Growth hormone-releasing peptide-2 infusion synchronizes growth hormone, thyrotrophin and prolactin release in prolonged critical illness

OBJECTIVE: During prolonged critical illness, nocturnal pulsatile secretion of GH, TSH and prolactin (PRL) is uniformly reduced but remains responsive to the continuous infusion of GH secretagogues and TRH. Whether such (pertinent) secretagogues would synchronize pituitary secretion of GH, TSH and/or PRL is not known. DESIGN AND METHODS: We explored temporal coupling among GH, TSH and PRL release by calculating cross-correlation among GH, TSH and PRL serum concentration profiles in 86 time series obtained from prolonged critically ill patients by nocturnal blood sampling every 20 min for 9 h during 21-h infusions of either placebo (n=22), GHRH (1 microg/kg/h; n=10), GH-releasing peptide-2 (GHRP-2; 1 microg/kg/h; n=28), TRH (1 microg/kg/h; n=8) or combinations of these agonists (n=8). RESULTS: The normal synchrony among GH, TSH and PRL was absent during placebo delivery. Infusion of GHRP-2, but not GHRH or TRH, markedly synchronized serum profiles of GH, TSH and PRL (all P< or =0.007). After addition of GHRH and TRH to the infusion of GHRP-2, only the synchrony between GH and PRL was maintained (P=0.003 for GHRH + GHRP-2 and P=0.006 for TRH + GHRH + GHRP-2), and was more marked than with GHRP-2 infusion alone (P=0.0006 by ANOVA). CONCLUSIONS: The nocturnal GH, TSH and PRL secretory patterns during prolonged critical illness are herewith further characterized to include loss of synchrony among GH, TSH and PRL release. The synchronizing effect of an exogenous GHRP-2 drive, but not of GHRH or TRH, suggests that the presumed endogenous GHRP-like ligand may participate in the orchestration of coordinated anterior pituitary hormone release.     

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 μg i.v.), GHRP-6 (1 μg/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-I by radioimmunoassay. RESULTS Hypothyroid patients had markedly lower peak GH values (mean ± SE μg/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.     

The combined administration of GH-releasing peptide-2 (GHRP-2), TRH and GnRH to men with prolonged critical illness evokes superior endocrine and metabolic effects compared to treatment with GHRP-2 alone

OBJECTIVE: Central hyposomatotrophism, hypothyroidism and hypogonadism are present concomitantly in men with prolonged critical illness. This study evaluated the impact of combined treatment with GH-releasing peptide-2 (GHRP-2), TRH and GnRH for 5 days compared with GHRP-2 + TRH and with GHRP-2 alone. PATIENTS AND DESIGN: Thirty-three men with prolonged critical illness participated at baseline compared to 50 age- and body mass index (BMI)-matched controls. Patients were randomly assigned to 5 days of placebo (n = 7), GHRP-2 (1 microg/kg/h; n = 9), GHRP-2 + TRH infusion (1 + 1 microg/kg/h; n = 9) or pulsatile GnRH (0.1 microg/kg every 90 min) together with GHRP-2 + TRH infusion (n = 8). MEASUREMENTS: GH, TSH and LH secretion were quantified by deconvolution analysis of serum concentration time series obtained by sampling every 20 min from 2100 to 0600 h at baseline and on nights 1 and 5 of treatment. Serum concentrations of IGF-I, IGFBPs, thyroid hormones, gonadal and adrenal steroids, proinflammatory cytokines and selected metabolic and inflammation markers were measured daily. RESULTS: Patients revealed suppressed pulsatile GH, TSH and LH secretion in the face of low serum concentrations of IGF-I, IGFBP-3 and the acid-labile subunit (ALS) (P < 0.0001 each), thyroid hormones (P < 0.0001) and total and estimated free testosterone (P < 0.0001) levels, whereas free oestradiol (E2) estimates were normal. Serum dehydroepiandrosterone sulphate (DHEAS) levels were also suppressed whereas morning cortisol was normal. Serum levels of type I procollagen (PICP) and bone alkaline phosphatase (sALP) were elevated whereas osteocalcin (OC) was low (P = 0.03). Ureagenesis (P < 0.0001) and breakdown of bone tissue (P < 0.0001) were increased. Baseline serum TNF-alpha, IL-6 and C-reactive protein level and white blood cell (WBC) count were elevated; serum lactate was normal. Only low T4 and high IGFBP-1 levels independently predicted mortality. GHRP-2 infusion reactivated GH secretion and normalized serum IGF-I, IGFBP-3 and ALS. GHRP-2 + TRH infusion reactivated both the GH axis and the thyroid axis, with normal levels of T4 and T3 reached within 1 day. Only GHRP-2 + TRH infusion combined with GnRH pulses reactivated the GH and TSH axis and at the same time increased pulsatile LH secretion compared to placebo. Only GnRH pulses together with GHRP-2 + TRH infusion increased testosterone significantly from day 2 (peak increase of + 312%) through day 5 and serum E2 with > 80% from day 1 through day 3 (all P = 0.05). Ureagenesis was reduced by GHRP-2 + TRH + GnRH (P = 0.01) and by GHRP-2 + TRH (P = 0.009) but not by GHRP-2 alone. Serum OC levels were increased only by GHRP-2 + TRH + GnRH (P = 0.03), with a trend for GHRP-2 + TRH (P = 0.09), but not by GHRP-2 alone. On day 5, serum lactate levels and WBC count were increased by GHRP-2 infused alone and in combination with TRH but not by GHRP-2 + TRH + GnRH. CONCLUSIONS: Coadministration of GHRP-2, TRH and GnRH reactivated the GH, TSH and LH axes in prolonged critically ill men and evoked beneficial metabolic effects which were absent with GHRP-2 infusion alone and only partially present with GHRP-2 + TRH. These data underline the importance of correcting the multiple hormonal deficits in patients with prolonged critical illness to counteract the hypercatabolic state.     

Growth hormone secretion after the administration of GHRP-6 or GHRH combined with GHRP-6 does not decline in late adulthood

OBJECTIVE Growth hormone (GH) secretion in middle and late adulthood declines with age. However, the precise mechanisms causing this impairment in OH release are unknown. HIs-D-Trp-Ala-Trp-D-Phe-Lys-NH₂(GHRP-6) Is a synthetic compound that releases OH in a dose related and specific manner in several species, including man. In order to gain a further Insight Into disrupted GH secretion in late adulthood, we evaluated GH responses to GHRP-6 or GHRH, administered either alone or in combination, in healthy young and late adulthood groups of subjects. DESIGN All subjects underwent three different tests carried out in random order and separated by at least one week. Tests were performed at 0900 h after an overnight fast. GHRH (100 μg), GHRP-6 (90 μg) either alone or In combination were administered as an I.v. bolus. SUBJECTS Groups of healthy young (mean ± SEM 22 ± 1·1 years, n= 9) and older adult subjects (59·5 ± 1·7 years, n= 9) were studied. MEASUREMENTS Serum GH levels were measured by radioimmunoassay. RESULTS In the group of young adult subjects the combined administration of GHRH and GHRP-6 elicited a greater GH increase than GHRH alone (F= 21·9, P < 0·001) or GHRP-6 alone (F= 6·2, P= 0·01). Similarly, the response to the combined stimuli was also greater than with GHRH alone (F= 21·8, P < 0·001) or GHRP-6 alone (F= 23·9, P < 0·001) In the late adulthood group of subjects. OH responses to GHRH were greater in younger than in older subjects (F= 3·45, P= 0·03). In contrast, OH responses to either GHRP-6 (F= 0·71, P= NS) or combined GHRH plus GHRP-6 administration (F= 0·68, P= NS) were not significantly different between the two groups. CONCLUSIONS These data show that OH responses to GHRP-6 are much greater than to GHRH In late adulthood. The marked Increase of plasma GH levels observed after administration of GHRP-6 alone or in combination with GHRH Indicates that Impaired GH secretion in late adulthood is a functional and potentially reversible state.     

Endocrine and metabolic effects of growth hormone (GH) compared with GH-releasing peptide, thyrotropin-releasing hormone, and insulin infusion in a rabbit model of prolonged critical illness

Treatment with recombinant human GH (rhGH) increases the mortality of critical illness. We postulated that combined GH-releasing peptide-2 (GHRP-2), TRH, and insulin infusion is a less toxic anabolic strategy through a putative inability to overstimulate the GH axis and a capacity to normalize thyroid hormone concentrations while foregoing excessive hyperglycemia. Burn-injured, parenterally fed, New Zealand White rabbits were randomized to receive 4-d treatment with saline (n=8); 60 microg/kg.h GHRP-2 and 60 microg/kg.h TRH, i.v. (n=9); or 3.5 mg/kg rhGH, s.c. (n=7). In the GHRP-2+TRH group, insulin was adjusted to maintain blood glucose below 180 mg/dl. Endocrine function and biochemical organ system function markers were studied. Animals were killed for assay of deiodinase activity in snap-frozen liver. Mortality, organ system function, hyperglycemia, and insulin requirement were equal in the three groups. GHRP-2+TRH increased pulsatile rabbit GH (rGH) and TSH release on d 1. After 4 d, rGH secretion and T4 and T3 concentrations were elevated, with a significant increase in hepatic activity of type 1 deiodinase and a decrease in type 3 deiodinase. Exogenous rhGH suppressed endogenous rGH secretion and increased IGF-I more than GHRP-2+TRH without altering thyroid hormone levels. Unlike GHRP-2+TRH, rhGH down-regulated liver type 3 deiodinase and did not affect type 1 deiodinase. We conclude that in experimentally induced critical illness, GHRP-2+TRH reactivated the GH and TSH axes and altered liver deiodinase activity, driving T4 to T3 conversion. In contrast to the human model, high dose rhGH was not rapidly lethal in this rabbit model. Whether this is explained by lack of rhGH-induced insulin resistance and hyperglycemia remains unclear.     

Growth hormone-releasing peptide-2 stimulates GH secretion in GH-deficient patients with mutated GH-releasing hormone receptor.

GH-releasing peptides (GHRPs) are synthetic peptides that bind to specific receptors and thereby stimulate the secretion of pituitary GH. In vivo it is uncertain whether these peptides act directly on somatotroph cells or indirectly via release of GHRH from the hypothalamus. In this study we compared the pituitary hormone response to GHRP-2 in 11 individuals with isolated GH deficiency (GHD) due to a homozygous mutation of the GHRH receptor (GHRH-R) gene and in 8 normal unrelated controls. Basal serum GH levels were lower in the GHD group compared with controls [0.11 +/- 0.11 (range, <0.04 to 0.38) vs. 0.59 +/- 0.76 microg/L (range, 0.04-2.12 microg/L); P = 0.052]. After GHRP-2 administration there was a 4.5-fold increase in serum GH relative to baseline values in the GHD group (0.49 +/- 0.41 vs. 0.11 +/- 0.11 microg/L; P = 0.002), which was significantly less than the 79-fold increase in the control group (46.8 +/- 17.6 vs. 0.59 +/- 0.76 microg/L; P = 0.008). Basal and post-GHRP-2 serum levels of ACTH, cortisol, and PRL were similar in both groups. Basal levels of serum TSH were significantly higher in the GHD group than in the control group (3.23 +/- 2.21 vs. 1.37 +/- 0.34 microIU/mL; P = 0.003). TSH levels in both groups did not change after GHRP-2 administration. These results suggest that an intact GHRH signaling system is not an absolute requirement for GHRP-2 action on GH secretion and that GHRP-2 has a GHRH-independent effect on pituitary somatotroph cells.     

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.     

Effect of simultaneous administration of GHRH (1-40) and TRH on GH, PRL and TSH secretion in normal man

The GHRH test represents a new tool in the study of secretion in man. Nine normal fasting males received on separate occasions in random order 1) GHRH 1-40 (1 microgram/Kg bw) iv at time 0; 2) TRH (6 micrograms/min) infusion between -30 and +120 min; 3) GHRH 1-40 (1 microgram/Kg bw) iv at time 0 plus TRH (6 micrograms/min) infusion between -30 and +120 min. Blood samples were drawn for GH, PRL and TSH at -90, -60, -30, 0 min and then every 15 min for 2 h. GHRH significantly increased GH in all subjects. The same GH response was found during GHRH plus TRH test. No effect was found either on PRL and TSH secretion after GHRH administration, or on GH pattern after TRH administration. A significant decrease of TSH, but not of PRL response was observed after GHRH plus TRH administration in comparison to TRH alone. These results underline that the inhibitory effect exerted by TRH on GH secretion during some experimental conditions is not linked to a pituitary interference between GHRH and TRH. The difference in TSH secretion, following GHRH plus TRH in comparison with TRH alone, could be due to a GHRH-induced central inhibitory mechanism, probably GHRH-related.     

Arginine stimulates growth hormone secretion by suppressing endogenous somatostatin secretion

To determine how arginine (Arg) stimulates GH secretion, we investigated its interaction with GHRH in vivo and in vitro. Six normal men were studied on four occasions: 1) Arg-TRH, 30 g arginine were administered in 500 mL saline in 30 min, followed by an injection of 200 micrograms TRH; 2) GHRH-Arg-TRH, 100 micrograms GHRH-(1-44) were given iv as a bolus immediately before the Arg infusion, followed by 200 micrograms TRH, iv; 3) GHRH test, 100 micrograms GHRH were given as an iv bolus; and 4) TRH test, 200 micrograms TRH were given iv as a bolus dose. Blood samples were collected at 15-min intervals for 30 min before and 120 min after the start of each infusion. Anterior pituitary cells from rats were coincubated with Arg (3, 6, 15, 30, and 60 mg/mL) and GHRH (0.05, 1, 5, and 10 nmol/L) for a period of 3 h. Rat GH was measured in the medium. After Arg-TRH the mean serum GH concentration increased significantly from 0.6 to 23.3 +/- 7.3 (+/- SE) micrograms/L at 60 min. TRH increased serum TSH and PRL significantly (maximum TSH, 11.1 +/- 1.8 mU/L; maximum PRL, 74.6 +/- 8.4 micrograms/L). After GHRH-Arg-TRH, the maximal serum GH level was significantly higher (72.7 +/- 13.4 micrograms/L) than that after Arg-TRH alone, whereas serum TSH and PRL increased to comparable levels (TSH, 10.2 +/- 3.0 mU/L; PRL, 64.4 +/- 13.6 micrograms/L). GHRH alone increased serum GH to 44.9 +/- 9.8 micrograms/L, significantly less than when GHRH, Arg, and TRH were given. TRH alone increased serum TSH to 6.6 +/- 0.6 mU/L, significantly less than the TSH response to Arg-TRH. The PRL increase after TRH only also was lower (47.2 +/- 6.8 micrograms/L) than the PRL response after Arg-TRH. In vitro Arg had no effect on basal and GHRH-stimulated GH secretion. Our results indicate that Arg administered with GHRH led to higher serum GH levels than did a maximally stimulatory dose of GHRH or Arg alone. The serum TSH response to Arg-TRH also was greater than that to TRH alone. We conclude that the stimulatory effects of Arg are mediated by suppression of endogenous somatostatin secretion.     

Growth hormone/insulin-like growth factor-1 response to acute and chronic growth hormone-releasing peptide-2, growth hormone-releasing hormone 1-44NH2 and in combination in older men and women with decreased growth hormone secretion

To better appreciate the interactions of GHRP-2 and GHRH 1-44NH₂ on the release of GH in normal adult men and women with decreased GH secretion and low serum IGF-1 levels, a series of acute and chronic studies have been performed (n=5 men, 5 women). The acute iv bolus GH responses of these subjects to the two peptides alone and together suggest that the decreased GH secretion may be primarily due to a deficiency of the natural endogenous GHRP, ghrelin, rather than a decreased secretion of endogenous GHRH or excess secretion of SRIF. To determine whether the low GH response to GHRH was due to a limited capacity of pituitary to release GH, higher dosages of GHRP-2 alone were administered. At a dose of 1 μg/kg GHRP-2 the GH response was essentially the same as that elicited by 1 μg/kg GHRH+0.1μg/kg GHRP-2 while the GH response to 10 μg/kg GHRP-2 sc was about twice as high in both men and women. Although these subjects have a limited pituitary capacity to release GH, which is also an indication of decreased GH secretion in the presence of low serum IGF-1 levels, this alone would not explain the low GH response to GHRH. Furthermore, the finding that a low dose of 0.1 μg/kg GHRP-2 augments the GH response to 1 μg/kg GHRH is strongly against an excess secretion of SRIF. Twenty-four hour profiles of GH secretion during placebo, GHRP-2, and various doses of GHRH alone and together with GHRP-2 were studied. In addition, 1 μg/kg/h GHRP-2 was infused continuously sc to these subjects for 30 d. The normal pulsatile secretion of GH as well as the serum IGF-1 level was increased after 24 h and remained elevated for 30 d. With a deficiency of endogenous GHRH, the GH response of GHRP-2 would be little to none, while in subjects with a deficiency of the natural GHRP, the GH response to GHRH would be more attenuated. Thus, in chronic deficiency the GH response would be expected to depend on the degree of the capacity of the pituitary to release GH as well as the type(s) of hormonal deficiency.     

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.     

GH responses to intravenous bolus infusions of GH releasing hormone and GH releasing peptide 2 separately and in combination in adult volunteers

OBJECTIVE Synthetic growth hormone releasing peptides (GHRP) have potent GH-releasing activity in vivo and in vitro. The nature of the Interaction of GHRP and naturally occurring GH releasing hormone (GHRH) is still far from clear. We investigated GH release in response to individual peptide doses or combined doses of GHRH1–29NH₂ and GHRP-2, a novel GH-releasing peptide, in normal adults. DESIGN Subjects underwent three tests in a randomized order: (1) i.v. bolus of GHRH1-29NH₂ (1 μg/kg BW), (2) i.v. bolus of GHRP-2 (1 μg/kg BW), (3) i.v. bolus of GHRH1-29NH ₂ combined with GHRP-2 (same dosages). SUBJECTS Eight healthy non-obese male volunteers, aged 25–34 years. MEASUREMENTS Serum GH concentrations were measured by IRMA at ?15,0, + 10, 20,30,45,60,75,90 and120 minutes after the boluses. RESULTS Peak GH levels in response to GHRH1-29NH ₂, GHRP-2 and the combined GHRH1-29NH ₂ and GHRP-2 administrations were observed between 20 and 45 minutes. Peak GH levels at30 minutes were 32.8 ± 27.3 (mean ± SD), 109.7 ± 56.1 and 140.9 ± 80.6mU/l, respectively. The area under the curve for GH levels (GH AUC) calculated for the first 90 minutes after the GHRH1-29NH ₂ test (2061.2 ± 1601.9mU/1 min) was significantly lower than those after GHRP-2 (6205.1 ± 3216.9mU/l min) and the combined GHRH1-29NH ₂ and GHRP-2 challenge (9788.3 ± 5530.4mU/l min) (P = 0.0003 and P = 0.00005, respectively; palred Student's t-test for log transformed data). Although the GH AUC of the GHRP-2 test and the combined GHRHl-29NH ₂ and GHRP-2 test differed significantly (P = 0.016, t-test), the latter was not signlflcantly dlfferent from the sum of the GH AUCs of each subject after the separate tests. CONCLUSION Although the GH releasing potency of GHRP-2 significantly exceeded that of GHRH1-29NH ₂, we were not able to demonstrate synergy between the two substances. It is possible that GHRP-2 given in our study in higher molar quantities than GHRH1-29NH ₂ masked the effect of the latter.     

Effects of ghrelin, growth hormone-releasing peptide-6, and growth hormone-releasing hormone on growth hormone, adrenocorticotropic hormone, and cortisol release in type 1 diabetes mellitus

In type 1 diabetes mellitus (T1DM), growth hormone (GH) responses to provocative stimuli are normal or exaggerated, whereas the hypothalamic-pituitary-adrenal axis has been less studied. Ghrelin is a GH secretagogue that also increases adrenocorticotropic hormone (ACTH) and cortisol levels, similarly to GH-releasing peptide-6 (GHRP-6). Ghrelin's effects in patients with T1DM have not been evaluated. We therefore studied GH, ACTH, and cortisol responses to ghrelin and GHRP-6 in 9 patients with T1DM and 9 control subjects. The GH-releasing hormone (GHRH)-induced GH release was also evaluated. Mean fasting GH levels (micrograms per liter) were higher in T1DM (3.5 ± 1.2) than in controls (0.6 ± 0.3). In both groups, ghrelin-induced GH release was higher than that after GHRP-6 and GHRH. When analyzing Δ area under the curve (ΔAUC) GH values after ghrelin, GHRP-6, and GHRH, no significant differences were observed in T1DM compared with controls. There was a trend (P = .055) to higher mean basal cortisol values (micrograms per deciliter) in T1DM (11.7 ± 1.5) compared with controls (8.2 ± 0.8). No significant differences were seen in ΔAUC cortisol values in both groups after ghrelin and GHRP-6. Mean fasting ACTH values were similar in T1DM and controls. No differences were seen in ΔAUC ACTH levels in both groups after ghrelin and GHRP-6. In summary, patients with T1DM have normal GH responsiveness to ghrelin, GHRP-6, and GHRH. The ACTH and cortisol release after ghrelin and GHRP-6 is also similar to controls. Our results suggest that chronic hyperglycemia of T1DM does not interfere with GH-, ACTH-, and cortisol-releasing mechanisms stimulated by these peptides.     

Prolactin response to growth hormone-releasing hormone during chronic thyrotropin-releasing hormone infusion in the treatment of amyotrophic lateral sclerosis

In six patients suffering from amyotrophic lateral sclerosis we evaluated changes of T4, T3, TSH, PRL, and GH during treatment by continuous iv infusion of TRH for at least 15 days. No clinical improvement was detected. A significant rise of thyroid hormone levels was observed, as well as an upward trend of basal TSH levels and no change of basal PRL and GH levels. TRH acute test-induced TSH and PRL responses became blunted. Treatment provoked also the onset of a responsiveness of PRL to GHRH. The reduced TSH and PRL responses to acute TRH test during treatment could be explained by a down-regulation of TRH pituitary receptors. On the contrary, the onset of PRL responsiveness to GHRH is at present without a satisfactory explanation.     

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 β-adrenoceptor blockade on the GH response to GHRH In these patients. DESIGN In 18 hyperthyroid patients and In 12 control subjects, GHRH (100μg) was administered as an i.v. bolus injection. Eight hyperthyroid patients and 8 control subjects received 50 μg GHRH i.v. Seven hyperthyroid patients were reevaluated after β-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 14 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 absorptfometer. RESULTS The GH response to 100 μg GHRH in hyper thyroid 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 μg/l and 820 ± 113 vs 1879 ± 355 μg/l 120 min, respectively; P <0.01). IGF-I levels were significantly reduced in hyperthyroid patients compared to controls (131 ± 10 vs 201 ± 16 μg/l, respectively; P <0.01). Ideal body weight, serum albumin levels and the lean body mass were also reduced In hyperthyroid patients. After β-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.     

Inhibition of growth hormone release after the combined administration of CHRH and GHRP-6 in patients with Cushing's syndrome

OBJECTIVE In patients with Cushing's syndrome there is a blunted OH response to all types of stimuli. Although Inferential data point towards a direct perturbation in the pituitary exerted by glucocorticoids, the bask mechanism is unknown. His-d -TRP-ALA-TRP-d -Phe-Lys-NH₂ (GHRP-6) is a synthetic hexapeptlde which releases GH by a direct pituitary effect through receptors other than GHRH receptors. Furthermore, the combined administration of GHRH and GHRP-6 is able to induce a large OH discharge even in some pathological states such as obesity, associated with GH blockade. To gain further insight into the disrupted mechanisms of GH secretion, Cushing's syndrome patients were challenged with either GHRH, GHRP-6 or GHRH together with GHRP-6. A group of normal subjects was included for control purposes. DESIGN Three different tests were undertaken: (a) GHRH 100 μg I.v.; (b) GHRP-6 100 μg I.v. and (c) GHRH plus GHRP-6 100 μg I.v. of each; administered to each subject on different days, at least 4 days apart. PATIENTS Ten patients (8 women, 2 men) with untreated Cushing's syndrome, 9 Cushing's disease and 1 adrenal adenoma. Five healthy volunteers (3 women, 2 men) of similar ages served as a control group. MEASUREMENTS Plasma OH levels were measured by immunoradiometric assay. RESULTS The areas under the curve (AUC) of OH secretion (mean ± SEM In μ/1/120 mi) in the control subjects after each test were: GHRH, 1420 ± 330; GHRP-6, 2278 ± 290 and GHRH plus GHRP-6,7332 ± 592 (P < 0·05 vs each compound alone). The AUCs for Cushing's syndrome patients were: GHRH, 248 ± 165; GHRP-6 530 ±170 and for GHRH plus GHRP-6, 870 ± 258 (P < 0·05 vs GHRH alone). After the combined stimulus only one out of the ten patients with hypercortisolism showed a GH peak over 20 μ/l, while ail the controls had a peak over 04mU/l. CONCLUSIONS GHRP-6 induced OH secretion as well as the OH discharge elicited by GHRH and GHRP-6 are considerably reduced in Cushing's syndrome patients. This suggests that the main impairment of GH secretion in that pathological state resides at pituitary level.     

A paradoxical gender dissociation within the growth hormone/insulin-like growth factor I axis during protracted critical illness

Female gender appears to protect against adverse outcome from prolonged critical illness, a condition characterized by blunted and disorderly GH secretion and impaired anabolism. As a sexual dimorphism in the GH secretory pattern of healthy humans and rodents determines gender differences in metabolism, we here compared GH secretion and responsiveness to GH secretagogues in male and female protracted critically ill patients. GH secretion was quantified by deconvolution analysis and approximate entropy estimates of 9-h nocturnal time series in 9 male and 9 female patients matched for age (mean +/- SD, 67+/-11 and 67+/-15 yr), body mass index, severity and duration of illness, feeding, and medication. Serum concentrations of PRL, TSH, cortisol, and sex steroids were measured concomitantly. Serum levels of GH-binding protein, insulin-like growth factor I (IGF-I), IGF-binding proteins (IGFBPs), and PRL were compared with those of 50 male and 50 female community-living control subjects matched for age and body mass index. In a second study, GH responses to GHRH (1 microg/kg), GH-releasing peptide-2 (GHRP-2; 1 microg/ kg) and GHRH plus GHRP-2 (1 and 1 microg/kg) were examined in comparable, carefully matched male (n = 15) and female (n = 15) patients. Despite identical mean serum GH concentrations, total GH output, GH half-life, and number of GH pulses, critically ill men paradoxically presented with less pulsatile (mean +/- SD pulsatile GH fraction, 39+/-14% vs. 67+/-20%; P = 0.002) and more disorderly (approximate entropy, 0.946+/-0.113 vs. 0.805+/-0.147; P = 0.02) GH secretion than women. Serum IGF-I, IGFBP-3, and acid-labile subunit (ALS) levels were low in patients compared with controls, with male patients revealing lower IGF-I (P = 0.01) and ALS (P = 0.005) concentrations than female patients. Correspondingly, circulating IGF-I and ALS levels correlated positively with pulsatile (but not with nonpulsatile) GH secretion. Circulating levels of GH-binding protein and IGFBP-1, -2, and -6 were higher in patients than controls, without a detectable gender difference. In female patients, PRL levels were 3-fold higher, and TSH and cortisol tended to be higher than levels in males. In both genders, estrogen levels were more than 3-fold higher than normal, and testosterone (2.25+/-1.94 vs. 0.97+/-0.39 nmol/L; P = 0.03) and dehydroepiandrosterone sulfate concentrations were low. In male patients, low testosterone levels were related to reduced GH pulse amplitude (r = 0.91; P = 0.0008). GH responses to GHRH were relatively low and equal in critically ill men and women (7.3+/-9.4 vs. 7.8+/-4.1 microg/L; P = 0.99). GH responses to GHRP-2 in women (93+/-38 microg/L) were supranormal and higher (P<0.0001) than those in men (28+/-16 microg/L). Combining GHRH with GHRP-2 nullified this gender difference (77+/-58 in men vs. 120+/-69 microg/L in women; P = 0.4). In conclusion, a paradoxical gender dissociation within the GH/ IGF-I axis is evident in protracted critical illness, with men showing greater loss of pulsatility and regularity within the GH secretory pattern than women (despite indistinguishable total GH output) and concomitantly lower IGF-I and ALS levels. Less endogenous GHRH action in severely ill men compared with women, possibly due to profound hypoandrogenism, accompanying loss of the putative endogenous GHRP-like ligand action with prolonged stress in both genders may explain these novel findings.     

The impact of cranial irradiation on GH responsiveness to GHRH plus GH-releasing peptide-6

Patients treated with cranial radiation are at risk of GH deficiency (GHD). We evaluated somatotroph responsiveness to maximal provocative tests exploring the GH releasable pool in relation to the impact of radiation damage to the hypothalamic-pituitary unit. The GH-releasing effect of GHRH plus GH secretagogue [GH-releasing peptide (GHRP)-6] (GHRH+GHRP-6) was studied in 22 adult patients (age, 23.2 +/- 1.4 yr; 8 female and 14 male; mean body mass index, 22.6 +/- 0.7 kg/m(2)) who received cranial radiation for primary brain tumor distant from hypothalamic-pituitary region 7.6 +/- 0.7 yr before GH testing. Two stimulatory tests for GH secretion were employed: insulin tolerance test (ITT, 0.15 IU/kg regular insulin i.v. bolus); and GHRH+GHRP-6 test: GHRH (Geref Serono, Madrid, Spain; l microg/kg) plus GHRP-6 (CLINALFA, Laufelingen, Switzerland; 1 microg/kg) as i.v. bolus. Serum GH was measured (Delphia; Perkin Elmer, Wallac, Turku, Finland) at -30, -15, 0, 15, 30, 45, 60, 90, and 120 min. Anterior pituitary function was normal in all except in 1 female with hyperprolactinemia. Twelve out of 22 irradiated patients were GH-deficient (GHD) with both tests. Eleven out of 22 (50%) irradiated patients were severely GHD, according to the ITT (GH < 3 microg/liter; mean GH peak, 1.5 +/- 0.5 microg/liter). In 9 irradiated patients, in whom ITT was performed as well, mean peak GH after the GHRH+GHRP-6 test was 6.2 +/- 0.8 microg/liter, which is considered as severe GHD, according to our own cut-off for the test (peak GH < 10 microg/liter). GH responses to both tests were highly concordant, but the differential in the GH peak concentrations between GHD and non-GHD irradiated patients was significantly larger for the GHRH+GHRP-6 test than that for the ITT. The 2 discordant responses, i.e. poor response to the ITT and good response to the GHRH+GHRP-6 test, were found in 1 hyperprolactinemic female patient and in 1 other female. One irradiated patient was diagnosed as GHD only with the combined test, because ITT was contraindicated because of epilepsy. PRL and cortisol responses to ITT were normal in all irradiated patients and did not depend on the GH status. IGF-I levels were not informative or discriminative between the GHD and non-GHD irradiated adult patients. In conclusion, the use of GH secretagogues plus GHRH is an easy, reliable and accurate way of assessing GH secretion in cranially irradiated patients. Impairment of the GH releasable pool in the irradiated patients, with a maximal provocative test, reflects alterations in the hypothalamic-pituitary unit caused by radiotherapy.     

Effects of ghrelin,GH-releasing peptide-6 (GHRP-6) and GHRH on GH,ACTH and cortisol release in hyperthyroidism before and after treatment

In thyrotoxicosis GH responses to stimuli are diminished and the hypothalamic–pituitary–adrenal axis is hyperactive. There are no data on ghrelin or GHRP-6-induced GH, ACTH and cortisol release in treated hyperthyroidism. We, therefore, evaluated these responses in 10 thyrotoxic patients before treatment and in 7 of them after treatment. GHRH-induced GH release was also studied. Peak GH (μg/L; mean ± SE) values after ghrelin (22.6 ± 3.9), GHRP-6 (13.8 ± 2.3) and GHRH (4.9 ± 0.9) were lower in hyperthyroidism before treatment compared to controls (ghrelin: 67.6 ± 19.3; GHRP-6: 25.4 ± 2.7; GHRH: 12.2 ± 2.8) and did not change after 6 months of euthyroidism (ghrelin: 32.7 ± 4.7; GHRP-6: 15.6 ± 3.6; GHRH: 7.4 ± 2.3), although GH responses to all peptides increased in ~50% of the patients. In thyrotoxicosis before treatment ACTH response to ghrelin was two fold higher (107.4 ± 26.3) than those of controls (54.9 ± 10.3), although not significantly. ACTH response to GHRP-6 was similar in both groups (hyperthyroid: 44.7 ± 9.0; controls: 31.3 ± 7.9). There was a trend to a decreased ACTH response to ghrelin after 3 months of euthyroidism (35.6 ± 5.3; P = 0.052), but after 6 months this decrease was non-significant (50.7 ± 14.0). After 3 months ACTH response to GHRP-6 decreased significantly (20.4 ± 4.2), with no further changes. In hyperthyroidism before treatment, peak cortisol (μg/dL) responses to ghrelin (18.2 ± 1.2) and GHRP-6 (15.9 ± 1.4) were comparable to controls (ghrelin: 16.4 ± 1.6; GHRP-6: 13.5 ± 0.9) and no changes were seen after treatment. Our results suggest that the pathways of GH release after ghrelin/GHRP-6 and GHRH are similarly affected by thyroid hormone excess and hypothalamic mechanisms of ACTH release modulated by ghrelin/GHSs may be activated in this situation.     

A novel in vivo rabbit model of hypercatabolic critical illness reveals a biphasic neuroendocrine stress response

High doses of GH, used to induce anabolism in prolonged critically ill patients, unexpectedly increased mortality. To further explore underlying mechanisms, a valid animal model is needed. Such a model is presented in this study. Seven days after arterial and venous cannulae placement, male New Zealand White rabbits were randomly allocated to a control or a critically ill group. To induce prolonged critical illness, a template controlled 15% deep dermal burn injury was imposed under combined general and regional (paravertebral) anesthesia. Subsequently, critically ill rabbits received supplemental analgesia and were parenterally fed with glucose, insulin, amino acids, and lipids. On d 1 and d 8 after randomization, acute and chronic spontaneous hormonal profiles of GH, TSH, and PRL secretion were obtained by sampling blood every 15 min for 7 h. Furthermore, GH, TSH, and PRL responses to an iv bolus of GH-releasing peptide 2 (GHRP-2) + TRH were documented on d 0, 1, and 8. Hemodynamic status and biochemical parameters were evaluated on d 0, 1, 3, 5, and 8, after which animals were killed and relative wet weight and water content of organs was determined. Compared with controls, critically ill animals exhibited transient metabolic acidosis on d 1 and weight loss, organ wasting, systolic hypertension, and pronounced anemia on d 8. On d 1, pulsatile GH secretion doubled in the critically ill animals compared with controls, and decreased again on d 8 in the presence of low plasma IGF-I concentrations from d 1 to d 8. GH responses to GHRP-2 + TRH were elevated on d 1 and increased further on d 8 in the critically ill animals. Mean TSH concentrations were identical in both groups on d 1 and 8, in the face of dramatically suppressed plasma T(4) and T(3) concentrations in the critically ill animals. PRL secretion was impaired in the critically ill animals exclusively on d 8. TSH and PRL responses to GHRP-2 and TRH were increased only on d 1. In conclusion, this rabbit model of acute and prolonged critical illness reveals several of the clinical, biochemical, and endocrine manifestations of the human counterpart.