Glucagon Secretion In Response Research Articles

Neurotransmitters partially restore glucose sensitivity of insulin and glucagon secretion from perfused streptozotocin-induced diabetic rat pancreas

To elucidate the mechanisms of insensitivity of hormone secretion to glucose in streptozotocin-induced diabetic rat islets, we investigated the effects of acetylcholine (ACh) and norepinephrine on insulin and glucagon secretion in response to changes in glucose concentration, using perfused pancreas preparations. Basal insulin secretion at a blood glucose level of 5.6 mmol/l was significantly higher and basal glucagon secretion significantly lower in streptozotocin-induced diabetic rats than in controls, and neither high (16.7 mmol/l) nor low (1.4 mmol/l) blood glucose concentrations influenced insulin or glucagon secretion. Addition of 10(-6) mol/l ACh to the perfusate increased glucose-stimulated insulin secretion. Also, 10(-6) mol/l ACh, 10(-7) mol/l norepinephrine, as well as a combination of both, induced marked glucagon secretion, this was suppressed by high blood glucose level. Although simultaneous addition of 10(-6) mol/l ACh and 10(-7) mol/l norepinephrine induced only a slight increase in glucagon secretion in response to glucopenia, there was a significant increase in glucagon secretion in conjunction with an ambient decrease in insulin. Histopathological examination revealed a marked decline in acetylcholinesterase and monoamine-oxidase activities in the islets of streptozotocin-induced diabetic rats. We speculate that reduction of the potentiating effects of ACh and norepinephrine lessens glucose sensitivity of islet beta and alpha cells in this rat model of diabetes.

Glucagon secretory response to hypoglycaemia, adrenaline and carbachol in streptozotocin-diabetic rats.

Glucagon response to insulin-induced hypoglycaemia is impared in diabetes, but the mechanism is not established. Pancreatic A cell hyporesponsiveness to adrenergic or cholinergic stimulation could contribute to the impairment. We therefore compared the plasma glucagon responses to intravenous infusion of adrenaline (1200 ng kg(-1) min(-1) for 20 min) or to intravenous injection of the cholinergic agonist carbachol (50 micrograms kg(-1)) in chloral hydrate-anaesthetized rats made diabetic with the use of streptozotocin (80 mg kg(-1) subcutaneously) 6 weeks before and in anaesthetized control rats. Insulin was infused intravenously to reduce plasma glucose levels to below 1.8 mmol L(-1). As expected, the plasma glucagon response was reduced by approximately 45% in streptozotocin-diabetic rats compared with controls (P = 0.045). During adrenaline infusion, plasma glucagon levels increased by 277 +/- 92 pg mL(-1) in controls (P = 0.009) and by 570 +/- 137 pg mL(-1) in the diabetic rats (P = 0.002). Thus, the plasma glucagon response to adrenaline was approximately doubled in the diabetic rats (P = 0.045). Following carbachol injection, plasma glucagon levels were raised by 1211 +/- 208 pg mL(-1) (P < 0.001) in controls but only by 555 +/- 242 pg mL(-1) in the diabetic rats (P = 0.049). Thus, the plasma glucagon response to carbachol was impared by approximately 58% in the diabetic rats (P = 0.028). We conclude that carbachol-stimulated glucagon secretion is impared concomitantly with the impared glucagon response to hypoglycaemia in streptozotocin-diabetic rats, whereas adrenaline-induced glucagon secretion is exaggerated. We suggest that a reduced pancreatic A cell responsiveness to cholinergic stimulation could contribute to the impairment of the glucagon response to insulin-induced hypoglycaemia in diabetes.

Involvement of nitric oxide in neuroglycopenia-induced insulin and glucagon secretion in the mouse

Neuroglycopenia induced by administration of 2-deoxy- d-glucose is known to stimulate the secretion of both insulin and glucagon in mice by a mechanism that is dependent on neural activity. In the present study, we examined whether the neurotransmitter nitric oxide (NO) is involved in this process. Therefore, 2-deoxy- d-glucose (500 mg/kg) was injected intravenously alone or together with the inhibitor of NO synthase, N G-nitro- l-arginine methyl ester (50 mg/kg) to conscious mice. It was found that N G-nitro- l-arginine methyl ester inhibited the increased plasma levels of both insulin (by 26%; P = 0.039) and glucagon (by 45%; P < 0.001) at 10 min after injection of 2-deoxy- d-glucose. Similarly, the NO synthase inhibitor, N G-nitro- l-arginine, which is devoid of the anticholinergic property of N G-nitro- l-arginine methyl ester, inhibited the responses of both insulin (by 53%; P = 0.026) and glucagon (by 57%; P = 0.003) to 2-deoxy- d-glucose. In contrast, the stereoisomer of N G-nitro- l-arginine methyl ester, N G-nitro- d-arginine methyl ester, which is devoid of NO synthase inhibitory activity, was without effect on 2-deoxy- d-glucose-induced insulin and glucagon secretion. Plasma levels of adrenaline and noradrenaline after administration of 2-deoxy- d-glucose were also reduced by N G-nitro- l-arginine methyl ester. In contrast, the insulin and glucagon secretory responses to intravenous injection of arginine (250 mg/kg), glucose (500 mg/kg) or the cholinergic agonist, carbachol (30 μg/kg), were not influenced by N G-nitro- l-arginine methyl ester, N G-nitro- d-arginine methyl ester or N G-nitro- l-arginine. We conclude that the increased secretion of both insulin and glucagon during neuroglycopenia in the mouse is partially mediated by NO.

Pancreas Transplantation in Diabetic Humans Normalizes Hepatic Glucose Production During Hypoglycemia

Although successful pancreas transplantation in humans with type I diabetes mellitus restores glucose-induced insulin secretion, provides freedom from insulin treatment, and normalizes fasting glucose levels, much less is known about its effects on counterregulation of hypoglycemia. To determine whether pancreas transplantation normalizes glucagon secretion and hepatic glucose production (HGP) during hypoglycemia, we performed hyperinsulinemic hypoglycemic clamps in successful recipients of pancreas allografts. Recipients were found to have glucagon secretory responses during hypoglycemia that were similar to those of control subjects (incremental glucagon response: recipients, 147 +/- 34 ng/L; control subjects, 161 +/- 43 ng/L, NS) but were significantly higher than those of matched subjects with type I diabetes (23 +/- 9 ng/L, P < 0.01). HGP rates at the end of 120 min of hypoglycemia were also significantly higher in recipients and control subjects than in subjects with diabetes (pancreas recipients, 1.92 +/- 0.33 mg.kg-1.min-1; control subjects, 2.05 +/- 0.18 mg.kg-1.min-1; subjects with type I diabetes, 0.58 +/- 0.12 mg.kg-1.min-1). A comparison with a third group of nondiabetic kidney transplant recipients demonstrated that the beneficial effects on glucose counterregulation were a result of pancreas transplantation and not the associated immunosuppressive therapy. We conclude that pancreas transplantation restores hypoglycemia-induced glucagon secretion and HGP, thereby allowing for normalization of glucose recovery from hypoglycemia.

Glucagon, insulin and somatostatin secretion in response to sympathetic neural activation in streptozotocin-induced diabetic rats. A study with the isolated perfused rat pancreas in vitro.

Changes in glucagon, insulin and somatostatin secretion induced by electrical splanchnic nerve stimulation were examined in rats treated with streptozotocin as neonates and as adults. In order to study the direct neural effects we used the isolated perfused rat pancreas with intact left splanchnic nerve in vitro. In normal rats splanchnic nerve stimulation causes significant decreases in insulin (30-40%) and somatostatin (30-50%) secretion at both 16.7 mmol/l and 1 mmol/l glucose concentrations. In the neonatal streptozotocin-diabetic rat splanchnic nerve stimulation at 16.7 mmol/l glucose decreased insulin secretion (14%) further than in the control rats (30%), however, somatostatin secretion did not decrease to the same extent. Similar results were also observed at the low (1 mmol/l) glucose concentration. On the other hand, percent decreases of insulin and somatostatin secretion induced by splanchnic nerve stimulation in the streptozocin-diabetic rats were similar to the values observed in the normal control rats. The glucagon secretion in response to splanchnic nerve stimulation at 16.7 mmol/l glucose from pancreatic Alpha cells in both types of induced diabetes is exaggerated, and the degree of exaggeration seems to parallel the severity of the hyperglycaemia. However, the splanchnic nerve stimulation-induced glucagon secretion at 1 mmol/l glucose was impaired in the streptozotocin-diabetic rats, but not in the neonatal streptozotocin-diabetic rats. These data suggest that the sensitivity of diabetic Alpha and Delta cells to sympathetic neural activation are blunted, whereas the sensitivity of Beta cells is enhanced in the diabetic animal model.

Effect of physiological hyperinsulinemia on counterregulatory hormone responses during hypoglycemia in humans.

We evaluated the effect of continuous physiological hyperinsulinemia on counterregulatory hormone responses in seven healthy subjects, each studied on two occasions. Hormone responses were measured during identical 2-h periods of hypoglycemia (plasma glucose target 3.5 mmol/L) at insulin levels of 350 pmol/L or 640 pmol/L. During hypoglycemia, there were significant (50-1400%) increases in glucagon, epinephrine, norepinephrine, GH, and cortisol which were comparable in the two groups. We further evaluated the influence of the duration of mild hyperinsulinemia on the responses in an additional group of normal subjects (n = 7). Brief (30 min) exposure to insulin was compared to a prolonged (3.5 h) insulin infusion, each followed by identical hypoglycemia. Plasma insulin (approximately 350 pmol/L) and plasma glucose (target 3.3 mmol/L) were similar in both groups. The increases in epinephrine, norepinephrine, GH, and cortisol during hypoglycemia were virtually identical in the two groups. However, the secretion of glucagon was blunted following prolonged hyperinsulinemia, increasing to levels of 249 +/- 17 ng/L in the brief studies and to only 185 +/- 20 ng/L in the prolonged studies (P < 0.005). The insulin-induced decrement in plasma amino acids were similar in the two studies and could not account for the impaired glucagon secretory response. 1) Brief exposure to even high physiological levels of insulin do not alter the magnitude of counterregulatory hormone secretion during hypoglycemia; 2) prolonged hyperinsulinemia results in a selective blunting of the plasma glucagon response to hypoglycemia, perhaps due to a direct suppressive effect of insulin on alpha-cell secretion.

Progressive and concurrent deterioration of vagus-stimulated and hypoglycemia-induced glucagon secretion in streptozotocin-diabetic rats

The effects of left cervical vagus nerve stimulation on glucagon secretion were studied in streptozotocin-diabetic and age-matched control adult male rats. At two-week intervals, after the induction of streptozotocin-diabetes, streptozotocin-diabetic and age-matched control rats were anesthetized with chloral hydrate (350 mg/kg, ip). Left cervical vagus nerves were electrically stimulated via a Grass stimulator with 5-volt monophasic pulses of 3 msec duration at a frequency of 20 Hz for 1, 2, and 4 min. Arginine-induced glucagon secretion was also determined. Vagus nerve-stimulated (2 and 4 min) glucagon secretion deteriorated as the duration of streptozotocin-diabetes increased. Glucagon secretion in response to vagus nerve stimulation was virtually absent by 12 weeks of streptozotocin-diabetes. However, arginine-induced glucagon secretion was unaffected. Subsequent experiments showed that the defect in glucagon secretion from vagal stimulation occurred concurrently with that seen from insulin-induced hypoglycemia. These results indicate that the impaired hypoglycemia-induced glucagon secretion in long-term streptozotocin-diabetic rats may be correlated with the deterioration of the parasympathetic nervous system transmission in streptozotocin-diabetes.

Islet Allografts in the Cryptorchid Testes of Spontaneously Diabetic BB/Wor dp Rats: Response to Glucose, Glipizide, and Arginine

The aim of this study was to examine insulin and glucagon secretory patterns in successfully transplanted spontaneously diabetic BB/Wor dp rats. Diabetic, BB/Wor dp rats received abdominal, intratesticular islet grafts of MHC-compatible BB/Wor dr donor rats without immunosuppression. After a period of 74 +/- 15 days of normoglycemia, they were given the following challenges: 1) glucose, by mouth, 2) a single oral dose of glipizide, with glucose, and 3) arginine, by iv infusion. The pertinent results included the mean fasting plasma glucose levels of control, Sprague-Dawley (C), of transplanted BB/Wor dp (T), and nontransplanted, insulin treated, diabetic BB/Wor dp (D), and they were, respectively, 97 +/- 4 mg/dl, 110 +/- 3 mg/dl, and 350 +/- 40 mg/dl. Fasting plasma insulin levels in C and T rats were 21.9 +/- 3 microU/ml, and 20.4 +/- 2 microU/ml, respectively. Fasting plasma glucagon levels in C, T, and D, were 37.8 +/- 5.7 pg/ml, 43.4 +/- 4.6 pg/ml, and 47.4 +/- 4.9 pg/ml, respectively. During oral glucose tolerance test, the pattern of insulin secretion in the C and T rats was identical with a peak attained at 15 min. Glucose caused a 70% suppression of plasma glucagon levels in C rats (P less than 0.01); T rats suppressed 14%, but this was not statistically significant; D rats failed to suppress. Glipizide plus glucose caused an improved glucose tolerance in T rats without significantly affecting insulin levels. In the same rats, glipizide resulted in a significant suppression of glucagon compared with levels in the presence of glucose alone. Arginine caused a minimal release of insulin in T rats and a major glucagon secretory response in D rats. Pancreatic glucagon content was significantly (P less than 0.03) lower in C and T, compared with D rats. Furthermore, the transplanted testes of T contained substantial amounts of glucagon. In summary, these data suggest that grafted testes in spontaneously diabetic BB/Wor dp rats contain both beta and alpha-cells and that these cells have the capacity to respond to specific secretagogues independently.

Attenuated Anomeric Difference of Glucose-Induced Insulin Release in the Perfused Pancreas of Diazoxide-Treated Rats

Mild hyperglycemia was induced in normal rats by oral administration of both diazoxide and D-glucose. After 48 hours of such a treatment, the insulin and glucagon secretory responses of the perfused pancreas to alpha- and beta-D-glucose (3.3 mM) were examined in the presence of 10.0 mM L-leucine. The output of insulin, but not that of glucagon, and the perfusion pressure were higher in treated than control rats. The alpha-anomer of D-glucose was a more potent insulin secretagogue than beta-D-glucose in both control and treated rats. However, the alpha/beta ratio in insulin output was twice higher in control than treated rats. By analogy with other experimental models of diabetes, the attenuation in the anomeric difference of glucose-stimulated insulin output in the treated rats could reflect an altered secretory response to alpha- rather than beta-D-glucose. These findings suggest that hyperglycemia provokes, as a function of its severity and duration, first attenuation and then suppression, if not inversion, of the anomeric preference for alpha-D-glucose in insulin release. They are also compatible with the hypothesis that the anomeric malaise, associated with B-cell glucotoxicity, is caused by a progressive accumulation of glycogen in this cell.

Pancreatic endocrine function in cystic fibrosis

To characterize pancreatic endocrine secretion and to examine interrelationships among alterations in alpha, beta, and pancreatic polypeptide cell function in patients with cystic fibrosis (CF), we studied 19 patients with exocrine insufficiency (EXO), including 9 receiving insulin therapy (EXO-IT); 10 patients with no exocrine insufficiency (NEXO); and 10 normal control subjects. First-phase C-peptide response to intravenously administered glucose was significantly impaired in CF patients with exocrine insufficiency (EXO-IT = 0.02 +/- 0.01; EXO = 0.11 +/- 0.02; NEXO = 0.25 +/- 0.05; control subjects = 0.30 +/- 0.04 nmol/L). Lowering fasting glucose levels with exogenous insulin administration in EXO-IT did not improve beta cell responsivity to glucose. The C-peptide response to arginine was less impaired (EXO-IT = 0.12 +/- 0.02; EXO = 0.15 +/- 0.02; NEXO = 0.23 +/- 0.06; control subjects = 0.28 +/- 0.04 nmol/L). Alpha cell function, measured as peak glucagon secretion in response to hypoglycemia, was diminished in EXO but not NEXO (EXO-IT = 21 +/- 10; EXO = 62 +/- 19; NEXO = 123 +/- 29; control subjects = 109 +/- 12 ng/L). Despite diminished glucagon response, EXO patients recovered normally from hypoglycemia. Peak pancreatic polypeptide response to hypoglycemia distinguished CF patients with exocrine insufficiency from those without exocrine insufficiency (EXO-IT = 3 +/- 2; EXO = 3 +/- 1; NEXO = 226 +/- 68; control subjects = 273 +/- 100 pmol/L). Thus CF patients with exocrine disease have less alpha, beta, and pancreatic polypeptide cell function than CF patients without exocrine disease. These data suggest either that exocrine disease causes endocrine dysfunction in CF or that a common pathogenic process simultaneously and independently impairs exocrine and endocrine function.

GLP-1 and GLP-17-36 Amide

We studied the cellular distribution of glucagon-like peptide-1 (GLP-1) in the pancreas and gut and the effects of GLP-1 and its truncated form, GLP-1(7-36) amide, on basal and stimulated insulin and glucagon secretion in the mouse. Immunofluorescence staining showed that GLP-1 immunoreactivity occurred within peripheral islet cells and in cells located mainly distally in the small intestine and in the entire large intestine. Double-immunostaining revealed that the GLP-1-immunoreactive cells were identical to the glucagon/glicentin cells. Experiments in vivo revealed that basal insulin secretion was stimulated by GLP-1(7-36) amide at the dose levels of 8 and 32 nmol/kg, and by GLP-1 at 32 nmol/kg. Furthermore, GLP-1(7-36) amide showed additive stimulatory influence with glucose (2.8 mmol/kg), the cholinergic agonist carbachol (0.16 mumol/kg), and the C-terminal octapeptide of cholecystokinin (CCK-8, 5.3 nmol/kg), when injected at 8 or 32 nmol/kg. In contrast, stimulated insulin secretion was unaffected by GLP-1. Moreover, the glucagon secretory responses to carbachol and CCK-8 were inhibited by GLP-1(7-36) amide but were unaffected by the entire GLP-1. We conclude that GLP-1(7-36) has the potential for being a modulator of islet hormone secretion.

Islet Hormonal Regulation of Glucose Turnover during Exercise in Type 1 Diabetes*

A decline in plasma insulin and an increase in glucagon are known to occur during intense and/or prolonged exercise. However, it is not established whether changes in insulin and glucagon secretion are involved in the precise matching of hepatic glucose production to the enhanced glucose uptake by muscle during brief, low intensity exercise. We studied the effects of 30-min cycle exercise at 40% of maximal aerobic capacity in healthy subjects and C-peptide-deficient subjects with type 1 diabetes (IDDM) using [3-3H]glucose to estimate glucose turnover. Diabetic subjects were studied during continuous iv insulin infusion, which normalized glucose kinetics before experimental perturbations. In control (saline-infused) experiments, endogenous glucose appearance (Ra) increased by 80-90% above baseline to match the increase in glucose disappearance in both normal and IDDM subjects, even though the latter exercised at fixed levels of plasma free insulin, averaging 203 +/- 19 pmol/L. In other experiments, somatostatin was infused, and glucagon (1.0 ng/kg.min) and insulin (at two different rates) were maintained at constant levels. Infusion of insulin in normal subjects at doses sufficient to maintain constant peripheral plasma insulin was associated with no apparent effect on glucose turnover (plasma insulin, 80 +/- 21 pmol/L, compared to 52 +/- 5 pmol/L during saline; P = NS). However, insulin infusion at doses that normalized the portal insulin concentration (approximately 208 pmol/L) together with glucagon replacement inhibited the rise in glucose production in both normal and IDDM subjects. There were similar 45-55% reductions (P less than 0.03) of the increase in Ra seen with exercise in control experiments. When peripheral plasma free insulin (and presumably portal levels as well) were increased by about 20% in this experimental setting in IDDM (278 +/- 43 pmol/L), the suppression of Ra was even more profound, and Ra failed to increase at all with exercise. We conclude that the hormonal regulation of Ra in brief duration exercise in man does not necessitate the decrements in portal venous insulin observed under more intense exercise conditions as long as an exercise-induced glucagon secretory response can occur. Glucagon secretion alone cannot prevent hypoglycemia when portal venous insulin concentrations are increased by minimal amounts, such as in insulin-treated diabetics.

Insulin and glucagon secretory responses to arginine, glucagon, and theophylline during perifusion of human fetal islet-like cell clusters.

The secretory responsiveness of human fetal pancreatic endocrine cells was studied by perifusion of cultured islet-like cell clusters (ICC). ICC were obtained from 7 fetuses at 13-15 weeks gestation and 21 fetuses at 17-22 weeks gestation. The ICC were challenged with glucose (20 mmol/L), arginine (10 mmol/L), glucagon (1.4 mumol/L), and theophylline (10 mmol/L) combined with zero, low (2 mmol/L), or high (20 mmol/L) glucose. At 13-15 weeks, glucose and arginine enhanced insulin release in some experiments, whereas glucagon and theophylline were always potent stimuli (mean response, 4-fold regardless of the glucose concentration). At 17-22 weeks, both glucose alone (20 mmol/L) and arginine (10 mmol/L, with 2 mmol/L glucose) induced a small (1.4- to 1.5-fold) increase in insulin release. When arginine was combined with 20 mmol/L glucose, the response was potentiated to become a 2.3-fold increase. In contrast, glucagon was equally effective in 2 and 20 mmol/L glucose (2.9- and 2.6-fold responses, respectively) and produced a half-maximal response even in the absence of glucose. In this age range the most potent stimulus for insulin release was clearly theophylline. The effect of theophylline was also remarkably independent of the glucose concentration of the perifusate (5.6-, 8.1-, and 8.6-fold responses at 0, 2, and 20 mmol/L glucose, respectively). Glucagon release from the ICC of the 17- to 22-week-old fetuses was low (mean basal glucagon release, 2.9; insulin, 24.8 fmol/100 ICC/min). Glucagon release was not affected by 20 mmol/L glucose, but was stimulated by arginine and theophylline. These findings suggest that in the fetal pancreas, in contrast to the adult organ, insulin release results from elevation of intracellular cAMP concentrations (by glucagon or theophylline) relatively independent of the exogenous glucose concentration. Therefore, glucagon may have an important role in regulating insulin release during the early development of human fetal B-cells.

Pancreatic endocrine function in cirrhotic rats

Pancreatic endocrine function in liver cirrhosis was examined in rats both in vivo and in vitro. Experimental liver cirrhosis was induced by subcutaneous injections of 50% carbon tetrachloride in a dose of 2 mL/kg body weight twice a week for 16 weeks. Control rats received a similar dose of olive oil during the same period. In cirrhotic rats, immunoreactive insulin contents in the pancreas were significantly lower, whereas immunoreactive glucagon contents were about threefold higher than those of control rats. In the first part of this study, insulin and glucagon concentrations in both jugular and portal venous blood at basal conditions and after oral glucose loading were simultaneously determined in vivo. Peripheral insulin levels, both before and after glucose loading, were higher, whereas portal insulin concentrations were lower in cirrhotic rats than in the control rats. In contrast, glucagon levels in both the peripheral and portal veins were significantly higher in cirrhotic rats than in control rats. In the second part, isolated perfused pancreata were prepared from cirrhotic and control rats to further characterize the endocrine function of cirrhotic rat pancreas. Insulin secretion in response to 16.7 mmol/L glucose and 100 pmol/L cholecystokinin-octapeptide both were 40% lower in cirrhotic rats than in controls. In contrast, there was no significant difference in arginine-stimulated insulin release between the two groups. However, glucagon secretion in response to 20 mmol/L arginine was 40% higher in cirrhotic rats. If sensitivity is defined as the hormone release proportional to the pancreatic contents, then A and B cells in the cirrhotic rats had normal sensitivity to both glucose and cholecystokinin-octapeptide. In contrast, the responsiveness of B cells to arginine was greatly increased in cirrhotic rats, although that of A cells remained normal. These results suggest that the abnormal pancreatic endocrine secretory function in liver cirrhosis might be due mainly to alterations in the pancreatic hormone contents rather than an altered hormone release mechanism.

Alpha-adrenoceptor blockade by phentolamine inhibits beta-adrenergically and cholinergically induced glucagon secretion in the mouse.

Glucagon secretion is known to be stimulated by activation of the alpha-adrenoceptors. In this study, we investigated whether alpha-adrenoceptor blockade by phentolamine affects basal and stimulated glucagon secretion in the mouse. Phentolamine was injected intraperitoneally to mice at dose levels varying from 2.6 to 260 mumol/kg. It was found that, while decreasing plasma glucose levels, phentolamine did not over this wide dose range affect basal glucagon concentrations indicating an inhibition of the hypoglycaemia-induced glucagon secretion. Further, phentolamine clearly inhibited the glucagon secretory response to beta-adrenergic or cholinergic stimulation. Thus, phentolamine (2.6 mumol/kg), impaired the glucagon secretory response to the beta 2-adrenoceptor agonist terbutaline by 51% (P less than 0.01), and to the cholinergic agonist carbachol by 44% (P less than 0.02). We conclude that alpha-adrenoceptor blockade by phentolamine inhibits the glucagon secretion following hypoglycaemia or stimulation by beta-adrenergic and cholinergic agonists. Thus, the alpha-adrenoceptors seem to be of great importance for glucagon secretion in the mouse.

Anomeric specificity of the insulin and glucagon secretory responses to D-glucose in lean and obese Zucker rats.

The perfused pancreas of lean and obese Zucker rats was exposed, in the presence of L-leucine, to the anomers of D-glucose, which were administered on four successive occasions in the alpha/beta/alpha/beta or beta/alpha/beta/alpha sequence. In 5 lean and 6 obese rats, alpha-D-glucose was more efficient than beta-D-glucose in both stimulating insulin secretion and suppressing glucagon release. Although D-glucose evoked a greater release of insulin and, during prolonged exposure to the hexose, a less pronounced suppression of glucagon secretion in obese than lean rats, the anomeric specificity of these secretory responses was not different in the two groups of animals. In one lean rat, however, alpha-D-glucose, while efficiently stimulating insulin release, failed on two occasions to inhibit glucagon secretion. This isolated observation raises the possibility that the anomeric specificity of functional events evoked by D-glucose in the endocrine pancreas may occasionally be perturbed.

Fluctuations of alpha cell calcium, potassium and sodium during amino acid perifusion of rat pancreatic islets.

Perifusion of rat pancreatic islets with a physiologic, 6-mM amino acid mixture resulted in typical acute and second phase glucagon secretion over 30 min. At various intervals, islets were acutely fixed and processed for scanning electron microscopy, identification of alpha cells, and measurements of single alpha cell content of calcium (Ca), potassium (K) and sodium (Na) with energy-dispersive x-ray analysis. Biphasic glucagon secretion was attended by corresponding biphasic Ca accumulation and a reciprocal, biphasic suppression of K content and acute phase suppression of Na in alpha cells. All secretory and cellular events were preceded by an evanescent upward spike in alpha cell K at 1 min. These results indicate that alpha cell glucagon secretion in response to amino acid mixtures may be initiated by a K signal and is coupled subsequently to phasic changes in alpha cell Ca content. Fluctuations of alpha cell K and Na appear to relate inversely to Ca, suggesting that transmembrane fluxes of the three cations are interrelated.

Restoration of the A cell response to glucose in isolated rat islets of Langerhans

This study investigated the modulatory effects of forskolin, phorbol 12-myristate 13-acetate (PMA) and arginine on pancreatic glucagon secretion in response to changes in glucose concentrations. Glucose, on its own (0, 5, 9 and 18 mM), did not modify glucagon secretion from A cell-rich isolated rat islets of Langerhans. In the presence of 20 μM forskolin, glucagon release was stimulated dose-dependently on lowering the external glucose concentration to 0 mM. Sensitivity to glucose was achieved in the presence of either PMA or arginine; both agents also significantly enhanced glucagon release at all glucose concentrations tested. The responses of the B cells in these experiments were as expected from the available literature. These results indicate that (a) the endogenous rate of glucagon secretion in the isolated islet preparation was minimal and was insensitive to glucose, (b) sensitivity of the A cells to glucose could be restored by either arginine or agents which alter the concentration or activity of proposed cellular second messengers.

Effect of ammonia on glucagon secretion from the perfused pancreas of cirrhotic rats

Effects of ammonia on glucagon and insulin secretion from the perfused pancreas of cirrhotic rats were investigated to clarify the occurring mechanism of hypersecretion of pancreatic glucagon in liver cirrhotics. The results were as follows: During ammonia loading, insulin secretion was inhibited in a dose-related manner, whereas glucagon secretion was gradually increased at high concentrations of ammonia (2 mM) in control rats; this tendency was augmented in the presence of alpha-ketoglutarate in cirrhotic rats. On cessation of ammonia loading, a transient but definite increase in glucagon and insulin secretion was observed. Basal plasma glucagon and ammonia levels as well as basal glucagon secretion from the perfused pancreas of cirrhotic rats were significantly higher than in control rats. Basal insulin secretion from the perfused pancreas of cirrhotic rats was not different in spite of high levels of plasma insulin. Glucagon secretory response to glucose and arginine from the perfused pancreas of cirrhotic rats was higher than in the control pancreas, whereas insulin secretion was lower. In these cirrhotic rats, an increase in the number of islet cells, particularly A cells, was observed. These data suggested that hypersecretion of pancreatic glucagon which was responsible for hyperglucagonemia in cirrhotic rats might be attributed to high levels of ammonia and alpha-ketoglutarate in blood as well as to the fluctuation of abnormal ammonia concentration in blood and to the hypertrophy of islets, particularly of the A cell group due to hypersecretion.

Mechanism of the blunted glucagon response to insulin-induced hypoglycemia in diabetics

It has been widely reported that dysfunctions of pancreatic A-cell occur in diabetics. Since these pancreatic A-cell dysfunctions are not normalized by conventional insulin injection treatment, they were thought to be a primary defect of diabetes mellitus. Recently it was found that paradoxic glucagon secretion to oral glucose and excessive glucagon response to i.v. arginine could be perfectly normalized if strict blood glucose regulations were achieved with appropriate insulin treatment. However, there has been no report on the perfect normalization of glucagon secretion in response to insulin-induced hypoglycemia in diabetics. In this report, to elucidate the precise significance of A-cell function in hypoglycemia in diabetics, the effect of long-term strict glycemic regulations and the importance of intact autonomic nerve function on hypoglycemia-induced glucagon secretion were studied. In experiments on hypoglycemia-induced glucagon secretion in diabetics, 0.2 to 0.3 U/kg of regular insulin injection were usually employed to overcome the hyperglycemia and insulin resistance. However, hyperinsulinemia has been demonstrated to suppress A-cell function in experiments using the euglycemic clamp technique. Therefore, the effect of plasma insulin concentrations after insulin injections was first studied in 7 healthy volunteers by injecting insulin at doses of 0.1 U/kg and 0.3 U/kg. In this experiment with 0.3 U/kg of insulin, the rate of fall in glycemia and the nadir of blood glucose were made similar to that with 0.1 U/kg of insulin by using glucose clamp technique with artificial endocrine pancreas. The plasma glucagon response after 0.3 U/kg of insulin was significantly suppressed as compared to that after 0.1 U/kg of insulin. From these experiments, it was concluded that not only hypoglycemic stimuli but also plasma insulin concentrations are important factors for demonstrating significant glucagon secretion in response to insulin-induced hypoglycemia. Second, the effects of strict glycemic control and autonomic nerve function on hypoglycemia-induced glucagon secretion were studied. Regular insulin at a dose of 0.1 U/kg was injected in an i.v. bolus form into 21 insulin-dependent (IDDM) and 22 noninsulin-dependent (NIDDM) diabetics before and one to three months after strict glycemic control with multiple insulin injection therapy or continuous subcutaneous insulin infusion therapy. To reduce fasting blood glucose level and to obtain the same hypoglycemic stimuli, overnight insulin infusion at a basal dose was undertaken in IDDM who showed hyperglycemia before strict glycemic regulations.(ABSTRACT TRUNCATED AT 400 WORDS)