GHRH Release Research Articles

Expression of growth hormone secretagogue-receptors by growth hormone-releasing hormone neurons in the mediobasal hypothalamus.

A novel class of synthetic compounds, termed GH-secretagogues (GHSs), have been shown to be potent stimulators of GH release, although their mechanism of action and functional significance remains obscure. The recent cloning of the rat GHS receptor (GHS-R) permitted the identification of numerous sites of expression of GHS-R in brain, but nothing is yet known about the cell types that express this receptor. We performed dual chromogenic and autoradiographic in situ hybridization to test the hypothesis that GHRH neurons in the hypothalamus coexpress GHS-R mRNA. GHS-R-hybridizing cells showed extensive overlap with GHRH-expressing neurons in both the arcuate (Arc) and ventromedial (VMN) hypothalamic nuclei. Quantification of the double-labeled cells revealed that approximately 27% of GHRH-hybridizing neurons in the Arc, and 22% of those in the VMN, expressed the GHS-R gene. These studies are the first to colocalize the GHS-R to any neurochemical cell type in rat brain. The results provide evidence that the GHSs may directly modulate GHRH release, and thereby stimulate GH secretion, through interaction with the GHS-R on hypothalamic GHRH mRNA-containing neurons.

Interrelations Between Sleep and the Somatotropic Axis

In the human as in other mammals, growth hormone (GH) is secreted as a series of pulses. In normal young adults, a major secretory episode occurs shortly after sleep onset, in temporal association with the first period of slow-wave (SW) sleep. In men, approximately 70% of the daily GH output occurs during early sleep throughout adulthood. In women, the contribution of sleep-dependent GH release to the daily output is lower and more variable. Studies involving shifts of the sleep-wake cycle have consistently shown that sleep-wake homeostasis is the primary determinant of the temporal organization of human GH release. Effects of circadian rhythmicity may occasionally be detected. During nocturnal sleep, the sleep-onset GH pulse is caused by a surge of hypothalamic GHRH release which coincides with a circadian-dependent period of relative somatostatin disinhibition. Extensive evidence indicates the existence of a consistent relationship between SW sleep and increased GH secretion and, conversely, between awakenings and decreased GH release. There is a linear relationship between amounts of SW sleep--whether measured by visual scoring or by delta activity--and amounts of concomitant GH secretion, although dissociations may occur, most likely because of variable levels of somatostatin inhibition. Pharmacological stimulation of SW sleep results in increased GH release, and compounds which increase SW sleep may therefore represent a novel class of GH secretagogues. During aging, SW sleep and GH secretion decrease with the same chronology, raising the possibility that the peripheral effects of the hyposomatotropism of the elderly may partially reflect age-related alterations in sleep-wake homeostasis. While the association between sleep and GH release has been well documented, there is also evidence indicating that components of the somatotropic axis are involved in regulating sleep. The studies are most consistent in indicating a role for GHRH in promoting NREM and/or SW sleep via central, rather than peripheral, mechanisms. A role for GH in sleep regulation is less well-documented but seems to involve REM, rather than NREM, sleep. It has been proposed that the stimulation of GH release and the promotion of NREM sleep by GHRH are two separate processes which involve GHRH neurons located in two distinct areas of the hypothalamus. Somatostatinergic control of GH release appears to be weaker during sleep than during wake, suggesting that somatostatinergic tone is lower in the hypothalamic area(s) involved in sleep regulation and sleep-related GH release than in the area controlling daytime GH secretion. While the concept of a dual control of daytime and sleep-related GH secretion remains to be directly demonstrated, it allows for the reconciliation of a number of experimental observations.

Growth hormone-releasing hormone neurons in the arcuate nucleus express both Sst1 and Sst2 somatostatin receptor genes.

Several lines of evidence suggest that somatostatin (SRIF) regulates GH release through central control of hypothalamic GHRH neurons. A possible mechanism is through interaction with SRIF binding sites previously shown to be associated with a subpopulation of GHRH-containing neurons in the arcuate nucleus (Arc), although the molecular identity of these binding sites is not yet known. We performed dual chromogenic and autoradiographic in situ hybridization to determine whether GHRH neurons coexpress either the sst1 and/or sst2 SRIF receptor mRNAs. Computerized image analysis revealed that approximately 15% of GHRH-hybridizing neurons in the Arc expressed the sst1 receptor gene, whereas 15% coexpressed sst2 mRNA. These studies are the first to colocalize any SRIF receptor subtype in GHRH mRNA-containing neurons in brain. The results suggest that, in the Arc, SRIF may directly modulate GHRH release into the hypophyseal portal blood, and thereby influence GH secretion, through interaction with both sst1 and sst2 receptor subtypes.

Somatostatin and Growth Hormone-Releasing Hormone in Normal and Tumoral Human Breast Tissue: Endogenous Content, in Vitro Pulsatile Release, and Regulation

Endogenous production of SRIH and GHRH was analyzed in human breast tissue. SRIH precursor (pro-SRIH) was identified after Sephadex G-50 filtration of acetic acid extracts of normal and tumoral human breast samples. SRIH-(1–14) or -(1–28) could not be detected in breast tissue, whereas the immunoreactive SRIH released in vitro was characterized as SRIH-(1–28). Endogenous production of GHRH was assessed by identification of GHRH messenger ribonucleic acid by PCR followed by sequencing of the amplified complementary DNA and by high performance liquid chromatographic characterization of immunoreactive GHRH contained in the tissue and released in vitro. There were no differences in pro-SRIH or GHRH-(1–44) tissue contents between normal and tumoral samples. The release of both peptides was evidenced in perifusion and static incubation. Perifusion of normal breast tissue (n = 3) showed pulsatile release of SRIH and GHRH. Perifusion of tumors (n = 4) showed SRIH release in 50% of the cases. SRIH release was pulsatile in one case. GHRH release was observed in the four tumoral samples analyzed, but was pulsatile in only one case. In static incubation, tumors (n = 6) secreted 13 times more GHRH than did normal samples (n = 3; 383 ± 92 vs. 29.6 ± 4.6 fmol/mg protein; P < 0.05). Stimulation of GHRH release by exogenous SRIH was observed only with the normal tissue. Together these data provide evidence for the existence of local production of SRIH and GHRH by human breast. Hypersecretion of GHRH by breast tumors indicates that this peptide could play a role in maintaining epithelial cell proliferation as is the case for other peptides produced locally.

Somatostatin and growth hormone-releasing hormone in normal and tumoral human breast tissue: endogenous content, in vitro pulsatile release, and regulation.

Endogenous production of SRIH and GHRH was analyzed in human breast tissue. SRIH precursor (pro-SRIH) was identified after Sephadex G-50 filtration of acetic acid extracts of normal and tumoral human breast samples. SRIH-(1-14) or -(1-28) could not be detected in breast tissue, whereas the immunoreactive SRIH released in vitro was characterized as SRIH-(1-28). Endogenous production of GHRH was assessed by identification of GHRH messenger ribonucleic acid by PCR followed by sequencing of the amplified complementary DNA and by high performance liquid chromatographic characterization of immunoreactive GHRH contained in the tissue and released in vitro. There were no differences in pro-SRIH or GHRH-(1-44) tissue contents between normal and tumoral samples. The release of both peptides was evidenced in perifusion and static incubation. Perifusion of normal breast tissue (n = 3) showed pulsatile release of SRIH and GHRH. Perifusion of tumors (n = 4) showed SRIH release in 50% of the cases. SRIH release was pulsatile in one case. GHRH release was observed in the four tumoral samples analyzed, but was pulsatile in only one case. In static incubation, tumors (n = 6) secreted 13 times more GHRH than did normal samples (n = 3; 383 +/- 92 vs. 29.6 +/- 4.6 fmol/mg protein; P < 0.05). Stimulation of GHRH release by exogenous SRIH was observed only with the normal tissue. Together these data provide evidence for the existence of local production of SRIH and GHRH by human breast. Hypersecretion of GHRH by breast tumors indicates that this peptide could play a role in maintaining epithelial cell proliferation as is the case for other peptides produced locally.

Identification of growth hormone-releasing hormone and somatostatin neurons projecting to the median eminence in normal and growth hormone-deficient Ames dwarf mice.

In the spontaneous mutant Ames dwarf mouse, GH deficiency coincides with a dramatic increase in the expression of both mRNA and peptide for stimulatory GHRH and reduced expression of GH-inhibitory somatostatin (SRIH) mRNA and peptide. However, both GHRH and SRIH are markedly reduced in the dwarf median eminence (ME), suggesting that ME innervation by GHRH and SRIH neurons may be aberrant in the absence of GH. In order to test this hypothesis, the number of GHRH and SRIH ME-projecting neurons was evaluated in normal and dwarf mice using a combination of retrograde tract-tracing and neuron phenotype identification by immunocytochemistry (ICC). Adult animals were injected intraperitoneally with the fluorescent tract-tracer fluorogold (FG), which, in the brain, is taken up only by axons terminating in areas deprived of the blood-brain barrier, such as the ME. Visualization of FG was achieved by either UV illumination or ICC, and was combined as appropriate with fluorescence or bright-field ICC for GHRH or SRIH. Cells immunoreactive for GHRH or SRIH and labeled with FG were quantified at each 180-microns rostral-to-caudal level through the hypothalamus. As reported previously, the total number of hypophysiotropic GHRH neurons was markedly increased in dwarf compared with that in normal mice. However, a similar percentage of ME-innervating GHRH cells was estimated in dwarf (73 +/- 4%) and normal (76 +/- 3%) animals. Such a percentage in dwarfs thus represents a larger population of ME-projecting GHRH cells (749 +/- 53) than in normal mice (128 +/- 15). Increased numbers of FG-labeled GHRH neurons in dwarfs were located at the middle and posterior levels of the arcuate nucleus (2.08, 2.26 and 2.44 mm posterior to bregma). The percentage of FG-labeled SRIH neurons was also similar for dwarf (83 +/- 2%) and normal (87 +/- 2%) mice. Because the total SRIH-immunoreactive neuronal population in dwarfs is significantly reduced compared to that in normal animals, the similar FG-labeled percentage reflects a reduced number of SRIH cells projecting to ME in dwarf (1,376 +/- 104) compared with normal (3,192 +/- 267) mice. Fewer FG-labeled SRIH cells were found in dwarfs at every anterior-to-posterior level of the periventricular nucleus (p < 0.01 for comparisons at 0.28, 0.46, 0.64, and 1.0, and p < 0.05 for comparison at 1.18 mm posterior to the bregma). The present study indicates that the reduction in GHRH and SRIH immunoreactivity in the dwarf ME may result from different phenomena for each neuronal population. The reduction in GHRH immunostaining in the ME, despite a marked increase in the total ME-projecting GHRH neurons, may be interpreted as increased GHRH release, with consequent depletion of the ME stores. In contrast, the deficit in ME SRIH may be proportional to the deficit in the number of detectable SRIH periventricular nucleus neurons.

Desensitisation of somatostatin, TRH and GHRH responses to glucose in the diabetic (Goto-Kakizaki) rat hypothalamus.

We have studied the effects of glucose on the release of somatostatin (SS), TRH and GHRH from incubated hypothalami of normal and genetically diabetic, Goto-Kakizaki (GK) rats. The active isomer D-glucose caused a dose-related inhibition of SS, TRH and GHRH from normal rat hypothalami over a 20-min incubation period in vitro. In contrast, in GK rats the effects of glucose on TRH and SS were significantly reduced and the effects on GHRH were abolished. These data indicate that the sensitivity of SS-, TRH- and GHRH-producing hypothalamic neurones is reduced in diabetic rats. The effect is most pronounced for GHRH release as there was no change in the release of this peptide with increasing glucose concentrations. In conclusion, it appears that the diabetic state in GK rats causes differential desensitisation (GHRH > TRH and SS) of neuronal responses to subsequent changes in glucose concentrations in vitro. This may be due to alterations in the neurotransmitter control and/or a reduction in number, affinity or function of glucose transporters on these peptidergic neurones or other intermediary neuronal pathways.

Effect of intravenous or intracerebroventricular injections of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 on GH release in conscious, freely moving male rats.

The present study was designed to examine the effects of intracerebroventricular injection of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP) on GH secretion in freely moving conscious male rats and to determine whether the central action of GHRP is mediated by increased GHRH and/or somatostatin (SRIF) release. A significant and dose-related suppression of natural fluctuations in plasma GH occurred immediately after intracerebroventricular injection of GHRP at the doses of 1, 3 and 10 mu g/rat and was sustained for more than 1 h. An intracerebroventricular injection of GHRP also suppressed plasma GH rises after an intravenous injection of [D-Ala2, Nle27] human GHRH (1-28)agmatine (hGHRH analog), but the GH suppression by intracerebroventricular GHRP was significantly blunted in rats treated with antirat SRIF gamma-globulin (SRIF-ab). In addition, the suppression by intracerebroventricular GHRP of hGHRH-analog-induced GH release was observed even in rats treated with anti-rat GHRH goat gamma-globulin (GHRH-ab) which does not cross-react with hGHRH analog, and again its suppression was significantly attenuated by simultaneous treatment with SRIF-ab and GHRH-ab. Furthermore, intracerebroventricularly injected GHRP was effective enough to inhibit the central-nervous-system (endogenous GHRH)-driven natural GH-secretory surges in SRIF-ab-treated rats. When 10 mu g/kg GHRP was injected intravenously every 1 h during a 7-hour observation period in control rats without antibody treatment, plasma GH levels were increased with the peak 10 min after each injection during the surge period of the GH-secretory rhythm, but the plasma GH response to GHRP was frequently absent during its trough period. In addition, in SRIF-ab-treated rats, basal GH levels as well as plasma GH peaks after GHRP injections were markedly elevated and the obvious plasma GH rises after GHRP could be observed even during the trough period. In GHRH-ab-treated rats, basal GH levels were lower, and the plasma GH response to GHRP was significantly attenuated as compared with those in control rats. Plasma GH peaks after GHRP injections in rats simultaneously treated with SRIF-ab and GHRH-ab were higher than those in rats given GHRH-ab alone, but its difference in GH response was obviously smaller than that observed between SRIF-ab-treated and control rats, suggesting a crucial role of endogenous GHRH in GHRP-induced GH release. To further clarify the interaction of GHRH and GHRP in GH release, hGHRH analog was injected intravenously alone or simultaneously with GHRP in rats treated with SRIF-ab and GHRH-ab. The peak GH values after simultaneous injection of hGHRH analog and GHRP were significantly higher than the sum of GH peaks after each injection of hGHRH analog and GHRP. In conclusion, an intracerebroventricular injection of GHRP suppresses GH secretion by a central mechanism which is likely to include increased SRIF release, reduced GHRH release or both. We have confirmed that intravenous injection of GHRP can stimulate GH release by itself in an immunologically nullified condition of circulating endogenous GHRH and SRIF in rats, and that endogenous SRIF modestly suppresses the GH-stimulating effect of intravenously injected GHRP while both endogenous and exogenous GHRH markedly enhance GH release by intravenously injected GHRP in a synergistic manner.

Reproducibility and Comparison of Growth Hormone Secretion Tests

To assess the degree of reproducibility of spontaneous GH secretion and pharmacological tests we studied 15 prepubertal children with short stature and abnormal growth rate. In all children, spontaneous overnight GH secretion was measured followed by a clonidine test in 8 children and an arginine test in the remaining 7. The same protocol was repeated a week later. Intra-individual variability of GH secretion in both physiological and pharmacological tests was expressed as the coefficient of variation (CV%). No significant difference was found between the first and second value of parameters of spontaneous GH secretion. Maximum spontaneous GH peak (MS) and mean 12-h GH concentration (MGH) correlated significantly (r = 0.78, p < 0.001). Mean CV% of all parameters of repeated GH profiles (around 30%) were lower than those of provocative tests (around 70%) (p < 0.05). No significant difference was found between CV% of clonidine and arginine tests. There was no correlation between MGH or MS and GH response to provocative test in the same child. We found a significant correlation between the log transformed maximum provocative GH response to the arginine test and the length of the time interval (in min) from the end of the last GH peak in the previous profile to the time zero of the provocative test (r = 0.60, p < 0.05). This relationship was not found for the clonidine test. We conclude that spontaneous GH secretion in children with short stature is more reproducible than stimulated GH response with a week's difference. Perhaps the higher variability of provocative GH secretion may be related to the state of the endogenous hypothalamic rhythm of both GHRH and somatostatin release at the time of the test.

Influence of thyroid hormones on the regulation of growth hormone secretion.

The effects of thyroid hormones on GH secretion and the mechanisms underlying their action are very similar in man and the laboratory animal. We feel that it is possible to organize the available data into a unique pathophysiological model explaining these complex interactions (Table 1). In summary, physiological levels of circulating thyroid hormones are necessary to maintain normal pituitary GH secretion owing to their direct stimulatory actions. When the serum concentrations of thyroid hormone increase above the normal range there is an increase in hypothalamic somatostatin tone, which in turn suppresses pituitary GH secretion and overrides any stimulatory effects. The suppression of GH secretion by thyroid hormones may be mediated at the hypothalamic level also by a decrease in GHRH release.

Hypothalamic α2A-adrenoceptors stimulate growth hormone release in the rat

Oxymetazoline, the relatively selective α 2A-adrenoceptor agonist (with more than 60-fold selectivity over the α 2B-adrenoceptor subtype), was administered into the lateral ventricle (i.c.v.) of rats and plasma growth hormone (GH) levels were measured. Oxymetazoline was more potent to release GH after i.c.v. administration than was clonidine; 0.01 μg i.c.v. oxymetazoline already caused a significant release of GH, while at least 0.1 μg clonidine had to be administered to cause a similar response. The dose-response curve was of an inverted U shape since with 10 μg of oxymetazoline the plasma GH did not rise. When oxymetazoline was injected i.c.v. to rats with somatostatin fibres to the median eminence transected by an anterolateral cut in the hypothalamus there was a significant rise in plasma GH, suggesting that oxymetazoline stimulated GHRH rather than inhibited somatostatin release. Pretreatment with CH-38083 (7,8-(methylenedioxy)-14-α-hydroxy-alloberban HCl, selective for α 2-adrenoceptors but not differentiating between α 2A and α 2B subtypes), prevented the plasma GH rise normally elicited by 1 μg i.c.v. oxymetazoline. The α 2A- and α 1-selective adrenoceptor antagonist, WB-4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane hydrochloride), prevented the GH rise normally induced by oxymetazoline while prazosin, the α 2B- and α 1-selective adrenoceptor antagonist, prolonged the elevation occurring in the control rats between 30 and 60 min after oxymetazoline injection. Since both prazosin and WB-4101 are α 1-adrenoceptor antagonists but differ in their action on α 2A and α 2B subtypes as well as in their action on oxymetazoline-induced GH secretion, the antagonist studies suggest that oxymetazoline stimulates GH release through activation of α 2A-adrenoceptors stimulatory to GHRH release, and not by an action through α 2B- or α 2C- or α 1-adrenoceptors. Since WB-4101 also antagonized clonidine action on GH release we also suggest that the major component may be the stimulation of the α 2A-adrenoceptors in the clonidine action on GH release.

Relationship between hypophyseal portal GHRH and somatostatin and peripheral GH levels in the conscious sheep

The mechanisms involved in the genesis of pulsatile GH secretion are not well understood. Recently, methods for hypophyseal portal blood collection in conscious sheep became available. Using this method, GHRH and SRIH secretion into hypophyseal portal blood (HPB) and GH release from the pituitary gland were simultaneous assessed and the relationship between GHRH and SRIH changes in HPB and GH in peripheral blood was investigated. In 23 rams (9-11 month old, 35-45 kg bw), 126 hours of HPB were analysed. Fifty-four spontaneous GH peaks were detected. The majority of GH peaks (48.1%) was associated with an increased portal GHRH and a fall in somatostatin concentrations. A simultaneous increase in GHRH and somatostatin levels was observed in 18.5% of GH peaks while 12.9% of peaks occurred with a fall in SRIH and no modification in GHRH concentrations. Finally, 5/54 (9.3%) GH peaks occurred without any modification in portal GHRH and SRIH release. Our data indicate that the GHRH/SRIH interplay is complex. The occurrence of spontaneous GH peaks may be due not only to a coordinate increase in GHRH and reduction in SRIH release similar to male rat, but also to other patterns of GHRH/SRIH secretion.

Effects of metoclopramide on the growth hormone response to galanin in normal man.

One of the most prominent metabolic effects of the systemic administration of the synthetic neuropeptide galanin in man is the increase in growth hormone (GH) secretion. This stimulating action of galanin is thought to occur directly at the hypothalamic level through the release of GHRH. Recently, it has been shown that also dopaminergic drugs may elicit GH secretion through an increase in hypothalamic GHRH secretion. The aim of this study was to investigate if the action of galanin on GHRH and consequently on GH release may be mediated via dopaminergic pathways evaluating the effects of a potent central dopaminergic receptor blocker, metoclopramide (MCP), on the galanin-induced growth hormone (GH) secretion in normal subjects. We studied seven young non obese healthy subjects (three females and four males). GH secretion was evaluated after 45 min iv infusion of porcine galanin (0.5 mg in 100 ml of saline) from 0 to 45 min combined with a 60 min iv infusion of a) saline (100 ml) or b) MCP (10 mg in 100 ml of saline) from -15 to 45 min. In all the seven subjects, during galanin infusion, GH values increased with respect to baseline with peaks occurring between 30 and 60 min after the beginning of galanin infusion. Peak GH values ranged between 3.5 and 15.4 micrograms/l (mean 10.4 +/- 1.6 micrograms/l). During MCP infusion no significant differences in the GH response to galanin with respect to saline were observed both when absolute GH levels and GH AUC were examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neurotensin on growth hormone release in vivo

In order to investigate the mechanisms involved in the in vivo Growth Hormone (GH) response to Neurotensin (NT) we assessed the influence of estrogen status as well as the effect of passive immunization with antisomatostatin and anti-Growth Hormone-Releasing Hormone (GHRH) on NT-induced GH secretion in pentobarbital anesthetized rats. We found that, contrary to GH responses to GHRH, estrogen-treated rats (one single injection of 200 μg s.c. of estradiol valerate, 3 days before the experiment), exhibited markedly increased GH responses to different doses of NT (7.5, 15 and 30 μg/kg, i.v.). The stimulatory effect of NT (30 μ/kg) on estrogen-treated rats was similar in rats that received normal rabbit serum or passively immunized with antisomatostatin or anti-GHRH serum. In conclusion, estrogens play a facilitatory role on NT-induced GH release in the rat, which is exerted through a mechanism independent of hypothalamic GHRH or somatostatin release.

Hypothalamic regulation of growth hormone secretion during food deprivation in the rat

Suppressed pulsatile GH secretion in food-deprived rats has been hypothesized to be due to an increase in hypothalamic somatostatin secretion. We investigated this hypothesis and the role of GHRH in regulating GH secretion during food deprivation using two different models. In experiment one, rats were food deprived for 72h during which time they received a saline infusion (n=5). At the same time rats were normal fed for 72h during which time they received a somatostatin infusion (5 μg/h, n=7). After the 72h infusion period, all rats received two iv injections of GHRH (1 μg/rat) at 2h intervals. GH concentrations in food-deprived rats rose from approximately 10 ng/ml to 400–800 ng/ml in response to both GHRH injections. This increase was significantly greater (p<0.01) than the GH response (100–400 ng/ml) observed in somatostatin-infused animals. The significantly higher GH response observed in food-deprived rats as compared to somatostatin-infused, normal-fed rats suggests that somatostatin concentrations may decrease during food deprivation. In experiment two, rats were infused for 5h with either saline (n=6) or GHRH (10 μg/h, n=9) at the end of a 72h fast. GH concentrations did not change in saline-infused animals. In contrast, GH concentrations significantly increased (p<0.01) upon initiation of the continuous GHRH infusion. Yet, this release of GH was pulsatile in nature. Pulsatile GH secretion in the presence of a constant GHRH infusion suggests that pulsatile somatostatin release from the hypothalamus is maintained during food deprivation. These studies suggest that during food deprivation in the rat 1) absolute concentrations of somatostatin decrease, but its pattern of secretion remains pulsatile, and 2) decreased GHRH release may be responsible for the absence of spontaneous GH pulses.

Neuroendocrine control of growth hormone secretion in humans

The episodic secretion of growth hormone (GH) depends on the rhythmic alternation in the hypothalamic release of GHRH and somatostatin (SS) into the hypophyseal portal system. In turn, GH appears to maintain this rhythm by stimulating SS and inhibiting GHRH secretion. Central adrenergic pathways, by modulating SS secretion, seem to be the final mediator for most stimuli, including other neurotransmitters, modifying GH release. Glucocorticoids enhance GH gene transcription and facilitate GH gene expression; the latter effect also depends on physiologic plasma levels of thyroid hormones. Sex steroids mainly act on SS neurons, most likely by affecting the α 2-adrenergic transmission to them. Metabolic intermediates can also affect GH secretion: arginine and hypoglycemia inhibit SS release, whereas hyperglycemia and free fatty acids (FFA) stimulate it. In addition, a strong inhibitory effect of FFA on the somatotrophs also occurs.

Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men

Serum GH concentrations are increased in fasted or malnourished human subjects. We investigated the dynamic mechanisms underlying this phenomenon in nine normal men by analyzing serum GH concentrations measured in blood obtained at 5-min intervals over 24 h on a control (fed) day and on the second day of a fast with a multiple-parameter deconvolution method to simultaneously resolve endogenous GH secretory and clearance rates. Two days of fasting induced a 5-fold increase in the 24-h endogenous GH production rate [78 +/- 12 vs. 371 +/- 57 micrograms/Lv (Lv, liter of distribution volume) or 0.24 +/- 0.038 vs. 1.1 +/- 0.16 mg/m2 (assuming a distribution volume of 7.9% body weight), P = 0.0001]. This enhanced GH production rate was accounted for by 2-fold increases in the number of GH secretory bursts per 24 h (14 +/- 2.3 vs. 32 +/- 2.4, P = 0.0006) and the mass of GH secreted per burst (6.3 +/- 1.2 vs. 11 +/- 1.6 micrograms/Lv, P = 0.002). The latter was a result of increased secretory-event amplitudes (maximal rates of GH release attained within a burst) with unchanged secretory burst durations. GH was secreted in complex volleys composed of multiple discrete secretory bursts. These secretory volleys were separated by shorter intervals of secretory quiescence in the fasted than fed state (respectively, 88 +/- 4.2 vs. 143 +/- 14 min, P = 0.0001). Similarly, within volleys of GH release, constituent individual secretory bursts occurred more frequently during the fast [every 33 +/- 0.64 (fasted) vs. every 44 +/- 2.0 min (fed), P = 0.0001]. The t1/2 of endogenous GH was not significantly altered by fasting [18 +/- 2.2 (fasted) vs. 20 +/- 1.5 min (fed), P = 0.47]. Serum insulin-like growth factor I concentrations were unchanged after 56 h of fasting. In conclusion, the present data suggest that starvation-induced enhancement of GH secretion is mediated by an increased frequency of GHRH release, and longer and more pronounced periods of somatostatin withdrawal.

Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men.

Serum GH concentrations are increased in fasted or malnourished human subjects. We investigated the dynamic mechanisms underlying this phenomenon in nine normal men by analyzing serum GH concentrations measured in blood obtained at 5-min intervals over 24 h on a control (fed) day and on the second day of a fast with a multiple-parameter deconvolution method to simultaneously resolve endogenous GH secretory and clearance rates. Two days of fasting induced a 5-fold increase in the 24-h endogenous GH production rate [78 +/- 12 vs. 371 +/- 57 micrograms/Lv (Lv, liter of distribution volume) or 0.24 +/- 0.038 vs. 1.1 +/- 0.16 mg/m2 (assuming a distribution volume of 7.9% body weight), P = 0.0001]. This enhanced GH production rate was accounted for by 2-fold increases in the number of GH secretory bursts per 24 h (14 +/- 2.3 vs. 32 +/- 2.4, P = 0.0006) and the mass of GH secreted per burst (6.3 +/- 1.2 vs. 11 +/- 1.6 micrograms/Lv, P = 0.002). The latter was a result of increased secretory-event amplitudes (maximal rates of GH release attained within a burst) with unchanged secretory burst durations. GH was secreted in complex volleys composed of multiple discrete secretory bursts. These secretory volleys were separated by shorter intervals of secretory quiescence in the fasted than fed state (respectively, 88 +/- 4.2 vs. 143 +/- 14 min, P = 0.0001). Similarly, within volleys of GH release, constituent individual secretory bursts occurred more frequently during the fast [every 33 +/- 0.64 (fasted) vs. every 44 +/- 2.0 min (fed), P = 0.0001]. The t1/2 of endogenous GH was not significantly altered by fasting [18 +/- 2.2 (fasted) vs. 20 +/- 1.5 min (fed), P = 0.47]. Serum insulin-like growth factor I concentrations were unchanged after 56 h of fasting. In conclusion, the present data suggest that starvation-induced enhancement of GH secretion is mediated by an increased frequency of GHRH release, and longer and more pronounced periods of somatostatin withdrawal.

Autocrine and paracrine effects of peptides on human pituitary cells.

In contrast to normal human pituitaries, GH-secreting adenomas cannot process in vivo ProSRIH whereas they do it in vitro. The existence of an endogenous factor able to inhibit ProSRIH processing in vivo was postulated and such a role was analyzed for GHRH. Results showed that when GH adenomas are incubated in vitro with GHRH 10(-8) M, their ProSRIH contents are decreased, percent inhibition being negatively correlated to the amount of endogenously released GHRH. When incubation is performed in the presence of GHRH antibody in order to block the effect of endogenous GHRH, Pro-SRIH content is increased. The same effects are observed on SRIH release: inhibition by GHRH, stimulation by GHRH antibody. Normal rabbit serum had no effect. It may therefore be concluded that the absence of ProSRIH maturation observed in adenomas in vivo may be the consequence of the GHRH release that is known to be higher from GH adenomas than from normal pituitaries.

Interleukin-βModulates the Acute Release of Growth Hormone-Releasing Hormone and Somatostatin from Rat Hypothalamusin Vitro, Whereas Tumor Necrosis Factor and Interleukin-6 Have No Effect

There is clear evidence for communication between the immune and neuroendocrine systems. However, the effect of cytokines as major immune mediators on the hypothalamic growth peptides, GHRH and somatostatin (SRIH), is not well established. To investigate a possible hypothalamic action of the cytokines interleukin-1 beta (IL-1), interleukin-6, and tumor necrosis factor-alpha, on the release of GHRH and SRIH, we used a previously validated acute rat hypothalamic explant system. IL-1 caused a pronounced dose-dependent stimulation of SRIH in the dose-range 1-100 U/ml (P less than 0.01). GHRH showed a slight, but significant, increase in response to IL-1 tested in the dose-range 10-100 U/ml. Similar studies with mediobasal hypothalamic (GHRH and SRIH) or median eminence (SRIH) fragments produced no change in either GHRH or SRIH release. The effects of IL-1 were antagonized by the cyclo-oxygenase inhibitor, indomethacin (10 micrograms/ml). Stimulation of GHRH and SRIH could not be blocked by the CRH-antagonist alpha-helical CRH (9-41) at 10(-6) M. Interleukin-6, in the dose range 10-100 U/ml, and tumor necrosis factor-alpha, in the dose range 10-10,000 U/ml, had no effect on the acute hypothalamic release of either GHRH or SRIH. It is concluded that IL-1 stimulates the acute hypothalamic release of GHRH and SRIH, and that this effect is mediated by cyclo-oxygenase products. The marked IL-1 stimulation of hypothalamic SRIH release may override the minor increase of GHRH increase, and may thus contribute to disturbances in growth seen in the presence of chronic inflammation.