Pyridostigmine blocks the inhibitory effect of glucocorticoids on growth hormone-releasing hormone stimulated growth hormone secretion in normal man

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

Growth hormone deficiency in treated acromegaly and active Cushing's syndrome

Growth hormone deficiency (GHD) in adults is characterized by reduced quality of life and physical fitness, skeletal fragility, increased weight and cardiovascular risk. It may be found in (over-) treated acromegaly as well as in active Cushing's syndrome. Hypopituitarism may develop in patients after definitive treatment of acromegaly, although the exact prevalence of GHD in this population is still uncertain because of limited awareness, and scarce and conflicting data so far available. Because GHD associated with acromegaly and Cushing's syndrome may yield adverse consequences on similar target systems, the final outcomes of some complications of both acromegaly and Cushing's syndrome may be further affected by the occurrence of GHD. It is still largely unknown, however, whether GHD in patients with post-acromegaly or active Cushing's syndrome (e.g. pharmacologic glucocorticoid treatment) may benefit from GH replacement. We review the diagnostic, clinical and therapeutic aspects of GHD in adults treated for acromegaly and in those with active Cushing's syndrome.

Glucocorticoids and the regulation of growth hormone secretion

Glucocorticoids modulate the secretion of growth hormone (GH) by various and competing effects on the hypothalamus and pituitary gland. The final effects of this modulation depend on hormone concentrations and the duration of exposure. The traditional hypothesis is that chronically raised levels of glucocorticoids suppress the secretion of GH. However, a functional impairment of the GH reserve might also be observed in patients with low levels of glucocorticoids, such as those with secondary hypoadrenalism, which is consistent with the model of biphasic dose-dependent effects of glucocorticoids on the somatotropic axis. This Review updates our current understanding of the mechanisms underlying the effects of glucocorticoids on the secretion of GH and the clinical implications of the dual action of glucocorticoids on the GH reserve in humans. This Review will also address the potential diagnostic and therapeutic implications of GH for patients with a deficiency or excess of glucocorticoids.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.

Growth hormone response to growth hormone-releasing hormone is reduced in adult asthmatic patients receiving long-term inhaled corticosteroid treatment

BACKGROUND

Some studies have demonstrated that the function of the growth hormone (GH)-insulin-like growth factor (IGF)-1 axis is significantly impaired in patients with oral corticosteroid (CS)-induced osteoporosis. The aim of study was to investigate the effects of long-term therapy with inhaled CSs (ICSs) on the hypothalamic-pituitary-GH axis by the GH response to GH-releasing hormone (GHRH), as well as bone turnover, in adult asthmatic patients.

DESIGN

Cross-sectional study.

PATIENTS

Twenty-seven adult subjects with mild-to-moderate persistent asthma (long-term ICS therapy [ie, > 1 year], 20 patients; naive to ICS treatment, 7 patients) and 10 control subjects.

MEASUREMENTS

Each subject underwent testing with an IV bolus (1 mug/kg) injection of human GHRH, and samples of GH were taken 15 min before the GHRH injection, at 0 min (ie, at the time of GHRH injection), and at 15, 30, 45, 60, and 90 min after injection to obtain values for peak GH and DeltaGH. At baseline, samples of serum IGF-1 and blood-urine were collected for bone turnover markers.

RESULTS

The GH response to GHRH was significantly reduced in asthmatic patients receiving ICSs (peak GH, p < 0.05; and DeltaGH, p < 0.01) in comparison with control subjects and asthmatic patients who were naive to ICS therapy (peak GH and DeltaGH, p < 0.01). Baseline IGF-1 levels were similar in the three groups. Serum osteocalcin, a marker of bone formation, was significantly reduced (p < 0.01) and correlated with GH peak (r(2) = 0.34; p = 0.007) in asthmatic patients who were treated with ICSs.

CONCLUSIONS

We conclude that GH secretion in response to GHRH is significantly reduced in adult asthmatic patients receiving therapy with ICS and that such inhibition could play a negative role in bone metabolism.

Characterization of new selective somatostatin receptor subtype-2 (sst2) antagonists, BIM-23627 and BIM-23454. Effects of BIM-23627 on GH release in anesthetized male rats after short-term high-dose dexamethasone treatment

We here report a pharmacological characterization of two new somatostatin (SS) receptor subtype-2 (sst2) selective antagonists by evaluating their GH-releasing activity when administered, by different routes, in anesthetized adult rats and in freely moving 10-d-old rats. Moreover, we describe the effect of these SS antagonists on the GH response to GHRH after short-term high-dose dexamethasone (DEX) treatment in young male rats. BIM-23454 and BIM-23627, given iv, were able to counteract the SS-induced inhibition of GH secretion occurring after urethane anesthesia in a dose-dependent manner. In DEX-treated animals, the GH response to GHRH was partially blunted (5-min peak values, 270 +/- 50 ng/ml in saline-treated vs. 160 +/- 10 ng/ml in DEX-treated, P < 0.05); however, the simultaneous administration of BIM-23627 (0.2 mg/kg, iv) restored higher amplitude GH pulse, leading to a significantly higher overall mean GH response (area under the curve, 4200 +/- 120 ng/ml/30 min vs. 2800 +/- 100 ng/ml/30 min after GHRH alone; P < 0.05). The SS antagonists showed a reduced GH-releasing effect when administered sc or ip, likely attributable to decreased bioavailability, as compared with the iv route. SS antagonist administration also increased plasma glucagon, insulin, and glucose levels. Based on prior reports that sst2 tonically suppresses glucagon secretion, the antagonist most likely increased glucagon secretion from the pancreatic alpha-cells, with resultant increases in plasma glucose and then insulin.

GH therapy in juvenile chronic arthritis: results of a two-year controlled study on growth and bone

Disturbance of growth frequently occurs in children suffering from juvenile chronic arthritis (JCA). Recognition of growth impairment is important because reduced final height is one of the permanent consequences. The aim of this study was to evaluate the efficacy and safety of human GH (hGH) in growth-retarded prepubertal children with JCA. Thirty-five children were tested for GH deficiency (GHD) and randomly assigned to a study and an untreated control group; five were GH deficient and were part of the GHD group. All received glucocorticoids. The study group was treated with 1 IU/kg BW.wk hGH; the GHD group was given 0.5 IU. During 2 yr of hGH treatment growth velocity and height SD score increased compared with baseline values. There was a marked increase in growth velocity in the treated groups, but also some increase in the control group. Plasma levels of IGF-I and IGF-binding protein-3 increased with GH treatment. These results suggest that hGH might be useful in the treatment of growth impairment in JCA. GH may counteract the adverse effects of glucocorticoid therapy, but its effect is dependent on the disease activity. Long-term controlled studies are needed to determine the risks and benefits of GH therapy in JCA.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Direct effects of corticotrophin-releasing hormone on stimulated growth hormone secretion

OBJECTIVE

This study evaluated the effect of corticotrophin-releasing hormone (CRH) on growth hormone releasing hormone (GHRH)-stimulated growth hormone (GH) release in man.

DESIGN

Six healthy adult volunteers (age 20-35 years) were studied. On different occasions they each received an intravenous bolus of saline, CRH(1-41) (100 micrograms), adrenocorticotrophic hormone (ACTH) [Synacthen (500 ng/m2)] or hydrocortisone (50 mg), followed 30 minutes later by an intravenous bolus of either GHRH-(1-29)-NH2 (1.0 microgram/kg) or saline.

MEASUREMENT

Serum GH concentrations were measured using an immunoradiometric assay, and cortisol concentrations were measured by commercial radioimmunoassay. TSH concentrations were measured using a solid phase immunoradiometric assay kit.

RESULTS

Pretreatment with CRH(1-41) attenuated the GH response to GHRH [saline/GHRH-(1-29)-NH2 20.2 +/- 6.2 mU/l; CRH(1-41)/GHRH-(1-29)-NH2 10.9 +/- 2.8 mU/l (P = 0.01)]. This effect was not due to the rise in ACTH or cortisol induced by CRH(1-41), since pretreatment with either ACTH or hydrocortisone significantly augmented the GH response to GHRH-(1-29)-NH2 in the same subjects [ACTH/GHRH-(1-29)-NH2 30.3 +/- 8.8 mU/l (P = 0.01); hydrocortisone/GHRH-(1-29)-NH2 36.4 +/- 11.2 mU/l (P = 0.02)].

CONCLUSION

Our data suggest that the inhibitory effect of CRH(1-41) on GHRH-(1-29)-NH2-induced GH release is not a result of ACTH or cortisol release but reflects a direct action of CRH on GH secretion, possibly via stimulation of somatostatin release. The acute rise in GH following glucocorticoid administration could be explained in part by a rapid suppression of endogenous CRH.

Cortisol enhances non-REM sleep and growth hormone secretion in elderly subjects

Aging is accompanied by a continuous decline in slow wave sleep (SWS) and in growth hormone (GH) secretion, particularly during the sleeping period. Because short-term pulsatile administration of cortisol increases GH release and SWS in young adults, we wondered whether similar effects can be induced also in elderly men. Hourly injections of cortisol between 1700 and 600 h increased stage 2 and SWS and decreased rapid eye movement sleep. Spectral analysis revealed significant increases in delta and theta power. Cortisol infusions increased the GH secretion prior to sleep onset, but remained largely unchanged during sleep. Thus, sleep EEG and GH release are modulated by cortisol administration in a manner similar to that in young subjects, but to a lesser extent. The stimulatory effect of cortisol on both GH release and SWS points to a mechanism involving glucocorticoid-enhanced production and release of GH-releasing hormone that activates pituitary GH release and simultaneously antagonizes the effects of corticotropin-releasing hormone and somatostatin.

Glutamate decarboxylase autoimmunity and growth hormone secretion in type I diabetes mellitus

Insulin-dependent (type I) diabetic patients are known to have an exaggerated growth hormone (GH) response to GH-releasing hormone (GHRH), which is hypothesized to be due to a decrease in somatostatin tone. The aim of the study was to ascertain the influence of the presence and activity of the autoimmune process involving a key enzyme (glutamic acid decarboxylase [GAD]) in the synthetic pathway of a neurotransmitter regulating somatostatin secretion, ie, gamma-aminobutyric acid (GABA), on the GH response to GHRH alone or combined with an acetylcholinesterase inhibitor, pyridostigmine (PD), in patients with type I diabetes mellitus. Twenty non-obese type I diabetic patients and 17 normal subjects underwent an intravenous (IV) injection of 100 micrograms GHRH(1-29)NH2. Twelve of 20 diabetic subjects and all of the control subjects also underwent a second experimental procedure, administration of 120 mg oral PD 60 minutes before IV injection of 100 micrograms GHRH. Diabetic subjects with serum GAD antibody (GADA) levels more than 3 U (n = 10) showed significantly higher serum GH levels after GHRH injection as compared both with diabetic patients with GADA less than 3 U (n = 10) and with normal controls, whether expressed as absolute or peak values. GH peaks after GHRH were significantly (rs = .46, P < .05) correlated with the level of GADA in the whole population of type I diabetic subjects studied. PD significantly enhanced the GH response to GHRH, in terms of both absolute and peak values, in patients without GADA (n = 6) and in normal subjects. On the contrary, PD failed to enhance the GH response to GHRH in diabetic patients with GADA (n = 6). Our findings suggest that autoimmunity may play a key role in determining the exaggerated GH response to GHRH in type I diabetes mellitus. The mechanism underlying this effect is hypothesized to be the production of antibodies to GAD, a key enzyme in the synthesis of GABA, and in turn a reduced GABAergic stimulatory tone on somatostatin production at the hypothalamic level.

Adrenergic and cholinergic involvement in basal and growth hormone-releasing hormone-stimulated growth hormone secretion in glucocorticoid-treated rats

Glucocorticoids are known to inhibit GH secretion via somatostatin. The aim of our study was to elucidate the involvement of somatostatin in the GH-releasing action of the alpha 2 agonist clonidine and the cholinergic agent pyridostigmine in conscious, freely-moving rats chronically treated with dexamethasone. After seven days of chronic glucocorticoid treatment, animals received an i.v. injection of either saline (1 ml/kg) or clonidine (150 micrograms/kg) or pyridostigmine (100 micrograms/kg) at -15 min. Three blood samples were then drawn (-10 min, -5 min, and 0 min) to assess the GH response to either clonidine or pyridostigmine alone. After the 0 min sample, saline (1 ml/kg) or GNRH (500 ng/kg) was injected i.v. and additional blood samples were drawn from 5 to 30 min. The GH response to clonidine alone or combined with GNRH in rats treated with dexamethasone was significantly lower (p < 0.05) as compared to vehicle-treated rats. The GH response to pyridostigmine alone or combined with GNRH did not significantly differ between vehicle- and dexamethasone-treated rats. These data suggest that in the rat the mechanism of action of clonidine is mainly to stimulate endogenous GNRH secretion, while pyridostigmine appears to predominantly act by decreasing hypothalamic somatostatin.

Hexarelin, a novel GHRP-6 analog, counteracts the inhibitory effect of hydrocortisone on growth hormone secretion in acromegaly

Hexarelin (His-D-2-Methyl-Trp-Ala-Trp-D-Phe-Lys-NH2) is a GHRP-6 analog with the substitution of D-tryptophan with its 2-methyl derivative. The aim of our study was to ascertain whether hexarelin was able to counteract the glucocorticoid-mediated increase in hypothalamic somatostatin tone and consequent inhibition on serum GH levels in acromegalic patients. Ten patients (5 males, 5 females; age range 27-71 years; BMI range 23.3-35 kg/m2) with active acromegaly underwent: 1) hydrocortisone alone: a bolus iv injection of 100 mg hydrocortisone succinate in 2 mL saline, at time -60 followed by a 120 min iv infusion of 250 mg hydrocortisone succinate in 250 mL saline, from -60 to 60 min; 2) hexarelin+hydrocortisone: a bolus iv injection of hexarelin 100 micrograms, 60 min after initiation of a 2-hour hydrocortisone infusion; 3) hexarelin alone: a bolus iv injection of hexarelin at time 0, 60 min after initiation of a 2-hour saline infusion. The mean GH peak, expressed as percent change with respect to baseline level (mean of -75 and -60 minute samples), after hexarelin (1750 +/- 1157%) did not differ significantly with respect to that observed after hexarelin+hydrocortisone (1120 +/- 770%). After hydrocortisone alone the patients showed a mean decrease in GH levels as compared to baseline levels, of 47 +/- 7%. Our data show that the GH response to hexarelin in acromegaly is resistant to the inhibitor action of an acute and sustained elevation of serum cortisol levels. That hexarelin counteracts the glucocorticoid-mediated inhibition of GH secretion supports the hypothesis of an hexarelin-induced decrease in endogenous somatostatin tone.

Growth hormone responses to growth hormone-releasing hormone and clonidine in dogs with Cushing's syndrome

The effects of clonidine, a growth hormone (GH) secretagogue, acting at the hypothalamic level and synthetic GH-releasing hormone (GHRH), a physiological stimulus of somatotrophs, on GH secretion were measured in 11 dogs with Cushing's syndrome. Eight healthy dogs served as controls. After the administration of GHRH the dogs with hyperadrenocorticism had a mean (SEM) GH peak of 11.2 (2.5) ng ml-1 which was significantly lower than the peak of 48.6 (13.4) ng ml-1 observed in the healthy dogs. Similarly, the GH response to clonidine was inhibited in the dogs with hyperadrenocorticism, the mean GH peak being 9.3 (3.3) ng ml-1 compared with 135.6 (43.3) ng ml-1 in the control dogs. No significant difference between the GH responses to GHRH and clonidine was observed in the dogs with Cushing's syndrome, the areas under the response curves being 567.9 (78.2) and 478.0 (102.6) ng.min ml-1, respectively. These results demonstrate that the function of somatotrophs is abnormal in dogs with Cushing's syndrome. There is evidence that the likely action of clonidine in dogs is to inhibit the release of somatostatin and the results therefore suggest that the effect of an excess of glucocorticoid in the dog is probably not mediated through an increase in somatostatin tone.

Effect of GHRP-6 and GHRH on GH secretion in rats following chronic glucocorticoid treatment

The aim of our study was to investigate the effects of His-DTrp-Ala-Trp-Phe-Lys-NH2 (GHRP-6) on baseline and growth hormone-releasing hormone (GHRH) stimulated growth hormone (GH) release in conscious, freely-moving rats receiving chronic glucocorticoid treatment. Animals were treated daily for seven days with either vehicle or dexamethasone (dex, 40 micrograms/day). On the day of experimentation, rats received an i.v. injection of saline or GHRP-6 followed 15 min later by an i.v. injection of saline or rat GHRH. Three doses of GHRP-6 were evaluated, 1 microgram, 4 micrograms and 25 micrograms/kg; one dose of GHRH was evaluated, 500 ng/kg. GHRP-6 increased plasma GH levels over baseline concentrations in a dose-dependent fashion both in vehicle- and dex-treated rats. The GH response to GHRP-6 and GHRH was significantly less in dex-treated rats as compared to vehicle-treated rats. The combined administration of GHRP-6 and GHRH did not result in any change in plasma GH levels which could not be predicted from the administration of either peptide alone. Our results show that GHRP-6 is able to stimulate GH secretion in glucocorticoid-treated rats but it is unable to counteract the glucocorticoid-induced inhibition of GH secretion.

Effect of pyridostigmine on the growth hormone response to growth hormone-releasing hormone in lean and obese type II Diabetic patients

A suppressed growth hormone (GH) response to GH-releasing hormone (GHRH) in both lean and overweight type II diabetics has been reported. Pyridostigmine (PD), an acetylcholinesterase inhibitor, elicits GH secretion when administered alone and enhances the GH response to GHRH in normal subjects. The aim of our study was to evaluate the effect of PD on GHRH-stimulated GH secretion in both lean and obese type II diabetic patients. We studied 16 patients with type II diabetes mellitus (seven lean and nine obese). Eleven nondiabetic subjects (six lean and five obese) served as controls. Each subjects underwent treatment with (1) 120 mg PD orally or (2) 2 tablets of placebo orally, 60 minutes before intravenous (IV) injection of 100 micrograms GHRH-(1-29)NH2. We have found no significant differences in GH responses to GHRH between obese diabetics and obese controls. On the other hand, the absolute GH levels were significantly suppressed in lean type II diabetics compared with lean controls at 15 and 30 minutes after GHRH injection. Obese diabetic subjects had slightly but not significantly decreased GH responses to GHRH+PD compared with obese nondiabetic subjects (8.36 +/- 1.62 v 14.4 +/- 7.62 micrograms/L). Lean type II diabetics showed a blunted GH release after GHRH+PD compared with normal-weight healthy subjects (GH peaks, 15.77 +/- 2.17 v 40.88 +/- 6.17 micrograms/L, P < .05). PD enhanced significantly the GH response to GHRH in obese diabetics, obese controls, and non-obese controls (P < .05), but not in non-obese type II diabetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the combined administration of galanin and clonidine on serum growth hormone levels in normal subjects and in patients under chronic glucocorticoid treatment

Aim of our study was to investigate the effect of clonidine and galanin (alone or in combination) on growth hormone (GH) secretion in normal subjects and in adult patients with increased somatostatin tone due to chronic daily immunosuppressive glucocorticoid treatment. We studied 7 adult patients undergoing long-term (no less than 6 months) immunosuppressive glucocorticoid treatment for non endocrine diseases (4F, 3M; age 49.7 +/- 6.3 years). Six normal adult nonobese subjects (3F, 3M; age 34 +/- 2.7 years) served as controls. All subjects underwent the following three tests in random order: 1) iv infusion of clonidine, 150 micrograms in 10 mL of saline, from time 0 to 10 min; 2) iv infusion of synthetic porcine galanin, 500 micrograms in 100 mL of saline from -15 to 30 min; 3) iv infusion of clonidine from 0 to 10 min combined with synthetic porcine galanin iv infusion from -15 to 30 min. Blood samples for GH assay were taken at -15, 0, 15, 30, 45, 60, 90, 120 min. No significant differences in GH absolute values were observed at any time between the three different tests within each group of subjects. Normal subjects showed significantly (p < 0.05) higher GH peaks and GH absolute values from 15 to 90 min after galanin alone, clonidine alone and clonidine+galanin with respect to the glucocorticoid-treated patients. The absence of any either synergistic or at least additive effect on GH secretion of galanin and clonidine in conditions of both normal and increased somatostatin tone suggests that also in man, as well as in the rat, the action of galanin on the GH axis may be mediated through alpha-adrenergic pathways.

Glucocorticoids and the genesis of depressive illness. A psychobiological model

Abnormalities in the hypothalamic-pituitary-adrenal axis (HPA) have been the most consistently demonstrated biological markers in depressive illness. Numerous other neuroendocrine disturbances have also been described, including blunted clonidine-induced growth hormone release and blunted fenfluramine-induced prolactin release. These disturbances are generally interpreted in terms of monoaminergic receptor dysfunction. The theory presented here suggests that chronic stress which activates the HPA will in certain susceptible people produce changes in central monoamines. The high level of glucocorticoid receptors on such central neurons is postulated as mediating the alterations. Thus monoamine abnormalities, rather than being a core aetiological feature of depression, are seen as secondary to HPA overdrive.

Acute effects of a single administration of dexamethasone on basal and growth hormone-releasing hormone stimulated GH secretion in acromegaly

A single administration of dexamethasone causes both an early stimulatory and a late inhibitory effect on GH secretion in normal subjects.

OBJECTIVE

We investigated the effects of a single administration of dexamethasone on basal and GH-releasing hormone-stimulated GH secretion in eight patients with active acromegaly.

DESIGN

On three different days the patients received 4 mg i.v. dexamethasone, 1 microgram/kg body weight GH-releasing hormone 1-29, or matched placebos in different order.

PATIENTS

Eight subjects with active acromegaly, five of whom had not been treated previously, while the other three had received octreotide therapy which was stopped at least 7 days before testing.

MEASUREMENTS

Serum GH levels were measured in duplicate by a commercially available RIA kit.

RESULTS

Dexamethasone administration caused a significant decline of mean +/- SE GH levels from 51.8 +/- 13.8 to 30.0 +/- 9.2 mU/I at 180 minutes, that was not influenced by placebo administration at 180 minutes. On the contrary, when GH-releasing hormone substituted placebo administration, GH levels increased from 34.0 +/- 9.8 mU/I at 180 minutes to 56.0 +/- 15.6 mU/I at 195 minutes. The GH increase was higher when GH-releasing hormone was given without dexamethasone pretreatment (from 52.4 +/- 13.0 mU/I at 180 minutes to 86.4 +/- 25.4 mU/I at 195 minutes). Analysis of the GH area under the curve confirmed the significant inhibition of GH secretion after dexamethasone administration and the significant reduction of the GH response to GH-releasing hormone in the study with dexamethasone pretreatment.

CONCLUSIONS

At variance with data in normal subjects, acute i.v. administration of dexamethasone inhibits basal GH secretion and partially suppresses the GH response to GH-releasing hormone in acromegaly. Both alterations in the regulatory mechanism of adenomatous cells and perturbations of hypothalamic regulatory influences, induced by the state of chronic GH hypersecretion, are likely explanations of the different response to dexamethasone.

Physiological role of galanin in the regulation of anterior pituitary function in humans

The aim of our study was to elucidate the physiological role of the neuropeptide galanin in the regulation of anterior pituitary function in human subjects. Six healthy men (age range 26-35 yr, body mass index range 20-24 kg/m2) underwent in random order 1) an intravenous bolus injection of growth hormone-releasing hormone (GHRH)-(1-29)-NH2 (100 micrograms) + thyrotropin-releasing hormone (TRH, 200 micrograms) + luteinizing hormone-releasing hormone (LHRH, 100 micrograms) + corticotropin-releasing hormone (CRH, 100 micrograms), and 2) intravenous saline (100 ml) at time 0 plus either human galanin (500 micrograms) in saline (100 ml) or saline (100 ml) from -15 to +30 min. Human galanin determined a significant increase in serum GH (GH peak: 11.3 +/- 2.2 micrograms/l) from both baseline and placebo levels. No significant differences were observed between GH values after galanin and those after GHRH alone (24.3 +/- 5.2 micrograms/l). Human galanin significantly enhanced the GH response to GHRH (peak 49.5 +/- 10 micrograms/l) with respect to either GHRH or galanin alone. Human galanin caused a slight decrease in baseline serum adrenocorticotropic hormone (ACTH; 16.3 +/- 2.4 pg/ml) and cortisol levels (8 +/- 1.5 micrograms/dl). Galanin also determined a slight reduction in both the ACTH (peak 27 +/- 8 pg/ml) and cortisol (peak 13.8 +/- 1.3 micrograms/dl) responses to CRH. Baseline and releasing hormone-stimulated secretions of prolactin, thyroid-stimulating hormone, LH, and follicle-stimulating hormone were not altered by galanin. Our data suggest a physiological role for the neuropeptide galanin in the regulation of GH secretion in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone secretion: the role of glucocorticoids

Glucocorticoids (GCs) modulate the somatotropic axis at a genomic and a non-genomic level. Critical concentrations of steroids not only determine somatotrope differentiation but also enhance growth hormone (GH) gene expression. At a cellular level GH-releasing hormone (GHRH) and somatostatin (SS) are the two principal neuropeptides involved in the release of GH. In vitro data indicates that steroids enhance GH release by altering the affinity and the density of GHRH receptors. In addition, they reduce the sensitivity of the somatotrope to SS and decrease IGF-1 induced negative feedback on GH secretion. The net effect is an enhancement of GH release.

Arginine blocks the inhibitory effect of hydrocortisone on circulating growth hormone levels in patients with acromegaly

In patients with acromegaly, circulating growth hormone (GH) levels and GH responses to GH-releasing hormone (GHRH) are decreased by long-term administration of pharmacological doses of glucocorticoids. The aim of our study was to investigate the acute effects of intravenous (i.v.) infusion of hydrocortisone combined either with saline or arginine infusion on circulating GH levels in acromegaly. We studied five adult patients with acromegaly, two men and three women aged 54.6 +/- 4 years having a body mass index of 25.9 +/- 1.2 kg/m2. On two randomized occasions, patients underwent a bolus i.v. injection of 100 mg hydrocortisone succinate at time 0 followed by a 120-minute i.v. infusion of 250 mg hydrocortisone in 250 mL saline, combined with a 90-minute (from -15 to 75 minutes) i.v. infusion of (1) 60 g arginine hydrochloride in 200 mL saline, or (2) 200 mL saline. In all of the acromegalic patients during the infusion of hydrocortisone alone, serum GH levels clearly decreased (nadir range, 26.4% to 68.1%) with respect to GH levels before hydrocortisone administration (mean of time -15 and 0, basal level), with a nadir between 90 and 180 minutes after the beginning of the infusion. After arginine pretreatment, GH levels were significantly enhanced compared with levels attained with hydrocortisone saline, and they were also significantly increased (peak, 167.5% +/- 27.7%) with respect to basal levels. Our data show that arginine blocks the inhibitory effect of acute and sustained hypercortisolism on circulating GH levels in acromegaly.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of galanin on the growth hormone (GH) response to GH-releasing hormone in patients with Cushing's disease

Attenuated plasma GH secretion during sleep and blunted GH responses to provocative stimuli have been observed in patients with Cushing's disease. Synthetic porcine galanin elicits GH secretion when given alone, and enhances the GH response to GHRH in normal human subjects. The aim of our study was to investigate the effects of galanin on the GH response to GHRH in patients with Cushing's disease. We studied 5 female subjects with untreated active Cushing's disease caused by micro-pituitary adenomas (age 43 +/- 6.7 years; BMI 30 +/- 0.7 kg/m2). Four normal adult females, matched for age and body weight with the patients with Cushing's disease, were studied as controls. Subjects underwent in random order: (1) infusion of synthetic porcine galanin IV, 500 micrograms in 100 mL; (2) infusion of saline, IV, 100 mL. A bolus of human GHRH(1-29)NH2 (Geref, Serono, Italy), 100 micrograms in 1 mL saline, was injected IV at 0 minutes. Patients with Cushing's disease showed blunted GH peaks after GHRH (1.2 +/- 0.4 micrograms/L) during saline infusion, as compared to normal controls (24.6 +/- 4.6 micrograms/L; p < 0.05). During galanin infusion a significantly enhanced GH response to GHRH, as compared with saline infusion, was observed in control subjects (GH peak: 51.4 +/- 9.8 micrograms/L; p < 0.05), but not in patients with Cushing's disease (GH peak: 2.3 +/- 0.6 micrograms/L). GH levels were significantly lower both after saline and after galanin in patients with Cushing's disease as compared to normal controls. Our data demonstrate that galanin is not able to enhance the GH response to GHRH in patients with Cushing's disease. That galanin cannot reverse this effect suggests that the mechanism of action of galanin is not via a decrease in somatostatin release by the hypothalamus.

Effect of galanin on the growth hormone response to growth hormone-releasing hormone in acromegaly

Galanin enhances growth hormone (GH)-releasing hormone (GHRH)-stimulated GH secretion in normal man. In acromegaly, circulating GH levels are increased and the GH response to GHRH may be exaggerated. Galanin has been recently shown to decrease circulating GH levels in acromegaly. The aim of our study was to investigate the effects of galanin on the GH response to GHRH in acromegalic subjects. Five acromegalic patients (three men and two women) and seven healthy adult subjects (five men and two women) were studied. GHRH-induced GH secretion was evaluated during a 40-minute intravenous (IV) infusion of saline (100 mL) or porcine galanin (12.5 micrograms/min in 100 mL saline). In normal subjects, delta GH levels after GHRH+porcine galanin administration (47 +/- 7.5 micrograms/L) were significantly higher in comparison to levels obtained with GHRH+saline (21.7 +/- 3.5 micrograms/L, P < .05). In acromegalic patients, GH responses to GHRH (delta GH, 18.8 +/- 8.6 micrograms/L) were not altered by galanin infusion (delta GH, 17.6 +/- 5 micrograms/L). Our results give the first evidence that the same dose of galanin that induces a significant enhancement of the GH response to GHRH in normal subjects has no effect on the GH response to GHRH in acromegalic patients. It can be hypothesized that galanin may interact at the pituitary level with its own receptors expressed by somatotropes independent of GHRH. Failure of galanin to enhance GH response to GHRH in acromegalic patients could be due to a change in function of the galanin receptor on GH-secreting adenomatous cells.

The role of glucocorticoids in the regulation of Growth Hormone secretion: mechanisms and clinical significance

Glucocorticoids are well known to inhibit growth and GH secretion in humans and animals, yet in vitro these steroids stimulate GH synthesis and secretion. These opposite actions appear to be mediated at different sites. The inhibition involves modulation of hypothalamic somatostatin and the stimulation involves direct actions on the pituitary. Current evidence suggests that the predominant action in vivo is through the inhibitory influences of somatostatin.

Effect of galanin on growth hormone-releasing hormone-stimulated growth hormone secretion in adult patients with nonendocrine diseases on long-term daily glucocorticoid treatment

Glucocorticoids are thought to inhibit growth hormone (GH) secretion through an enhancement of endogenous somatostatin tone. The aim of our study was to evaluate the effect of galanin, a neuropeptide that stimulates GH secretion, on GH-releasing hormone (GHRH)-induced GH secretion in adult patients with nonendocrine diseases who were under daily immunosuppressive glucocorticoid therapy. Six normal subjects (four men, two women) and seven steroid-treated subjects (three men, four women) were studied. GHRH-induced GH secretion was evaluated during a 40-minute intravenous (i.v.) infusion of saline or porcine galanin (12.5 micrograms/min). During saline infusion, steroid-treated patients showed a blunted GH response to GHRH (GH peak, 8.1 +/- 2.8 micrograms/L), as compared with normal subjects (GH peak, 23.8 +/- 3.9 micrograms/L). During galanin infusion, the GH response to GHRH was significantly enhanced (GH peak, 46.6 +/- 9.4 micrograms/L, P less than .05), as compared with saline infusion in normal subjects. In contrast, galanin infusion did not enhance the GH response to GHRH (GH peak, 16.6 +/- 6.5 micrograms/L), as compared with saline infusion in steroid-treated patients. The area under the GH-response curves was also significantly (P less than .05) lower in steroid-treated subjects, as compared with normal subjects. Thus, galanin failed to normalize or enhance the GH response to GHRH in patients treated long-term with glucocorticoids. It can be hypothesized that galanin does not elicit GH secretion by decreasing hypothalamic somatostatin tone.

Effects of pyridostigmine on spontaneous and growth hormone-releasing hormone stimulated growth hormone secretion in children on daily glucocorticoid therapy after liver transplantation

OBJECTIVES

We aimed to investigate both nocturnal spontaneous and morning growth hormone (GH)-releasing hormone (GHRH)-induced GH secretion in children on daily glucocorticoid treatment after liver transplantation and to evaluate the effect of pyridostigmine (an acetylcholinesterase inhibitor thought to reduce hypothalamic somatostatin tone) on GH secretion in these patients.

DESIGN

We performed a randomized, single-blind, cross-over study.

PATIENTS

We studied three male and three female juvenile patients, within a year of orthotopic liver transplantation and under immunosuppressive glucocorticoid therapy (mean dose +/- SEM, 5.92 +/- 0.63 mg/day) and five normal children (four males, one female).

MEASUREMENTS

Both nocturnal spontaneous and morning GHRH-induced GH secretion were evaluated after administration of placebo, 1 tablet p.o., or pyridostigmine, 2 mg/kg p.o.

RESULTS

Spontaneous GH. Placebo: in liver transplanted children nocturnal GH secretion (mean GH level 10.8 +/- 2.0 mU/l) was not significantly different with respect to normal children (mean GH level 12.8 +/- 1.2 mU/l); pyridostigmine: nocturnal GH secretion was significantly increased as compared to placebo in subjects with liver transplantation but not in normal children. GHRH test. Placebo: liver transplanted patients showed a blunted GH response to GHRH with respect to normal children; pyridostigmine: the GH responses to GHRH (P less than 0.05) increased as compared to placebo and did not differ significantly in the two groups.

CONCLUSIONS

Our data suggest a steroid-mediated increase in hypothalamic somatostatin tone in liver transplanted children.

The role of cholinergic tone in modulating the growth hormone response to growth hormone-releasing hormone in normal man

Growth hormone-releasing hormone (GHRH) increases serum GH levels in a dose-dependent manner. Pyridostigmine (PD), an acetylcholinesterase inhibitor, is able to elicit GH secretion when administered alone and to enhance the GH response to GHRH in normal subjects, probably via a decrease in the hypothalamic release of somatostatin. The aim of the present study was to investigate if an enhancement of the cholinergic tone was able to influence the dose-response relationship between GHRH and GH in normal adult subjects. Six healthy adult volunteers underwent 10 experimental protocols. They were: human GHRH (1-29)NH2, 1 micrograms/kg injected as an intravenous (IV) bolus 60 minutes after (a) PD, 120 mg administered orally, or (b) placebo, two tablets administered orally; GHRH, 0.3 micrograms/kg injected as an IV bolus 60 minutes after (c) PD or (d) placebo; GHRH, 0.1 micrograms/kg injected as an IV bolus 60 minutes after (e) PD or (f) placebo; GHRH, 0.01 micrograms/kg injected as an IV bolus 60 minutes after (g) PD or (h) placebo; saline, 1 mL injected as an IV bolus 60 minutes after (i) PD or (l) placebo. The GH response in placebo-treated subjects was similar after 1 microgram/kg and 0.3 microgram/kg GHRH, while the 0.1 microgram/kg dose elicited a lower response. The 0.01 microgram/kg dose of GHRH did not significantly increase GH levels as compared with saline. After PD, the GH responses to GHRH were greatly enhanced at all doses tested: 1.0, 0.3, and 0.1 microgram/kg GHRH all elicited similar GH responses; the GH response to 0.01 microgram/kg GHRH was lower, but was still higher than that observed after saline.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcitonin on GH response to pyridostigmine in combination with hGHRH (1-29) NH2 in normal adult subjects

Studies in man demonstrated that salmon calcitonin (sCT) administration blunts the pituitary GH response to GH-releasing hormone (GHRH). However, the mechanisms underlying this inhibitory action of CT in man are unclear. Pyridostigmine (PD), an acetylcholinesterase inhibitor, is hypothesized to enhance the GH response to GHRH in normal subjects probably via a decrease in the somatostatinergic tone. The aim of the present study was to investigate the mechanism of the inhibitory action of sCT on the GH response to human GHRH (1-29) NH2 by concomitant PD administration in normal humans. The GH response to GHRH was significantly suppressed by prior administration of sCT. Pretreatment of subjects with PD significantly enhanced the GH response to GHRH but did not alter the inhibitory actions of sCT. We conclude that sCT is able to inhibit GHRH-stimulated GH secretion in man without influencing the hypothalamic somatostatinergic tone.