Growth hormone (GH) autofeedback on GH response to GH-releasing hormone. Role of free fatty acids and somatostatin

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Diagnosing growth hormone deficiency in adults

Adult growth hormone (GH) deficiency is a recognised syndrome associated with adverse phenotypic, metabolic, and quality-of-life features which improve in many patients when GH is substituted. The appropriate selection of patients at risk of growth hormone deficiency (GHD) is the crucial first step in arriving at a correct diagnosis. Although multiple diagnostic modalities are available including a 24-hour serum GH profile, stimulated GH levels, and insulin-like growth factor-1 (IGF-1) levels, the use of dynamic tests for GH reserves is required in most cases. This paper discusses the utility and drawbacks of the various testing modalities with reference to international guidelines. Regardless of the test chosen, clinical pitfalls including age and obesity must be taken into account. In addition, there is considerable analytical variation in the biochemical measurements of GH and IGF-1 which must be considered before making a diagnosis of GHD in adulthood.

Influence of body mass index on growth hormone responses to classic provocative tests in children with short stature

BACKGROUND

The diagnosis of growth hormone (GH) deficiency is based on a reduced peak GH response to provocative tests. However, the provocative tests are poorly reproducible and GH secretion is regulated by physiological parameters, such as body weight and puberty. The aim of this study was to assess the influence of body mass index (BMI) on GH response to provocative testing and to analyze the reproducibility of GH stimulation test.

METHODS

Clinical data were collected retrospectively by chart review from the Pediatric Endocrine Unit at the Ajou University Hospital. A total of 187 subjects with short stature who completed a GH stimulation testing between 2003 and 2009 were included in the study.

RESULTS

Of the 187 subjects, 66 (35.3%) had GH deficiency, while 121 (64.7%) were categorized as having idiopathic short stature. Reliability was calculated for 48 patients with idiopathic short stature who underwent the GH stimulation test twice. A GH response ≥10 ng/ml after retesting was found in 39 patients (81.3%) and a GH response <10 ng/ml was found in 9 patients (18.7%). In a stepwise multivariate analysis, BMI was a significantly independent predictor of peak GH. Elevated BMI was negatively associated with peak plasma GH levels.

CONCLUSIONS

The lack of reliability of GH values in response to pharmacological stimuli should be taken into account in the diagnosis of GH deficiency. Also, higher BMI is associated with lower GH secretion. BMI should be measured and GH results appropriately interpreted for all subjects undergoing GH stimulation testing.

Acute norepinephrine reuptake inhibition decreases performance in normal and high ambient temperature

Combined inhibition of dopamine (DA)/norepinephrine (NE) reuptake improves exercise performance and increases core temperature in the heat. A recent study demonstrated that this effect may primarily be related to increased DA activity. NE reuptake inhibition (NERI), however, has received little attention in humans, certainly in the heat, where central fatigue appears to be a main factor influencing performance. Therefore the present study examines the effect of NERI (reboxetine) on exercise capacity, thermoregulation, and hormonal response in normal and high temperature. Nine healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (Pla; 2 x 8 mg) or reboxetine (Rebox; 2 x 8 mg). Subjects exercised in temperate (18 degrees C) or warm (30 degrees C) conditions and cycled for 60 min at 55% W(max) immediately followed by a time trial (TT; Pla18/Rebox18; Pla30/Rebox30) to measure exercise performance. Acute NERI decreased power output and consequently exercise performance in temperate (P = 0.018) and warm (P = 0.007) conditions. Resting heart rate was significantly elevated by NERI (18 degrees C: P = 0.02; 30 degrees C: P = 0.018). In Rebox18, heart rate was significantly higher than in the Pla18, while in the heat no effect of the drug treatment was reported during exercise. In Rebox30, all hormone concentrations increased during exercise, except for growth hormone (GH), which was significantly lower during exercise. In Rebox18, prolactin (PRL) concentrations were significantly elevated; GH was significantly higher at rest, but significantly lower during exercise. In conclusion, manipulation of the NE system decreases performance and modifies hormone concentrations, thereby indicating a central NE effect of the drug. These findings confirm results from previous studies that predominantly increased DA activity is important in improving performance.

Impaired growth hormone secretion after glucose loading in non-obese women with polycystic ovary syndrome, possibly related to androgen but not insulin and free fatty acids

BACKGROUND

Secretion of growth hormone (GH) is known to be suppressed in women with polycystic ovary syndrome (PCOS). The suppression may be implicated in the pathogenesis of PCOS.

OBJECTIVE

To investigate the cause of suppressive GH secretion in PCOS.

METHODS

In order to eliminate the influence of obesity on the variables, all subjects were non-obese. Standard oral glucose tolerance tests (OGTT) were performed in 13 normal women and 15 patients with PCOS.

RESULTS

The serum GH level decreased progressively during the OGTT, and the mean area under the curve (AUC) for the GH response was significantly smaller in the PCOS group than in the normal group. A reduction in the level of serum free fatty acids (FFAs) was observed during OGTT, but only the fasting level of FFAs was significantly higher in the PCOS group than in the normal group. The mean AUCs for glucose, insulin and FFA responses in the PCOS group were significantly greater than those in the normal group. There was a significant negative correlation only between AUC(GH) and testosterone (r = -0.4557, p = 0.012).

CONCLUSION

Our findings suggest that the cause of susceptibility for GH secretion to be suppressed after glucose loading in non-obese PCOS patients may be hyperandrogenemia.

Effect of metformin on the growth hormone response to growth hormone–releasing hormone in obese women with polycystic ovary syndrome

OBJECTIVE

Obese women with polycystic ovary syndrome (PCOS) show a marked growth hormone (GH) hyporesponsiveness to several stimuli. We aimed to evaluate the impact of insulin metabolism on the GH secretion impairment in these subjects in relation to food ingestion.

DESIGN

Prospective clinical study.

SETTING

Academic research center.

PATIENT(S)

Nine obese women with PCOS.

INTERVENTION(S)

Metformin (1,500 mg/daily) was administered for three months. The study protocol, which was performed before and after therapy, included hormonal and lipid assays, oral glucose tolerance test (75 g), euglycemic hyperinsulinemic clamp, and growth hormone-releasing hormone (GHRH) test (50 microg/ev), both on fasting and after a standard meal.

MAIN OUTCOME MEASURE(S)

Growth hormone response to GHRH (expressed as the area under the curve) in different experimental conditions.

RESULT(S)

The preprandial GH response to GHRH was not modified by the therapy, whereas a significant increase (P<.05) occurred in the postprandial GH secretion, thus resembling the response of obese normal persons. This change was accompanied by a trend towards improvement, though not statistically significant, of all the evaluated glycoinsulinemic parameters. A significant reduction in cholesterol (P<.01) and androstenedione (P<.05) and an increase in sex hormone-binding globulin (P<.05) were also achieved.

CONCLUSION(S)

These data suggest that metformin is able to affect GH secretion in obese women with PCOS, even with minimal metabolic modifications.

Human growth hormone responses to repeated bouts of sprint exercise with different recovery periods between bouts

This study examined the growth hormone (GH) response to repeated bouts of sprint cycling. Eight healthy men completed three trials consisting of two 30-s sprints on a cycle ergometer separated by either 60 min (Trial A) or 240 min (Trial B) of recovery and a single 30-s sprint carried out the day after Trial B (Trial C). Trials A and B were separated by at least 7 days. Blood samples were obtained at rest and during recovery from each sprint. In Trial A, GH was elevated immediately before sprint 2, and there was no further increase in GH following the second sprint [area under the curve: 460 (SD 348) vs. 226 min.mug(-1).l(-1) (SD 182), P = 0.05]. Free insulin-like growth factor I tended to be lower immediately before sprint 2 than sprint 1 (P = 0.06). Serum free fatty acids were not different immediately before each of the sprints. In Trial B, there was a trend for a smaller GH response to the second sprint [GH area under the curve: 512 (SD 396) vs. 242 min.mug(-1).l(-1) (SD 190), P = 0.09]. Free insulin-like growth factor I tended to be lower (P = 0.06), and serum free fatty acids were higher (P = 0.01) immediately before sprint 2 than sprint 1. There was no difference in the GH response to sprinting on consecutive days (Trials B and C). In conclusion, repeated bouts of sprint cycling on the same day result in an attenuation or even ablation of the exercise-induced increase in GH, depending on the recovery interval between sprints.

Influence of body mass index and gender on growth hormone (GH) responses to GH-releasing hormone plus arginine and insulin tolerance tests

The aim of this study is to assess whether gender and body mass index (BMI) should be considered in developing thresholds to define GH deficiency, using GH responses to GHRH + arginine (ARG) stimulation and insulin tolerance test (ITT). Thirty-nine healthy subjects (19 males, 20 females; ages 21-50 yr) underwent GHRH + ARG, and another 27 subjects (19 males, 8 females; ages 20-49 yr) underwent ITT. Peak GH response was significantly higher (P = 0.005) after GHRH + ARG than with ITT, and this difference could not be explained by age, gender, or BMI. Peak GH response was negatively correlated with BMI in both tests (GHRH + ARG, r = -0.76; and ITT, r = -0.65). Peak GH response to GHRH + ARG was higher in females than males (P = 0.004; ratio = 2.4), but it was attenuated after eliminating the influence of BMI (P = 0.13; ratio = 1.6). No significant gender differences were found in peak GH responses to ITT, which could be due to the smaller number of female subjects studied. GH response to GHRH + ARG and ITT stimulation is sensitive to BMI differences and less so to gender differences. A higher BMI is associated with a depressed GH response to both stimulation tests. BMI should therefore be considered as a factor when defining the diagnostic cut-off points in the assessment of GH deficiency, whereas whether gender should be likewise used is inconclusive from this study.

Endocrine control of body composition in infancy, childhood, and puberty

Body composition exhibits marked variations across the early human lifetime. The precise physiological mechanisms that drive such developmental adaptations are difficult to establish. This clinical challenge reflects an array of potentially confounding factors, such as marked intersubject differences in tissue compartments; the incremental nature of longitudinal intrasubject variations in body composition; technical limitations in quantitating the unobserved mass of mineral, fat, water, and muscle ad seriatim; and the multifold contributions of genetic, dietary, environmental, hormonal, nutritional, and behavioral signals to physical and sexual maturation. From an endocrine perspective (reviewed here), gonadal sex steroids and GH/IGF-I constitute prime determinants of evolving body composition. The present critical review examines hormonal regulation of body composition in infancy, childhood, and puberty.

Body mass index determines evoked growth hormone (GH) responsiveness in normal healthy male subjects: diagnostic caveat for adult GH deficiency

GH secretion is decreased in obese subjects, whereas age-adjusted IGF-I concentrations are normal. This study was undertaken to rigorously delineate the extent of obesity [elevated body mass index (BMI)] associated with decreased somatotrope secretory function resulting in apparent adult GH deficiency. The peak GH response evoked by combined arginine (0.5 g/kg infused iv over 30 min) and GHRH (1 microg/kg iv bolus) was measured in 59 healthy male subjects with BMIs ranging from normal to obese. BMI correlated with the peak evoked GH response (Pearson r = -0.59; P < 0.01), and the percentage of subjects exhibiting an abnormal evoked GH response, i.e. less than 9 ng/ml, increased from 5% for those with a BMI less than 25 (normal), to 13% for those with a BMI of 25-26.9 (mildly overweight), to 33% for those with a BMI of 27-29.9 (moderately overweight), and to 64% for those with a BMI of 30 or more (obese). BMI is a major determinant of evoked adult GH response to provocative testing. The diagnosis of adult GH deficiency using the evoked GH response in patients with even mild BMI elevation does not accurately distinguish normal from deficient responses and may result in the erroneous classification of obese subjects as GH deficient and thus unnecessarily requiring GH replacement.

Effect of 6 weeks of sprint training on growth hormone responses to sprinting

This study examined the effect of 6 weeks of prescribed sprint training on the human growth hormone (hGH) response to cycle ergometer sprinting. Sixteen male subjects were randomly assigned to a training (n=8) or a control (n=8) group. Each subject completed two main trials, consisting of two all-out 30-s cycle-ergometer sprints separated by 60 min of passive recovery, once before, and once after a 6-week training period. The training group completed three supervised sprint-training sessions per week in addition to their normal activity, whilst control subjects continued with their normal activity. In the training group, peak and mean power increased post-training by 6% (P<0.05) and 5% (P<0.05), respectively. Post-exercise blood pH did not change following training, but the highest post-exercise blood lactate concentrations were greater [highest measured value: 13.3 (1.0) vs 15.0 (1.1) mmol l(-1)], with lower blood lactate concentrations for the remainder of the recovery period (P<0.05). Post-exercise plasma ammonia concentrations were lower after training [mean highest measured value: 184.1 (9.8) vs 139.0 (11.7) micromol l(-1), P<0.05]. Resting serum hGH concentrations did not change following training, but the peak values measured post-exercise decreased by over 40% in the training group [10.3 (3.1) vs 5.8 (2.5) microg l(-1), P<0.05], and mean integrated serum hGH concentrations were 55% lower after training [567 (158) vs 256 (121) min microg l(-1), P<0.05]. The hGH response to the second sprint was attenuated similarly before and after training. This study showed that 6 weeks of combined speed- and speed-endurance training blunted the human growth hormone response to sprint exercise, despite an improvement in sprint performance.

The exercise-induced growth hormone response in athletes

Human growth hormone (hGH) is secreted in a pulsatile fashion, generally following a circadian rhythm. A number of physiological stimuli can initiate hGH secretion, the most powerful, non-pharmacological of which are sleep and exercise. hGH has many varied roles throughout life, from growth itself, including the turnover of muscle, bone and collagen, to the regulation of selective aspects of metabolic function including increased fat metabolism and the maintenance of a healthier body composition in later life. The exercise-induced growth hormone response (EIGR) is well recognised and although the exact mechanisms remain elusive, a number of candidates have been implicated. These include neural input, direct stimulation by catecholamines, lactate and or nitric oxide, and changes in acid-base balance. Of these, the best candidates appear to be afferent stimulation, nitric oxide and lactate. Resistance training results in a significant EIGR. Evidence suggests that load and frequency are determining factors in the regulation of hGH secretion. Despite the significant EIGR induced by resistance training, much of the stimulus for protein synthesis has been attributed to insulin-like growth factor-1 with modest contributions from the hGH-GH receptor interaction on the cell membrane. The EIGR to endurance exercise is associated with the intensity, duration, frequency and mode of endurance exercise. A number of studies have suggested an intensity 'threshold' exists for EIGR. An exercise intensity above lactate threshold and for a minimum of 10 minutes appears to elicit the greatest stimulus to the secretion of hGH. Exercise training above the lactate threshold may amplify the pulsatile release of hGH at rest, increasing 24-hour hGH secretion. The impact of chronic exercise training on the EIGR remains equivocal. Recent evidence suggests that endurance training results in decreased resting hGH and a blunted EIGR, which may be linked to an increased tissue sensitivity to hGH. While the potential ergogenic effects of exogenous GH administration are attractive to some athletes, the abuse of GH has been associated with a number of pathologies. Identification of a training programme that will optimise the EIGR may present a viable alternative. Ageing is often associated with a progressive decrease in the volume and, especially, the intensity of exercise. A growing body of evidence suggests that higher intensity exercise is effective in eliciting beneficial health, well-being and training outcomes. In a great many cases, the impact of some of the deleterious effects of ageing could be reduced if exercise focused on promoting the EIGR. This review examines the current knowledge and proposed mechanisms for the EIGR, the physiological consequences of endurance, strength and power training on the EIGR and its potential effects in elderly populations, including the aged athlete.

Effect of acute reduction of free fatty acids by acipimox on growth hormone-releasing hormone-induced GH secretion in type 1 diabetic patients

BACKGROUND

In type 1 diabetes mellitus (DM1), high GH basal levels and exaggerated responses to several stimuli have been described. Acipimox is an antilipolytic drug that produces an acute reduction of free fatty acids (FFA). The aim of this study was to evaluate the effect of the reduction of plasma FFA with acipimox, alone or in combination with GHRH, on GH secretion in DM1.

METHODS

Six type 1 diabetic patients were studied (three women, three men), mean age of 30 +/- 2.1 years, body mass index (BMI) 23.1 +/- 1.5 kg/m2. As a control group, six normal healthy subjects of similar age, sex and weight were studied. Each patient and control received GHRH [1 microg/kg intravenously (i.v.) at min 180], acipimox (250 mg orally at min 0 and 120) and GHRH plus acipimox on three separated days. Subjects served as their own control. Blood samples were taken at appropriate intervals for determination of GH, FFA and glucose.

RESULT

In control subjects, the GH area under the curve (AUC; microg/l x 120 min) was for acipimox-treated 1339 +/- 292 and 1528 +/- 330 for GHRH-induced secretion. The GH AUC after the administration of GHRH plus acipimox was 3031 +/- 669, significantly greater than the response after acipimox alone (P<0.05) or GHRH alone (P<0.05). In diabetic patients, the GH AUC was for acipimox-treated 2516 +/- 606 and 1821 +/- 311 for GHRH-induced secretion. The GH AUC after the administration of GHRH plus acipimox was 7311 +/- 1154, significantly greater than the response after acipimox alone (P<0.05) or GHRH alone (P<0.05). The GH response after acipimox was increased in diabetic when compared with normal (P<0.05), with a GH AUC of 1339 +/- 292 and 2515 +/- 606 for normal subjects and diabetic patients, respectively. The GH response after acipimox plus GHRH was increased in diabetic when compared with normal (P<0.05), with a GH AUC of 3031 +/- 669 and 7311 +/- 1154 for normal subjects and diabetic patients, respectively. The administration of acipimox induced a FFA reduction during the entire test.

CONCLUSIONS

Reduction of free fatty acids with acipimox is a stimulus for GH secretion in DM1. The combined administration of GHRH plus acipimox induces a markedly increased GH secretion in type 1 diabetic patients when compared with normal subjects. These data suggest that patients with DM1 exhibit a greater GH secretory capacity than control subjects, despite the fact that endogenous FFA levels seems to exert a greater inhibitory effect on GH secretion in these patients.

Growth hormone responses to sub-maximal and sprint exercise

Exercise is a potent stimulus for growth hormone (GH) release and a single bout of exercise can result in marked elevations in circulating GH concentrations. The magnitude of the GH response to exercise will vary according to the type, intensity and duration of exercise as well as factors such as the age, gender, body composition and fitness status of the individual performing the exercise. However, the mechanisms regulating GH release in response to exercise are not fully understood. This review considers the GH responses to sub-maximal and sprint exercise and discusses the factors that might affect GH release along with the mechanisms that have been proposed to regulate exercise-induced GH release.

Reduced expression of the growth hormone and type 1 insulin-like growth factor receptors in human somatotroph tumours and an analysis of possible mutations of the growth hormone receptor

OBJECTIVE

Clinical acromegaly is characterized by elevated GH secretion in the presence of high circulating IGF-I levels. We hypothesized that the physiological IGF-I/GH negative feedback loop may be reset in somatotroph adenomas, specifically in terms of the level of expression of these receptors or mutations of the GH receptor (GH-R) in such tumours.

METHODS

We therefore investigated the full coding sequence of the GH-R in a series of somatotroph and other pituitary adenomas. We also investigated the mRNA expression of these putative feedback receptors in a series of pituitary adenomas and normal pituitary tissue, and their protein expression by immunostaining. Real-time RT-PCR assay was used for the quantification of the type 1 IGF receptor (IGF-R) and GH receptor (GH-R) mRNA, and sequence analysis was performed on the coding region of the GH-R gene.

RESULTS

No somatic mutations of the GH-R mRNA were detected in 18 GH-secreting tumours or two nonfunctioning pituitary adenomas (NFPAs). However, the levels of GH-R mRNA were significantly lower in both somatotroph tumours and NFPAs compared to the normal pituitary (P < 0.05 for both). Immunostaining for GH-R also showed significantly less GH-R expression in somatotroph adenomas compared to normal pituitary tissue (P < 0.0001). IGF-R mRNA levels were significantly lower in somatotroph tumours compared to normal pituitary (P = 0.005), and trended lower in corticotroph tumours (P = 0.07), while the other tumour types showed no significant difference from normal pituitary. Immunostaining for IGF-R also showed less IGF-R protein in the somatotroph adenomas compared to the normal pituitary tissue (P < 0.01).

CONCLUSIONS

Our findings suggest that decreased feedback inhibition of GH because of somatic mutations of the coding region of the GH-R are unlikely to be a common factor in the pathogenesis of these tumours. Nevertheless, decreased expression of the GH-R and of IGF-R in somatotroph tumours (both at the mRNA and protein level) may, at least in part, help explain the continuous secretion of GH from the tumour despite the high circulating levels of IGF-I and GH.

Aspectos neuroendócrinos da síndrome metabólica

A síndrome metabólica (SM) é caracterizada por alterações no metabolismo glicídico, obesidade, hipertensão e dislipidemia. Estas alterações metabólicas interrelacionam-se com diversos eixos endócrinos controlados pelo hipotálamo e pela hipófise. A obesidade central parece relacionar-se a uma hiperativação do eixo hipotálamo-hipófise-adrenal, como também do sistema nervoso simpático, que poderia levar a um quadro de hipercortisolismo sub-clínico e hipertensão arterial. A SM é também um estado de hipo-somatotropismo relativo relacionado à gordura visceral. Além disso, níveis elevados de ácidos graxos livres e a hiperinsulinemia, secundários à resistência insulínica, estão relacionadas a um bloqueio do eixo somatotrófico. Em homens, a SM relaciona-se a um hipogonadismo tanto por diminuição de gonadotrofinas como por inibição direta da produção de testosterona. Já nas mulheres, existe um excesso de produção de androgênios, principalmente relacionado à hiperinsulinemia, aumento da atividade da aromatase e da liberação de LH. Desta forma, a SM é um estado relacionado a importantes modificações nos mecanismos de feedback responsáveis pelo correto funcionamento dos eixos neuroendócrinos.

The inhibition of growth hormone secretion presented in obesity is not mediated by the high leptin levels: a study in human leptin deficiency patients

GH secretion is regulated by hypothalamic and peripheral hormones under a very complex interplay. Superimposed on this regulation, signals of a metabolic nature connect GH secretion with the metabolic and energetic homeostasis of a given individual. GH secretion is enhanced in malnutrition and is severely impeded in obesity, but no information is available to explain why GH secretion is severely impeded or blocked in excess adiposity. Obesity is associated with high plasma levels of leptin, and leptin participates at the hypothalamic and pituitary levels in the regulation of GH secretion. Thus, it has been postulated that the inhibitory action of obesity on GH discharge may be mediated by excess leptin levels. The only situation in which obesity does not parallel leptin values is the rare case of morbid obesity due to leptin deficiency caused by missense mutation of the leptin gene. To understand the causes of GH blockade presented in obesity, patients with both homozygous and heterozygous mutations of the leptin gene and matched controls for both sex and body mass index (BMI) were studied. Three homozygous and 5 heterozygous patients with leptin gene mutations as well as 13 control subjects were studied. In all subjects basal levels of leptin and GH values stimulated by the combined administration of GHRH plus GH-releasing peptide-6 (GHRP-6) were analyzed. To analyze the effects of obesity and leptin levels, 5 groups were designed, all them matched by sex and adiposity. The number of subjects (n), leptin levels in micrograms per liter, and adiposity in BMI were as follows: nonobese subjects: n = 5, BMI = 22.1 +/- 0.9 kg/m2, leptin = 5.4 +/- 0.9; heterozygous patients: n = 5, BMI = 27.0 +/- 1.0 kg/m2, leptin = 2.3 +/- 0.1; controls for the heterozygous group: n = 5, BMI = 24.7 +/- 1.1 kg/m2, leptin = 5.7 +/- 1.2; homozygous patients: n = 3, BMI = 54.4 +/- 0.2 kg/m2, leptin = 1.0 +/- 0.2; and controls for the homozygous group: n = 3, BMI = 50.3 +/- 2.0 kg/m2, leptin = 35.0 +/- 6.6. In these matched groups, the GHRH- and GHRP-6-stimulated GH secretion (mean peak +/- SE; micrograms per liter) was: nonobese, 86.8 +/- 8.9 [significantly higher than heterozygous (28.6 +/- 4.9) and control for heterozygous (39.9 +/- 10.4)]; homozygous group, 9.4 +/- 3.0; control for homozygous, 9.3 +/- 1.0 (significantly lower than the heterozygous, control for heterozygous, and nonobese groups). Hence, it appeared that GH discharge was negatively conditioned by adiposity and was not influenced by leptin levels. To further analyze this observation, a correlation analysis showed that GH peaks were negatively correlated with BMI in the 13 control subjects as well as in the 8 leptin-deficient patients. On the contrary, the GH peaks were negatively correlated with leptin levels in controls, but showed the opposite pattern in homo- and heterozygous patients. In conclusion, the GH secretion blockade, which is characteristic of obese states, is due to adiposity or some factor linked to adiposity, but not to elevated plasma leptin levels.

No effect of a noradrenergic reuptake inhibitor on performance in trained cyclists

INTRODUCTION

According to the central fatigue hypothesis, serotonin (5-HT) is related to fatigue, whereas the noradrenergic system is primarily concerned with arousal and motivation, and therefore hypothesized to enhance performance. The purpose of the present study was to examine the effects of a selective noradrenergic reuptake inhibitor (reboxetine 2 x 4 mg REB-NARI) on exercise performance.

METHODS

Seven healthy well-trained male cyclists (age: 23 +/- 1.7 yr, height: 182 +/- 5.8 cm, weight: 73.5 +/- 8.5 kg, VO2max: 73.5 +/- 6.4 mL x kg(-1) x min(-1), Watt(max): 376 +/- 11.7 W) participated to the study. Subjects completed two endurance tests (time trials) starting at 65% Wmax in a double-blind randomized cross-over design. Blood samples were collected for adrenocorticotropin, prolactin, cortisol, growth hormone (GH), beta-endorphins, and catecholamines and were taken at 30-min time intervals until the end of exercise. Performance was analyzed with a paired t-test, whereas data for hormonal and metabolic differences during the trials were analyzed using an ANOVA repeated measures design and an LSD-planned comparisons test. Significance level was set at P < 0.05.

RESULTS

Performance was not influenced by the NARI (REB: 97 min +/- 3 min, placebo (PLAC): 92 min +/- 1 min). All hormones increased during exercise except for GH in the REB trial, which was significantly lower than PLAC. The other hormones were significantly higher in the REB trial versus the PLAC trial at the end of exercise and during recovery.

CONCLUSION

In conclusion, the results demonstrate that the drug had a central effect. In particular, the higher resting GH concentrations indicated a marked and selective noradrenergic effect of REB. However, performance was not influenced by a selective NARI in well-trained endurance athletes.

Growth hormone responses to repeated maximal cycle ergometer exercise at different pedaling rates

The present study examined the growth hormone (GH) response to repeated bouts of maximal sprint cycling and the effect of cycling at different pedaling rates on postexercise serum GH concentrations. Ten male subjects completed two 30-s sprints, separated by 1 h of passive recovery on two occasions, against an applied resistance equal to 7.5% (fast trial) and 10% (slow trial) of their body mass, respectively. Blood samples were obtained at rest, between the two sprints, and for 1 h after the second sprint. Peak and mean pedal revolutions were greater in the fast than the slow trial, but there were no differences in peak or mean power output. Blood lactate and blood pH responses did not differ between trials or sprints. The first sprint in each trial elicited a serum GH response (fast: 40.8 +/- 8.2 mU/l, slow: 20.8 +/- 6.1 mU/l), and serum GH was still elevated 60 min after the first sprint. The second sprint in each trial did not elicit a serum GH response (sprint 1 vs. sprint 2, P < 0.05). There was a trend for serum GH concentrations to be greater in the fast trial (mean GH area under the curve after sprint 1 vs. after sprint 2: 1,697 +/- 367 vs. 933 +/- 306 min x mU(-1) x l(-1); P = 0.05). Repeated sprint cycling results in an attenuation of the GH response.

Effect of acute pharmacological modulation of plasma free fatty acids on GH secretion in acromegalic patients

OBJECTIVES

In acromegaly GH secretion is markedly increased due in most cases to a GH secreting pituitary adenoma. GH secretion is modulated by variations in the levels of free fatty acids (FFA). Recent studies in different clinical situations, have shown that reduction in FFA with acipimox (A) modifies somatotroph cell responsiveness. The aim of the present study was to evaluate the effect of acute pharmacological reduction of plasma FFA on both basal GH levels and GHRH-mediated GH secretion in acromegalic patients.

PATIENTS

Six acromegalic patients (four female, two male) aged 57 +/- 4 years., with active disease due to pituitary adenomas were studied. Four of the patients had been treated previously by surgery and/or radiotherapy. The diagnosis of active acromegaly was established by clinical assessment, increased serum IGF-I and impaired GH suppression after oral glucose.

MEASUREMENTS

Four tests were performed: placebo, A (250 mg, orally, - 210 minutes and - 60 minutes), GHRH (100 microg, iv, 0 minutes) and GHRH plus A. The different tests on each subject were performed in random order one week apart, each subject served as their own control. Serum GH was measured by RIA at appropriate intervals. The area under the curve (AUC) was calculated by the trapezoidal

METHOD

Statistical analysis was performed by Wilcoxon test. P < 0.05 was considered significant.

RESULTS

The administration of A induced a FFA reduction during the entire test both when administered with placebo and with GHRH: AUC (mmol/l x 90 minutes): placebo plus placebo: 88.2 +/- 7.3. Placebo plus A: 23.2 +/- 4.6 (P < 0.05). Placebo plus GHRH: 85.4 +/- 6.9. A plus GHRH: 21.8 +/- 3.8 (P < 0.05). Mean peak GH level (microg/l) after placebo plus placebo was 5.0 +/- 1.8 not significantly different than after placebo plus A with a mean peak of 6.2 +/- 2 (P = ns). Mean peak GHRH-induced GH secretion was 26.0 +/- 15.4 and was not modified by previous A administration with mean peak of 24.4 +/- 11.8 (P = ns).

CONCLUSIONS

In acromegalic patients acute pharmacological reduction of FFA with acipimox did not modify basal GH levels or GHRH-induced GH secretion, suggesting that the adenomatous somatotroph cell is unresponsive to physiological signals such as FFA which act at a pituitary level. These data support the hypothesis of an intrinsic neoplastic pituitary defect for the pathogenesis of acromegaly.

Effects of free fatty acids and acipimox, a lipolysis inhibitor, on the somatotroph responsiveness to GHRH in anorexia nervosa

OBJECTIVE

Anorexia nervosa is characterized by low IGF-1 and high GH and free fatty acid (FFA) levels. As FFA exerts an inhibitory feedback action on GH secretion in physiological conditions, we hypothesized that somatotroph cells could be less sensitive to the negative feedback action of FFA in anorexia nervosa.

PATIENTS

Fifteen patients with anorexia nervosa (AN, age: mean +/- SEM: 20.8 +/- 1.2 years, BMI: 15.9 +/- 0.3 kg/m2) and 12 normal female controls (NW, age 27.2 +/- 2.1 years, BMI 21.2 +/- 2.2 kg/m2).

MEASUREMENTS

We studied the effects of lipid-heparin emulsion (Li-He, Intralipid 10% 250 ml + heparin 2500 U iv from -60 to + 90 minutes in seven AN and six NW) or acipimox (ACI, 250 mg p.o. at -60 minutes in eight AN and six NW), a lipolysis inhibitor, on the GH response to GHRH (1 microg/kg iv as a bolus at 0 minutes).

RESULTS

Basal IGF-1 levels were lower (P < 0.05) while GH levels were higher (P < 0.05) in AN than in NW. On the other hand, basal FFA levels in the two groups were not significantly different. In both groups Li-He increased FFA levels (P < 0.05), which became higher (P < 0. 02) in AN than in NW. Li-He infusion inhibited (P < 0.05) basal GH levels in AN to levels overlapping those in NW. The GH response to GHRH in the whole AN group was higher than in NW (P < 0.03). Li-He inhibited the somatotroph responsiveness to GHRH in AN (P < 0.03) as well as in NW (P < 0.03) and during Li-He the GH response to GHRH in AN became similar to that in NW. Whilst ACI pretreatment enhanced the GH response to GHRH in AN (P < 0.02), it did not significantly increase that in NW. Interestingly, after ACI administration, FFA levels were inhibited in both groups (P < 0.05) persisting higher in AN than in NW (P < 0.05).

CONCLUSION

Though GH hypersecretion in anorexia nervosa occurs in presence of enhanced lipolysis, our present findings indicate that the sensitivity of somatotroph cells to the inhibitory feedback action of free fatty acid is preserved.

GH response to exercise: assessment of the pituitary refractory period, and relationship with circulating components of the GH-IGF-I axis in adolescent females

The concept of pituitary refractory period for GH secretion has been previously described. To measure the length of this refractory period we performed an exercise provocation test for GH secretion immediately following multiple overnight GH blood sampling. In addition, we correlated the magnitude of the GH response to a single exercise input with mean overnight GH, IGF-I and circulating IGFBP levels. 23 healthy adolescent females (15-17 yr) performed 10-min constant cycle ergometry at a power normalized to each subject's aerobic and anaerobic capacity. GH was measured every 10 min starting 10 min before exercise and then for 60 min after the exercise bout. Mean nocturnal GH was calculated from overnight values obtained every 20 min over a 12-h period. Pre-exercise GHBP, IGF-I and IGFBPs 1-5 were assessed using standard techniques. In five subjects, a spontaneous GH peak had preceded the exercise test by 1 hour or less, and no response to exercise was found. In the remaining 18 subjects, a GH peak (6.8 +/- 1.3 ng/ml, p < 0.0001) was observed at 32 +/- 4 min after the onset of exercise. The GH response to exercise was not correlated with fitness, mean GH or IGF-I but was correlated with IGFBP-3 (r = 0.65, p < 0.05). Spontaneous GH pulses may acutely render the pituitary refractory to exercise stimuli. The length of this refractory period is approximately 1 hour. The data corroborate the idea that while relationships exist among the various components of the GH-IGF-I axis, no single factor identified to date fully reflects GH-IGF-I "tone".

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Interaction between body composition, leptin and growth hormone status

Administration of growth hormone (GH) induces changes in body composition, namely, increases in both bone and lean mass and a decrease in fatty tissue. However, the contrary issue, i.e. the way in which body composition affects the secretion of GH, is highly controversial. Disease states such as obesity and chronic hypercortisolism are associated with increased adiposity and/or the central distribution of fat. Ageing, characterized by excess adiposity, is also associated with impaired secretion of GH. In these states, both spontaneous and stimulated secretion of GH is severely impeded. At the other extreme, malnutrition and fasting are both associated with increased secretion of GH when confronted with most, if not all, stimuli. As the common factor in all of these situations is the increased or decreased adiposity, or the changes in energy homeostasis, it has been postulated that adipose tissue exerts a relevant role in the control of GH secretion in man. The link between adipose tissue and GH seems to be exerted through at least two signals produced by adipocytes: free fatty acids (FFA) and the recently cloned protein, leptin. An increase in FFA blocks secretion of GH, while a decrease in FFA enhances secretion. Leptin, a hormone whose main role is to regulate the intake of food and energy expenditure, seems to regulate GH secretion by acting at the hypothalamic level. In summary, body composition affects GH secretion by way of the degree of adiposity, and free fatty acids and leptin would appear to be the messages through which adipocytes participate in the regulation of GH secretion. This framework clarifies the metabolic control of GH, a hormone with profound metabolic activities.

Peak oxygen uptake, muscle volume, and the growth hormone-insulin-like growth factor-I axis in adolescent males

PURPOSE

The growth effects of exercise appear to be mediated in part by central neuroendocrine control reflected in circulating levels of growth hormone (GH), insulin-like growth factor-I (IGF-I), and their binding proteins (BP). In previous studies positive correlations between peak VO2 and circulating IGF-I have been demonstrated. The relationship between peak oxygen uptake and these potential regulating factors has not been examined in adolescent males where patterns of GH pulsatility and levels of IGF-I are rapidly changing.

METHODS

Forty-three healthy adolescent males (age 16 +/- 0.7 yr, 70% at Tanner V) performed cycle ergometry to determine p oxygen uptake (peak VO2), and magnetic resonance images to determine the thigh muscle volume. Baseline blood samples were collected for GHBP, the extracellular portion of the GH tissue receptor (by ligand mediated immunofunctional assay), IGF-I (by RIA), and IGFBPs 1-5 (by RIA). Mean GH was determined from samples obtained every 20 min overnight.

RESULTS

Peak VO2/kg was positively correlated with mean overnight GH levels (r = 0.41, P < 0.005). Both peak VO2/kg and thigh muscle volume/kg were negatively correlated with GHBP (r = -0.33, P < 0.02) and IGFBP-4 (r = -0.52, P < 0.005). There were no correlations between peak VO2/kg and IGF-I or IGFBPs 1-3, and 5.

CONCLUSIONS

GH pulsatility is increased adolescent males who have higher peak VO2, but this did not translate into increases in IGF-I. We speculate that in the fitter males, lower GHBP levels may reduce hepatic sensitivity to GH. Thus, circulating IGF-I was unchanged despite higher mean GH in subjects with higher peak VO2. IGFBP-4 which is known to inhibit IGF-I was negatively correlated with peak VO2 leading, possibly, to increased IGF-I bioactivity. Fitness (as assessed by muscle mass and peak VO2) does modulate the GH-IGF-I axis, but not solely through circulating IGF-I; both GHBP and IGFBPs play important roles.

Neuropharmacological assessment of growth hormone secretion

The biochemical diagnosis of individuals who are either deficient in growth hormone (GH) or who have alterations in the normal pattern of GH secretion is difficult. The uncertainty surrounding diagnosis reflects the lack of a thorough understanding of the physiology of GH secretion and of the hypothalamic hormones involved. At least three hormones are implicated: GH-releasing hormone (GHRH), somatostatin and the endogenous ligand of the GH secretagogue receptor, although the role that each plays in the release of GH is not clear from the available experimental evidence. In such a situation, most of the dynamic tests of GH secretory capacity in humans need to undergo a 'trial and error' process before being validated. The search for the 'gold standard' test of GH secretion is ongoing, and the combination of GHRH plus GH secretagogues will probably play an important role in future clinical diagnosis.

Effect of aging on the sensitivity of growth hormone secretion to insulin-like growth factor-I negative feedback

To determine the effect of aging on the suppression of GH secretion by insulin-like growth factor (IGF)-I, we studied 11 healthy young adults (6 men, 5 women, mean +/- SD: 25.2 +/- 4.6 yr old; body mass index 23.7 +/- 1.8 kg/m2) and 11 older adults (6 men, 5 women, 69.5 +/- 5.8 yr old; body mass index 24.2 +/- 2.5 kg/m2). Saline (control) or recombinant human IGF-I (rhIGF-I) (2 h baseline then, in sequence, 2.5 h each of 1, 3, and 10 micrograms/kg.h) was infused iv during the last 9.5 h of a 40.5-h fast; serum glucose was clamped within 15% of baseline. Baseline serum GH concentrations (mean +/- SE: 3.3 +/- 0.7 vs. 1.9 +/- 0.5 micrograms/L, P = 0.02) and total IGF-I concentrations (219 +/- 15 vs. 103 +/- 19 micrograms/L, P < 0.01) were higher in the younger subjects. In both age groups, GH concentrations were significantly decreased by 3 and 10 micrograms/kg.h, but not by 1 microgram/kg.h rhIGF-I. The absolute decrease in GH concentrations was greater in young than in older subjects during the 3 and 10 micrograms/kg.h rhIGF-I infusion periods, but both young and older subjects suppressed to a similar GH level during the last hour of the rhIGF-I infusion (0.78 +/- 0.24 microgram/L and 0.61 +/- 0.16 microgram/L, respectively). The older subjects had a greater increase above baseline in serum concentrations of both total (306 +/- 24 vs. 244 +/- 14 micrograms/L, P = 0.04) and free IGF-I (8.5 +/- 1.4 vs. 4.2 +/- 0.6 micrograms/L, P = 0.01) than the young subjects during rhIGF-I infusion, and their GH suppression expressed in relation to increases in both total and free serum IGF-I concentrations was significantly less than in the young subjects. We conclude that the ability of exogenous rhIGF-I to suppress serum GH concentrations declines with increasing age. This suggests that increased sensitivity to endogenous IGF-I negative feedback is not a cause of the decline in GH secretion that occurs with aging.

Effect of chronic treatment with biosynthetic growth hormone (GH) on the GH response to double GH-releasing hormone administration in children with short stature

Growth hormone (GH) exerts a negative feedback on its own secretion through direct and indirect mechanisms. Normal children, unlike adults, display consecutive GH responses after repeated GH-releasing hormone (GHRH) administration. In this study, we investigated the effects of long-term GH administration on this peculiar secretory pattern. Eight children with severe short stature and impaired GH responses to suprapituitary provocative stimuli associated with normal GH responsiveness to GHRH, underwent, while on chronic treatment with biosynthetic GH and on the 10th day following drug withdrawal, a double bolus GHRH test (two GHRH injections of 1 microg/kg b.w. given as i.v. boluses two hours apart). During GH therapy the GH response to the first GHRH bolus appeared to be reduced, though not significantly so, compared to that observed at diagnosis; after treatment withdrawal, this response returned quite similar to pretreatment. Both during and after treatment, the GH response to the second GHRH bolus was comparable for magnitude to that evoked by the first application. In conclusion, even prolonged treatments with rhGH do not induce persisting alterations of the physiological mechanisms subserving GH regulation.

The endocrinology of obesity

Numerous endocrine alterations are associated with obesity (Table 1). The majority of the alterations are secondary to obesity and must be considered simply associated and potentially in the pathogenesis of the complications of obesity. The discovery of new endocrine peptides such as leptin that signal body fat content will increase our understanding of the regulation of body fat content. As a result, therapies will most certainly be developed that are directly targeted at the alterations in endocrine function.

Differential effect of insulin-like growth factor-1 and growth hormone on hypothalamic regulation of growth hormone secretion in the rat

Pharmacological administration of either growth hormone (GH) or insulin-like growth factor 1 (IGF-1) were reported to inhibit endogenous GH release in humans and in the laboratory animal. We have evaluated the short-term differential mechanisms whereby the two hormones affect hypothalamic regulation of GH secretion. Wistar male rats (90 days old) were injected i.p. with either GH (recombinant GH NIAMDD, Baltimore, MD, USA), rIGF-1 (Fujisawa Pharmaceutical Co. Ltd., Osaka, Japan) or saline. Animals were sacrificed at 15, 30, 60 and 120 minutes following injection. Hypothalami were dissected and extracted immediately and the levels of growth hormone-releasing hormone (GHRH) and somatostatin were determined using specific antisera. Trunk blood was collected for GH and IGF-1 determination by RIA. Administration of IGF-1 or GH markedly decreased hypothalamic somatostatin stores by 77% and 54% respectively, within 15 minutes. Concomitantly, the wide range of GH levels found in the control group was reduced in the IGF-1 treated group suggesting that the pulsatile pattern of GH secretion was suppressed. Growth hormone administration induced an increase in hypothalamic GHRH stores (60% at 120 minutes). During this period serum IGF-1 levels were not altered. It is suggested that short term modulation of hypothalamic neurohormones by GH and IGF-1 is mediated by rapid stimulation of somatostatin release by both hormones, and inhibition of GHRH release is induced only by GH.

Growth hormone response to L-dopa and pyridostigmine in women with polycystic ovarian syndrome

OBJECTIVE

To evaluate the GH secretion and clarify the factors influencing the GH secretion in women with polycystic ovarian syndrome (PCOS).

DESIGN

Comparison of the GH response to L-dopa with or without pyridostigmine (inhibitor of acetylcholinesterase) pretreatment and insulin response to oral glucose tolerance test in patients with PCOS and matched controls.

SETTING

Outpatients and healthy volunteers studied at a clinical research unit of a university hospital.

PATIENTS, PARTICIPANTS

Ten women with PCOS and 9 controls with regular cycles were recruited.

INTERVENTIONS

After an overnight fast, each subject underwent a GH stimulation test with L-dopa with or without pyridostigmine pretreatment. Plasma insulin and glucose levels were measured after a 75-g glucose load.

MAIN OUTCOME MEASURES

Plasma GH, insulin-like growth factor I (IGF-I), insulin, and nonesterified fatty acids.

RESULTS

Growth hormone responses and GH area under the response curve (AUC) to L-dopa were significantly lower in PCOS than those in controls. Pyridostigmine enhanced the GH response to L-dopa significantly in PCOS. Insulin responses and insulin AUC to oral glucose load were significantly higher in PCOS than those in controls. Plasma IGF-I levels of PCOS were significantly higher than controls. Insulin AUC had a positive correlation with plasma IGF-I levels but an inverse correlation with GH AUC in PCOS and controls.

CONCLUSION

Our result indicated that decreased GH secretion of PCOS may be associated with a high somatostatin activity and a high plasma IGF-I level.

Influence of hyperthyroidism on growth hormone secretion

OBJECTIVE

Hyperthyroidism is associated with altered GH secretion. Whether this is due to changes of somatotroph responsiveness or reflects an alteration in negative feedback signals at the hypothalamic level is unknown. We therefore performed a series of studies to shed some light onto this issue.

DESIGN

Study 1: GHRH (1 microgram/kg b.w.) was injected i.v. in 38 hyperthyroid patients and in 30 normal subjects; in 11 of the patients the GHRH test was repeated following methimazole-induced remission of hyperthyroidism. Study 2: hGH (2 U i.v.) or saline were administered 3 hours prior to GHRH; six hyperthyroid patients and six normal subjects were studied. Study 3: ten normal subjects and ten hyperthyroid patients were given 75 g oral glucose or water 30 minutes before GHRH. Study 4: 11 normal subjects and eight hyperthyroid patients were studied. TRH or vehicle were dissolved in 250 ml of saline solution and infused at a rate of 400 micrograms/h for 150 minutes. Thirty minutes after the beginning of the infusions, L-arginine (30 g infused over 45 min i.v.) was administered.

PATIENTS

Hyperthyroid patients were compared to normal subjects.

MEASUREMENTS

Growth hormone was measured by RIA at 15-minute intervals.

RESULTS

GH responses to GHRH were subnormal in hyperthyroid patients. Following antithyroid drug treatment with methimazole, GH responses to GHRH increased in these patients in comparison to pretreatment values. Serum IGF-I levels, which were elevated before treatment, decreased after methimazole administration. Exogenous GH administration induced a clear decrease of GH responses to GHRH in both control and hyperthyroid subjects. On the other hand, oral glucose load decreased the GH responses to GHRH in normal but not in hyperthyroid subjects. TRH administration did not modify the GH responses to arginine in either normal subjects or hyperthyroid patients.

CONCLUSIONS

Hyperthyroidism is associated with increased serum IGF-I levels and marked alterations in the neuroregulation of GH secretion. These changes involve decreased GH responsiveness to GHRH at the pituitary level and, at the hypothalamic level, a lack of suppressive effect of an oral glucose load. The normal inhibitory effect of exogenous GH administration but not of an oral glucose load in hyperthyroid patients suggests that these two feedback signals act through different mechanisms. The lack of effect of a TRH infusion on GH responses to L-arginine in normal and hyperthyroid patients makes an inhibitory role for TRH in GH secretion unlikely, at least in Caucasian subjects.

Physiological levels of growth hormone fail to suppress growth hormone releasing hormone (1-29) NH2-stimulated growth hormone secretion in man

We studied 11 normal adult males. Six subjects (Study A) received a bolus of saline or of 50 mU biosynthetic human growth hormone (r-hGH) or a one hour iv infusion of r-hGH (80 mU/h) in random order. On each occasion this was followed by an iv bolus of GHRH (1-29) NH2 (100 micrograms) 90 minutes after the first event. Five subjects (Study B) received a bolus iv injection of saline or of 500 mU r-hGH followed by iv GHRH (1-29) NH2 (100 micrograms) 90 minutes later. There was no significant difference in the serum GH concentrations achieved following the 50 mU bolus or iv infusion of r-hGH (range 5.6-67.0 mU/l). Higher concentrations of GH (mean +/- SE, 238.4 +/- 21.3 mU/l) were achieved with the 500 mU bolus of r-hGH. The peak GH responses to iv GH-RH (1-29) NH2 were similar in all instances. The most important factor determining the response to exogenous GHRH (1-29) NH2 was the serum GH concentration at the time that the GHRH (1-29) NH2 was administered and the mode of r-hGH administration (iv bolus or iv infusion). These data demonstrate that within the range of physiological serum GH concentrations the mode of presentation of GH (bolus or infusion) and GH secretory status are the most important factors in determining GH responsivity to GHRH. Under these circumstances GH would appear not to participate in a rapid-acting short-loop negative feedback mechanism in man as the response to exogenous GHRH was not attenuated.

Pituitary reserve after repeated administrations of releasing hormones in young and in elderly men: reproducibility on different days

Combined testing of the pituitary gland by administration of GHRH, CRH, GnRH and TRH has been proposed for clinical studies, although some reports indicate that individual endocrine responses can be different when releasing hormones are used alone or in combination. Aims of this study were to evaluate: 1) the reproducibility of the combined test on different days; 2) the endocrine responses to the combined test applied twice at 3h and at 5h interval; 3) differences of endocrine responses in young and in elderly men. Six healthy young men (aged 25 to 35 yr) and 6 healthy elderly men (aged 65 to 75 yr) were evaluated: elderly men had lower testosterone, free T3, and somatomedin-c levels than young men, while 17 beta-estradiol and inhibin were not significantly different, all values being within normal laboratory limits. The 12 men were tested on day 1 with iv GHRH (50 micrograms) CRH (50 micrograms), GnRH (100 micrograms) and TRH (200 micrograms) at 08:00h and again at 11:00h; on day 8, the same men were tested at 08:00h and again at 13:00h. At 08:00h, the endocrine responses were similar on day 1 and on day 8. The second GH (young and elderly men) and PRL (only young men) response was blunted on day 1, when the interval between two consecutive stimuli was 3h, but not on day 8, when the interval was 5h. Elderly men differed from young men only for GH and for PRL release on day 1 at 08:00h.(ABSTRACT TRUNCATED AT 250 WORDS)