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

Neuroendocrine tumors secreting growth hormone-releasing hormone: Pathophysiological and clinical aspects

Hypothalamic GHRH is secreted into the portal system, binds to specific surface receptors of the somatotroph cell and elicits intracellular signals that modulate pituitary GH synthesis and/or secretion. Moreover, GHRH is synthesized and expressed in multiple extrapituitary tissues. Excessive peripheral production of GHRH by a tumor source would therefore be expected to cause somatotroph cell hyperstimulation, increased GH secretion and eventually pituitary acromegaly. Immunoreactive GHRH is present in several tumors, including carcinoid tumors, pancreatic cell tumors, small cell lung cancers, endometrial tumors, adrenal adenomas, and pheochromocytomas which have been reported to secrete GHRH. Acromegaly in these patients, however, is uncommon. The distinction of pituitary vs. extrapituitary acromegaly is extremely important in planning effective management. Regardless of the cause, GH and IGF-1 are invariably elevated and GH levels fail to suppress (<1 microg/l) after an oral glucose load in all forms of acromegaly. Dynamic pituitary tests are not helpful in distinguishing acromegalic patients with pituitary tumors from those harbouring extrapituitary tumors. Plasma GHRH levels are usually elevated in patients with peripheral GHRH-secreting tumors, and are normal or low in patients with pituitary acromegaly. Unique and unexpected clinical features in an acromegalic patient, including respiratory wheezing or dyspnea, facial flushing, peptic ulcers, or renal stones sometimes are helpful in alerting the physician to diagnosing non pituitary endocrine tumors. If no facility to measure plasma GHRH is available, and in the absence of MRI evidence of pituitary adenoma, a CT scan of the thorax and abdominal ultrasound could be performed to exclude with good approximation the possibility of an ectopic GHRH syndrome. Surgical resection of the tumor secreting ectopic GHRH should be the logical approach to a patient with ectopic GHRH syndrome. Standard chemotherapy directed at GHRH-producing carcinoid tumors is generally unsuccessful in controlling the activated GH axis. Somatostatin analogs provide an effective option for medical management of carcinoid patients, especially those with recurrent disease. In fact, long-acting somatostatin analogs may be able to control not only the ectopic hormonal secretion syndrome, but also, in some instances, tumor growth. Therefore, although cytotoxic chemotherapy, pituitary surgery, or irradiation still remain available therapeutic options, long-acting somatostatin analogs are now preferred as a second-line therapy in patients with carcinoid tumors and ectopic GHRH-syndrome.

The reduced release of GH by GHRH in 8 subjects aged 65-69 years is augmented considerably by rivastigmine, a drug for Alzheimer's disease

BACKGROUND

The growth hormone (GH) secretion declines by 14% with each decade of adult life. Several attempts have been made to reverse the manifestations of the senile GH deficiency, termed somatopause, but GH substitution treatment in old age has not yet developed an established regimen. Cholinesterase inhibitors like pyridostigmine are able to elicit GH secretion when administered alone and to enhance the GH response to growth hormone releasing hormone (GHRH), but its clinical use is limited due to the strong peripheral cholinergic side effects.

OBJECTIVE

The aims of our experiments were to find out whether the GH response to GHRH can be augmented by rivastigmine, a new orally applicable and well-tolerated selective inhibitor of cerebral acetylchoinesterase.

METHODS

Eight healthy volunteers (age range 65-69 years) were studied. After an overnight fast, GHRH tests were done: 1 microg/kg GHRH was injected as an intravenous bolus. Blood samples for an immunoradiometric GH assay were taken at the time of GHRH injection (time 0) and after 15, 30, 45, 60, and 120 min. First, the baseline experiment was done: it consisted of two subsequent GHRH tests which were carried out within an interval of 120 min. Four weeks later the rivastigmine experiment was done identically, but 60 min before performing the second GHRH test, rivastigmine (4.5 mg) was administered orally. The GH secretory responses were expressed as areas under the curve (AUC; median, interquartile range), Wilcoxon's signed-rank test was used for statistical comparisons.

RESULTS

Baseline experiment: The GH AUC of the first GHRH test was 1040 (range 420-1250) ng/ml/h. The repeated GHRH stimulation after 120 min (second GHRH test) showed a 13-fold decrease to 80 (range 60-130) ng/ml/h. Rivastigmine experiment: The GH AUC of the first GHRH test was 950 (range 540-1430) ng/ml/h and, therefore, similar to that of the baseline experiment. 60 min after ingestion of the single oral dose of rivastigmine (4.5 mg), the following GHRH stimulation (second GHRH test) nearly doubled the GH AUC to 1580 (range 860-3330) ng/ml/h. Comparing the DeltaGH AUC values (DeltaGH AUC = GH AUC of the first GHRH test minus GH AUC of the second GHRH test), baseline experiment versus rivastigmine experiment, there was a 20-fold (p = 0.018) increase in GH AUC after rivastigmine pretreatment.

CONCLUSIONS

Rivastigmine is a powerful drug to enhance GH release to repeated GHRH stimulation in healthy elderly human subjects. Future investigations are necessary to find out whether rivastigmine can restore the senile decline of the circadian GH secretion in the long term and, therefore, has new implications for patient treatment.

Impaired growth hormone secretion in fibromyalgia patients: evidence for augmented hypothalamic somatostatin tone

OBJECTIVE

To determine whether female fibromyalgia (FM) patients exhibit a normal growth hormone (GH) response to an acute exercise stressor, and to assess the importance of somatostatin tone in the generation of this GH response.

METHODS

Twenty female FM patients were compared with 10 healthy female controls. All subjects exercised to volitional exhaustion on a treadmill. A standard metabolic cart was used to monitor pulse, blood pressure, electrocardiography, oxygen uptake, carbon dioxide output, anaerobic threshold, and maximal workload. Blood was drawn for GH and cortisol measurements 1 hour before exercise, immediately before exercise, immediately after exercise, and 1 hour after exercise. One month later, testing that was exactly similar was performed, except all subjects were given pyridostigmine bromide (Mestinon; 30 mg orally) 1 hour before exercise.

RESULTS

Compared with controls, FM patients failed to exhibit a GH or cortisol response to acute exercise (P = 0.003). After administration of pyridostigmine, 1 hour before exercise, the GH levels of FM patients increased 8-fold (P = 0.001), to a value comparable with that of controls. Pyridostigmine did not increase the cortisol response to exercise in FM patients. Pyridostigmine alone did not stimulate GH secretion in FM patients, nor did it improve exercise-induced GH secretion in controls. FM patients with normal insulin-like growth factor 1 (IGF-1) levels had an impaired GH response to exercise.

CONCLUSION

Three new findings are reported: 1) FM patients have a reduced GH response to exercise, 2) pyridostigmine reverses this impaired response, and 3) defective GH secretion in FM can occur in patients with normal IGF-1 levels. Because pyridostigmine is known to reduce somatostatin tone, it is surmised that the defective GH response to exercise in FM patients probably results from increased levels of somatostatin, a hypothalamic hormone that inhibits GH secretion.

Acute dexamethasone administration enhances GH responsiveness to GH releasing peptide-6 (GHRP-6) in man

OBJECTIVE

Acute administration of glucocorticoids stimulates GH secretion probably by a decrease in hypothalamic somatostatin release. GHRP-6 is a synthetic hexapeptide that increases GH secretion by a mechanism of action not yet fully known, but apparently not by inhibition of hypothalamic somatostatin release. The aim of this study was to evaluate the effect of acute dexamethasone administration on GH responsiveness to GHRP-6 in man.

DESIGN

One group of subjects received iv GHRP-6 (1 microg/kg), GH-releasing hormone (GHRH; 100 microg), GHRH plus GHRP-6 or saline 3.5 h after oral acute dexamethasone administration (4 mg; at 0600 h). A second study group was treated with GHRP-6, GHRH or GHRP-6 plus GHRH after placebo ingestion, following the same protocol.

PATIENTS

Sixteen normal subjects (mean age: 29 +/- 3.3 years), with normal BMI (22.4 +/- 2.0 kg/m2), were studied. Eight subjects received dexamethasone and the other eight were treated with placebo.

MEASUREMENTS

Serum GH was measured by a two site monoclonal antibody immunofluorometric assay.

RESULTS

In the placebo-treated subjects, mean peak GH (mU/l; mean +/- SE) and AUC (mU.min/l) values after GHRP-6 administration (peak: 43.8 +/- 9.0; AUC: 2262.0 +/- 459. 2) did not differ from those observed after GHRH injection (peak: 49. 8 +/- 12.0; AUC: 2903.4 +/- 872.6). The association of the two peptides markedly increased GH levels (peak: 172.4 +/- 34.2; AUC: 10393.0 +/- 1894.8) compared with the isolated administration of GHRP-6 or GHRH. In the subjects who received dexamethasone 3.5 h before saline injection, GH baseline values were significantly higher than those observed after 90 min of sampling (12.4 +/- 9.4 vs. 4.6 +/- 2.0). Mean GH peak and AUC values after GHRP-6 (peak: 78.8 +/- 11.0; AUC: 4114.6 +/- 588.2) and after GHRH administration (peak: 46.8 +/- 16.0; AUC: 3006.8 +/- 1010.0) did not differ significantly in the dexamethasone-treated subjects. In this study group, the administration of the two peptides together caused a significant increase in both peak (119.2 +/- 16.0) and AUC values (7377.0 +/- 937.2) compared with the response obtained after each peptide alone. When the two groups were compared, a significant increase in GH responsiveness to GHRP-6 was observed after dexamethasone administration compared with placebo. No differences in GH response to GHRH, or to the administration of the two peptides together, were seen between the two groups.

CONCLUSIONS

Oral dexamethasone, at a dose of 4 mg, enhances GH releasing peptide-6-induced GH release when administered 3.5 h earlier. These results suggest that dexamethasone and GHRP-6 could act at different sites of GH releasing mechanisms. Further studies are necessary to elucidate these findings.

Age-related blunting of growth hormone secretion during exercise may not be soley due to increased somatostatin tone

Age-related declines in growth hormone (GH) secretion may result from augmented somatostatin (SRIH) tone and/or diminished GH-releasing hormone (GHRH) secretion. We assessed GH release during exercise without and with pyridostigmine (PYR), which indirectly suppresses SRIH. GH levels were measured throughout exercise and recovery in 12 young men (mean +/- SEM, 20.8 +/- 0.4 years) and seven old men (66.1 +/- 1.9). The area under the GH curve (GH-AUC) was greater in young versus old men during a short-term maximal exercise test (12.9 +/- 2.8 v 1.5 +/- 0.2 ng x min(-1) x mL(-1), P = .002) and a 1-hour 60% maximal (submaximal, 10.0 +/- 1.5 v 3.0 +/- 1.0 ng x min(-1) x mL(-1), P = .001) cycle exercise bout. PYR increased the GH-AUC in young and old men during maximal (20.9 +/- 5.2 v 4.9 +/- 1.8) and submaximal (12.3 +/- 1.6 v 4.7 +/- 1.5) exercise (P < .05). The greater GH response to maximal versus submaximal exercise suggests a role for adrenergic modulation of GHRH during exercise. However, the failure of PYR to restore the responses of the old to those of the young suggests that increased SRIH tone does not completely explain the age difference in GH secretion during exercise.

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.

Hypothalamic control of growth hormone (GH) secretion in type I diabetic men: effect of the combined administration of GH-releasing hormone and hexarelin, a novel GHRP-6 analog

Insulin dependent (type I) diabetic patients show abnormal growth hormone (GH) secretion. Hexarelin is an analog of GHRP-6 which releases GH in part via somatostatin inhibition. The aim of our study was to evaluate the effects of hexarelin and GHRH, administered either alone or in combination, on GH secretion in 10 type I diabetic and 7 normal men. All the subjects were administered: 1) human GHRH (1-29) NH2 100 micrograms i.v. bolus at 0 min; 2) hexarelin 100 micrograms i.v. bolus at 0 min; 3) hexarelin 100 micrograms + hGHRH 100 micrograms i.v. bolus at 0 min. In type I diabetic patients significantly greater GH responses to GHRH and hexarelin have been observed with normal subjects. Hexarelin caused a significantly (p < 0.05) greater GH response as compared to GHRH in both diabetic and control subjects. After the administration of hexarelin+GHRH, a significant increase in both GH absolute and peak levels as compared to hexarelin or GHRH alone was found in all the subjects. However, the GH responses to the combined stimuli were not significantly different in diabetics as compared to normals; moreover, the interaction of GHRH and hexarelin was synergistic in controls and additive in diabetics. We hypothesize that a reduction in the hypothalamic somatostatin inhibitory tone combined with increased pituitary GH production may be responsible for the pattern of the GH responses to hexarelin and GHRH observed in our type I diabetic patients.

Lack of involvement of the cholinergic mechanism in vasoactive intestinal peptide- and peptide-histidine methionine-induced growth hormone (GH) responses in acromegaly: comparison with the GH responses to thyrotropin-releasing hormone and GH-releasing hormone

We examined whether the cholinergic mechanism is involved in the paradoxical GH responses to vasoactive intestinal peptide (VIP) and peptide histidine methionine (PHM) in acromegaly. 28 patients with active acromegaly underwent i.v. bolus injections of thyrotropin-releasing hormone (TRH, 500 micrograms), GH-releasing hormone (GHRH, 100 micrograms), VIP (100 micrograms), and PHM (100 micrograms) with or without a prior atropine treatment (1 mg, i.m., 30 min before). Blood samples were collected before and at intervals up to 120 min after the injection, and plasma GH levels were measured. In response to TRH, GHRH, VIP and PHM, 23 (82%), 24 (86%), 13 (46%) and 7 (25%) patients, respectively, responded with a significant GH increase (> 50% and 6 micrograms/l above the basal level). The effect of atropine pretreatment was examined in only these responders to the respective peptides. When the GH responses were estimated by the area under the response curve, the atropine pretreatment was able to significantly suppress the GH response to GHRH, but not to TRH, VIP, or PHM. Although the lack of cholinergic involvement in the TRH-induced GH release in acromegaly is confirmatory to previous reports, the same results with the VIP- and PHM-induced GH release are novel. The present study may suggest that in acromegaly the physiological GH response is mediated by the cholinergic mechanism, but the paradoxical ones are not.

Inhibitory effect of galanin on growth hormone release from rat pituitary tumor cells (GH1) in culture

The growth hormone (GH) releasing effect of GH-releasing hormone (GHRH) and galanin, a 29-amino acid peptide widely distributed in mammalian CNS, was investigated in cultured rat pituitary tumor cells (GH1) as compared to normal rat somatotrophs. GHRH stimulated dose-dependently GH secretion in normal somatotrophs but did not affect GH secretion in GH1 cells. Galanin (1-10 microM) stimulated GH release in a concentration-dependent manner, but with lower potency as compared to GHRH, in normal rat pituitaries but was inhibitory in rat GH1 cells. The results of this study indicate that while galanin has the ability to stimulate GH release from dispersed pituitary cells of normal rats it has potent direct inhibitory effects on GH release from tumor rat cells.

Effects of metoclopramide on the paradoxical growth hormone response to galanin in acromegaly

Galanin is able to enhance growth hormone (GH)-releasing hormone stimulated GH secretion in normal man. In acromegaly circulating GH levels are elevated and the GH response to GHRH may be exaggerated. Galanin has been recently shown to decrease circulating GH levels in acromegaly. Dopaminergic drugs were the only previously known agents able to cause a paradoxical GH fall in acromegaly. Aim of our study was to investigate the effects of a potent central dopaminergic receptor blocker, metoclopramide (MCP), on the galanin-induced paradoxical GH secretion in acromegalic subjects. Two male and three female patients with active acromegaly (age range 44-66 years, body mass index range 24.6-28 Kg/m2) were studied after 45 min i.v. infusion of porcine galanin (0.5 mg in 100 ml of saline) from 0 to 45 min combined with a 60 min i.v. infusion of a) saline (100 ml) or b) MCP (10 mg in 100 ml of saline) from -15 to 45 min. After galanin, GH values fell from baseline (27.5 +/- 10 micrograms/L) to a mean nadir of 16.4 +/- 6.1 micrograms/L; after galanin + MCP, circulating GH levels were also decreased (mean nadir 17.3 +/- 8.1 micrograms/L) in all the patients with respect to baseline (23.6 +/- 9.7 micrograms/L). No significant differences were found in absolute or percent of baseline GH levels after galanin+saline vs galanin + MCP. Our results suggest that the paradoxical GH fall after galanin in acromegalic patients is not mediated through dopaminergic receptor. It can be hypothesized that galanin may interact at the pituitary level with its own receptors expressed by GH-secreting adenomatous cells.

Variability in the growth hormone response to growth hormone-releasing hormone alone or combined with pyridostigmine in type 1 diabetic patients

In man the GH response to GHRH is variable within and between subjects. Pyridostigmine (PD), an acetylcholinesterase inhibitor, has been shown to reduce the variability of the GH response to GHRH in normal subjects. The aim of this study was to assess the existence of either inter- or intraindividual variability in the GH response to GHRH in type 1 diabetic patients. Moreover, we investigated the effect of PD on such variability in the same patients. Seven (4 females-3 males) nonobese type 1 diabetic patients underwent two experiments performed in consecutive days according to a single-blind protocol: 1) 120 mg oral PD 60 min before iv injection of human (h) GHRH-(1-29) NH2, 100 micrograms in 2 ml of sterile water; 2) oral placebo 60 min before iv injection of 100 micrograms hGHRH. The two experiments were then repeated, following the same procedure, one and two weeks after the start of the study. The GH peaks after GHRH were variable within different subjects but also in the same subject on different occasions. However, the mean GH peak levels after GHRH in the three tests were not significantly different (14.2 +/- 3.5, 15.3 +/- 3, 16.5 +/- 6.4 micrograms/L, respectively), the coefficient of variation for each test was 65%, 51.8%, 102.4%, respectively (mean 73.1 +/- 15.1%). The GH response to GHRH was always significantly enhanced by PD administration: the mean GH peak levels in the three tests were 31.9 +/- 7.1, 44.8 +/- 10.4, 49.9 +/- 13.1 micrograms/L, respectively, without significant differences between tests.(ABSTRACT TRUNCATED AT 250 WORDS)

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 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.

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.