Assignment of the human growth hormone-releasing hormone receptor gene (GHRHR) to 7p14 by in situ hybridization.
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Animals↗
Biomedical subjects
M O Thorner
Publications and source records attributed to M O Thorner.
Growth hormone and erythropoietin differentially activate DNA-binding proteins by tyrosine phosphorylation.
Binding of growth hormone (GH) and erythropoietin (EPO) to their respective receptors results in receptor clustering and activation of tyrosine kinases that initiate a cascade of events resulting not only in the rapid tyrosine phosphorylation of several proteins but also in the induction of early-response genes. In this report, we show that GH and EPO induce the tyrosine phosphorylation of cellular proteins with molecular masses of 93 kDa and of 91 and 84 kDa, respectively, and that these proteins form DNA-binding complexes which recognize an enhancer that has features in common with several rapidly induced genes such as c-fos. Assembly of the protein complexes required tyrosine phosphorylation, which occurred within minutes after addition of ligand. The activated complexes translocated from the cytoplasm to the nucleus. The protein activated by GH is antigenically similar to p91, a protein common to several transcription complexes that are activated by interferons and other cytokines. In contrast, the proteins activated by EPO are distinct from p91. These findings establish the outlines for a cytokine-induced intracellular signaling pathway, which begins with ligand-induced receptor clustering that activates one or more tyrosine kinases. These data are the first to demonstrate that GH- and EPO-activated tyrosine-phosphorylated proteins can specifically recognize a well-defined enhancer and therefore provide a mechanism for rapidly transducing signals from the membrane to the nucleus.
Base Sequence↗
Enhanced basal and disorderly growth hormone secretion distinguish acromegalic from normal pulsatile growth hormone release.
Pulses of growth hormone (GH) release in acromegaly may arise from hypothalamic regulation or from random events intrinsic to adenomatous tissue. To distinguish between these possibilities, serum GH concentrations were measured at 5-min intervals for 24 h in acromegalic men and women with active (n = 19) and inactive (n = 9) disease and in normal young adults in the fed (n = 20) and fasted (n = 16) states. Daily GH secretion rates, calculated by deconvolution analysis, were greater in patients with active acromegaly than in fed (P < 0.05) but not fasted normal subjects. Significant basal (nonpulsatile) GH secretion was present in virtually all active acromegalics but not those in remission or in fed and fasted normal subjects. A recently introduced scale- and model-independent statistic, approximate entropy (ApEn), was used to test for regularity (orderliness) in the GH data. All but one acromegalic had ApEn values greater than the absolute range in normal subjects, indicating reduced orderliness of GH release; ApEn distinguished acromegalic from normal GH secretion (fed, P < 10(-12); fasted, P < 10(-7)) with high sensitivity (95%) and specificity (100%). Acromegalics in remission had ApEn scores larger than those of normal subjects (P < 0.0001) but smaller than those of active acromegalics (P < 0.001). The coefficient of variation of successive incremental changes in GH concentrations was significantly lower in acromegalics than in normal subjects (P < 0.001). Fourier analysis in acromegalics revealed reduced fractional amplitudes compared to normal subjects (P < 0.05). We conclude that GH secretion in acromegaly is highly irregular with disorderly release accompanying significant basal secretion.
Acromegaly↗
Photoaffinity cross-linking to the pituitary receptor for growth hormone-releasing factor.
Photoaffinity cross-linking methods presented here demonstrate a 55-kilodalton (kDa) GH-releasing factor (GRF) receptor in ovine pituitary membranes and in cell lines expressing the cloned human pituitary receptor complementary DNA. Covalent cross-linking of photoprobe to this high affinity site is strongly competed by 1 nM GRF. Competition shows strong specificity for GRF over related peptides. Reduced cross-linking in the presence of guanosine 5'-O-(3-thiotriphosphate) suggests that this is a G-protein-coupled receptor. Detection of cross-linking to this receptor required detergent extraction to reduce high nonspecific binding of GRF photoprobe. Partial deglycosylation of the cross-linked receptor with neuraminidase caused a shift in apparent size to 52 kDa. Complete deglycosylation with N-glycosidase caused a shift to 45 kDa, demonstrating that this receptor is an N-linked glycoprotein and agreeing with the protein size and single glycosylation site predicted from the cloned complementary DNA sequence. These sizes differ from those found in previous reports which used chemical cross-linking to identify GRF receptor. This photoaffinity cross-linking method will facilitate studies of receptor function and tissue distribution. Photoaffinity cross-linking can also be used to map regions of the receptor molecule and bound GRF that are in close proximity.
Animals↗
Enhanced sensitivity growth hormone (GH) chemiluminescence assay reveals lower postglucose nadir GH concentrations in men than women.
Modifications were made to a commercially available human (h) GH chemiluminescence assay (Nichols Luma Tag hGH assay), which improved its sensitivity to 0.002 micrograms/L. The results of this assay had a high correlation with those of the Nichols hGH immunoradiometric assay (IRMA; r = 0.91; P < 0.001). The addition of recombinant hGH-binding protein (0.1-10 nmol/L) to standards and serum samples caused a dose-responsive reduction in measured GH in both the chemiluminescence assay and the IRMA; at physiological concentrations of hGH-binding protein, a 10-20% reduction was observed. Fifteen normal young adults (nine men and six women) underwent a standard 100-g oral glucose tolerance test, and plasma GH was measured from 30 min before until 5 h after glucose ingestion. GH was measurable in all samples with the chemiluminescence assay, but fell below the sensitivity of the IRMA in 59% of the samples. There was no difference between baseline or peak glucose levels in male and female subjects, but serum GH concentrations (mean +/- SD) measured by the enhanced sensitivity chemiluminescence assay were lower in male than female subjects at both baseline (0.12 +/- 0.08 vs. 2.3 +/- 2.3 micrograms/L; P < 0.01) and the postglucose GH nadir (0.029 +/- 0.014 vs. 0.25 +/- 0.23 micrograms/L; P < 0.01). The high correlation between baseline and nadir GH (r = 0.82; P < 0.001) and the equivalent fractional decline in mean GH levels in men and women after glucose administration (67 +/- 17% vs. 84 +/- 8%; P = 0.06) suggest that the lower GH levels in men after glucose treatment are due to lower baseline values and not to a greater suppressive effect of glucose.
Adolescent↗
Neuroendocrine responses to a novel growth hormone secretagogue, L-692,429, in healthy older subjects.
L-692,429 (L), a novel nonpeptide mimic of GH-releasing peptide (GHRP), is a potent GH secretagogue in animals and young men. To assess the safety and efficacy of L in stimulating GH release in healthy older men and women, 16 subjects were admitted to a randomized, double blind, cross-over comparison of i.v. administered placebo, GH-releasing hormone [GHRH-(1-29)-NH2; 1 microgram/kg] and two doses of L (0.2 and 0.75 mg/kg). Blood samples were obtained at 5-min intervals for 60 min before and 240 min after each dose for measurement of GH; cortisol, PRL, and insulin-like growth factor-I (IGF-I) were measured less frequently. Peak and integrated GH concentrations increased significantly after L in a dose-dependent manner. Responses to L at either dose were significantly greater than the response to GHRH: peak GH responses in older men and women were (mean +/- SE; micrograms per L): after placebo, 1.2 +/- 0.2; L (0.2 mg/kg), 16.5 +/- 1.8; L (0.75 mg/kg), 32.2 +/- 3.9; and GHRH, 7.6 +/- 1.3 (P < 0.05, L vs. placebo or GHRH). Serum cortisol and PRL concentrations increased after both doses of L, but to values within the respective normal ranges. Serum IGF-I values did not change consistently in any group. The GH responses to GHRH and L (0.75 mg/kg) were highly correlated (r2 = 0.61; P < 0.0004). Deconvolution analysis demonstrated that the increase in serum GH concentrations stimulated by L and GHRH resulted from enhanced GH secretion rates, with no change in the half-life of GH disappearance. Amplitudes of GH secretory pulses were increased 11-, 18-, and 4-fold after L (0.2 mg/kg), L (0.75 mg/kg), and GHRH treatments, respectively. The number of GH secretory pulses was significantly increased by L (0.75 mg/kg; 4.6 +/- 0.4) and GHRH (4.4 +/- 0.3) compared to placebo (2.6 +/- 0.5), but the interval between pulses was shorter after L (0.75 mg/kg; 28.6 +/- 3.6 min) than after GHRH (50.7 +/- 7.7 min; P < 0.05). Adverse experiences were limited to brief episodes of flushing or a warm sensation about the upper body. L-692,429 is a potent GH secretagogue that is well tolerated in healthy older men and women. At the doses employed in this study, L elicited greater increases in GH secretion rates and serum GH concentrations than GHRH. L-692,429 may have therapeutic advantages over peptide GH secretagogues to restore endogenous GH secretion in GH deficiency states or the hyposomatotropism of aging.
Aged↗
Growth hormone induces tyrosine phosphorylation but does not alter insulin-like growth factor-I gene expression in human IM9 lymphocytes.
GH induces hepatic IGF-I synthesis by increasing transcription of its gene. IGF-I is synthesized, however, in many other tissues where the effect of GH on its gene expression is less well characterized. IGF-I and GH are produced by human lymphocytes and may function as autocrine regulators of lymphoproliferation. We have therefore used the human IM9 lymphocyte cell line to (A) define the IGF-I gene transcripts expressed and (B) investigate the effect of GH on early (protein tyrosine phosphorylation) and late (changes in IGF-I mRNA levels) events in intracellular signal transduction. Multiple IGF-I mRNA species, ranging in size from 0.9 to 5.8 kb, were detected by Northern hybridization of poly(A)+ mRNA from IM9 cells. The human IGF-I gene contains at least six exons and alternative splicing produces a number of transcripts. Solution hybridization with exon-specific riboprobes and amplification by PCR using exon-specific primers revealed that multiple transcripts were expressed in IM9 cells, and that exon 2 was the dominant leader exon. Treatment of IM9 cells with 200 ng recombinant human (rh)GH/ml led to the specific tyrosine phosphorylation of three intracellular proteins (93, 120 and 134 kDa), which are involved in the initial signalling of the GH transduction pathway. However a solution hybridization assay using the IGF-IA specific riboprobe on IM9 cell RNA from similar experiments revealed that GH treatment did not change IGF-I gene expression. This study has demonstrated (A) that the IGF-I gene is expressed in human IM9 lymphocytes, (B) that in contrast to other human tissue, exon 2 is the major leader exon, and (C) that rhGH induces tyrosine phosphorylation of 93, 120 and 134 kDa proteins but does not alter IGF-I gene expression. The IM9 cell may form an important model to investigate a GH transduction pathway not coupled to the IGF-I gene.
Cell Line↗
The somatotrophic axis and the reproductive process in health and disease.
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Female↗
Sleep-promoting effects of growth hormone-releasing hormone in normal men.
Growth hormone-releasing hormone (GHRH) promotes rapid-eye-movement (REM) and non-REM sleep in animals, but there is little direct evidence for a hypnogenic action of GHRH in humans. In the present study, the possible somnogenic effects of intravenous bolus injections of a dose of GHRH eliciting physiological elevations of GH secretion in healthy young men were investigated. GHRH (0.3 micrograms/kg body wt) was given in early sleep [i.e., 1st slow-wave (SW) period], late sleep (i.e., 3rd REM period), and early sleep after sleep deprivation until 0400 h (i.e., 1st SW period). In the absence of sleep deprivation, injection of GHRH in early sleep did not modify SW sleep but increased REM sleep. GHRH administration in the third REM period was followed by a marked decrease of wake and an almost 10-fold increase in SW sleep. When GHRH was administered during the first SW period after sleep deprivation until 0400 h, the duration of wake decreased. Thus GHRH has sleep-promoting effects in young adults, particularly when given at a time of decreased sleep propensity.
Adult↗
Normal control of growth hormone secretion.
Growth hormone (GH) is secreted by the anterior pituitary gland in a pulsatile fashion under the regulation of two hypothalamic peptides: GH-releasing hormone (GHRH) stimulates GH synthesis and secretion while somatostatin inhibits GH release. Studies in rats, sheep and humans indicate that whereas GHRH is required for the initiation of GH pulses, the amplitude of GH pulses is modulated by somatostatin. In humans, these interactions result in a pattern of volleys of GH-secretory pulses with intervening periods of relative secretory quiescence. The amplitude and frequency of GH-secretory pulses are regulated by a complex array of external and internal stimuli including age, gender, menstrual cycle phase, pubertal status, nutrition, sleep, body composition and exercise. Changes in plasma concentrations of gonadal hormones, insulin and insulin-like growth factor-I likely mediate the effects of several of these factors. A greater understanding of the physiology of GH secretion will enable the development of future strategies to enhance GH secretion in GH-deficient states including the use of GH secretagogues and modification of nutrition and exercise habits.
Aging↗
A low dose euglycemic infusion of recombinant human insulin-like growth factor I rapidly suppresses fasting-enhanced pulsatile growth hormone secretion in humans.
To determine if insulin-like growth factor I (IGF-I) inhibits pulsatile growth hormone (GH) secretion in man, recombinant human IGF-I (rhIGF-I) was infused for 6 h at 10 micrograms.kg-1.h-1 during a euglycemic clamp in 10 normal men who were fasted for 32 h to enhance GH secretion. Saline alone was infused during an otherwise identical second admission as a control. As a result of rhIGF-I infusion, total and free IGF-I concentrations increased three- and fourfold, respectively. Mean GH concentrations fell from 6.3 +/- 1.6 to 0.59 +/- 0.07 micrograms/liter after 120 min. GH secretion rates, calculated by a deconvolution algorithm, decreased with a t 1/2 of 16.6 min and remained suppressed thereafter. Suppression of GH secretion rates occurred within 60 min when total and free IGF-I concentrations were 1.6-fold and 2-fold above baseline levels, respectively, and while glucose infusion rates were < 1 mumol.kg-1.min-1. During saline infusion, GH secretion rates remained elevated. Infusion of rhIGF-I decreased the mass of GH secreted per pulse by 84% (P < 0.01) and the number of detectable GH secretory pulses by 32% (P < 0.05). Plasma insulin and glucagon decreased to nearly undetectable levels after 60 min of rhIGF-I. Serum free fatty acids, beta-hydroxybutyrate, and acetoacetate were unaffected during the first 3 h of rhIGF-I but decreased thereafter to 52, 32, and 50% of levels observed during saline. We conclude that fasting-enhanced GH secretion is rapidly suppressed by a low-dose euglycemic infusion of rhIGF-I. This effect of rhIGF-I is likely mediated through IGF-I receptors independently of its insulin-like metabolic actions.
3-Hydroxybutyric Acid↗
Stimulation of tyrosine phosphorylation in human cells by activation of the growth hormone receptor.
To investigate the possibility that tyrosine phosphorylation of cellular proteins might play a role in GH receptor signaling, we have studied tyrosine phosphorylation induced by GH and by GH mutants in the human lymphocyte line IM-9, a homologous cell system which is known to respond to GH by increased proliferation. IM-9 cells were treated with physiological concentrations of recombinant human GH (rhGH). Protein lysates from these cells were then analyzed by polyacrylamide gel electrophoresis, transferred to nitrocellulose, and probed with an antibody specific for phosphotyrosine. rhGH stimulated the tyrosine phosphorylation of two proteins having M(r) of approximately 93,000 and 120,000. Tyrosine phosphorylation of these proteins was time and dose dependent. At 2 nM rhGH tyrosine phosphorylation of these two proteins was evident by 5 min, maximal at 15 min, and decreasing by 45 min of treatment. At doses of 10 and 100 nM rhGH, tyrosine phosphorylation was stimulated by 1 min of GH treatment. IM-9 cells were also treated with genetically engineered mutant forms of the GH protein. Previous biophysical analysis of these mutant GH proteins has shown that the GH protein contains two distinct binding sites which interact in a sequential manner with the extracellular domains of two distinct GH receptor molecules, thus forming a dimeric complex. By treating IM-9 cells with these same GH mutants and analyzing tyrosine phosphorylation, we found that tyrosine phosphorylation was inhibited under conditions which prevent receptor dimerization, thus providing evidence that formation of a dimeric GH:(GH receptor)2 complex may be important for intracellular signaling by the GH receptor.
Binding Sites↗
Human growth hormone (GH) receptor is characterized as the 134-kilodalton tyrosine-phosphorylated protein activated by GH treatment in IM-9 cells.
Activation of the GH receptor (GHR) results in tyrosine phosphorylation of cellular proteins, and this tyrosine phosphorylation is believed to be important in GH action. We have shown previously that GH rapidly stimulates the tyrosine phosphorylation of 134-, 120-, and 93-kilodalton (kDa) proteins in the human IM-9 lymphocyte line. We now provide three lines of evidence indicating that the 134-kDa tyrosine-phosphorylated protein, not the 120- or 93-kDa proteins, is the GHR. 1) A monoclonal antibody that interacts with the extracellular domain of the GHR (Mab263) immunoprecipitated a 134-kDa phosphotyrosine-containing protein, but not a 120- or 93-kDa protein, from GH-treated IM-9 cells. 2) The GHR contains N-linked carbohydrates. When total cell lysates from GH-treated IM-9 cells were treated with endoglycosidases to remove these carbohydrates, the majority of the 134-kDa phosphotyrosine-containing protein was no longer detected. Furthermore, the immunoprecipitated 134-kDa protein was completely deglycosylated and resulted in a single band of approximately 100 kDa. Neither the 120- nor 93-kDa tyrosine-phosphorylated proteins were affected by endoglycosidase treatment. 3) The Mab263 antibody immunoprecipitated a 134-kDa phosphotyrosine-containing protein from GH-treated 293 cells (human embryonic kidney cell line) that stably express the full-length rabbit GHR. This protein was not detected in control cells expressing the neomycin resistance gene alone. We conclude that the 134-kDa protein that is tyrosine phosphorylated upon GH stimulation of IM-9 cells is the GHR.
Antibodies, Monoclonal↗
Amplitude suppression of the pulsatile mode of immunoradiometric luteinizing hormone release in fasting-induced hypoandrogenemia in normal men.
In the male rodent and primate, fasting or severe caloric restriction significantly decreases serum testosterone concentrations, putatively via inducing secondary hypogonadotrophism. To clarify this presumptive pathophysiology, we have used: 1) a high sensitivity immunoradiometric assay, which correlates well with an in vitro Leydig cell bioassay of LH; 2) blood sampling every 5 min for 24 h basally and every 10 min for 3 h after GnRH injection before and after a 5-day (water only) fast in eight healthy young men; and 3) deconvolution analysis to evaluate in vivo LH secretory burst frequency, amplitude, duration, and mass, and LH half-life simultaneously. We documented a 50% fall in serum total and free testosterone concentrations, and a 30% decrease in 24-h mean serum LH concentrations (viz., fed 3.0 +/- 0.47 vs. fasted 2.1 +/- 0.39 U/L, P = 0.043). Deconvolution analysis revealed preservation of LH secretory pulse frequency (fed 12.9 +/- 0.48 vs. fasted 12.6 +/- 0.78 secretory bursts/day, P = NS) during fasting-induced hypogonadotropism. The duration of computer-resolved LH secretory bursts, the interburst interval, and the calculated endogenous half-life of LH also did not change, whereas LH secretory burst mass declined significantly; viz. from 28 +/- 5 in the fed to 14 +/- 3.2 U/L of distribution volume/day in the fasted state (P = 0.034). In contrast, LH release after a 10 micrograms pulse of GnRH iv was enhanced during fasting in seven of the eight men. Fasting also decreased mean (24 h) serum TSH and PRL, increased cortisol, dehydroepiandrosterone sulfate and GH, and did not affect FSH concentrations or the radioiodinated albumen distribution space. In summary, in young men 5 days of nutrient deprivation selectively attenuates the mass of LH secreted per burst without altering LH secretory event frequency or LH half-life. We infer that decreased LH release per burst is due to decreased hypothalamic GnRH impulse strength, since LH release induced by a submaximally effective pulse of exogenous GnRH is amplified rather than attenuated.
Adult↗
Twenty-four-hour growth hormone (GH)-releasing peptide (GHRP) infusion enhances pulsatile GH secretion and specifically attenuates the response to a subsequent GHRP bolus.
GH-releasing peptide (GHRP; SK&F 110679) is a synthetic hexapeptide that specifically stimulates GH release through nonopiate non-GH-releasing hormone (non-GHRH) receptors. To determine the effects of a 24-h GHRP infusion, eight normal young men received infusions of saline for 2 h, then saline (on two occasions) or GHRP (1.0 micrograms/kg.h; on two occasions) for 24 h, followed by an iv bolus of GHRP or GHRH (1.0 micrograms/kg) and a 2.5-h saline infusion. Serum GH was measured every 10 min throughout the 28.5-h period. GH secretion rates [per L distribution volume (Lv)] were determined by deconvolution analysis; attributes of pulsatile GH release were assessed by Cluster analysis. GH secretion was enhanced and remained pulsatile during GHRP infusions. The two GHRP infusions increased GH secretion rates (micrograms per Lv/h) 8-fold compared to saline (GHRP, 12 +/- 2.1 and 12 +/- 2.2; saline, 1.5 +/- 0.34 and 1.4 +/- 0.27; P < 0.05). The number of GH pulses, pulse duration and height, incremental pulse amplitude, interpeak valley concentration, and individual pulse areas were significantly greater during GHRP infusions than during saline treatment. Attributes of pulsatile GH release on the two GHRP infusion days were significantly correlated, indicating that enhancement of GH secretion by GHRP is highly reproducible. Mean plasma insulin-like growth factor-I (IGF-I) concentrations increased 12% and 22% on GHRP infusion days, whereas IGF-I levels declined 18% and 20% during saline infusions (P < 0.05). GHRP infusion significantly attenuated the GH response to a subsequent GHRP bolus injection; both GH secretion rates (GHRP, 4.1 +/- 1.6; saline, 19 +/- 3.0 micrograms/Lv.h; P < 0.05) and peak GH concentrations (GHRP, 7.9 +/- 2.9; saline, 25 +/- 2.9 micrograms/L; P < 0.05) were decreased. In contrast, peak GH concentrations in response to GHRH were significantly increased after GHRP infusion compared to those after saline treatment (24 +/- 4.7 vs. 11 +/- 2.7 micrograms/L; P < 0.05). We conclude that 24-h GHRP infusions augment pulsatile GH release and increase plasma IGF-I concentrations without significant adverse effects. Attenuation of the GH response to a subsequent GHRP bolus is not caused by depletion of pituitary GH, since the response to a GHRH bolus was enhanced by prior infusion of GHRP.
Adult↗
Molecular cloning and expression of a human anterior pituitary receptor for growth hormone-releasing hormone.
GH-releasing hormone (GHRH), acting through the GHRH receptor (GHRH-R), plays a pivotal role in the regulation of GH synthesis and secretion in the pituitary. It is possible that GHRH may serve other roles in other tissues. Here we report the cloning of a cDNA encoding a human GHRH-R from an acromegalic pituitary cDNA library. The isolated cDNA encodes a 423-amino acid protein that has seven putative transmembrane domains characteristic of G-protein-coupled receptors. It is a member of the secretin family of G-protein-coupled receptors and has 47%, 42%, 35%, and 28% identity with receptors for vasoactive intestinal peptide, secretin, calcitonin, and PTH, respectively. Transient expression of this cDNA in COS cells induced saturable, high affinity, GHRH-specific binding and also stimulated intracellular cAMP accumulation in response to physiological concentrations of GHRH. A specific GHRH antagonist blocked both binding and second messenger response. Northern analysis indicated that GHRH-R mRNA was most abundant in extracts of pituitary and was not detected in other tissues.
Acromegaly↗
On the discovery of growth hormone-releasing hormone.
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Amino Acid Sequence↗
Pulsatile growth hormone release in normal women during the menstrual cycle.
OBJECTIVE: We sought to characterize pulsatile growth hormone (GH) release in normal women during the menstrual cycle and to document possible relationships between such characteristics and concentrations of 17 beta-oestradiol and progesterone. SUBJECTS: Fifteen women with ostensibly normal menstrual function were studied during the early follicular phase, 15 during the late follicular phase and 15 during the mid-luteal phase of the menstrual cycle. DESIGN: The phase of the menstrual cycle having been documented, blood samples were obtained from each woman every 10 minutes for 24 hours. MEASUREMENTS: Serum GH was measured in each sample by immunoradiometric assay. Pulsatile GH release was appraised utilizing the objective, statistically-based pulse detection algorithm Cluster. RESULTS: The mean (+/- SEM) integrated serum GH concentration (mU/l min) in late follicular phase women (5335 +/- 848) was higher than that observed in early follicular phase women (3156 +/- 322; P = 0.032). The integrated GH concentration calculated for mid-luteal phase women (3853 +/- 788) was intermediate between but not statistically different from that observed in early follicular (P = 0.48) and late follicular (P = 0.14) phase women. No differences in GH pulse frequency (pulses/24 hours) were found among early follicular (8.27 +/- 0.55), late follicular (7.93 +/- 0.91) or mid-luteal (8.47 +/- 0.66) phase women. Mean maximal GH pulse amplitude (mU/l) was higher in late follicular phase (8.93 +/- 1.00) than early follicular phase (5.74 +/- 0.67; P = 0.008) and mid-luteal phase (5.76 +/- 0.74; P = 0.008) women. Similarly, incremental GH pulse amplitude (mU/l) was higher in late follicular phase (7.33 +/- 0.83) than early follicular phase (4.68 +/- 0.58; P = 0.005) and mid-luteal phase (4.36 +/- 0.39; P = 0.002) women. No differences in mean pulse widths or in the interpeak valley mean GH concentrations were found among the groups. Multiple regression of each pulse parameter against serum concentrations of testosterone, 17 beta-oestradiol and progesterone revealed a significant (P = 0.045) positive correlation between maximum GH pulse amplitude and oestradiol and a significant (P = 0.04) negative correlation between maximal GH pulse amplitude and progesterone (r = 0.41). CONCLUSION: These results suggest that late follicular phase concentrations of oestradiol may enhance circulating GH via an amplitude-modulated rather than a frequency-modulated effect on the endogenous GH pulse. Progesterone may blunt this oestrogen-associated effect, thus resulting in the observed mid-luteal phase concentrations of GH. Whether these gonadal hormones act primarily at the hypothalamus and/or anterior pituitary gland remains to be clarified, but the present observations indicate that pulsatile GH release throughout the normal menstrual cycle is significantly amplitude regulated.
Adult↗