Interfering effects of insulin, growth hormone and glucose on adipokine secretion.

INTRODUCTION: Cell culture media with high glucose concentration are normally used. Data on the secretion of the adipokines adiponectin and resistin from adipocytes in response to insulin and growth hormone (GH) both under normo- and hyperglycemic conditions are not available. It was the aim of the study to investigate the impact of standard metabolic conditions (normo-/hyperglycemia, normo-/hyperinsulinemia) and of GH on the secretion of adiponectin and resistin. MATERIAL AND METHODS: 3T3-L1 preadipocytes were differentiated into adipocytes and then incubated under normoglycemia (100 mg/dl), hyperglycemia (450 mg/dl), in combination with insulin (0, 0.2, 2.0 nM) and/or GH (1 nM). Adiponectin and resistin secretion was measured by ELISA. RESULTS: Insulin significantly stimulates adiponectin and resistin secretion under normo- and hyperglycemia. Hyperglycemia PER SE stimulates adiponectin and resistin secretion both in the absence and presence of low or high insulin concentrations. GH stimulates adiponectin secretion both under normoglycemic and hyperglycemic conditions. Whereas insulin does not modulate GH-induced adiponectin secretion under normoglycemia, insulin augments adiponectin release under hyperglycemia. GH stimulates resistin secretion only under normoglycemia, but not under hyperglycemic conditions. Since scavenger receptor B-I expression did not change, these effects are specific and not caused by a simple enhancement of adipocyte differentiation. DISCUSSION: Glucose, insulin and growth hormone have significant and interfering effects on the secretion of resistin and adiponectin. Several of the well-known in vivo phenomena such as diurnal variation or effects of re-feeding and weight-loss might be explained by direct effects of these hormones on adipocytes. Finally, when effects of hormones on adipocyte function are investigated, it is a prerequisite to take glucose levels of the cell culture media into account.     

Effects of growth hormone on insulin sensitivity and forearm metabolism in normal man

To elucidate the short-term actions of growth hormone on insulin sensitivity and forearm metabolism, we have studied six normal male subjects receiving a 6-h hyperinsulinaemic euglycemic clamp with and without a concomitant 4-h growth hormone infusion. When infused, serum growth hormone rose to 25 +/- 4 mU/l and during administration of insulin serum insulin increased by 11 +/- 1 mU/l. During euglycemic clamp, administration of growth hormone decreased forearm glucose uptake after 180 min and onward (240 min 0.216 +/- 0.031 vs 0.530 +/- 0.090 mg/100 ml/min, p less than 0.05). Glucose infusion rate (240 min 2.83 +/- 0.24 vs 4.35 +/- 0.28 mg.kg-1.min-1, p less than 0.05) and glucose disposal rate (240 min 3.57 +/- 0.17 vs 4.00 +/- 0.15 mg.kg-1.min-1, p less than 0.05) also decreased. Growth hormone persistently increased hepatic glucose production after 120 min. After 210 min, all circulating lipid intermediates increased slightly. The decrease in forearm glucose uptake and glucose infusion rate and the increase in hepatic glucose production was observed before there was any detectable increase in circulating levels and forearm uptake of lipid intermediates. These data suggest that growth hormone induces insensitivity to insulin in liver, muscle and fat after 120, 180 and 210 min respectively. The early effects of growth hormone on glucose metabolism seems independent of changes in the rate of lipolysis.     

Inhibitory effect of calcitonin on growth hormone and insulin secretion in man

The effect of calcitonin administration on basal and arginine-stimulated growth hormone and insulin plasma levels was investigated. The intramuscular injection of synthetic salmon calcitonin (100 U MRC) in five normal subjects produced a significant decrease (p less than 0.05) in insulin concentration. The same amount of calcitonin given 15 min before an arginine infusion test in seven normal subjects significantly reduced the response of growth hormone (p less than 0.025) and insulin (p less than 0.005) to the stimulus.     

Influence of acetylsalicylic acid on plasma glucose, insulin, glucagon, and growth hormone levels following tolbutamide stimulation in man.

The effects of acetylsalicylic acid (ASA), a known inhibitor of prostaglandin (PG) synthesis, on plasma glucose, insulin, glucagon and growth hormone (GH) responses to tolbutamide were examined in ten normal volunteers. Treatment with 3.2 g ASA daily for 3 days caused a significant reduction in basal plasma glucose levels (p less than 0.05); by contrast, basal insulin rose from 23 +/- 2 to 31 +/- 2 microU/ml (p less than 0.01). No significant changes in the basal concentrations of glucagon and GH were found after ASA. Insulin response to tolbutamide was significantly augmented after ASA (p less than 0.01) while GH response to hypoglycemia was reduced (p less than 0.05). The pattern of plasma glucose and glucagon was not significantly modified by the treatment. Since ASA seems to have an action opposite to PGE on insulin and GH secretion, it is possible that the ASA may work through inhibition of PG synthesis.     

Short-term effects of growth hormone on fuel oxidation and regional substrate metabolism in normal man

Primarily by increasing the availability of lipid intermediates, GH is likely to have profound effects on substrate consumption rates. To examine the short term actions of GH on glucose turnover, fuel oxidation and regional forearm metabolism, six normal volunteers were each studied twice for 5.5 h after having received a 4-h infusion of GH (20 ng/kg.min) or saline. GH induced slightly falling plasma glucose levels, acute 40-50% decreases in forearm glucose uptake, and no change in glucose turnover. Furthermore, substantial increases in circulating concentrations and forearm uptake of nonesterified fatty acids and 3-hydroxybutyrate were recorded. Although GH infusion was followed by a 50% reduction of forearm alanine release hepatic nitrogen excretion seemed unaffected. Energy expenditure was not influenced by GH, but the non-protein respiratory exchange ratio decreased from a basal value of 0.778 +/- 0.008 to 0.732 +/- 0.007 after GH treatment (P less than 0.05). Correspondingly, lipid oxidation increased from 1.20 +/- 0.06 to 1.48 +/- 0.09 mg/kg.min, and glucose oxidation decreased from 0.97 +/- 0.12 to 0.39 +/- 0.06 mg/kg.min (P less than 0.05). Nonoxidative glucose utilization tended to increase. These data indicate that GH, by promoting lipid utilization and decreasing glucose oxidation, diminishes the need for gluconeogenesis and, therefore, could be protein preserving in the long term. Overall, we found no evidence of GH having acute insulin-like effects on glucose metabolism. GH appears to increase glucose storage, leaving total energy expenditure unaffected.     

Effects of gut hormone on insulin and glucagon secretion

Summary To examine the possible influences of gastrointestinal hormones upon the secretion of the hormones of islets of Langerhans, highly purified preparations of gastrin, secretin and pancreozymin were injected endoportally in anesthetized dogs. All three hormones were found to cause an immediate rise in the concentration of insulin in the pancreaticoduodenal vein. The effect of gastrin on insulin release was quantitatively trivial, while that of secretin was more substantial and of longer duration; however, pancreozymin appeared to be the most potent insulin stimulator and, in addition, caused a parallel rise in pancreatic glucagon secretion. Furthermore, pancreozymin was shown to augment both the insulin and the glucagon response to hyperaminoacidemia. Intraduodenal administration of amino acids, known to be the most potent stimulator of endogenous pancreozymin, was found to elicit a greater and more rapid release of insulin and glucagon than the intravenous administration of amino acids, suggesting that endogenous pancreozymin plays a physiologic role in augmenting the islet cell hormone response to ingested amino acids. The physiologic augmenter of the insular response to ingested glucose remains unidentified, however.Supported by USPHS, Grant AM-02700-08.     

Acute suppressive effect of human growth hormone on basal insulin secretion in man.

The acute effects of human growth hormone (GH) on the basal levels of glucose and insulin in blood were investigated in 11 healthy men. GH doses of 5, 10, 20, and 40 mug/kg body weight were given iv as a constant-rate infusion over 30 min, and resulted in peak hormone levels (30 min) of 20.5 plus or minus 1.0, 48.5 plus or minus 2.2, 108.2 plus or minus 4.5, and 229.2 plus or minus 14.6 ng/ml, respectively. There was a small (max 9.8 plus or minus 2.6%) but significant decrease in the blood glucose level, observed already at 15 min after the beginning of the GH infusion and persisting up to 90 min. The highest dose of GH induced the most marked changes, but there was otherwise no clear correlation between dose and effect. The basal plasma insulin levels showed a more marked (max 16.0 plus or minus 4.7%) decrease which was not correlated, in time or in magnitude, with the changes in blood glucose. In some subjects, in whom no significant decrease in blood glucose was observed, plasma insulin still demonstrated a similar fall (max 20.2 plus or minus 7.6%). Neither were these changes in plasma insulin correlated to the dose of GH within the range used in this study. The findings suggested that the early, insulin-like effect of GH on blood glucose is distinct from its effect on the pancreas. The latter is a suppressive one, consistent with earlier findings on glucose-induced insulin release.     

Effect of clofibrate on arginine-stimulated glucagon and insulin secretion in man

Insulin and glucagon have been reported to have opposing effects upon the mechanisms regulating serum triglyceride concentration. Glucagon in excess of insulin will lower serum lipids in man. In the present studies, we have examined the possibility that a change in glucagon and insulin regulation might contribute to the hypolipemic action of the drug clofibrate. Control insulin and glucagon secretion were evaluated in 24 normal subjects by intravenous arginine infusion, which resulted in a prompt rise in both serum immunoreactive insulin and glucagon concentration. During the maximum rise in concentration of these hormones, plasma triglyceride concentration was acutely reduced from basal levels of $\text{104 ± 6}\text{mg}\text{100}\text{ml}$ to $\text{75 ± 5}\text{mg}\text{100}\text{ml}$ (p ≤ 0.001). Following 7 days of clofibrate therapy, basal plasma triglyceride concentration attained a new mean level of $\text{78 ± 5}\text{mg}\text{100}\text{ml}$, while basal insulin and glucagon concentrations remained unchanged. However, arginine infusion now resulted in a reduction of the insulin secretory response to 56% of the preclofibrate studies with an associated normal glucagon secretory response. Serum triglyceride concentration was further reduced during arginine infusion to $\text{46 ± 3}\text{mg}\text{100}\text{ml}$, demonstrating this minimum level as maximum plasma glucagon levels were attained, representing an excess of this hormone relative to the reduced insulin concentration. These observations are consistent with an effect of clofibrate on the hormonal regulation of triglyceride physiology in man. Glucose tolerance was unimpaired by clofibrate therapy in these normal subjects, in spite of an apparent reduction in glucose-stimulated insulin secretion.     

Differential effects of arginine on growth hormone releasing hormone and insulin induced growth hormone secretion

OBJECTIVE: We wished to investigate the interaction of arginine, GHRH and insulin stress on GH secretion. DESIGN: Six healthy, non-obese volunteers underwent seven separate studes in random order. They received (1) insulin alone at 0 minutes; (2) GHRH alone at 15 minutes; (3) arginine alone at 0-30 minutes; (4) arginine at 0-30 minutes and GHRH at 15 minutes; (5) insulin at 0 minutes and arginine at 0-30 minutes; (6) insulin at 0 minutes, GHRH at 15 minutes and arginine at 0-30 minutes; (7) insulin at 0 minutes and GHRH at 15 minutes. MEASUREMENTS: GH and PRL were measured from -30 to 150 minutes at intervals of 15 minutes. RESULTS: Arginine increased GH responses to GHRH and decreased GH responses to hypoglycaemia, but this inhibitory effect of arginine was reversed by GHRH. CONCLUSIONS: The findings suggest that arginine-induced GH release is mainly mediated by a decrease in somatostatinergic tone, while GH responses to insulin stress are probably mediated by both an increase in hypothalamic GHRH release and inhibition of somatostatin.     

The inhibitory effect of somatostatin on growth hormone, insulin and glucagon secretion in diabetes mellitus.

The inhibitory effect of somatostatin on insulin, glucagon and growth hormone secretion was studied in 5 patients with diabetes mellitus. In three maturity onset diabetics, somatostatin infusion abolished the insulin rise induced by breakfast and oral glucose, and in 2 of them, inhibited the basal insulin secretion by 50% seen during control studies. Concomitantly, there was a marked and prompt reduction of glucagon levels (50%) with a sustained effect. The plasma glucose levels were either unchanged or slightly increased. Following the somatostatin infusion, there was a prompt rebound increase in both insulin and glucagon levels with a relatively stable plasma glucose concentration. In contrast, a drastic reduction of plasma glucose in face of a relatively small fall in plasma glucagon in response to somatostatin infusion was observed in 2 insulin-dependent diabetics. In all patients, the episodic release of growth hormone seen during the control day was abolished during somatostatin infusion.     

Modulation by verapamil of insulin and glucagon secretion in man

Summary The present investigation was designed to evaluate the effect of acute and protracted verapamil administration on insulin and glucagon secretion in man. For this purpose, 14 normal subjects received two consecutive glucose pulses (5 g i.v. in less than 20 sec or 20 g i.v. in less than 1 min, 7 subjects for each group), 70 or 90 min apart, before and during an infusion of verapamil (160 g/min). Seven additional normal subjects received two consecutive arginine pulses (5 g i.v.), 70 min apart. In 14 inpatients with coronary heart disease, we investigated the effect of protracted verapamil administration. Seven of these subjects underwent two oral glucose tolerance tests (100 g) and the other 7 two arginine tests (30 g) before and after a 10-day treatment with verapamil, 240 mg/die p.o. divided into three doses; the last dose, 80 mg, was given orally 1 h before the performance of the post-treatment test. Verapamil significantly inhibited the acute insulin response (AIR, mean change from 3–10 min) to glucose (5 g), as well as the AIR and AGR (acute glucagon response) to arginine (5 g). By contrast, verapamil failed to alter significantly the AIR to the higher glucose pulse. There was no significant change of oral glucose tolerance after verapamil, nor was there a change in insulin response to oral glucose. By contrast, insulin and glucagon responses to arginine infusion were significantly reduced by the drug.     

The effect of intraportal and peripheral infusions of glucagon on insulin and glucose concentrations and glucose tolerance in normal man

Summary The effect of peripheral and intraportal infusions of 0.86 pmol/kg · min^–1 of glucagon on plasma glucose, plasma insulin, and glucose tolerance was examined in four normal subjects. Peripheral glucagon concentrations increased by 60–90 pmol/l during intraportal and 70–180 pmol/l during peripheral infusions. The infusions caused increases in plasma glucose levels of approximately 1 mmol/l, and in plasma insulin levels of 75–100%, regardless of route of administration. Intravenous glucose tolerance tests carried out during the glucagon infusions showed that glucose tolerance remained within the normal range and was uninfluenced by the route of administration.     

The effect of glucose on the growth hormone response to glucagon and propranolol-glucagon in normal subjects.

The mean (+/- SE) peak level of serum growth hormone (GH) after intramuscular injection of glucagon in ten normal adult men was 15.1 +/- 2.1 ng/ml; glucose infusion suppressed the mean peak GH to 9.6 +/- 3.7 ug/ml (p less than 0.05). Pretreatment of eight of these subjects with propranolol caused a modest increase in the mean peak GH after glucagon (19.4 +/- 2.8 ng/ml) but did not improve the mean peak GH after glucagon when glucose was infused (8.7 +/- 2.8 ng/ml). Individual analysis of the peak GH showed that glucose infusion did not uniformly suppress the peak GH after glucagon; in seven subjects the peak GH was suppressed but in three it was not. Conclusions: (1) The GH response after glucagon is usually due to a fall in serum glucose after the initial rise in serum glucose induced by glucagon. (2) Nevertheless, since glucose does not consistently inhibit the GH response after glucagon, a second mechanism probably exists by which glucagon stimulates GH secretion. (3) Glucose completely suppresses the propranolol-induced increase in the GH response to glucagon; an adrenergic mechanism may be involved in the control of GH secretion by glucose.     

Effects of corticotropin-releasing hormone on insulin and glucagon secretion in mice

Besides in the brain, corticotropin-releasing hormone occurs in the pancreas. Therefore, its effects on plasma levels of insulin and glucagon were investigated in vivo in the mouse. At 2 min after CRH injection (0.5-8.0 nmol/kg), plasma insulin was lowered: by 4.0 nmol/kg from 38 +/- 4 to 28 +/- 2 mU/l (P less than 0.05). Plasma insulin was lowered also at 6 min, whereas at 10 min, plasma insulin levels were elevated (P less than 0.05). Plasma glucagon levels were slightly lowered (P less than 0.05) at 10 min after CRH injection, whereas plasma glucose was slightly elevated (P less than 0.05) at 6 min after injection but not at 2 or 10 min. The effects of CRH on the plasma insulin and glucagon response to iv injections of half-maximal dose levels of glucose (2.8 mmol/kg) or the cholinergic agonist carbachol (0.16 mumol/kg) were also investigated. CRH, 4.0 nmol/kg, however, could not influence the plasma insulin or glucagon levels after the iv injection of either glucose or carbachol. Thus, CRH slightly affects basal plasma levels of insulin and glucagon in mice. In contrast, stimulated insulin and glucagon secretions are not affected by CRH. Peripheral CRH may therefore be of slight importance for the regulation of basal plasma levels of insulin and glucagon in the mouse.     

Effect of insulin on glucagon secretion mediated via glucose metabolism of pancreatic a cells in ducks

Summary A possible action of insulin via glucose metabolism on the pancreatic A cell response to glucose, was studied in ducks. 2-Deoxyglucose, a non-metabolizable analogue of glucose was used. In normal ducks, the hyperglycaemia induced by 2-deoxyglucose (IV: 0.5 g/kg) resulted in hyperglucagonaemia, while the same degree of hyperglycaemia, induced by glucose infusion (IV injection 25 mg/kg, and infusion 5 mg/kg/min) immediately suppressed glucagon secretion. In diabetic ducks, two days after subtotal pancreatectomy, glucose responsiveness of the A cell was abolished, but could be restored by insulin treatment before (IM 0.2 U/kg insulin+8 g/kg glucagon every 6 h) and during (IV 3.6 mU/kg+infusion 0.9 mU/kg/min) the glucose test (IV: 0.5 g /kg). The normal response of the A cell to glucose was not observed in diabetic insulin-treated ducks after the administration of 2-deoxyglucose (IV: 0.5 g/kg). These data suggest an inhibitory effect of the metabolism of glucose on the release of glucagon. In addition, the action of insulin on the A cell may be mediated by its effect on glucose metabolism within the A cell.     

Interactions of galanin and arginine on growth hormone, prolactin, and insulin secretion in man.

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)