Induced GH Secretion Research Articles

Neuropeptides of Anterior Pituitary Origin: Autocrine or Paracrine Functions?

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary cell functions. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover, neuropeptides could interact with each other. Indeed DA stimulated TRH release while PRL secretion decrease at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that observed in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of acute ethanol exposure on clonidine-induced growth hormone release in male rats

The influence of acute ethanol (ETOH) on the regulation of GH release has not been clearly elucidated. In this study the effect of ETOH on clonidine (CLON)-induced GH secretion was examined. To test the effect of ETOH on CLON-induced GH release doses of 1, 2, and 3 g/kg ETOH in a 25% ETOH/saline solution or 10 ml/kg saline were injected IP 24 and 1 h before CLON (30 micrograms/kg BW IV). Blood samples were withdrawn through a chronic jugular cannula. ETOH was found to alter the GH surge, which occurred 15 min after CLON administration in controls, in a dose-dependent manner. The 1 g/kg dose reduced the GH surge slightly but not significantly. The 2 g/kg dose suppressed the GH surge which was significantly lower than in controls. The 3 g/kg dose eliminated the GH surge completely. To determine if pituitary GH release is directly influenced by ETOH, animals were injected with GRH (250 ng/kg IV) one hour after the second dose of ETOH (3 g/kg IP). There was no difference in the GH surge in control or ETOH-injected animals. Anti-SRIF administered 30 minutes before ETOH or saline did not alter the response to GRH. These results suggest that ETOH reduces the GH responsiveness to alpha 2-GRH stimulation.

Failure of pyridostigmine to increase both basal and GHRH-induced GH secretion in the night

The aim of this study was to verify that the stimulatory effect of cholinergic agonists on both basal and stimulated GH release observed in the morning persists in the night. The effects of pyridostigmine (120 mg orally), a cholinesterase inhibitor, on both basal and GHRH (1 micrograms/kg iv)-induced GH secretion were studied in 8 healthy volunteers, aged 22-30 years. In the morning, administration of pyridostigmine induced a significant increase in basal GH levels compared with saline (area under the response curve, mean +/- SEM: 277.0 +/- 54.0 vs 49.7 +/- 8.2 micrograms.l-1.h-1, p less than 0.02) as well as a strong potentiation of the GHRH-induced GH release (2117.6 +/- 353.0 vs 427.9 +/- 87.0 micrograms.l-h-1, p less than 0.02). In the night, GH secretion after pyridostigmine did not differ from saline (194.5 +/- 21.9 vs 89.4 +/- 28.7 micrograms.l-1.h-1). Moreover pyridostigmine failed to potentiate the GHRH-induced GH increase (1071.9 +/- 170.4 vs 740.2 +/- 150.9 micrograms.l-1.h-1). The pyridostigmine + GHRH-induced GH rise during the night was lower (p less than 0.05) than in the morning. All together, these data seem to indicate that cholinergic neurons controlling GH secretion are already maximally stimulated at night. As cholinergic activity negatively modulates SRIH secretion, our findings suggest that a reduced somatostatinergic tone in the hypothalamus is present during the night.

Somatostatin inhibition of thyrotropin-releasing hormone- and growth hormone-releasing factor-induced growth hormone secretion in young and adult anesthetized chickens

The present communication examines the influence of somatostatin (SRIF 14) on basal and thyrotropin-releasing hormone (TRH)- or growth hormone-releasing factor (GRF)-induced GH secretion in young (6 week old) and adult male chickens. Studies were performed in sodium pentobarbitone-anesthetized chickens where basal plasma concentrations of GH are low and both TRH and GRF consistently stimulate GH release. In both young and adult chickens, basal GH secretion was reduced by SRIF 14 infusion (3 μg/kg/min). Similarly, in adult and young birds, the GH secretory response to a challenge with a GRF bolus (10 μg/kg) was inhibited by the concomitant intravenous infusion of SRIF 14 (0.3 or 3.0 μg/kg/min). In adult chickens, the GH response to TRH (10 μg/kg) was suppressed (by 93%) by SRIF 14 infusion (3 μg/kg/min) and tended to be inhibited (by 29%) by a lower dose of SRIF 14 (0.3 μg/kg/min). In contrast, in young chicks, GH release following TRH challenge (either 1 or 10 μg/kg) was only partially inhibited by SRIF 14 infusion (3 μg/kg/min). The response to a TRH challenge (10 μg/kg) was unaffected by the low dosage of SRIF 14 infusion (0.3 μg/kg/min). It is concluded that SRIF 14 inhibits both GRF- and TRH-stimulated GH release in young and adult chickens.

ACUTE DEXAMETHASONE SUPPRESSION OF ACTH SECRETION STIMULATED BY HUMAN CORTICOTROPHIN RELEASING HORMONE, AVP AND HYPOGLYCAEMIA

In order to obtain more insight into the mechanisms regulating endogenous ACTH secretion in humans we studied the inhibitory effect of acute i.v. dexamethasone administration on ACTH release under various conditions. Six male volunteers were subjected to six different protocols. After combined i.v. injection of 100 micrograms corticotrophin releasing hormone (CRH) and 100 micrograms growth hormone releasing hormone (GRH) there was the expected rise in ACTH (area under the curve, 1053 +/- 204 (SE) (pmol/l) min) and cortisol (59788 +/- 10098 (nmol/l) min) rise which was suppressed by prior i.v. injection of 2 mg dexamethasone (ACTH: 444 +/- 63 (pmol/l) min; cortisol: 28528 +/- 2152 (nmol/l) min). Insulin hypoglycaemia (IH) led to a more pronounced ACTH and cortisol rise compared with CRH (6307 +/- 817 (pmol/l) min and 82080 +/- 21934 (nmol/l) min, respectively) which was not completely suppressed by prior pretreatment with dexamethasone (ACTH, 580 +/- 103 (pmol/l) min; cortisol: 55649 +/- 5821 (nmol/l) min). Combined AVP/CRH injection (10 IU/100 micrograms) after pretreatment with dexamethasone (344 +/- 41 (pmol/l) min for ACTH; 32832 +/- 3173 (nmol/l) min for cortisol) could not reproduce the ACTH secretion following IH after pretreatment with dexamethasone (579 +/- 103 (pmol/l) min for ACTH and 55649 +/- 5821 (nmol/l) min for cortisol). In all subjects a saline control with 2 mg dexamethasone was performed. These findings confirm the acute inhibitory effect of glucocorticoids on CRH-stimulated ACTH secretion. Since CRH-induced ACTH secretion is almost completely abolished by administration of dexamethasone the ACTH rise following IH after dexamethasone can not be mediated by endogenous CRH alone. Moreover, since the addition of AVP to CRH (after dexamethasone suppression) could not reproduce the ACTH rise during IH after dexamethasone pretreatment, an additional, yet unknown factor stimulating ACTH secretion may be involved. In the same protocols, no significant difference could be observed comparing IH and GRH induced GH secretion (4948 +/- 1172 (mU/l) min vs 3596 +/- 820 (mU/l) min, NS); furthermore, in contrast to results obtained by chronic steroid administration, acute i.v. dexamethasone pretreatment did not affect IH or GRH-induced GH secretion (4110 +/- 666 (mU/l) min vs 2916 +/- 462 (mU/l) min, NS). The GRH-stimulated GH secretion (3596 +/- 820 (mU/l) min) was not suppressed by prior intravenous treatment with dexamethasone (2916 +/- 504 (mU/l) min, NS).

HYPOTHALAMIC REGULATION OF GROWTH HORMONE (GH) SECRETION

A physiological role of somatostatin (SRIF) on GH-releasing factor (GRF) -induced GH secretion was examined by utilizing in vitro perifusion systems. The following three experiments were carried out.1. GRF-induced GH release was examined in whole anterior pituitaries from rats subjected to the following treatments : 1) anterolateral hypothalamic (ALH) deafferentation, 2) lesions of the hypothalamic periventricular nucleus (Pe) and 3) passive immunization with anti-SRIF serum. The response of these pituitaries to human (h) GRF (0.1 μM) stimulation was markedly attenuated within a day, accompanied by a significant decrease in SRIF content in the stalk-median eminence from rats with ALH deafferentation and Pe lesions. Also SRIF in the SME significantly decreased after the ALH cut or Pe lesions, whereas hGRF remained constant even 10 days after the operations. These data suggest that SRIF in vivo plays a significant role in the anterior pituitary in maintaining hGRF-GH response in vitro.2. The effects of SRIF infusion on successive GH release in response to GRF were examined in an in vitro system as in Exp. 1. Two hours after the first GRF (0.1 μM) stimulation, the response to the second stimulation was markedly attenuated. But this was not observed when the pituitaries were subjected to the second stimulation after a 3 hour interval. In contrast, when SRIF (0.1 μM) was infused for 50 min 1 h after the first stimulation to lower basal GH release, the second response to hGRF was restored to the level of the first response. To determine whether SRIF exerts a direct action on the GH response, a prestimulatory perifusion with SRIF (0.1 μM) for 50 min was tested. The treatment tended to facilitate the pituitary response to hGRF. When pretreated with SRIF for 50 min at a lower concentration (0.05 μM), the response to the first hGRF (0.05 μM) stimulation was significantly facilitated. These data suggest that 1) SRIF perifusion rapidly restores the attenuated response to a second hGRF challenge by lowering GH release to basal levels and 2) SRIF pretreatment facilitates the GH response to the first hGRF stimulation.3. The facilitation by SRIF of hGRF-induced GH response of dispersed pituitary cells was further characterized and analyzed. SRIF-pretreatment (1.0 nM) also caused a marked facilitation of the hGRF (1.0 nM) -GH response which continued for 30 min after SRIF withdrawal. Although a significant “rebound increase” in GH release was observed within 15 min after SRIF withdrawal, the amount was much less than in the hGRF-GH response. Both DbcAMP (1.0 mM) -and KCl (15 mM) -induced GH release response were significantly facilitated by SRIF pretreatment. However, cAMP production by hGRF was not increased but significantly decreased by SRIF pretreatment. Thus SRIF facilitation of hGRF-GH release is thought to take place in the process after cAMP formation.From the results of these experiments, it is concluded that 1) SRIF is required for the maintenance of the pituitary response to GRF, 2) SRIF pretreatment facilitates GRF-induced GH release.

Insulin suppresses triiodothyronine-induced growth hormone secretion by GH3 rat pituitary cells.

Physiological doses of insulin were shown by us to suppress basal and hydrocortisone (HCT)-induced GH secretion and GH mRNA levels in GH3 rat pituitary cells. Because T3 stimulates GH gene expression, the effects of semisynthetic human insulin were tested on T3-induced GH secretion. Cells were first incubated for 48 h in medium containing insulin and fetal calf serum (10%) depleted of T3 and T4 by ion exchange resin. Insulin suppression of basal GH secretion was independent of T3, as insulin (0.7 nM) suppressed basal secretion of GH by 40% in T3-depleted cells. Medium glucose concentrations and cell proliferation did not differ in control or insulin-treated wells. The 4-fold increase in GH secretion induced by added T3 (0.5 nM) during 72 h was suppressed up to 40% by insulin (P less than 0.001). This suppression was maximal with 3.5 nM insulin and occurred after a lag period of 48 h. The previously described 20-fold synergistic stimulation of GH by T3 (0.5 nM) together with HCT (1 microM) was also suppressed by insulin (3.5 nM) by 80% during 72 h of incubation. The selectivity of the inhibitory effects of insulin on GH were also shown when the T3-induced suppression of PRL secretion was reversed by insulin treatment. The data show that physiological doses of insulin antagonize T3-induced stimulation of GH secretion and also partially block the synergistic stimulation of GH by T3 and HCT. As T3 and HCT probably stimulate GH gene transcription at different sites, insulin may suppress GH gene expression at a more distal site. Alternatively, the inhibitory effects of insulin on T3-induced GH secretion in these cells may be posttranscriptional.

177 DECREASE IN GROWTH HORMONE (GH) AND SOMATOMEDIN C (SOMC) DURING LHRH AGONIST (LHRHa) TREATMENT OF CENTRAL PRECOCIOUS PUBERTY (CPP)

LHRHa suppresses both gonadotropin pulsations and sex steroids and slows height velocity (HV) in patients with CPP. To evaluate GH's role in this slowing of growth, sleep induced GH secretion (20 min. samples, 10 PM-2 AM) and SOMC (10 PM) were measured in 9 pre-adrenarchal girls with CPP (ages at onset 1.6-6.3 yrs, M=3.8; bone ages advanced a mean of 3.7 years). Therapy suppressed gonadotropin pulsations, response to LHRH, estradiol and maturation indices in all patients. Growth velocity fell from 14.4 ± 1.7 cm/yr (Mean ± SEM, all measurements) pretherapy to 8.7 ± 1.4 after 6 mos of therapy (p=0.0005). SOMC levels decreased from 3.1 ± 0.7 IU/ml pretherapy to 1.3 ± 0.2 at 3 mos (p=0.009) and 1.4 ± 0.2 at 6 mos (p=0.007). Nocturnal peak levels fell from 25.4 ± 6.8 ng/ml pretreatment to 9.1 ± 1.7 at 3 mos (p < 0.01) and 7.5 ± 1.1 at 6 mos of therapy (p < 0.01). Total GH secreted during the 4 hr interval decreased a mean of 58% at 3 mos and 48% at 6 mos compared to pretreatment. The accelerated HV in CPP patients under 6 years is associated with increased GH and SOMC production. With LHRHa induced suppression of gonadal steroids, both HV and GH and SOMC production are decreased.

489 DECREASE IN GROWTH HORMONE (GH) AND SOMATOMEDIN (SOMC) DURING LHRH AGONIST (LHRHa) TREATMENT OF CENTRAL PRECOCIOUS PUBERTY (CPP)

LHRHa suppresses both gonadotropin pulsations and sex steroids and slows height velocity (HV) in patients with CPP. To evaluate GH's role in this slowing of growth, sleep induced GH secretion (20 min samples, 10 PM – 2 AM) and SOMC (10 PM) were measured in 9 pre-adrenarchal girls with CPP (ages at onset 1.6–6.3 yrs, M=3.8; bone ages advanced a mean of 3.7 yrs). Therapy suppressed gonadotropin pulsations, response to LHRH, estradiol and maturation indices in all patients. Growth velocity fell from 14.4 ± 1.7 cm/yr (Mean ± SEM, all measurements) pretherapy to 8.7 ± 1.4 after 6 mos of therapy (p=0.0005). SOMC levels decreased from 3.1 ± 0.7 IU/ml pretherapy to 1.3 ± 0.2 at 3 mos (p=0.009) and 1.4 ± 0.2 at 6 mos (p=0.007). Nocturnal peak GH levels fell from 25.4 ± 6.8 ng/ml pretreatment to 9.1 ± 1.7 at 3 mos (p < 0.01) and 7.5 ± 1.1 at 6 mos of therapy (p <0.01). Total GH secreted during the 4 hr interval decreased a mean of 58% at 3 mos and 48% at 6 mos compared to pretreatment. The accelerated HV in CPP patients under 6 yrs is associated with increased GH and SOMC production. With LHRHa induced suppresion of gonadal steroids, both HV and GH and SOMC production are decreased.

Stimulation of growth hormone release by vasoactive intestinal polypeptide from human pituitary adenomas in vitro.

The effects of vasoactive intestinal polypeptide (VIP), TRH, dopamine, and rat median eminence extract on GH release from GH-secreting pituitary adenomas were studied in vitro using a sensitive superfusion method. Dispersed pituitary tumor cells obtained from three patients with acromegaly were placed in a superfusion column, and the amounts of GH in the superfusate were determined. The addition of VIP (10(-6) M) to the perfusion system resulted in a marked increase in GH release in all three cases, and a dose-response relationship in VIP (10(-8) 10(-6) M) induced GH secretion was observed in one case studied. TRH (10(-7) M) and median eminence extract (1 equivalent/ml) also caused an abrupt and marked increase in GH release in all of the experiments. The infusion of either dopamine (10(-7) M) or bromocriptine (10(-7) M) inhibited GH secretion. These results suggest that VIP as well as TRH stimulate GH secretion by a direct action on GH-secreting pituitary tumor cells in at least some acromegalic patients.

Differential neuroendocrine responses to thyrotropin-releasing hormone during rapid eye movement and slow wave sleep in man.

The evidence that both TRH and somatostatin (SRIF) control TSH secretion prompted this study of the hypothesis that slow wave sleep (SWS)-induced GH secretion may be related to a decrease in hypothalamic SRIF release. Ten adult men spent 1 accommodation night, followed by 2 experimental nights, in the sleep laboratory, during which electroencephalogram monitoring and blood sampling by iv catheter were performed. Subjects received TRH (50µg, iv) in 0.5 ml saline or saline alone during SWS and rapid eye movement sleep (REM) on 1 night and a reversed injection protocol on the other night. Each subject received only one TRH injection per night, during either SWS or REM. Blood was sampled every 30 min during sleep (lights off from 2300–0700 h), except immediately after each injection, when samples were taken at 0, 5, 10, 15, 30, 45, 60, 90, and 120 min. Serum TSH, GH, PRL, and cortisol levels were determined by RIA. Characteristic sleep patterns of GH and cortisol secretion were observed in these subjects, wit...

407 ORAL CLONIDINE AS A TEST FOR GROWTH HORMONE RELEASE IN SHORT STATURE (SS) AND TOURETTE'S SYNDROME (TS)

The administration of C, an α2 adrenergic agonist, induces GH secretion, sedation and hypotension. To test the hypothesis that these effects are independent of one another, we measured GH responses to oral C (4 μg/kg) in 14 SS patients whose post-exercise GH was <7 ng/ml and in 5 TS patients. Plasma C levels, blood pressure and degree of sedation were recorded as were plasma levels of 3-methoxy-4 hydroxyphenyl glycol (MHPG). To determine the persistence of GH responsiveness the 5 TS were restudied after 3 mos Rx with C. GH peaks (mean 17.4±3 ng/ml) occurred at 60-90 min. in all TS and 9/14 SS patients. There was no tendency for responsiveness to diminish after Rx with C with mean post Rx GH peaks of 22±5ng/ml in the 5 TS. In 5/14 SS, all GH deficient on insulin testing, peak GH was <3 ng/ml. C levels rose to a peak of 1.2±3ng/ml at 60′, preceding the GH peak by 30′. An observed 12% fall in MHPG levels (attributable to a C-mediated reduction in central nervous system (CNS) noradrenergic activity) was not related to the timing or extent of the GH peaks. These peaks also bore little temporal relationship to the slight hypotension or marked sedation occurring after oral C. We conclude that C is a highly effective tool for the diagnosis of GH deficiency and that the GH response to C is sustained during chronic Rx with C. Our data support animal studies which suggest that C exerts its various effects at multiple sites in the CNS.

Effects of GABA-ergic drugs on nigrostriatal and hypothalamic-pituitary dopaminergic systems

Baclofen and sodium valproate are ineffective in the treatment of tardive dyskinesia when given alone but in combination with small doses of haloperidol are effective in controlling the dyskinetic movements, at least over short periods of time. Neither baclofen nor sodium valproate affected basal PRL secretion, haloperidol-induced PRL secretion, or apomorphine induced GH secretion. Baclofen antagonized the L-dopa-induced GH response in each of 3 subjects investigated.

Studies on growth hormone secretion IX. Prostaglandins do not act like ionophores

To gain further insight on the mechanism of GH secretion in general and on the stimulation of this process by prostaglandins in particular, we compared the effects of PGE 1 and PGE 2 on hormone release and cyclic nucleotide levels with those of the ionophores A23187 and X537A under a variety of experimental conditions. All these substances (in the presence but not in the absence of calcium) enhanced GH release in incubated rat anterior pituitaries in vitro , prostaglandins being considerably more potent than ionophores. However, while PGE 2 caused a dose-dependent rise in pituitary cyclic AMP levels (from doubling at 10 −7 M to a two-hundred fold increase at 10 −5 M), the ionophores had no effect on the concentrations of this nucleotide. Neither PGE 2 nor the ionophores had any measurable effect on cyclic GMP levels. Exposure of tissues to ionophores for 60 minutes rendered them refractory to subsequent stimulation by PGE 1 or to ionophores themselves, whereas preincubation with PGE 1 did not diminish GH responses during a second incubation period. On the other hand, 60-minute preincubation of hemipituitaries in the presence of ionophores (10 −5 M) did not suppress subsequent PGE 1-promoted cyclic AMP accumulation. Metabolic blockers inhibited PGE 2 and A23187-promoted GH-release but failed to suppress GH-response to X537A. Verapamil partially inhibited PGE 2 but not ionophore induced GH secretion. Ionophores particularly X537A, accelerated 45Ca efflux while PGE 1 did not influence this. Electronmicroscopy revealed extensive vacuolization localized chiefly at the Golgi apparatus when tissues were incubated with X537A. PGE 1 and A23187 had no such morphological effect. It is concluded that prostaglandins E and ionophores promote GH secretion by different mechanisms.

STUDIES OF GROWTH HORMONE (GH) SECRETION IN JUVENILE DIABETES (J.Dbt)

From studies of exercise induced GH secretion, HANSEN (Scand.J. Clin.Lab.Invest. 28, 195, 1971) and PASSA et al (Lancet ii, 72, 1974) concluded that GH hypersecretion was frequent in J.Dbt and was a possible etiological factor of diabetic retinopathy. To further investigate the role of GH in J.Dbt, blood glucose (BG) and plasma GH were determined during controled exercise performed 1rst in basal condition, 2nd under glucose infusion in 7 controls and 15 J.Dbt aging 12 to 30 years, 4 of them having a retinopathy. All diabetics received their daily insulin injection. None were ketotic during the study. Before exercise the J.Dbt had GH levels higher than normal. During both tests, when compared with control values, the pattern of GH rise appeared to be different in the J.Dbt. with BG lower or in the same range than the controls, and in the J.Dbt with abnormally high BG: in basal condition significantly higher GH rise (p<0,05) were found only in the J.Dbt with normal or low BG. Glucose infusion blocked the GH rise in the controls and in the hyperglycemic J.Dbt but did not in those with BG curves similar to the controls. The 4 patients with retinopathy figured in the hyperglycemic groups and showed an exercise induced GH rise similar to the controls. This study confirmed that GH hypersecretion is present in J.Dbt; however, it gives no evidence that GH hypersecretion is a factor contributing to diabetic retinopathy.

Blockade of serotonin induced GH secretion in rats by melatonin and cyproheptadine.

Blockade of serotonin induced GH secretion in rats by melatonin and cyproheptadine.