M. F. Boghen

Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH.

GHRP-2 (D-Ala-D-beta Nal-Trp-D-Phe-Lys-NH2) and Hexarelin (HEX) (His-D-2-methylTrp-Ala-Trp-DPhe-Lys-NH2) are synthetic, non-natural super-analogs of GHRP-6 endowed with potent stimulatory effect on GH secretion and slight stimulatory effect on PRL, ACTH and cortisol levels. Their GH-releasing activity ahs never been compared each other and their effects on PRL, ACTH and cortisol have never been compared with that of other stimuli. To clarify these points, in 6 normal young adults (22-27 yr) we studied the GH, PRL, ACTH and cortisol responses to 1 and 2 micrograms/kg i.v. GHRP-2 and HEX comparing them with that after 1 micrograms/kg i.v. GHRH and 400 micrograms i.v. TRH + 2 micrograms/kg i.v. hCRH. The Gh responses to 2 micrograms/kg i.v. GHRP-2 or HEX, compared with those to 1 microgram/kg GHRH, were also studied in 6 normal elderly subjects (66-73 yr). In young adults 1 microgram/kg i.v. GHRP-2 and HEX induced a similar, strong GH response, which was higher (p < 0.05) than that to GHRH. The administration of 2.0 micrograms/kg i.v. GHRP-2 and HEX again elicited a similar GH response, which was higher (p < 0.05) than that after the 1.0 microgram/kg dose. In elderly subjects, the GH those in young subjects. In young adults, the PRL responses to all doses of GHRP-2 or HEX were similar and lower (p < 0.01) responses were similar to those to hCRH. In conclusion, our results demonstrate that, in man, GHRP-2 and HEX have similar, 2 and HEX is not fully specific, as they induce similar increases in PRL, ACTH and cortisol levels. The PRL-releasing activity of GHRPs is lower than that of TRH while their ACTH/cortisol-releasing activity is similar to that of hCRH.

Acute cardiovascular and hormonal effects of GH and hexarelin, a synthetic GH-releasing peptide, in humans

Reduced cardiac mass and performances are present in GH deficiency and are coun-teracted by rhGH replacement. GH and IGF-I possess specific myocardial receptors and have been reported able to exert an acute inotropic effect. Synthetic GH secretagogues (GHS) possess specific pituitary and hypothalamic but even myocar-dial receptors. In 7 male volunteers, we studied cardiac performance by radionuclide angiocardiography after iv administration of rhGH or hexare-lin (HEX), a peptidyl GHS. The administration of rhGH or HEX increased circulating GH levels to the same extent (AUC: 1594.6±88.1 vs 1739.3±262.2 μg/l/min for 90 min) while aldosterone and cate-cholamine levels did not change; HEX, but not rhGH, significantly increased cortisol levels. Left ventricular ejection fraction (LVEF), mean blood pressure (MBP) and heart rate (HR) were unaffected by rhGH (62.4±2.1 vs 62.1±2.3%, 90.6±3.4 vs 92.0±2.5 mm Hg, 62.3±1.8 vs 66.7±2.7 bpm). HEX increased LVEF (70.7±3.0 vs 64.0±1.5%, p<0.03) without significant changes in MBP and HR (92.8±4.7 vs 92.4±3.2 mmHg, 63.1±2.1 vs 67.0±2.9 bpm). LVEF significantly raised at 15 min, peaked at 30 min and lasted up to 60 min after HEX. These findings suggest that in man, the acute administration of Hexarelin exerts a short-lasting, positive inotropic effect. This effect seems GH-in-dependent and might be mediated by specific GHS myocardial receptors.

Cardiac effects of hexarelin in hypopituitary adults.

Growth hormone (GH)-releasing peptides possess specific pituitary, hypothalamic, and myocardial receptors. Seven adult male patients with GH deficiency (GHD) (age, mean+/-S.E.M.: 42.0+/-4.0 year) were studied by equilibrium radionuclide angiocardiography after i.v. administration of hexarelin, a peptide GH secretagogue. Data for these patients were compared with those for nine adult male controls (37.0+/-2.7 year). The GH response to hexarelin was negligible in patients with GHD compared to control subjects (CS) (peak: 1.9+/-0.9 vs. 45.7+/-3.6 microg/l, P<0.001). Basal left ventricular ejection fraction (LVEF) in patients with GHD was lower than that in CS (50+/-1% vs. 63+/-2%, P<0.001). Hexarelin administration increased LVEF both in patients with GHD and in CS (peak: 57+/-2 vs. 70+/-2, respectively, P<0.05 vs. baseline) without changing catecholamine levels, mean blood pressure (MBP), or cardiac output in either group. In conclusion, the acute administration of hexarelin exerts a short-lasting positive inotropic effect in humans, probably GH-independent and mediated by specific myocardial receptors for GH secretagogues.

Mechanisms underlying the negative growth hormone (GH) autofeedback on the GH-releasing effect of hexarelin in man

The growth hormone (GH) response to GH-releasing hormone (GHRH) is strongly inhibited by previous administration of recombinant human GH (rhGH), likely as a consequence of a somatostatin-mediated GH negative autofeedback. Hexarelin (HEX), a synthetic hexapeptide belonging to the GH-releasing peptide (GHRP) family, possesses a GH-releasing activity greater than that of GHRH both in animals and in man. The mechanism of action of GHRPs is yet to be completely clarified, although concomitant actions at the pituitary and hypothalamic level have been hypothesized. To further clarify the mechanisms of action underlying the GH-releasing activity of HEX, in six normal young volunteers we studied the effects of rhGH (2 U intravenously [IV]) on the GH response either to GHRH (2 microg/kg IV) or to HEX (2 microg/kg IV) alone or combined with GHRH and/or pyridostigmine ([PD], 120 mg orally). The GH-releasing effect of HEX was higher than that of GHRH (area under the curve [AUC], 2,200.8 +/- 256.9 v 792.2 +/- 117.6 microg/L/h, P < .001), whereas combined administration of the two substances induced a true synergistic effect, with GH release after HEX plus GHRH (4,259.2 +/- 308.0 microg/L/h) being higher (P < .02) than the arithmetic sum of the GH increases induced by each compound separately administered. After rhGH administration, the GH-releasing effect of HEX was blunted (1,468.9 +/- 193.7 microg/L/h, P < .04; inhibition of 32.1%), whereas that of GHRH was nearly abolished (102.0 +/- 7.8 microg/L/h, P < .02; inhibition of 86.1%). The GH response to combined administration of HEX and GHRH was also blunted by the previous rhGH bolus (3,070.6 +/- 481.8 microg/L/h, P < .02; inhibition of 26.7%). PD did not modify the GH-releasing effect of HEX either alone (2,456.8 +/- 317.5 microg/L/h) or combined with GHRH (4,009.1 +/- 360.8 microg/L/h). rhGH was again able to blunt the GH response to HEX combined with PD (1,619.3 +/- 237.9 microg/L/h, P < .02), but failed to modify the GH response to HEX combined with GHRH and PD (4,548.4 +/- 698.0 microg/L/h). In conclusion, these results demonstrate that rhGH administration only blunts the GH-releasing activity of HEX, but abolishes that of GHRH. The blunting effect of rhGH on the GH response to HEX is probably mediated by a concomitant reduction in the activity of GHRH-secreting neurons and an increase of somatostatinergic tone.

Arginine enhances the growth hormone-releasing activity of a synthetic hexapeptide (GHRP-6) in elderly but not in young subjects after oral administration.

In man the GH-releasing hexapeptide His-DTrp-Ala-Trp-DPhe-Lys-NH2 (GHRP-6) has been shown to be active even after oral administration. On the other hand, it has been shown that arginine (ARG) totally restores the reduced somatotropic responsiveness to GHRH observed in aging. Based on the foregoing, in this study we verified the GH-releasing activity of oral GHRP-6 (300 µg/kg) in normal aging and the possible enhancing effect of 8 g oral ARG on the GH-releasing effect of GHRP-6. Eight young (age 24–32 yr) and 8 elderly (age 66–85 yr) subjects were studied. In all the GH response to GHRH (1 µg/kg iv) was also studied. Both IGF-I levels and the GH response to iv GHRH were lower in elderly than in young subjects (mean±SE, IGF-I: 65.1±9.1 vs 142.9±9.4 µg/L, p<0.0001; GH peak: 5.4±1.0 vs 13.6 ± 0.8 µg/L, p<0.0001). Oral GHRP-6 administration induced a GH rise in elderly which was lower, though not significantly, than that in young subjects (GH peak: 9.9±2.0 vs 16.2±5.4 µg/L). Oral ARG administration enhanced the GHRP-6-induced GH rise in elderly (GH peak: 22.1±3.3 µg/L, p<0.01 vs GHRP-6 alone) while failed to modify it in young subjects (GH peak: 13.5±3.4 µg/L). The GH response to oral ARG+GHRP-6 in elderly was higher than that to all stimuli in young adults (p<0.05). In conclusion, present results show that in humans oral GHRP-6 administration induces a clear GH increase also in aging. The finding that ARG enhanced the activity of GHRP-6 in elderly but not in young subjects suggests that ARG is able to enhance the somatotropic responsiveness to GHRP-6 only when somatostatinergic activity is increased, i.e. in aging. Treatment with oral GHRP-6 alone or combined with low oral ARG may be hypothesized to restore GH secretion in aging.

A new test for the diagnosis of growth hormone deficiency due to primary pituitary impairment: combined administration of pyridostigmine and growth hormone-releasing hormone

The diagnosis of growth hormone (GH) deficiency (GHD) is currently based on failure to increase plasma GH levels to an arbitrary cutoff point of 7 or 10 μg/l in response to two provocative stimuli. False negative responses to these tests, however, frequently occur thus reducing their diagnostic reliability. The aim of this study was to assess a combination of pyridostigmine (PD) and GH-releasing hormone (GHRH) (60 mg oral PD 60 min before 1 μg/Kg GHRH iv) as a reliable test probing pituitary somatotropic function. In fact PD, an acetylcholinesterase inhibitor, strikingly potentiates GH response to GHRH likely by inhibiting somatostatin release. The combination PD + GHRH was tested in normal children and adolescents (NS, n = 27) and in a large group of short children classified as having familial short stature (FSS, n = 24), constitutional growth delay (CGD, n = 34) and GH deficiency (organic, oGHD, n = 6; idiopathic, iGHD, n = 10). In all groups results obtained by PD + GHRH were compared with those obtained by testing with GHRH, Clonidine (CLON) and PD alone and by studying spontaneous nocturnal GH secretion over 8 hours. Assuming 7 μg/l as minimum normal GH peak, a positive response occurred in only 18/24, 11/12 and 12/13 NS for GHRH, CLON, and PD, respectively. In contrast even assuming a minimum normal GH peak as high as 20 μg/l, PD + GHRH induced a positive response in 27/27 NS all having a nocturnal GH mean concentration (MC) ≥ 3 ug/l. Therefore PD + GHRH test gave no false negative responses and this was true not only in NS but even in all FSS and CGD having a GH MC ≥ 3 μg/l. On the other hand, PD + GHRH induced a negative GH response in all oGHD and in 8/10 iGHD patients. In the remaining two iGHD patients, PD + GHRH demonstrated a normal pituitary GH reserve in spite of a GH MC < 3 μg/l and low IGF-I level, thus pointing to a hypothalamic pathogenesis for the GHD. Considering FSS and CGD children having a GH MC < 3 μg/l, PD + GHRH showed a primary pituitary GH deficiency in 3/12 CGD with low plasma IGF-I levels. In conclusion, in slowly growing children PD + GHRH test is the most reliable provocative test for the diagnosis of primary pituitary GH deficiency being capable to discriminate between an unequivocally normal and impaired somatotropic function. The study of spontaneous GH secretion is mandatory only for those patients having low height velocity but a normal GH response to PD + GHRH in order to look for the existence of a GHD due to hypothalamic dysfunction.

A neuroendocrinological approach to evidence an impairment of central cholinergic function in aging.

A hypothalamic pathogenesis for the reduced GH secretion in aging has been reported for both animal and man. To further address this issue we studied in 31 elderly normal subjects (6 males and 25 females, aged 66–90 yr) and in 22 young healthy controls (13 males and 9 females, aged 20–35 yr) the GH responses to GHRH test (GHRH29, 1 μg/kg iv as a bolus at 0 min) alone and combined with pyridostigmine, a cholinesterase inhibitor (PD, 120 mg po 60 min before GHRH), or with arginine (ARG, 30 g in 100 ml infused from 0 to 30 min). Serum IGF-I levels were lower in elderly than in young subjects (mean±SE: 86.9±7.2 vs 288.7±22.1 μg/L, p<0.01). The GHRH-induced GH increase was lower in elderly than in young subjects (p<0.01). PD increased the GH response to GHRH in both groups (p<0.001), but in elderly subjects this response persisted lower (p<0.0001) than that ob-served in young adults. Also ARG coadministration potentiated the GHRH-induced GH release in both groups (p<0.0001) but in this case the elderly’s responses overlapped with the young’s. The GH in-crease observed after combined administration of ARG and GHRH was higher (p<0.0001) than that elicited by PD plus GHRH in elderly but not in young subjects. Analyzing individual GH responses, a GH peak below the limit of normality for young adults was observed in 19 (61.3%) elderly subjects after PD plus GHRH administration while ARG plus GHRH test elicited a normal GH peak in all but one. Taking into account that the GH-releasing effect of both PD and ARG is likely mediated by inhibition of hypothalamic somatostatin release, our data show that in elderly subjects the acutely releasable GH pool is preserved and give support to the hypothesis that a somatostatin hypertone underlies the reduced GH secretion of aged individuals. PD but not ARG fails to potentiate the GH response to GHRH in about 60% of aged subjects. These findings are suggestive for an impairment of the hypothalamic cholinergic system which, in turn, would be responsible for somatostatin hyperactivity and GH hyposecretion.

Activity of GH/IGF-I axis in trauma and septic patients during artificial nutrition: Different behavior patterns?

The aim of this study was to compare several parameters of GH/IGF-I axis activity in septic and trauma patients during Intensive Care Unit (ICU) stay. To this goal, 13 patients with sepsis (SEP) and 16 with trauma (TRA) were studied. Thirty-three adult subjects (AS) were studied as controls. Serum IGF-I and -II, IGFBP-1, -2 and -3, GH and GHBP levels were studied on day 1, 3, 5 and 7 after ICU admission, during comparable artificial nutrition in SEP and TRA and basally in AS. In 5 patients with SEP and 6 with TRA, the GH response to GHRH was evaluated on day 3. On ICU day 1, IGF-I and -II and IGFBP-3 in SEP were lower (p<0.05) than in TRA which, in turn, were lower (p<0.01) than in AS. IGF-I increased (p<0.05) both in SEP and TRA, but, on ICU day 7, those in SEP persisted lower than in TRA, which became similar to those in AS. IGF-II levels increased (p<0.05) in SEP only, persisting lower (p<0.05) than in TRA. On ICU day 1, GH in SEP and TRA were similar and did not vary until day 7, overlapping those in AS. The GH response to GHRH in SEP and TRA was similar and lower (p<0.01) than in AS. These findings indicate that IGF-I activity is impaired more in septic than in trauma patients. Reduced IGF-I activity probably reflects peripheral GH resistance though basal and GHRH-induced GH levels were not increased in these conditions.

Normal IGF-I and enhanced IGFBP-3 response to very low rhGH dose in patients with dilated cardiomyopathy

Well-nourished patients with dilated cardiomyopathy (DCM) show slight reduction of mean basal IGF-I levels which, however, display a response to a rhGH dose as low as 5.0 μg/kg/day similar to that of age-matched control subjects (CS). To further investigate peripheral GH sensitivity, we studied the IGF-I and IGFBP-3 responses to 4-day sc 2.5 μg/kg/day rhGH administration, the lowest effective dose able to increase IGF-I levels in normal subjects, in 10 DCM patients [age (mean±SE): 57.6±1.0 yr, body mass index (BMI): 24.0±1.2 kg/m2, left ventricular ejection fraction: 26.2±3.2%, NYHA (New York Heart Association): I/0, II/4, III/4, IV/2] and in 9 age-matched healthy CS (age: 55.3±1.2 yr, BMI: 23.7±1.8 kg/m2). Basal IGF-I levels in DCM were lower though not significantly than those in CS (147.7±9.8 vs 174.7±17.0 μg/l). Basal IGFBP-3 levels in DCM were similar to those in CS (3.1±0.3 vs 2.7±0.2 mg/l). In CS 4-day rhGH increased IGF-I levels (222.4±14.9 μg/l; p<0.01 vs baseline) but did not modify IGFBP-3 levels (3.0±0.2 mg/l). In DCM IGF-I levels were increased by 4-day rhGH administration (175.7±11.0 μg/l; p<0.05 vs baseline) with a similar percent extent than in CS. On the other hand, in DCM, but not in CS, 4-day rhGH significantly increased IGFBP-3 levels (3.5±0.3 mg/l; p<0.05 vs baseline). Therefore, in conclusion, testing with the lowest effective rhGH dose further suggest that peripheral GH sensitivity in well-nourished DCM is preserved. On the other hand, DCM patients show enhanced IGFBP-3 sensitivity to stimulation by rhGH.