Search

Objective: The diagnosis of growth hormone (GH) deficiency (GHD) in adults is based on a reduced peak GH response to provocative tests, such as the insulin tolerance test (ITT) and the GH-releasing hormone-arginine (GHRH-ARG) test. However, the cut-off limits of peak GH response in lean subjects are not reliable in obese patients; this is noteworthy since adult GHD is often associated with obesity. Aim of this study was to evaluate the diagnostic cut-off limits of peak GH response to the GHRH-ARG test in overweight and obese as well as in lean population., Design and Methods: The GH responses to the GHRH-ARG test were studied in 322 patients with organic hypothalamic-pituitary disease and in 318 control subjects. Patients were subdivided into two groups on the basis of the number of pituitary hormone deficits, except for GH deficiency: (a) patients with total pituitary hormone deficit (TPHD) and (b) patients without or with no more than two pituitary hormone deficits (PHD). Both patients and control subjects were divided into three subgroups according to body mass index (BMI): lean (BMI <25 kg/m(2)), overweight (BMI > or = 25 and <30 kg/m(2)) and obese (BMI > or =30 kg/m(2)). TPHD patients were assumed to be GH deficient, whereas PHD patients may include subjects with either normal or impaired GH secretion. The statistical analysis was carried out by the Receiver-Operating Characteristic curve analysis (Medcalc 7.2). The diagnostic cut-off points were calculated for lean, overweight and obese subjects to provide optimal separation of GH-deficient patients and control subjects according to two criteria: (1) a balance between high sensitivity and high specificity; (2) to provide the highest pair of sensitivity/specificity values for GH deficiency., Results: In the lean population the best pair of values, with highest sensitivity as 98.7% and highest specificity as 83.7%, was found using a peak GH cut-off point of 11.5 mug/l. In the overweight population the best pair of values, 96.7 and 75.5%, respectively, was found using a peak GH cut-off point of 8.0 mug/l. In the obese population the best pair of values, 93.5 and 78.3%, respectively, was found using a peak GH cut-off point of 4.2 mug/l. Applying the above mentioned cut-off points, among PHD patients we found that 80 subjects (72%) were GHD whereas 31 (28%) had normal GH secretion., Conclusions: In conclusion the GHRH-ARG test is a reliable tool for the diagnosis of adult GH deficiency in lean, overweight and obese patients, provided that specific BMI-related cut-off limits are assumed.

Ghrelin possesses central and peripheral endocrine actions including influence on the endocrine pancreatic function. To clarify this latter ghrelin action, in seven normal young subjects [age (mean +/- SEM), 28.3 +/- 3.1 yr; body mass index, 21.9 +/- 0.9 kg/m(2)), we studied insulin and glucose levels after acute ghrelin administration (1.0 microg/kg i.v.) alone or combined with glucose [oral glucose tolerance test (OGTT), 100 g orally], arginine (ARG, 0.5 g/kg i.v.) or free fatty acid (FFA, Intralipid 10%, 250 ml). Ghrelin inhibited (P < 0.05) insulin and increased (P < 0.05) glucose levels. OGTT increased (P < 0.01) glucose and insulin levels. FFA increased (P < 0.05) glucose but did not modify insulin levels. ARG increased (P < 0.05) both insulin and glucose levels. Ghrelin did not modify both glucose and insulin responses to OGTT as well as the FFA-induced increase in glucose levels; however, ghrelin administration was followed by transient insulin decrease also during FFA. Ghrelin blunted (P < 0.05) the insulin response to ARG and enhanced (P < 0.05) the ARG-induced increase in glucose levels. In all, ghrelin induces transient decrease of spontaneous insulin secretion and selectively blunts the insulin response to ARG but not to oral glucose load. On the other hand, ghrelin raises basal glucose levels and enhances the hyperglycemic effect of ARG but not that of OGTT. These findings support the hypothesis that ghrelin exerts modulatory action of insulin secretion and glucose metabolism in humans.

Objective: Ghrelin, a 28 amino acid peptide purified from the stomach and showing a unique structure with an n-octanoyl ester in serine-3 residue, is a natural ligand of the GH secretagogue (GHS) receptor (GHS-R) and strongly stimulates GH secretion. In humans, ghrelin is more potent than growth hormone-releasing hormone (GHRH) and non-natural GHS such as hexarelin. Moreover, ghrelin shows a true synergism with GHRH, has no interaction with hexarelin and, similarly to non-natural GHS, is partially refractory to the inhibitory effect of exogenous somatostatin (SS). Despite this evidence, the mechanisms underlying the GH-releasing effect of ghrelin in humans have not been fully clarified., Subjects: To this aim we enrolled six normal young volunteers [age (mean +/- SEM) 28.9 +/- 3.1 year; body mass index 22.3 +/- 1.0 kg/m2)., Design and Measurements: In all subjects we studied the effects of glucose (OGTT, 100 g oral glucose at -45 min) or free fatty acids (FFA) load [lipid-heparin emulsion, Li-He, Intralipid 10% 250 ml + heparin 2500 U i.v. from -30 to +120 min] as well as of arginine (ARG, 0.5 g/kg infused from 0 to +30 min) on the GH response to human ghrelin (1.0 micro g/kg i.v. at 0 min) administration. These results were compared with those obtained by studying the effects of OGTT, Li-He and ARG on the GH response to GHRH-29 (1.0 micro g/kg i.v. at 0 min)., Results: The GH response to ghrelin (auc 5452.4 +/- 991.3 micro g/l/h) was higher (P < 0.05) than that after GHRH (1519.4 +/- 93.3 micro g/l/h). The GH response to GHRH was inhibited by OGTT (450.7 +/- 81.1 micro g/l/h, P < 0.05) and almost abolished by Li-He (230.0 +/- 63.6 micro g/l/h, P < 0.05) while was markedly potentiated by ARG (2520.4 +/- 425.8 micro g/l/h, P < 0.05). The GH response to GHRH + ARG, however, was lower (P < 0.05) than that to ghrelin alone. The GH response to ghrelin was blunted by OGTT (2153.1 +/- 781.9 micro g/l/h, P < 0.05) as well as by Li-He (3158.8 +/- 426.7 micro g/l/h, P < 0.05) but these responses remained higher (P < 0.05) than that to GHRH alone. On the other hand, ARG did not modify the GH response to ghrelin (6324.3 +/- 1275.5 micro g/l/h). For GH 1 micro g/l = 2 mU/l., Conclusions: In humans, ghrelin exerts a strong stimulatory effect on GH secretion which is partially refractory to the inhibitory effect of both glucose and FFA load and is not enhanced by ARG. These factors almost abolish and potentiate, respectively, the GH response to GHRH, at least partially, via modulation of hypothalamic SS release. Thus, our findings agree with the hypothesis that ghrelin as well as non-natural GHS acts, at least partially, by antagonizing SS activity.

Insulin-like growth factor I (IGF-I) exerts a negative feedback effect on GH secretion via either direct actions at the pituitary level or indirect ones at the hypothalamic level, through stimulation of somatostatin (SS) and/or inhibition of GHRH release. In fact, recombinant human IGF-I (rhIGF-I) in humans inhibits spontaneous GH secretion as well as the GH response to GHRH and even more to GH/GH-releasing peptides, whose main action is on the hypothalamus, antagonizing SS and enhancing GHRH activity. The aim of the present study was to further clarify in humans the mechanisms underlying IGF-I-induced inhibition of somatotroph secretion. In six normal young volunteers (all women; mean +/- SEM: age, 28.3+/-1.2 yr; body mass index, 21.3+/-1.2 kg/m2) we studied the GH response to GHRH (1 microg/kg, iv, at 0 min), both alone and combined with arginine (ARG; 0.5 g/kg, iv, from 0-30 min), which probably acts via inhibition of hypothalamic SS release, after pretreatment with rhIGF-I (20 microg/kg, sc, at -180 min) or placebo. rhIGF-I increased circulating IGF-I levels (peak at -60 vs. -180 min: 54.9+/-3.9 vs. 35.9+/-3.3 mmol/L; P < 0.05) to a reproducible extent, and these levels remained stable and within the normal range until 90 min. The mean GH concentration over 3 h (from -180 to 0 min) before ARG and/or GHRH was not modified by placebo or rhIGF-I. After placebo, the GH response to GHRH (peak, 23.6+/-2.9 microg/L) was strikingly enhanced (P < 0.05) by ARG coadministration (69.6+/-9.9 microg/L). rhIGF-I blunted the GH response to GHRH (13.1+/-4.5 microg/L; P < 0.05), whereas that to GHRH plus ARG was not modified (59.5+/-8.9 microg/L), although it occurred with some delay. Mean glucose and insulin concentrations were not modified by either placebo or rhIGF-I. In conclusion, ARG counteracts the inhibitory effect of rhIGF-I on somatotroph responsiveness to GHRH in humans. These findings suggest that the acute inhibitory effect of rhIGF-I on the GH response to GHRH takes place on the hypothalamus, possibly via enhancement of SS release, and that ARG overrides this action.

Background: Diagnosing GH deficiency in adults is difficult due to the age-related variations of GH/IGF-I axis and the influence of nutrition. Nowadays, GH replacement is allowed for patients with GH peak to provocative stimuli < 3 micrograms/L. Somatotrope insufficiency is present in hypopituitarism but also in obesity and hypercortisolism. However, to evaluate GH insufficiency in adults is difficult due to variations of GH and IGF-I levels as function of age and nutrition status., Methods: We aimed to verify the GH response to GHRH (1 mg/kg i.v.) combined with pyridostigmine (PD, 120 mg p.o.) or arginine (ARG, 0.5 g/kg i.v.), in 26 hypopituitaric patients (GHD), in 11 obese women (OB), in 8 women with Cushing's syndrome (CS), and in 72 control subjects (NS)., Results: IGF-I levels in GHD were lower than those in OB (p < 0.01) and in CS (p < 0.01) which, in turn, were lower to those in NS (p < 0.02). In NS, the GH peak responses to GHRH + PD and GHRH + ARG were similar and the minimum normal GH peak was 16.5 mg/L. GHD had GH responses similar, lower than those in NS (p < 0.01) and always below the normal limit. However, only 12/20 and 8/14 had peaks < 3 micrograms/L; conventionally, below this limit severe GH deficiency is shown and rhGH replacement is allowed. In OB, the GH responses to GHRH + PD and GHRH + ARG were similar, lower (p < 0.01) and higher (p < 0.01) than those in NS and GHD, respectively. Six out of 11 OB had GH peaks below the normal limits but nobody < 3 micrograms/L. In CS, the GH response to GHRH + PD was lower than that to GHRH + ARG (p < 0.01); both these responses were lower than those in NS (p < 0.01) and even in OB (p < 0.01) but higher than those in GHD (p < 0.01). All and 7/8 CS had GH peaks lower than normal limits after PD + GHRH and ARG + GHRH, respectively while 6/8 showed GH peak < 3 micrograms/L after PD + GHRH but only 1 after ARG + GHRH., Conclusions: Present data demonstrate that the maximal somatotrope secretory capacity is reduced in OB and even more in CS. From a diagnostic point of view, PD + GHRH and ARG + GHRH tests distinguish OB from severe GHD. As hypercortisolism impairs the activity of cholinesterase inhibitors, only ARG + GHRH, but not PD + GHRH is a reliable test to explore the maximal somatotrope secretory capacity in CS. Notably, even with the ARG + GHRH test, in CS the maximal somatotrope secretory capacity is sometimes so reduced as to overlap with that of severe GHD.

The GH response to provocative stimuli in obese is often as low as in panhypopituitaric patients with severe GHD; however, IGF-I levels are normal or slightly reduced. In 53 patients with simple obesity (11 M and 42 F, age: 40.3+/-1.6 yr, BMI: 39.1+/-1.0 Kg/m2), we evaluated the GH response to GHRH (1 microg/kg iv)+arginine (ARG, 0.5 g/kg iv), and total IGF-I levels. The mean (+/-SE) GH peak after GHRH+ARG was markedly lower (74% reduction, p<0.0001) in obese (16.8+/-2.0 microg/l) than in normal subjects (62.7+/-4.3 microg/l). IGF-I levels in obese patients (134.0+/-7.6 microg/l) were lower (33% reduction, p<0.001) than in normal subjects (200.8+/-5.7 microg/l). Taking into account the 3rd centile limit of normal response, the GH response to GHRH+ARG was reduced in 62.3% (33/53) of the obese patients, and 21.2% (7/33) of them had low IGF-I levels. Assuming the 1st centile limit, it was reduced in 33.9% (18/53) obese subjects, and 22% (4/18) of them had low IGF-I levels. Considering 3.0 microg/L as arbitrary cut-off, the GH response was reduced in 5.7% (3/53) of the obese patients, and still one of them had low IGF-I levels. Our findings: a) confirm that the secretory capacity of somatotroph cells is often deeply impaired in obesity; b) demonstrate that IGF-I assay generally rules out severe impairment of GH/IGF-I axis in obese patients with marked reduction of the GH secretion; c) indicate that the percentage of obese patients with concomitant reduction of GH secretion and IGF-I levels is not negligible. Thus, IGF-I assay should be routinely performed in obese patients; those presenting with low IGF-I levels should undergo further evaluation of their hypothalamo-pituitary function and morphology, particularly in the presence of empty sella.

Arginine has well-known stimulatory effects on GH, PRL and insulin secretion in man but the mechanisms underlying these effects are still unclear. More recently, it has been demonstrated that arginine is the precursor of nitric oxide (NO) which mediates its vasodilatatory effect. Thus, it has been hypothesized that NO could also mediate the hormonal effects of arginine. To clarify this point, in seven normal young volunteers (7 normal male subjects, age 26-35 yr) we compared the effects of arginine hydrochloride (ARG, 0.5 g/kg iv over 30 min) on GH, PRL, insulin and glucose levels as well as on blood pressure, with those of isosorbide dinitrate (ISDN, 5 mg po) and molsidomine (MOLS, 4 mg po), two NO donors which possess well-known vasodilatatory effects. ARG infusion elicited a clear-cut GH increase (peak vs baseline 17.6 +/- 4.7 vs 2.7 +/- 0.8 (g/L, p < 0.01), PRL (20.6 +/- 2.8 vs 6.9 +/- 0.5 (g/L, p < 0.01) and insulin levels (31.4 +/- 5.7 vs 4.5 +/- 2.1 (U/L, p < 0.01) while induced a biphasic variation of plasma glucose levels with early increase (p < 0.01), followed by late decrease below basal values (p < 0.01). On the other hand, blood pressure was decreased by ARG (nadir vs baseline; systolic: 103 +/- 6 vs 112 +/- 3, p < 0.02 and diastolic 61 +/- 4 vs 72 +/- 2 mmHg, p < 0.02, respectively). ISDN and MOLS did not modify basal GH, PRL and insulin as well as glucose levels while induced a clear reduction in blood pressure (ISDN: nadir vs baseline; systolic: 94 +/- 4 vs 112 +/- 2, p < 0.02; diastolic 69 +/- 3 vs 80 +/- 2, p < 0.02; MOLS: systolic: 94 +/- 3 vs 113 +/- 2 p < 0.02; diastolic 63 +/- 4 vs 72 +/- 2, p < 0.02). The lowering effect of both ISDN and MOLS on both systolic and diastolic blood pressure levels was higher than that induced by ARG. The effect of the latter was, in turn, significantly different from that of placebo on diastolic levels only. In conclusion, our present date are against the hypothesis that NO mediates the stimulatory effects of arginine on GH, PRL and insulin secretion. On the other hand, our findings agree with the hypothesis that ARG has NO-mediated vasodilatatory effect able to decrease blood pressure in man.

Parasympathetic nervous system is known to affect insulin secretion in animal and man and there is evidence that it is involved in the outcome of spontaneous and stimulated insulin hypersecretion observed in animal obesity. In human obesity, there are contradictory data. We studied the effect of 150 mg orally administered pirenzepine (PNZ), a muscarinic receptor antagonist, on the insulin response to glucose (75 g p.o. or 0.33 g/kg i.b.w. i.v.) or arginine (0.5 g/kg infused in 30 min) in 18 obese subjects normotolerant to glucose. PNZ did not modify basal serum insulin and the hormone response to either intravenous glucose (AUC: 5221.6 +/- 1177:6 vs 5309.8 +/- 1534.8 mU/L.min) or arginine load (4257.9 +/- 832.7 vs 3952.8 +/- 549.3 mU/L.min). Calculated as AUC the insulin response to oral glucose load was unaffected by PNZ (6601.5 +/- 1218.6 vs 8614.3 +/- 1095.2 mU/L.min). Actually, the insulin rises at +30 min after oral glucose load was significantly blunted by PNZ (37.0 +/- 3.4 vs 81.6 +/- 16.9 mU/L; p < 0.03). However, after statistical evaluation by ANCOVA assuming basal insulin and +30 min glucose levels as covariates, this significant disappeared. Our present data do not agree with the hypothesis that the cholinergic system plays a role in the exaggerated insulin secretion of obesity. Nevertheless, these findings confirm that acetylcholine positively influences insulin secretion in humans, likely via indirect mechanisms.

In hyperprolactinemic patients an exaggerated glucose-induced insulin secretion has been reported, but these results have not been confirmed by other researchers. On the other hand, there are few data concerning somatotrope secretion in this condition. In order to clarify these points, in seven normal weight hyperprolactinemic female patients (HP: age 18-46 years, body mass index = 21.8 +/- 0.6 kg/m(2), basal prolactin = 91.7 +/- 16.5 micrograms/l) we studied the effects of glucose load (100 g orally) and/or arginine (0.5 g/kg infused over 30 min) on insulin glucose and growth hormone (GH) levels. These results were compared with those obtained in seven patients with simple obesity (OB: age 23-48 years, body mass index = 38.3 +/- 2.6 kg/m(2)) in whom exaggerated insulin and low GH secretion are well known. Seven normal women (NS: age 26-32 years, body mass index = 20.6 +/- 1/9 kg/m(2)) were studied as controls. The insulin response to glucose in HP (area under curve = 11,460.8 +/- 1407.5 mU x min x l(-1)) was not significantly different from NS (7743.7 +/- 882.9 mU x min x l(-1)) and OB (14,504.8 +/- 1659.9 mU x min x l(-1)). The arginine-induced insulin release in HP and OB was similar (4219.4 +/- 631.7 and 4107.3 +/- 643.2 mU x min x l(-1), respectively), both being higher (p < 0.02) than in NS (2178.1 +/- 290.9 mU x min x l(-1). Glucose and arginine had an additive effect on insulin release in HP and NS (19,769.1 +/- 3249.6 and 10,996.6 +/- 1201.0 mU x min 1(-1), respectively) and a synergistic effect in OB (28 117.3 +/- 5224.7 mU x min x l(-1)). In HP the insulin response to the combined administration of glucose and arginine was not significantly different from the one in OB, and both were higher (p < 0.05) than in NS. The increase in glucose levels after glucose administered on its own or combined with arginine was higher (p < 0.02) and longer lasting in OB than in NS and HP. After arginine in OB, the glucose levels did not show the late decrease under baseline values observed in HP and NS. Glucose inhibited GH secretion both in HP and NS (p < 0.05), while arginine stimulated it in all groups, although the GH response in HP and NS was higher (p < 0.03) than in OB. The arginine-induced GH secretion was inhibited by glucose in HP and NS but not in OB. These results demonstrate that both in hyperprolactinemic patients and in obesity there is a clear increase in insulin secretion. The insulin hyperresponsiveness in hyperprolactinemia is more clearly demonstrated by combined stimulation with glucose and arginine. In spite of similar insulin hypersecretion in hyperprolactinemic and obese patients, GH secretion is reduced only in the latter; with these data the hypothesis that somatotrope insufficiency in obesity is due to hyperinsulinism is unlikely.

Dopaminergic system seems to influence the regulation of insulin secretion, although in man conflicting data are reported. Furthermore, bromocriptine (BRC), a dopaminergic agonist, has been recently found to inhibit the seasonally occurring hyperinsulinemia and the increase in body weight in the hamster. On this basis, we investigated the effect of BRC on spontaneous and stimulated insulin secretion in human obesity. Six obese (BMI: 33.2 +/- 1.6 Kg/m2) underwent the administration of: 1) arginine (ARG, 0.5 g/Kg iv in 30 min), 2) BRC (2.5 mg po), 3) ARG+BRC. In each test plasma glucose and serum insulin, growth hormone (GH) and prolactin levels were determined. BRC did not significantly reduce spontaneous and ARG-induced insulin release. Baseline and stimulated glucose levels were also unchanged. BRC determined an increase in GH levels (3.7 +/- 1.3 vs 0.5 +/- 0.3 microgram/l, p < 0.05), but failed to modify the somatotrope responsiveness to ARG. On the other hand, both spontaneous and stimulated PRL secretion were reduced by BRC (2.5 +/- 0.4 vs 6.7 +/- 1.1 micrograms/l, p < 0.05 and 0.8 +/- 1.9 vs 11.0 +/- 2.1 micrograms/l, p < 0.05, respectively). Our results show that in obese patients the acute activation of dopaminergic receptors by bromocriptine fails to modify both basal and ARG-induced insulin release, while inhibits spontaneous and stimulated PRL secretion. Our data also show that the low GH response to arginine in obesity is not improved by the coadministration of bromocriptine, in agreement with the hypothesis that both substances act by the same mechanism, i.e. inhibition of endogenous somatostatin release.

Increased free fatty acid (FFA) levels of obese patients are likely involved in the pathogenesis of the growth hormone (GH) hyposecretion of obesity. To clarify their role, we studied the influence of inhibition of plasma FFA levels, induced by 500 mg oral acipimox (ACX), an antilipolytic drug, on the GH response to GH-releasing hormone (GHRH) alone or combined with arginine ([ARG] study A) in six normal women ([NS] aged 24 to 37 years; body mass index, 22.4 +/- 0.9 kg/m2) and six obese women ([OB] aged 21 to 40 years; body mass index 39.5 +/- 3.2 kg/m2). In a group of seven OB patients (aged 18 to 58 years; body mass index, 35.8 +/- 1.3 kg/m2), the effect of ACX on either GHRH- or GHRH+ARG-stimulated GH increase was also studied after a 4-day treatment with the same drug at 250 mg three times daily (study B). OB patients had baseline FFA levels higher than NS (0.77 +/- 0.06 v 0.44 +/- 0.09 mmol/L, P<.05). In study A, ACX reduced FFA levels to the same nadir in both groups (0.11 +/- 0.02 and 0.12 +/- 0.03 mmol/L, NS and OB subjects, respectively). In NS, ACX failed to significantly potentiate the GH response to either GHRH (1,371.9 +/- 425.2 v 1,001.8 +/- 229.0 micrograms/L x min) or GHRH+ARG (3558.4 +/- 1,513.7 v 3,045.9 +/- 441.8 micrograms/L x min), while in OB patients it increased the GH response to GHRH (797.6 +/- 277.3 v 353.8 +/- 136.7 micrograms/L x min, P<.01) and did not modify the response to ARG+GHRH (1,010.5 +/- 253.1 v 821.1 +/- 222.0 micrograms/L x min). In study B, ACX reduced FFA levels in OB patients (nadir, 0.09 +/- 0.04 mmol/L). This treatment strikingly increased the GH response to GHRH (1,734.0 +/- 725.4 v 271.5 +/- 112.8 micrograms/L x min, P<.01) and significantly potentiated that to ARG+GHRH (2,371.9 +/- 571.3 v 1,020.0 +/- 343.2 micrograms/L x min, P<.05). In conclusion, our present findings indicate that an acute reduction of plasma FFA levels in OB patients restores their somatotrope responsiveness, whereas it does not affect GH secretion in lean subjects. After prolonged treatment, ACX further improves GHRH-stimulated GH secretion in OB patients, suggesting that elevated FFA levels play a leading role in the GH hyposecretory state of obesity.

Objective: Fasting is known to clearly increase both spontaneous and GHRH-stimulated GH secretion in normal subjects and this effect is likely to be due to hypothalamic mechanism(s). Our aim was to clarify the effect of a 3 or 4-day fast, on the GH response to GHRH alone or combined with arginine, an amino acid probably acting via inhibition of hypothalamic somatostatin release., Design: Two tests with GHRH (1 microgram/kg i.v.), administered either alone or in combination with arginine (ARG, 0.5 g/kg i.v.) were performed, in a randomized order at least 3 days apart. In obese women the two tests were repeated after a 3 or 4-day fast., Patients: Seven obese women (OB, aged 17-54 years, BMI 42.4 +/- 3.6 kg/m2, waist-hip ratio (WHR) 0.85 +/- 0.01) and ten healthy women, as control subjects (CS, aged 20-44 years, BMI 23.1 +/- 1.1 kg/m2, WHR 0.79 +/- 0.01) were studied., Measurements: Serum GH and IGF-I levels were measured by radioimmunoassay. The GH secretory responses were expressed either as absolute values (mU/l) or as areas under the curve (AUC, mU/l/h) calculated by trapezoidal integration. IGF-I concentrations were expressed as absolute values (microgram/l) with reference to a pure recombinant IGF-I preparation. Results are expressed as mean +/- SEM., Results: Basal GH and IGF-I levels in OB were lower than in CS (0.8 +/- 0.2 vs 4.8 +/- 1.0 mU/l, P < 0.0001 and 120.1 +/- 21.4 vs 188.7 +/- 13.1 micrograms/l, P < 0.02, respectively). The GHRH-induced GH rise in OB was lower (P < 0.00001) than in CS (AUC 340.2 +/- 81.0 vs 2125.0 +/- 199.6 mU/l/h). ARG increased the GHRH-induced GH rise in both groups, but in OB the GH response to ARG+GHRH (1458.4 +/- 439.0 mU/l/h, P < 0.03 vs GHRH alone) remained lower (P < 0.0001) than in CS (6396.2 +/- 772.2 mU/l/h, P < 0.01 vs GHRH alone). In spite of a reduction in body weight and IGF-I, insulin and glucose levels, in OB fasting failed to modify both the basal GH levels and the somatotroph responsiveness to GHRH when administered either alone or combined with ARG. An increase in free fatty acids (FFA) was also found after fasting., Conclusions: The results of this study demonstrate that in obesity the somatotroph hyporesponsiveness to GHRH, either alone or combined with arginine, is not improved by short-term fasting. As fasting is considered a CNS mediated stimulus to GH secretion, its ineffectiveness in obesity does not support a hypothalamic pathogenesis and suggests that long standing metabolic alterations, such as hyperinsulinaemia and/or elevated free fatty acids, could play a major role in causing GH insufficiency in obese patients.

It is known that spontaneous and stimulated GH secretion is reduced in obesity. On the other hand, it has been recently reported that, in obese subjects, plasma GH levels did not change during a hyperglycemic clamp. To further study the sensitivity of somatotrope cells to inhibitory influences in obesity, we studied the effect of somatostatin, pirenzepine, or glucose on the GH response to GHRH or arginine in 32 obese patients and 30 controls. Basal GH levels were lower in obese than in normal subjects (1.0 +/- 0.6 vs. 4.8 +/- 0.7 micrograms/L, P < 0.05), while insulin-like growth factor-I levels were similar in both groups (137.3 +/- 13.2 vs. 138.8 +/- 12.2 micrograms/L). In obese as well as in control subjects pirenzepine abolished the GH response to either GHRH (AUC0-120: 43.7 +/- 9.6 vs. 258.3 +/- 59.9 micrograms/L/h, P < 0.04 and 113.0 +/- 75.0 vs. 870.5 +/- 255 micrograms/L.h, P < 0.01, respectively) or arginine (6.5 +/- 2.5 vs. 118.7 +/- 55.9 micrograms/L.h, P < 0.05 and 47.7 +/- 7.3 vs. 334.0 +/- 157.5 micrograms/L.h, P < 0.01, respectively). Differently from pirenzepine, glucose blunted the GH response to either GHRH or arginine in control subjects (260.8 +/- 38.3 vs. 479.5 +/- 83.9 micrograms/L.h, P < 0.03 and 294.8 +/- 46.3 vs. 625.1 +/- 139.1 micrograms/L.h, P < 0.05, respectively), but failed to modify it in obese patients (193.7 +/- 39.4 vs. 172.4 +/- 33.6 micrograms/L.h and 121.1 +/- 43.4 vs. 155.1 +/- 39.7 micrograms/L.h, respectively). On the other hand, somatostatin deeply blunted the GHRH-induced GH release in obese patients (58.5 +/- 25.4 vs. 548.7 +/- 196.6 micrograms/L.h, P < 0.05) as well as in controls (181.4 +/- 44.4 vs. 759.7 +/- 46.6 micrograms/L.h, P < 0.04). In conclusion, our results show that, in obesity, the stimulated GH release is refractory to the inhibitory effect of glucose but not of pirenzepine, in spite of their likely common mechanism of action, i.e. increase of hypothalamic somatostatin release. Exogenous somatostatin is able to abolish GH secretion both in normal and obese subjects. These data suggest the existence of a peculiar inhability of hyperglycemia to trigger somatostatinergic release in obesity.

Objectives: The aim of this work was to clarify the mechanisms underlying growth hormone (GH) hyposecretion in Cushing's syndrome (CS) and in obesity. We studied the GH response to GH-releasing hormone (GHRH) alone and combined with arginine or pyridostigmine, two substances likely to inhibit hypothalamic somatostatin release., Design: Three tests with GHRH alone (1 microgram/kg i.v. at 0 min) and combined with arginine (ARG, 0.5 g/kg infused over 30 min) or pyridostigmine (PD, 120 mg orally at -60 min) were performed 3 days apart and in random order in eight women with CS (five with ACTH-dependent and three with ACTH-independent hypercortisolism, age 18-56, BMI 26.1 +/- 1.5 Kg/m2) and 11 with OB (age 17-54, BMI 42.9 +/- 2.2 Kg/m2). Eleven normal women (age 23-50, BMI 21.9 +/- 0.3 Kg/m2) were studied as controls (C)., Measurements: Serum GH and IGF-I levels were measured by radioimmunoassay. The GH secretory responses were expressed either as absolute values (micrograms/L) or as areas under the curve (AUC, micrograms/L/h) calculated by trapezoidal integration. IGF-I concentrations were expressed as absolute values (micrograms/L) with reference to a pure recombinant IGF-I preparation., Results: Basal GH levels in CS were similar to those registered in OB (mean +/- s.e.m. 0.7 +/- 0.1 vs 0.9 +/- 0.2 microgram/L) and lower than in C (3.4 +/- 0.5 microgram/L, P < 0.00001). On the other hand, IGF-I levels were similar in all groups. The GHRH-induced GH rise in CS was lower, though not significantly, to that observed in OB (AUC: 65.6 +/- 13.2 vs 192.5 +/- 61.7 microgram/L/h) and both GH responses were significantly lower than that of C (1029.9 +/- 98.0 micrograms/L/h, P < 0.00001). ARG enhanced the GHRH-induced GH release in CS (331.9 +/- 51.9 micrograms/L/h vs GHRH alone, P < 0.0001), OB (852.4 +/- 162.1 micrograms/L/h, P < 0.0001) and C (3362.6 +/- 386.0 micrograms/L/h, P < 0.0002). However, the GH response to GHRH plus ARG in CS was lower (P < 0.002) than that in OB which, in turn, was lower than that in C (P < 0.00001). Pyridostigmine significantly enhanced the GHRH-induced GH rise in C (2808.5 +/- 221.2 micrograms/L/h, P < 0.00001) and, to a lesser extent, in OB (627.3 +/- 84.7 micrograms/L/h, P < 0.0002) but not in CS (102.9 +/- 25.0 micrograms/L/h)., Conclusions: Our results indicate that the GH releasable pool is reduced in obesity and, to an even greater extent, in Cushing's syndrome. The inability of pyridostigmine and arginine to restore a normal GH response to GHRH in these conditions makes the existence of a hypothalamic somatostatinergic hyperactivity unlikely.

An increased spontaneous and stimulated growth hormone (GH) secretion is well documented in insulin-dependent diabetes mellitus. On the contrary, in non-insulin-dependent diabetes mellitus (NIDDM) conflicting results arise from literature. In 14 patients with NIDDM, 7 normal weight (NWD) and 7 obese (OD), we investigated the somatotrope responsiveness to GHRH (1 microgram/kg) alone or combined with arginine (ARG, 0.5 g/kg), which is able to enhance the GH response to GHRH, probably by inhibiting somatostatin release from hypothalamus. Baseline IGF-I, IRI FFA and glucose levels were also determined. Twelve healthy normal subjects (NS) and 12 obese patients (OP) were evaluated as control groups. GH but not IGF-I levels were higher (p < 0.05) in NS than in OP (1.5 +/- 0.5 vs 0.5 +/- 0.2 microgram/l). Insulin levels were higher (p < 0.05) in OP than in NS, NWD and OD (18.7 +/- 1.8 vs 8.7 +/- 0.5, 6.4 +/- 1.9 and 11.8 +/- 1.2 microU/l). FFA were higher (p < 0.05) in NWD. OD and OP than in NS (0.69 +/- 0.04, 0.70 +/- 0.04 and 0.65 +/- 0.06 vs 0.39 +/- 0.03 mmol/l). Plasma glucose was higher (p < 0.05) in diabetic patients than in normal and obese subjects. GH responses to GHRH in NWD, OD and OP were similar (AUC: 221.6 +/- 33.3, 206.0 +/- 35.9 and 177.2 +/- 57.3 micrograms/l/min, respectively) and all lower (p < 0.05) than that in NS (776.7 +/- 206.5 micrograms/l/min). ARG determined a significant increase of GHRH-induced GH release in all groups (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Objective: It is well known that acetylcholine and arginine stimulate GH secretion while activation of beta-adrenergic receptors inhibits GH secretion in man. We aimed therefore to ascertain whether or not the inhibitory influence of beta-adrenergic receptors on GH secretion would over-ride the stimulatory one of acetylcholine and arginine., Design: We studied the interaction of salbutamol, a beta 2-adrenergic agonist (SAL, 0.08 mg/kg orally) with pyridostigmine, a cholinesterase inhibitor (PD, 120 mg orally), or arginine (0.5 g/kg i.v.) on both basal and GHRH (1 microgram/kg i.v.)-stimulated GH secretion., Subjects: Fourteen healthy male volunteers, aged 20-35 years, were studied., Measurements: Serum GH was measured in duplicate by immunoradiometric assay., Results: In study A, SAL inhibited the GH response both to GHRH (P < 0.01) and ARG (P < 0.002). ARG enhanced the GHRH-induced GH rise (P < 0.01) but its effect was abolished (P < 0.02) by SAL pretreatment. In study B, SAL inhibited the GH response both to GHRH (P < 0.01) and PD (P < 0.02). PD enhanced the GH response to GHRH (P < 0.001) but its effect was abolished (P < 0.05) by SAL pretreatment. In both studies, the GH response to GHRH alone was similar to that to the neurohormone when combined with ARG + SAL or PD + SAL., Conclusion: Our results show that beta 2-adrenergic activation by salbutamol is able to inhibit not only the GH rise induced by GHRH, arginine and pyridostigmine, but even the potentiating effect of both arginine and pyridostigmine on the GH response to GHRH. They indicate that catecholamines, acetylcholine and arginine play a major role in GH secretion having opposite influences aimed to balance the function of the hypothalamus-GH-IGF-I axis in man.

We studied the interaction between free fatty acids (FFAs) and arginine (ARG) on basal and growth hormone (GH)-releasing hormone (GHRH)-stimulated GH secretion in 14 normal subjects. Compared with placebo, ARG induced a significant increase of GH secretion (334.0 +/- 157.5 v 36.9 +/- 27.6 micrograms/L/h, P < .05). The increased levels of FFAs (1.9 +/- 0.4 mEq/L), obtained by the infusion of a lipid-heparin emulsion, abolished the effect of ARG (55.8 +/- 45.6 v 334.0 +/- 157.5 micrograms/L/h, P < .05). GHRH-induced GH secretion was potentiated by ARG (2,009.9 +/- 463.2 v 922.0 +/- 244.4 micrograms/L/h, P < .05) and suppressed by lipid-heparin infusion (106.2 +/- 28.3 v 922.0 +/- 244.4 micrograms/L/h, P < .01). Moreover, the lipid-heparin infusion inhibited the potentiating effect of ARG on the GHRH-induced GH increase (527.9 +/- 113.6 v 2,009.9 +/- 463.2 micrograms/L/h, P < .01). These results confirm the strong inhibitory effect of FFAs on GH secretion, showing that they are even able to inhibit the potentiating effect of ARG on the GH response to GHRH. Since ARG likely acts via inhibition of hypothalamic somatostatin release, the inhibitory effect of FFAs on GH secretion could take place directly at the pituitary level and/or at the hypothalamic level, counteracting the effect of ARG.

Aim of this study was to verify whether arginine (ARG), which likely inhibits hypothalamic somatostatin release, has an enhancing effect on the GHRH-induced GH rise, even when administered orally at low dose. To this goal we studied the effects of 4 g orally administered ARG, either hydrochloride (ARG-H) or aspartate (ARG-A), on both basal and GHRH (1 microgram/Kg i.v.)-stimulated GH secretion in 31 children with familial short stature (11 males and 20 females, aged 5.5-13.8 yr, pubertal stage I-III, and compared the results with those of i.v. infusion of 0.5 g/kg ARG-H. Oral ARG-H (Group A, n = 11) induced a significant increase of basal GH levels (4.2 +/- 1.3 vs 1.0 +/- 0.4 micrograms/L, p < 0.02) and enhanced the GH response to GHRH (41.1 +/- 8.6 vs 25.3 +/- 6.7 micrograms/L, p < 0.02). Oral ARG-A (Group B, n = 10) induced a slight, but not statistically significant increase in serum GH levels (3.4 +/- 1.5 vs 1.0 +/- 0.3 micrograms/L) and enhanced the GHRH-induced GH rise (49.7 +/- 9.8 vs 26.1 +/- 8.4 micrograms/L, p < 0.05). Intravenous ARG-H (Group C, n = 10) stimulated basal GH levels (6.2 +/- 1.2 vs 1.2 +/- 0.3 micrograms/L, p < 0.005) and increased the GHRH-induced GH rise (46.7 +/- 5.0 vs 17.1 +/- 2.3 micrograms/L, p < 0.005). This response was similar to those after oral ARG-H or ARG-A plus GHRH. No variation was observed in PRL levels after oral ARG (either ARG-H or ARG-A) and/or GHRH.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of pyridostigmine (60 mg orally), arginine (0.5 g/kg iv) and propranolol (PROP, 40 mg orally) on GHRH (1 microgram/kg iv)-induced GH release was studied in seven short children. Pyridostigmine and arginine induced a similar potentiating effect on GHRH-induced GH rise (Peak, mean +/- SEM: 56.9 +/- 12.8 and 48.6 +/- 8.5 micrograms/L, respectively vs 12.3 +/- 1.6 micrograms/L; p < 0.05). Combination of GHRH with propranolol induced an increase of GH that was significant only with regard to peak (28.9 +/- 8.4 micrograms/L) but not to AUC. However, GH rises observed after GHRH combined with PD or ARG did not significantly differ from that recorded after coadministration of GHRH and PROP both for peak and AUC. Our results confirm that pyridostigmine and arginine have a striking potentiating effect on the GHRH-induced GH rise in children and show that the tests with GHRH + PD and GH + H + ARG are ore reliable than that with GHRH + PROP to explore the secretory capacity of somatotroph cells.

A blunted growth hormone (GH) response to several stimuli, including growth hormone-releasing hormone (GHRH), has been shown in obesity. Arginine (ARG) has been demonstrated to potentiate the GHRH-induced GH increase in normal subjects, likely acting via inhibition of hypothalamic somatostatin release. To shed further light onto the mechanisms underlying the blunted GH secretion in obesity, we studied the effect of ARG (0.5 g/kg infused intravenously [IV] over 30 minutes) on both basal and GHRH (1 micron/kg IV)-stimulated GH secretion. Eight obese subjects (aged 26.4 +/- 3.9 years; body mass index, 39.0 +/- 1.9 kg/m2) and eight normal control volunteers (aged 27.0 +/- 1.7 years; body mass index, 22.3 +/- 0.5 kg/m2) were studied. In obese subjects, the GH response to both GHRH and ARG was lower (P less than .01 and P less than .002, respectively) than in controls. ARG potentiated the GH response to GHRH in obese patients (P less than .0003). However, in these patients, the GH secretion elicited by GHRH, even when coadministered with ARG, persisted at reduced levels (P less than .005) when compared with controls. Basal insulin-like growth factor-1 (IGF-1) levels did not significantly differ in obese subjects and in normal subjects (161.1 +/- 37.0 v 181.0 +/- 12.8 micrograms/L). In conclusion, ARG enhances the blunted GHRH-induced GH increase in obese patients, but the GH responses to ARG alone and to ARG + GHRH persist at lower levels than in normals. Thus, our results suggest the existence of a reduced pituitary GH pool in obesity.

Galanin (GAL), a 29 amino acid neuropeptide, is known to increase both basal and growth hormone-releasing hormone (GHRH)-induced growth hormone (GH) secretion while not significantly increasing prolactin (PRL) secretion in man. GAL is also endowed with an inhibiting effect on glucose-stimulated insulin release in animals, but not in man. We studied the effect of GAL (80 pmol/kg/min infused over 60 minutes) on the arginine- (ARG, 30 g infused over 30 minutes) stimulated GH, PRL, insulin, and C-peptide secretion in eight healthy volunteers (age, 20 to 30 years). GAL induced an increase of GH (GAL v saline, area under curve [AUC], mean +/- SEM: 316.5 +/- 73.9 v 93.2 +/- 20.9 micrograms/L/h, P less than .05), but failed to modify both PRL and insulin secretion. GAL enhanced the ARG-induced stimulation of both GH (1,634.1 +/- 293.1 v 566.9 +/- 144.0 micrograms/L/h, P less than .02) and PRL secretion (1,541.9 +/- 248.8 v 1,023.8 +/- 158.7 micrograms/L/h, P less than .02). On the contrary, GAL blunted the ARG-stimulated insulin (816.3 +/- 87.7 v 1,322.7 +/- 240.9 mU/L/h, P less than .05), as well as C-peptide secretion (105.1 +/- 9.8 v 132.8 +/- 17.3 micrograms/L/h, P less than .02). ARG administration induced a transient increase of glucose levels (P less than .01 v baseline) followed by a significant decrease (P less than .05 v baseline). This latter effect was prevented by the coadministration of GAL. In conclusion, these results show that in man GAL potentiates the GH response to ARG, suggesting that these drugs act at the hypothalamic level, at least in part, via different mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

It is well known that in normal adults the growth hormone (GH) response to GH-releasing hormone (GHRH) is inhibited by previous administration of the neurohormone. In 7 healthy volunteers (age 20-34 years) we studied the GH responses to two consecutive GHRH boluses (1 microgram/kg i.v. every 120 min) alone or coadministered with arginine (30 g i.v. over 30 min). The GH response to the first GHRH bolus (area under the curve, mean +/- SEM: 506.3 +/- 35.1 micrograms/l/h) was higher (p = 0.0001) than that to the second one (87.1 +/- 14.6 micrograms/l/h). The latter response was clearly increased (p = 0.0001) by coadministering arginine (980.5 +/- 257.5 micrograms/l/h). When every GHRH bolus was combined with arginine a marked potentiation of GH response to both boluses was found. However, the second combined administration of arginine and GHRH induced a GH increase which was lower compared to the first one (p = 0.016). In conclusion, our results show that arginine potentiates the GHRH-induced GH secretion preventing the lessening of somatotrope responsiveness to the neurohormone alone. As there is evidence that this phenomenon is due to an enhanced somatostatin release, these findings give further evidence of a somatostatin-suppressing effect of arginine.

OBJECTIVE: To evaluate the maximal secretory capacity of somatotrope cells in obesity and to compare it with that in hypopituitaric patients with GH deficiency. DESIGN: Stimulation with GHRH. (1 μg/kg iv) combined with arginine (ARG, 0.5 g/kg iv), which strongly potentiates the GH response to the neurohormone, likely inhibiting hypothalamic somatostatin. The reproducibility of the GH response to GHRH + ARG was evaluated in a second session. SUBJECTS: Forty-five patients with simple obesity (OB 11 male and 34 female, age 40.5 ± 1.8 y, BMI 38.8 ± 1.1 kg/m²), 49 patients with hypopituitarism (GHD, 23 male and 26 female, 43.6± 2.4y, 24.7 ± 0.7 kg/m²) and 44 normal young volunteers (NS, 25 male and 19 female, 33.8 ± 1.0 y, 21.6 ± 0.3 kg/m²) were studied. MEASUREMENTS: GH levels were assayed by IRMA method, basally at - 60 and 0 min, and than every 15 min up to + 120 min. Basal IGF-I levels were assayed by RIA method, after acid-ethanol extraction. RESULTS: IGF-I levels in OB were lower (P < 0.005) than those in NS but higher (P < 0.005) than those in GHD. Mean peak GH response to GHRH + ARG in OB was clearly lower than that in NS (P < 0.005) and higher (P < 0.005) than that in GHD. Sixty-percent OB and 100% GHD showed peak GH responses lower than the minimum normal limit in NS (16.5 μg/I) while 4% OB and only 53% GHD with GH responses lower than 3 μg/I, the limit under which GH replacement therapy of severe deficiency is allowed. Good intraindividual reproducibility of the GH response to GHRH + arginine test was present in all groups (OB: r=0.78, P < 0.0001; GHD: r= 0.57, P< 0.003; NS: r=0.74, P< 0.0001;. CONCLUSIONS: The maximal secretory capacity of somatotrope cells is clearly less than normal in the obese but still more than is seen in GHD subjects. However, in about 50% of obese patients, the pituitary GH releasable pool overlaps with that of hypopituitaric patients with GH deficiency. Thus, even... [ABSTRACT FROM AUTHOR]