except one, were acylated at the N terminus with different hydrophobic acids--e.g., isobutyric acid (Ibu) or 1-naphthylacetic acid (Nac) in order to study the effect of N-terminal acylation on the antagonistic activity. In the superfused rat pituitary cell system, all the analogs inhibited more powerfully the GHRH-induced growth hormone (GH) release than the standard GHRH antagonist [Ac-Tyr1,D-Arg2]hGHRH-(1-29)NH2. Antagonists [Ibu0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]hGHRH-(1-28) Agm (MZ-4-71), [Nac0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]hGHRH-(1-28) Agm (MZ-4-243), [Nac0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]hGHRH-(1-29) NH2 (MZ-4-169), [Nac0-His1,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]-hGH RH-(1-29)NH2 (MZ-4-181), and [Nac0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27,Asp28]hGH RH-(1-28)Agm (MZ-4-209) inhibited GH release at 3 x 10(-9) M. Among these peptides, MZ-4-243, MZ-4-169, and MZ-4-181 were also long acting in vitro. Antagonist MZ-4-243 inhibited GH release 100 times more powerfully than the standard antagonist and was the most potent in vitro among GHRH antagonists synthesized. Analogs with high inhibitory effects in vitro were also found to have high affinities to rat pituitary GHRH receptors. In experiments in vivo, antagonists [Ibu0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]-hGHRH-(1-28 )Agm (MZ-4-71), [Nac0,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]hGHRH-(1-29) NH2 (MZ-4-169), and [Nac0-His1,D-Arg2,Phe(4-Cl)6,Abu15,Nle27]hGHR H-(1-29)NH2 (MZ-4-181) induced a significantly greater inhibition of GH release than the standard antagonist. In view of their high antagonistic activity and prolonged duration of action, some of these antagonists of GHRH may find clinical applications, including treatment of certain endocrine disorders and insulin-like growth factor I-dependent tumors.

Effect of excitatory amino acid receptor agonists on secretion of growth hormone as assessed by the reverse hemolytic plaque assay.

Recent findings indicate that excitatory amino acids (EAAs) can modulate growth hormone (GH) secretion in several mammalian species in vivo and in vitro. In this study, we examined the effects of EAA receptor antagonists [N-methyl-D,L-aspartate (NMDA), kainic acid, L-glutamate] on GH secretion by the reverse hemolytic plaque assay (RHPA). Anterior pituitary cells of adult male Sprague-Dawley rats were enzymatically dispersed and subjected to RHPA. EAA receptor agonists increased the mean plaque area in a dose-dependent manner: the maximal increase was observed at 10 microM and increased the fraction of somatotrophs forming large plaques. NMDA (10 microM) did not increase the mean plaque area in the presence of the NMDA receptor antagonists 10 microM AP-7 and 10 microM MK-801. Coincubation of kainic acid with the non-NMDA receptor antagonist CNQX blocked the kainic-acid-stimulated increase in GH secretion. The addition of MK-801, AP-7 or CNQX to glutamate caused a partial reduction of the mean plaque area. Ten micromoles per liter glutamate with 10 nM GH-releasing hormone (GHRH) produced an additive effect on GHRH-induced GH release. Somatostatin suppressed the stimulatory action of glutamate. We speculate that glutamate plays a role in the regulation of GH secretion.

Lack of evidence for acute effects of growth hormone-releasing hormone on serum insulin and glucose levels in normal and hypophysectomized rats.

Growth hormone (GH) is diabetogenic but has also been suggested to stimulate insulin release. Our aim was to study whether acute effects of GH on serum insulin and glucose concentrations can be observed when GH release is physiologically induced by GH-releasing hormone (GHRH), which is known to stimulate insulin secretion. We also studied the acute effects of GHRH as such on insulin release in vivo and in vitro. Female Sprague-Dawley rats were hypophysectomized (hypox) at 40 days of age. At day 188, rats received 1 microgram GHRH(1-29)/100 g body weight i.v. In sham-operated rats, GHRH induced a rise in serum GH from 36.5 +/- 1.9 ng/ml (time 0 min) to 560.0 +/- 13.3 ng/ml (5 min;) (n = 6; p < 0.001). In hypox rats, no measurable serum levels of GH were detected either before or after GHRH treatment. Serum insulin was unaffected by GHRH in both groups of animals. Serum glucose was higher in sham operated than in hypox rats (7.3 +/- 0.4 vs. 6.3 +/- 0.1 mM at 0 min; p < 0.01). Serum glucose did not rise following GHRH injection. In isolated rat islets, cultured for 3 days at 11.1 mM glucose, 0.1-10 nM GHRH stimulated basal insulin release at 3 mM glucose. To summarize, GHRH at high concentrations stimulates insulin release in vitro, but neither GHRH, at a concentration where GH secretion is greatly stimulated, nor GHRH-induced GH release, have any acute effects on serum insulin or glucose levels.

Analysis of growth hormone release from rat anterior pituitary cells by reverse hemolytic plaque assay: influence of interleukin-1

Interleukin-1 (IL-1) has been suggested to directly affect pituitary growth hormone (GH) release, although other investigators have failed to observe this effect. We examined the effects of IL-1β on GH secretion from single somatotrophs by means of reverse hemolytic plaque assay (RHPA). Anterior pituitary cells of adult male rats were enzymatically dispersed and subjected to RHPA. IL-1β at 100 pM and 1 nM, increased both the mean plaque area and the fraction of somatotrophs forming large plaques. IL-1β did not increase the mean plaque area in the presence of the IL-1 receptor antagonist (IL-1ra). IL-1β (1 nM) added together with GH-releasing hormone (GHRH; 10 nM), showed no additive effect on GHRH-induced GH release. The stimulatory action of IL-1β on the release of GH was suppressed by somatostatin. In conclusion, our data show that IL-1β stimulates GH-secretion through direct action on the pituitary.

Pyridostigmine potentiates growth hormone (GH)-releasing hormone-induced GH release in both men and women.

It has been recently reported that pyridostigmine (PD), an indirect cholinergic agonist, probably acting via inhibition of hypothalamic somatostatin, potentiates the GH-releasing hormone (GHRH)-induced GH rise in men, but not in women. The aim of this study was to verify the sex-related, if any, GH response to GHRH (1 microgram/kg, i.v., as a bolus) both alone and preceded by two different doses of PD (120 mg, group A, and 60 mg, group B, given orally 60 min before GHRH) in a large group of volunteers (36 women, aged 18-35 yr, and 48 men, aged 18-35 yrs). In group A, 120 mg oral PD potentiated the GH response to GHRH in both men [area under the curve (AUC), 2579.3 +/- 264.5 vs. 806.2 +/- 99.7 micrograms/L.h; P < 0.00001] and women (AUC, 2273.2 +/- 248.7 vs. 792.6 +/- 72.7 micrograms/L.h; P < 0.00001). Similarly, in the group B, 60 mg oral PD potentiated the GH response to GHRH in both men (AUC, 1929.6 +/- 157.2 vs. 568.2 +/- 81.3 micrograms/L.h; P < 0.01) and in women (AUC, 1655.9 +/- 146.9 vs. 738.2 +/- 105.7 micrograms/L.h; P < 0.01). The GH responses to GHRH, both alone and after 120 and 60 mg oral PD, did not significantly differ in men and women. No sex-related difference was observed in the cholinergic side-effects (mild abdominal pain and muscle fasciculations) that occurred in nearly 30% of the subjects. In conclusion, our results clearly show that there is no sex-related difference in the potentiating effect of PD on GHRH-induced GH release, ruling out the suggestion that women have increased cholinergic activity, leading to reduced somatostatinergic tone.

GH secretion status in myotonic dystrophy

Frequent endocrine alterations and abnormal growth hormone (GH) secretion have been reported in myotonic dystrophy (MD). To evaluate GH secretion status in MD, GH response to 100 μg of growth hormone releasing hormone (GHRH) with or without pyridostigmine pretreatment and its relation with insulin-induced hypoglycemia was investigated in MD patients and compared with normal controls. The mean peak plasma GH response to GHRH was 27.8 ± 19.2 μg/l normal subjects and 11.4 ± 8.7 μg/l in MD patients. In five of seven patients GH reached a mean peak of 12.6 ± 4.2 μg/l after insulin-induced hypoglycemia, compared with 5 ± 2.8 μg/l after GHRH. Conversely, in two patients GH reached a peak of 16.1 and 32 μg/l after GHRH, and only 2.5 and 5.3, respectively, after hypoglycemia. Pretreatment with pyridostigmine in nine patients tested potentiated GHRH-induced GH release with a peak of 17.6 ± 12.5 μg/l, compared with 10.05 ± 6.7 μg/l after GHRH alone; IGF-I levels were normal in all patients.

Effect of Enhancement of Cholinergic Tone on the Growth Hormone Response to Acute Hyperglycaemia or Thyrotropin-Releasing Hormone in Dogs

The effect of enhancement of cholinergic tone by pyridostigmine on the growth hormone (GH) response to thyrotropin-releasing hormone (TRH) or glucose-induced acute hyperglycaemia was tested in six adult unanaesthetized beagle dogs. Both TRH (5μg/ kg iv) and glucose (2 g/kg orally) did not significantly alter baseline GH levels but reduced the GH response to GH-releasing hormone (GHRH) (2 μg/kg iv), although this effect was more clear-cut with TRH than with glucose. Pretreatment with pyridostigmine (2 mg/kg orally) counteracted the inhibitory effect of hyperglycaemia on the GHRH-induced GH release, but had no effect on the inhibition induced by TRH. In summary, these results indicate that: 1) acute hyperglycaemia and TRH play an inhibitory role on GHRH-stimulated GH secretion in dogs; 2) the inhibitory effect of acute hyperglycaemia is mediated via hypothalamic cholinergic neurotransmission, whereas other neurotransmitter pathways would be. involved in the effect of TRH.

Effects of sex and age on pyridostigmine potentiation of growth hormone-releasing hormone-induced growth hormone release.

Previous studies have shown that pyridostigmine (PD) is capable of increasing the growth hormone (GH) response to GH-releasing hormone (GHRH) in young healthy subjects. In order to investigate the influence of age and sex on the PD potentiation of GHRH-induced GH release, we have studied the GH response to GHRH (50 micrograms i.v.) 1 h after oral administration of placebo or PD (60 mg) in 8 young healthy men (aged 19-28 years) and 8 age-matched young women (aged 18-25 years) during the follicular phase of the menstrual cycle, as well as in 8 postmenopausal women (aged 57-62 years) and 8 age-matched elderly men (aged 56-64 years). In the same subjects the effect of PD alone (60 mg p.o.) was also studied. Furthermore, in 6 postmenopausal women and 6 elderly men, the effect of a 30-mg PD oral dose on GH secretion and GH response to GHRH was evaluated with a similar protocol. The GH responses (mean +/- SE) to GHRH + placebo were similar in young men (peak 20.1 +/- 2 ng/ml, AUC 1,250 +/- 113 ng/ml/min) and women (peak 29.3 +/- 2.3 ng/ml, AUC 1,769 +/- 305 ng/ml/min). PD 60 mg was capable of significantly increasing the GH response to GHRH in young men (peak 43.5 +/- 5.1 ng/ml, AUC 3,734 +/- 472 ng/ml/min, p less than 0.005) but not in women (peak 39 +/- 2.3 ng/ml, AUC 2,479 +/- 205 ng/ml/min).(ABSTRACT TRUNCATED AT 250 WORDS)