Novel mechanisms of growth hormone regulation: growth hormone-releasing peptides and ghrelin.

Growth hormone secretion is classically modulated by two hypothalamic hormones, growth hormone-releasing hormone and somatostatin. A third pathway was proposed in the last decade, which involves the growth hormone secretagogues. Ghrelin is a novel acylated peptide which is produced mainly by the stomach. It is also synthesized in the hypothalamus and is present in several other tissues. This endogenous growth hormone secretagogue was discovered by reverse pharmacology when a group of synthetic growth hormone-releasing compounds was initially produced, leading to the isolation of an orphan receptor and, finally, to its endogenous ligand. Ghrelin binds to an active receptor to increase growth hormone release and food intake. It is still not known how hypothalamic and circulating ghrelin is involved in the control of growth hormone release. Endogenous ghrelin might act to amplify the basic pattern of growth hormone secretion, optimizing somatotroph responsiveness to growth hormone-releasing hormone. It may activate multiple interdependent intracellular pathways at the somatotroph, involving protein kinase C, protein kinase A and extracellular calcium systems. However, since ghrelin has a greater ability to release growth hormone in vivo, its main site of action is the hypothalamus. In the current review we summarize the available data on the: a) discovery of this peptide, b) mechanisms of action of growth hormone secretagogues and ghrelin and possible physiological role on growth hormone modulation, and c) regulation of growth hormone release in man after intravenous administration of these peptides.

Novel mechanisms of growth hormone regulation: growth hormone-releasing peptides and ghrelin

Growth hormone secretion is classically modulated by two hypothalamic hormones, growth hormone-releasing hormone and somatostatin. A third pathway was proposed in the last decade, which involves the growth hormone secretagogues. Ghrelin is a novel acylated peptide which is produced mainly by the stomach. It is also synthesized in the hypothalamus and is present in several other tissues. This endogenous growth hormone secretagogue was discovered by reverse pharmacology when a group of synthetic growth hormone-releasing compounds was initially produced, leading to the isolation of an orphan receptor and, finally, to its endogenous ligand. Ghrelin binds to an active receptor to increase growth hormone release and food intake. It is still not known how hypothalamic and circulating ghrelin is involved in the control of growth hormone release. Endogenous ghrelin might act to amplify the basic pattern of growth hormone secretion, optimizing somatotroph responsiveness to growth hormone-releasing hormone. It may activate multiple interdependent intracellular pathways at the somatotroph, involving protein kinase C, protein kinase A and extracellular calcium systems. However, since ghrelin has a greater ability to release growth hormone in vivo, its main site of action is the hypothalamus. In the current review we summarize the available data on the: a) discovery of this peptide, b) mechanisms of action of growth hormone secretagogues and ghrelin and possible physiological role on growth hormone modulation, and c) regulation of growth hormone release in man after intravenous administration of these peptides.

Acute decrease in circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis.

In thyrotoxicosis there is an impaired GH response to GHRH, normal GH responsiveness to GHRP-6 and lack of synergistic GH response after simultaneous administration of both peptides. We have previously shown that the GHRH-induced GH release in these patients increases after an acute reduction of circulating T3 values with administration of iopanoic acid, a compound that inhibits peripheral conversion of T4 to T3. We have now studied the effect of a decrease in serum T3 levels on the GH response to GHRP-6 (1 microg/kg) plus GHRH (100 microg) in 9 hyperthyroid patients before and after 15 days of treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (600 mg/day). Nine normal subjects were also studied. In all hyperthyroid patients iopanoic acid induced a rapid decrease and normalisation of serum T3 levels. In these subjects peak GH (microg/l; mean +/- SE) and AUC (microg/l x 120 min) values after GHRP-6 plus GHRH were significantly higher on day 15 compared to pretreatment values (peak, 18.3 +/- 3.0 vs 13.4 +/- 1.9; AUC, 1227.9 +/- 212.9 vs 968.5 +/- 160.4; p<0.05). Despite the significant enhancement of the GH responsiveness to GHRP-6 plus GHRH after treatment with iopanoic acid, this response remained significantly blunted when compared to controls both in terms of peak GH (18.3 +/- 3.0 vs 83.7 +/- 15.2; p<0.05) and AUC values (1227.9 +/- 212.9 vs 4956.5 +/- 889.3; p<0.05). In conclusion, our results show that an acute decrease of circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis. This could suggest that circulating T3 does not have a major role in the mechanisms involved in the synergistic effect of these peptides.

GHRP-6 is able to stimulate cortisol and ACTH release in patients with Cushing's disease: comparison with DDAVP.

It has been shown that hexarelin stimulates ACTH and cortisol secretion in patients with Cushing's disease. The ACTH release induced by this peptide is 7-fold greater than that obtained by hCRH. The mechanism of action of hexarelin on the hypothalamic-pituitary-adrenal axis has not been fully elucidated. Although controversial, there is evidence that it might be mediated by arginine vasopressin (AVP). The aim of this study was to evaluate the ACTH and cortisol releasing effects of GHRP-6 in patients with Cushing's disease and to compare them with those obtained with DDAVP administration. We studied 10 patients with Cushing's disease (8 female, 2 male; age: 36.7 +/- 4.2 yr), 9 with microadenomas, who were submitted to both GHRP-6 (2 microg/kg iv) and DDAVP (10 micro g i.v.) in bolus administration on 2 separate occasions. ACTH was measured by immunochemiluminometric assay and cortisol by radioimmunoassay. The sensitivities of the assays are 0.2 pmol/l for ACTH, and 11 nmol/l for cortisol. GHRP-6 was able to increase significantly both ACTH (pmol/l, mean +/- SE; basal: 15.5 +/- 1.7 vs peak: 45.1 +/- 9.3) and cortisol values (nmol/l, basal: 583.0 +/- 90.8 vs peak: 1013.4 +/- 194.6). ACTH AUC (pmol/l min(-1)) and cortisol AUC (nmol/l min(-1)) values were 1235.4 and 20577.2, respectively. After DDAVP administration there was a significant increase in ACTH (basal: 13.0 +/- 1.4 vs peak: 50.5 +/- 16.2) and cortisol levels (basal: 572.5 +/- 112.7 vs peak: 860.5 +/- 102.8. AUC values for ACTH and cortisol were 1627.6 +/- 639.8 and 18364.7 +/- 5661.4, respectively. ACTH and cortisol responses to GHRP-6 and DDAVP did not differ significantly (peak: 45.1 +/- 9.3 vs 50.5 +/- 16.2; AUC: 1235.4 +/- 424.8 vs 1627.6 +/- 639.8). There was a significant positive correlation between peak cortisol values after GHRP-6 and DDAVP administration (r = 0.87, p = 0.001). Our results show that GHRP-6 is able to stimulate ACTH and cortisol release in patients with Cushing's disease. These responses are similar to those obtained after DDAVP injection. These findings could suggest the hypothesis that both peptides act by similar mechanisms, either at hypothalamic or pituitary level.

GH-releasing peptide (GHRP-6)-induced ACTH release in patients with addison's disease: effect of glucocorticoid withdrawal.

GH releasing peptide (GHRP-6) is a synthetic hexapeptide with potent GH releasing activity both in man and in animals. This peptide is also able to stimulate ACTH and cortisol (F) release. It has been suggested that the ACTH responsiveness to GHRP-6 is modulated by circulating glucocorticoid levels. To further clarify this hypothesis, we studied the effect of GHRP-6 (1 ug/kg, iv) on ACTH and F release in patients with Addison's disease (no.=6) during replacement therapy and after 72 h of glucocorticoid withdrawal. Seven controls were also submitted to a single GHRP-6 test. In control subjects, ACTH values (pmol/l; mean +/- SE) increased from 2.9 +/- 0.8 to 4.7 +/- 1.4 (peak). AUC (pmol.min/l) values were 170.3 +/- 48.8. F (nmol/l) values increased from 257.0 +/- 42.9 to 367.0 +/- 50.8. In patients with Addison's disease there was an increase in ACTH levels from 38.1 +/- 17.1 to 174.9 +/- 79.4 after GHRP-6 administration. AUC values were 5490.4 +/- 2269.1. After 72 h withdrawal of glucocorticoid, there was an increase in basal ACTH values (191.2 +/- 97.3), and a trend toward an increase in ACTH levels after GHRP-6 (p=0.053). Patients with Addison's disease on therapy showed a significantly higher ACTH response to GHRP-6 when compared to controls. Our results show that in patients with Addison's disease on replacement there is an increased ACTH release after GHRP-6 administration, compared to controls. After 72 h glucocorticoid withdrawal, this enhanced responsiveness is not maintained. Our data suggest that circulating glucocorticoids modulate GHRP-6-induced ACTH release and that multiple mechanisms may be involved in this process.

Dexamethasone does not increase IGF-I and IGFBP-3 levels in man in the absence of endogenous GH.

It has been previously shown that short-term glucocorticoid administration increases circulating IGF-I and IGFBP-3 levels both in men and rats. An increase in endogenous GH secretion or a direct hepatic effect have been suggested as possible mechanisms. The aim of this study was to investigate the effect of short-term dexamethasone administration (3 mg orally during 7 days) in 8 patients with Sheehan's syndrome in replacement therapy. All patients had GH values <2.5 pg/l after clonidine administration. Before treatment IGF-I values were 9.3 3.6 microg/l (mean +/- SE) and IGFBP-3 levels were 1,195 +/- 208 microg/l. After dexamethasone administration there were no significant changes either in IGF-I or IGFBP-3 values (10.7 +/- 4.1 and 1,110 +/- 214 microg/l, respectively). A significant increase in insulin values was observed after dexamethasone administration (before: 120 +/- 10 micromol/l; after: 175 +/- 27 pmol/l; p<0.05), while glucose levels did not reach statistical significance (before: 4.6 +/- 0.3 mmol/l; after: 5.9 +/- 1.0 mmol/l). Our data suggest that dexamethasone is unable to increase circulating IGF-I and IGFBP-3 levels in man in the absence of endogenous GH.

Effects of short-term glucocorticoid deprivation on growth hormone (GH) response to GH-releasing peptide-6: studies in normal men and in patients with adrenal insufficiency.

There are no data in the literature about the effects of glucocorticoid deprivation on GH-releasing peptide-6 (GHRP-6)-induced GH release. The aims of this study were to evaluate GH responsiveness to GHRP-6 1) after metyrapone administration in normal men, and 2) in patients with chronic hypocortisolism after glucocorticoid withdrawal for 72 h. In normal subjects, metyrapone ingestion did not alter significantly GH responsiveness to GHRP-6 [n = 8; peak, 39.3 +/-7.1 microg/L; area under the curve (AUC), 1958.8 +/- 445.7 microg/min x L; mean +/- SE] compared to placebo (n = 8; peak, 21.9 +/- 4.5; AUC, 1131.0 +/- 229.6). In patients with chronic hypocortisolism (n = 8), GH responses to GHRP-6 were similar both during replacement therapy (peak, 11.8 +/- 3.9; AUC, 563.2 +/- 208.7) and after withdrawal of prednisone (peak, 14.4 +/- 4.5; AUC, 695.6 +/- 272.9) and did not differ from those in controls. Interestingly, after glucocorticoid withdrawal, GH responsiveness to GHRP-6 in patients with chronic hypocortisolism was significantly lower than that in normal subjects pretreated with metyrapone. Our data suggest that short term glucocorticoid deprivation does not have a major impact on GHRP-6-dependent GH-releasing mechanisms. However, in long standing hypocortisolism, subtle changes in GHRP-6 secretory pathways may be present.

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.

Iopanoic acid-induced decrease of circulating T3 causes a significant increase in GH responsiveness to GH releasing hormone in thyrotoxic patients.

OBJECTIVE: Thyroid hormones participate in GH synthesis and secretion, and an impaired GH response to many pharmacological stimuli, including GH releasing hormone (GHRH), has been found in thyrotoxicosis. Although the mechanisms involved in this process have not been fully elucidated, there is evidence that thyroid hormones could act at both hypothalamic and pituitary levels. There are no data in the literature about the effect of an acute reduction of circulating T3 levels on GH secretion in hyperthyroidism. The GH responsiveness to GHRH was therefore evaluated in a group of hyperthyroid patients during short-term treatment with iopanoic acid. Iopanoic acid is a compound that induces a rapid decrease in serum T3 levels, mainly by inhibition of peripheral conversion of T4 to T3. To the authors' knowledge, there is no evidence of a direct effect of iopanoic acid on GH secretion. DESIGN: Hyperthyroid patients were submitted to a GHRH test (100 microg, i.v.) before (day 0), and on days 4, 7 and 15 after oral treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (200 mg every 8 h). A group of normal control subjects was also submitted to a single GHRH test (100 microg, i.v.). PATIENTS: Nine patients with thyrotoxicosis (eight women, one man), with a mean age of 34 years, were studied. All patients had high serum levels of total T3 and total T4, and suppressed TSH levels. None of them had taken any medication for at least 3 months before the study. The patients were compared with a group of nine control subjects (five women, four men) with a mean age of 31 years. MEASUREMENTS: GH and TSH were measured by immunofluorometric assays. Total T3, total T4 and IGF-I were determined by radioimmunoassay. Albumin levels were measured by a colorimetric method. RESULTS: Iopanoic acid induced a rapid and maintained decrease in serum T3 concentrations, with a significant reduction on days 4, 7 and 15 compared with pre-treatment values. In hyperthyroidism, peak GH levels (mean +/- SE mU/l) after GHRH were significantly higher on day 15 (24.4 +/- 3.8) than those observed on days 0 (14.2 +/- 1.6), 4 (15.2 +/- 3.0) and 7 (19.6 +/- 5.0). There was a 79% increase in this response on day 15 compared with the pre-treatment period. Hyperthyroid patients had a blunted GH response to GHRH on days 0, 4 and 7 in comparison with control subjects. However, on day 15, no differences were observed between the area under the curve (mean +/- SE mU/l.120 min) in thyrotoxic patients (1770 +/- 306) and in the control group (3300 +/- 816). IGF-I and albumin levels did not change during iopanoic acid administration. CONCLUSIONS: The results show that an acute reduction in serum T3 levels elicits an increase in GH responsiveness to GHRH in hyperthyroidism. Although the mechanisms involved in this process are still unknown, it is possible that T3 influences GH responsiveness to GHRH via hypothalamic somatostatin release. Alternatively, T3 could have a direct effect at the pituitary somatotroph, modulating GHRH intracellular pathways.

IGF-I levels rise and GH responses to GHRH decrease during long-term prednisone treatment in man.

Glucocorticoid excess is associated with a blunted GH response to GHRH. IGF-I levels in hypercortisolism are controversial and have been reported as low, normal or high. The aim of this study was to evaluate longitudinally time-dependent changes in the GH response to GHRH, IGF-I, IGFBP-3 and albumin values in patients during corticotherapy. Six patients received GHRH before and after one week and one month of prednisone administration (20-60 mg/d, orally). IGF-I, IGFBP-3 and albumin were determined in each test, at time 0. Ten normal controls were also evaluated in one occasion. There were no differences in basal GH values, GH response to GHRH, IGF-I and IGFBP-3 levels between controls and patients before starting corticotherapy. Albumin (g/l; mean+/-SE) values were lower in patients before treatment (31+/-4) than in controls (43+/-1). After one week of prednisone administration there was a significant decrease in peak GH (microg/l) levels (before: 18.8+/-7.4; 1 week: 5.0+/-1.3), which was maintained after one month (8.1+/-3.5). IGF-I (microg/l) levels increased significantly, from 145+/-23 to 205+/-52 after one week of therapy, reaching levels of 262+/-32 after one month. IGFBP-3 (mg/l) values did not increase significantly (before: 2.1+/-0.2; 1 week: 2.5+/-0.3; 1 month: 2.8+/-0.2). Albumin levels showed a significant rise both after one week (36+/-4) and one month (42+/-3) of corticotherapy. In summary, we observed a marked decrease in the GH response to GHRH after one week and one month of prednisone administration associated with an increase in circulating IGF-I and albumin values. The physiological implications of these findings are still uncertain. It is possible that glucocorticoids increase hepatic IGF-I and albumin synthesis, although other mechanisms may have a role.

Growth hormone (GH) response to GH-releasing peptide-6 in patients with insulin-dependent diabetes mellitus.

In insulin-dependent diabetes mellitus (IDDM), inappropriate growth hormone (GH) responses to several stimuli, including GH-releasing hormone (GHRH), have been described. A decreased hypothalamic somatostatinergic tone is one of the most likely explanations for these findings. His-DTrp-Ala-Trp-DPhe-Lys-NH2 [GH-releasing peptide-6 [GHRP-6]] is a synthetic hexapeptide that stimulates GH release in vitro and in vivo. The mechanism of action of GHRP-6 is unknown, but it probably does not inhibit hypothalamic somatostatin secretion. Also, GHRH and GHRP-6 apparently activate different intracellular pathways to release GH. The aim of this study was to evaluate whether there is a differential effect of IDDM on GHRP-6- and GHRH-induced GH secretion. Six patients with IDDM and seven control subjects were studied. Each subject received GHRP-6 (1 microgram/kg intravenously [IV]), GHRH (100 micrograms IV), and GHRP-6 + GHRH on 3 separate days. GH peak values (mean +/- SE in micrograms per liter) were similar in controls and diabetics after GHRH (22.5 +/- 7.8 v 24.0 +/- 9.7) and after GHRP-5 (20.5 +/- 5.3 v 24.4 +/- 6.3). The association of GHRP-6 and GHRH induced a significantly higher GH release than administration of the isolated peptides in both groups. The synergistic GH response to combined administration of GHRP-6 and GHRH was not different in controls (70.5 +/- 20.0) and diabetics (119.0 +/- 22.2). In summary, the effectiveness of GHRP-6 in IDDM could reinforce the evidence that this peptide probably does not release GH through a decrease in hypothalamic somatostatin secretion. Moreover, our data suggest that both GHRH and GHRP-6 releasing mechanisms are unaltered in IDDM.

Different effects of growth hormone releasing peptide (GHRP-6) and GH-releasing hormone on GH release in endogenous and exogenous hypercortisolism.

OBJECTIVE: Chronic hypercortisolism is associated with decreased GH responsiveness to GHRH. GHRP-6 is a synthetic hexapeptide that releases GH in several species, including man. As GHRH and GHRP-6 apparently stimulate GH release by different mechanisms, we evaluated the GH responses to these peptides in patients with endogenous and exogenous glucocorticoid excess and also in control subjects. DESIGN: Six patients with endogenous hypercortisolism, nine with exogenous glucocorticoid excess and 10 normal controls were submitted to three tests, in random order, with GHRH (100 micrograms), GHRP-6 (1 microgram/ kg) or GHRP+GHRP-6, in the same doses, i.v., on separate days. MEASUREMENTS: GH was measured by immunofluorometric assay. IGF-I was determined by radioimmunoassay. Plasma glucose was measured by the glucose-oxidase technique. RESULTS: Peak GH values (mean +/- SE; microgram/l) after GHRH were significantly blunted in endogenous (2.0 +/- 0.7) and exogenous (3.6 +/- 1.2) hypercortisolaemic patients compared to controls (24.9 +/- 6.1). The endogenous group had lower peak GH values after GHRP-6 alone (7.7 +/- 1.9) or together with GHRH (18.8 +/- 5.8) than those observed in controls (GHRP-6: 22.1 +/- 3.6; GHRH+GHRP-6: 77.4 +/- 15.0) and in exogenous hypercortisolism (27.4 +/- 6.2 and 78.1 +/- 19.9). There were no differences in the GH responses to GHRP-6 alone or in combination with GHRH when controls were compared to the exogenous group. No changes in plasma IGF-I and glucose levels were observed. CONCLUSIONS: Our results suggest that hypercortisolism had a different effect on the GH-releasing mechanisms stimulated by GHRH and GHRP-6. Moreover, in endogenous hypercortisolism both GHRH and GHRP-6 pathways are affected, while in the exogenous group GHRP-6 releasing mechanisms are apparently preserved.

Growth hormone responses to GH-releasing peptide (GHRP-6) in hypothyroidism.

OBJECTIVE: Both spontaneous and stimulated GH secretion are reduced in patients with hypothyroidism. The mechanisms involved in these alterations are not yet fully understood. GHRP-6 is a synthetic hexapeptide that releases GH both in vivo and in vitro. Its mechanism of action is unknown, but there is evidence that this peptide acts as a functional somatostatin antagonist at pituitary level. The aim of this study was to evaluate the GH response to GHRP-6 in patients with primary hypothyroidism and in normal controls. DESIGN: Patients with hypothyroidism and normal controls were randomly submitted to 3 tests with GHRH (100 micrograms i.v.), GHRP-6 (1 microgram/kg i.v.) and GHRH + GHRP-6, on separate days. PATIENTS: Eleven patients with primary hypothyroidism were compared with 10 control subjects. MEASUREMENTS: GH, TSH and free T4 were measured by immunofluorometric assay and IGF-1 by radioimmunoassay. RESULTS: Hypothyroid patients had markedly lower peak GH values (mean +/- SE micrograms/l) after GHRH administration (4.1 +/- 0.9) compared to control subjects (24.9 +/- 5.1). After GHRP-6 injection hypothyroid patients had a significantly higher GH release (12.6 +/- 1.9) than that obtained with GHRH, while in control subjects GH values were similar (22.1 +/- 3.6). No significant differences in peak GH responses were observed following the administration of either GHRP-6 alone (controls 22.1 +/- 3.6; patients 12.6 +/- 1.9) or in combination with GHRH (controls 77.4 +/- 15.0; patients 52.8 +/- 10.9), despite the trend to smaller responses in hypothyroid patients. CONCLUSION: We have shown that patients with primary hypothyroidism have higher GH responses to GHRP-6 than to GHRH, which are markedly blunted. When GHRP-6 was associated with GHRH, a significant increase in the GH response was observed in these patients, which could suggest a role for somatostatin in this process. Our data suggest that thyroid hormones modulate GH release induced by GHRH and GHRP-6 through different mechanisms. However, additional studies are necessary to further elucidate this hypothesis.

Effect of low-dose oral and intravenous dexamethasone administration on growth hormone secretion in children.

Acute dexamethasone administration (2 mg/m2 i.v. and 4 mg orally) increases growth hormone (GH) release in children. We evaluated the effect of a low intravenous dose (1 mg/m2) of dexamethasone on GH secretion in 8 short normal children and in 6 GH-deficient children. There was a significant GH increase at 120, 150 and 180 min in short normal children (maximal value: 18.9 +/- 2.1 micrograms/l; mean +/- EP), compared to placebo administration. In contrast, no significant GH elevation was seen in GH-deficient children (1.3 +/- 0.4 micrograms/l). There was no difference in the GH response after intravenous dexamethasone and oral clonidine in these same 8 short normal children and 6 GH-deficient children. Although no significant GH release was observed after dexamethasone or clonidine in GH deficiency, an increase in GH after GH-releasing hormone was seen (6.1 +/- 1.9 micrograms/l). There was a significant GH increase (18.5 +/- 3.3 micrograms/l) after low-dose (2-mg) oral dexamethasone administration in another 8 short normal children, which was similar to values after intravenous injection. No side effects were noted after intravenous or oral dexamethasone. In conclusion, low-dose intravenous or oral dexamethasone administration causes a marked GH release in short normal children, probably mediated by hypothalamic structures.

Different growth hormone (GH) response to GH-releasing peptide and GH-releasing hormone in hyperthyroidism.

Altered GH responses to several pharmacological stimuli, including GHRH, have been found in hyperthyroidism. The mechanisms underlying these disturbances have not been fully elucidated. GH-releasing peptide-6 (GHRP-6) is a synthetic hexapeptide that specifically stimulates GH release both in vitro and in vivo. The mechanism of action of GHRP-6 is unknown, but it probably acts by inhibiting the effects of somatostatin on GH release. The aim of this study was to evaluate the effects of GHRP-6 on GH secretion in patients with hyperthyroidism (n = 9) and in control subjects (n = 9). Each subject received GHRP-6 (1 microg/kg, iv), GHRH (100 microg, iv), and GHRP-6 plus GHRH on 3 separate days. GH peak values (mean +/- SE; micrograms per L) were significantly lower in hyperthyroid patients compared to those in control subjects after GHRH alone (9.0 +/- 1.3 vs. 27.0 +/- 5.2) and GHRP-6 plus GHRH (22.5 +/- 3.5 vs. 83.7 +/- 15.2); a lack of the normal synergistic effect of the association of both peptides was observed in thyrotoxicosis. However, a similar GH response was seen in both groups after isolated GHRP-6 injection (31.9 +/- 5.7 vs. 23.2 +/- 3.9). In summary, we have shown that hyperthyroid patients have a normal GH response to GHRP-6 together with a blunted GH responsiveness to GHRH. Our data suggest that thyroid hormones modulate GH release induced by these two peptides in a differential way.

A prolactin-secreting tumor in a patient with Klinefelter's syndrome: a case report.

We report the case of a patient with Klinefelter's syndrome who developed a prolactin (PRL)-secreting tumor. The patient developed headaches, visual alterations and also symptoms of hypogonadism despite appropriate testosterone (T) replacement therapy. The diagnosis of hyperprolactinemia was then suspected. The laboratory findings confirmed the hypothesis, showing high levels of serum PRL. The patient was initially treated with oral bromocriptine, and afterwards with the injectable form. There was a marked decrease in PRL levels and in tumor size. Although some neoplasms, like breast carcinoma and germ cell tumors, are known to occur more frequently in patients with Klinefelter's syndrome, an association with PRL-secreting tumor has not been reported yet. In conclusion, symptoms of hypogonadism in patients with Klinefelter's syndrome receiving appropriate T replacement therapy can suggest the presence of hyperprolactinemia.

Growth hormone axis in cushing's syndrome.

All levels of the growth hormone (GH), GH binding protein (GHBP), insulin-like growth factor (IGF) and IGF binding protein (IGFBP) axis are influenced by chronic hypercortisolism. Thus, there is a blunted response to GHRH alone or together with other stimuli associated with a marked suppression of endogenous GH secretion but accompanied by normal GHBP, normal to low IGF-1 and GHBPs 1 and 3 with the correspondent 41.5 and 38.5-kD molecular forms of the latter presenting values similar to normal. These findings may suggest enhanced GH sensitivity with normal or increased IGF-1 bioavailability to the correspondent tissue receptors. In conclusion, the glucocorticoid (GC)-induced target tissue resistance can neither be attributed to the suppression of the GH axis nor to changes in circulating GHBPs 1 and 3. However, it may be related either to the described 12-to-20-kD inhibitor(s) which antagonizes postbinding IGF-1 bioactivity (gene expression) and/or by the downmodulation of activator protein-1 (Fos/Jun) activity by the GC-GC receptor complex.

Low circulating IGF-I levels in hyperthyroidism are associated with decreased GH response to GH-releasing hormone.

OBJECTIVE: Several abnormalities in the GH response to pharmacological stimuli have been described in hyperthyroidism. Both normal and high serum IGF-I levels have been reported, as well as a decrease in IGF-I bioactivity. We have evaluated the GH response to GH-releasing hormone (GHRH) in hyperthyroid patients and the effects of hyperthyroidism on serum IGF-I levels. The possible relations between nutritional status, thyroid hormones and IGF-I levels were also investigated. We also studied the influence of long-term beta-adrenoceptor blockade on the GH response to GHRH in these patients. DESIGN: In 18 hyperthyroid patients and in 12 control subjects, GHRH (100 micrograms) was administered as an i.v. bolus injection. Eight hyperthyroid patients and 8 control subjects received 50 micrograms GHRH i.v. Seven hyperthyroid patients were reevaluated after beta-adrenoceptor blockade. IGF-I and albumin levels were measured initially in all hyperthyroid patients and control subjects. Body composition was determined in 11 hyperthyroid patients and in a group of 33 matched normal controls. PATIENTS: Hyperthyroid patients were compared to control subjects. MEASUREMENTS: GH, TSH and free T4 were measured by immunofluorometric assay. IGF-I, total T3 and total T4 were measured by radioimmunoassay. Body composition was determined using a dual-energy X-ray absorptiometer. RESULTS: The GH response to 100 micrograms GHRH in hyperthyroid patients was blunted compared to control subjects. The mean peak GH levels and the area under the curve were significantly lower in hyperthyroid patients compared to control subjects (11 +/- 1 vs 27 +/- 5 micrograms/l and 820 +/- 113 vs 1879 +/- 355 micrograms/l 120 min, respectively; P < 0.01). IGF-I levels were significantly reduced in hyperthyroid patients compared to controls (131 +/- 10 vs 201 +/- 16 micrograms/l, respectively; P < 0.01). Ideal body weight, serum albumin levels and the lean body mass were also reduced in hyperthyroid patients. After beta-adrenoceptor blockade there were no changes in the blunted GH response to GHRH in hyperthyroid patients. CONCLUSION: Our data suggest that the blunted GH response to GHRH in hyperthyroidism is apparently not related to circulating IGF-I levels. It is possible that nutritional factors could play a role in the reduced circulating IGF-I levels found in these patients.

Different effects of pyridostigmine on growth hormone (GH) response to GH-releasing hormone in endogenous and exogenous hypercortisolemic patients.

1. Somatostatin may play a role in the inhibition of growth hormone (GH) response to GH-releasing hormone (GHRH) in hypercortisolism. To examine this hypothesis we studied the effect of pyridostigmine, a cholinergic agonist that decreases hypothalamic somatostatin, on the GH response to GHRH in 8 controls, in 6 patients with endogenous hypercortisolism (3 with Cushing's disease and 3 with adrenal adenomas) and in 8 patients with exogenous hypercortisolism (lupus erythematosus chronically treated with 20-60 mg/day of prednisone). Each subject received GHRH(1-29)NH2,100 micrograms iv twice, preceded by pyridostigmine (120 mg) or placebo, orally. 2. The GH response to GHRH was significantly blunted in all hypercortisolemic patients compared to controls both after placebo (GH peak, 5.8 +/- 1.6 vs 46.2 +/- 15.9 micrograms/l, mean +/- SEM) and after pyridostigmine (15.7 +/- 5.6 vs 77.2 +/- 19.8 micrograms/l). 3. The GH response was absent in endogenous hypercortisolemic patients compared to the exogenous group, both after placebo (2.2 +/- 0.3 vs 8.5 +/- 2.4 micrograms/l) and after pyridostigmine (4.9 +/- 2.5 vs 23.8 +/- 8.7 micrograms/l). The GH release after GHRH/pyridostigmine for the exogenous group was similar to the response of controls treated with GHRH/placebo. 4. These results confirm that the GH response to GHRH is blunted in hypercortisolism, although more pronounced in the endogenous group. Pyridostigmine partially reversed this inhibition in the exogenous group. Therefore, somatostatin may play a role in the inhibition of GHRH-induced GH release in exogenous hypercortisolemic states.

Different effects of pyridostigmine on growth hormone (GH) response to GH-releasing hormone in endogenous and exogenous hypercortisolemic patients

1. Somatostatin may play a role in the inhibition of growth hormone (GH) response to GH-releasing hormone (GHRH) in hypercortisolism. To examine this hypothesis we studied the effect of pyridostigmine, a cholinergic agonist that decreases hypothalamic somatostatin, on the GH response to GHRH in 8 controls, in 6 patients with endogenous hypercortisolism (3 with Cushing's disease and 3 with adrenal adenomas) and in 8 patients with exogenous hypercortisolism (lupus erythematosus chronically treated with 20-60 mg/day of prednisone). Each subject received GHRH(1-29)NH2,100 micrograms iv twice, preceded by pyridostigmine (120 mg) or placebo, orally. 2. The GH response to GHRH was significantly blunted in all hypercortisolemic patients compared to controls both after placebo (GH peak, 5.8 +/- 1.6 vs 46.2 +/- 15.9 micrograms/l, mean +/- SEM) and after pyridostigmine (15.7 +/- 5.6 vs 77.2 +/- 19.8 micrograms/l). 3. The GH response was absent in endogenous hypercortisolemic patients compared to the exogenous group, both after placebo (2.2 +/- 0.3 vs 8.5 +/- 2.4 micrograms/l) and after pyridostigmine (4.9 +/- 2.5 vs 23.8 +/- 8.7 micrograms/l). The GH release after GHRH/pyridostigmine for the exogenous group was similar to the response of controls treated with GHRH/placebo. 4. These results confirm that the GH response to GHRH is blunted in hypercortisolism, although more pronounced in the endogenous group. Pyridostigmine partially reversed this inhibition in the exogenous group. Therefore, somatostatin may play a role in the inhibition of GHRH-induced GH release in exogenous hypercortisolemic states