Pyridostigmine enhances even if it does not normalize the growth hormone responses to growth hormone-releasing hormone in patients with Cushing's disease.

Subjects with Cushing's disease have diminished growth hormone (GH) response to growth hormone-releasing hormone (GHRH). The aim of our study was to investigate the underlying mechanism of this diminished GH response in these patients using pyridostigmine (PD), an acetylcholinesterase inhibitor, which is reported to increase GH secretion by reducing somatostatin tone. Eight subjects with untreated Cushing's disease (caused by a pituitary adenoma) and 6 control subjects received GHRH 100 micrograms in 1 ml of saline, as intravenous bolus injection 60 min after (1) placebo (2 tablets, p.o.) or (2) PD (120 mg, p.o.). After GHRH plus placebo, the GH peak (mean +/- SEM) was significantly lower in subjects with Cushing's disease (2.4 +/- 0.5 micrograms/l) compared to control subjects (25.1 +/- 1.8 micrograms/l, p less than 0.05). After GHRH plus PD, the GH peak was significantly enhanced both in subjects with Cushing's disease (7.1 +/- 2.3 micrograms/l, p less than 0.05) and in control subjects (42.3 +/- 4.3 micrograms/l, p less than 0.05). In patients with Cushing's disease, the GH response to GHRH plus PD was lower with respect to the GH response to GHRH alone in normal subjects. We conclude that hypercortisolism may cause a decrease in central cholinergic tone which is in turn hypothesized to be responsible of an enhanced somatostatin release from the hypothalamus. However, other metabolic or central nervous system alterations may act synergistically with hypercortisolism in causing GH inhibition in patients with Cushing's disease.     

Role of food intake in the modulation of hexarelin-induced growth hormone release in normal human subjects.

Hexarelin (HEX) is a new synthetic analog of the Growth Hormone releasing peptides and is stronger than GHRH in releasing GH in vivo. No information is available on the effect of food ingestion on HEX-induced GH secretion. On the other hand, we have previously demonstrated that food intake at lunchtime in normal subjects has an inhibitory effect on the GH response to GHRH. The aim of the present study was to investigate the effect of food ingestion on GH secretion induced by HEX as compared to GHRH in six normal men (aged 23-29 years) and six normal women (aged 24-29 years). The body weights for all subjects were within 120% of their ideal body weight, according to their sex and age. Our data confirm that HEX is much more powerful than GHRH in inducing GH release in humans, both in the fasting state (GH-AUC: 3010 +/- 695 after HEX, vs. 1339 +/- 281 after GHRH, microg/L/120 min; p<0.06) and after a meal (GH-AUC: 1523 +/- 121, after HEX, vs. 309 +/- 61, after GHRH, microg/L/120 min; p<0.06). Moreover, our study shows that food intake partially blunts the fasting GH response to HEX (GH-AUC: 3010 +/- 695 after HEX, in fasting state, vs. 1523 +/- 121 after HEX, after meal, microg/L/120 min; p<0.06; mean inhibition of AUC 41.02 +/- 7.96%), whereas it nearly abolishes the GH response to GHRH in the same subjects (GH-AUC: 1339 +/- 281 after GHRH, in fasting state, vs. 309 +/- 61 after GHRH, after meal, microg/L/120 min; p<0.06; mean inhibition of AUC 70.31 +/- 6.22%). In conclusion, our study confirms that HEX acts differently from GHRH; the GH releasing effect of HEX could be only partially influenced by the physiological metabolic or neuroendocrine food-related modifications.     

Effect of arginine on the GHRH-stimulated GH secretion in patients with hyperthyroidism.

Patients with hyperthyroidism have reduced GH responses to pharmacological stimuli and reduced spontaneous nocturnal GH secretion. The stimulatory effect of arginine on GH secretion has been suggested to depend on a decrease in hypothalamic somatostatin tone. The aim of our study was to evaluate the effects of arginine on the GH-releasing hormone (GHRH)-stimulated GH secretion in patients with hyperthyroidism. Six hyperthyroid patients with recent diagnosis of Graves' disease [mean age +/- SEM, 39.2 +/- 1.4 years; body mass index (BMI) 22 +/- 0.4 kg/m2] and 6 healthy nonobese volunteers (4 males, 2 females; mean age +/- SEM, 35 +/- 3.5 years) underwent two experimental trials at no less than 7-day intervals: GHRH (100 micrograms, i.v.)-induced GH secretion was evaluated after 30 min i.v. infusion of saline (100 ml) or arginine (30 g) in 100 ml of saline. Hyperthyroid patients showed blunted GH peaks after GHRH (13.2 +/- 2.9 micrograms/l) as compared with normal subjects (23.8 +/- 3.9 micrograms/l, p < 0.05). GH peaks after GHRH were only slightly enhanced by arginine in hyperthyroid subjects (17.6 +/- 2.9 micrograms/l), whereas, in normal subjects, the enhancement was clear cut (36.6 +/- 4.4 micrograms/l; p < 0.05). GH values after arginine + GHRH were still lower in hyperthyroid patients with respect to normal subjects. Our data demonstrate that arginine enhances but does not normalize the GH response to GHRH in patients with hyperthyroidism when compared with normal subjects. We hypothesize that hyperthyroxinemia may decrease GH secretion, both increasing somatostatin tone and acting directly at the pituitary level.     

Effects of passive immunization of growth hormone-releasing hormone and somatostatin on growth hormone secretion under conditions of high somatostatin tone.

Pulsatile GH secretion decreases during food-deprivation in the rat. It has been hypothesized that this decrease is due to elevated hypothalamic somatostatin secretion. This is based on the observation that GH increases in food-deprived rats following removal of endogenous somatostatin using passive immunization techniques. Cognizant of the important stimulatory effects of growth hormone-releasing hormone (GHRH) on GH secretion, we sought to determine if this neuropeptide plays any role in mediating GH secretion in food-deprived rats. Male rats were prepared with indwelling venous catheters using sodium pentobarbital anesthesia seven days prior to experimentation. Animals were food-deprived for 72 h, after which control blood samples were drawn from -60 to 0 min. One group was then treated with normal rabbit serum (NRS), while a second group was treated with GHRH antiserum (GHRHab). At 55 min all animals received somatostatin antiserum (SSab). No animal exhibited any spontaneous GH peak during the one hour control period or in the subsequent one hour period following the administration of GHRHab or NRS. Absence of GH pulsatility during food-deprivation, coupled with no decrease in GH levels in food-deprived rats treated with GHRHab suggest that diminished GHRH pulsatility is likely during food-deprivation. Subsequent treatment of these animals with SSab resulted in an identical 2.5 fold increase in GH concentrations. This result suggests that GHRH is not involved in the GH rebound following somatostatin withdrawal in food-deprived rats.     

Nocturnal ghrelin pulsatility and response to growth hormone secretagogues in healthy men

The physiological importance of endogenous ghrelin in the regulation of growth hormone (GH) secretion is still unknown. To investigate the regulation of ghrelin secretion and pulsatility, we performed overnight ghrelin and GH sampling every 20 min for 12 h in eight healthy male subjects [age 37 +/- 5 (SD) years old, body mass index 27.2 +/- 2.9 kg/m2]. Simultaneous GH and ghrelin levels were assessed to determine the relatedness and synchronicity between these two hormones in the fasted state during the overnight period of maximal endogenous GH secretion. Pulsatility analyses were performed to determine simultaneous hormonal dynamics and investigate the relationship between GH and ghrelin by use of cross-approximate entropy (X-ApEn) analyses. Subjects demonstrated 3.0 +/- 2.1 ghrelin pulses/12 h and 3.3 +/- 0.9 GH pulses/12 h. The mean normalized ghrelin entropy (ApEn) was 0.93 +/- 0.09, indicating regularity in ghrelin hormone secretion. The mean normalized X-ApEn was significant between ghrelin and GH (0.89 +/- 0.12), demonstrating regularity in cosecretion. In addition, we investigated the ghrelin response to standard GH secretagogues [GH-releasing hormone (GHRH) alone and combined GHRH-arginine] in separate testing sequences separated by 1 wk. Our data demonstrate that, in contrast to GHRH alone, which had little effect on ghrelin, combined GHRH and arginine significantly stimulated ghrelin with a maximal peak at 120 min, representing a change of 66 +/- 14 pg/ml (P = 0.001 by repeated-measures ANOVA and P = 0.02 for GHRH vs. combined GHRH-arginine by MANOVA). We demonstrate relatedness between ghrelin and GH pulsatility, suggesting either that ghrelin participates in the pulsatile regulation of GH or that the two hormones are simultaneously coregulated, e.g., by somatostatin or other stimuli. Furthermore, the differential effects of GHRH alone vs. GHRH-arginine suggest that inhibition of somatostatin tone may increase ghrelin. These data provide further evidence of the physiological regulation of ghrelin in relationship to GH.     

alpha-Glycerylphosphorylcholine administration increases the GH responses to GHRH of young and elderly subjects.

Growth hormone (GH) secretion is decreased during aging in humans and in rodents. This decrease may be due to increased hypothalamic somatostatin release, which is inhibited by cholinergic agonists, or to decreased secretion of GHRH. Alpha-glyceryl-phosphorylcholine (alpha-GFC) is a putative acetylcholine precursor used in the treatment of cognitive disorders in the elderly. In order to learn what effect alpha-GFC had on GH secretion, GH-release hormone (GHRH) was given to young and old human volunteers, with or without the addition of alpha-GFC. GH secretion was greater in the younger subjects than in the old individuals, and both groups had a greater GH response to the GHRH+alpha-GFC than to GHRH alone. The potentiating effect of alpha-GFC on GH secretion was more pronounced in the elderly subjects. These findings confirm the observation that aged individuals respond less well to GHRH than younger subjects, and provides further evidence that increased cholinergic tone enhances GH release.     

Somatostatin-14 neurons in the ovine hypothalamus: colocalization with estrogen receptor alpha and somatostatin-28(1-12) immunoreactivity,and activation in response to estradiol

Pituitary gland growth hormone (GH) secretion is influenced by two hypothalamic neuropeptides: growth hormone-releasing hormone (GHRH) and somatostatin. Recent data also suggest that estrogen modulates GH release, particularly at the time of the preovulatory luteinizing hormone surge, when a coincident surge of GH is observed in sheep. The GHRH neurons do not possess estrogen receptor alpha (ERalpha), suggesting that estrogen does not act directly on GHRH neurons. Similarly, few somatotropes express ERalpha, suggesting a weak pituitary effect of estradiol on GH. It was hypothesized, therefore, that estradiol may affect somatostatin neurons to modulate GH release from the pituitary. Using immunocytochemical approaches, the present study revealed that although somatostatin neurons were located in several hypothalamic sites, only those in the arcuate nucleus (13% +/- 2%) and ventromedial nucleus (VMN; 29% +/- 1%) expressed ERalpha. In addition, we found that all neurons immunoreactive for somatostatin-14 were also immunoreactive for somatostatin-28(1-12). To determine whether increased GH secretion in response to estradiol is through modulation of GHRH and/or somatostatin neuronal activity, a final study investigated whether c-fos expression increased in somatostatin- and GHRH-immunoreactive cells at the time of the estradiol-induced LH surge in intact anestrous ewes. Estradiol significantly (P < 0.05) increased the percentage of GHRH (estradiol, 75% +/- 3%; no estradiol, 19% +/- 2%) neurons expressing c-fos in the hypothalamus. The percentage of somatostatin-immunoreactive neurons coexpressing c-fos in the estradiol-treated animals was significantly (P < 0.05) higher (periventricular, 44% +/- 3%; arcuate, 72% +/- 5%; VMN, 81% +/- 5%) than in the control animals (periventricular, 22% +/- 1%; arcuate, 29% +/- 3%; VMN, 31% +/- 3%). The present study suggests that estradiol modulates the activity of GHRH and somatostatin neurons but that this effect is most likely mediated through an indirect interneuronal pathway.     

Anatomy of the Hypophysiotropic Somatostatinergic and Growth Hormone-releasing Hormone System Minireview

The central control of growth hormone (GH) secretion from the pituitary gland is ultimately achieved by the interaction between two hypothalamic neurohormones, somatostatin which inhibits and growth hormone-releasing hormone (GHRH) which stimulates GH release. The regulation of the somatostatin and GHRH release from the hypothalamus is regulated by a range of other neuropeptides, neurotransmitters, neurohormones. In this mini review we attempt to provide a short summary covering the anatomy and chemical characteristics of the various cell populations regulating GH secretion as a tribute to Miklós Palkovits who pioneered the field of functional neuroanatomy of hypothalamic networks.Special Issue Dedicated to Miklós Palkovits.     

Growth hormone (GH) profiles with successive provocation by GH-releasing hormone and arginine in children: a clinical appraisal

To establish a single and reliable test for evaluating growth hormone (GH) secretion, we examined successive GH provocation by two agents with different modes of action, GH releasing-hormone (GHRH) and arginine (Arg) in 60 children of short stature, 6 patients with pituitary dwarfism and 9 normal young adults. Their GH profiles were qualitatively classified into 4 types: 25 children and 7 adults responded to both stimuli with 2 GH peaks (48.7 +/- 4.3 [SEM] micrograms/L for GHRH and 32.2 +/- 2.6 micrograms/L for Arg in children; 25.8 +/- 7.6 micrograms/L and 30.1 +/- 9.2 micrograms/L respectively in adults) (type A). A single peak for GHRH (57.7 +/- 4.6 micrograms/L) without an Arg-induced peak was obtained in 29 younger children (type B), which is considered to be a GHRH-dominant pattern. Two of them were diagnosed as hypothalamic GHRH deficiency based on a low nocturnal plasma GH and good response to GH treatment. Six adolescents and 2 adults showed a blunted response to GHRH (9.0 +/- 1.1 micrograms/L) but a normal response to Arg (40.6 +/- 9.5 micrograms/L) (type C), which appears to be caused by somatostatin (SRIH) hypertonicity. None with pituitary dwarfism responded to both stimuli (4.5 +/- 1.3 and 2.3 +/- 0.5 micrograms/L). Thus, the GHRH-Arg test makes it possible to evaluate the counterbalance between GHRH and SRIH as well as to differentiate pituitary GH deficiency from hypothalamic GHRH dysfunction.     

Effect of long-term GHRH and somatostatin administration on GH release and body weight in prepubertal female rats

In order to find a chronic GHRH administration capable of stimulating growth rate without depleting pituitary GH content, prepubertal female rats were subcutaneously (sc) treated with GHRH (1-29)-NH2 and somatostatin (SS). In experiment 1, the rats received sc injections of GHRH and cyclic natural SS for 19 days. In the second study, female rats were continuously treated during 21 days with GHRH, using a slow release pellet, alone or combined with one daily injection of long acting SS (octreotide). In experiment 1, body weight was significantly increased when GHRH was administered at the highest daily dosage (1200 microg/day), accompanied by an slight increment in pituitary GH content. Hypothalamic SS concentrations decreased when GHRH or SS were administered alone whereas the combined treatment with both peptides did not modify this parameter, which suggests the existence of a balance between the chronic actions of both peptides on hypothalamus. In experiment 2, the continuous infusion of GHRH increased plasma GH levels and tended to enhance pituitary GH content. Nevertheless, GHRH effect was not effective enough to increase body weight. By adding one daily injection of SS both GHRH effects on the pituitary gland were abolished. Our study indicates that female rats retain responsiveness to chronic GHRH and SS treatments at both pituitary and hypothalamic levels.     

Growth hormone response to long-term GH-RH administration in lambs

The pattern of long-term GHRH administration capable of stimulating GH release without depleting pituitary GH content has been investigated using two experimental approaches. In experiment 1, recently weaned male lambs were treated for 3 weeks as follows: Group A) control; B) subcutaneous (sc) continuous infusion of GHRH (1200 mg/day) using a slow release pellet; C) the same as B plus 1 daily sc injection of long acting somatostatin (SS) (octreotide, 20 mg) ; D) 3 daily sc GHRH (250 mg) injections ; E) 2 daily sc injections of GHRH (250 mg) and 2 of natural SS (250 mg). In experiment 2, recently weaned male lambs were continuously GHRH-treated using sc osmotic minipumps (900 mg/day) alone or combined with a daily sc injection of octreotide (20 mg) for 4 weeks. Basal plasma GH levels were increased after chronic pulsatile GHRH treatment but not after any kind of continuous GHRH administration. This increment was maintained during the 3 weeks of experimentation and appeared accompanied by a pituitary GH content similar to controls. A marked GH response to the iv GHRH challenge was observed in controls and in lambs receiving both types of continuous sc GHRH infusions, whereas pulsatile sc GHRH-treated animals did not respond to the iv GHRH challenge in the first and second weeks of the study but did so in the third week of treatment. These data demonstrate that long-term pulsatile GHRH administration is capable of stimulating GH release in growing male lambs, without producing pituitary desensitization.     

The effect of cyproheptadine on plasma growth hormone (GH) and on somatostatin response to GH-releasing hormone in man

Cyproheptadine (CPH)--a putative serotonin antagonist--is known to inhibit growth hormone (GH) response to various pharmacological stimuli, as well as during sleep. To elucidate the possible site at which this drug takes effect, we examined plasma GH and somatostatin response to i.v. GHRH1-44 (1 microgram/kg body wt.) before and after CPH treatment in 10 healthy volunteers. The oral administration of CPH (8-12 mg daily for 5 days; total dose 56 mg) significantly curbed GH response to GHRH as expressed in peak plasma GH values (32.0 +/- 6.1 micrograms/l vs. 12.6 +/- 3.2 micrograms/l; P less than 0.01) and in integrated GH response area (2368 +/- 517 micrograms x l-1 x 2 h vs. 744 +/- 172 micrograms x l-1 x 2 h; P less than 0.01). Plasma somatostatin levels did not change in response to GHRH.     

Comparative effect of clonidine and growth hormone (GH)-releasing hormone on GH secretion in adult patients on chronic glucocorticoid therapy.

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 effects of GH-releasing hormone (GHRH) and clonidine, an alpha-2-adrenergic agonist which increases GH secretion acting at the hypothalamic level with an unknown mechanism, on GH secretion in seven adult patients (3M, 4F) with non endocrine diseases and on daily immunosuppressive glucocorticoid therapy. Eleven normal subjects (7M, 4F) served as controls. Steroid-treated patients showed a blunted GH response to GHRH (GH peak 8.3 +/- 3 micrograms/L) with respect to normal subjects (GH peak 19.3 +/- 2.4 micrograms/L). The GH responses to clonidine were also blunted (p less than 0.05) in steroid-treated patients (GH peak 5.8 +/- 2.8 micrograms/L) with respect to normal subjects (GH peak 17.6 +/- 2.3 micrograms/L). No significant differences between the GH responses to GHRH and clonidine were observed either in steroid-treated or in normal subjects. Clonidine is not able to enhance GH secretion similar to GHRH in patients chronically treated with steroids. It can be hypothesized that clonidine does not elicit GH secretion decreasing hypothalamic somatostatin tone.     

Effects of acute intravenous injection of two growth hormone-releasing hormones (GHRH 1-40 and 1-29) on serum growth hormone and other pituitary hormones in short children with pulsatile growth hormone secretion

We administered two different growth hormone-releasing hormones (GHRH) to 20 short, prepubertal children who had spontaneous secretion of growth hormone (GH), assessed from 24-hour GH secretion profiles (72 sampling periods of 20 min). We compared one i.v. injection of 1 microgram/kg of GHRH 1-40 with that of GHRH 1-29 regarding serum concentrations of GH, prolactin, luteinizing hormone, follicle-stimulating hormone and IGF-I. The children were allocated to two groups without statistical randomization. Both groups were given both peptides, with at least 1 week in between. The first group started with GHRH 1-40, the other with GHRH 1-29. The peptides both induced an increased serum concentration of GH of the same magnitude: mean maximal peak of 89 +/- 12 mU/l after GHRH 1-40 and 94 +/- 10 mU/l after GHRH 1-29 (n.s.). The mean difference in maximum serum GH concentration in each child after injection was 52 +/- 9 mU/l, range 1-153 mU/l. GHRH 1-29 also induced a short-term, small increase in the concentrations of prolactin (p less than 0.05), luteinizing hormone (p less than 0.01) and follicle-stimulating hormone (p less than 0.05). We conclude that the shorter sequence GHRH 1-29, when given in a dose of 1 microgram/kg, gives a rise in serum concentration of GH similar to that after the native form GHRH 1-40.     

Influence of chronic treatment with the growth hormone secretagogue Ipamorelin,in young female rats: somatotroph response in vitro

Growth hormone (GH) is secreted in the anterior pituitary gland by the somatotroph cells. Secretion is regulated by growth hormone releasing hormone (GHRH) and somatostatin. Morever, GH secretagogues (GHS) can exert a considerable effect on GH secretion. In order to determine the effects of chronic treatment with the GHS Ipamorelin on the composition of the somatotroph cell population and on somatotroph GH content, an in vitro analysis was performed of the percentage of somatotroph cells (% of total), the ratio of different GH cell types (strongly/weakly-staining) and individual GH content, in pituitary cell cultures obtained from young female rats receiving Ipamorelin over 21 days (Ipamorelin group) and the effects were compared with those of GHRH (GHRH group) or saline (saline group). The ultrastructure of somatotroph cells did not change, but the volume density of secretion granules was increased (P<0.05) by previous in vivo Ipamorelin or GHRH treatment. In 3-day basal pituitary cell monolayer cultures, the percentage of somatotroph cells showed no modifications between groups, nor was there any change in the ratio of strongly/weakly immunostaining GH cells. In the Ipamorelin group alone, in vitro treatment with Ipamorelin (10(-8) M), or GHRP 6 (10(-8) M), or GHRH (10(-8) M) for 4 hours, increased the percentage of somatotroph cells, without modifying the ratio of strongly/weakly immunostained GH cells. Basal intracellular GH content in somatotroph cells over 4 hours was lower in the Ipamorelin group and the GHRH group than in the saline group. Only in the Ipamorelin group did Ipamorelin (10(-8) M), GHRP 6 (10(-8) M) and GHRH (10(-8) M) prompt increased intracellular GH content. These data suggest that, at least in the young female rat, the GHS Ipamorelin is able to exert a dynamic control effect on the somatotroph population and on GH hormone content.     

Putative GH pulse renewal: periventricular somatostatinergic control of an arcuate-nuclear somatostatin and GH-releasing hormone oscillator

Growth hormone (GH) pulsatility requires periventricular-nuclear somatostatin(SRIF(PeV)), arcuate-nuclear (ArC) GH-releasing hormone (GHRH), and systemic GH autofeedback. However, no current formalism interlinks these regulatory loci in a manner that generates self-renewable GH dynamics. The latter must include in the adult rat 1) infrequent volleys of high-amplitude GH peaks in the male, 2) frequent discrete low-amplitude GH pulses in the female, 3) disruption of the male pattern by severing SRIF(PeV) outflow to ArC, 4) stimulation of GHRH and GH secretion by central nervous system delivery of SRIF, 5) inhibition of GH release by central exposure to GHRH, and 6) a reboundlike burst of GHRH secretion induced by stopping peripheral infusion of SRIF. The present study validates by computer-assisted simulations a simplified ensemble formulation that predicts each of the foregoing six outcomes, wherein 1) blood-borne GH stimulates SRIF(PeV) secretion after a long time latency, 2) SRIF(PeV) inhibits both pituitary GH and ArC GHRH release, 3) ArC GHRH and SRIF(ArC) oscillate reciprocally with brief time delay, and 4) SRIF(PeV) represses and disinhibits the putative GHRH-SRIF(ArC) oscillator. According to the present analytic construction, time-delayed feedforward and feedback signaling among SRIF(PeV), ArC GHRH, and SRIF(ArC) could endow the complex physiological patterns of GH secretion in the male and female.     

GH responses to GHRH and GHRP-6 in Streptozotocin (STZ)-diabetic rats

GH responses to GHRH, the physiologic hypothalamic stimulus, and GHRP-6, a synthetic hexapeptide that binds the Ghrelin receptor, were studied in rats treated with streptozotocin (STZ), an experimental model of diabetes. Sprague-Dawley male rats received a single injection either of STZ (70 mg/Kg in 0.01 M SSC, i.p.) or of the vehicle (0.01 M SSC). GH responses were challenged with two different doses of GHRH (1 and 10 microg/kg) or GHRP-6 (3 and 30 microg/kg) and with a combination of both at low (1 + 3 microg/kg) or high (10 + 30 microg/kg) doses, respectively. We observed a dose-dependent effect for GH responses to GHRH both in STZ-treated rats and in controls. However, we could not find significant differences between STZ-rats and controls. GH responses to GHRP-6 occurred in a dose-dependent manner in STZ-rats, but not in controls. GH responses to GHRP-6 in both groups were clearly lower than those elicited by GHRH. GH responses to 30 microg/Kg of GHRP-6 were significantly greater in STZ-rats than in controls (AUC: 3549.9 +/- 1001.4 vs. 2046.4 +/- 711.7; p<0.05). The combined administration of GHRH plus GHRP-6 was the most potent stimuli for GH in both groups. The administration of doses in the lower range (1 + 3 microg/Kg, GHRH + GHRP-6 respectively) induced a great peak of GH in STZ-rats and in control rats, revealing a synergistic effect of GHRH and GHRP-6 in both groups. When the higher doses were administered (10 + 30 microg/kg), GH levels in time 5, and AUC were significantly higher in control rats. In addition, a negative correlation between WT (weight tendency) values and GH responses, represented as AUC, could be established in STZ-rats (r2=-0.566, p=0.004 for GHRH; r2=-0.412, p=0.028 for GHRP-6). Thus, the more negative the values of WT were, the more severe the metabolic alteration and, therefore, the higher the GH response to GHRH and GHRHP-6. In conclusion, our results do not support the existence of a functional hypothalamic hypertone of SS in diabetic rats, as GH responses were not usually reduced in STZ-rats, except when both secretagogues were administered together at the higher doses. Besides, GH responses to GHRH and GHRP-6 were inversely correlated with the severity of the metabolic alteration in STZ-rats, meaning that worse glycaemic control promoted higher GH secretion. These results resemble those found in humans, where GH responses to secretagogues are increased in type-1 diabetes and depend on hyperglycaemia, and are representative of not well-controlled insulin-dependent diabetic status.     

Change in the growth hormone (GH) response to GH-releasing factor (GRF) caused by exogenous GH in short children

To determine whether exogenous GH induces feedback of GH release in children, growth hormone-releasing factor (GRP) tests were performed before and after 10-day GH administration. Sixteen non-obese short boys, aged 5-14 yr, with normal GH response to pharmacological tests were studied. Mean basal and peak serum GH levels in GRF tests before and after exogenous GH were not significantly different. The subjects were divided into two groups, A and B, according to the percent change in integrated areas under the GH curves in GRF tests (GH AUC) before and after 10-day GH administration. Group A consisted of 6 boys with decreased GH AUC and group B consisted of 10 boys with increased GH AUC. Mean peak GH in GRF tests and mean GH AUC were significantly higher before exogenous GH in group A than in group B. The boys in group A were all prepubertal, while 4 boys in group B had begun their early pubertal change. The mean age in group A (7.8 +/- 1.8 yr) was significantly lower than that of group B (11.9 +/- 2.4 yr). GH AUC before exogenous GH showed a significant correlation with the percent change in AUC (= -0.742, p less than 0.01). These data demonstrated that the exogenous GH suppressed the GH response to GRF in prepubertal children with good response to GRF before exogenous GH, while it exaggerated the GH response to GRF in older children with relatively poor response before GH.     

Ectopic growth hormone-releasing hormone (GHRH) syndrome in a case with multiple endocrine neoplasia type I

A 36-yr-old man with multiple endocrine neoplasia (MEN) type I had an ectopic growth hormone-releasing hormone (GHRH) syndrome due to a GHRH-secreting pancreatic tumor. The immunoreactive (IR)-GHRH concentration in his plasma ranged from 161 to 400 pg/ml (299 +/- 61 pg/ml, mean +/- SD; normal, 10.4 +/- 4.1 pg/ml), and a significant correlation was found between his plasma IR-GHRH and GH (r = 0.622, p less than 0.02). After removal of the pancreatic tumor, the high plasma GH concentration returned to nearly the normal range (42.2 +/- 31.3 to 9.6 +/- 3.8 ng/ml). These changes paralleled the normalization of his plasma IR-GHRH (16.1 +/- 3.8 pg/ml) and some of his symptoms related to acromegaly improved. However, plasma GH (7.7 +/- 1.3 ng/ml) and IGF-I (591 +/- 22 ng/ml) concentrations were high at 12 months after surgery, suggesting adenomatous changes in the pituitary somatotrophs. Before surgery, exogenous GHRH induced a marked increase in plasma GH, and somatostatin and its agonist (SMS201-995) completely suppressed GH secretion, but not IR-GHRH release. No pulsatile secretion of either IR-GHRH or GH was observed during sleep. An apparent increase in the plasma GH concentration was observed in response to administration of TRH, glucose, arginine or insulin, while plasma IR-GHRH did not show any fluctuation. However, these responses of plasma GH were reduced or no longer observed one month and one year after surgery. These results indicate that 1) a moderate increase in circulating GHRH due to ectopic secretion from a pancreatic tumor stimulated GH secretion resulting in acromegaly, and evoked GH responses to various provocative tests indistinguishable from those in patients with classical acromegaly, and 2) the ectopic secretion of GHRH may play an etiological role in the pituitary lesion of this patient with MEN type I.     

Comparative effect of galanin and pyridostigmine on the growth hormone response to growth hormone-releasing hormone in normal aged subjects.

The aim of our study was to investigate the effects of aging on the growth hormone (GH) response to growth hormone-releasing hormone (GHRH) alone and in combination with either the neuropeptide galanin or the acetylcholinesterase inhibitor pyridostigmine (PD) in normal subjects. In protocol 1 (GHRH/galanin), 9 old healthy volunteers, ranging in age from 68 to 97 years, and 6 young subjects, ranging in age from 25 to 31 years, received: (a) human GHRH (1-29)NH2, 100 micrograms in 1 ml saline, as an intravenous bolus, and (b) porcine galanin, 500 micrograms in 100 ml saline, as an intravenous infusion from -10 to 30 min combined with GHRH, 100 micrograms i.v. at time 0. In protocol 2 (GHRH/PD), 14 old healthy volunteers, ranging in age from 65 to 91 years, and 11 young subjects, ranging in age from 19 to 34 years, received: (a) GHRH (1-29)NH2, 100 micrograms in 1 ml saline, as an intravenous bolus, and (b) PD, 120 mg administered per os 60 min before GHRH, 100 micrograms as an intravenous bolus. Blood samples for GH were drawn at -75, -60 (time of PD administration), -45, -30, -15, -10 (time of beginning of galanin infusion), 0 (time of GHRH injection), 15, 30, 45, 60, 90, and 120 min. The GH response to GHRH was significantly (< 0.05) enhanced either by galanin or PD pretreatment both in young and old subjects. However, the GH response to GHRH alone or combined with either galanin or PD was significantly greater in the young subjects as compared to the old subjects.(ABSTRACT TRUNCATED AT 250 WORDS)