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The effect of a 6 hour continuous infusion of Hexarelin (100 micrograms/hour) on GH peak frequency, amplitude and duration, GH trough concentrations, the interval between successive peaks and the pituitary responsiveness to GHRH, as well as GH axis and galanin mRNA contents, were examined in conscious adult male rats. Plasma GH concentrations peaked within 15 minutes after the initiation of Hexarelin infusion, but returned to baseline levels by 60 minutes. No significant differences between Hexarelin and saline infused rats were noted for any of the parameters of pulsatile GH release analyzed. However, following a 6 hour infusion, rats treated with Hexarelin demonstrated a greater GH responsiveness to GHRH (delta GH: 57 +/- 16 ng/ml for Hexarelin infused; 21 +/- 7 ng/ml for saline infused; p0.05). Furthermore, the rats infused with Hexarelin demonstrated decreased GHRH and increased hypothalamic galanin mRNA contents as compared to the saline infused rats, while hypothalamic somatostatin and pituitary GH mRNA contents appeared unchanged. Rats infused with Hexarelin had lower pituitary galanin mRNA content than did the rats which were infused with saline. Collectively, these results suggest that Hexarelin may not act via alteration of somatostatin synthesis and that suppression of somatostatin's action at the pituitary can not be excluded. The current study also suggests that other hypothalamic pathways aside from those currently defined for the growth axis may be involved in the mechanism by which Hexarelin and the other GH-releasing peptides elicit GH release.

To study the onset of the action of gonadal sex steroids on the GH axis in spontaneous puberty, which is prolonged and sparingly predictable, we present a clinical investigative paradigm in which six previously untreated boys with isolated hypogonadotropic hypogonadism were exposed to progressively higher testosterone levels designed to mimic the androgen environment recognized during the early stages of puberty. We administered three incremental doses of testosterone (25-, 50-, and 100-mg im injections), each over a period of 4 weeks. Studies of overnight pulsatile GH secretion and GH responses to GHRH alone or combined with L-arginine (a functional somatostatin antagonist) were performed before testosterone administration and after each dose of testosterone. Serum testosterone, but not estrogen, levels increased progressively in all subjects during therapy. Deconvolution analysis of GH release profiles disclosed that GH secretory burst mass was stimulated significantly even by 25 mg testosterone. This parameter was not altered further by higher doses of testosterone. Spontaneous GH secretory burst number and amplitude increased significantly only after the 50- and 100-mg testosterone treatments, after which the serum GH response to GHRH and arginine also rose significantly. In contrast, the GH response to GHRH alone was not significantly affected by any dose of testosterone. Serum testosterone levels correlated significantly with the primary parameters of nocturnal GH secretion. In summary, our experimental model suggests that in males even very small increases in circulating testosterone occurring during the earliest stages of puberty are able to amplify pulsatile GH secretion. Our concomitant secretagogue data further suggest that testosterone exerts its action at different sites in the hypothalamo-somatotropic axis, i.e. directly at the pituitary level, and also at hypothalamic loci, possibly increasing both GHRH and somatostatin release.

The aim of this study was to examine the effect of recombinant human leptin on growth hormone (GH) secretion in perifused anterior pituitary slices from adult pigs. Anterior pituitary slices from sows were perifused and treated with recombinant human leptin (10 nM) and GH-releasing hormone (GHRH; 1 nM). In some experiments, pituitary slices were coincubated with stalk median eminence (SME). In a subset of the coincubation experiments, immunoneutralization of endogenous GHRH and somatostatin (SRIH) release was performed with antisera to GHRH and SRIH. Leptin increased GH secretion in pituitary slices alone (up to 100% vs. control at 40 min) as well as in pituitary slices coincubated with SME (up to 122% vs. control at 40 min). A significant difference was observed in GH secretion from pituitary slices when the tissue was coincubated with leptin and GHRH at a low concentration (0.1 nM), but not when GHRH was used at 1 and 10 nM. Furthermore, anti-SRIH antiserum increased GH release from pituitary slices in coincubation experiments with SME. Finally, SRIH secretion was significantly reduced by leptin (down by 35% vs. control from 0 to 30 min of treatment) in cultured SME. These data show that leptin is effective in stimulating GH secretion by acting at two different levels: (1) it stimulates GH secretion directly from pituitary slices, and (2) it reduces SRIH tone from the median eminence and, indirectly, increases GH secretion from the pituitary. These results support the hypothesis that leptin may be an interesting hormonal mediator of growth and related metabolic effects by acting directly on the hypothalamic-pituitary axis.

We have previously shown that hexarelin, a novel GH-releasing peptide (GHRP), is able to elicit GH release when administered i.v., s.c. or by mouth and that it is a more potent GH secretagogue than GHRP-6. In the current study, we investigated the effects of hexarelin administered as repeated doses at 2 h intervals or as a continuous 6, 30 or 174 h infusion to conscious male rats. In the first experiment, adult male Sprague-Dawley rats were prepared with dual indwelling jugular catheters. On the day of experimentation, these animals received three 25 micrograms/kg i.v. boluses of hexarelin at 2 h intervals with blood sampling at 5, 10, 15, 30, 60, 90 and 120 min after each dose. The mean peak GH response and the mean area under the GH response curve (AUC) for the 30 min after each administration were calculated and are reported as the mean +/- S.E.M. For both the peak and AUC results there was a significant (P < 0.05) difference in the GH response noted between the first (peak 301 +/- 37 ng/ml; AUC 5585 +/- 700 ng/ml per 30 min) and second (peak 149 +/- 47 ng/ml; AUC 3056 +/- 908 ng/ml per 30 min) injections of hexarelin, but not between the first and third (peak 214 +/- 49 ng/ml; AUC 3862 +/- 844 ng/ml per 30 min). In a second series of experiments, adult male Sprague-Dawley rats received continuous infusions (100 micrograms/h) of hexarelin or saline (1 ml/h) for 6, 30 or 174 h. Blood samples were collected every 20 min for the duration of the 6 h infusion and for the last 6 h of the two longer hexarelin infusions. Plasma GH concentrations peaked within 40 min of the initiation of infusion, but soon returned to basal levels. Mean plasma GH concentrations did not differ between any of the treatment groups, nor did any of the parameters of pulsatile hormone release analyzed. No significant differences in plasma corticosterone concentrations were noted between any of the treatment groups. On the other hand, while neither the 6 h (941 +/- 70 ng/ml) nor the 30 h (954 +/- 70 ng/ml) hexarelin infusions resulted in a significant increase in the plasma IGF-I concentrations over those noted in the saline controls (935 +/- 65 ng/ml), a 174 h hexarelin infusion did elicit a significant increase (1289 +/- 42 ng/ml; P < 0.05). Thus it appears that, while continuous exposure to hexarelin does not disrupt normal GH cycling, it may (after up to 174 h of exposure) alter other components of the growth axis. In addition, since the character of pulsatile GH release remained unaltered in response to the hexarelin infusion, it appears that this GHRP may not act by suppression of functional somatostatin tone as has been suggested previously.

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 (p0.05) 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.

One of the most prominent metabolic effects of the systemic administration of the synthetic neuropeptide galanin in man is the increase in growth hormone (GH) secretion. This stimulating action of galanin is thought to occur directly at the hypothalamic level through the release of GHRH. Recently, it has been shown that also dopaminergic drugs may elicit GH secretion through an increase in hypothalamic GHRH secretion. The aim of this study was to investigate if the action of galanin on GHRH and consequently on GH release may be mediated via dopaminergic pathways evaluating the effects of a potent central dopaminergic receptor blocker, metoclopramide (MCP), on the galanin-induced growth hormone (GH) secretion in normal subjects. We studied seven young non obese healthy subjects (three females and four males). GH secretion was evaluated after 45 min iv infusion of porcine galanin (0.5 mg in 100 ml of saline) from 0 to 45 min combined with a 60 min iv infusion of a) saline (100 ml) or b) MCP (10 mg in 100 ml of saline) from -15 to 45 min. In all the seven subjects, during galanin infusion, GH values increased with respect to baseline with peaks occurring between 30 and 60 min after the beginning of galanin infusion. Peak GH values ranged between 3.5 and 15.4 micrograms/l (mean 10.4 +/- 1.6 micrograms/l). During MCP infusion no significant differences in the GH response to galanin with respect to saline were observed both when absolute GH levels and GH AUC were examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppressed pulsatile GH secretion in food-deprived rats has been hypothesized to be due to an increase in hypothalamic somatostatin secretion. We investigated this hypothesis and the role of GHRH in regulating GH secretion during food deprivation using two different models. In experiment one, rats were food deprived for 72h during which time they received a saline infusion (n = 5). At the same time rats were normal fed for 72h during which time they received a somatostatin infusion (5 micrograms/h, n = 7). After the 72h infusion period, all rats received two iv injections of GHRH (1 microgram/rat) at 2h intervals. GH concentrations in food-deprived rats rose from approximately 10 ng/ml to 400-800 ng/ml in response to both GHRH injections. This increase was significantly greater (p < 0.01) than the GH response (100-400 ng/ml) observed in somatostatin-infused animals. The significantly higher GH response observed in food-deprived rats as compared to somatostatin-infused, normal-fed rats suggests that somatostatin concentrations may decrease during food deprivation. In experiment two, rats were infused for 5h with either saline (n = 6) or GHRH (10 micrograms/h, n = 9) at the end of a 72h fast. GH concentrations did not change in saline-infused animals. In contrast, GH concentrations significantly increased (p < 0.01) upon initiation of the continuous GHRH infusion. Yet, this release of GH was pulsatile in nature. Pulsatile GH secretion in the presence of a constant GHRH infusion suggests that pulsatile somatostatin release from the hypothalamus is maintained during food deprivation. These studies suggest that during food deprivation in the rat 1) absolute concentrations of somatostatin decrease, but its pattern of secretion remains pulsatile, and 2) decreased GHRH release may be responsible for the absence of spontaneous GH pulses.

The secretion of growth hormone (GH) in both male and female rats is controlled by two main neuropeptides, GH-releasing hormone (GHRH), which is stimulatory, and somatostatin, which is inhibitory. Recently, it has been shown that galanin (GAL) also stimulates GH secretion, although the underlying mechanism is still unknown. It was the aim of this study to begin to elucidate if and how GAL regulates its own production at the hypothalamic and pituitary level. Rats underwent the following experimental trials. In experiment 1, adult male and female rats had blood samples collected at -15 minutes, -7.5 minutes, and immediately preceding a subcutaneous (s.c.) injection of GAL at a dose of either 50 or 200 microg/kg. Blood samples were collected at 5, 10, 15, 30, and 60 minutes, and the GH concentration was measured using a radioimmunoassay. The tissues were collected and analyzed for mRNA levels of hypothalamic and pituitary GAL. In experiment 2, adult male and female rats were treated long-term with 200 microg/kg GAL for 7 days s.c., and the pituitary and hypothalamus were analyzed for GAL mRNA. Serum GH concentrations were significantly increased in acutely dosed male and female rats regardless of the dosage level. For the male and female animals acutely dosed with both 50 and 200 microg/kg GAL, hypothalamic GAL mRNA was decreased, whereas pituitary GAL mRNA was affected by 200 microg/kg GAL only in females (increased). For the animals treated long-term with GAL, hypothalamic GAL mRNA was decreased while mRNA for pituitary GAL was increased. We conclude that regardless of the dosage and duration of treatment, administration of GAL negatively regulates hypothalamic GAL mRNA in a non-gender-specific way. Pituitary GAL synthesis appears to be stimulated particularly during chronic SCGAL administration.

Neuroendocrine mechanisms governing growth hormone (GH) secretion are sensitive to nutritional status since the normal pulsatile pattern of GH release is disrupted during conditions of food deprivation or malnutrition. A reasonable hypothesis for this occurrence is the alteration of somatostatin and GH-releasing hormone (GHRH) synthesis, storage and secretion. In this study, we investigated the effects of food deprivation on GH, GHRH, hypothalamic and pituitary galanin (GAL), and somatostatin through immunocytochemical and mRNA analysis. Adult male rats were subjected to 72 h of food deprivation, during which an average of 18% total body weight was lost. ICC studies were performed on brain sections from the rostral, middle and caudal regions of the median eminence of the hypothalamus using the avidin-biotin-peroxidase method. Immunocytochemical results were generated for the percent area and optical density (intensity) of immunostaining in the median eminence. Messenger RNA analyses were performed using sense and antisense riboprobes produced from cDNA clones for GH, GHRH, somatostatin and GAL. Food deprivation decreased somatostatin immunostaining in middle and caudal regions of the median eminence; similarly, food deprivation resulted in decreased GHRH immunostaining in rostral and middle sections of the median eminence of the hypothalamus. mRNA levels for somatostatin, GHRH and GH and GAL were also reduced by food deprivation. Our data suggest that suppressed GH secretion in food-deprived rats may reflect a general downregulation of the neuroendocrine and pituitary GH axis.

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 (p0.05) 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.

Glucocorticoids are potent inhibitors of linear growth and growth hormone (GH) secretion when secreted or administered in pharmacological amounts in vivo. The mechanisms involved require further clarification although enhanced somatostatin tone has been suggested to play a role. In this study, we investigated the effects of excess glucocorticoids on pituitary GH, hypothalamic GHRH and hypothalamic somatostatin through immunocytochemical (ICC) and mRNA analysis. Twelve adult male rats were injected daily with dexamethasone (40 micrograms/day, i.p.) or saline for 4 days. ICC studies were performed on brain sections from the rostral, middle and caudal regions of the median eminence of the hypothalamus using the avidin-biotin-peroxidase method. Messenger RNA analyses were performed using sense and antisense riboprobes produced from GH, GHRH and somatostatin cDNAs. Immunocytochemical results were generated for the percent area and intensity (optical density) of immunostaining in the median eminence. Glucocorticoids increased somatostatin immunostaining of the rostral, middle and caudal regions of the median eminence while GHRH staining was only reduced in the rostral region of the median eminence and unchanged in the other hypothalamic regions. GH and somatostatin mRNA levels dramatically increased following glucocorticoid treatment concomitantly with a decrease in GHRH mRNA levels. Our data suggest that increased somatostatin synthesis and storage and a decrease in GHRH mRNA synthesis play a major role in the GH inhibitory effects of glucocorticoids.

The effects of thyroid hormones on GH secretion and the mechanisms underlying their action are very similar in man and the laboratory animal. We feel that it is possible to organize the available data into a unique pathophysiological model explaining these complex interactions (Table 1). In summary, physiological levels of circulating thyroid hormones are necessary to maintain normal pituitary GH secretion owing to their direct stimulatory actions. When the serum concentrations of thyroid hormone increase above the normal range there is an increase in hypothalamic somatostatin tone, which in turn suppresses pituitary GH secretion and overrides any stimulatory effects. The suppression of GH secretion by thyroid hormones may be mediated at the hypothalamic level also by a decrease in GHRH release.

The aim of our study was to investigate the effect of hexarelin, a novel GH-releasing peptide-6 analog, and GH-releasing hormone (GHRH) (alone or in combination) on GH secretion in adult patients with increased somatostatin tone due to chronic glucocorticoid excess. We studied seven adult patients undergoing long-term (no less than 6 months) immunosuppressive glucocorticoid treatment for non-endocrine diseases (six females and one male, age range 42–68 years) and one subject (female, age 31 years) with endogenous hypercortisolism due to adrenal adenoma. Six normal subjects (four females and two males) matched for sex and age with the patients and not undergoing any therapy served as controls. All the subjects underwent the following three tests in random order: (1) human GHRH (1–29)NH2 (100 μg in 1 ml saline) injected as an i.v. bolus at 0 min, (2) hexarelin (100 μg in 1 ml saline) injected as an i.v. bolus at 0 min and (3) hexarelin (100 μg in 1 ml of saline) plus GHRH (100 μg in 1 ml saline) injected as an i.v. bolus at 0 min. After GHRH alone the patients with glucocorticoid excess showed a blunted GH response as compared with normal subjects (median delta GH: 0·9, range 0–5·6 μg/l vs 7·1, range 0·3–14·9 μg/l). No significant differences were observed in the steroid-treated group with respect to normal subjects after hexarelin alone (median delta GH: 15·5, range 1·9–45·2 μg/l vs 17·9, range 5·5–53·9 μg/l). The GH responses after the combined stimuli were significantly decreased in the patients with glucocorticoid excess as compared with normal subjects (median delta GH: 43·5, range 21–56·9 μg/l vs 73·7, range 19·6-116·8 μg/l). The two stimuli acted in a synergistic fashion in both groups of subjects. It may be hypothesized that hexarelin influences the GH inhibitory effect of glucocorticoids in humans, acting at the hypothalamic somatostatin level. Journal of Endocrinology (1995) 146, 227–232

Glucocorticoids, when administered over prolonged periods of time, cause protein wasting, osteoporosis, elevation of total cholesterol, and carbohydrate intolerance. Human GH is a potent anabolic agent known to stimulate protein synthesis and osteoblast activity. Chronic hypercortisolemia is associated with impaired GH secretion. The aim of our study was to evaluate the effects of short term administration of human recombinant GH on bone and fuel metabolism in patients receiving chronic glucocorticoid treatment and with suppressed GHRH-stimulated GH peaks (10 micrograms/L). We studied nine nonobese adult patients more than 70 yr of age (seven females and two males; age range, 41-68 yr; body mass index, 26 +/- 1.3 kg/m2) undergoing long term glucocorticoid therapy for nonendocrine diseases. After a 3-day stabilization period in the hospital, several parameters were evaluated in all patients: 1) protein, 2) bone, 3) lipid, 4) carbohydrate metabolism, and 5) immune system function under baseline conditions. At 1800 h on the fifth day of hospitalization, the patients began treatment with a daily sc injection of 0.1 IU/kg (0.037 mg/kg) recombinant human GH (Humatrope, Eli Lilly Co.) for 7 days. GH administration caused a significant increase in nitrogen balance (from -0.12 +/- 0.04 to -0.03 +/- 0.02 g/kg.day; P0.05), osteocalcin, carboxy-terminal propeptide of type I procollagen, and carboxy-terminal telopeptide of type I collagen with respect to basal levels. After GH administration, total, high density lipoprotein, and low density lipoprotein cholesterol levels were significantly lowered, and serum triglyceride levels were increased in all patients. Normal blood glucose levels during GH administration were observed in our patients concomitantly with a slight increase in insulin secretion. After GH treatment, the T-helper/T-suppressor cell ratio significantly increased with respect to basal levels (2.5 +/- 0.4 vs. 2.2 +/- 0.3; P0.05). Our data suggest that in patients receiving chronic glucocorticoid treatment, GH administration may significantly antagonize several side-effects of long term glucocorticoid administration, such as protein wasting, osteoporosis, and hyperlipidemia.

The aim of our study was to investigate the effect of galanin on baseline and growth hormone (GH)-releasing hormone (GHRH)-stimulated GH concentrations in conscious, freely moving rats receiving long-term glucocorticoid treatment. Animals were treated for 7 days with an intraperitoneal injection of either vehicle or dexamethasone ([dex] 40 micrograms/d). Rats underwent the following experimental trials: at -15 minutes animals received an intravenous injection of saline or galanin (12.5 micrograms/kg), and at 0 minutes rats received a second intravenous injection of saline or rat GHRH (500 ng/kg). Blood samples were drawn every 5 minutes from -15 to +15 minutes and then at 30 minutes. The GH response to saline + GHRH alone was significantly higher (P < .05) in chronically vehicle-treated rats as compared with chronically dex-treated ones. In contrast, galanin + saline increased serum GH levels in a similar fashion in both chronically vehicle- and dex-treated rats. The response to galanin + GHRH was similar to galanin + saline in chronically vehicle-treated rats, but was significantly enhanced in chronically dex-treated rats. These results suggest that galanin-mediated GH release in rats may involve somatostatinergic pathways.

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.

The aim of our study was to elucidate the relationship between the level of circulating cortisol and the GH responsiveness to GHRH in six hypoadrenal patients (one male and five females; age range, 35-67 yr; body mass index range, 18-31 kg/m2). Twenty-four hours after taking the last dose of replacement therapy, each patient underwent the following experimental trials on nonconsecutive days: 1) saline, and 2) 12.5 mg, or 3) 25 mg, or 4) 250 mg hydrocortisone hemisuccinate in 250 mL saline constant iv infusion from 0-180 min. On each occasion, 1 micrograms/kg human GHRH-(1-29)NH2 was injected as an iv bolus at 60 min. During GHRH and saline infusion, serum cortisol levels were always less than the detection limit of the assay (55 nmol/L). During 12.5-, 25-, and 250-mg hydrocortisone infusions (from 15-180 min), serum cortisol averaged 413.8 +/- 19.3, 772.5 +/- 46.9, and 1520.2 +/- 110.4 nmol/L, respectively. The GH peaks after GHRH treatment during the various infusions of hydrocortisone were compared to the GH peaks observed after saline, which were normalized to 100% in each subject. GH peaks after GHRH and 25 mg hydrocortisone (70 +/- 11%) and GHRH and 250 mg hydrocortisone (69 +/- 7%) were significantly (P < 0.05) lower than the GH peaks after GHRH and saline or GHRH and 12.5 mg hydrocortisone (83 +/- 15%). No significant differences were observed between the GH peaks after GHRH and 12.5 mg hydrocortisone or GHRH and saline. Our data demonstrate that in hypoadrenal patients, the acute absence of circulating cortisol does not impair the GH secretory response to GHRH with respect to the eucortisolemic state. Moreover, our data suggest that 700 nmol/L is the approximate threshold serum cortisol concentration above which a decrease in the GH responsiveness to GHRH is observed in humans. Further increases in serum cortisol levels above this threshold value do not cause a proportional decrease in the GH responsiveness to GHRH.

Our recent data show that acute and sustained hypercortisolism decreases circulating growth hormone (GH) levels in acromegaly with respect to saline infusion. It has been hypothesized that in acromegalic patients, as well as in normal subjects, short-term increases in serum cortisol levels may be able to cause an enhancement of hypothalamic somatostatin secretion, which in turn may be responsible for the glucocorticoid mediated GH inhibition. The aim of our study was to investigate the acute effects of an intravenous infusion of hydrocortisone on the GH response to growth hormone-releasing hormone (GHRH) in acromegaly. We studied 6 adult patients with active acromegaly (3 M, 3 F; mean age 60.5 +/- 4.1 years; mean body mass index 27.1 +/- 0.6 kg/m2). All the patients underwent: (1) a bolus intravenous injection of 100 mg hydrocortisone succinate in 2 ml saline, at time -60 followed by a 120-min intravenous infusion of 250 mg hydrocortisone succinate in 250 ml saline, from -60 to 60 min; (2) a bolus intravenous injection of human GHRH 1-29NH2 100 micrograms in 1 ml saline, 60 min after initiation of a 2-hour hydrocortisone infusion; (3) a bolus intravenous GHRH injection 60 min after initiation of a 2-hour saline infusion. In all of the acromegalic patients during hydrocortisone succinate infusion, GH values clearly decreased with respect to basal levels (mean nadir 47 +/- 8.6%, p0.05 with respect to basal levels). After GHRH injection and saline infusion all the patients showed a significant increase in GH levels (mean peak 231.5 +/- 52.8%, p0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The aims of our study were to investigate the effect of the acetylcholinesterase inhibitor pyridostigmine (PD) administration on growth hormone (GH) secretion in acromegaly and to investigate the effects of PD on GH levels following an i.v. infusion of hydrocortisone in acromegaly. We studied five adult patients with active acromegaly, three men and two women with a mean age of 60 +/- 5 years (range 47-71 years) and a mean BMI of 27 +/- 0.7 kg/m2 (range 24-28 kg/m2). All the patients underwent: 1) placebo, 2 tablets po or 2) PD, 120 mg po, at time -60 plus a bolus i.v. injection of 100 mg hydrocortisone succinate in 2 ml saline at time 0 followed by an i.v. infusion of 250 mg hydrocortisone succinate in 250 ml saline from 0 to 120 min, or 3) PD, po or 4) placebo, po at time -60 plus a bolus i.v. injection of 2 ml saline followed by an i.v. infusion of 250 ml saline from 0 to 120 min. Serum GH values did not significantly change after PD administration compared to those during placebo treatment and with respect to baseline levels. In all of the acromegalic patients during hydrocortisone succinate infusion, GH values clearly decreased with respect to basal levels in varying degrees, with a nadir between 90 and 180 minutes after the beginning of hydrocortisone infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

GH secretion is markedly altered in diabetic rats and humans. Diabetes in the rat, whether occurring spontaneously or after streptozotocin administration, results in depressed GH secretion. This defect is likely caused by an increase in hypothalamic somatostatin tone and decreased pituitary GH. The effects of diabetes in humans depend upon the etiology of the disease. In type-1 diabetes, GH secretion is increased and in type 2 it is decreased. Again, these changes are hypothesized to be due to opposite alterations in hypothalamic somatostatin. Current evidence suggests that GH hypersecretion in human type-1 diabetes may be relevant to important metabolic and angiopathic complications of the 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; p0.05). 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; p0.05), 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.

In patients with acromegaly, circulating growht 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 (IV) 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/m 2 . On two randomized occasions, patients underwent a bolus IV injection of 100 mg hydrocortisone succinate at time 0 followed by a 120-minute IV infusion of 250 mg hydrocortisone in 250 mL saline, combined with a 90-minute (from −15 to 75 minutes) IV 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. It also seems likely that in both acromegalic patients and normal subjects, acute increases in serum cortisol levels may cause an enhancement of hypothalamic somatostatin secretion, which in turn may be responsible for the glucocorticoid-mediated GH inhibition.

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 μg in 2 ml of sterile water; 2) oral placebo 60 min before iv injection of 100 μg 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 μg/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 μg/L, respectively, without significant differences between tests. After PD+GHRH the interindividual variability in the GH response was still present but significantly lower than after GHRH alone. The coefficient of variation for each test was 58.7%, 61.3%, 69.3%, respectively (mean 63.1±3.2%). It can be hypothesized that PD may reduce the interindividual variability of the GH response to GHRH in the diabetic population by decreasing somatostatin tone only in diabetic patients with normal-high hypothalamic somatostatin.

We investigated the short-term effect of the TXB inhibitor picotamide on albuminuria induced by exercise in 15 microalbuminuric (i.e., with UAE at rest between 20 and 200 μg/min) type II diabetic patients (12 men and 3 women, age 56 ± 2, BMI 28 ± 1 kg/m2) and in six normal age-matched control subjects. The diabetic subjects performed five submaximal exercise tests (90% of theoretical heart rate) on a cycle ergometer: the first two under basal conditions; the third and fifth after subjects had received picotamide (900 mg/day) or placebo (3 tablets/day) for 10 days; the fourth exercise always was performed after 10 days of wash-out. Control subjects performed two exercises: the first in baseline conditions and the second after 10 days of picotamide administration (900 mg/day). When diabetic patients were untreated, a significant (P < 0.05) increase in UAE with respect to baseline levels was observed immediately after and 1 h after the exercise test. After picotamide administration, UAE significantly decreased (P < 0.05) immediately after and 1 h after exercise, as compared with diabetic patients given a placebo. In normal subjects, exercise was followed by a slight increase in UAE, which was not significantly affected by picotamide administration. Our results show that short-term administration of picotamide is associated with a reduction in UAE after exercise in type II diabetes patients with microalbuminuria while at rest. Picotamide, a TXB synthetase and receptor inhibitor, may decrease exercise-induced albuminuria in diabetic patients through a reduction in circulating TXB levels andinhibition of TXB action, which in turn may act by lowering glomerular capillary hydraulic pressure.

Glucocorticoids are thought to inhibit GH secretion through an enhancement of endogenous somatostatin tone. The aim of our study was to evaluate the effect of arginine, a secretagogue that increases GH secretion acting at the hypothalamic level, probably by decreasing somatostatin tone, on GH-releasing hormone (GHRH)-induced GH secretion in three male and five female adult patients with nonendocrine disease who were receiving daily immunosuppressive glucocorticoid therapy. Six normal subjects (four males and two females) served as controls. GHRH-induced GH secretion was evaluated after 30-min iv infusion of saline (100 mL) or arginine (30 g) in 100 mL saline. After saline administration, steroid-treated patients showed a blunted GH response to GHRH (GH peak, 8.7 +/- 2.4 micrograms/L) compared to that of normal subjects (GH peak, 23.8 +/- 3.9 micrograms/L). The GH responses to GHRH increased (P less than 0.05) after pretreatment with arginine compared to saline pretreatment in both normal subjects (GH peak, 36.6 +/- 4.0 micrograms/L) and steroid-treated patients (GH peak, 28.4 +/- 5.5 micrograms/L). The GH responses to GHRH plus arginine were not significantly different in steroid-treated and normal subjects. Thus, arginine is able to normalize the GH response to GHRH in patients receiving chronic glucocorticoid treatment. Our data are evidence that the stimulatory action of arginine and the inhibitory action of glucocorticoids on GH secretion are mediated by opposite effects on hypothalamic somatostatin tone.

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.

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)

Patients with hyperthyroidism have reduced growth hormone (GH) responses to pharmacological stimuli and reduced spontaneous nocturnal GH secretion. The stimulatory effect of clonidine on GH secretion has been suggested to depend on an enhancement of hypothalamic GH-releasing hormone (GHRH) release. The aim of our study was to evaluate the effects of clonidine and GHRH on GH secretion in patients with hyperthyroidism. Eight hyperthyroid females with recent diagnosis of Graves' disease (age range 20-55 years, body mass index range 19.2-26.2 kg/m2) and 6 healthy female volunteers (age range 22-35 years, body mass index range 19-25 kg/m2) underwent two experimental trials at no less than 7-day intervals: (a) an intravenous infusion of clonidine 150 micrograms in 10 ml of saline, or (b) a bolus intravenous injection of human GHRH (1-29)NH2, 100 micrograms in 1 ml of saline. Hyperthyroid patients showed blunted GH peaks after clonidine (7.1 +/- 1.7 micrograms/l) as compared to normal subjects receiving clonidine (28.5 +/- 4.9 micrograms/l, p less than 0.05). GH peaks after GHRH were also significantly lower in hyperthyroid subjects (8.0 +/- 1.7 micrograms/l) as compared to normal subjects receiving GHRH (27.5 +/- 4.4 micrograms/l, p less than 0.05). No significant differences in the GH values either after clonidine or GHRH were observed in the two groups of subjects examined. Our data demonstrate that the GH responses to clonidine as well as to GHRH in patients with hyperthyroidism are inhibited in a similar fashion with respect to normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Subjects with Cushing's disease have diminished growth hormone (GH) response to growth hormone-releasing hormone (GHRH). The aim of our study was to investigate the underlying mechanism of this diminished GH response in these patients using pyridostigmine (PD), an acetylcholinesterase inhibitor, which is reported to increase GH secretion by reducing somatostatin tone. Eight subjects with untreated Cushing's disease (caused by a pituitary adenoma) and 6 control subjects received GHRH 100 micrograms in 1 ml of saline, as intravenous bolus injection 60 min after (1) placebo (2 tablets, p.o.) or (2) PD (120 mg, p.o.). After GHRH plus placebo, the GH peak (mean +/- SEM) was significantly lower in subjects with Cushing's disease (2.4 +/- 0.5 micrograms/l) compared to control subjects (25.1 +/- 1.8 micrograms/l, p less than 0.05). After GHRH plus PD, the GH peak was significantly enhanced both in subjects with Cushing's disease (7.1 +/- 2.3 micrograms/l, p less than 0.05) and in control subjects (42.3 +/- 4.3 micrograms/l, p less than 0.05). In patients with Cushing's disease, the GH response to GHRH plus PD was lower with respect to the GH response to GHRH alone in normal subjects. We conclude that hypercortisolism may cause a decrease in central cholinergic tone which is in turn hypothesized to be responsible of an enhanced somatostatin release from the hypothalamus. However, other metabolic or central nervous system alterations may act synergistically with hypercortisolism in causing GH inhibition in patients with Cushing's disease.

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

In the present study we investigated the effects of the acetylcholinesterase inhibitor pyridostigmine (PD), which is hypothesized to decrease hypothalamic somatostatin tone, alone and in association with GH-releasing hormone (GHRH) on GH secretion in 18 type 1 diabetic patients and 12 normal subjects using a randomized double blind placebo-controlled protocol. All subjects received either 120 mg oral PD or placebo 60 min before iv injection of either human GHRH-(1-29) NH2 (100 micrograms) or sterile water (2 mL). In normal subjects both PD alone and GHRH alone caused a significant increase in GH. PD and GHRH acted in a synergistic fashion when combined. In diabetic patients the GH response to GHRH was variable. To segregate the responses, the ratio between the GH increase after GHRH plus PD and after GHRH alone was calculated for each subject. In 10 diabetic patients (group A) the ratio was lower than 2 SD (P less than 0.05) from the mean response of normal subjects. These patients showed an exaggerated GH increase after GHRH and a lower GH increase after PD with respect to normal subjects. Eight diabetic patients (group B) showed a ratio similar to that in normal subjects and similar GH responses to the stimuli. No significant differences were found between groups A and B with respect to age, body mass index, and blood glucose levels. Duration of diabetes was longer and basal GH levels were higher in group A. Hemoglobin-A1c was higher in group A, but of only borderline statistical significance (P = 0.052). Our data demonstrate that in diabetic patients with exaggerated GH responses to GHRH an increase in cholinergic tone does not affect GH secretion. These data suggest that in some type 1 diabetic patients an altered somatostatinergic control of GH secretion may contribute to their abnormal GH response to GHRH.

Clinically, glucocorticoids are important immunosuppressive hormones. Yet, these steroids are also potent inhibitors of growth. We hypothesized that glucocorticoids may partially inhibit growth by increased somatostatin (SRIF) secretion. We tested this hypothesis using passive immunization techniques. Male rats (approximately 50 g) were treated daily (ip) for 33 days with saline (SAL) or dexamethasone (DEX, 40 micrograms/kg) and every fourth day with normal sheep serum (NSS) or SRIF antiserum (SRIF-ab, 0.25 ml). Body wts were recorded daily. Groups (n = 6) were: 1) SAL + NSS, 2) SAL + SRIF-ab, 3) DEX + NSS, and 4) DEX + SRIF-ab. Regression analysis of the growth curves clearly demonstrated differences in body wt gain for the four treatment groups (P less than 0.01). Final body wt of SAL + NSS treated rats was 285 +/- 5 g and 285 +/- 12 g in SAL + SRIF-ab treated rats. DEX + NSS treated rats weighed significantly less (227 +/- 4 g, P less than 0.01) than SAL-treated rats. This glucocorticoid-induced decrease in body wt was partially reversed by the concomitant treatment of rats with SRIF-ab (241 +/- 6 g, P less than 0.05). This observation leads to the conclusion that the inhibitory effect of glucocorticoids on growth may be mediated, in part, by increased SRIF secretion.

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.

In vitro studies have demonstrated that thyroid hormones can enhance basal and stimulated growth hormone secretion by cultured pituitary cells. However, both in man and in the rat the effects of high thyroid hormone levels on GH secretion are unclear. The aim of our study was to test the GH response to human GHRH in hyperthyroid patients and to evaluate the effects on GH secretion of short- and long-term pharmacological decrease of circulating thyroid hormones. We examined 10 hyperthyroid patients with recent diagnosis of Graves' disease. Twelve healthy volunteers served as controls. All subjects received a bolus iv injection of GHRH(1-29)NH2, 100 μg. Hyperthyroid patients underwent a GHRH test one and three months after starting antithyroid therapy with methimazole, 10 mg/day po. GH levels at 15, 30, 45, 60 min and GH peak after stimulus were significantly lower in hyperthyroid patients than in normal subjects. The GH peak was also delayed in hyperthyroid patients. After one month of methimazole therapy, most of the hyperthyroid patients had thyroid hormone levels in the normal range, but they did not show significant changes in GH levels after GHRH, and the GH peak was again delayed. After three months of therapy with methimazole, the hyperthyroid patients did not show a further significant decrease in serum thyroid hormone levels. However, mean GH levels from 15 to 60 min were significantly increased compared with the control study. The GH peak after GHRH was also earlier than in the pretreatment study. In conclusion, the GH response to GHRH is inhibited and delayed by hyperthyroidism and returns to the normal pattern after long-term euthyroidism has been achieved with methimazole.

SUMMARY 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.