Growth hormone (GH) response to GH-releasing peptide-6 and GH-releasing hormone in normal-weight and overweight patients with non-insulin-dependent diabetes mellitus

The growth hormone (GH) response to GH-releasing hormone (GHRH) in patients with non-insulin-dependent diabetes mellitus (NIDDM) was found to be either decreased or normal. The recent introduction of a new and potent GH stimulus, GH-releasing peptide-6 (GHRP-6), allowed further investigation of the functional properties of somatotropes in a variety of metabolic diseases. The aim of the present study was to investigate the response of GH to GHRP-6, GHRH, and GHRP-6 + GHRH in NIDDM patients. Twenty-one patients with NIDDM were divided into two groups: group A, normal weight (body mass index [BMI], 23.31+/-0.62 kg/m2); and group B, overweight (BMI, 27.62+/-0.72 kg/m2). Eight normal-weight control subjects (group C) were studied. Each subject received GHRP-6 (90 microg intravenously [i.v.]), GHRH (100 microg i.v.), and GHRP-6 + GHRH on three separate occasions. There was no difference between the GH response after GHRP-6 in groups A, B, and C in terms of the GH peak (50.95+/-11.55, 51.96+/-7.71, and 70.07+/-15.59 mU/L, P>.05) and the area under the curve (AUC) for GH (2,340.06+/-617.36, 2,684.54+/-560.57, 3,462.78+/-1,223.53 mU/L/120 min, P>.05). A decreased GH response to GHRH was found in group B in comparison to group A (B v A: peak GH response, 8.25+/-1.90 v 22.19+/-8.81, P<.05; AUC GH, 479.62+/-84.0 v 1,443.21+/-743.76, P<.05). There was no difference in the GH response between group A and group C (peak GH response, 22.19+/-8.81 v 26.42+/-6.71, P>.05; AUC, 1,443.21+/-743.76 v 1,476.51+/-386.56, P>.05). There was a significant difference between the same parameters in group B versus group C (8.25+/-1.90 v 26.42+/-6.71, P<.05; AUC, 479.62+/-84.0 v 1,476.51+/-386.56, P<.05). The combined administration of GHRP-6 + GHRH elicited a synergistic GH response in NIDDM patients and controls. There was a significant difference between groups A and B for the GH peak (96.49+/-9.80 v 68.38+/-8.25, P<.05), whereas there was no difference for the AUC (5,111.13+/-703.77 v 3,425.95+/-459.67, P>.05). There was no difference in the peak GH after the combined test between group A and group C (96.49+/-9.80 v 139.82+/-24.16, P>.05), whereas the peak GH in the same test was significantly decreased in group B in comparison to group C (68.38+/-8.25 v 139.82+/-24.16, P<.05). The AUC for GH after combined GHRP-6 + GHRH in group A versus group C was not significantly different (5,111.13+/-703.77 v 9,274.71+/-1,541.46, P>.05), whereas there was a significant difference for the same test between group B and group C (3,425.95+/-459.67 v 9,274.71+/-1,541.46, P<.05). Our results demonstrate that normal-weight NIDDM patients have a preserved GH response to GHRP-6, GHRH, and GHRP-6 + GHRH, and overweight NIDDM patients have a blunted response to GHRH and GHRP-6 + GHRH. The preserved GH response to GHRP-6 in both diabetic groups suggests that the secretory potential of somatotropes is preserved in NIDDM patients. The impairment of the GH response to GHRH in overweight NIDDM patients could be a functional defect due to the obesity, since it could be overridden by administration of GHRP-6.     

Growth hormone (GH)-releasing peptide stimulates GH release in normal men and acts synergistically with GH-releasing hormone

The acute GH release stimulated by the synthetic hexapeptide, His-DTrp-Ala-Trp-DPhe-Lys-NH2 [GH releasing peptide (GHRP)], was determined in 18 normal men and compared with the effects of GH-releasing hormone, GHRH-(1-44)-NH2. Specificity of effect was assessed by measurement of serum PRL, LH, TSH, and cortisol. GHRP was administered at doses of 0.1, 0.3, and 1.0 microgram/kg by iv bolus. GHRH at a dose of 1.0 microgram/kg was administered alone and together with various does of GHRP. No adverse clinical effects of laboratory abnormalities were observed in response to GHRP. A side-effect of mild facial flushing of 1- to 3-min duration occurred in 16 of the 18 subjects who received GHRH-(1-44)-NH2. Mean (+/- SEM) peak serum GH levels after injection of placebo and 0.1, 0.3, and 1.0 microgram/kg GHRP were 1.2 +/- 0.3, 7.6 +/- 2.5, 16.5 +/- 4.1, and 68.7 +/- 15.5 micrograms/L, respectively. The submaximal dosages of 0.1 and 0.3 microgram/kg GHRP plus 1 microgram/kg GHRH stimulated GH release synergistically. Serum PRL and cortisol levels rose about 2-fold above basal levels only at the 1 microgram/kg dose of GHRP, and there were no changes in serum LH and TSH over the first hour after administration of the peptide(s). GHRP is a potent secretagogue of GH in normal men. Since GHRP and GHRH together stimulate GH release synergistically, these results suggest that GHRP and GHRH act independently. This supports our hypothesis that the GH-releasing activity of GHRP reflects a new physiological system in need of further characterization in animals and man.     

Pattern-dependent suppression of growth hormone (GH) pulsatility by ghrelin and GH-releasing peptide-6 in moderately GH-deficient rats

The peptide hormone ghrelin binds to the GH secretagogue receptor (GHS-R), stimulates GH secretion, and promotes adipogenesis. However, continuous GHS infusion does not stimulate skeletal growth and is associated with desensitization to further GH secretagogue treatment. In this study, 7-d intermittent (i.e. every 3 h) infusion of ghrelin, or the GH secretagogue, GH-releasing peptide-6, in the moderately GH- deficient transgenic growth-retarded rat, augmented GH secretion, leading to a sustained acceleration in skeletal growth. In contrast, continuous infusion of ghrelin, or GH-releasing peptide-6, suppressed the amplitude of spontaneous GH secretory episodes and produced only a transient increase in body weight gain. The reduction in GH secretion seen with continuous GHS-R activation was not associated with a desensitization of the pituitary to GH-releasing factor or to down-regulation of hypothalamic GHS-R mRNA expression. Continuous ghrelin treatment elicited an increase in somatostatin mRNA expression in the periventricular nuclei. Thus, exposure to continuously elevated circulating ghrelin may be responsible for the suppression of GH secretion reported in rats after prolonged starvation.     

E2 supplementation selectively relieves GH's autonegative feedback on GH-releasing peptide-2-stimulated GH secretion.

Female gender confers resistance to GH autonegative feedback in the adult rat, thereby suggesting gonadal or estrogenic modulation of autoregulation of the somatotropic axis. Here we test the clinical hypothesis that short-term E2 replacement in ovariprival women reduces GH's repression of spontaneous, GHRH-, and GH-releasing peptide (GHRP)-stimulated GH secretion. To this end, we appraised GH autoinhibition in nine healthy postmenopausal volunteers during a prospective, randomly ordered supplementation with placebo vs. E [1 mg micronized 17 beta-E2 orally twice daily for 6-23 d]. The GH autofeedback paradigm consisted of a 6-min pulsed i.v. infusion of recombinant human GH (10 microg/kg square-wave injection) or saline (control) followed by i.v. bolus GHRH (1 microg/kg), GHRP-2 (1 microg/kg), or saline 2 h later. Blood was sampled every 10 min and serum GH concentrations were measured by chemiluminescence. Poststimulus GH release was quantitated by multiparameter deconvolution analysis using published biexponential kinetics and by the incremental peak serum GH concentration response (maximal poststimulus value minus prepeak nadir). Outcomes were analyzed on the logarithmic scale by mixed-effects ANOVA at a multiple-comparison type I error rate of 0.05. E2 supplementation increased the (mean +/- SEM) serum E2 concentration from 43 +/- 1.8 (control) to 121 +/- 4 pg/ml (E2) (158 +/- 6.6 to 440 +/- 15 pmol/liter; P < 0.001), lowered the 0800 h (preinfusion) serum IGF-I concentration from 127 +/- 7.7 to 73 +/- 3.6 microg/liter (P < 0.01), and amplified spontaneous pulsatile GH production from 7.5 +/- 1.1 to 13 +/- 2.3 microg/liter per 6 h (P = 0.020). In the absence of exogenously imposed GH autofeedback, E2 replacement enhanced the stimulatory effect of GHRP-2 on incremental peak GH release by 1.58-fold [95% confidence interval, 1.2- to 2.1-fold] (P = 0.0034) but did not alter the action of GHRH (0.83-fold [0.62- to 1.1-fold]). In the E2-deficient state, bolus GH infusion significantly inhibited subsequent spontaneous, GHRH-, and GHRP-induced incremental peak GH responses by, respectively, 33% (1-55%; P = 0.044 vs. saline), 79% (68-86%; P < 0.0001), and 54% (32-69%; P = 0.0002). E2 repletion failed to influence GH autofeedback on either spontaneous or GHRH-stimulated incremental peak GH output. In contrast, E2 replenishment augmented the GHRP-2-stimulated incremental peak GH response in the face of GH autoinhibition by 1.7-fold (1.2- to 2.5-fold; P = 0.009). Mechanistically, the latter effect of E2 mirrored its enhancement of GH-repressed/GHRP-2-stimulated GH secretory pulse mass, which rose by 1.5-fold (0.95- to 2.5-fold over placebo; P = 0.078). In summary, the present clinical investigation documents the ability of short-term oral E2 supplementation in postmenopausal women to selectively rescue GHRP-2 (but not spontaneous or GHRH)-stimulated GH secretion from autonegative feedback. The secretagogue specificity of E's relief of GH autoinhibition suggests that this sex steroid may enhance activity of the hypothalamopituitary GHRP-receptor/effector pathway.     

Growth hormone (GH) response to GH-releasing hormone in depression

To explore the GHRH-GH-somatomedin axis integrity in major depressive disorder, 11 drug-free patients and normal subjects matched for age, sex, ovarian status, and body weight received 1 microgram/kg synthetic human GHRH-44 amide as an iv bolus dose. Compared to the normal subjects, the depressed patients had reduced mean basal serum GH levels [2.2 +/- 0.5 (+/- SE) vs. 1.1 +/- 0.2 ng/mL (micrograms/L); P less than 0.05] and a significant attenuation of the net GH response to GHRH [1346 +/- 499 vs. 217 +/- 46 ng.min/mL (micrograms.min/L); P less than 0.01]. The blunted GH responses occurred in the face of significantly increased plasma somatomedin C (Sm-C) levels [1.1 +/- 0.2 vs. 0.6 +/- 0.1 U/mL; P less than 0.05]. The magnitude of GH responses to GHRH did not differ between men and women and was not significantly correlated with age, body weight, baseline serum GH levels, or plasma Sm-C levels in either individual groups or both groups combined. The increased plasma Sm-C levels in the depressed patients could have resulted from diurnal hypersecretion of GH, and the diminished GH responses to GHRH may reflect normal Sm-C-mediated feedback at the level of the pituitary. The presumed GH hypersecretion may be due to decreased hypothalamic somatostatin release and/or hyperactivity of GHRH-containing neurons. Thus, the pathological process resulting in abnormal GH secretory patterns associated with depression may occur primarily at a suprapituitary site.     

Selective lack of growth hormone (GH) response to the GH-releasing peptide hexarelin in patients with GH-releasing hormone receptor deficiency

The mechanism of the synergistic relationship between GH-releasing peptide (GHRP) and GHRH with respect to GH secretion is poorly understood. We report the response to hexarelin, a potent GHRP, in patients affected with a homozygous mutation in the GHRH receptor gene, with consequent GHRH resistance and GH-deficient dwarfism. This newly described syndrome is the human homolog of the little (lit/lit) mouse. Intravenous administration of hexarelin (2 microg/kg) to four male adult patients (dwarfs of Sindh) resulted in a complete lack of elevation in plasma GH levels (< 1 ng/mL), an at least 50- to 100-fold deviation from the normal response. In contrast, plasma PRL, ACTH, and cortisol levels rose in a normal manner in response to hexarelin. We conclude that an intact GHRH signaling system is critical for GHRPs to exert their effect on GH release, but that the GHRH system is not necessary for the effect of GHRP on PRL and ACTH secretion. Hexarelin (and probably other GHRPs) are not effective agents for the treatment of patients with GHRH resistance due to GHRH receptor deficiency.     

Corticoid-induced growth hormone (GH) secretion in GH-deficient and normal children.

Acute administration of corticoids is a potent stimulus of GH secretion in man. To ascertain their mechanism and point of action as well as the suitability of this novel test in the diagnosis of GH-deficient states, normal controls and GH-deficient children were studied. They were selected based on auxological criteria and the GH response to provocative stimuli. The GH-deficient children presented a blunted GH (mean +/- SEM; microgram/L) discharge after insulin-induced hypoglycemia (2.9 +/- 0.4), propranolol-exercise (7.4 +/- 1.5), and clonidine (6.5 +/- 0.8) compared with values in the normal children (7.2 +/- 2.2, 15.8 +/- 2.4, and 15.6 +/- 1.8, respectively). As expected, GH-releasing hormone (GHRH)-induced GH discharge in GH-deficient children (33.2 +/- 4.9) was similar to that in the control children (35.1 +/- 6.0). Administration of 2 mg/m2 body surface dexamethasone, iv, to normal children induced, 3 h later, a mean GH peak of 14.1 +/- 1.2 micrograms/L. This was significantly higher that the corticoid-induced GH peak in GH-deficient children (6.7 +/- 1.1 micrograms/L). The corticoid-induced areas under the secretory curve were 1130 +/- 55 and 616 +/- 54 for the control and GH-deficient children, respectively. GH release in children after dexamethasone administration followed the pattern previously described for adults, i.e. there were no modifications of basal GH levels in the first 2 h, the GH peak appeared around the third hour, and the GH levels remained increased until the fourth hour after dexamethasone administration. Individually considered, practically all control children, but only 2 of 12 GH-deficient children, presented a dexamethasone-induced GH peak over the 10 micrograms/L level. In both normal and GH-deficient patients, corticoids appeared just as potent a stimulus as propranolol-exercise and clonidine, and more potent than hypoglycemia. This new stimulus showed a pattern similar to that of the hypothalamic stimuli, but clearly different with respect to the pituitary one (GHRH), suggesting that corticoids activate GH secretion by acting at hypothalamic level. In conclusion, acute administration of corticoids could be a suitable test in the diagnostic armamentaria of GH-deficient states.     

Pituitary responsiveness to GH-releasing hormone, GH-releasing peptide-2 and thyrotrophin-releasing hormone in critical illness

OBJECTIVE Protein hypercatabolism and preservation of fat depots are hallmarks of critical illness, which is associated with blunted pulsatile GH secretion and low circulating IGF-I, TSH, T4 and T3. Repetitive TRH administration is known to reactivate the pituitary-thyroid axis and to evoke paradoxical GH release in critical illness. We further explored the hypothalamic-pituitary function in critical illness by examining the effects of GH-releasing hormone (GHRH) and/or GH-releasing peptide-2 (GHRP-2) and TRH administration. PATIENTS AND DESIGN Critically ill adults (n=40; mean age 55 years) received two i.v. boluses with a 6-hour interval (0900 and 1500 h) within a cross-over design. Patients were randomized to receive consecutively placebo and GHRP-2 (n=10), GHRH and GHRP-2 (n=10), GHRP-2 and GHRH+GHRP-2 (n=10), GHRH+GHRP-2 and GHRH+GHRP-2+TRH (n=10). The GHRH and GHRP-2 doses were 1μg/kg and the TRH dose was 200μg. Blood samples were obtained before and 20, 40, 60 and 120 minutes after each injection. MEASUREMENTS Serum concentrations of GH, T4, T3, rT3, thyroid hormone binding globulin (TBG), IGF-I, insulin and cortisol were measured by RIA; PRL and TSH concentrations were determined by IRMA. RESULTS Critically ill patients presented a striking GH response to GHRP-2 (mean±SEM peak GH 51±9 μg/l in older patients and 102±2μg/l in younger patients; P=0.005 vs placebo). The mean GH response to GHRP-2 was more than fourfold higher than to GHRH (P=0.007). In turn, the mean GH response to GHRH+GHRP-2 was 2.5-fold higher than to GHRP-2 alone (P=0.01), indicating synergism. Adding TRH to the GHRH+GHRP-2 combination slightly blunted this mean response by 18% (P=0.01). GHRP-2 had no effect on serum TSH concentrations whereas both GHRH and GHRH+GHRP-2 evoked an increase in peak TSH levels of 53 and 32% respectively. The addition of TRH further increased this TSH response < ninefold (P=0.005), elicited a 60% rise in serum T3 (P=0.01) and an 18% increase in T4 (P=0.005) levels, without altering rT3 or TBG levels. GHRH and/or GHRP-2 induced a small increase in serum PRL levels. The addition of TRH magnified the PRL response 2.4-fold (P=0.007). GHRP-2 increased basal serum cortisol levels (531±29nmol/l) by 35% (P=0.02); GHRH provoked no additional response, but adding TRH further increased the cortisol response by 20% (P=0.05). CONCLUSIONS The specific character of hypothalamic-pituitary function in critical illness is herewith extended to the responsiveness to GHRH and/or GHRP-2 and TRH. The observation of striking bursts of GH secretion elicited by GHRP-2 and particularly by GHRH+GHRP-2 in patients with low spontaneous GH peaks opens the possibility of therapeutic perspectives for GH secretagogues in critical care medicine.     

Growth hormone (GH) response to GH-releasing hormone in children with subnormal integrated concentrations of GH

We determined the GH responses to human GH-releasing hormone-40 (GHRH) in poorly growing children who had either normal or deficient GH secretion, as measured by pharmacological stimulation and integrated concentration of GH (IC-GH). Ten patients had both normal pharmacologically stimulated GH and IC-GH (GH-normal), 15 patients had normal pharmacologically stimulated GH but deficient IC-GH [GH neurosecretory dysfunction (GHND)], and the remaining 7 patients had both subnormal stimulated GH and IC-GH [GH deficiency (GHD)]. The mean peak plasma GH response to GHRH was 11.7 +/- 8.5 (+/- SD) ng/ml in GHD patients, significantly lower than the responses of both the GHND (49.2 +/- 39.2 ng/ml; P less than 0.0001) and GH-normal (51.8 +/- 44 ng/ml; P less than 0.0001) groups. The range of peak GH responses to GHRH in GHD patients overlapped the lower end of the range of responses in the GHND and GH-normal patients. Three GH-normal and eight GHND patients had greatly enhanced GH responses to GHRH (greater than 50 ng/ml); no GHD patients had a response over 24.2 ng/ml. There was no difference between the GH responses of male and female patients within groups to GHRH. There was a significant correlation between the log of the peak GH response to GHRH and the log of the maximal GH response to standard pharmacological stimuli (r = 0.51; P less than 0.005). Because of the variability of GH responses to GHRH encountered among the patients, the response to GHRH cannot be used as a test for identifying patients with inadequate spontaneous GH secretion. The IC-GH is the only method that can identify children with GHND.     

Growth hormone (GH) release after administration of GH-releasing hormone in relation to endogenous 24-h GH secretion in short children

Endogenous GH secretion was measured every 20 min for 24 h in 36 short children. This was immediately followed by an i.v. injection of GH-releasing hormone (GHRH)(1-29)-NH2 (1 microgram/kg), and GH was estimated every 15 min for the following 2 h. The aim was to determine whether endogenous pulsatile GH secretion had any relation to, or influence on, the GH release induced by GHRH. A high variability was found both in the 24-h GH secretion expressed as area under the curve above the baseline (0-1588 mU/l x 24 h) and the maximal GH response to GHRH (5-296 mU/l), as well as after an arginine-insulin tolerance test (4-59 mU/l). We found a positive correlation (correlation coefficient of Spearman (rs) = 0.49; P less than 0.01) between the GH response to GHRH and the spontaneous GH secretion over a 24-h period, in spite of a negative correlation (rs = -0.80; P less than 0.01) with the GH secretion during the preceding 3 h. We conclude that the GH response to a GHRH test correlates with endogenous GH secretion in short children, and may be helpful in estimating the ability to release GH. It is important, however, to be aware of the influence of the spontaneous GH secretion during the 3 h immediately preceding administration of GHRH.     

Atenolol enhances nocturnal growth hormone (GH) release in GH-deficient children during long term GH-releasing hormone therapy

The effect of the selective beta 1-adrenergic blocking agent atenolol (50 or 100 mg, orally) on spontaneous and GH-releasing hormone (GHRH)-stimulated GH release was evaluated in six GH-deficient children during long term therapy with GHRH. Nocturnal GH concentrations were determined every 20 min for 12 h under the following four conditions: 1) control, 2) atenolol administration only, 3) sc GHRH administration only, and 4) combined GHRH and atenolol administration. The mean 12-h nocturnal GH concentrations after administration of atenolol alone [2.4 +/- 0.6 microgram/L (mean +/- SEM)] or GHRH alone (2.7 +/- 1.0 micrograms/L) were indistinguishable from baseline values (2.0 +/- 0.5 microgram/L; P greater than 0.05). In contrast, the addition of atenolol to ongoing GHRH therapy caused a clear augmentation of 12-h overnight GH release compared to that during all other study periods (5.0 +/- 1.3 micrograms/L; P less than 0.05). In a subset of three subjects for whom GH pulse characteristics were determined, the primary mode of the enhanced GH release was through an increase in the amplitude of serum GH pulses. These results are consistent with the hypothesis that beta-adrenergic blocking compounds enhance the responsivity of the pituitary gland to agents that permit GH release by inhibiting hypothalamic somatostatin secretion or action. They suggest that atenolol may have potential as an adjunctive therapy in some children with abnormalities of GH secretion when GHRH is the primary therapeutic agent.     

Impaired growth hormone (GH) response to pyridostigmine in type 1 diabetic patients with exaggerated GH-releasing hormone-stimulated GH secretion

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.     

Growth hormone-releasing hormone stimulates mitogen-activated protein kinase

GH-releasing hormone (GHRH) can induce proliferation of somatotroph cells. The pathway involving adenylyl cyclase/cAMP/protein kinase A pathway in its target cells seems to be important for this action, or at least it is deregulated in some somatotroph pituitary adenomas. We studied in this work whether GHRH can also stimulate mitogen-activated protein (MAP) kinase. GHRH can activate MAP kinase both in pituitary cells and in a cell line overexpressing the GHRH receptor. Although both protein kinase A and protein kinase C could activate MAP kinase in the CHO cell line studied, neither protein kinase A nor protein kinase C appears to be required for GHRH activation of MAP kinase in this system. However, sequestration of the betagamma-subunits of the G protein coupled to the receptor inhibits MAP kinase activation mediated by GHRH. This pathway also involves p21ras and a phosphatidylinositol 3-kinase, probably phosphatidylinositol 3-kinase-gamma. Despite the involvement of p21ras, the protein kinase Raf-1 is not hyperphosphorylated in response to GHRH, contrary to what usually occurs when the Ras-Raf-MAP kinase pathway is activated. In summary, this work describes for the first time the activation of MAP kinase by GHRH and outlines a path for this activation that is different from the cAMP-dependent mechanism that has been traditionally described as mediating the mitogenic actions of GHRH.     

Blockade of the growth hormone (GH) receptor unmasks rapid GH-releasing peptide-6-mediated tissue-specific insulin resistance

The roles of GH and its receptor (GHR) in metabolic control are not yet fully understood. We studied the roles of GH and the GHR using the GHR antagonist pegvisomant for metabolic control of healthy nonobese men in fasting and nonfasting conditions. Ten healthy subjects were enrolled in a double blind, placebo-controlled study on the effects of pegvisomant on GHRH and GH-releasing peptide-6 (GHRP-6)-induced GH secretion before and after 3 days of fasting and under nonfasting conditions (n = 5). Under the condition of GHR blockade by pegvisomant in the nonfasting state, GHRP-6 (1 microg/kg) caused a increase in serum insulin (10.3 +/- 2.1 vs. 81.3 +/- 25.4 mU/L; P < 0.001) and glucose (4.2 +/- 0.3 vs. 6.0 +/- 0.6 mmol/L; P < 0.05) concentrations. In this group, a rapid decrease in serum free fatty acids levels was also observed. These changes were not observed under GHR blockade during fasting or in the absence of pegvisomant. We conclude that although these results were obtained from an acute study, and long-term administration of pegvisomant could render different results, blockade of the GHR in the nonfasting state induces tissue-specific changes in insulin sensitivity, resulting in an increase in glucose and insulin levels (indicating insulin resistance of liver/muscle), but probably also in an increase in lipogenesis (indicating normal insulin sensitivity of adipose tissue). These GHRP-6-mediated changes indicate that low GH bioactivity on the tissue level can induce changes in metabolic control, which are characterized by an increase in fat mass and a decrease in lean body mass. As a mechanism of these GHRP-6-mediated metabolic changes in the nonfasting state, direct nonpituitary-mediated GHRP-6 effects on the gastroentero-hepatic axis seem probable.     

A simple diagnostic test using GH-releasing peptide-2 in adult GH deficiency

OBJECTIVE: The international, first-line diagnostic test for adult GH deficiency is the insulin tolerance test (ITT), which is contraindicated in some patients due to severe adverse events. Alternatives such as GH-releasing hormone combined with arginine or GH-releasing peptides (GHRP) have been proposed. We validated the use of GHRP-2 for diagnosing adult GH deficiency (GHD). METHODS: Seventy-seven healthy subjects and 58 patients with peak GH<3 microg/l by ITT were enrolled. After overnight fasting, a 100 microg dose of GHRP-2 was administered intravenously; blood samples were taken during the subsequent 2 h and GH measured by immunoradiometric assay. RESULTS: Serum GH peak occurred within 60 min after GHRP-2 administration in all subjects. GH responses to GHRP-2 were not affected by gender, but were slightly lower in elderly subjects and those with adiposity, although these did not influence diagnosis of GHD. Repeated tests showed favourable reproducibility. Peak GH concentrations after GHRP-2 were significantly (P<0.001) lower in patients (1.36+/-2.60 microg/l) than the healthy group (84.6+/-60.9 microg/l) with no difference between hypothalamic and pituitary diseases. Serum GH concentration at the point where sensitivity of response crossed with specificity ranged from 15 to 20 microg/l. A cut-off value of 15 microg/l for diagnosing GHD with GHRP-2 corresponded to the diagnostic value of 3 microg/l in the ITT. CONCLUSIONS: The GHRP-2 provocative test showed favourable reproducibility and was mildly influenced by age and adiposity. Severe GH deficiency could be diagnosed with high reliability using a 15 microg/l (9 microg/l when GH calibrated with recombinant World Health Organization 98/574 standard) cut-off for peak GH concentration.     

Preservation of growth hormone secretion in response to growth hormone-releasing peptide-2 during prednisone therapy.

Children who require long-term glucocorticoid treatment often demonstrate poor growth. Growth hormone (GH) secretion is decreased during glucocorticoid treatment, and this decrease may be due to a relative excess of the hypothalamic hormone somatostatin (SRIF). GH-releasing peptide-2 (GHRP-2) is a GH secretagogue that acts via multiple mechanisms at multiple sites. One of its proposed mechanisms is the ability to bypass SRIF blockade of GH secretion. We measured the ability of GHRP-2 to release GH before and during prednisone therapy (20 mg orally three times daily for 4 days). The degree of preservation of GH secretion and the pattern of GH release in response to GHRP-2 were compared with those observed in response to arginine, a known SRIF inhibitor. GH release in response to GHRP-2 and arginine was measured in the same eight subjects before and during prednisone therapy. Before prednisone, peak GH levels in response to arginine and GHRP-2 were 8.8 +/- 2.8 and 80.8 +/- 21.2 microg/L. During prednisone therapy, the peak GH level in response to arginine and to GHRP-2 was 20.1 +/- 8.3 and 71.3 +/- 18.4 microg/L, respectively. The difference in peak values before and after prednisone was not significant. The time to the peak GH level during prednisone therapy occurred sooner for both arginine and GHRP-2. The pattern of GH release to arginine and to GHRP-2 was not identical, and the mean area under the curve for GH release to GHRP-2 decreased significantly with steroid treatment (P = .04), suggesting that GHRP-2 acts by mechanisms additional to the removal of SRIF inhibition. GHRP-2 elicited a 10-fold greater GH response than arginine at baseline, and the GH response was threefold greater versus arginine even in the face of prednisone therapy. GH release occurred earlier for both arginine and GHRP-2 during steroid treatment. We propose that this may suggest an increased storage phenomenon due to the blockade of GH secretion by glucocorticoids and then a sudden release with SRIF inhibition. If GHRP-2 can indeed counteract the inhibitory effect of glucocorticoids on GH secretion, then a new form of therapy may be available to support growth in children who must receive long-term steroid treatment.     

A low dose of ghrelin stimulates growth hormone (GH) release synergistically with GH-releasing hormone in humans

The synergistic relationship between GH-releasing secretagogue (GHS) and GH-releasing hormone (GHRH) with respect to GH secretion is well known. In the present study, we report a similar relationship between GHRH and ghrelin, a recently identified endogenous ligand for the GHS receptor. In normal male adults, various doses of ghrelin were intravenously administered alone or together with 1.0 microg/kg GHRH. At small doses of 0.08 and 0.2 microg/kg ghrelin, combined administration of the two peptides significantly stimulated GH release in a synergistic manner; the mean GH response values of the two peptide combinations were more than the summed mean GH response values of each peptide alone (P < 0.05). In addition, at 1.0 microg/kg ghrelin, the tendency of the synergistic effect was observed, although the comparison was not statistically significant probably due to a submaximal dose ceiling effect. No synergistic effects with respect to ACTH or prolactin secretion were observed. In conclusion, the synergistic interaction between ghrelin and GHRH was clearly shown and might be useful for a provocation test to diagnose GH deficiency.     

Growth hormone (GH) secretion during continuous infusion of GH-releasing peptide: partial response attenuation

The synthetic hexapeptide GH-releasing peptide (GHRP; SK&F 110679) specifically stimulates GH release in man. To determine the effect of a continuous GHRP infusion and whether response attenuation occurs in man, we administered to six healthy subjects a 6-h infusion of saline and three doses of GHRP, each followed by a 1.0 micrograms/kg bolus injection. GH was measured every 10 min using an immunoradiometric assay. During the saline infusion, spontaneous GH peaks occurred at variable times in four of the six subjects. During the continuous GHRP infusion, a single burst of GH release occurred with the two lower doses (0.1 and 0.3 micrograms/kg.h). With the highest dose of 1.0 micrograms/kg.h, a primary burst of GH release was followed by sporadic secretory episodes of lesser magnitude during the infusion; the GH concentrations remained above baseline before administration of the iv GHRP bolus in all six subjects. The mass of GH secreted was indirectly determined using waveform-independent deconvolution analysis. Mean GH secretion rates (micrograms per L distribution volume/h) were calculated by dividing the GH mass by the time interval. The GH secretion rates during the infusion period (0900-1430 h) were 2.40 +/- 0.68, 2.47 +/- 0.61, 7.67 +/- 1.86, and 14.75 +/- 2.32 on the saline and GHRP (0.1, 0.3, and 1.0 micrograms/kg.h) infusion days, respectively (P less than 0.05, 1.0 micrograms/kg.h vs. saline). The GH secretion rates after the iv GHRP bolus were 18.28 +/- 3.81, 19.01 +/- 2.03, 11.70 +/- 2.55, and 7.86 +/- 0.80 on the saline and GHRP (0.1, 0.3, and 1.0 micrograms/kg.h) infusion days, respectively (P less than 0.05, 1.0 micrograms/kg.h vs. saline). Compared with the saline infusion, the GH response to GHRP infusions was dose dependent (r = 0.81; P less than 0.001). The GH response to the iv bolus was inversely related to the dose of the preceding 5.5-h continuous GHRP infusion (r = -0.58; P = 0.003), and the total amount of GH secreted (constant infusion plus the bolus infusion periods) was not different among the GHRP doses. Constant GHRP infusion stimulates GH release in man, and partial response attenuation occurs with a subsequent 1.0 micrograms/kg GHRP bolus. We hypothesize that GHRP is active at multiple sites and may act as a functional somatostatin antagonist. Further studies are needed to better determine the site(s) of GHRP action and its potential use as a diagnostic and therapeutic agent.