Evidence that the regulation of growth hormone secretion is mediated predominantly by a growth hormone releasing factor

Spontaneous pulsatile growth hormone (GH) secretion and stress-induced suppression of GH was examined in chronically cannulated male rats with electrolytic lesions of the periventricular preoptic anterior hypothalamic area (PO/AHA) where somatostatin neurons innervating the median eminence are known to be located. Median eminence somatostatin was depleted in lesioned animals by 85%. Bursts of GH secretion occurred more frequently than in sham-lesioned animals (1.9 +/- 0.2 vs. 2.6 +/- 0.2 h, respectively, p less than 0.025), however, average concentrations of GH were reduced by lesions (118.4 +/- 11.6 vs 192.3 +/- 28.4 ng/ml, p less than 0.025). Suppression of GH by stress was unaffected by PO/AHA lesions. It is concluded that somatostatin plays only minor roles in the generation of GH troughs during rhythmic GH secretion, and in the suppression of GH during stress. Inhibition of GH releasing factor secretion, therefore, is presumed to be the likely method by which GH suppression is achieved in these physiological situations.     

Involvement of nitric oxide in the regulation of growth hormone secretion in dogs

Nitric oxide (NO) is a highly reactive gas that has been suggested to function as a neurotransmitter in the neuroendocrine system. In this work, we have evaluated the role of NO pathways in growth hormone (GH) secretion by assessing the effect of L-arginine infusion, a precursor of NO formation, and L-NAME, a nitric oxide synthase (NOS) inhibitor. The experiments were carried out on 7 adult beagle dogs. A saline infusion was carried out on all the dogs as a control test. L-arginine (infusion i.v. 10 g in 100 ml of saline, from t = 0 to 30 min) and L-nitro-arginine-methyl ester, L-NAME (infusion of 300 microg/kg in 120 ml of saline, from t = -30 to 45 min) were administered alone and together with growth hormone-releasing hormone (GHRH) (i.v. bolus at 0 min, at a dose of 100 microg), the synthetic GH secretagogue GHRP-6 (i.v. bolus at 0 min, at a dose of 90 microg), and the 5-HT1D serotonin receptor agonist sumatriptan, SUM (s.c. injection at the dose of 3 mg). Plasma cGH was determined by RIA. Results were evaluated by one-way analysis of variance, followed by the Newman-Keuls test for multiple comparisons. L-arginine administration resulted in a slight increase in plasma cGH in comparison with saline controls. Combined administration of L-arginine and GHRH enhanced cGH release in comparison with GHRH alone. L-NAME alone did not modify baseline cGH levels, but completely suppressed the GH release induced by GHRH or GHRP-6. It also strongly reduced, but did not abolish the effect of the two peptides (GHRH plus GHRP-6) administered together. Finally, administration of the 5-HT1D agonist SUM induced a significant cGH secretion in all dogs, a response which was not modified when L-NAME was administered in combination with SUM. In conclusion, our data show that inhibition of NO blunts both GHRH or GHRP-6-induced cGH release, and are compatible with the hypothesis that it acts by decreasing hypothalamic somatostatin release.     

Ghrelin-induced growth hormone release from goldfish pituitary cells is nitric oxide dependent

Ghrelin (GRLN) is an important neuroendocrine regulator of growth hormone (GH) release in vertebrates. Previous studies show goldfish (g)GRLN₁₉-induced GH from the goldfish pituitary involves voltage sensitive Ca²⁺ channels, increases in intracellular Ca²⁺ and the PKC signalling pathway. We set out to examine the role of the nitric oxide (NO) pathway in gGLRN₁₉-induced GH release from primary cultures of goldfish pituitary cells using pharmacological regulators in cell column perifusion systems. The NO scavenger PTIO abolished gGRLN₁₉-induced GH release and co-treatment with the NO donor SNP and GRLN did not produce additive GH release responses. Nitric oxide synthase (NOS) inhibitors 1400 W and 7-Ni abolished GRLN-induced GH release while treatment with another NOS inhibitor, AGH, had no significant effect. Taken together, these results demonstrate that the NOS/NO is an integral component of gGRLN₁₉-induced signalling within the goldfish pituitary cells, and given the relative specificity of AGH for inducible NOS and endothelial NOS isoforms, suggests that neuronal NOS is the likely NOS isoform utilized in goldfish somatotropes by this physiological regulator.     

一氧化氮在L-精氨酸促生长激素分泌中的作用

氨基酸是促进垂体生长激素(GH)分泌的潜在刺激因子.在所有氨基酸中,左旋精氨酸(L-精氨酸)促GH分泌作用最为明显.口服或静脉滴注精氨酸均可刺激垂体GH的分泌.L-精氨酸促GH的分泌可能涉及多种机制,但一氧化氮(NO)依赖的可溶性鸟苷酸环化酶-环鸟苷酸-蛋白激酶G(sGC-cGMP-PKG)信号转导通路在此过程中起着重...     

Evidence that gonadotropin-releasing hormone also functions as a growth hormone-releasing factor in the goldfish

The present study examined the influence of GnRH on the in vivo and in vitro secretion of GH in the goldfish (Carassius auratus). Intraperitoneal injection of several GnRH peptides, including a form native to goldfish, salmon GnRH (sGnRH), elevated circulating GH levels in female goldfish. An analog of mammalian GnRH (mGnRH), [D-Ala6,Pro9-NEt] mGnRH (mGnRH-A), at a dosage of 0.1 microgram/g BW increased serum GH levels for up to 48 h after a single ip injection. Goldfish receiving a series of injections of this dose of mGnRH-A also displayed an increased rate of body growth, indicating that the mGnRH-A-induced increase in the circulating GH level was sufficient to accelerate body growth. In vitro experiments using perifused pituitary fragments found that sGnRH stimulated the secretion of GH from the goldfish pituitary in a potent, dose-dependent, and reversible manner. The time course of response and half-maximally effective dose of sGnRH were very similar for both GH and gonadotropin (GTH) secretion in vitro, suggesting that the mechanism(s) mediating the stimulatory actions of GnRH in the goldfish may be similar for both GH and GTH secretion. However, GnRH-induced GH and GTH secretion from the goldfish pituitary can occur independently of each other, as demonstrated by the finding that somatostatin inhibited the GnRH stimulation of GH secretion in vitro, without influencing the GTH response, whereas the dopamine agonist apomorphine inhibited GnRH-induced GTH secretion in vitro, without influencing the GH response. Furthermore, the dopamine antagonist pimozide did not influence serum GH levels, although pimozide potentiated the stimulatory effect of GnRH on GTH secretion in vivo by blocking the endogenous GTH release inhibitory action of dopamine. Results of the present study suggest that the secretion of GH and GTH in the goldfish are regulated, at least in part, through a common releasing factor, GnRH, whereas somatostatin and dopamine appear to act independently as GH and GTH release inhibitory factors, respectively.     

Further evidence that thyrotropin-releasing hormone participate in the regulation of growth hormone secretion in the rat

Effects of thyrotropin-releasing hormone (TRH) on growth hormone (GH) secretion were investigated in vivo (on intact or mediobasal hypothalamic lesioned rats tested under either anesthesia or free moving conditions) as well as in vitro (in incubation or perifusion systems of anterior pituitary tissue). The peptide induced a rapid, dose-dependent increase of plasma GH levels in free moving animals bearing an extensive lesion of the mediobasal hypothalamus including the median eminence. Under comparable conditions, TRH was ineffective in intact animals. After chloral hydrate anesthesia a GH response to TRH was recorded in both groups, but lesioned rats exhibited a better responsiveness to all doses tested. In vitro TRH increased GH release from incubated or perifused pituitaries sampled from both intact and lesioned rats in a transient and concentration-dependent manner. A similar effect was obtained with the (3 Me His2) analogue of TRH. These findings indicate that TRH can affect GH secretion at the pituitary level under specific experimental conditions and support the hypothesis that either peripheral hormones or other, still unidentified hypothalamic neurohormones may modulate this effect.     

Evidence that pulsatile follicle-stimulating hormone secretion is independent of endogenous luteinizing hormone-releasing hormone

Although LHRH can stimulate the release of both LH and FSH from the pituitary, there are a number of instances in which the secretion of LH and FSH are divergent. Previous studies from our laboratory have indicated that pulsatile LH and FSH secretion are independently regulated by gonadal factors. We have, therefore, reexamined the role of LHRH in regulating pulsatile gonadotropin secretion by evaluating the effect of passive LHRH immunoneutralization on LH and FSH secretion in castrate adult male rats. Injection of 500 microliters ovine anti-LHRH serum no. 772 (LHRH-AS) into 2-week-castrate rats caused an 85% suppression of mean plasma LH levels by 2 h, which lasted through 48 h. Mean plasma FSH, however, was reduced by only 19% after 2 h and by only 59% after 48 h. When cannulated 2-week-castrate rats were bled every 10 min, both LH and FSH were secreted in a pulsatile manner. Injection of 500 microliters LHRH-AS caused an immediate abolishment of LH pulses and a rapid reduction in mean plasma LH through 24 h. Pulsatile FSH secretion, as characterized by the parameters of pulse frequency and amplitude, was unaffected by LHRH-AS, although mean plasma FSH levels were significantly reduced. Collectively, the results suggest that pulsatile FSH secretion is regulated by a separate factor(s) distinct from LHRH, but that LHRH is required for the maintenance of elevated FSH levels.     

Evidence that reduced growth hormone secretion observed in monosodium glutamate-treated rats is the result of a deficiency in growth hormone-releasing factor

The present experiments were designed to examine various aspects of GH secretion in adult male rats given monosodium glutamate (MSG; 4 mg/g BW, sc) during the neonatal period. MSG-treated animals sustained lesions localized to the hypothalamic arcuate nuclei (ARC) and had reduced nasal-anal lengths and body weights. Anterior pituitary (AP) weights were decreased, but AP concentrations of GH and PRL were not significantly altered. Analysis of pulsatile GH secretion showed depressed GH pulses and prolonged GH trough periods. Mean 5-h plasma GH levels were reduced, whereas PRL levels were not affected. Morphine sulfate (MS) at doses of 0.01, 0.1, 1.0, and 3.0 mg/kg induced a prompt rise in GH during the 45 min after drug administration in controls. MSG-treated animals showed a significant rise in GH only with 1.0 and 3.0 mg/kg MS. A significant elevation in PRL was found in both control and MSG-treated animals after 1.0 and 3.0 mg/kg MS. The pentobarbital-induced rise in GH was also blunted in MSG-treated animals. MSG-treated animals which were administered antisomatostatin serum showed elevated GH trough and mean GH levels, with no apparent effect on GH peak levels. In view of the mechanisms by which MS and pentobarbital act to increase GH secretion, the present data suggest that the GH regulatory deficit observed in MSG-treated rats is due to a relative loss of GH-releasing factor secondary to ARC damage.     

Differential involvement of nitric oxide signaling in dopamine and PACAP stimulation of growth hormone release in goldfish

Previous studies in goldfish pituitary cells have shown that nitric oxide synthase (NOS)/nitric oxide (NO) signaling is involved in mediating the growth hormone (GH) release response to gonadotropin-releasing hormones. In this study, the involvement of this signaling pathway in mediating the action of two cAMP-mobilizing neuroendocrine stimulators of GH release, pituitary adenylate cyclase-activating polypeptide (PACAP) and dopamine (DA), was investigated in cell column perifusion experiments with primary cultures of dispersed pituitary cells. GH responses to PACAP were unaffected by three NOS inhibitors, aminoguanidine hemisulfate, 1400W and 7-nitroindazole (7-Ni). PACAP-stimulated GH release was also not reduced by two NO scavengers, rutin hydrate and PTIO, but NO-donor sodium nitroprusside (SNP)-elicited GH release was additive to the GH response to PACAP. In contrast, DA-induced GH secretion was reduced by 7-Ni, rutin hydrate and PTIO while not being additive to the GH response induced by SNP. These results indicate that although both PACAP and DA stimulation of acute GH release involve activation of adenylate cyclase/cAMP, DA- but not PACAP-signaling also utilizes the NOS/NO second messenger system.     

L-arginine-induced growth hormone secretion is not influenced by co-infusion of the nitric oxide synthase inhibitor N-monomethyl-L-arginine in healthy men.

In animals, it has been demonstrated that nitric oxide (NO) is a potent neuroregulatory substance. By intravenous infusion, L-arginine is converted to NO and citrulline, but it is unknown whether NO is responsible for the GH stimulating effect of L-arginine in humans. We investigated whether intravenous infusion of the NO synthase inhibitor N-monomethyl-L-arginine (L-NMMA) influenced L-arginine stimulated GH secretion. Ten healthy men, aged 28.6 +/- 1.9 (mean +/- SEM) years were examined twice. L-arginine was infused intravenously in a dose of 0.5 g/kg, max 35 g, from 0 to 30 min, accompanied by either: (1) L-NMMA from -5 to 0 min, in a dose of 3 mg/kg, max 250 mg, and in a dose of 3.5 mg/kg, max 250 mg from 0 to 60 min; or (2) a saline infusion. Heart rate increased (P = 0.032), and diastolic blood pressure decreased (P < 0.001) in the two situations. Plasma cGMP was unchanged and identical in the two situations (P = 0.679). Urine cGMP/creatinine ratio increased during both examinations (P = 0.041). Growth hormone secretion increased significantly during L-arginine infusion (P = < 0.001) without any effect of L-NMMA (P = 0.848). We did not find evidence that NO influences GH secretion. It remains to be tested, however, whether a higher dose of L-NMMA may influence L-arginine stimulated GH secretion.     

Noradrenergic regulation of growth hormone secretion in the baboon

We have investigated the effects of iv administered noradrenergic agonists and antagonists on plasma GH concentration in the adolescent baboon, Papio papio, with the aim of defining the relative roles of adrenergic receptor subtypes (alpha 1, alpha 2, beta 1, and beta 2) in the regulation of GH release. Clonidine (0.02 mg/kg) or UK-14,304 (0.02 mg/kg), potent centrally acting alpha 2 noradrenergic agonists, were infused into 24 animals pretreated with either saline, or selective alpha 1 and alpha 2 noradrenergic antagonists. Both agonists potently augment plasma GH, producing peak levels of 30-60 ng/ml 15 min post infusion. These responses can be prevented by the prior infusion of the alpha 2 antagonist, piperoxane (1.0 mg/kg), but not by the alpha 1 antagonist, prazosin (2.0 mg/kg). Log dose response curves of the 2 agonists demonstrate a greater potency for UK-14,304 vs. clonidine on a molar basis. In animals pretreated with monoamine depleting agents (reserpine and alpha-methyl paratyrosine) the plasma GH response to an infusion of clonidine (0.02 mg/kg) is significantly enhanced (P less than 0.001). Beta-Adrenoreceptor antagonism by propranolol (0.02 or 1.0 mg/kg) or the more selective beta 2-adrenoreceptor antagonist, ICI 118,551 (0.02-1.0 mg/kg), results in a rapid and significant (P less than 0.01) increase in plasma GH. The beta 1-antagonist, practolol (0.2-2.0 mg/kg), does not alter plasma GH levels. It is proposed that in the baboon, noradrenaline acts on alpha 2-noradrenergic receptors to stimulate GH release and on beta 2-noradrenergic receptors to inhibit GH release.     

Evidence that the N-terminus of human growth hormone is involved in expression of its growth promoting, diabetogenic, and insulin-like activities.

Recent work with various point and deletion mutants of human GH (hGH) has suggested that the proximal N-terminal end of the hormone molecule is important for its growth promoting action. This study was conducted to examine the growth promoting, diabetogenic, and insulin-like activities of two N-terminal mutants of hGH, the deletion mutant Des-7 hGH (met8, ala11), and a chimeric mutant of bovine GH (bGH) and hGH containing the N-terminal 13 amino acids of bGH (met, ala 1-13/14-191, asp11). The CD spectra of these mutants are similar to that of wild-type hGH and they retain lactogenic activity on Nb2 lymphoma cells, whereas their ability to bind to somatogenic receptors on IM-9 lymphocytes and bovine liver membranes is markedly reduced. In this study, growth promoting activity of the mutants was assessed using the 9-day weight gain test in hypophysectomized rats. Des-7 hGH had a potency of 0.03 IU/mg protein in this assay, whereas the potency of the bGH/hGH chimera was 0.71 IU/mg. Diabetogenic activity was tested in the ob/ob mouse, using the elevation of fasting blood glucose and the worsening of glucose tolerance after a 3-day course of treatment as end-points. Both Des-7 hGH and the bGH/hGH chimera had reduced diabetogenic activity compared to that of biosynthetic wild-type hGH, consistent with their reduced growth activity. Insulin-like activity was assessed by testing the in vitro ability of the mutants to stimulate [14C] glucose oxidation by epididymal adipose tissue of hypophysectomized rats. Des-7 hGH had about 1% the activity of wild-type hGH, whereas the chimera was about 20% as active. When Des-7 hGH was added to the incubation medium along with wild-type hGH in ratios of 5, 12.5, or 25:1 (Des-7 hGH:hGH), the insulin-like action of hGH was significantly inhibited, indicating that the mutant is a modest antagonist of the insulin-like action of hGH. When the ability of Des-7 hGH to compete with [125I] hGH for binding to isolated rat adipocytes was tested, the mutant was about 10% as effective as wild-type hGH. Thus, Des-7 hGH appears to be more effective in binding to adipocyte GH receptors than in triggering an insulin-like response, perhaps accounting for its modest antagonistic activity. The results of this study suggest that the proximal N-terminal end of the hGH molecule is involved in the expression of the growth promoting, diabetogenic and insulin-like activities of GH.     

Nitric oxide synthase in the rat anterior pituitary gland and the role of nitric oxide in regulation of luteinizing hormone secretion.

By using immunohistochemistry and in situ hybridization, we have demonstrated that the nitric oxide (NO)-synthesizing enzyme NO synthase is present in gonadotrophs and in folliculo-stellate cells of the anterior pituitary gland of male and female rats. A marked increase in levels of NO synthase protein and mRNA was observed after gonadectomy. In vitro studies on dispersed anterior pituitary cells suggest that NO inhibits gonadotropin-releasing-hormone-stimulated luteinizing hormone release. An inhibitory effect of NO has also been shown on growth-hormone-releasing-hormone-stimulated release of growth hormone [Kato, M. (1992) Endocrinology 131, 2133-2138]. Thus these findings support a dual mechanism for NO in the control of anterior pituitary hormone secretion, an autocrine mediation of luteinizing hormone release on gonadotrophs, and a paracrine effect on growth hormone secretion involving folliculo-stellate cells closely related to somatotrophs. We speculate that NO may participate in producing the pulsatile secretion patterns of these two pituitary hormones.     

Dietary regulation of human growth hormone secretion

The effect of diet on the secretion of human growth hormone (HGH) was investigated in eight normal subjects—each studied before and after four separate dietary regimens. A high-carbohydrate (525 g) diet of 3600 cal containing 75 g of protein for 23 days suppressed completely arginine-initiated HGH secretion. While subjects were ingesting a control diet, the mean maximal response to arginine was 21.5 ± 3.5 mμg/ml vs. 4.6 ± 0.9 mμg/ml (p < 0.01) following the experimental diet ($\text{X}$ ± SEM). The latter value did not differ from the mean maximal HGH response occurring “spontaneously” after control infusion of saline (3.5 ± 0.9 mμg/ml). This suppression of HGH secretion appeared related to the amount of carbohydrate rather than the total caloric level. An identical pattern occurred when carbohydrate was proportionately the same, but total intake was reduced to 2300 cal. With a high-carbohydrate diet of 2300 cal, the mean maximal HGH response to arginine was 4.7 ± 1.1 mμg/ml vs. a control response of 21.4 ± 4.7 mμg/ml (p < 0.01). Twenty-four-hour secretory patterns of HGH were assessed in three subjects befor and after the high-carbohydrate diet of 2300 cal. Overall HGH secretion was reduced significantly at the termination of the 23-day experimental period in two of these three subjects. HGH secretion was, likewise, significantly reduced by high-carbohydrate diets containing less protein. The change of HGH secretion could not be related in any study to differences in plasma-free fatty acid (FFA) or glucose concentration. FFA concentrations did decline in the first 3–7 days of each experimental period, but returned to basal values 7–8 days prior to reassessment of HGH secretion. Plasma glucose concentrations did not change significantly at any time.     

Evidence for alpha-adrenergic regulation of episodic growth hormone and prolactin secretion in the undisturbed male rat

The present experiments were designed to study the effect of the centrally active alpha-adrenergic receptor agonist, clonidine, on episodic GH and PRL secretion in male rats after selective blockade of norepinephrine (NE) and epinephrine (EP) synthesis with the dopamine-beta-hydroxylase inhibitor, FLA-63. Freely behaving, chronically cannulated rats were maintained on a constant light-dark cycle in isolation test chambers. Beginning at 1000 h, blood samples were removed every 20 min for 5-h periods without disturbing the animal. FLA-63 was administered (10 or 20 mg/kg ip) at 0845 h. Clonidine (15 or 150 microgram/kg iv) was given at times that coincided with the spontaneous occurrence of episodic GH peaks or troughs observed in control animals. Results of the present study are summarized as follows: 1) selective blockade of NE and EP synthesis with FLA-63 (20 mg/kg) caused complete suppression of episodic GH but had no significant effect on PRL release; 2) clonidine (150 microgram/kg) restored the pulsatile pattern of GH secretion in FLA-63-treated rats, and 3) clonidine (15 and 150 microgram/kg) stimulated PRL release in a dose-dependent manner. These findings suggest a major stimulatory role of alpha-adrenergic receptors in episodic GH and PRL secretion.     

Evidence that changes in tonic luteinizing hormone secretion determine the growth of preovulatory follicles in the rat

Physiological concentrations of progesterone (20-100 ng/ml), maintained by the insertion of implants into 30-day-old rats, delayed first ovulation, and withdrawal of progesterone on day 47 of age synchronized first ovulation in rats. Inhibition of ovulation involved negative feedback regulation of tonic LH and FSH secretion, blockage of gonadotropin surges, and suppression of preovulatory, but not antral, follicular growth. Removal of implants resulted in a rapid decline in serum progestrone from 100 to 5 ng/ml within 0-12 h. Between 0-36 h there were progressive increases in serum concentrations of LH and FSH, enhanced accumulation of estradiol by individual follicles incubated in vitro with or without exogenous substrate, and marked progressive increases in the content of LH (but not FSH) receptors in both thecal and granulosa cells. These events were followed by gonadotropin surges at 48 h (1800 h on day 49), ovulation, and morphological and biochemical signs of luteinization, including decreases in follicular gonadotropin receptor content and estradiol accumulation, evident by 60 h. With the exception of changes in basal LH, this sequence of events is remarkably similar in time and pattern to that after the decline of progesterone on diestrous day 2 and ovulation on proestrus of a 5-day cycle. Although a direct effect of progesterone on ovarian follicular cell function cannot be excluded, the data suggest that subtle but sustained increases in LH (and possibly FSH) are required for the enhanced follicular accumulation of estradiol and LH-binding activity occurring between diestrus and proestrus of the rat estrous cycle. Thus, perhaps some of the mystery surrounding the endocrine events between diestrus and proestrus can be ascribed to changes in serum LH that have been too small and/or variable for current nonserial sampling methods and RIAs to detect reliably.