The effect of intravenous glucose infusion on the exercise-induced serum growth hormone rise in normals and juvenile diabetics

Exercise-induced growth hormone response in euglycaemia and hyperglycaemia in patients with Type 1 diabetes mellitus

AIMS

To compare exercise-induced growth hormone (GH) response in patients with Type 1 diabetes during stable euglycaemic and hyperglycaemic conditions.

METHODS

We conducted a randomized, controlled, single-blinded cross-over trial in seven male patients with well-controlled Type 1 diabetes. The patients cycled twice for 120 min at a level of 55-60% maximal oxygen uptake. Euglycaemia was at 5.0 mmol/l, hyperglycaemia at 11.0 mmol/l.

RESULTS

Area under the curve of GH (AUC(GH)) during exercise was significantly higher during euglycaemia [1430 ng ml(-1) min, 95% confidence interval (CI) 703-2910] compared with hyperglycaemia (1061 ng ml(-1) min, 95% CI 538-2091, P = 0.02).

CONCLUSIONS

In patients with Type 1 diabetes, GH concentrations during moderate aerobic exercise during stable hyperglycaemic conditions are significantly lower compared with euglycaemia. These findings are compatible with preserved glucose-mediated GH regulation during exercise in individuals with well-controlled Type 1 diabetes.

The inhibitory effect of glucose on growth hormone secretion is lost in obesity but not in hypertension

In obesity there is a clear reduction of both spontaneous and stimulated GH secretion. Furthermore, in obese patients the somatotrope responsiveness to provocative stimulation is selectively refractory to the inhibitory effect of glucose load. It has been hypothesized that hyperinsulinism of obese patients could play a role in the pathogenesis of these alterations. Aim of the present study was to verify the GH response to GHRH and the ability of glucose load to inhibit it in patients with essential hypertension in whom hyperinsulinism and insulin resistance are frequently present. To this goal, 7 patients with essential hypertension (HP, age, mean +/- SE: 29.6 +/- 2.4 yr, 3 females and 4 males, BMI: 21.7 +/- 1.2 kg/m2), 7 obese (OB, 4 females and 3 males, 31.9 +/- 4.1 yr, 35.6 +/- 2.0 kg/m2) and 7 normal subjects (NS, 4 females and 3 males, 28.3 +/- 3.9 yr, 21.0 +/- 1.6 kg/m2) underwent the following tests: GHRH (1 microgram/kg i.v. at time 0) alone and preceded by oral glucose load (OGTT, 100 g po at -45 min). Basal insulin levels were similar in HP and OB (11.3 +/- 0.5 and 12.7 +/- 2.2 microU/ml, respectively); these, in turn, were higher (p < 0.005) than those in NS (6.8 +/- 0.8 microU/ml). Basal plasma glucose levels in HP were similar to those in OB and NS (80.3 +/- 3.6, 86.9 +/- 6.7 and 84.4 +/- 1.7 mg/dl, respectively). In HP and OB and NS basal GH (1.0 +/- 0.5, 1.0 +/- 0.6 and 0.3 +/- 0.1 micrograms/l, respectively) and IGF-I levels (132.6 +/- 14.8, 137.3 +/- 13.2 and 138.8 +/- 12.2 micrograms/l, respectively) were similar. In HP the GH response to GHRH (AUC: 1058.8 +/- 347.8 micrograms/l/min) was similar to that observed in NS (959.0 +/- 167.8 micrograms/l/min) and higher than that in OB (344.8 +/- 67.2 micrograms/l/min, p < 0.01). OGTT clearly blunted (p < 0.01) the GHRH-induced GH response in HP as well as in NS (401.8 +/- 104.4 and 521.6 +/- 76.6 (g/l/min, respectively) but not in OB (387.4 +/- 78.8 (g/l/min). The OGTT-induced insulin levels in HP did not differ from those of OB, both being higher (p < 0.05) than those recorded in NS. Glucose levels after OGTT were similar in the three groups. In conclusion, this study demonstrates that, like in normal subjects but differently from in obese patients the GH response to GHRH is normal in patients with essential hypertension and it is normally inhibited by oral glucose load even when these patients show high insulin levels. Thus, it is unlikely that the low somatotrope secretion and its refractoriness to inhibition by glucose load in obesity is due to hyperinsulinism.

Arginine abolishes the inhibitory effect of glucose on the growth hormone response to growth hormone-releasing hormone in man

Acute hyperglycemia inhibits the growth hormone (GH) response to several stimuli including growth hormone-releasing hormone (GHRH), likely acting by stimulation of endogenous somatostatin release. The aim of our study was to verify whether arginine ([Arg] 30 g intravenously [IV] in 30 minutes), a well-known GH secretagogue likely acting via inhibition of hypothalamic somatostatin release, counteracts the inhibitory effect of oral glucose (OG) administration (100 mg orally) on the GH response to GHRH (1 micrograms/kg IV bolus) in seven normal subjects (aged 20 to 30 years). The GH response to GHRH (peak, 11.6 +/- 1.8 micrograms/L) was inhibited by previous OG load (peak, 7.4 +/- 0.8 micrograms/L; P less than .02 v GHRH alone) and potentiated by Arg coadministration (peak, 36.2 +/- 8.8 micrograms/L; P less than .03 v GHRH alone). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 +/- 6.9 micrograms/L). In conclusion, our results show that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. These data, although indirect, suggest that both acute hyperglycemia and Arg act at the hypothalamic level, stimulating and inhibiting, respectively, the release of somatostatin.

Effect of enhancement of cholinergic tone on the growth hormone response to acute hyperglycaemia or thyrotropin-releasing hormone in dogs

The effect of enhancement of cholinergic tone by pyridostigmine on the growth hormone (GH) response to thyrotropin-releasing hormone (TRH) or glucose-induced acute hyperglycaemia was tested in six adult unanaesthetized beagle dogs. Both TRH (5μg/ kg iv) and glucose (2 g/kg orally) did not significantly alter baseline GH levels but reduced the GH response to GH-releasing hormone (GHRH) (2 μg/kg iv), although this effect was more clear-cut with TRH than with glucose. Pretreatment with pyridostigmine (2 mg/kg orally) counteracted the inhibitory effect of hyperglycaemia on the GHRH-induced GH release, but had no effect on the inhibition induced by TRH. In summary, these results indicate that: 1) acute hyperglycaemia and TRH play an inhibitory role on GHRH-stimulated GH secretion in dogs; 2) the inhibitory effect of acute hyperglycaemia is mediated via hypothalamic cholinergic neurotransmission, whereas other neurotransmitter pathways would be. involved in the effect of TRH.

Inappropriate growth-hormone (GH) response to thyrotropin-releasing hormone (TRH) occurs infrequently in well-regulated diabetes mellitus

We randomly administered thyrotropin-releasing hormone (200 micrograms, as an i.v. bolus) or control saline (in isovolumic amount) to 30 male diabetic subjects (23 IDDM, 7 NIDDM) in fair metabolic control (HbA1 9.7 +/- 0.3%, means +/- SEM) and to 12 healthy male controls on two different mornings. While GH in the basal state was similar in IDDM, NIDDM and normal subjects, TRH administration evoked a significant GH release only in a single IDDM individual. The only GH-responder to TRH was a newly-diagnosed (two weeks) IDDM patient, still with a high glycated hemoglobin level (HbA1 11.1%), despite normal plasma glucose levels. Saline infusion did not affect GH concentrations either in normals or in diabetics. Exaggerated GH responses to TRH are uncommon in diabetic patients in good metabolic conditions.

Inappropriate growth hormone response to luteinizing hormone-releasing hormone in diabetes mellitus

We randomly administered luteinizing hormone-releasing hormone (LHRH) or thyrotropin releasing hormone (TRH) (25 micrograms and 200 micrograms, respectively, as a bolus), to 16 diabetic male subjects (9 type I, 7 type II) and to 9 healthy male controls in two different mornings. While GH in the basal state was similar in type I, type II, and normal subjects, LHRH administration surprisingly evoked a significant GH release in 7 (5 type 1, 2 type II) diabetic patients. GH-responders had higher glycated hemoglobin than non-responders (11 +/- 1 nu 8.3 +/- 0.5%) but superimposable fasting and intratest average glucose levels. Only one patient among the GH-responders to LHRH showed a GH release also after TRH. These data support the hypothesis that GH secretion in diabetes, especially when poorly controlled, is abnormal.

Diabetes and exercise

This review describes (1) the metabolic and hormonal response to exercise in normal and diabetic man, and (2) the potential benefits of physical training in diabetes. Whereas in normal man plasma glucose varies little during exercise, the insulin-dependent diabetic subject may experience an increase in plasma glucose, a modest decrease or a marked decrease which can result in symptomatic hypoglycemia. Evidence is reviewed that the glycemic response depends on the ambient plasma concentration of insulin and that this may be influenced by an effect of exercise on the absorbtion of insulin from its site of injection. The response to exercise of noninsulin-dependent diabetic subjects and of diabetic subjects with autonomic neuropathy is also described. Physical training improves glucose tolerance in some noninsulin-dependent diabetic subjects and in insulin-dependent patients, it may diminish insulin requirements. It may also have a role in retarding the development of cardiovascular complications. Physical training is not totally innocuous, however, and in many patients with diabetes special precautions are required.

The effect of different diets and of insulin on the hormonal response to prolonged exercise

The importance of carbohydrate availability during exercise for metabolism and plasma hormone levels was studied. Seven healthy men ran on a treadmill at 70% of individual maximal oxygen uptake having eaten a diet low (F) or high (CH) in carbohydrate through 4 days. At exhaustion the subjects were encouraged to continue to run while glucose infusion increased plasma glucose to preexercise levels. Forearm venous blood, biopsies from vastus muscle and expiratory gas were analyzed. Time to exhaustion was longer in CH- (106 +/- 5 min (S.E.)) than in F-expts. (64 +/- 6). During exercise, overall carbohydrate combustion rate, muscular glycogen depletion and glucose and lactate concentrations, carbohydrate metabolites in plasma, and estimated rate of hepatic glucose production were higher, fat metabolites lower, and the decrease in plasma glucose slower in CH- than in F-expts. Plasma norepinephrine increased and insulin decreased similarly in CH- and F-expts., whereas the increase in glucagon, epinephrine, growth hormone and cortisol was enhanced in F-expts. Glucose infusion eliminated hypoglycemic symptoms but did not substantially increase performance time. During the infusion epinephrine decreased markedly and glucagon even to preexercise levels. Infusion of insulin (to 436% of preexercise concentration) in addition to glucose in F-expts. did not change the plasma levels of the other hormones more than infusion of glucose only but reduced fat metabolites in plasma. At exhaustion muscular glycogen depletion was slow, and the glucose gradient between plasma and sarcoplasma as well as the muscular glucose 6-phosphate concentration had decreased.

CONCLUSIONS

The preceding diet modifies the energy depots, the state of which (as regards size, receptors and enzymes) is of prime importance for metabolism during prolonged exercise. Plentiful carbohydrate stores favor both glucose oxidation and lactate production. During exercise norepinephrine increases and insulin decreases independent of plasma glucose changes whereas receptors sensitive to glucose privation but not to acute changes in insulin levels enhance the exercise-induced secretion of glucagon, epinephrine, growth hormone and cortisol. Abolition of cerebral hypoglycemia does not inevitably increase performance time, because elimination of the hypoglycemia may not abolish muscular energy lack.

Variation of blood glucose and serum growth hormone, prolactin and insulin in subjects with insulin-dependent diabetes, after OGTT and pretreatment with 2-Br-alpha-ergocryptine

In insulin-dependent diabetics high GH values are usually observed. The OGTT does not modify these levels. It was observed that CB 154 (2-Br-alpha ergocryptine) seems capable of lowering GH in acromegalics. We thought it pertinent to check whether or not CB 154 could reduce GH also in diabetics. OGTT was performed in insulin-dependent diabetic patients; 5 days later the same test was repeated after pretreatment with 2.5 mg of CB 154. Serum IRI, GH, hPRL levels and blood glucose values were checked during the test. Significant GH and hPRL variations were not observed after OGTT in the diabetic subjects examined; IRI was always absent, and glycemia followed the model of the diabetic curve. On the contrary, after pretreatment with CB 154, we observed: 1) hPRL reduction; 2) GH increase, sometimes as early as 30 min after OGTT; 3) IRI was never present and blood glucose levels were lower than the values observed with glucose alone. Contrary to what is observed in acromegalics, CB 154 is unable to lower the increased GH levels normally present in insulin-dependent diabetics. After pretreatment with CB 154, OGTT seems to release GH, which is not observed after glucose alone. This can be accounted for only by postulating an antagonism at the hypothalamic level between the adrenergic and serotoninergic pathways of GH stimulation.