Adrenal responsiveness to high, low and very low ACTH 1-24 doses in obesity.

OBJECTIVE: To investigate adrenal activity in visceral obesity in which adrenal hyperactivity has been hypothesized. This could reflect hypothalamus-pituitary alterations leading to slight hyperfunction of the adrenal. Primary adrenal hypersensitivity to ACTH drive in obesity has also been suggested. However, it has also been reported that dehydroepiondrosterone (DHEA) levels in obesity are reduced and it has been hypothesized that this could play a role in the increased cardiovascular risk in obese patients. SUBJECTS: We have studied seven obese women with visceral adiposity (OB, age: 33.6+/-3.3 years, BMI: 33.8+/-1.3 kg/m2, WHR: 0.88+/-0.01). The results in OB were compared with those recorded in a group of age-matched normal women (NS, age: 30+/-1.3 years, BMI: 19.9+/-0.4 kg/m2, WHR: 0.76+/-0.02). METHODS: We have studied the cortisol (F), aldosterone (A) and DHEA responses to ACTH 1-24 administered at low (LD, 0.5 microg/m2) or very low (VLD, 0.125 microg/m2) dose followed by a second challenge with supramaximal dose (HD, 250 microg). RESULTS: Basal F, A and DHEA levels in OB were similar to those in NS. The peak F responses to ACTH were dose-related in both groups. At each dose the F peaks in OB (VLD: 495.6+/-43.9 nmol/l, HD: 722.3+/-67.7 nmol/l; LD: 519.2+/-46.0 nmol/l, HD: 729.6+/-44.7 nmol/l) were similar to those in NS (VLD: 556.7+/-45.9 nmol/l, HD: 704.8+/-20.7 nmol/l; LD: 511.8+/-22.8 nmol/l, HD: 726.7+/-26.5 nmol/l). The peak A responses to ACTH were dose-related in both groups. At each dose, the A peaks in OB (VLD: 0.55+/-0.03 pmol/l, HD: 0.79+/-0.09 pmol/l; LD: 0.63+/-0.04 pmol/l, HD: 0.78+/-0.09 pmol/l) were similar to those in NS (VLD: 0.8+/-0.10 pmol/l, HD: 0.86+/-0.09 pmol/l; LD: 0.8+/-0.10 pmol/l, HD: 0.95+/-0.12 pmol/l). The peak DHEA responses to ACTH were dose-related in both groups. At each dose the DHEA peaks in OB (VLD: 58.6+/-13.3 nmol/l, HD: 61.9+/-13.1 nmol/l; LD: 55.18+/-6.4 nmol/l, HD: 72.3+/-9.8 nmol/l) were similar to those in NS (VLD: 54.3+/-8.2 nmol/l, HD: 57.8+/-8.2 nmol/l; LD: 42.2+/-3.7 nmol/l, HD: 56.9+/-4.3 nmol/l). CONCLUSIONS: This study shows that the cortisol, aldosterone and dehydroepiondrosterone responses to high, low and very low ACTH doses in obese women overlap with those in age-matched lean controls; these findings suggest normal sensitivity of the different zones of the adrenal cortex to ACTH in obesity.

Comparisons among old and new provocative tests of GH secretion in 178 normal adults.

Classical provocative stimuli of GH secretion such as insulin-induced hypoglycaemia, arginine, clonidine, glucagon and levodopa have been widely used in clinical practice for approximately 30 years. On the other hand, in the last 10 years new potent stimuli of GH secretion have been proposed, but an extensive comparison with the classical ones has rarely been performed, at least in adults. In order to compare the GH-releasing activity of old and new provocative stimuli of GH secretion, and to define the normative values of the GH response, in 178 normal adults (95 males, 83 females; age range: 20-50 years, all within +/-15% of their ideal body weight), we studied the GH response to: insulin-induced hypoglycaemia (ITT, 0.1IU/kg i.v.), arginine (ARG, 0.5g/kg i.v.), clonidine (CLO, 300 microg/kg p.o.), glucagon (GLU, 1mg i.m.), pyridostigmine (PD, 120mg p.o.), galanin (GAL, 80pmol/kg per min), GH-releasing hormone (GHRH, 1 microg/kg i.v.), GHRH+ARG, GHRH+PD, hexarelin, a GH-releasing protein (HEX, 2 microg/kg i.v.) and GHRH+HEX (0.25 microg/kg i.v.). The mean (+/-s.e.m.) peak GH response to ITT (21.8+/-2.8, range: 3.0-84.0 microg/l) was similar to those to ARG (18.0+/-1.6, range: 2.9-39.5 microg/l) or GLU (20. 5+/-2.2, range: 10.6-36.9 microg/l) which, in turn, were higher (P<0. 001) than those to CLO (8.2+/-1.6, range: 0.3-21.5 microg/l), PD (9. 6+/-1.1, range: 2.2-33.0 microg/l) and GAL (9.3+/-1.1, range: 3.9-18. 3 microg/l). The GH response to GHRH (19.1+/-1.5, range: 2.7-55.0 microg/l) was similar to those after ITT, ARG or GLU but clearly lower than those after GHRH+ARG (65.9+/-5.5, range: 13.8-171.0 microg/l) and GHRH+PD (50.2+/-4.6, range: 17.7-134.5 microg/l) which, in turn, were similar. The GH response to HEX (55.3+/-5.5, range: 13.9-163.5 microg/l) was similar to those after GHRH+ARG and GHRH+PD but lower (P<0.001) than that after GHRH+HEX (86.0+/-4.3, range: 49. 0-125.0 microg/l) which was the most potent stimulus of GH secretion. In this adult population the third centile limits of peak GH response to various stimuli were the following: ITT: 5.3; ARG: 2.9; CLO: 1.5; GLU: 7.6; PD: 2.2; GAL: 4.0; GHRH: 5.0; GHRH+ARG: 17.8; GHRH+PD: 17.9; HEX: 21.6; GHRH+HEX: 57.1. These results confirm that, among classical provocative tests of GH secretion, ITT followed by ARG and GLU are the most potent ones and possess clear limits of normality. GHRH+ARG or PD and HEX are strong stimuli of GH secretion which, however, is maximally stimulated by a combination of GHRH and a low dose of HEX. It is recommended that each test is used with appropriate cut-off limits.

Glucagon is an ACTH secretagogue as effective as hCRH after intramuscolar administration while it is ineffective when given intravenously in normal subjects.

It is widely accepted that glucagon stimulates GH, ACTH and cortisol release in humans, though the mechanisms underlying these effects are unclear. Aim of the present study was to evaluate the stimulatory effect of intramuscolar (i.m.) and intravenous (i.v.) glucagon (GLU) administration on ACTH, cortisol (F) and GH release in normal adult subjects and to compare its effect on hypothalamo-pituitary adrenal (HPA) axis with that of hCRH. To this goal, in 6 normal young women (26-32 yrs, 50-58 kg) we studied the ACTH and F responses to either i.m. or i.v. GLU (1 mg, approximately 0.017 mg/kg in subjects of 54.1 +/- 1.6 kg) administration as well as to i.v. hCRH (2.0 micrograms/kg) or placebo administration. The GH and glucose variations after GLU administration were also studied. I.v. GLU did not modify the spontaneous decrease of ACTH and cortisol levels observed after placebo. Conversely, i.m. GLU elicited clear-cut ACTH and F responses (peak vs baseline, mean +/- SEM: 53.0 +/- 15.2 vs 19.0 +/- 1.5 pg/ml, p < 0.05 and 222.3 +/- 23.8 vs 158.3 +/- 7.0 micrograms/l, p < 0.05) which were higher than those recorded after hCRH (28.1 +/- 4.6 vs 17.4 +/- 3.1 pg/ml, p < 0.02 and 182.7 +/- 22.8 vs 114.8 +/- 12.3 micrograms/l p < 0.02), though this difference did not attain statistical significance. Also GH rise was recorded after i.m. but not after i.v. GLU administration (11.6 +/- 3.4 vs 3.3 +/- 0.7 micrograms/l, p < 0.05). Thirty min after both i.v. and i.m. GLU administration glucose levels showed a similar increase followed by similar decrease. The intramuscular administration of GLU induced negligible side-effects in some subject (mild and transient nausea) which, on the contrary, were clear in all subjects after its intravenous administration (nausea, vomiting, tachycardia). In conclusion, glucagon "per se" is not an ACTH, cortisol and GH secretagogue. After intramuscular administration glucagon is a stimulus of HPA axis at least as effective as hCRH. The mechanisms underlying the ACTH, cortisol and GH responses to i.m. glucagon unlikely include glucose variations or stress.

Human aging and the GH-IGF-I axis.

The activity of the GH-IGF-I axis undergoes an age-related reduction and in the elderly both spontaneous GH secretion and IGF-I levels are frequently low overlapping with those usually recorded in GH deficient patients. Hypoactivity of the GH-IGF-I axis could explain age-related changes in body composition, function and metabolism, as also indicated by evidence that treatment with rhGH reverses these alterations. The mechanisms underlying the hypoactivity of the GH-IGF-I axis in the aged likely include changes in nutrition and lifestyle, e.g. reduction of physical exercise. However, alterations of neurohormonal hypothalamic control of GH secretion, including reduced activity of GHRH-secreting neurons and somatostatinergic hyper-activity, seem to play a major role. The exaggerated somatostatinergic hyperactivity could be due, in turn, to the impairment of cholinergic activity found in the aging brain. Age-related variations in the activity of other neurotransmitters, such as catecholamines, amino acids, e.g. arginine, neuropeptides, e.g. galanin and/or a putative natural GHRP-like ligand, could play a key role in causing the reduced activity of the GH-IGF-I axis. It is still unclear whether it is of benefit to restore GH secretion in aging. As the pituitary GH releasable pool is preserved in the elderly, it would be more appropriate to increase GH by GH secretagogues such as the new synthetic GH-releasing peptides (GHRPs) or non-peptidyl GHRP mimetics which are active even with oral administration.

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.