Growth Hormone-releasing Factor Secretion Research Articles

Neurological manifestations following clonidine test for growth hormone reserve in children: case report and review of the literature

Introduction administration of a single dose of clonidine 4 μg/kg or 0.15 mg/m2.[6] However, the use of clonidine for this Clonidine is an α2-adrenergic agonist usually used for purpose is not authorised in the manufacturer’s prescribing treatment of hypertension, phaeochromocytoma, pain and information. other disorders. α2-Adrenergic agonists modulate growth hormone secretion by inhibiting the release of Case report somatostatin by the hypothalamus and also by stimulating growth hormone releasing factor secretion.[1-3] Due to its An 11-year-old girl with short stature (1.25m) and agonist action in the CNS, clonidine is used in the delayed puberty was referred to the Paediatric diagnosis of delayed growth in adults and children with Endocrinology Department at the Severo Ochoa Hospital lack of growth hormone (somatotropin).[1,4] in Madrid, Spain to have her growth hormone reserve Growth hormone has a direct effect on growth, but assessed. Levels of thyroid stimulating hormone (TSH) most of its effect is mediated by an insulin-like growth and IGFBP-3 were normal, while levels of IGF-1 were factor (IGF) and other peptides, such as somatomedin. low (89 ng/mL; normal range 117–771 ng/mL). The IGF is closely linked to six specific plasmatic proteins following month, the IGF-1 level test was repeated, giving referred to as IGF binding protein (IGFBP). The levels of results of 51 ng/mL. The growth hormone insulin IGF and IGFBP-3 depend on growth hormone secretion, stimulation test was considered negative (i.e. growth which increases in patients with acromegaly and decreases hormone levels were <5 ng/mL after insulin stimulation). in patients with hypopituitarism or growth hormone Consequently, it was decided to use the clonidine deficiency. At high levels, insulin also stimulates growth stimulation test to assess her growth hormone levels. She due to its anabolic action.[5] received 0.11mg of clonidine. During the test, she suffered The growth disorders are diagnosed by physical from hypotension, loss of consciousness and hypertonia. examination, bone age measurement, thyroid function She was initially diagnosed as having syncope. Two hours tests, and indirect (IGF-I and IGFBP-3 later, she was admitted to the Paediatric Emergency radioimmunoassays) and direct (growth hormone release Department with hallucinations, psychomotor agitation, stimulation test) growth hormone tests. The randomised pallor and sphincter relaxation. IGF-I and IGFBP-3 quantifications accurately reflect the That evening, after being discharged, she presented average growth hormone blood levels, as they have a long with dysarthria, drowsiness, paraesthesia and carpopedal half life. When IGF-I and IGFBP-3 levels are low, direct spasm. She returned to the Paediatric Emergency determinations of growth hormone levels are required. As Department and was kept under medical observation for the secretion of growth hormone is episodic, the 24 hours. Blood ion tests and physical and neurological measurement of this hormone is determined after explorations were normal. stimulation by various substances, such as insulin or clonidine.[5] If adequate increments in growth hormone This adverse reaction was communicated to the levels do not occur after stimulation by at least two Pharmacovigilance Surveillance Programme. The results substances, the stimulation test is considered negative and, of the clonidine stimulation test were considered positive, therefore, the patient has a growth hormone deficiency. with growth hormone levels of 6.90, 7.90, 11.10 and 13.50 Using the clonidine test, growth hormone levels are ng/mL at 0, 60, 90 and 120 minutes, respectively, after measured at 60, 90 and 120 minutes after the clonidine administration.

Lead attenuation of episodic growth hormone secretion in male rats.

The purpose of this study was to characterize the effects of chronic lead exposure on growth hormone and insulin-like growth factor-1 status in growing male rats. Pituitary growth hormone content, episodic growth hormone release, plasma insulin-like growth factor-1 levels, and growth hormone response to exogenous growth hormone-releasing factor were quantified in young rats given lead nitrate. Twenty male Sprague-Dawley weanling rats were given lead nitrate (1000 ppm lead) in drinking water for a period of 6 weeks. Lead treatment significantly reduced body weight gain. Pituitary growth hormone content was not altered by lead treatment. Mean plasma growth hormone levels were reduced 44.6% by lead treatment (46.41 +/- 6.2 ng/ml; p = .003) as compared to controls (83.82 +/- 10 ng/ml). Lead treatment reduced mean growth hormone peak amplitude by 37.5%, mean nadir concentration by 60%, and growth hormone peak area by 35%. These findings are consistent with decreased hypothalamic growth hormone-releasing factor secretion or reduced somatotrope responsiveness. Exogenous growth hormone-releasing factor increased plasma growth hormone in lead-treated and control rats. However, this response was blunted by the lead treatment (lead treated: 485.6 +/- 57.8 vs. controls: 870.2 +/- 127 ng/ml; p = .03). Plasma insulin-like growth factor-1 concentration was not significantly affected by lead treatment. These results demonstrate that lead intoxication attenuates growth hormone release without abolishing the hypothalamic endocrine mechanisms driving growth hormone pulsatility. This suggests that lead acts at the level of the pituitary somatotroph rather than at the level of the hypothalamus.

Middle-age alterations in the sexually dimorphic plasma growth hormone profiles: involvement of growth hormone-releasing factor and effects on cytochrome p450 expression.

Rat liver, as well as other species, contains numerous sex-dependent isoforms of cytochrome P450 (P450) that are regulated by the sexually dimorphic profiles of circulating growth hormone. During puberty, young adulthood, and senescence, changes in the hormonal profiles appear to be responsible for alterations in age-associated expression levels of selective P450 isoforms. In contrast, little is known about the growth hormone secretory profiles and their P450-dependent expression levels during middle age. In the present study, we observed subtle changes in the hormonal concentrations, and frequencies of peaks and interpulse periods in the sexually dimorphic growth hormone profiles of 1-year-old male and female rats correlated to suppression of male-specific isoforms CYP2C11 and CYP2C13 and female-predominant CYP2C7. To identify possible causes for the age-associated changes in the circulating growth hormone profiles, the responsiveness of the hypothalamic-pituitary axis to growth hormone secretagogues clonidine and growth hormone-releasing factor (GRF) were examined in middle-aged male and female rats. In spite of the same sexually dimorphic response in young adult and middle-aged rats to both secretogogues (males > females), the pituitary somatotrophs in the older animals exhibited a dramatic decrease in sensitivity to clonidine, characterized by subnormal growth hormone release levels and an inordinate delay in pituitary response to clonidine stimulation. Results from similar studies conducted on middle-aged arcuate nucleus-lesioned rats suggest that a decline in GRF secretion is a possible contributor to the age-associated alterations in plasma growth hormone profiles during middle age. These changes in GRF-induced, sexually dimorphic secretory growth hormone profiles and the accompanying decline in P450 expression levels may anticipate similar, but more profound, changes to occur during senescence.

Somatostatin-Stimulated Growth Hormone-Releasing Factor Secretion In Vitro Is Modified by Fetal Ethanol Exposure

Exposure to ethanol (ETOH) in utero has been found to alter the growth hormone (GH)/insulin-like growth factor axis and retard growth. GH release is regulated by the interaction of the hypothalamic hormones somatostatin [somatotropin release inhibiting factor (SRIF)] and GH-releasing factor (GRF). Communication between these factors occurs at both the hypothalamic and pituitary levels. In this study, we examined the effect of fetal ETOH exposure on the postnatal development of SRlF regulation of GRF release at the hypothalamic level. The studies were conducted on hypothalami from 30- and 60-day-old pups in an in vitro perfusion system. SRlF was tested against K+-induced GRF release. Basal GRF release, not in the presence of external stimuli, from the hypothalami of ETOH-exposed pups (both male and female) was greater than GRF release from tissue of offspring of both pair-fed and control dams. GRF release was also greater in female, compared with male hypothalami, and the effect of ETOH was more pronounced at 30, compared with 60, days of age. The heightened release of GRF may reflect reduced sensitivity to feedback regulation by GH in ETOH-exposed pups. Furthermore, fetal ETOH exposure reversed the effect of SRlF modulation on K+-induced GRF release. In hypothalami from offspring of both pair-fed (except 30-day-old females) and ad libitum-fed controls, SRlF enhanced the K+-induced GRF release. In tissue from ETOH-exposed pups (except 60-day-old males), GRF release was inhibited by SRIF. We suggest that this pattern may reflect a change in the characteristics of SRlF connections with GRF neurons by ETOH. In summary, this data indicates that both basal and SRIF-stimulated GRF is altered by ETOH and that effect remains significantly altered at least until puberty.

Thyroid Hormone Regulation of Gene Expression of the Pituitary Growth Hormone-Releasing Factor Receptor

To examine thyroid hormone regulation of the pituitary receptor for hypothalamic growth hormone (GH)-releasing factor (GRF), we studied effects of hypothyroidism on the pituitary GRF receptor (GRF-R) mRNA and its related parameters in rats. Thyroidectomy (Tx) induced a 61-65% reduction in GRF-R mRNA levels, which was significantly reversed with thyroxine (T4) replacement for 5 days at a dose of 1 μg/100 g/day. Pituitary GH contents changed parallel to GRF-R mRNA levels following the Tx and T4 replacement. In contrast, Tx enhanced GRF release > 2 fold, which was not reversed with the regime of T4 replacement. These results indicate that thyroid hormone promotes pituitary GRF-R gene expression, not by modulating GRF secretion, but by acting on the pituitary directly. The decline in GRF receptor expression would contribute to somatotroph failure by rendering the pituitary refractory to the increased GRF signal in hypothyroidism.

Mouse growth hormone-releasing factor secretion is activated by inhibin and inhibited by activin in placenta.

We investigated the effects of activin and inhibin on the regulation of mouse growth hormone-releasing factor (mGHRF) secretion by primary placental cells harvested at Day 12 of pregnancy. Activin-A, an activator of FSH secretion, inhibited mGHRF secretion. In contrast, inhibin, an inhibitor of FSH secretion, activated mGHRF secretion. The lowest concentrations of activin-A and inhibin that significantly affected mGHRF secretion were 2 nM. Follistatin, a binding protein of activin, completely eliminated the ability of activin to inhibit mGHRF secretion. The steady-state level of mGHRF mRNA, as assessed by Northern analysis, was reduced by incubation of placental cells with activin-A. All activin and inhibin subunit mRNAs were expressed in mouse placenta, and their expressions increased during gestation. These findings suggest that activin and inhibin have opposite effects on mGHRF secretion as compared with FSH secretion and that they regulate mGHRF secretion in an autocrine or paracrine manner in the mouse placenta in vivo.

Patterns of expression of SSTR1 and SSTR2 somatostatin receptor subtypes in the hypothalamus of the adult rat: relationship to neuroendocrine function

The neuropeptide somatostatin is the major physiological inhibitor of growth hormone secretion. With the aim of identifying the receptor subtypes through which this neuropeptide may be exerting its neuroendocrine actions in the brain, we have examined by in situ hybridization the distribution of the messenger RNA for SSTR1 and SSTR2 isoforms in the hypothalamus of adult male and female rats. Both receptor subtypes were highly expressed in the medial preoptic area, suprachiasmatic nucleus and arcuate nucleus. High SSTR1, but low SSTR2, expression was evident in the para- and periventricular nuclei as well as in the ventral premammillary nucleus. Conversely, moderate to high SSTR2, but low SSTR1, messenger RNA levels were detected in the anterior hypothalamic nucleus, ventromedial and dorsomedial nuclei and medial tuberal nucleus. Taken together, these distributional patterns conform to those of somatostatin binding sites as visualized by in vitro autoradiography, suggesting that an important proportion of SSTR1 and SSTR2 receptors in the hypothalamus are associated with the perikarya and dendrites of intrinsic neurons. The distribution of SSTR1-expressing cells within the periventricular, paraventricular and suprachiasmatic nuclei was similar to that of neurons previously reported to contain and/or express somatostatin in the brain suggesting that some of the SSTR1 receptors may correspond to autoreceptors. Within the arcuate nucleus, the distribution of SSTR1 and SSTR2 messenger RNA-expressing cells was comparable to that of neurons previously found to selectively bind somatostatin-14 within this area. Given that over one third of these cells also contain and express growth hormone-releasing factor, the present findings suggest that both of these receptor subtypes are involved in the central regulation of growth hormone-releasing factor secretion by somatostatin. Taken together, the present results suggest that SSTR1 and SSTR2 somatostatin receptor messenger RNAs are heavily expressed in those neurons containing somatostatin and/or growth hormone-releasing factor and thereby imply a role for both SSTR1 and SSTR2 somatostatin receptor subtypes in neuroendocrine regulation of growth hormone secretion in both sexes of this species.

Growth hormone releasing factor and vasoactive intestinal peptide stimulate rat granulosa cell plasminogen activator activity in vitro during follicular development

Growth hormone-releasing factor (GRF) and vasoactive intestinal peptide (VIP) are two structurally homologous peptides sharing common target cell receptor and known to enhance FSH-induced steroidogenesis of undifferentiated granulosa cell in vitro. Although VIP, has been reported to stimulate plasminogen activator (PA) activity in rat granulosa cells, our knowledge on the actions and interactions of these two peptides with FSH in the regulation of rat granulosa cell PA system during follicular development remains incomplete. Undifferentiated and differentiated rat granulosa cells from pre-antral (DES-treated rats) and antral (eCG-treated rats) follicles, respectively, were cultured in a chemically defined medium in the absence and presence of FSH (400 ng/ml), GRF (10 −8–10 −5 M) and/or VIP (10 −9–10 −5 M). Net secreted (PAs) and cell-associated (PAc) PA activities was measured by the fibrinolysis assay and characterized by the fibrin overlay method. Granulosa cell differentiative (progestin secretion) and proliferative (DNA synthesis) responses were analyzed by radioimmunoassay and [ 3H]thymidine incorporation, respectively. Both GRF and VIP stimulated PAs and PAc activities in a concentration-dependent manner in 24-h cultures of granulosa cells from the two stages of follicular development. They (10 −5 M) enhanced FSH-stimulated PAs activity in granulosa cell cultures of pre-antral follicles, with GRF being more effective than VIP. On the contrary, only GRF (10 μM) potentiated FSH-induced PAs and PAc activities in cultures of granulosa cell from antral follicles. The stimulation of PA activity by these agonists decreased with the duration of culture irrespective of the stage of follicular development. This decrease in PA activity was not associated with a change in PA inhibitor activity. Whereas two enzyme activities corresponding to 55 and 30 kDa were observed in culture of undifferentiated granulosa cells, only the high molecular weight species was detectable in the differentiated cells and was regulated by the gonadotropin and the peptides. Whereas GRF and VIP (1 μM) enhanced basal and FSH-stimulated progesterone (P) and 20α-dihydroprogesterone (20α-OH-P) secretion by undifferentiated granulosa cells, only basal progestin production were stimulated by the peptides in differentiated cells. GRF, but not VIP, stimulated granulosa cell DNA synthesis, a response independent of the stage of follicular development and markedly suppressed by FSH. The earlier demonstration of GRF and VIP secretion by ovarian cells and the presently observed influence on the proliferative and differentiative responses of granulosa cells to these peptides in vitro suggest that they play an important role in the intra-ovarian regulation of follicular development.

Transforming Growth Factor-β1 Post-transcriptionally Inhibits Mouse Growth Hormone Releasing Factor Secretion in Placenta

The aim of this study was to investigate whether TGF-β1 regulates mouse GHRF secretion by primary cultures of placental cells from day 12 of pregnancy. Ten ng/ml TGF-β1 significantly inhibited mouse GHRF secretion by the third day of culture. The lowest concentration of TGF-β1 that significantly inhibited mouse GHRF secretion was 1 ng/ml. The inhibitory effect of TGF-β1 on mouse GHRF secretion was completely eliminated by addition of an anti-TGF-β1 antibody. Steady-state levels of mouse GHRF mRNA as assessed by Northern analysis was not reduced by incubation of placental cells from day 12 of pregnancy with 10 ng/ml TGF-β1 for 5 days. Both placenta and decidua expressed TGF-β1 mRNA, and the level of TGF-β1 mRNA in decidua increased during gestation while the level of TGF-β1 mRNA was constant throughout gestation. These findings suggest that TGF-β1 is one of the potent regulators of mouse GHRF secretion and that TGF-β1 regulates mouse GHRF secretion in an autocrine or paracrine manner in the mouse placenta in vivo.

Thyroid hormone modulation of the hypothalamic growth hormone (GH)-releasing factor-pituitary GH axis in the rat.

Both thyroid hormone and hypothalamic growth hormone (GH)-releasing factor (GRF) facilitate pituitary somatotroph function. However, the pathophysiological role of thyroid hormone in GRF secretion is less well understood. Thyrotoxicosis, induced by administration of thyroxine (T4) in rats, inhibited both pituitary GH levels and immunoreactive GRF secretion from incubated hypothalamus. At the highest dose of T4 given for 12 d, GRF secretion and pituitary GH decreased by 50 and 39%, respectively. Hypothyroidism induced by thyroidectomy (Tx) enhanced GRF secretion approximately twofold while depleting pituitary GH by greater than 99%. Both of these hypothalamic and pituitary effects were reversed by replacement of T4 but not human GH for 7 or 14 d. Human GH was as potent as T4 in restoring decreased body weight gains or serum insulin-like growth factor-1 levels in Tx rats. These results indicate that at both physiological and pathological concentrations in serum, thyroid hormone acts as an inhibitory modulator of GRF secretion, probably not involving a feedback mechanism through GH. A biphasic effect of thyroid hormone on pituitary GH levels appears to derive from the difference in primary target tissues of hyper- and hypothyroidism, the hypothalamus and the pituitary, respectively.

The neurophysiological regulation of growth hormone secretion

With the advent of genetic engineering, the importance of GH in the regulation of growth and metabolism in domestic species has been clearly demonstrated. Ample evidence of an integral role for GH in the processes of growth and lactation exists in dairy cattle (1,2), sheep (3), beef cattle (4) and swine (5). For example, circulating GH levels are high during the period of rapid growth in several species including cattle (6), swine (7) and poultry (8). Endogenous GH secretion is primarily controlled by the central nervous system (CNS) via two specific hypothalamic neurohormones, growth hormone-releasing factor (GRF) and somatostatin (SRIF), an inhibitor of GH release. The secretion of GRF and SRIF is governed by a host of neuropeptides and neurotransmitters which provide a functional link between higher CNS centers and hypophysiotropic neurons. This review will focus on the CNS regulation of GH secretion and circulating factors which feedback to either stimulate or inhibit its release.

Galanin stimulates immunoreactive growth hormone-releasing factor secretion from rat hypothalamic slices perifused in vitro

The effect of galanin (GAL) on the release of GH-releasing factor (GRF) and somatostatin (SRIF) was examined in an in vitro perifusion system of rat hypothalamic slices. GAL at doses at 10 −7 and 10 −6M stimulated the release of immunoreactive GRF while it failed to affect SRIF release. Therefore, in vivo stimulation of GH release by GAL may be explained in part by the GRF-releasing effect of this peptide.

Catecholaminergic innervation of GRF-containing neurons in the rat hypothalamus revealed by electron-microscopic cytochemistry

The catecholaminergic innervation of neurons containing growth hormone-releasing factor (GRF) was examined by use of a method which combined either 5-hydroxydopamine (5-OHDA) uptake or autoradiography after intraventricular injection of 3H-noradrenaline with immunocytochemistry for GRF in the same tissue sections at the electron-microscopic level. In the ventrolateral part of the arcuate nucleus of the rat hypothalamus a large number of immunonegative axon terminals were found to make synaptic contact with GRF-like immunoreactive (GRF-LI) cell bodies and processes. 3H-noradrenaline autoradiography or 5-OHDA-labeling combined with GRF immunocytochemistry revealed that axon terminals labeled with 3H-noradrenaline or 5-OHDA make synaptic contact with the GRF-LI nerve cell bodies and processes. These findings indicate that catecholamine-containing neurons innervate GRF neurons to regulate GRF secretion via synapses in the rat arcuate nucleus.

Combined Autoradiographic and Immunohistochemical Evidence for an Association of Somatostatin Binding Sites with Growth Hormone‐Releasing Factor‐Containing Nerve Cell Bodies in the Rat Arcuate Nucleus

Abstract The regulation of growth hormone secretion depends upon the complex interplay between two hypothalamic hypophysiotropic factors: growth hormone-releasing factor and somatotropin release inhibiting factor or somatostatin. Interactions between these two neurohormones appear to be exerted both distally, at the level of pituitary somatotropes, and proximally, within the hypothalamus. In an attempt to detect a possible anatomical substrate for central interactions between the two neurohormones, we compared the autoradiographic distribution of specifically labeled somatostatin binding sites with the immunohistochemical distribution of growth hormone-releasing factor-containing neurons in the hypothalamus of adult rats. Somatostatin binding sites were labeled in vitro by incubating serial brain sections with [(125)l]TyrO-DTrp8-somatostatin. Growth hormone-releasing factor-immunoreactive neurons were visualized in a second set of animals, using an antiserum raised against synthetic rat growth hormone-releasing factor (1-29) NH(2). In light microscopic autoradiograms of sections incubated with [(125)l]somatostatin the label was found to be concentrated over small, round or oval neuronal perikarya clustered within the ventrolateral aspect of the arcuate nucleus. The topographic distribution of these [(125)l]somatostatin-labeled cells was similar to that of growth hormone-releasing factor-immunoreactive neurons detected within the same region. Moreover, the number of [(125)l]somatostatin-labeled cells was found to vary in parallel with that of growth hormone-releasing factor-immunoreactive neurons throughout the rostro-caudal extent of the arcuate nucleus (coefficient of correlation r = 0.80). These results suggest that somatostatin binding sites may be directly associated with the perikarya of arcuate growth hormone-releasing factor neurons. Such an association would provide an anatomical substrate for a direct regulation of growth hormone-releasing factor secretion by somatostatin at the hypothalamic level.

Growth Hormone-Releasing Factor Secretion from Fetal Hypothalamic Cell Cultures Is Modulated by Forskolin, Phorbol Esters, and Muscimol*

The regulation of GRF secretion was studied using a fetal rat hypothalamic cell culture system. The cells were subjected to short term release experiments on days 10-18 after plating, and GRF secretion was assessed by RIA. The identity of GRF immunoreactivity in the incubation medium was confirmed by reverse phase liquid chromatographic analysis. Depolarization of the cells with 56 mM K+ evoked a 4-fold increase in basal GRF release. When cultures were pretreated for 6 days with the adenylate cyclase activator forskolin, basal GRF release was augmented in subsequent release experiments to levels 2-fold greater than those in the control cultures. In nonpretreated cultures, forskolin (1-100 microM) and the protein kinase C activator phorbol 12-myristate 13-acetate (10 nM-1 microM), stimulated basal GRF release in a dose-dependent fashion. The Ca2+ channel blocker verapamil (100 microM) significantly inhibited the GRF response to both forskolin and phorbol 12-myristate 13-acetate. The gamma-aminobutyric acid (GABA) agonist muscimol (0.1-10 microM) inhibited forskolin-stimulated, but not K+ stimulated, GRF release in a dose-dependent manner. This inhibition was reversed by the GABA antagonists bicuculline and picrotoxinin. Muscimol (10 microM) slightly suppressed basal GRF release. The present findings suggest that GRF secretion can be evoked by agents known to increase intracellular cAMP levels or activate protein kinase-C. They also support a role for GABA in the inhibitory control of GRF secretion.

Withdrawal of endogenous somatostatin induces secretion of growth hormone-releasing factor in rats.

Secretion of GH occurs in episodic bursts under the dual control of two hypothalamic peptides, GH-releasing factor (GRF) and GH-inhibiting factor (somatostatin, SRIF). Recent studies in rats suggest that episodic GH secretion is generated by the periodic release of GRF, which is associated with the simultaneous withdrawal of SRIF secretion. To test the possibility that GRF discharge is functionally coupled with the withdrawal of SRIF, we investigated whether acute withdrawal of SRIF can induce GRF release by the rat hypothalamus using highly specific antisera against SRIF and rat GRF. In conscious unrestrained rats, i.v. administration of SRIF antiserum at the period of the GH trough induced a rapid onset of the GH secretory surge with a peak value of 309 +/- 67 micrograms/l (mean +/- S.E.M.) 30 min after injection. Pretreatment with antiserum to rat GRF resulted in a approximately 83% suppression of the GH surge induced by SRIF antiserum without affecting the trough GH values. GRF antiserum also significantly inhibited the spontaneous GH surge. In urethane-anaesthetized rats, as in conscious rats, an acute phasic GH release was caused by SRIF antiserum despite the interference of anaesthesia with spontaneous GH secretion. A further surge-like GH secretion was not restored during the next several hours, however, with the GH secretory profile being characterized by a tonic increase in the baseline levels of GH. In the anaesthetized rat antiserum to rat GRF, having no effect on basal GH levels, similarly inhibited by approximately 66% the acute GH surge induced by SRIF antiserum and decreased by about 30% the later sustained rise in GH.(ABSTRACT TRUNCATED AT 250 WORDS)

TRH axon terminals in synapsis with GRF neurons in the arcuate nucleus of the rat hypothalamus as revealed by double labeling immunocytochemistry

Synaptic input to neurons containing growth hormone-releasing factor (GRF) by axon terminals containing thyrotropin-releasing hormone (TRH) in the arcuate nucleus (AN) of the rat hypothalamus was examiined using a method combining pre-embedding peroxidase-anti-peroxidase for GRF with postembedding immunocolloidal gold staining for TRH. The TRH-like immunoreactive axon terminals were found to make synaptic contact with GRF-like immunoreactive neurons with unlabeled axon terminals. From these findings, TRH-containing neurons in the hypothalamic AN of the rat may be considered to innervate GRF neurons, to regulate GRF secretion or to have some other functions via synapses.

Ectopic corticotrophin-releasing-factor and growth hormone releasing factor secretion: diagnosis using human pituitary cell culture.

Cell culture of human pituitary tissue has been used to diagnose a patient with Cushing's syndrome due to ectopic secretion of corticotrophin-releasing factor (CRF; case 1) and a case of acromegaly associated with ectopic secretion of a growth-hormone releasing factor (GRF; case 2). In both patients a pituitary tumour was not detected. Case 1 had a small cell carcinoma and symptoms of the ectopic ACTH syndrome, but in culture the carcinoma failed to secrete detectable ACTH. However, the culture medium used to maintain this carcinoma in vitro was found to contain a substance which stimulated ACTH secretion by human pituitary corticotrophs in cell culture. Radioimmunoassays and HPLC indicated that this substance had similar elution characteristics to human CRF and cross-reacted with antiserum to ovine CRF. Case 2 was found to have a lung tumour, the removal of which led to regression of her acromegalic symptoms. In culture, this tumour did not secrete GH, but did secrete a GRF. We conclude that the Cushing's syndrome and acromegaly, in cases 1 and 2, respectively, were due to ectopic secretion of CRF and GRF leading to hyperstimulation of the pituitary gland.

Exogenous growth hormone inhibits growth hormone-releasing factor-induced growth hormone secretion in normal men.

Previous studies from this laboratory and by others in rats, monkeys, and humans support the concept that growth hormone (GH) can regulate its own secretion through an autofeedback mechanism. With the availability of human growth hormone-releasing factor (GRF), the possible existence of such a mechanism was reexplored by examining the effect of exogenous GH on the GH response induced by GRF-44-NH2 in six normal men (mean age, 32.4 yr). In all subjects the plasma GH response evoked by GRF-44-NH2 (1 microgram/kg i.v. bolus) was studied before and after 5 d of placebo (1 ml normal saline i.m. every 12 h), and then before and 12 h after 5 d of biosynthetic methionyl human GH (5 U i.m. every 12 h). The GH response to GRF (maximal increment over time 0 value) was significantly inhibited after GH treatment (0-1.3 vs. 2.3-11.2 ng/ml before treatment, P = 0.05), but was not significantly affected by placebo. This impaired pituitary response to GRF persisted for at least 24 h following exogenous GH treatment in two subjects who underwent further study. Serum somatomedin-C concentrations were significantly increased after 5 d of GH treatment (2.66-5.00 vs. 0.92-1.91 U/ml before treatment, P = less than 0.01). The impaired pituitary response to GRF may be mediated indirectly through somatomedin, somatostatin, by a direct effect of GH on the pituitary somatotropes, or by all of these mechanisms. These data suggest that after GH treatment, the blunted GH response to synthetic GRF is not solely a consequence of the inhibition of hypothalamic GRF secretion.

Patterns of Growth Hormone-Eeleasing Factor and Somatostatin Secretion into the Hypophysial-Portal Circulation of the Rat

The interrelation between the secretion of two hypophysiotropic peptides, growth hormone-releasing factor (GRF) and somatostatin (SRIF), in the generation of episodic growth hormone (GH) secretion was inferred from direct measurements of immunoreactive GRF and immunoreactive SRIF concentrations in the hypophysial-portal plasma of the rat. Secretion of immunoreactive GRF was found to be episodic, with maximal concentrations present during periods of expected GH secretory episodes. Secretion of immunoreactive GRF was accompanied by a moderate reduction in portal plasma levels of immunoreactive SRIF. Passive immunoneutralization of SRIF was associated with increased concentrations of immunoreactive GRF in hypophysial-portal plasma. On the basis of these observations, it appears that each GH secretory episode is initiated by pulsatile secretion of immunoreactive GRF into the portal circulation, which is preceded by or is concurrent with a moderate reduction of inhibitory tone provided by portal immunoreactive SRIF. These experiments provide direct insights into central and adenohypophysial mechanisms by which GRF and SRIF interact to generate episodic secretion of GH.