Effect of combined administration of growth hormone (GH)-releasing hormone, GH-releasing peptide-6, and pyridostigmine in normal and obese subjects

Treatment with Growth Hormone for Adults with Growth Hormone Deficiency Syndrome: Benefits and Risks

Pharmacological treatment of growth hormone deficiency (GHD) in adults began in clinical practice more than 20 years ago. Since then, a great volume of experience has been accumulated on its effects on the symptoms and biochemical alterations that characterize this hormonal deficiency. The effects on body composition, muscle mass and strength, exercise capacity, glucose and lipid profile, bone metabolism, and quality of life have been fully demonstrated. The advance of knowledge has also taken place in the biological and molecular aspects of the action of this hormone in patients who have completed longitudinal growth. In recent years, several epidemiological studies have reported interesting information about the long-term effects of GH replacement therapy in regard to the possible induction of neoplasms and the potential development of diabetes. In addition, GH hormone receptor polymorphism could potentially influence GH therapy. Long-acting GH are under development to create a more convenient GH dosing profile, while retaining the excellent safety, efficacy, and tolerability of daily GH. In this article we compile the most recent data of GH replacement therapy in adults, as well as the molecular aspects that may condition a different sensitivity to this treatment.

The Safety and Efficacy of Growth Hormone Secretagogues

INTRODUCTION

Growth hormone (GH) increases lean body mass, decreases fat mass, increases exercise tolerance and maximum oxygen uptake, enhances muscle strength, and improves linear growth. Long-term studies of GH administration offer conflicting results on its safety, which has led to strict Food and Drug Administration criteria for GH use. The potential drawbacks of exogenous GH use are believed to be due in part to impaired regulatory feedback.

AIM

To review the literature on GH secretagogues (GHSs), which include GH-releasing peptides and the orally available small-molecule drug ibutamoren mesylate.

METHODS

Review of clinical studies on the safety and efficacy of GHSs in human subjects.

MAIN OUTCOME MEASURE

Report on the physiologic changes from GHS use in human subjects including its safety profile.

RESULTS

GHSs promote pulsatile release of GH that is subject to negative feedback and can prevent supra-therapeutic levels of GH and their sequelae. To date, few long-term, rigorously controlled studies have examined the efficacy and safety of GHSs, although GHSs might improve growth velocity in children, stimulate appetite, improve lean mass in wasting states and in obese individuals, decrease bone turnover, increase fat-free mass, and improve sleep. Available studies indicate that GHSs are well tolerated, with some concern for increases in blood glucose because of decreases in insulin sensitivity.

CONCLUSION

Further work is needed to better understand the long-term impact of GHSs on human anatomy and physiology and more specifically in the context of a diversity of clinical scenarios. Furthermore, the safety of these compounds with long-term use, including evaluation of cancer incidence and mortality, is needed. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sex Med Rev 2018;6:45-53.

[Benefits and risks of growth hormone in adults with growth hormone deficiency]

Adult growth hormone (GH) deficiency is a well-recognized clinical syndrome with adverse health consequences. Many of these may improve after replacement therapy with recombinant GH. This treatment induces an increase in lean body mass and a decrease in fat mass. In long-term studies, bone mineral density increases and muscle strength improves. Health-related quality of life tends to increase after treatment with GH. Lipid profile and markers of cardiovascular risk also improve with therapy. Nevertheless, GH replacement therapy is not without risk. According to some studies, GH increases blood glucose, body mass index and waist circumference and may promote long-term development of diabetes and metabolic syndrome. Risk of neoplasia does not appear to be increased in adults treated with GH, but there are some high-risk subgroups. Methodological shortcomings and difficulties inherent to long-term studies prevent definitive conclusions about the relationship between GH and survival. Therefore, research in this field should remain active.

Increased adiposity and insulin correlates with the progressive suppression of pulsatile GH secretion during weight gain

Pathological changes associated with obesity are thought to contribute to GH deficiency. However, recent observations suggest that impaired GH secretion relative to excess calorie consumption contributes to progressive weight gain and thus may contribute to the development of obesity. To clarify this association between adiposity and GH secretion, we investigated the relationship between pulsatile GH secretion and body weight; epididymal fat mass; and circulating levels of leptin, insulin, non-esterified free fatty acids (NEFAs), and glucose. Data were obtained from male mice maintained on a standard or high-fat diet. We confirm the suppression of pulsatile GH secretion following dietary-induced weight gain. Correlation analyses reveal an inverse relationship between measures of pulsatile GH secretion, body weight, and epididymal fat mass. Moreover, we demonstrate an inverse relationship between measures of pulsatile GH secretion and circulating levels of leptin and insulin. The secretion of GH did not change relative to circulating levels of NEFAs or glucose. We conclude that impaired pulsatile GH secretion in the mouse occurs alongside progressive weight gain and thus precedes the development of obesity. Moreover, data illustrate key interactions between GH secretion and circulating levels of insulin and reflect the potential physiological role of GH in modulation of insulin-induced lipogenesis throughout positive energy balance.

Effect of acute ghrelin administration on glycaemia and insulin levels in obese patients

OBJECTIVES

Ghrelin is a 28-amino-acid peptide, predominantly produced by the stomach. There are several studies that suggest the importance of ghrelin in obesity. However, the pancreatic endocrine response to ghrelin in obesity is unclear at present. The aim of this study was to clarify whether ghrelin administration influences glucose and insulin levels in obese patients.

PATIENTS AND METHODS

Six obese female patients (31 +/- 3.4 year) with a BMI of 36.1 +/- 7.7 kg/m(2) were studied. Three tests were done: placebo, ghrelin (1 microg/kg, intravenously) and growth hormone-releasing hormone (GHRH; 1 microg/kg, iv) plus ghrelin (1 microg/kg, iv). Blood samples were taken at appropriate intervals for determination of glucose and insulin. Statistical analyses were performed by Wilcoxon and Mann-Whitney tests.

RESULTS

Glucose (mean peak, mmol/l) level after placebo administration was 4.9 +/- 0.2. Glucose level after the administration of ghrelin was 5.1 +/- 0.2, not significantly different from the response after placebo (p = NS). Glucose level after the administration of ghrelin plus GHRH was 5.1 +/- 0.2, not significantly different from placebo (p = NS). Insulin (mean peak, mU/l) level after placebo administration was 16.1 +/- 6.1. Insulin level after the administration of ghrelin was 12.3 +/- 1.6, not significantly different from placebo (p = NS). Insulin level after the administration of ghrelin plus GHRH was 11.1 +/- 2.7, not significantly different from the response after placebo (p = NS).

CONCLUSIONS

In female obese patients, we did not find significant differences in glucose or insulin levels following ghrelin or GHRH plus ghrelin administration.

Cardiovascular risk in aging and obesity: is there a role for GH

GH has significant impact in adults. In fact, patients with the GH deficiency (GHD) syndrome are now recognized as having an increased cardiovascular risk. The effects of human aging on GH secretion have been evaluated by a number of researchers. Studies of 24 h secretion of GH have shown variable reductions in most 24-h GH secretory parameters in middle-aged and in older men and women, resulting in a decrease in plasma levels of its anabolic mediator IGF-I. Obesity is also associated with several endocrine and metabolic abnormalities. These include decreased serum GH concentrations, reduced GH half-life, frequency of GH secretory episodes and daily GH production rate. The mechanism of the low GH in obesity is not completely understood nor is it clear whether its relationship with visceral adiposity is causal. The aim of this article will be to review the available clinical data concerning the potential involvement of "subclinical" or perhaps better "functional" GHD, which is observed in aging and obesity, in the increase in cardiovascular risk which characterizes these two conditions.

The GHRH/GHRP-6 test for the diagnosis of GH deficiency in elderly or severely obese men

OBJECTIVE AND DESIGN

Ageing and obesity result in decreased activity of the GH/IGF-I axis and concomitant impaired GH responses to secretory stimuli. We therefore determined the validity of the GH cut-off value of 15.0 microg/l in the GH-releasing hormone (GHRH)/GH releasing peptide-6 (GHRP-6) test for the diagnosis of GH deficiency in elderly or severely obese men.

METHODS

We performed a combined GHRH/GHRP-6 test in ten elderly men (mean age 74 years; mean body mass index (BMI) 24.6 kg/m(2)), nine obese men (mean age 47 years; mean BMI 40.6 kg/m(2)) and seven healthy male controls (mean age 51 years, mean BMI 24.3 kg/m(2)). After assessment of fasting plasma GH, IGF-I and IGF-binding protein-3 (IGFBP-3), GHRH (100 microg) and GHRP-6 (93 microg) were given intravenously as a bolus injection. Repeated GH measurements were performed for two hours.

RESULTS

Both peak GH levels and areas under the curve (AUC) were significantly lower in the obese than in the controls (peak 13.2 vs 53.4 microg/l, P = 0.001; AUC 707 vs 3250 microg/l x 120 min; P = 0.001). Mean GH response in the elderly was lower than in the controls (peak 35.0 microg/l; AUC 2274 microg/l x 120 min), but this was not statistically significant. In contrast, GH peak levels in seven obese men remained below the cut-off level of 15.0 microg/l associated with severe GH deficiency. All others had GH peak levels exceeding this threshold. IGFBP-3 levels were significantly lower in the elderly than in the controls (1.35 vs 2.05 mg/l, P = 0.001). Baseline GH or IGF-I did not differ significantly between groups.

CONCLUSIONS

GH responses following GHRH/GHRP-6 administration were significantly reduced in severely obese men, but were not significantly reduced in elderly men, despite a negative trend. Our data indicate that the cut-off GH level of 15.0 microg/l after GHRH + GHRP-6 administration for the diagnosis of severe GH deficiency cannot be used in severely obese men.

Marked GH secretion after ghrelin alone or combined with GH-releasing hormone (GHRH) in obese patients

OBJECTIVES

Ghrelin is a 28-amino-acid peptide, predominantly produced by the stomach. It displays a strong GH-releasing activity mediated by the hypothalamus-pituitary GH secretagogue (GHS)-receptor (GHS-R). There are different studies that suggest the importance of ghrelin in feeding and weight homeostasis. In obesity there is a markedly decreased GH secretion. For both children and adults, the greater the body mass index (BMI), the lower the GH response to provocative stimuli, including the response to GHRH. However, the response to the natural GH secretaogogue ghrelin is unclear at the present time. The aim of the present study was to evaluate the GH response to ghrelin alone or combined with GHRH in a group of obese patients, in order to further understand the deranged GH secretory mechanisms in obesity and to clarify the mechanism of action of ghrelin.

PATIENTS AND MEASUREMENTS

Six obese female patients (31 +/- 3.4 years) with a BMI of 36.1 +/- 7.7 kg/m(2) were studied. As a control group, six normal nonobese female subjects of similar age and sex were studied. Four tests were performed: placebo, GHRH [1 micro g/kg, no more than 100 micro g, intravenous (i.v.)], ghrelin (1 micro g/kg, no more than 100 micro g, i.v.) and GHRH (1 micro g/kg, no more than 100 micro g, i.v.) plus ghrelin (1 micro g/kg, no more than 100 micro g, i.v.). Blood samples were taken at appropriate intervals for determination of GH. Statistical analyses were performed by Wilcoxon and by Mann-Whitney tests.

RESULTS

After GHRH, the median peak GH secretion in obese patients was 2.4 micro g/l (range 0.9-8.9 micro g/l). Ghrelin-induced GH secretion showed in obese patients a median peak of 24.4 micro g/l (range 7.4-85.0 micro g/l), significantly greater than the response after GHRH (P < 0.05). After the combined administration of GHRH plus ghrelin in obese patients the median peak GH secretion was 39.9 micro g/l (range 19.2-120.0 micro g/l), significantly greater than the response after GHRH (P < 0.05) or ghrelin (P < 0.05). GHRH-induced GH secretion in normal control subjects showed a median peak of 25.0 micro g/l (range 16.5-33.4 micro g/l). Ghrelin-induced GH secretion in normal showed a median peak of 68.5 micro g/l (range 22.5-119.5 micro g/l), significantly greater than the response after GHRH (P < 0.05). After the combined administration of GHRH plus ghrelin, in normal subjects the median peak GH secretion was 117.8 micro g/l (range 77.5-280.1 micro g/l), significantly greater than the response after GHRH or ghrelin alone (P < 0.05). When we compare the response of normal and obese patients, after GHRH alone, it was markedly decreased in obese people when compared with normal patients (P < 0.05) with a median GH peak of 25.0 micro g/l (range 16.5-33.4 micro g/l) and 2.4 micro g/l (range 0.9-8.9 micro g/l) for normal and obese patients, respectively. When we compare the response of normal and obese patients, after ghrelin alone or GHRH plus ghrelin, it was only blunted in obese subjects when compared with normal subjects with a median GH peak of 68.5 micro g/l (range 22.5-119.5 micro g/l) and 24.4 micro g/l (range 7.4-85 micro g/l) for normal and obese subjects, respectively, after ghrelin alone (P < 0.05) and a median GH peak of 117.8 micro g/l (range 77.5-280.1 micro g/l) and 39.9 micro g/l (range 19.2-120.0 micro g/l) for normal and obese patients, respectively, after GHRH plus ghrelin (P < 0.05).

CONCLUSIONS

This study has demonstrated a massive GH response to ghrelin alone or combined with GHRH in obese patients, suggesting that altered ghrelin secretion could play a major role in the blunted GH secretion present in obese patients.

Diagnostic reliability of a single IGF-I measurement in 237 adults with total anterior hypopituitarism and severe GH deficiency

OBJECTIVE

Within an appropriate clinical context, GH deficiency (GHD) in adults must be demonstrated biochemically by a single provocative test. Insulin-induced hypoglycaemia (ITT) and GH-releasing hormone (GHRH) + arginine (ARG) are indicated as the tests of choice, provided that appropriate cut-off limits are defined. Although IGF-I is the best marker of GH secretory status, its measurement is not considered a reliable diagnostic tool. In fact, considerable overlap between GHD and normal subjects is present, at least when patients with suspected GHD are considered independently of the existence of other anterior pituitary defects. Considering the time and cost associated with provocative testing procedures, we aimed to re-evaluate the diagnostic power of IGF-I measurement.

DESIGN

To this goal, in a large population [n = 237, 139 men, 98 women, age range 20-80 years, body mass index (BMI) range 26.4 +/- 4.3 kg/m2] of well-nourished adults with total anterior pituitary deficit including severe GHD (as shown by a GH peak below the 1st centile limit of normal response to GHRH + ARG tests and/or ITT) we evaluated the diagnostic value of a single total IGF-I measurement. IGF-I levels in hypopituitary patients were evaluated based on age-related normative values in a large population of normal subjects (423 ns, 144 men and 279 women, age range 20-80 years, BMI range 18.2-24.9 kg/m2).

RESULTS

Mean IGF-I levels in GHD were lower than those in normal subjects in each decade, but not the oldest one (74.4 +/- 48.9 vs. 243.9 +/- 86.7 micro g/l for 20-30 years; 81.8 +/- 46.5 vs. 217.2 +/- 56.9 micro g/l for 31-40 years; 85.8 +/- 42.1 vs. 168.5 +/- 69.9 micro g/l for 41-50 years; 82.3 +/- 39.3 vs. 164.3 +/- 60.3 micro g/l for 51-60 years; 67.5 +/- 31.8 vs. 123.9 +/- 50.0 micro g/l for 61-70 years; P < 0.0001; 54.3 +/- 33.6 vs. 91.6 +/- 53.5 micro g/l for 71-80 years, P = ns). Individual IGF-I levels in GHD were below the age-related 3rd and 25th centile limits in 70.6% and 97.63% of patients below 40 years and in 34.9% and 77.8% of the remaining patients up to the 8th decade, respectively.

CONCLUSIONS

Total IGF-I levels are often normal even in patients with total anterior hypopituitarism but this does not rule out severe GHD that therefore ought to be verified by provocative testing of GH secretion. However, despite the low diagnostic sensitivity of this parameter, very low levels of total IGF-I can be considered definitive evidence of severe GHD in a remarkable percentage of total anterior hypopituitary patients who could therefore skip provocative testing of GH secretion.

Impairment of GH responsiveness to GH-releasing hexapeptide (GHRP-6) in Prader-Willi syndrome

The aim of this study was to evaluate the GH-releasing activity of a synthetic hexapeptide, GHRP-6, in the Prader-Willi syndrome (PWS). Sixteen PWS patients (7 males and 9 females, aged 12.7-38.3 yr), 15 with essential obesity (OB) (7 males and 8 females, aged 12.9-42.9 yr), and 8 short normal children (SN; 3 males and 5 females, aged 10.2-14.3 yr) underwent 2 tests on separate occasions, being challenged with GHRP-6 (1 microg/kg, iv) or GHRH (1 microg/kg, iv)+PD (60 or 120 mg for children or adults, po). Moreover, in 11 patients with PWS and in the group of SN, the GH response to at least 2 stimulation tests had been previously determined. GH was analyzed either as mean peak values (GHp, mcg/l), or as the area under the curve (AUC, mcg/l/h) and the net incremental area under the curve (nAUC, mcg/l/h). In the group of PWS subjects, GH responses to both GHRP-6 (GHp: 11.4+/-2.0; AUC: 588+/-113; nAUC: 483+/-108) and GHRH+PD (GHp: 7.3+/-1.8; AUC: 486+/-122; nAUC: 371+/-250) were significantly lower than those observed either in OB (GHRP-6: GHp: 25.7+/-3.2, p<0.003; AUC: 1833+/-305, p<0.005; nAUC: 1640+/-263, p<0.0001. GHRH+PD: GHp: 15.1+/-2.4, p<0.009; AUC: 1249+/-248, p<0.003; nAUC: 918+/-230, p<0.006) or in SN patients (GHRP-6: GHp: 39.1+/-3.1, p<0.0001; AUC: 2792+/-158, p<0.0001; nAUC: 2705+/-165, p<0.00005. GHRH+PD: GHp: 27.5+/-3.7, p<0.0001; AUC: 1873+/-251, p<0.0001; nAUC: 1692+/-219, p<0.0005). Unlike control groups, in PWS patients GH levels after GHRP-6 did not differ from those obtained after GHRH+PD. Interestingly, low IGF-I values were present in all PWS subjects. Furthermore, no patient with PWS showed normal GH response to the previously performed GH stimulation tests. As already reported, GH release after GHRP-6 or GHRH+PD was significantly lower in OB than in SN subjects. In conclusion, our data indicate that: 1) GH response to GHRP-6 is clearly impaired in PWS; 2) the blunted GH responses to the provocative stimuli in PWS are not an artifact of obesity; 3) short stature in PWS is caused by a complex dysfunction of the hypothalamo-pituitary structures.

Synergy of L-arginine and growth hormone (GH)-releasing peptide-2 on GH release: influence of gender

We test the hypotheses that 1) growth hormone (GH)-releasing peptide-2 (G) synergizes with L-arginine (A), a compound putatively achieving selective somatostatin withdrawal and 2) gender modulates this synergy on GH secretion. To these ends, 18 young healthy volunteers (9 men and 9 early follicular phase women) each received separate morning intravenous infusions of saline (S) or A (30 g over 30 min) or G (1 microg/kg) or both, in randomly assigned order. Blood was sampled at 10-min intervals for later chemiluminescence assay of serum GH concentrations. Analysis of covariance revealed that the preinjection (basal) serum GH concentrations significantly determined secretagogue responsiveness and that sex (P = 0.02) and stimulus type (P < 0.001) determined the slope of this relationship. Nested ANOVA applied to log-transformed measures of GH release showed that gender determines 1) basal rates of GH secretion, 2) the magnitude of the GH secretory response to A, 3) the rapidity of attaining the GH maximum, and 4) the magnitude or fold (but not absolute) elevation in GH secretion above preinjection basal, as driven by the combination of A and G. In contrast, the emergence of the G and A synergy is sex independent. We conclude that gender modulates key facets of basal and A/G-stimulated GH secretion in young adults.

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.

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.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Growth hormone-releasing peptides and their analogs

Growth hormone-releasing peptides (GHRPs) are a series of hepta (GHRP-1)- and hexapeptides (GHRP-2, GHRP-6, Hexarelin) that have been shown to be effective releasers of GH in animals and humans. More recently, a series of nonpeptidyl GH secretagogues (L-692,429, L-692,585, MK-0677) were discovered using GHRP-6 as a template. Some cyclic peptides as well as penta-, tetra-, and pseudotripeptides have also been described. This review summarizes recent developments in our understanding of the GHRPs, as well as the current nonpeptide pharmacologic analogs. GHRPs and their analogs have no structural homology with GHRH and act via specific receptors present at either the pituitary or the hypothalamic level. The GHRP receptor has recently been cloned and it does not show sequence homology with other G-protein-coupled receptors known so far. This evidence strongly suggests the existence of a natural GHRP-like ligand which, however, has not yet been found. Although the exact mechanism of action of GHRPs has not been fully established, there is probably a dual site of action on both the pituitary and the hypothalamus, possibly involving regulatory factors in addition to GHRH and somatostatin. Moreover, the possibility that GHRPs act via an unknown hypothalamic factor (U factor) is still open. The marked GH-releasing activity of GHRPs is reproducible and dose-related after intravenous, subcutaneous, intranasal, and even oral administration. The GH-releasing effect of GHRPs is the same in both sexes, but undergoes age-related variations. It increases from birth to puberty and decreases in aging. The GH-releasing activity of GHRPs is synergistic with that of GHRH and not affected by opioid receptor antagonists, while it is only blunted by inhibitory influences that are known to nearly abolish the effect of GHRH, such as neurotransmitters, glucose, free fatty acids, glucocorticoids, rhGH, and even exogenous somatostatin. GHRPs maintain their GH-releasing effect in somatotrope hypersecretory states, such as acromegaly, anorexia nervosa, and hyperthyroidism. On the other hand, GHRPs and their analogs have been reported to be effective in idiopathic short stature, in some situations of GH deficiency, in obesity, and in hypothyroidism, while in patients with pituitary stalk disconnection and in Cushing's syndrome the somatotrope responsiveness to GHRPs is almost absent. A potential role in the treatment of short stature, aging, catabolic states, and dilated cardiomyopathy has been envisaged.

Age-related growth hormone-releasing activity of growth hormone secretagogues in humans

Growth hormone-releasing peptides (GHRPs) are synthetic molecules with strong, dose-related and reproducible growth hormone (GH)-releasing activity in humans. GHRPs act at both the pituitary and the hypothalamic level, where specific receptors have been located. In adults, GHRPs release more GH than does GH-releasing hormone (GHRP), whilst their co-administration has a synergistic effect, indicating that they have, at least partially, different mechanisms of action. However, normal activity of GHRH-secreting neurones is needed to achieve the full GH-releasing effect of GHRPs. In contrast to GHRH, the GH-releasing activity of GHRPs is not further increased by substances acting via inhibition of hypothalamic somatostatin, and is only blunted by substances that stimulate hypothalamic somatostatin release. Even free fatty acids and exogenous somatostatin, which act directly on somatotrophs, do no more than blunt the effect of GHRPs. Thus, the GH-releasing activity of GHRPs is partially refractory to inhibitory influences, GHRPs act, at least in part, by antagonism of somatostatin activity, both at the pituitary and the hypothalamic level. The GH-releasing effect of GHRPs is not dependent on gender, but undergoes age-related variations. Gonadal steroids seem to influence the activity of GHRPs only in childhood. The reduced GH response to GHRPs in the elderly is probably due mainly to concomitant GHRH hypoactivity and somatostatinergic hyperactivity. A preserved GH-releasing effect of GHRPs has been reported in acromegaly, anorexia nervosa, hyperthyroidism and in critically ill patients. GHRPs have also been found to increase GH release in children with idiopathic short stature, in GH deficiency and in obese patients, in whom there is a well-known reduction of somatotroph function. The GH response to GHRPs is markedly reduced in hypothyroidism and Cushing's syndrome.