Insulin secretion in the hibernating edible dormouse (Glis glis): in vivo and in vitro studies

Plasma glucose and insulin have been studied during lethargy and spontaneous arousal of hibernating edible dormouse. During lethargy blood glucose was low while plasma insulin remained at the same level as in other seasons. Plasma glucose and insulin did not fluctuate along the phase of lethargy. During spontaneous arousal plasma insulin rose strongly from the 17 degrees C stage, reaching the higher values at 26 degrees C while blood glucose was only 85 mg/100 ml, then decreased at 37 degrees C. The effect of glucose and temperature on insulin secretion was studied using perfused pancreas preparation from hibernating edible dormice. During the rewarming of the edible dormouse pancreas the insulin release did not occur in response to the absolute extracellular glucose level but occurred in response to a B cell membrane phenomenon which was dependent on the changing rate of glucose level. The effect of glucose and temperature on insulin secretion from perfused pancreas was compared between edible dormouse and homeotherm permanent, the rat. The B cell response to glucose of the dormouse pancreas increased up to 15 degrees C whereas that of the rat only from 25 degrees C. The dormouse insulin secretion reached a peak value at the 30 degrees C of temperature, whereas that of the rat progressively increased until 37 degrees C. These results showed that some biochemical adjustment or process of acclimatization took place in the B cells of the hibernators.     

Glucose oxidation by adipose tissue of the edible dormouse (Glis glis) during hibernation and arousal: effect of insulin

1. The effect of insulin on U-14C-glucose oxidation by adipose tissue isolated from hibernating or arousing edible dormouse has been studied. 2. CO2 production derived from radioglucose was analysed point by point during in vitro rewarming (from 6 to 37 degrees C). 3. The rate of temperature increase was 2 degrees/5 min in order to mimic the rate of rewarming during the spontaneous arousal of the dormouse. 4. Insulin did not increase the glucose oxidation by the adipose tissue from hibernating dormouse whereas adipocytes from active animal present high insulin sensitivity. 5. These results suggest that insulin resistance occurs during hibernation.     

Hormonal control of lipolysis from the white adipose tissue of hibernating jerboa (Jaculus orientalis)

1. Plasma glucose, glycerol, free fatty acids and total lipid content of the white adipose tissue were measured in euthermic and hibernating jerboa. 2. During hibernation, plasma glucose and glycerol were low compared to the euthermic animals, whereas there was no obvious difference in plasma free fatty acids. The white adipose tissue lipid content was strongly reduced in the hibernating state. 3. The effect of lipolytic hormones (norepinephrine and glucagon) and antilipolytic hormone (insulin) on in vitro glycerol release by adipose tissue isolated from hibernating or euthermic jerboa has been studied. 4. The white adipose tissue from hibernating jerboa presented a higher sensitivity to norepinephrine and glucagon than that of euthermic jerboa; insulin did not modify either basal glycerol release or lipolysis induced by the two lipolytic hormones at low temperatures (7 degrees C) and during the rewarming (from 7 degrees C to 37 degrees C) of the tissue slices. 5. These results suggested that white adipose tissue constitutes an important source of substrates derived from lipolysis during hibernation.     

Factors influencing insulin and glucagon secretion in lean and genetically obese mice.

The control of insulin and glucagon secretion from isolated pancreatic islets of lean and genetically obese mice has been compared. The enlarged islets of obese mouse pancreas and islets of obese mouse pancreas and islets of obese mice maintained on a restricted diet manifested a greater response to glucose stimulation of insulin secretion than the lean mice islets. The glucagon content of the islets, the secretion of glucagon in a medium containing 150 mg% glucose and the stimulation of glucagon secretion by arginine did not differ significantly in the two groups. Adrenaline stimulated glucagon secretion in vitro from obese mice but not from lean mice. Antinsulin serum injections into obese mice increased the plasma glucagon levels about twofold and had no effect on glucagon levels in lean mice, although the level of hyperglycaemia was the same in both groups. It is suggested that the suppression of glucagon release by glucose requires a higher concentration of insulin in the obese mouse pancreas than in lean mice.     

Pancreatic A and B cell stimulation in euthermic and hibernating marmots (Marmota flaviventris): effects of glucose and arginine administration

Summary In euthermic and hibernating marmots (Marmota flaviventris), the pancreatic A and B cells respond in the appropriate secretory manner to glucose or arginine injection. Although reduced, this response, is clearly present in hibernating marmots. When glucose is administered to euthermic or hibernating marmots, plasma insulin concentrations rise and glucagon levels fall. While similar results are obtained in hibernation, the time period of the response is much longer due to the slowing of temperature dependent metabolic processes. Injection ofl-arginine stimulates an increase in plasma glucose, insulin, and glucagon as expected. Measurements of plasma glucose, insulin, and glucagon under basal conditions, suggest that there are no significant differences between any phase of hibernation (eg. entrance, deep hibernation, arousal) and euthermia. These results provide indirect evidence that the pancreatic A and B cells of hibernating marmots continue to function in order to help regulate plasma glucose concentration.     

The role of insulin, glucagon and growth hormone in the regulation of plasma glucose and free fatty acid levels in anorexia nervosa

It is well established that glucagon plays an important role in the regulation of fuel supplies as its plasma level increases during the first days of a complete fast. However, it is not certain that glucagon is involved in the adaptation to chronic starvation. In the present study, this problem was investigated by the determination of the changes in the plasma glucagon level elicited by an i.v. glucose tolerance test followed by an i.v. arginine perfusion in 26 self starved patients suffering from anorexia nervosa (AN) and 14 control patients having only minor neurotic disorders. The basal plasma glucagon level tended to be higher in the AN patients than in the controls; but the difference was not statistically significant. Glucagon responses to glucose and arginine observed in the AN patients were not significantly different from those seen in the control patients. In the AN patients, the insulin response to both loads was reduced and the plasma GH level increased paradoxically after the glucose load, whereas it rose normally after the arginine load. It may be concluded that in chronic starvation by AN the regulation of fuel supplies depends mainly on decreased insulin and increased growth hormone secretion. The role of glucagon seems to be of minor importance in this condition.     

Combined administration of sodium chloro-2-propionate and insulin on the secretion of glucagon by the pancreas in the diabetic rat

This work was designed to study the effects of sodium 2-chloropropionate (2CP) alone or combined with insulin, in vitro, on glucagon secretion from pancreas isolated from rats, made diabetic by streptozotocin (66 mg/kg i.p.). The pancreata were perfused with a physiological solution containing 2.8 mM glucose (0.5 g/l) and glucagon secretion was stimulated by an arginine infusion (5 mM) for 30 min. When 2CP (1 mM) and/or insulin (4 IU/l) were applied, they were infused from the start of the organ perfusion. In the presence of glucose alone, a marked decrease in glucagon output was observed in diabetic rat pancreas. The arginine perfusion induced a biphasic glucagon secretion both in normal and diabetic rat pancreas; this response was however clearly reduced in diabetic rat pancreas. In diabetic rat pancreas, the infusion of either 2CP or insulin had no effect on glucagon output in presence of glucose alone, nor did it modify the response to arginine. In contrast, the combined infusion of insulin and 2CP induced different effects depending on the conditions: whereas in presence of glucose alone it restored a glucagon output close to that recorded in normal rat pancreas, it did not modify the response to arginine.     

A comparative study of several serotonin receptor antagonists on basal and stimulus induced release of insulin and glucagon in the intact rat.

Several commonly used serotonin receptor antagonists were studied for their ability to influence basal plasma insulin and glucagon (using 30K antibody) levels as well as the response of these hormones to a glucose or arginine challenge administered systematically to overnight fasted rats. Cyproheptadine, in contrast to other antagonists employed, induced large increases of insulin, glucagon and glucose, although this hyperinsulinemia was of a smaller magnitude when compared with hormone levels observed during an equivalent hyperglycemia resulting from glucose administration. The pancreatic response to a glucose load (increased insulin and decreased glucagon release) and an arginine load (increased insulin and glucagon release) were prevented by cyproheptadine pretreatment. Basal insulin levels were bot consistently altered by methysergide or cinanserin and were slightly elevated by metergoline. Basal glucagon levels were unaffected by these drugs. These three agents potentiated the insulinotropic effect of an arginine load whereas only metergoline exerted a similar effect on the response to glucose loading. Glucagon release in response to these stimuli was not significantly altered by drug pretreatment.     

Inhibitory effect of rat amylin on the insulin responses to glucose and arginine in the perfused rat pancreas

Amylin, a 37-amino acid polypeptide, is the main component of amyloid deposits in the islets of Langerhans, and has been identified in the B-cell secretory granules. We have investigated the effect of rat amylin on the insulin and glucagon release by the isolated, perfused rat pancreas. Amylin infusion at 750 nM, markedly reduced unstimulated insulin release (ca. 50%, P less than 0.025), whereas it did not modify glucagon output. At the same concentration, amylin also blocked the insulin response to 9 mM glucose (ca. 80%, P less than 0.025) without affecting the suppressor effect of glucose on glucagon release. The inhibitory effect of amylin on glucose-induced insulin secretion was confirmed by lowering the amylin concentration (500 nM) and increasing the glucose stimulus (11 mM); again, no effect of amylin on glucagon release was observed. Finally, amylin, at 500 nM, reduced the insulin response to 3.5 mM arginine (ca. 40%, P less than 0.025) without modifying the secretion of glucagon elicited by this amino acid. It can be concluded that, in the rat pancreas, the inhibitory effect of homologous amylin on unstimulated insulin secretion, as well as on the insulin responses to metabolic substrates (glucose and arginine), favours the concept of this novel peptide as a potential diabetogenic agent.     

Effect of glucagon antiserum on somatostatin,glucagon and insulin release from the isolated perfused rat pancreas

In order to elucidate the effect of glucagon antiserum on the endocrine pancreas, the release of somatostatin, glucagon, and insulin from the isolated perfused rat pancreas was studied following the infusion of arginine both with and without pretreatment by glucagon antiserum. Various concentrations of arginine in the presence of 5.5 mM glucose stimulated both somatostatin and glucagon secretion. However, the responses of somatostatin and glucagon were different at different doses of arginine. The infusion of glucagon antiserum strongly stimulated basal secretion in the perfusate total glucagon (free + antibody bound glucagon) and also enhanced its response to arginine, but free glucagon was undetectable in the perfusate during the infusion. On the other hand, the glucagon antiserum had no significant effect on either insulin or somatostatin secretion. Moreover, electron microscopic study revealed degrannulation and vacuolization in the cytoplasm of the A cells after exposure to glucagon antiserum, suggesting a hypersecretion of glucagon, but no significant change was found in the B cells or the D cells. We conclude that in a single pass perfusion system glucagon antiserum does not affect somatostatin or insulin secretion, although it enhances glucagon secretion.     

Adenosine triphosphate (ATP) and glucose. Action on insulin and glucagon secretion]

ATP stimulates glucagon and insulin secretions from the isolated perfused rat pancreas. This effect is modulated by glucose. Glucagon secretion is stimulated by ATP only in the absence or in the presence of a low glucose concentration (0.5 g/1). As to insulin secretion, it is strongly stimulated only in the presence of a glucose concentration of 1.5 g/1.     

Effects of haematocrit value and glucagon on the metabolism of perfused rat liver

Liver from adult male rats were perfused in situ for 30 min with either undiluted, defibrinated rat blood (haematocrit value 38%) or the same blood diluted with buffer to give a haematocrit of 20%. Perfusion with diluted blood lowered the PO2 of the effluent perfusate but this was insufficient to prevent the fall in O2 consumption due to the reduction in haematocrit. Glucagon (5 X 10(-9) M) increased hepatic O2 consumption with whole blood but not with diluted blood. perfusate K+ was increased by perfusion with diluted blood and glucagon. Bile flow was depressed and biliary K+ increased by glucagon but only in experiments with whole blood. Perfusate glucose was raised by lowering of hepatic O2 consumption but the hormonal stimulation of glucose output was the same at both haematocrits. Net ketogenesis was increased with perfusion with diluted blood and by glucagon. In the absence of glucagon there was a net secretion of triacylglycerols which was depressed by lowering of the haematocrit. Glucagon inhibited triacylglycerol secretion and the effect was greater with whole blood so that there was net uptake. While effects of glucagon were obtained during perfusion at a lower haematocrit, it would appear that whole blood was the medium that allowed their fullest expression.     

Modulation of A and B cell functions by tolbutamide and arginine in the pancreas of thiamine-deficient rats

The effects of administration of glucose orally and tolbutamide or arginine intravenously on insulin and glucagon secretion and blood glucose level were studied in normal and thiamine-deficient rats. In thiamine deficiency, insulin secretion and glucose tolerance were impaired during glucose ingestion. Tolbutamide decreased the blood glucose level in both control and thiamine-deficient rats but its stimulatory effect on insulin secretion was minimal in thiamine-deficient rats unlike the control animals. Arginine did not alter substantially the blood glucose or insulin in thiamine-deficient rats, whereas it increased the insulin level in control rats. The fasting plasma glucagon level was high in thiamine deficiency. Tolbutamide increased the plasma glucagon in control rats, but did so only marginally in thiamine-deficient rats. Arginine also increased the glucagon secretion throughout the period of study in control rats. In thiamine-deficient rats the glucagon secretion was pronounced only after 20 min of arginine administration. These results suggest that an unimpaired glucose metabolism is a prerequisite to induce proper insulin secretion. Only proper insulin secretion can check the glucagon secretion rather than the increased glucose level. Hypoglycemia can induce glucagon secretion independent of the insulin level.     

Insulin, glucagon, and somatostatin secretion by cultured rat islet cell tumor and its clones

Cells derived from rat islet tumor and grown in culture (parent cells-RIN-m) and two clones obtained from them were used to study the effect of various secretagogues on insulin, glucagon, and somatostatin secretion. Parent cells secreted all three hormones in various quantities, while clone 5F secreted predominantly insulin and clone 14B secreted predominantly somatostatin. The secretory behavior of these cells were compared to each other and to that of normal islets. In general, as in the case of normal islets, insulin secretion was stimulated by calcium, potassium, tolbutamide, theophylline, and glucagon. It was inhibited by somatostatin. Glucagon secretion was stimulated by calcium, arginine, and theophylline. Somatostatin secretion was stimulated in clone 14B by arginine, tolbutamide, theophylline, and insulin. These cells differ from normal islets, in that they do not respond to glucose or arginine with increased insulin secretion. Also somatostatin failed to inhibit glucagon secretion. The similarity in insulin secretory responses of parent cells and clone 5F suggests that local or paracrine islet hormone secretion plays only a negligible role in the control of other hormone secretion in these cells.     

Zinc Supplementation Does Not Affect Glucagon Response to Intravenous Glucose and Insulin Infusion in Patients with Well-Controlled Type 2 Diabetes

Glucagon dysregulation is an essential component in the pathophysiology of type 2 diabetes. Studies in vitro and in animal models have shown that zinc co-secreted with insulin suppresses glucagon secretion. Zinc supplementation improves blood glucose control in patients with type 2 diabetes, although there is little information about how zinc supplementation may affect glucagon secretion. The objective of this study was to evaluate the effect of 1-year zinc supplementation on fasting plasma glucagon concentration and in response to intravenous glucose and insulin infusion in patients with type 2 diabetes. A cross-sectional study was performed after 1-year of intervention with 30 mg/day zinc supplementation or a placebo on 28 patients with type 2 diabetes. Demographic, anthropometric, and biochemical parameters were determined. Fasting plasma glucagon and in response to intravenous glucose and insulin infusion were evaluated. Patients of both placebo and supplemented groups presented a well control of diabetes, with mean values of fasting blood glucose and glycated hemoglobin within the therapeutic goals established by ADA. No significant differences were observed in plasma glucagon concentration, glucagon/glucose ratio or glucagon/insulin ratio fasting, after glucose or after insulin infusions between placebo and supplemented groups. No significant effects of glucose or insulin infusions were observed on plasma glucagon concentration. One-year zinc supplementation did not affect fasting plasma glucagon nor response to intravenous glucose or insulin infusion in well-controlled type 2 diabetes patients with an adequate zinc status.     

Stimulatory effect of xenin-8 on insulin and glucagon secretion in the perfused rat pancreas

Xenin is a 25-amino acid peptide of the neurotensin/xenopsin family identified in gastric mucosa as well as in a number of tissues, including the pancreas of various mammals. In healthy subjects, plasma xenin immunoreactivity increases after meals. Infusion of the synthetic peptide in dogs evokes a rise in plasma insulin and glucagon levels and stimulates exocrine pancreatic secretion. The latter effect has also been demonstrated for xenin-8, the C-terminal octapeptide of xenin. We have investigated the effect of xenin-8 on insulin, glucagon and somatostatin secretion in the perfused rat pancreas. Xenin-8 stimulated basal insulin secretion and potentiated the insulin response to glucose in a dose-dependent manner (EC(50)=0.16 nM; R(2)=0.9955). Arginine-induced insulin release was also augmented by xenin-8 (by 40%; p<0.05). Xenin-8 potentiated the glucagon responses to both arginine (by 60%; p<0.05) and carbachol (by 50%; p<0.05) and counteracted the inhibition of glucagon release induced by increasing the glucose concentration. No effect of xenin-8 on somatostatin output was observed. Our observations indicate that the reported increases in plasma insulin and glucagon levels induced by xenin represent a direct influence of this peptide on the pancreatic B and A cells.     

Influence of temperature on the secretion of insulin and glucagon induced by stimulation of cholinergic receptors in the presence of glucose]

In the presence of a glucose concentration of 1.5 g/1 the secretion of insulin from the isolated perfused rat pancreas is clearly weaker at 28 degrees C than at 37.5 degrees C. In response to cholinergic stimulation, the absolute increase of insulin secretion rate is less at 28 degrees C than at 37.5 degrees C. However, when evaluated in percentage in relation to the baseline value, this increase is more important at the lower temperature. As to glucagon secretion, lowering of the temperature from 37.5 degrees C to 28 degrees C modifies neither this secretion in the presence of glucose alone, nor the increased secretion provoked by the cholinergic stimulation.     

Combination of aerobic exercise and an arginine,alanine, and phenylalanine mixture increases fat mobilization and ketone body synthesis

During exercise, blood levels of several hormones increase acutely. We hypothesized that consumption of a specific combination of amino acids (arginine, alanine, and phenylalanine; A-mix) may be involved in secretion of glucagon, and when combined with exercise may promote fat catabolism. Ten healthy male volunteers were randomized in a crossover study to ingest either A-mix (3 g/dose) or placebo (3 g of dextrin/dose). Thirty minutes after ingesting, each condition subsequently performed workload trials on a cycle ergometer at 50% of maximal oxygen consumption for 1 h. After oral intake of A-mix, the concentrations of plasma ketone bodies and adrenalin during and post-exercise were significantly increased. The area under the curve for glycerol and glucagon was significantly increased in the post-exercise by A-mix administration. These results suggest that pre-exercise ingestion of A-mix causes a shift of energy source from carbohydrate to fat combustion by increasing secretion of adrenalin and glucagon.     

Glucose regulation of arginine-induced pancreatic somatostatin release from the isolated perfused rat pancreas

The effects of glucose alone, combinations of glucose with arginine or tolbutamide and either arginine or tolbutamide alone, on somatostatin, insulin, and glucagon secretion were investigated using the isolated perfused rat pancreas. When glucose alone was raised in graded increments at 15-min intervals from an initial concentration of 0 mM to a maximum of 16.7 mM, somatostatin as well as insulin in the perfusate increased with the glucose, while glucagon decreased. The similarity of the glucose stimulated somatostatin and insulin release was especially evident when the perfusate glucose was increased from an initial dose of 4.4 mM rather than 0 mM to 8.8 mM or 16.7 mM. In addition, glucose at concentrations varying from 4.4 mM to 11 mM dose-dependently enhanced arginine-induced somatostatin and insulin release and suppressed glucagon release dose-dependently as before. Arginine in the absence of glucose was not capable of stimulating somatostatin secretion whereas tolbutamide, in contrast, was capable of stimulating somatostatin secretion even in the absence of glucose.     

Glucagon and insulin secretion by dispersed islet cells: possible paracrine relationships

The ability of dispersed islet cells in a perifusion system to secret glucagon and insulin in response to physiologic stimuli was investigated. Normal hamster islets were isolated by collagenase digestion and the cells dispersed by sequential digestion with collagenase and trypsin. Following a 50-min period of equilibrium in buffer with high glucose concentrations (5.0 mg/ml), glucagon secretion was stimulated by glucopenia and subsequently, inhibited by increasing the concentration of glucose. The responsiveness to glucose inhibition was significantly less in dispersed islet cells than in intact islets. However, the dispersed islet cells showed significantly greater response to arginine. Glucagon secretion by dispersed islet cells was stimulated to tolbutamide and epinephrine but somatostatin had no effect. Dispersed islet cell preparations did not augment insulin secretion in response to glucose but did secrete more insulin in response to arginine. Intact islets secreted insulin in response to glucose but not arginine. We conclude that A cells in cell suspension do not need direct contact or an intact intra-islet environment in order to respond to glucose, arginine, epinephrine, or tolbutamide but the extent of response may be influenced by paracrine effects. However, paracrine relationships may be important in determining the response of B cells to secretagogues.