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

Glucocorticoids have been shown to inhibit GH secretion in normal man when acutely and chronically administered in pharmacological amounts. 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 the influence of glucocorticoids on the GH response to PD administered either alone or in combination with GHRH in normal adult subjects. Six healthy adult volunteers underwent six experimental protocols. They received 1) human (h) GHRH(1-29)NH2, 100 micrograms injected as an iv bolus; 2) cortisone acetate, 50 mg administered orally (po) 60 min before an hGHRH iv bolus injection; 3) PD, 120 mg administered po, 60 min before an hGHRH iv bolus injection; 4) PD and cortisone acetate, administered po 60 min before an hGHRH iv bolus injection; 5) PD, administered po 60 min before a saline iv bolus injection; 6) PD and cortisone acetate administered po 60 min before a saline iv bolus injection. Mean GH levels, peak GH levels, and GH area under the curves (AUCs) were significantly lower after GHRH + cortisone as compared to GHRH alone. However, these parameters were not significantly different after PD + GHRH + cortisone when compared to PD + GHRH and after PD + cortisone when compared to PD alone. We conclude that acute administration of pharmacological amounts of glucocorticoids cannot inhibit the GH response to PD alone or in combination with GHRH. Thus, we hypothesize that the inhibitory action of glucocorticoids on the GH response to GHRH in man may be mediated by an enhancement of either somatostatin release by the hypothalamus or somatostatin action on the pituitary.     

Beta-endorphin mediates clonidine stimulated growth hormone release

The role of beta-endorphin in the stimulation of growth hormone secretion elicited by administration of clonidine (Clon), an alpha2-adrenergic agonist, was investigated in awake, freely moving male rats. Animals were infused slowly with either 1 ml of normal rabbit serum (NRS), beta-endorphin antiserum (beta-end-AS) or ACTH antiserum (ACTH-AS) 2 h before the administration of Clon (100 microgram/kg body weight, intravenously). In addition, naloxone (Nal) (2.5 mg/kg body weight, intravenously) was given 15 min prior to Clon in some experiments. Blood samples were taken at 15-min intervals prior to and following Clon administration. Clon caused plasma GH levels to rise 15-fold to peak levels of 177 +/- 38 ng/ml (p less than 0.01) at 30 min. Pretreatment of both Nal or beta-end-AS significantly reduced Clon-stimulated GH secretion to 72 +/- 19 ng/ml (p less than 0.05) and 87 +/- 30 ng/ml (p less than 0.05) respectively. In contrast, the infusion of ACTH antiserum did not affect Clon-stimulated GH release. Our data suggest that beta-endorphin or a related opioid peptide is an important mediator of GH secretion induced by alpha2-adrenergic stimulation. Since blockade of opioid receptors blunted Clon-induced GH release only partially (approximately 50%), other mediators are most likely activated following alpha 2-adrenergic stimulation.     

Corticotropin-releasing hormone inhibition of growth hormone-releasing hormone-induced growth hormone release in man

Recent studies in the rat have shown that intracerebroventricular administration of CRH inhibited spontaneous pulsatile GH secretion and prevented GH-releasing hormone (GHRH)-induced GH release. We have studied the effect of CRH on GHRH-induced GH release in man. In the first study, CRH was injected iv at three different doses (100, 50, or 25 micrograms) at 0800 h together with 50 micrograms GHRH in six men and six women. In a second study, 100 micrograms CRH were given iv at 0800 h, 1 h before the administration of 50 micrograms GHRH in five men and five women. Each subject demonstrated a normal GH response after the administration of 50 micrograms GHRH plus saline. All doses of CRH administered simultaneously with GHRH significantly inhibited GHRH-induced GH release in women [peak value +/- SE after GHRH plus saline, 28.9 +/- 2.9 micrograms/L; after GHRH plus 100 micrograms CRH, 9.9 +/- 0.7 micrograms/L (P less than 0.001); after GHRH plus 50 micrograms CRH, 8.7 +/- 0.8 micrograms/L (P less than 0.001); after GHRH plus 25 microgram CRH, 9.5 +/- 1.6 microgram/L (P less than 0.001]). In contrast, in men, while a dose of 100 micrograms CRH was capable of suppressing GHRH-induced GH secretion (peak value +/- SE, 8.1 +/- 0.6 vs. 20 +/- 2.9 micrograms/L; P less than 0.001), no inhibition was observed after 50- and 25-micrograms doses. When 100 micrograms CRH were injected 1 h before the administration of 50 micrograms GHRH, it strongly inhibited GHRH-induced GH secretion in both men (peak value +/- SE, 6.2 +/- 2.8 vs. 24.6 +/- 5.9 micrograms/L; P less than 0.02) and women (peak value +/- SE, 14.2 +/- 4.5 vs. 37.8 +/- 6.7 micrograms/L; P less than 0.005), and this inhibition lasted up to 2 h post-CRH administration. These results demonstrate that CRH is capable of inhibiting GHRH-induced GH release in both men and women. Furthermore, the findings suggest that a sexual dimorphism in the neuroregulation of GH secretion may be present in man. In view of the inhibitory action of CRH on GH secretion, simultaneous administration of CRH and GHRH for testing should be avoided in clinical practice.     

Effects of osmotically stimulated endogenous vasopressin on basal and CRH-stimulated ACTH release in man

Two groups of six healthy young males participated in separate experiments to examine the physiological role of endogenous vasopressin in h-CRH-induced (100 micrograms iv) ACTH release: a) after drinking of 3500 ml of water; b) after thirsting for 23 h; c) after 0.9% saline infusion, and d) after 5.0% saline infusion (0.06 ml/kg per min for 120 min). AVP levels were markedly elevated (4 ng/l) after thirsting and 5% saline infusion when compared with water loading or infusion of physiological saline. Although basal and h-CRH-stimulated ACTH and cortisol levels tended to be higher during hypertonic saline infusion and dehydration, no significant difference was observed between states of high or low endogenous AVP levels. These results are not in accordance with previous studies using ovine CRH, which might be due to its longer half-time or the timing of the changes in AVP plasma levels in relation to the CRH injection. Our data suggest that the osmotic modulation performed in this study results in AVP concentrations in the adenohypophysis, which are in the threshold range for influencing ACTH release induced by exogenous h-CRH.     

Secretory patterns of growth hormone during basal periods in man

Human growth hormone (hGH) concentrations in plasma often fall to levels not detectable by RIA. These so-called basal levels prevail during the greater part of the day. Since hGH is involved in the homeostasis of several metabolic processes, it is important to examine its exact plasma concentration and secretory pattern during basal periods. We used an immunoadsorbent technique to extract hGH from large plasma samples to precisely measure basal hGH concentrations and their variation with time. Blood samples (20 mL) were drawn from 12 normal subjects in the fasted and rested state every 15 minutes over a three-hour period. Plasma hGH levels varied over three orders of magnitude (range, 34 to 60,000 pg/mL). During basal periods, episodes of secretory pulses, of moderate sustained secretion, and of complete secretory inactivity occurred. Women had significantly higher overall hGH levels as well as basal hGH levels than men, but no significant sex difference in the pulse frequency during basal periods could be detected in the limited time allotted for study. No convincing relationship was noted between variations in plasma glucose and the secretory pattern of hGH, or vice versa. We conclude that hGH is secreted in an episodic fashion during basal periods. Conceptually, basal and stimulated hGH secretion may be viewed as extremes of a continuous spectrum of pituitary activity, basal hGH levels are lower than heretofore appreciated, the known tendency of women to higher hGH levels is also evident in the basal range, and oscillations in plasma glucose do not affect the microsecretory pattern of hGH, nor are endogenous hGH pulses followed by acute changes in glycemia.     

The effect of exogenous melatonin on stimulated neurohypophysial hormone release in man

OBJECTIVES: To examine the effect of hormone replacement therapy (HRT) on endothelial function of forearm resistance vessels in women with Turner's syndrome. DESIGN: Subjects were studied on three occasions: on their usual HRT (study 1), then after 6 weeks off HRT (study 2) and finally after a further 6 weeks on HRT (study 3). PATIENTS: Seven young women with Turner's syndrome were studied. MEASUREMENTS: Forearm blood flow in response to intrabrachial infusion of bradykinin, 10, 30, 100 pmol/min (endothelium-dependent vasodilator), glyceryl trinitrate, 4, 8, 16 nmol/min (GTN; endothelium-independent vasodilator), noradrenaline, 60, 120, 240 pmol/min (NA, alpha-adrenergic receptor agonist) and NG-monomethyl-l-arginine, 1, 2, 4 micromol/min (L-NMMA; NO synthase inhibitor) was assessed by bilateral venous plethysmography. RESULTS: The vasodilator response to bradykinin, expressed as the within-subject mean difference in area under the dose-response curve between study 2 and study 1, was significantly diminished (-744.2 +/- 287.2, P = 0.04) but improved 6 weeks after HRT recommencement. However, there was no significant change in response to GTN (between study 2 and study 1, 189.5 +/- 247.8, P = 0.47). The vasoconstrictor response to L-NMMA was also diminished in study 2 when compared to study 1 (-100.4 +/- 35.4, P = 0.039) and was restored after HRT was recommenced (between study 3 and study 2, 117.5 +/- 69.3, P = 0.17) whereas there was no significant difference in response to NA between study 2 and study 1 (76.7 +/- 50.6, P = 0.18) or study 3 and study 2 (-70.8 +/- 71.1, P = 0.38). CONCLUSIONS: HRT improves endothelial function in women with Turner's syndrome.     

Dual action of dopamine on growth hormone release in vitro

The effect of dopamine (DA) on growth hormone (GH) release was studied in perifused freshly dispersed rat anterior pituitary cells. Pulses of DA (0.01-100 nmol/l), each applied for 30 min, resulted in a prompt rise in GH release. This effect was reversible, concentration-dependent and partially antagonized by metoclopramide, a DA antagonist. The effect of DA was further tested on GH-stimulated secretion by human GH-releasing factor (hGRF). Perifusion with hGRF (6.25 pmol/l) for 2 min elicited an immediate rapid increase in GH release which lasted 20 min. Pretreatment of cells with DA (100 nmol/l) for 10 min and a subsequent hGRF challenge during continuation of DA perifusion significantly reduced the effect of hGRF pulses on GH release. The present data suggest that DA has direct opposite actions at the somatotroph level, stimulating the basal GH release and inhibiting the hGRF-induced GH secretion, and may thus be an important modulator of GH release.     

Effects of intravenous corticotropin-releasing hormone upon sleep-related growth hormone surge and sleep EEG in man

Corticotropin-releasing hormone (CRH) plays a key role in coordinating neuroendocrine, metabolic and behavioral responses in stress and affective disorders. To further investigate the effects of enhanced pituitary-adrenocortical activity upon sleep-related phenomena we administered four intravenous injections of 50 micrograms human (h)-CRH or saline to 11 normal males at 10 p.m., 11 p.m., 12 p.m. and 1 a.m. and measured plasma levels of cortisol and growth hormone (GH) as well as sleep EEG recordings throughout the night. Treatment with h-CRH resulted in a significant increase of mean (+/- SEM) cortisol secretion between 11 p.m. and 3 a.m. (h-CRH: 100.6 +/- 9.5 ng/ml; saline: 39.0 +/- 1.5 ng/ml; p less than 0.01). This initial cortisol increase after repeated h-CRH stimulations was followed by a period of attenuated plasma cortisol between 3 and 7 a.m. (h-CRH: 70.3 +/- 7.0 ng/ml; saline: 115.5 +/- 8.0 ng/ml; p less than 0.01). Cortisol surges after h-CRH were associated with a significant blunting of sleep-related GH release expressed as areas under concentration curves (h-CRH: 1.245 +/- 0.32 ng/ml/min.10(3); saline: 2.462 +/- 0.92 ng/ml/min.10(3), p less than 0.01). In addition to these hormonal effects, h-CRH induced a decrease of REM and slow wave sleep (stages III and IV) while the amount of more shallow sleep (stages I and II) increased. These effects upon sleep structure were more pronounced during the second part of the night.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective beta 1-adrenergic receptor-blockade with atenolol enhances growth hormone releasing hormone and mediated growth hormone release in man

The growth hormone (GH) responses to a single bolus injection of the growth hormone releasing hormone (GRH) were examined in the basal state and in the presence of beta-adrenergic receptor blocking agents of different specificity in ten normal men. During a constant five-hour infusion of 56 micrograms/min of propranolol (nonselective beta-adrenergic receptor-blocker) in seven subjects studied, there was a significant augmentation of the GH release in response to exogenous GRH compared to the GH response during saline infusion, as measured by the peak serum GH concentrations after GRH (P = 0.019) and the integrated GH values (P = 0.019). A similar significant enhancement of GH responses to exogenous GRH as compared to the control day was observed with the specific beta 1-adrenergic receptor-blocker atenolol in all seven subjects studied (four of whom also participated in the propranolol study). Both the peak GH response to a GRH bolus and the integrated GH values were significantly greater with atenolol (P = 0.019 for both). There was no difference in serum GH concentrations after beta-adrenergic receptor-blocking drugs during a three-hour sampling period before GRH administration compared to placebo. Our results support the concept that beta-adrenergic receptors may modulate either the release or action of hypothalamic somatostatin in the control of GH secretion in man. We suggest the effect is mediated by specific beta 1-adrenergic receptors.     

Stimulation of growth hormone and corticotropin release by angiotensin II in man

The intravenous (IV) infusion of angiotensin II (AII) was administered to seven healthy male volunteers in a randomized placebo-controlled study. As expected, AII induced a significant increase in blood pressure and plasma aldosterone concentrations. AII caused a significant increase in corticotropin (ACTH) and growth hormone (GH) release, but had no effect on the release of thyrotropin (TSH) and prolactin (PRL). These findings suggest that peripherally circulating AII might influence ACTH and GH secretion in humans.     

Dopaminergic modulation of arginine mediated growth hormone and prolactin release in man

The mechanism of arginine (ARG) induced growth hormone (GH) and prolactin (PRL) release is poorly understood. Though dopamine (DA) is known to inhibit a number of GH and PRL secretagogues, it has been reported not to affect arginine mediated GH release. Our study was undertaken to study the interaction of DA and ARG further. Six healthy male volunteers received DA (4μg/kg/min) or saline from 0–240 min on two separate days. ARG (30 gms) was given 150–180 min on both days (Protocol I). In Protocol II 5 subjects received ARG on different days 30–60 min into a DA or saline infusion. On a third day only DA was given. Arginine alone was given in 2 separate infusions spaced 135 minutes apart to 5 volunteers (Protocol III). In Protocol I when ARG was given at 150 min, the maximal GH peak of 11.1 ± 1.3 ng/ml, which occurred 45 min later, was blunted by DA treatment (5.3 ± 1.1 ng/ml, p < 0.005). On the DA day prior to ARG, there was also a GH peak at 75 min of 8.1 ± 1.9 ng/ml. In Protocol II, when the ARG and DA responses coincided, the mean maximal GH response to both stimuli was 20.5 ± 3.3 ng/ml which was greater than to either DA (7.7 ± 2.2 ng/ml, p < 0.01) or ARG alone (14.6 ± 3.0 ng/ml, N.S.). Moreover, the area under the curve for DA and ARG together (1196 ± 186 ng/ml/150 min) was greater than for DA (484 ± 160 ng/ml/150 min, p < 0.001) or ARG separately (544 + 70 ng/ml/150 min, p < 0.05). During the double arginine, which mirrored temporally the pattern of GH release observed in Protocol I, the peak GH responses after each infusion were similar (9.0 ± 2.1 and 8.8 ± 2.1 ng/ml). The maximal PRL responses to ARG were 11.1 ± 3.8 ng/ml and 15.7 ± 4.8 ng/ml in Protocol I and II respectively, but these responses were abolished by the DA infusion in both studies. Our data, therefore, provide further evidence to support a dual role of DA in GH release. DA stimulates basal GH secretion and this effect is additive to that of ARG. However, when ARG administration is delayed, the GH response is now blunted. The inhibitory action of DA is not mediated by its prior release of GH, since the GH responses are similar after sequential ARG infusions. Consequently, the role of DA is directly related to the degree of secretion. This duality is not seen with PRL secretion, where DA is always inhibitory.     

Alpha-adrenergic control of growth hormone release during surgical stress in man

The mechanisms involved in the initial release of growth hormone (GH) during cholecystectomy have been studied after the administration of phentolamine in saline and in isotonic glucose, and after the administration of 10% glucose. Infusion of these substances was started 10 min before and terminated 30 min after skin incision. The serum GH levels 30 min after skin incision in a nontreated control group were raised to 14.4 ± 1.0 ng/ml. The alpha-adrenergic blockade by phentolamine (20 mg during 40 min) regardless of whether administered in saline or in isotonic glucose inhibited GH response to surgery (4.3 ± 2.1 ng/ml, or 2.2 ± 0.4 ng/ml). The administration of 10% glucose (40 g during 40 min) led to a diminished response in some, but not in all the patients (6.2 ± 1.2 ng/ml). It is concluded that the alpha-adrenergic mechanism participates in GH response to surgery.     

Testosterone inhibition of growth hormone release stimulated by a growth hormone secretagogue: studies in the rat and dog

Anabolic steroids are frequently taken by athletes and bodybuilders together with recombinant human GH (rhGH), though there is some scientific evidence that the use of anabolic steroids reverses the rhGH-induced effects. Recently, we have shown that treatment with rhGH (0.2 IU/kg s.c., daily x 12 days) in the dog markedly reduced the canine GH (cGH) responses stimulated by EP51216, a GH secretagogue (GHS), evaluated after 3 and 5 daily rhGH injections, and that the inhibition was still present a few days after rhGH discontinuation. The aim of the present study was to evaluate in the dog the GH response to EP51216 (125 mug/kg i.v.) in a condition of enhanced androgenic function (i.e. acute injection or 15-day treatment with testosterone at the dose of 2 mg/kg i.m. on alternate days), and in the hypophysectomized rat the hypothalamic and hippocampal expression of ghrelin, the receptor of GHSs (GHS-R), GH-releasing hormone (GHRH) and somatostatin (SS) after specific hormonal replacement therapies (testosterone, 1 mg/kg/day s.c.; hydrocortisone, 500 mug/kg/day s.c.; rhGH, 400 mug/kg/day s.c.; 0.9% saline 0.1 ml/kg/day s.c.; x11 days). In the dog experiments, under baseline conditions, a single injection of EP51216 elicited an abrupt rise of plasma cGH. Twenty-four hours from the acute bolus injection of testosterone, C(max) and AUC(0-90) of the GHS-stimulated cGH response were significantly lower than baseline cGH response; 5 days later, there was still a significant decrease of either parameter versus the original values. Short-term treatment with testosterone markedly reduced the GHS-stimulated cGH responses evaluated during (5th bolus) and at the end (8th bolus) of testosterone treatment. Four and 8 days after testosterone withdrawal, the EP51216-stimulated cGH response was still significantly reduced when compared with that under baseline conditions. Plasma concentrations of insulin-like growth factor 1 (IGF-1) were stable until the 5th bolus of testosterone and decreased progressively in the remaining time of the testosterone treatment; 4 and 8 days from treatment withdrawal, IGF-1 levels were still suppressed. In rat studies, hypothalamic mRNA levels of GHS-R were significantly reduced by treatments with testosterone and hydrocortisone, whereas hippocampal expressions of ghrelin, GHRH and SS were reduced by rhGH replacement therapy. In conclusion, these studies show that a single administration of testosterone can abrogate the cGH response ensuing acute stimulation by a GHS; the inhibitory effect of testosterone on the cGH response to GHS is present during and even 8 days after termination of a short-lived treatment with testosterone; these events occur via a Copyright (c) 2006 S. Karger AG, Basel.     

The impact of blood glucose levels on stimulated adrenocorticotropin hormone and growth hormone release in healthy subjects

Objective  In studies investigating the influence of glucose levels on the pituitary function the methods used have been variable and mainly focused on the change in function as a reaction to unphysiological low or high blood glucose levels. In the present study the impact of physiological and elevated blood glucose levels on adrenocorticotropin hormone (ACTH) and growth hormone release are investigated.
Design  The euglycaemic and hyperglycaemic clamp techniques were used to reach stable levels of 4, 8 and 12 mmol/l blood glucose levels. After a stabilization phase of 2 h, a corticotropin releasing hormone (CRH) or a growth hormone releasing hormone (GHRH) stimulation test was performed.
Subjects  Seven and eight healthy male volunteers, belonging to two groups, participated in this study.
Measurements  The area under the curve (AUC), peak values and time to peak of ACTH, cortisol and growth hormone were calculated to evaluate the response to the CRH and GHRH stimulation test.
Results  The peak values of ACTH, cortisol and growth hormone seemed to be the highest during the 4 mmol/l clamp sessions, compared with the 8 and 12 mmol/l clamps, although the differences were not statistically significant when analysed for every subject individually. The AUC and time to peak measurements were comparable during the three clamp procedures.
Conclusion  The pituitary reaction on CRH and GHRH was not significantly changed by various blood glucose levels.