S. Goffi

Effect of the potentiation of cholinergic activity on the variability in individual GH response to GH-releasing hormone

In man the GH response to GHRH is highly variable and some normal subjects may be completely unresponsive to the neuropeptide. On the other hand, the potentiation of cholinergic activity by pyridostigmine (PD), a cholinesterase inhibitor, increases the GH response to GHRH, probably by inhibiting somatostatin release. The aim of this study was to assess the existence of intrain-dividual variability in the GH response to GHRH and verify the effects of PD treatment on inter- and intraindividual variability. Twenty normal adults (17 M and 3 F) and 10 normal prepubertal children (9 M and 1 F) underwent 2–5 administrations of 1 µg/kg GHRH on different days. Seven adults and all children also underwent 1–5 other tests in which GHRH was preceded (60 min before) by oral PD (120 mg in adults and 60 mg in children). The GH responses to GHRH were highly variable, not only within subjects but also in the same subject on different occasions (peak range; adults: 0.4–49.0 ng/ml; children: 2.4–50.0 ng/ml). PD always markedly increased the GH response to GHRH, even unmasking this response in 3 adults and 4 children hyporesponsive to the neuropeptide alone. However the variability in the GH response was still present (adults: 27.2–108.5 ng/ml; children: 25.0–144.0 ng/ml), though reduced (adults: p = 0.0005; children: p = 0.0204). These data indicate that: i. A great inter- and intraindividual variability in the GH response to GHRH is present. ii. PD always potentiates this response, reducing the variability and abolishing false negative responses. The variability in the GH response to GHRH may be due, at least in part, to a different somatostatinergic tone that can be blunted by the enhancement of the cholinergic function.

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.

Cholinergic Enhancement by Pyridostigmine Potentiates Spontaneous Diurnal but not Nocturnal Growth Hormone Secretion in Short Children

It has been shown that enhanced cholinergic tone induced by pyridostigmine (PD) increases both basal and GHRH-stimulated GH levels in both adults and children. In this study the effects of PD (60 mg orally) on GH secretion were studied both in the morning (from 8.00 to 12.00) and in the night (from 23.00 to 3.00) in 7 short children previously shown as having a normal spontaneous nocturnal GH secretion. In the morning, PD induced a GH increase higher than saline (peak, mean +/- SEM: 17.4 +/- 3.4 vs. 5.5 +/- 3.0 ng/ml, p less than 0.02; area under curve (AUC): 360.8 +/- 71.4 vs. 109.4 +/- 44.7 ng/ml/h, p less than 0.01). In the night, no difference was observed between GH secretion after PD (peak: 16.7 +/- 2.4 ng/ml; AUC: 468.2 +/- 95.5 ng/ml/h) and saline (peak: 16.0 +/- 2.7 ng/ml; AUC: 409.1 +/- 97.7 ng/ml/h). Spontaneous GH secretion was higher during the night than in the morning (p less than 0.02) whereas nocturnal GH secretion overlapped with that in the morning after PD. The ability of PD to increase GH secretion during the morning but not GH hypersecretion occurring at night implies that the cholinergic tone in the central nervous system areas controlling GH secretion is already maximally stimulated at night. Since, reportedly, the cholinergic system negatively modulates somatostatin secretion, presence of a physiologically reduced somatostatinergic tone may be envisaged at night.

Prolactin and TSH responses to both domperidone and TRH in normal and hyperprolactinaemic women after dopamine synthesis blockade.

A study of the effect of α‐methyl‐1‐tryosine (metyrosine) blockade (2 g/d for 2 d) of dopamine (DA) synthesis on the PRL and TSH response to domperidone (DOM) and TRH in normal women and subjects with pathological hyperpro‐lactinaemia is reported.

Acute administration of pyridostigmine and clonidine has an additive stimulatory effect on GH release in normal children.

It has been shown in humans that both alpha2-adrenoceptor activation by clonidine (CLON) and cholinergic enhancement by pyridostigmine (PD) have a clear-cut stimulatory effect on GH release. As this effect is probably mediated by two different mechanisms, i.e. via increased endogenous GHRH for CLON and via inhibition of endogenous somatostatin for PD, in 8 normal children we studied the effect of both single and combined acute oral administration of CLON (150 μg/m2) and PD (60 mg). When administered alone, CLON and PD induced a similar GH increase (peak, mean ±SE: 14.6 ± 2.4 vs 14.2 ± 3.1 ng/ml; area under curve, AUC: 376.9 ± 57.6 vs 390.0 ± 74.3 ng/ml/h). Combined administration of CLON and PD had an additive effect on GH release (peak: 27.5 ± 4.5 ng/ml; AUC: 920.8 ± 153.3 ng/ml/h; p < 0.005 vs CLON and PD alone). In conclusion, presented data show that: i) CLON and PD have similar GH-releasing effect in normal children; ii) The additive stimulatory effect on GH release exerted by acute combined administration of CLON and PD agrees with the hypothesized different mechanism of action of these two drugs; iii) A therapeutic association of CLON and PD may be envisaged in the treatment of some children of short stature.

Effect of naloxone on gonadotropin secretion before and after testosterone in Klinefelter's syndrome.

A study was performed on eight subjects with Klinefelters syndrome to assess the relation between gonadal hormones and opioid inhibition of gonadotropin secretion through comparison of their gonadotropin response to naloxone (NAL) (0.3 mg/kg; 1/3 bolus iv. at time 0 and 2/3 iv. for 120 min) before and after testosterone propionate (TP) 100 mg/day im. for 5 days. Under basal conditions, NAL failed to induce a significant change in LH levels. After TP, however, despite unchanged basal LH levels (mean +/- S.E.M.: 27.0 +/- 3.4 vs 21.2 +/- 3.21 microU/ml), LH significantly increased in response to NAL. FSH did not respond to NAL either before or after TP administration, though FSH levels were significantly reduced by TP. These findings suggest that in man, as in animals, gonadal hormones regulate opioid inhibition of LH secretion. The negative feedback of testosterone and its ability to activate opioid inhibiting tone may be dissociated, in keeping with the view that gonadal hormones control gonadotropin secretion through the activation of distinct, albeit concomitant, mechanisms.