Interaction of salbutamol with pyridostigmine and arginine on both basal and GHRH-stimulated GH secretion in humans

Multipathway modulation of exercise and glucose stress effects upon GH secretion in healthy men

OBJECTIVE

Exercise evokes pulsatile GH release followed by autonegative feedback, whereas glucose suppresses GH release followed by rebound-like GH release (feedforward escape). Here we test the hypothesis that age, sex steroids, insulin, body composition and physical power jointly determine these dynamic GH responses.

METHODS

This was a prospectively randomized glucose-blinded study conducted in the Mayo Center for Advancing Translational Sciences in healthy men ages 19-77 years (N=23). Three conditions, fasting/rest/saline, fasting/exercise/saline and fasting/rest/iv glucose infusions, were used to drive GH dynamics during 10-min blood sampling for 6h. Linear correlation analysis was applied to relate peak/nadir GH dynamics to age, sex steroids, insulin, CT-estimated abdominal fat and physical power (work per unit time).

RESULTS

Compared with the fasting/rest/saline (control) day, fasting/exercise/saline infusion evoked peak GH within 1h, followed by negative feedback 3-5h later. The dynamic GH excursion was strongly (R(2)=0.634) influenced by (i) insulin negatively (P=0.011), (ii) power positively (P=0.0008), and (iii) E2 positively (P=0.001). Dynamic glucose-modulated GH release was determined by insulin negatively (P=0.0039) and power positively (P=0.0034) (R(2)=0.454). Under rest/saline, power (P=0.031) and total abdominal fat (P=0.012) (R(2)=0.267) were the dominant correlates of GH excursions.

CONCLUSION

In healthy men, dynamic GH perturbations induced by exercise and glucose are strongly related to physical power, insulin, estradiol, and body composition, thus suggesting a network of regulatory pathways.

Acute effects of acylated ghrelin on salbutamol-induced metabolic actions in humans

The aim of this study is to describe a potential modulatory effect of acute acylated ghrelin (AG) administration on the glucose, insulin, and free fatty acids (FFA) responses to salbutamol (SALBU). Six healthy young male volunteers underwent the following four testing sessions in random order at least 7 days apart: (a) acute AG administration (1.0 μg/kg i.v. as bolus at 0'); (b) SALBU infusion (0.06 μg/kg/min i.v. from -15' to +45'); (c) SALBU infusion+AG; and (d) isotonic saline infusion. Blood samples for glucose, insulin, and FFA levels were collected every 15 min. As expected, with respect to saline, SALBU infusion induced a remarkable increase in glucose (10.8±5.6 mmol/l×min; P<0.05), insulin (2436.8±556.9 pmol/l×min; P<0.05), and FFA (18.9±4.5 mmol/l×min; P<0.01) levels. A significant increase in glucose (7.4±3.9 mmol/l×min; P<0.05) and FFA levels (10.0±2.8 mmol/l×min; P<0.01) without significant variations in insulin levels were recorded after AG administration. Interestingly, the hyperglycemic effect of AG appeared to be significantly potentiated during SALBU infusion (26.7±4.8 mmol/l×min; P<0.05). On the other hand, the stimulatory effect of SALBU on insulin and FFA was not significantly modified by AG administration. The results of this study show that acute AG administration has a synergic effect with β2-adrenergic receptor activation by SALBU on blood glucose increase, suggesting that their pharmacological hyperglycemic action takes place via different mechanisms. On the other hand, AG has a negligible influence on the other pharmacological metabolic effects of SALBU infusion.

The metabolic response to the activation of the beta-adrenergic receptor by salbutamol is amplified by acylated ghrelin

BACKGROUND

It is well recognized that beta-adrenergic receptors mediate important endocrine and metabolic actions. In fact, beta-adrenergic receptor activation negatively influences GH secretion while exerting relevant metabolic actions such as the stimulation of insulin secretion, glycogenolysis, and lipolysis.

AIM

We have already shown that the activation of the GH secretagogue receptor (GHS-R)-1a by acylated ghrelin (AG) counteracts the inhibitory effect of salbutamol (SALB), a beta2-adrenergic agonist, on GH release. The aim of the present study in humans was to clarify whether the metabolic response to SALB is affected by the infusion of AG, also known to exert significant metabolic actions.

METHODS

Six healthy young male volunteers underwent the following testing sessions in random order at least 5 days apart: a) SALB (0.06 microg/kg/min iv from 0 to 60 min) alone; b) SALB in combination with AG (1.0 microg/kg/min iv from -60 to 60 min); c) isotonic saline. Insulin, glucose, and free fatty acids (FFA) levels were evaluated every 15 min.

RESULTS

As expected, with respect to saline, SALB administration tended to increase both insulin secretion [Delta area under the curve (DeltaAUC): 0.16+/-0.09 vs 0.003+/-0.077 x 10(3) microU/ml/min; p>0.05] and FFA levels (DeltaAUC: 8.0+/-7.3 vs -4.0+/-4.0 mEq/l/min; p>0.05), while glucose levels did not change. The metabolic response to SALB was significantly modified under the exposure of AG. In fact, under AG infusion, SALB elicited a more marked increase of FFA (DeltaAUC: 22.3+/-3.2 vs 8.0+/-7.3 mEq/l/min; p<0.05) as well as a slight elevation in insulin (DeltaAUC: 0.37+/-0.11 vs 0.16+/-0.09 x 10(3) microU/ml/min; p>0.05). Under AG, the baseline glucose levels were more elevated but, again, in combination with AG, SALB did not significantly modify glucose levels.

CONCLUSIONS

Beta-adrenergic receptors and AG are likely to interact at the metabolic level. In humans, the lypolitic response to a beta2-adrenergic agonist such as SALB is amplified by AG. Meanwhile, during the co-treatment, the marginal insulinotropic effect was not associated with an increase in glycemia.

Aging and the growth hormone/insulin like growth factor-I axis

Growth hormone release and IGF-I synthesis decrease with increasing age. The regulation of the GH/IGF-I system is dependent on the integrity of the hypothalamus, pituitary and liver. During aging there are several changes which contribute to the decline in GH/IGF-I including changes in signal to the somatotrophs from growth hormone releasing hormone, somatostatin and other factors such as body composition, exercise, diet and sleep. All of these factors are discussed in detail within this review. The phenotypic similarities between aging and adult growth hormone deficiency syndrome combined with this decrease in GH/IGF-I with aging have prompted the question whether aging is a GH deficient state. The advent of recombinant growth hormone has led to a number of studies treating elderly patients with GH alone or in combination with sex steroids or exercise. The results of these studies would not back up the use of GH in elderly non-hypopituitary patients as they did not show efficacy, showed high rates of adverse events and there is also some evidence associating GH/IGF-I and risk of neoplasia. If GH therapy is to be used in this cohort of patients further long term efficacy and safety studies are required.

Neuroendocrine control of GH release during acute aerobic exercise

GH secretion declines with aging and is decreased in conditions such as obesity. Several physiologic factors alter pulsatile GH secretion, including age, gender, body composition, regional distribution of fat and in particular abdominal visceral fat, sleep, nutrition, exercise and serum concentrations of gonadal steroids, insulin and IGF-I. Acute aerobic exercise is a powerful stimulus to GH release. Available studies suggest that intensity and duration of acute exercise, fitness, and training state may all influence, in part, the GH response to exercise. Intensity of exercise plays a key role in GH response to exercise. In the present paper we will discuss the GH response during acute aerobic exercise with a focus on exercise intensity and GH release. We will also provide an overview of the neuroendocrine control of exercise-induced GH release. Finally, information related to the effects of aging and gender on the GH response to exercise will be provided.

Synergy of L-arginine and GHRP-2 stimulation of growth hormone in men and women: modulation by exercise

We investigated the ability of exercise, a multipathway, potent, physiological stimulus for GH release, to alter the synergistic interaction of L-arginine (A) and GH-related peptide (GHRP)-2 (G) observed at rest and the ability of gender to further modulate this putative interaction. Subjects (9 men and 9 early follicular phase women) completed 30 min of constant load aerobic exercise in combination with intravenous infusions of saline (S), A (30 g over 30 min), G (1 microg/kg bolus), or both (AG) in separate study sessions in randomly assigned order. Measures of GH release were logarithmically transformed for statistical analysis. Similar to rest, exercise maintained the rank order (AG > G > A > S) of effective stimulation of GH release for the key response measures in men or women, a gender disparity in the time to reach the maximal serum GH concentration, the calculated endogenous GH half-life, and the observed effect of preinfusion (basal) serum GH concentrations on determining secretagogue responsiveness. Exercise potentiated the individual stimulatory actions of A and G, while blunting the relative magnitude of the synergistic (supra-additive) interaction observed at rest. We infer from the present data that 1) exercise is likely to induce release of both GHRH and somatostatin, 2) L-arginine may facilitate the effect of exercise by limiting somatostatin release, 3) GHRP-2 could further enhance the stimulatory impact of exercise by opposing central actions of somatostatin and/or heightening endogenous GHRH release, and 4) gender strongly controls the relative but not absolute magnitude of A/G synergy both at rest and after exercise.

Synergy of L-arginine and growth hormone (GH)-releasing peptide-2 on GH release: influence of gender

We test the hypotheses that 1) growth hormone (GH)-releasing peptide-2 (G) synergizes with L-arginine (A), a compound putatively achieving selective somatostatin withdrawal and 2) gender modulates this synergy on GH secretion. To these ends, 18 young healthy volunteers (9 men and 9 early follicular phase women) each received separate morning intravenous infusions of saline (S) or A (30 g over 30 min) or G (1 microg/kg) or both, in randomly assigned order. Blood was sampled at 10-min intervals for later chemiluminescence assay of serum GH concentrations. Analysis of covariance revealed that the preinjection (basal) serum GH concentrations significantly determined secretagogue responsiveness and that sex (P = 0.02) and stimulus type (P < 0.001) determined the slope of this relationship. Nested ANOVA applied to log-transformed measures of GH release showed that gender determines 1) basal rates of GH secretion, 2) the magnitude of the GH secretory response to A, 3) the rapidity of attaining the GH maximum, and 4) the magnitude or fold (but not absolute) elevation in GH secretion above preinjection basal, as driven by the combination of A and G. In contrast, the emergence of the G and A synergy is sex independent. We conclude that gender modulates key facets of basal and A/G-stimulated GH secretion in young adults.

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.

Effects of beta-adrenergic agonists and antagonists on the growth hormone response to growth hormone-releasing hormone in anorexia nervosa

BACKGROUND

In anorexia nervosa (AN), growth hormone (GH) hypersecretion and low insulin-like growth factor I (IGF-I) levels are present. It is unclear whether this is due to a peripheral GH resistance and a reduced IGF-I negative feedback on GH secretion or to a primary hypothalamic dysfunction. In AN, in contrast to normal subjects, cholinergic antagonists and agonists, whose action is somatostatin (SS)-mediated, have reduced and absent effects on the GH response to growth hormone-releasing hormone (GHRH). Since arginine, another substance acting via inhibition of SS, maintains its potentiating effect on GH secretion in AN, it has been hypothesized that somewhat specific alteration of the SS-mediated cholinergic influence may be present in this condition. To further clarify the neural control of AH secretion in AN, we evaluated the effects of beta-adrenergic agonists and antagonists, which are known to inhibit and increase, respectively, the GHRH-induced GH secretion in normal subjects.

METHODS

We studied the effect of atenolol (ATE), a beta 1-adrenergic antagonist, and salbutamol (SALB), a beta 2-adrenergic agonist, on the GHRH-induced GH release in 10 patients with AN and in 10 normal age-matched women (NW).

RESULTS

Basal GH levels were higher, whereas IGF-I were lower in AN than in NW. The GHRH-induced GH rise in AN was higher than that in NW. ATE significantly enhanced the GH response to GHRH in NW, but not in AN. The GH responses to GHRH after ATE pretreatment were similar in NW and in AN. The GH response to GHRH was inhibited by SALB in both NW and AN. The GH responses to GHRH after SALB pretreatment were similar in NW and AN.

CONCLUSIONS

These data reveal an exaggerated somatotrope responsiveness to GHRH in AN that is not further increased by beta-adrenergic blockade, while is abolished by beta-adrenergic activation. This suggests that an impairment of beta-adrenergic influence on GH secretion is present in anorexia nervosa.

Reduction of the somatotrope responsiveness to GHRH and Hexarelin but not to arginine plus GHRH in hyperprolactinemic patients

Aim of the present study was to verify the maximal secretory capacity of somatotrope cells in patients with pathological hyperprolactinemia (HPRL) comparing it with that in normal age-matched women (NW). To this goal in 12 HPRL normal weight patients (age 28.6 +/- 2.6 yr, BMI 23.1 +/- 1.1 kg/m2) and 8 NW (27.2 +/- 0.8 yr, 22.8 +/- 0.8 kg/m2) we studied the GH response to GHRH (1 microgram/kg i.v.), GHRH plus arginine (ARG, 0.5 g/kg i.v.), an amino acid probably acting at the hypothalamic level inhibiting somatostatin release, and Hexarelin (HEX, 2 micrograms/kg i.v.), a synthetic hexapeptide belonging to GHRP family, which acts concomitantly at the pituitary and the hypothalamic level. IGF-I levels in HPRL were similar to those in NW (179.2 +/- 16.5 micrograms/l and 218.5 +/- 30.8 micrograms/l). In NW the GH response to GHRH (AUC: 1299.5 +/- 186.9 micrograms 90 min/l) was lower (p < 0.02) than those to GHRH + ARG (5252.7 +/- 846.3 micrograms 90 min/l) and HEX 3216.6 +/- 462.3 micrograms 90 min/l) which, in turn, were similar. In HPRL the GH response to GHRH (894.7 +/- 242.4 micrograms 90 min/l) was lower (p < 0.03) than that to HEX (1586.5 +/- 251.3 micrograms 90 min/l) and both were lower (p < 0.03) than that to GHRH + ARG (4468.8 +/- 941.7 micrograms 90 min/l). In HPRL the GH responses to GHRH and HEX were lower than those that in NW (p < 0.03) while that to GHRH + ARG was similar in both groups. These results demonstrate that the somatotrope responsiveness to GHRH and HEX is clearly reduced in patients with pathological hyperprolactinemia. On the other hand, in this condition the GH response to GHRH + ARG is normal. As arginine likely acts via inhibition of hypothalamic somatostatin release, these findings show that the maximal secretory capacity of somatotrope cells in hyperprolactinemia is preserved and indicate that partial refractoriness of somatotrope cells to GHRH and HEX could be due to somatostatinergic hyperactivity.

Influence of beta-adrenergic agonists and antagonists on the GH-releasing effect of Hexarelin in man

Beta-adrenergic receptors mediate the inhibitory influence of cathecolamines on GH secretion, probably via the stimulation of hypothalamic somatostatin release. Accordingly, beta-adrenergic agonists and antagonists inhibit and increase, respectively, the GH response to many stimuli, including GHRH, in man. Aim of the present study was to verify the effect, if any, of beta-adrenergic drugs on the GH response to Hexarelin, a synthetic GH-releasing hexapeptide. Interestingly, the GH-releasing effect of Hexarelin has been reported to be partially refractory to neuroendocrine manipulations known to strongly enhance or abolish the GHRH-induced GH release. In 6 normal male volunteers (aged 22-27 yr) we studied the interaction of the maximally effective iv dose of Hexarelin (HEX, 2 micrograms/kg iv at 0 min) with atenolol (100 mg po at -60 min) or salbutamol (0.08 mg/kg po at -60 min), which are beta-adrenergic antagonist and agonist, respectively. HEX induced a marked GH rise (AUC, mean +/- SE: 4573.2 +/- 588.8 micrograms.min/L), which was unchanged by atenolol (4706.2 +/- 928.2 micrograms.min/L) but blunted by salbutamol (2792.8 +/- 618.0 micrograms.min/L, p < 0.03). In conclusion, present data show that, in man, the GH-releasing effect of Hexarelin is not enhanced by beta-adrenergic blockade while is only blunted by the activation of beta receptors. According to other data, these results indicate that the potent GH-releasing activity of Hexarelin is, at least partially, refractory to beta-adrenergic-mediated manipulations of somatostatinergic activity.

Effect of two beta 2-agonist drugs, salbutamol and broxaterol, on the growth hormone response to exercise in adult patients with asthmatic bronchitis

The aim of our study was to evaluate the effect of the iv administration of two different beta 2- receptors agonists, salbutamol and broxaterol, on the growth hormone (GH) response to maximal exercise in 11 patients (8 males and 3 females; age range 18-65 yr; mean +/- SE age 56 +/- 13 yr; BMI 26.2 +/- 1.4 kg/m2) with chronic asthmatic bronchitis. All the subjects underwent four cycloergometric exercise tests (incremental workload until maximal predicted heart rate). At baseline, at maximal exercise, at the end of the recovery period and 60 min after the end of each exercise, blood samples were drawn for the assay of GH, glucose, insulin, lactates, norepinephrine and epinephrine. Two exercises were performed without treatment while the remaining two were performed 60 min after the administration of 400 micrograms of either salbutamol or broxaterol (both diluted in 10 ml of saline) according to a randomized double blind cross-over design. Both exercise tests performed without treatment caused a significant (p < 0.05) and similar GH peak with respect to baseline values (from 0.3 +/- 0.1 micrograms/L to 2.8 +/- 1.3 micrograms/L, mean of the two exercise tests). Salbutamol pretreatment blunted the GH response to exercise which caused a no more significant serum GH peak over the baseline levels (from 0.6 +/- 0.2 micrograms/L to 1.4 +/- 0.6 micrograms/L,). Moreover, broxaterol completely abolished the GH response to exercise (baseline level 0.6 +/- 0.2 micrograms/L; peak levels 0.4 +/- 0.1 micrograms/L). The serum GH peak after exercise + broxaterol was significantly (p < 0.05) lower as compared to exercise + salbutamol. In conclusion, we have demonstrated for the first time that beta 2 stimulation blunts the physiological GH response to maximal exercise in adult human subjects. It can be suggested that changes in brain neurotransmitters, possibly an increase in the alpha-adrenergic tone, are likely to be involved in this endocrine effects of exercise.