The late growth hormone rise induced by oral glucose is enhanced by cholinergic stimulation with pyridostigmine in normal subjects

OBJECTIVE: We have investigated the late GH rise occurring 3-5 hours after oral glucose administration. We have assessed the effect of endogenous cholinergic enhancement with pyridostigmine on the delayed GH rise following oral glucose loading in normal subjects. DESIGN: Placebo or 75 g oral glucose was given to the normal subjects 3 hours before 120 mg oral pyridostigmine or placebo. Four tests were carried out at random. (0 min) + placebo (180 min); test 2: glucose (0 min) + placebo (180 min); test 3: placebo (0 min) + pyridostigmine (180 min); test 4: glucose (0 min) + pyridostigmine (180 min). SUBJECTS: We studied eight normal subjects (four male and four female), ages 19-29 years, body mass indices 18-22 kg/m2. MEASUREMENTS: Plasma glucose and serum GH concentrations were measured for 6 hours after oral glucose or placebo administration. RESULTS: Pyridostigmine treatment significantly enhanced the GH releasing effect of prior (3 h) oral glucose. Late GH peak obtained by oral glucose loading rose from (mean +/- SEM) 17.4 +/- 4.6 to 37.2 +/- 9.0 mU/l (P < 0.05) after pyridostigmine, while GH peak following placebo plus pyridostigmine was 12.4 +/- 2.0 mU/l (P < 0.05 vs glucose plus pyridostigmine). The analysis of GH area under curves (AUCs) in the second phase of the tests (180-360 min) confirmed that glucose plus pyridostigmine released a greater amount of GH (4128 +/- 764 mU/l/3h) than glucose (1694 +/- 494 mU/l/3h, P < 0.001) or pyridostigmine alone (1292 +/- 150 mU/I/3h, P < 0.001). CONCLUSIONS: Pyridostigmine, an indirect cholinergic drug likely to inhibit somatostatin secretion from the hypothalamus, enhanced the late GH releasing activity of oral glucose. There is evidence that glucose suppresses plasma GH initially by increasing hypothalamic somatostatin release. This would result in an increase in the pituitary stores of GH. We propose that the delayed GH rise after oral glucose occurs when there is a fall in hypothalamic somatostatinergic tone; this is further reduced by the administration of pyridostigmine. At this time the pituitary stores of GH are released as a consequence of resumption of hypothalamic GHRH activity. This leads to the late GH rise.     

Cholinergic modulation of growth hormone responses to growth hormone-releasing hormone in uraemic patients on peritoneal dialysis

BACKGROUND: Hypothalamic cholinergic neurotransmission plays a major role in the regulation of GH secretion. Pyridostigmine, a cholinesterase inhibitor, is able to decrease hypothalamic somatostatinergic tone and release GH in normal subjects. Blockade of muscarinic receptor with pirenzepine blunts the GH release in several clinical situations. However, little information is available on the role played by central cholinergic pathways in GH regulation in uraemic patients. OBJECTIVE: We aimed to assess GH responses to GHRH after pretreatment with pyridostigmine and pirenzepine in a group of uraemic patients undergoing peritoneal dialysis (PD). GH responses of the patients treated with recombinant human erythropeitin (rhEPO) were compared to patients without treatment. DESIGN: We studied 14 male patients on PD and nine control subjects. All subjects underwent three endocrine test in random order after an overnight fast. Each subject received GHRH (100 microg, i.v. in bolus at 0 minutes). Sixty minutes before the injection of GHRH subjects were given oral placebo, pyridostigmine (120 mg), or pirenzepine (100 mg). MEASUREMENTS: Blood samples for GH were collected at -60, 0, 15, 30, 45, 60 and 90 minutes The hormonal secretory responses were studied by a time-averaged (area under the curves, AUC) and time-independent (peak values) analysis. RESULTS: Baseline GH concentrations were similar in patients and controls. GH responses to placebo plus GHRH were also comparable in patients and controls (peak 26.6 +/- 3.8 vs. 33.2 +/- 4.4 mU/l, AUC 28.2 +/- 3.4 vs. 27.8 +/- 4.6 mU/h/l). Pyridostigmine administration induced a significant potentiation of GH responses to GHRH both in patients (peak 43.2 +/- 5.2 mU/l, AUC 47.6 +/- 6.0 mU/h/l; P < 0.01) and in control subjects (peak 79.2 +/- 8.6 mU/l, AUC 78.0 +/- 9.4 mU/h/l; P < 0.01). However, the increment in GH peak and AUC was significantly (P < 0.05) greater in controls in relation to values found in patients. Pirenzepine administration induced an abolishment of GH release after GHRH stimulation both in PD patients (peak 5.4 +/- 2.6 mU/l, AUC 6.0 +/- 2.4 mU/h/l; P < 0.01) and in healthy controls (peak 3.8 +/- 0.6 mU/l, AUC 4.0 +/- 0.4 mU/h/l; P < 0.05). Responses to pyridostigmine plus GHRH and pirenzepine plus GHRH were similar in patients on chronic therapy with recombinant human erythropeitin and in patients without rhEPO therapy. CONCLUSION: These results suggest that the cholinergic regulation of GH release is preserved in uraemic patients on peritoneal dialysis. The significantly lower increase in GH response to GHRH induced by pyridostigmine suggests that cholinergic stimulatory tone is attenuated in patients in relation to control subjects. Long-term therapy with rhEPO seems not to affect GH responses to cholinergic stimulation or blockade.     

Pyridostigmine partially reverses dexamethasone-induced inhibition of the growth hormone response to growth hormone-releasing hormone.

The GH response to insulin-induced hypoglycaemia and growth hormone-releasing hormone (GHRH) has been shown to be impaired in subjects with Cushing's syndrome and in healthy volunteers given oral glucocorticoids. Pyridostigmine is an anticholinesterase that stimulates GH secretion, probably by inhibition of hypothalamic somatostatin secretion. This work was designed to study the site of action of glucocorticoids in inhibiting the secretion of GH. Eight healthy male volunteers were studied on three occasions in random order. They took 2 mg oral dexamethasone or placebo at precisely 6-hourly intervals for 48 h before receiving 120 mg oral pyridostigmine or placebo, followed 60 min later by GHRH (100 micrograms) i.v. Samples for measuring GH were obtained at 15 min intervals for 2 h. The 'area under the curve' (AUC) for each of the treatments was significantly different: dexamethasone-pyridostigmine-GHRH (mean +/- S.E.M., 1938 +/- 631 mU/min per l), dexamethasone-placebo-GHRH (634 +/- 211) and placebo-placebo-GHRH (4267 +/- 1183) (P < 0.02, Wilcoxon test). In conclusion, dexamethasone given for 48 h significantly inhibited the AUC for GH following treatment with GHRH. However, pretreatment with pyridostigmine significantly reversed the inhibition although this was still partial. Our data suggested that this short-term suppressive effect of dexamethasone was independent of GHRH, and most probably relates to stimulation of the release of somatostatin.     

Arginine normalizes the growth hormone (GH) response to GH-releasing hormone in adult patients receiving chronic daily immunosuppressive glucocorticoid therapy.

Glucocorticoids are thought to inhibit GH secretion through an enhancement of endogenous somatostatin tone. The aim of our study was to evaluate the effect of arginine, a secretagogue that increases GH secretion acting at the hypothalamic level, probably by decreasing somatostatin tone, on GH-releasing hormone (GHRH)-induced GH secretion in three male and five female adult patients with nonendocrine disease who were receiving daily immunosuppressive glucocorticoid therapy. Six normal subjects (four males and two females) served as controls. GHRH-induced GH secretion was evaluated after 30-min iv infusion of saline (100 mL) or arginine (30 g) in 100 mL saline. After saline administration, steroid-treated patients showed a blunted GH response to GHRH (GH peak, 8.7 +/- 2.4 micrograms/L) compared to that of normal subjects (GH peak, 23.8 +/- 3.9 micrograms/L). The GH responses to GHRH increased (P less than 0.05) after pretreatment with arginine compared to saline pretreatment in both normal subjects (GH peak, 36.6 +/- 4.0 micrograms/L) and steroid-treated patients (GH peak, 28.4 +/- 5.5 micrograms/L). The GH responses to GHRH plus arginine were not significantly different in steroid-treated and normal subjects. Thus, arginine is able to normalize the GH response to GHRH in patients receiving chronic glucocorticoid treatment. Our data are evidence that the stimulatory action of arginine and the inhibitory action of glucocorticoids on GH secretion are mediated by opposite effects on hypothalamic somatostatin tone.     

Activation of cholinergic neurotransmission by pyridostigmine reverses the inhibitory effect of hyperglycemia on growth hormone (GH) releasing hormone-induced GH secretion in man: does acute hyperglycemia act through hypothalamic release of somatostatin?

Acute hyperglycemia blocks growth hormone (GH) secretion in response to provocative stimuli including growth hormone releasing hormone (GHRH) administration. However, the precise mechanism of glucose action is unknown. To determine if enhanced somatostatinergic stimulation accounts for the decreased GH secretion, we studied the effect of enhanced cholinergic tone by pyridostigmine on the hyperglycemia blockade of GH release in 7 normal subjects. Pyridostigmine, an acetylcholinesterase inhibitor, has been postulated as an inhibitor of somatostatin release. Each subject underwent 4 tests with GHRH injection (100 micrograms i.v. at 0 min). In the first (control) test, placebo was administered before GHRH. In the second test, 100 g of glucose was administered p.o. 45 min before GHRH. In the third test, pyridostigmine, 120 mg p.o., was administered 60 min before GHRH, and in the fourth test, pyridostigmine, glucose and GHRH were administered at -60, -45 and 0 min, respectively. GHRH-induced GH secretion of 25.8 +/- 4.5 ng/ml was significantly reduced by previous glucose administration (12.1 +/- 4.5 ng/ml) and significantly potentiated by previous pyridostigmine pretreatment (56.5 +/- 16.8 ng/ml). In the fourth test (pyridostigmine plus glucose plus GHRH) the GH peak of 42.4 +/- 9.2 ng/ml was significantly higher than after GHRH alone and not different to the pyridostigmine-GHRH test. In conclusion, central cholinergic activation by pyridostigmine reversed the hyperglycemic blockade of GHRH-induced GH secretion. In addition, hyperglycemia was unable to reduce the potentiating effect of pyridostigmine on GH secretion elicited by GHRH. Based on the reported actions of pyridostigmine, acute hyperglycemia might act over GH release by inducing hypothalamic somatostatin release.     

Cholinergic receptor activation by pyridostigmine restores growth hormone (GH) responsiveness to GH-releasing hormone administration in obese subjects: evidence for hypothalamic somatostatinergic participation in the blunted GH release of obesity

GH secretion in response to provocative stimuli is decreased in obese individuals. However, the precise mechanism causing this decrease is unknown. In an attempt to determine if reduced cholinergic stimulation accounts for the decreased GH secretion, we studied the effect of enhanced cholinergic tone induced by pyridostigmine on GHRH-stimulated GH secretion in a group of seven obese and seven normal subjects. When GHRH (100 micrograms, iv) was administered after placebo in the obese group, mean plasma GH rose from 0.5 +/- (0.1 (+/- SE) to 3.6 +/- 1.5 micrograms/L at 30 min. When the same obese subjects were given GHRH 60 min after pyridostigmine administration (120 mg, orally), the mean plasma GH level rose from 1.8 +/- 0.6 to 21.0 +/- 7.5 micrograms/L at 30 min. The responses to placebo and pyridostigmine were significantly different at 15, 30, 45, 60, and 90 min. In the normal subjects, a similar dose of GHRH induced a GH peak of 24.3 +/- 7.1 micrograms/L, and the GHRH-stimulated peak was significantly higher (56.2 +/- 16.8 micrograms/L) after pyridostigmine administration. To study the effect of pyridostigmine alone six other obese and six other normal subjects were tested with pyridostigmine or placebo on different days. In the normal subjects the mean peak plasma GH level after pyridostigmine was 12.5 +/- 3.1 micrograms/L, and in the obese subjects it was 4.6 +/- 1.3 micrograms/L. Thus, pyridostigmine potentiated the action of GHRH, rather than merely being additive. We conclude that pyridostigmine stimulates GH secretion in obese as well as normal subjects, although the response was less in the former group. Pyridostigmine potentiates the response to GHRH in both groups, but again, the response was less in the obese subjects. These results suggest that the impaired somatotroph responsiveness in obese subjects may be due to chronically decreased hypothalamic cholinergic tone, resulting in enhanced somatostatinergic tone.     

An inhibition of the hypothalamic somatostatinergic tone is not the cause of the enhanced growth hormone response to growth hormone-releasing hormone in patients with liver cirrhosis.

Animal models of liver cirrhosis (LC) display a reduced hypothalamic somatostatinergic tone. To test whether a similar mechanism could explain the enhanced Growth Hormone (GH) secretory response to GH-Releasing Hormone (GHRH), which is seen in human LC, we studied the effect of the cholinesterase inhibitor pyridostigmine (PD), which is able to reduce the release of hypothalamic somatostatin (SS), on the GHRH-stimulated GH secretion. We considered that if PD were unable to increase GH secretion, this would constitute evidence of an already inhibited endogenous somatostatinergic tone. If proved, this in turn could explain the enhanced GH response to GHRH seen in LC. Ten LC patients and nine controls were given GHRH (100 microg, intravenously), or PD (120 mg, orally) plus GHRH. After GHRH alone, the GH peak was four times higher in LC than in controls (40.85+/-15.7 ng/ml in LC and 9.35+/-2.5 ng/ml in controls). In LC, PD administration markedly increased the GH response to GHRH (GH peak: 98.0+/-19.7 ng/ml; +240% vs. GHRH alone). The ability of PD to increase the GH response in patients with LC suggests that in this condition the enhanced GH response to GHRH is not due to a completely inhibited endogenous somatostatinergic tone. SS appears instead to maintain its modulator role on GH secretion in human LC, in contrast with what observed in animal models.     

Radiation and neuroregulatory control of growth hormone secretion

OBJECTIVE Cranial irradiation frequently results In growth hormone (GH) deficiency. Patients with radiation-induced GH deficiency usually remain responsive to exogenous growth hormone releasing hormone, implying radiation damages the hypothalamus rather than the pituitary. Little is known about the effect of cranial irradiation on the neuroendocrine control of GH secretion. This study was to determine the effect of cranial irradiation on somatostatin tone. DESIGN Somatostatin tone was examined by manipulating cholinergic tone in young adults with radiation-induced GH deficiency and a control population. Each individual underwent three separate studies: the GH response to 100μg GHRH-(1–29)-NH₂ was assessed alone, and 60 minutes after pyridostigmine or pirenzepine. PATIENTS Eight young male adults with radiation induced GH deficiency following treatment in childhood for a brain tumour or acute lymphoblastlc leukaemla, and ten healthy adult men were studied. MEASUREMENTS Serum growth hormone was measured at 15-minute intervals throughout each of the three study periods. RESULTS One of 10 controls and four of eight irradiated subjects had a peak GH level to GHRH analogue of less than 20mU/l. After pretreatment with pyridostigmine, all subjects except one irradiated subject had a peak GH level of greater than 20 mU/l. Pretreatment with pyridostigmine and pirenzepine significantly modified the GH response to GHRH analogue within both groups (P < 0·0005). Pretreatment with pyridostigmine significantly enhanced the GH response to GHRH analogue (median (range) area under the curve, 9029 (1956–20940) mU/l/min in controls vs 1970 (628–3608) mU/l/min in the irradiated group) compared with GHRH analogue alone (1953 (512–16140) mU/l/min in control group vs 997 (266–3488) mU/l/min in the Irradiated group). Pretreatment with pirenzepine significantly attenuated the GH response to GHRH analogue (552 (64–1274) mU/l/min in controls vs 305 (134–2726) mU/l/min in Irradiated group). Between the groups there was no significant difference in GH area under the curve (AUC) after GHRH analogue alone. There was a significantly (P= 0·0014) greater increment of GH Secretion after pyridostigmine and GHRH analogue compared with GHRH analogue alone (difference in AUC of pyridostigmine + GHRH analogue and GHRH analogue alone 6348 (696–12856) mU/l controls vs 542 (120–1340) mU/l In the irradiated group) and significantly (P= 0·033) greater suppression of GH secretion after pirenzepine and GHRH analogue compared with GHRH analogue alone (difference in AUC of GHRH analogue alone and pirenzepine + GHRH analogue 1644 (222–15205) mU/l in controls vs 479 (469–1623) mU/l in the irradiated group) in the control population compared with those who had received cranial irradiation in childhood. CONCLUSIONS These data suggest that cranial irradiation reduces but does not abolish somatostatin (SRIH) tone and also reduces endogenous GHRH secretion. Although SRIH tone is reduced, it can be increased by cholinergic manipulation and is therefore not irreversibly fixed. This has possible implications if GHRH analogues were used to treat children with radiation induced GH deficiency.     

Pyridostigmine does not reverse dexamethasone-induced growth hormone inhibition

Glucocorticoids inhibit the growth hormone (GH) response to a variety of stimuli, including GH-releasing hormone (GHRH) in vivo, but they increase GHRH-stimulated GH secretion when added, in vitro, to animal and human pituitary cells. This discrepancy has led to the hypothesis that glucocorticoids act in vivo by increasing somatostatin secretion from the hypothalamus. To examine this hypothesis, we used a cholinergic drug, pyridostigmine (PD), which reduces hypothalamic somatostatin secretion. Eight normal volunteers were studied. They underwent four tests: (1) GHRH test; (2) Dex + GHRH (GHRH test after treatment the night before, with dexamethasone (Dex)); (3) PD + GHRH; (4) Dex + PD + GHRH. Dex significantly inhibited the GH response to GHRH expressed as area under the GH/time curve (AUC, microgram/1/min) (mean +/- SEM = 895.2 +/- 196.6 vs 1970.9 +/- 600.1, P less than 0.05). PD significantly increased the AUC of GH secretion in PD + GHRH compared with GHRH alone (3541.2 +/- 571.3 vs 1970.9 +/- 600.1, P less than 0.01) but by no means restored completely the normal GH response to GHRH, when given to Dex-pretreated subjects. Furthermore, the mean AUC of Dex + PD + GHRH was significantly lower than that of PD + GHRH (1621.7 +/- 500.6 vs 3541.2 +/- 571.3, P less than 0.01), demonstrating that Dex continues to exert its inhibitory effect on GH secretion in the presence of PD. These results suggest that glucocorticoid-induced GH inhibition does not act solely through an increase in hypothalamic somatostatin secretion.     

Melatonin stimulates growth hormone secretion through pathways other than the growth hormone-releasing hormone

OBJECTIVE There is evidence that melatonin plays a role in the regulation of GH secretion. The aim of this study was to investigate the neuroendocrine mechanisms by which melatonin modulates GH secretion. Thus we assessed the effect of oral melatonin on the GH responses to GHRH administration and compared the effects of melatonin with those of pyridostigmine, a cholinergic agonist drug which is likely to suppress hypothalamic somatostatin release. DESIGN The study consisted of four protocols carried out during the afternoon hours. Study 1: oral melatonin (10 mg) or placebo were administered 60 minutes prior to GHRH (100 μg i.v. bolus). Study 2: GHRH (100 μg i.v. bolus) or placebo were administered at 0 minutes; oral melatonin or placebo were given at 60 minutes and were followed by a second GHRH stimulus (100 μg i.v. bolus) at 120 minutes. Study 3: placebo; oral melatonin (10 mg); oral pyridostigmine (120 mg); melatonin (10 mg) plus pyridostigmine (120 mg) were administered on separate occasions. Study 4: placebo; oral melatonin (10 mg); oral pyridostigmine (120 mg); melatonin (10 mg) plus pyridostigmine (120 mg) were administered on separate occasions 60 minutes prior to a submaximal dose (3 μg i.v. bolus) of GHRH. SUBJECTS Four groups of eight normal male subjects, ages 22–35 years, were randomly assigned to each protocol. MEASUREMENTS Growth hormone was measured by RIA at 15-minute intervals. RESULTS Oral melatonin administration had a weak stimulatory effect on GH basal levels. Prior melatonin administration approximately doubled the GH release induced by supramaximal (100 μg) or submaximal (3 μg) doses of GHRH. Melatonin administration restored the GH response to a second GHRH challenge, given 120 minutes after a first GHRH i.v. bolus. The GH releasing effects of pyridostigmine, either alone or followed by GHRH, were greater than those of melatonin. However, the simultaneous administration of melatonin and pyridostigmine was not followed by any further enhancement of GH release, either in the absence or in the presence of exogenous GHRH. CONCLUSIONS Our data indicate that oral administration of melatonin to normal human males increases basal GH release and GH responsiveness to GHRH through the same pathways as pyridostigmine. Therefore it is likely that melatonin plays this facilitatory role at the hypothalamic level by inhibiting endogenous somatostatin release, although with a lower potency than pyridostigmine. The physiological role of melatonin in GH neuroregulation remains to be established.     

Effect of galanin on growth hormone-releasing hormone-stimulated growth hormone secretion in adult patients with nonendocrine diseases on long-term daily glucocorticoid treatment.

Glucocorticoids are thought to inhibit growth hormone (GH) secretion through an enhancement of endogenous somatostatin tone. The aim of our study was to evaluate the effect of galanin, a neuropeptide that stimulates GH secretion, on GH-releasing hormone (GHRH)-induced GH secretion in adult patients with nonendocrine diseases who were under daily immunosuppressive glucocorticoid therapy. Six normal subjects (four men, two women) and seven steroid-treated subjects (three men, four women) were studied. GHRH-induced GH secretion was evaluated during a 40-minute intravenous (i.v.) infusion of saline or porcine galanin (12.5 micrograms/min). During saline infusion, steroid-treated patients showed a blunted GH response to GHRH (GH peak, 8.1 +/- 2.8 micrograms/L), as compared with normal subjects (GH peak, 23.8 +/- 3.9 micrograms/L). During galanin infusion, the GH response to GHRH was significantly enhanced (GH peak, 46.6 +/- 9.4 micrograms/L, P less than .05), as compared with saline infusion in normal subjects. In contrast, galanin infusion did not enhance the GH response to GHRH (GH peak, 16.6 +/- 6.5 micrograms/L), as compared with saline infusion in steroid-treated patients. The area under the GH-response curves was also significantly (P less than .05) lower in steroid-treated subjects, as compared with normal subjects. Thus, galanin failed to normalize or enhance the GH response to GHRH in patients treated long-term with glucocorticoids. It can be hypothesized that galanin does not elicit GH secretion by decreasing hypothalamic somatostatin tone.     

Glucocorticoids: potent inhibitors and stimulators of growth hormone secretion

Excessive glucocorticoid concentrations are well recognized inhibitors of linear growth, due in part to their suppression of GH secretion. The mechanism of this inhibition has been unclear, especially since glucocorticoids enhance the in vitro GH response of pituitary cells to GH-releasing hormone (GHRH). We investigated the possibility that hypothalamic somatostatin might be mediating these dichotomous observations by using passive immunization techniques. The GH response to GHRH was significantly blunted in rats pretreated with the synthetic glucocorticoid, dexamethasone, compared to that in normal animals. In marked contrast, the immunoneutralization of somatostatin resulted in a significantly enhanced GH response to GHRH in dexamethasone-treated animals. These results suggest that the previously described inhibitory action of glucocorticoids on GH secretion in vivo are mediated via altered hypothalamic somatostatin tone.     

EFFECT OF ENHANCEMENT OF ENDOGENOUS CHOLINERGIC TONE WITH PYRIDOSTIGMINE ON GROWTH HORMONE (GH) RESPONSES TO GH-RELEASING HORMONE IN PATIENTS WITH CUSHING''S SYNDROME

Growth hormone (GH) secretion in patients with Cushing's syndrome is diminished to all the stimuli tested so far but the precise mechanisms through which this occurs are unknown. In order to investigate whether increased somatostatinergic tone might be responsible for this alteration, we studied the effect of pyridostigmine (120 mg p.o. at -60 min), which activates cholinergic synapses and thus suppresses hypothalamic somatostatin release on GH responses to GHRH (100 micrograms, i.v. at 0 min), in six patients with Cushing's syndrome. We found that while pyridostigmine markedly potentiated GH responses to GHRH, in all the normal subjects tested (n = 12), neither GHRH alone nor GHRH plus pyridostigmine elicited any increase in GH secretion in any of the patients with Cushing's syndrome. This suggests that chronic glucocorticoid excess induces marked alterations in the hypothalamic control of GH secretion.     

Corticosteroid-induced growth hormone secretion in normal and obese subjects

OBJECTIVE: In normal subjects, corticosteroids stimulate growth hormone (GH) secretion at 3 hours. Obesity is associated with blunted GH secretion. In order to clarify both the deranged mechanism of GH secretion in obesity and the corticosteroid mechanism of action we have assessed in normal and obese subjects the effects of dexamethasone, pyridostigmine (a drug capable of suppressing somatostatin release) and GHRH. We also compared in normal subjects the stimulatory effect of three different corticosteroids on plasma GH levels. DESIGN: In both normal and obese subjects the following tests were carried out: placebo; dexamethasone alone (4 mg i.v. at 0 minutes); and dexamethasone plus pyridostigmine (120 mg p.o. at 60 minutes). In normal subjects we also studied the effects of hydrocortisone (100 mg i.v. at 0 minutes) and deflazacort (a corticosteroid that does not cross the blood-brain barrier) (60 mg i.v. at 0 minutes). In obese subjects we also assessed the effect of dexamethasone plus GHRH (100 micrograms i.v. at 150 minutes) on plasma GH levels. PATIENTS: Ten normal subjects and 22 obese subjects were studied. Normal controls were within 10% of their ideal body weight. Obese subjects had a body mass index of 37.1 +/- 1.1 (mean +/- SEM). MEASUREMENTS: Plasma GH levels were measured by radioimmunoassay. RESULTS: Dexamethasone-induced GH secretion in normal subjects (28.6 +/- 7.8 millimicron/l, P less than 0.05). Corticosteroids did not alter GH levels in obese subjects. Pretreatment with pyridostigmine increased dexamethasone-induced GH release in normal subjects (40.8 +/- 6.8 millimicron/l) but this did not achieve statistical significance. Dexamethasone plus pyridostigmine did not alter GH levels in obese subjects (8.0 +/- 1.6 mU/l). In some subjects, dexamethasone pretreatment potentiated GHRH-stimulated GH secretion, while in half the subjects the basal GH levels were not altered. In control subjects, hydrocortisone and deflazacort caused GH release similar to dexamethasone. CONCLUSIONS: Corticosteroids are a new and selective stimulus of GH secretion. They do not cause GH release in obese subjects. Their relative independence from cholinergic control suggest that they act by reducing somatostatin secretion.     

Cholinergic enhancement of 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.     

GH FEEDBACK OCCURS THROUGH MODULATION OF HYPOTHALAMIC SOMATOSTATIN UNDER CHOLINERGIC CONTROL: STUDIES WITH PYRIDOSTIGMINE AND GHRH

We have studied the effect of increased cholinergic tone on the GH response to growth hormone-releasing hormone (GHRH) and on GH feedback, using pyridostigmine, an acetylcholinesterase inhibitor. In six healthy male adult volunteers 120 mg oral pyridostigmine increased basal GH secretion compared to placebo and augmented the GH response to 100 micrograms i.v. GHRH (1-29) NH2; the effect was more than the additive effect of pyridostigmine and GHRH when each was given alone. Pretreatment with 2 IU methionyl-hGH given i.v. abolished the serum GH response to GHRH given 3 h later, demonstrating a negative feedback loop of GH on the response to GHRH; this inhibited response to GHRH was restored in subjects given pyridostigmine as well as methionyl-hGH. The data demonstrate that enhanced cholinergic tone releases GH, augments the serum GH response to GHRH and unblocks the negative feedback effect of methionyl-hGH pretreatment on the GH response to GHRH. These results suggest that GH negative feedback effects on its own secretion occur predominantly through increased hypothalamic somatostatin secretion; this somatostatin secretion is under inhibitory cholinergic control.     

Pyridostigmine fails to increase either spontaneous or GHRH-stimulated GH secretion during day or night in growth hormone-insufficient children

OBJECTIVE The aim of the study was to investigate whether pyridostigmine, a cholinesterase inhibitor which is thought to act at the hypothalamus to inhibit somatostatin secretion, would augment spontaneous or GHRH-stimulated serum GH levels in patients with GH-insufficiency. DESIGN Oral pyridostigmine 60 mg or placebo was administered at the start of a 9-h subcutaneous infusion of either GHRH (1-29)NH2 10 micrograms/kg/h or saline control. Studies were performed during the daytime (0900-1800 h) in five patients, and the night-time (2100-0600 h) in a further five. PATIENTS Ten short, pre-pubertal children (aged 6-11 years; eight boys) with growth hormone insufficiency were studied. MEASURES Blood for serum GH was sampled every 20 min, and analysed using the PULSAR program. RESULTS The subcutaneous infusion of GHRH 10 micrograms/kg/h increased mean serum GH levels (+/- SEM): by day 17.7(+/- 6.8) vs placebo 2.2(+/- 0.4) mU/l (P less than 0.01), and by night 26.9(+/- 3.3) vs 5.5(+/- 1.3) mU/l (P less than 0.05). There was a significant rise in mean 'baseline' GH concentration: by day 5.5(+/- 1.7) vs 1.0(+/- 0.0) mU/l (P less than 0.05); and night 8.2(+/- 2.7) vs 1.3(+/- 0.3) mU/l (P less than 0.05). Pyridostigmine failed to produce a significant overall increase in either spontaneous or GHRH-stimulated GH secretion by day or night, although there was a significant rise in mean GH levels during the 3 h following pyridostigmine administration in the morning: 4.4(+/- 1.1) vs 2.4(+/- 0.5) mU/l (P less than 0.001). GHRH or pyridostigmine given singly or in combination had no significant effect on the number of pulses. Side-effects attributable to pyridostigmine occurred in seven children. CONCLUSIONS Pyridostigmine, either on its own or as an adjuvant therapy in combination with GHRH, acts for only a brief time and does not offer any potential benefit in the management of children with short stature.     

Galanin abolishes the inhibitory effect of cholinergic blockade on growth hormone-releasing hormone-induced secretion of growth hormone in man

In five healthy normal male volunteers, pretreatment with the cholinergic muscarinic antagonist pirenzepine (30 mg i.v.) almost abolished the growth hormone (GH) response to a maximal dose (120 micrograms i.v.) of growth hormone-releasing hormone (GHRH) (GH response at 40 min 5.6 + 1.3 mU/l with GHRH and pirenzepine vs 40.8 +/- 5.3 mU/l with GHRH alone, P less than 0.02). Concomitant i.v. infusion of galanin (40 pmol/kg/min) with pirenzepine not only restored but significantly potentiated the GH response to GHRH (GH at 40 min 72.2 +/- 10.5 mU/l, P less than 0.001 vs GHRH and pirenzepine, P less than 0.02 vs GHRH alone). Previous studies have proposed that cholinergic pathways control GH release via somatostatin and this study suggests that galanin may act by modulating hypothalamic somatostatinergic tone either directly or, possibly, by facilitating cholinergic neurotransmission.     

Hypothalamic regulation of impaired growth hormone secretion in diabetic rats. 1. Studies in streptozotocin-induced diabetic rats.

Growth hormone (GH) secretion is blunted in diabetic rats. In the present experiment we observed that pituitary GH concentrations and the plasma GH response to an exogenous dose of growth hormone-releasing hormone (GHRH) is decreased in streptozotocin-induced diabetic rats (p less than 0.02) with respect to normal rats. In an attempt to determine if increased somatostatin (SRIF) secretion is responsible for the decreased GH secretion, we studied the effect of modulating SRIF tone on the GH response to GHRH in normal and streptozotocin-induced diabetic rats. Rats were pretreated with either normal sheep serum and saline (NSS+SAL), somatostatin antibodies (SRIF-Ab), or pyridostigmine (PD), an acetylcholinesterase inhibitor hypothesized to reduce hypothalamic SRIF secretion. Pretreatment of normal rats with SRIF-Ab or PD resulted in an increased GH response to exogenous GHRH in comparison to NSS+SAL-pretreated normal rats at 5 min postinjection. In contrast, pretreatment of diabetic rats with SRIF-Ab or PD did not alter the GH response to exogenous GHRH when compared to NSS+SAL-pretreated diabetic animals. These results suggest that the blunted GH response to exogenous GHRH observed in streptozotocin-induced diabetic rats may not be due to an increase of endogenous SRIF tone.     

Effect of cholinergic enhancement by pyridostigmine on growth hormone secretion in obese adults and children

In obesity the reduced growth hormone (GH) responses to several provocative stimuli including growth hormone-releasing hormone (GHRH) indicate a diminished somatotroph responsiveness but do not distinguish between primary pituitary and hypothalamic pathogenesis. However, it has been shown that the cholinergic system positively influences Gh secretion likely by modulating somatostatin release in a negative way. Thus, the effect of cholinergic activity enhancement by pyridostigmine (PD), an acetylcholinesterase inhibitor, on both basal and GHRH-induced GH secretion was studied in 14 obese subjects (eight adults and six children). Eighteen nonobese subjects (seven adults and 11 children) were studied as controls. In obese subjects the GHRH-induced GH increase was lower than in controls (peak, mean +/- SEM, adults, 9.2 +/- 2.7 v 16.8 +/- 5.7 ng/mL; children, 8.0 +/- 0.8 v 20.3 +/- 4.6 ng/mL) attaining statistical significance only in children group (P less than .02). The PD-induced GH response in the two obese groups was similar to that observed in relative controls (adults, 5.3 +/- 1.0 v 7.4 +/- 1.7 ng/mL; children, 9.6 +/- 1.6 v 13.3 +/- 1.4 ng/mL). PD clearly potentiated the GH response to GHRH in obese subjects, both adults (P less than .05 v GHRH alone) and children (P less than .0005 v GHRH alone). However, the GH responses to PD + GHRH was significantly reduced in obese subjects compared with controls (adults, 18.1 +/- 2.2 v 42.7 +/- 10.7 ng/mL, P less than .05; children, 28.3 +/- 4.5 v 58.2 +/- 7.7 ng/mL, P less than .01). In conclusion, PD is able to potentiate the blunted GH responses to GHRH in obese adults and children, inducing a GH increase similar to that observed after GHRH alone in normal subjects. This finding suggests that an alteration of somatostatinergic tone could be involved in the reduced GH secretion in obesity.