Glucagon secretion from the perfused rat pancreas. Studies with glucose and catecholamines

Glucagonotropic and Glucagonostatic Effects of KATP Channel Closure and Potassium Depolarization

The role of depolarization in the inverse glucose-dependence of glucagon secretion was investigated by comparing the effects of KATP channel block and of high potassium. The secretion of glucagon and insulin by perifused mouse islets was simultaneously measured. Lowering glucose raised glucagon secretion before it decreased insulin secretion, suggesting an alpha cell-intrinsic signal recognition. Raising glucose affected glucagon and insulin secretion at the same time. However, depolarization by tolbutamide, gliclazide, or 15 mM KCl increased insulin secretion before the glucagon secretion receded. In contrast to the robust depolarizing effect of arginine and KCl (15 and 40 mM) on single alpha cells, tolbutamide was of variable efficacy. Only when applied before other depolarizing agents had tolbutamide a consistent depolarizing effect and regularly increased the cytosolic Ca2+ concentration. When tested on inside-out patches tolbutamide was as effective on alpha cells as on beta cells. In the presence of 1 µM clonidine, to separate insulinotropic from glucagonotropic effects, both 500 µM tolbutamide and 30 µM gliclazide increased glucagon secretion significantly, but transiently. The additional presence of 15 or 40 mM KCl in contrast led to a marked and lasting increase of the glucagon secretion. The glucagon secretion by SUR1 knockout islets was not increased by tolbutamide, whereas 40 mM KCl was of unchanged efficiency. In conclusion a strong and sustained depolarization is compatible with a marked and lasting glucagon secretion. KATP channel closure in alpha cells is less readily achieved than in beta cells, which may explain the moderate and transient glucagonotropic effect.

Localized glucoprivation of hindbrain sites elicits corticosterone and glucagon secretion

Glucose is required for brain energy metabolism. Decerebration, aqueduct occlusion, and cannula mapping studies have established that glucose-sensing cells capable of eliciting feeding and adrenal medullary responses to glucoprivation are localized in the hindbrain. Glucoprivation also evokes corticosterone and glucagon secretion, but the location of receptors mediating these responses is unknown. To determine whether glucoreceptive sites controlling these responses are present in the hindbrain, we administered the antiglycolytic agent, 5-d-thioglucose (5TG, 24 microg in 200 nl) into brain stem sites through implanted cannulas and examined plasma concentrations of corticosterone and glucagon. Both hindbrain and hypothalamic sites were tested. Blood was collected remotely from intra-atrial catheters at 0, 30, 60, 90, 120, 180, and 240 min after 5TG or control injection. Caudal hindbrain 5TG injections potently increased circulating corticosterone and glucagon concentrations. For corticosterone, the mean peak response (maximum concentration minus time 0 concentration) elicited at positive sites (23 of 40 sites) was 391 ng/ml (SE = 16). For glucagon, the mean peak response at positive sites (27 of 40 sites) was 46 pg/ml (SE = 6). Glucoprivically evoked glucagon secretion was abolished by the ganglionic blocker, hexamethonium, but not by adrenal denervation. Six of twenty-five hypothalamic sites were positive for corticosterone secretion, yielding plasma levels of 279 +/- 23 ng/ml, but none of the hypothalamic injection sites elevated glucagon concentrations. Results demonstrate that receptor cells responsive to glucose deficit and capable of increasing corticosterone and glucagon concentrations exist within the hindbrain, thus further delineating central glucoregulatory neural circuitry.

Regulation of glucagon secretion by glucose transporter type 2 (glut2) and astrocyte-dependent glucose sensors

Ripglut1;glut2-/- mice have no endogenous glucose transporter type 2 (glut2) gene expression but rescue glucose-regulated insulin secretion. Control of glucagon plasma levels is, however, abnormal, with fed hyperglucagonemia and insensitivity to physiological hypo- or hyperglycemia, indicating that GLUT2-dependent sensors control glucagon secretion. Here, we evaluated whether these sensors were located centrally and whether GLUT2 was expressed in glial cells or in neurons. We showed that ripglut1;glut2-/- mice failed to increase plasma glucagon levels following glucoprivation induced either by i.p. or intracerebroventricular 2-deoxy-D-glucose injections. This was accompanied by failure of 2-deoxy-D-glucose injections to activate c-Fos-like immunoreactivity in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus. When glut2 was expressed by transgenesis in glial cells but not in neurons of ripglut1;glut2-/- mice, stimulated glucagon secretion was restored as was c-Fos-like immunoreactive labeling in the brainstem. When ripglut1;glut2-/- mice were backcrossed into the C57BL/6 genetic background, fed plasma glucagon levels were also elevated due to abnormal autonomic input to the alpha cells; glucagon secretion was, however, stimulated by hypoglycemic stimuli to levels similar to those in control mice. These studies identify the existence of central glucose sensors requiring glut2 expression in glial cells and therefore functional coupling between glial cells and neurons. These sensors may be activated at different glycemic levels depending on the genetic background.

beta-Cell adaptation to insulin resistance. Increased pyruvate carboxylase and malate-pyruvate shuttle activity in islets of nondiabetic Zucker fatty rats

The beta-cell biochemical mechanisms that account for the compensatory hyperfunction with insulin resistance (so-called beta-cell adaptation) are unknown. We investigated glucose metabolism in isolated islets from 10-12-week-old Zucker fatty (ZF) and Zucker lean (ZL) rats (results expressed per mg/islet of protein). ZF rats were obese, hyperlipidemic, and normoglycemic. They had a 3.8-fold increased beta-cell mass along with 3-10-fold increases in insulin secretion to various stimuli during pancreas perfusion despite insulin content per milligram of beta-cells being only one-third that of ZL rats. Islet glucose metabolism (utilization and oxidation) was 1.5-2-fold increased in the ZF islets despite pyruvate dehydrogenase activity being 30% lowered compared with the ZL islets. The reason was increased flux through pyruvate carboxylase (PC) and the malate-pyruvate and citrate-pyruvate shuttles based on the following observations (% ZL islets): increased V(max) of PC (160%), malate dehydrogenase (170%), and malic enzyme (275%); elevated concentrations of oxaloacetate (150%), malate (250%), citrate (140%), and pyruvate (250%); and 2-fold increased release of malate from isolated mitochondria. Inhibition of PC by 5 mm phenylacetic acid markedly lowered glucose-induced insulin secretion in ZF and ZL islets. Thus, our results suggest that PC and the pyruvate shuttles are increased in ZF islets, and this accounts for glucose mitochondrial metabolism being increased when pyruvate dehydrogenase activity is reduced. As the anaplerosis pathways are implicated in glucose-induced insulin secretion and the synthesis of glucose-derived lipid and amino acids, our results highlight the potential importance of PC and the anaplerosis pathways in the enhanced insulin secretion and beta-cell growth that characterize beta-cell adaptation to insulin resistance.

Miniglucagon (glucagon 19-29): a novel regulator of the pancreatic islet physiology

Miniglucagon, the COOH-terminal (19-29) fragment processed from glucagon, is a potent and efficient inhibitor of insulin secretion from the MIN 6 beta-cell line. Using the rat isolated-perfused pancreas, we investigated the inhibitory effect of miniglucagon on insulin secretion and evaluated the existence of an inhibitory tone exerted by this peptide inside the islet. Miniglucagon dose-dependently inhibited insulin secretion stimulated by 8.3 mol/l glucose, with no change in the perfusion flow rate. A concentration of 1 nmol/l miniglucagon had a significant inhibitory effect on a 1 nmol/l glucagon-like peptide 1 (7-36) amide-potentiated insulin secretion. A decrease in extracellular glucose concentration simultaneously stimulated glucagon and miniglucagon secretion from pancreatic alpha-cells. Using confocal and electron microscopy analysis, we observed that miniglucagon is colocalized with glucagon in mature secretory granules of alpha-cells. Perfusion of an anti-miniglucagon antiserum directed against the biologically active moiety of the peptide resulted in a more pronounced effect of a glucose challenge on insulin secretion, indicating that miniglucagon exerts a local inhibitory tone on beta-cells. We concluded that miniglucagon is a novel local regulator of the pancreatic islet physiology and that any abnormal inhibitory tone exerted by this peptide on the beta-cell would result in an impaired insulin secretion, as observed in type 2 diabetes.

Evidence that extrapancreatic GLUT2-dependent glucose sensors control glucagon secretion

GLUT2-/- mice reexpressing GLUT1 or GLUT2 in their beta-cells (RIPGLUT1 x GLUT2-/- or RIPGLUT2 x GLUT2-/- mice) have nearly normal glucose-stimulated insulin secretion but show high glucagonemia in the fed state. Because this suggested impaired control of glucagon secretion, we set out to directly evaluate the control of glucagonemia by variations in blood glucose concentrations. Using fasted RIPGLUT1 x GLUT2-/- mice, we showed that glucagonemia was no longer increased by hypoglycemic (2.5 mmol/l glucose) clamps or suppressed by hyperglycemic (10 and 20 mmol/l glucose) clamps. However, an increase in plasma glucagon levels was detected when glycemia was decreased to < or =1 mmol/l, indicating preserved glucagon secretory ability, but of reduced sensitivity to glucopenia. To evaluate whether the high-fed glucagonemia could be due to an abnormally increased tone of the autonomic nervous system, fed mutant mice were injected with the ganglionic blockers hexamethonium and chlorisondamine. Both drugs lead to a rapid return of glucagonemia to the levels found in control fed mice. We conclude that 1) in the absence of GLUT2, there is an impaired control of glucagon secretion by low or high glucose; 2) this impaired glucagon secretory activity cannot be due to absence of GLUT2 from alpha-cells because these cells do not normally express this transporter; 3) this dysregulation may be due to inactivation of GLUT2-dependent glucose sensors located outside the endocrine pancreas and controlling glucagon secretion; and 4) because fed hyperglucagonemia is rapidly reversed by ganglionic blockers, this suggests that in the absence of GLUT2, there is an increased activity of the autonomic nervous system stimulating glucagon secretion during the fed state.

Beta-cell dysfunction in 48-hour glucose-infused rats is not a consequence of elevated plasma lipid or islet triglyceride levels

The abnormal insulin secretion found in human diabetics and animal models of diabetes has been attributed to the deleterious effects of chronic hyperglycemia and/or elevated circulating levels of nonesterified fatty acids (NEFAs). In this study, abnormal glucose-induced insulin secretion (GIIS) was generated by a 48-hour infusion of glucose and assessed by the isolated perfused pancreas technique. In these hyperglycemic animals, abnormal GIIS is accompanied by a decrease in plasma NEFAs, while plasma and, more importantly, islet triglycerides remain at levels comparable to those in the controls. It is concluded that the abnormal insulin secretion in this glucose infusion model was likely caused by 48 hours of hyperglycemia and not by changes in circulating or islet lipids.

Stimulation by 2-deoxy-D-glucose tetraacetates of hormonal secretion from the perfused rat pancreas

The effects of alpha- and beta-2-deoxy-D-glucose tetraacetate (1.7 and 8.5 mM) on insulin, somatostatin, and glucagon secretion from isolated rat pancreases perfused in the presence of 8.3 mM D-glucose were compared with those of unesterified 2-deoxy-D-glucose tested at the same two concentrations. The unesterified glucose analog caused, in a concentration-related manner, inhibition of glucose-induced insulin and somatostatin release and augmentation of glucagon secretion. The two anomers of 2-deoxy-D-glucose tetraacetate, however, increased the secretion rate of all three hormones; this effect was also related to the concentration of the esters. No obvious anomeric specificity of the secretory response to 2-deoxy-D-glucose tetraacetate was observed. These findings indicate that the insulinotropic action of hexose esters cannot be accounted for solely by the metabolic effect of their glucidic moieties. They suggest that the A, B, and D cells of the endocrine pancreas are each equipped with a receptor system responsible for the direct recognition of monosaccharide esters as secretagogues. They further support the view that a paracrine effect of insulin on glucagon-producing cells does not represent a major component in the regulation of their secretory activity.

The changes in adenine nucleotides measured in glucose-stimulated rodent islets occur in beta cells but not in alpha cells and are also observed in human islets

Glucose metabolism by pancreatic beta and alpha cells is essential for stimulation of insulin secretion and inhibition of glucagon secretion. Studies using rodent islets have suggested that the ATP/ADP ratio serves as second messenger in beta cells. This study compared the effects of glucose on glucose oxidation ([U-14C]glucose) and adenine nucleotides (luminometric method) in purified rat alpha and beta cells. The rate of glucose oxidation at 1 mM glucose was higher in beta than alpha cells (4.5-fold, i.e. approximately 2-fold after normalization for cell size). It was more strongly stimulated by 10 mM glucose in beta cells (9-fold) than in alpha cells (5-fold). At 1 mM glucose, ATP levels were similar in both cell types, which corresponds to an approximately 2-fold higher concentration in alpha cells ( approximately 6.5 mM) than in beta cells ( approximately 3 mM). In beta cells, glucose dose-dependently increased ATP and decreased ADP levels, causing a rise in the ATP/ADP ratio from 2.4 to 11.6 at 1 and 10 mM, respectively. In alpha cells, glucose did not affect ATP and ADP levels, and the ATP/ADP ratio remained stable around 7.5. In human islets, the ATP/ADP ratio progressively increased between 1 and 10 mM glucose. In duct cells, which often contaminate human islet preparations, an increase in the ATP/ADP ratio sometimes occurred between 1 and 3 mM glucose. In conclusion, the present observations establish that the regulation of glucagon secretion by glucose does not involve changes in alpha cell adenine nucleotides and further support the role of the ATP/ADP ratio in the control of insulin secretion.

Insulin, but not glucose lowering corrects the hyperglucagonemia and increased proglucagon messenger ribonucleic acid levels observed in insulinopenic diabetes

The factors that regulate glucagon biosynthesis and proglucagon gene expression are poorly defined. We previously reported that insulin inhibits proglucagon gene expression in vitro. In vivo, however, the effects of insulin on the regulation of the proglucagon gene have been controversial. Furthermore, whether glucose plays any role alone or in conjunction with insulin on proglucagon gene expression is unknown. We investigated the consequences of insulinopenic diabetes on glucagon gene expression in the endocrine pancreas and intestine and whether insulin and/or glucose could correct the observed abnormalities. We show here that in the first 3 days after induction of hyperglycemia by streptozotocin, rats have levels of plasma glucagon and proglucagon messenger RNA comparable to those of normoglycemic controls despite hyperglycemia. With more prolonged diabetes, plasma glucagon and proglucagon messenger RNA levels increase; this increase is corrected by insulin treatment, but not by phloridzin despite normalization of the glycemia by both treatments. Proglucagon gene expression exhibits the same regulatory response to glucose and insulin in both pancreas and ileum. We conclude that insulin tonically inhibits proglucagon gene expression in the pancreas and ileum and that glucose plays a minor, if any, role in this regulation.

The effect of glucose and glucagon-like peptide-1 stimulation on insulin release in the perfused pancreas in a non-insulin-dependent diabetes mellitus animal model

This study was designed to investigate the effect of glucogon-like peptide-1 (GLP-1) on pancreatic beta-cell function in normal, Zucker diabetic fatty (ZDF) rats, a model for non-insulin-dependent diabetes mellitus (NIDDM or type II diabetes) and their heterozygous siblings. Pancreas perfusion and enzyme-linked immunosorbent assay (ELISA) were used to detect the changes in insulin release under fasting and hyperglycemic conditions and following stimulation with GLP-1. Animals from the ZDF/Gmi-fa rats (ZDF) were grouped according to age, sex, and phenotype (obese or lean), and compared with LA lean rats. Glucose stimulation (10 mmol/L) in obese rats showed repressed response in insulin release. Glucose plus GLP-1 stimulation caused increased insulin release in all groups. The degree of this response differed between groups: lean > obese; young > adult; female > male. The LA lean control group was most sensitive, while the ZDF overtly diabetic group had the lowest response. In addition, the pulsatile pattern of insulin secretion was suppressed in ZDF rats, especially in obese groups. These results support the hypothesis that GLP-1 can effectively stimulate insulin secretion. Insulin release was defective in ZDF obese rats and could be partially restored with GLP-1. ZDF lean rats also showed suppression of beta-cell function and there was a difference in beta-cell function related to sex in ZDF strain. This study documents the efficacy of GLP-1 to stimulate insulin release and contributes to our understanding of the pathophysiological mechanisms underlying NIDDM.

Impaired phasic insulin and amylin secretion in diabetic rats

We have proposed that a hyperstimulated insulin secretion causing beta-cell degranulation is the basis for the impaired glucose-potentiated insulin secretion in type 2 diabetes ("overworked beta-cell"). To confirm this idea, we previously investigated tolbutamide-infused euglycemic rats. Two novel kinds of beta-cell dysfunction were observed: altered phasic glucose-potentiated insulin secretion with preferential sparing of the first phase and a raised secreted ratio of amylin to insulin. The current study tested these parameters in 90% (intact beta-cell insulin stores) and 95% (markedly lowered insulin stores) pancreatectomized (Px) diabetic rats. Rats underwent pancreas perfusion 5-6 wk postsurgery. Controls showed nonchanging insulin secretion during a 20-min perfusion of 16.7 mM glucose + 10 mM arginine. In contrast, both Px groups showed an altered phasic pattern, with the first phase being supernormal (for the beta-cell mass) but the second phase reduced in tandem with the insulin content. Amylin secretion from control and 90% Px rats paralleled the insulin output, so that the amylin-to-insulin ratio averaged 0. 12 +/- 0.03% in the controls and 0.16 +/- 0.01% in the 90% Px rats over the two secretory phases. In contrast, the amylin-to-insulin ratio in 95% Px rats equaled that of controls during the first phase (0.12 +/- 0.1%) but was twice normal during the second phase (0.32 +/- 0.4%). These results confirm the validity of the overworked beta-cell schema by showing identical beta-cell functional defects in Px rats and tolbutamide-infused normoglycemic rats.

Effects of vagal blockade on the counterregulatory response to insulin-induced hypoglycemia in the dog

Our aim was to determine whether vagal transmission is required for the hormonal response to insulin-induced hypoglycemia in 18-h-fasted conscious dogs. Hollow coils were placed around the vagus nerves, with animals under general anesthesia, 2 wk before an experiment. On the day of the study they were perfused with -15 degrees C ethanol for the purpose of blocking vagal transmission, either coincident with the onset of insulin-induced hypoglycemia or after 2 h of established hypoglycemia. In a separate study the coils were perfused with 37 degrees C ethanol in a sham cooling experiment. The following parameters were measured: heart rate, arterial plasma glucose, insulin, pancreatic polypeptide, glucagon, cortisol, epinephrine, norepinephrine, glycerol, free fatty acids, and endogenous glucose production. In response to insulin-induced hypoglycemia (42 mg/dl), plasma glucagon peaked at a level that was double the basal level, and plasma cortisol levels quadrupled. Plasma epinephrine and norepinephrine levels both rose considerably to 2,135 +/- 314 and 537 +/- 122 pg/ml, respectively, as did plasma glycerol (330 +/- 60%) and endogenous glucose production (150 +/- 20%). Plasma free fatty acids peaked at 150 +/- 20% and then returned to basal levels by the end of the study. The hypoglycemia-induced changes were not different when vagal cooling was initiated after the prior establishment of hypoglycemia. Similarly, when vagal cooling occurred concurrently with the initiation of insulin-induced hypoglycemia (46 mg/dl), there were no significant differences in any of the parameters measured compared with the control. Thus vagal blockade did not prevent the effect on either the hormonal or metabolic responses to low blood sugar. Functioning vagal afferent nerves are not required for a normal response to insulin-induced hypoglycemia.

Time course of the defective alpha-cell response to hypoglycemia in diabetic BB rats

Although it is understood that patients with insulin-dependent diabetes mellitus (IDDM) lose the ability to release glucagon during a hypoglycemic challenge, the relationship of this defect to the disease onset and loss of beta-cell function is not well defined. To address this issue, we measured the counterregulatory response in three groups of BB/wor rats during sequential 90-minute euglycemic (7 mmol/L) and hypoglycemic (3 mmol/L) insulin clamps (180 pmol/kg.min). Group 1 (n = 8) consisted of nondiabetic BB rats (aged 84 +/- 3 days), and groups 2 and 3 were rats studied 1 day (n = 7) or 7 days (n = 6) after diabetes onset. Plasma glucagon concentrations were similar in all groups during euglycemia (244 +/- 47 ng/L for nondiabetic, 308 +/- 38 for 1 day of diabetes, and 277 +/- 30 for 7 days of diabetes). Moreover, after 1 day of diabetes, the increase in plasma glucagon during hypoglycemia was similar to that seen in controls (to 581 +/- 94 and 650 +/- 118 ng/L, respectively) even though insulin production by the pancreas was virtually absent. However, after 7 days of diabetes, plasma glucagon only increased to 339 +/- 59 ng/L during hypoglycemia (P = nonsignificant v basal), despite normal pancreatic glucagon content (11.5 +/- 1.2 v 10.8 +/- 0.6 micrograms/g in nondiabetic controls). In conclusion, the hypoglycemia-associated defect in glucagon release occurs early in the course of diabetes in BB rats and is not associated with decreased baseline plasma or pancreatic glucagon levels. This impairment, although not immediately linked to the decrease in pancreatic insulin content, occurs soon afterward, implying that the two events are related.

Vitamin A status affects the development of diabetes and insulitis in BB rats

BB/Wor rats develop autoimmune diabetes mellitus with many features in common with human insulin-dependent diabetes mellitus. Since retinoids are known to have effects on insulin secretion and immune function, these studies were designed to investigate the effects of retinoid deficiency on diabetes in BB/Wor rats and to identify a role for retinoid status in the pathogenesis of autoimmune diabetes mellitus. Litters of diabetes-prone (DP) and diabetes-resistant (DR) BB/Wor rats were divided at weaning and fed a diet either (1) devoid of retinoids and leading to clinical deficiency at approximately 60 days of age (A-def diet)-following 10 days of clinical deficiency, rats on the A-def diet were changed to a diet containing 2 microg/g retinoic (A-def/RA diet); (2) containing 2 microg/g retinoic acid but deficient in retinol (RA diet); or (3) replete in retinol with 4 microg/g retinyl palmitate (RP diet). Rats receiving RP or RA diets were pair-fed to rats on the A-def/RA diet. Diabetes by 120 days of age was greatly reduced (P < .01) in DP rats that received the A-def/RA diet (four of 27) or RA diet (four of 29) versus the RP diet (13 of 31). Insulitis progressed with age in nondiabetic DP rats receiving the RP diet (P < .02) or RA diet (P < .05), but not the A-def/RA diet (P > .22). Insulin secretion was measured in perfused pancreas of nondiabetic rats after age 120 days and correlated negatively with insulitis (P < .05). DP rats receiving the RP diet had reduced insulin secretion as compared with other DP and DR rats (P < .05). In DR rats, retinoid status had no effects on insulitis through 120 days of age or on insulin secretion after 120 days of age. In conclusion, retinol deficiency reduces diabetes and insulitis in DP BB/Wor rats, and retinoic acid can at least partly substitute for retinol in the development of insulitis.

Pancreatic sympathetic nerves contribute to increased glucagon secretion during severe hypoglycemia in dogs

To determine if pancreatic sympathetic nerves can contribute to increased glucagon secretion during hypoglycemia, plasma glucagon and pancreatic glucagon secretion in situ were measured before and during insulin-induced hypoglycemia in three groups of halothane-anesthetized dogs. All dogs were bilaterally vagotomized to eliminate the input from pancreatic parasympathetic nerves. One group of dogs received only vagotomy (VAGX). A second group was vagotomized and adrenalectomized (VAGX + ADX). A third group received vagotomy, adrenalectomy, plus surgical denervation of the pancreas (VAGX + ADX + NERVX) to prevent activation of pancreatic sympathetic nerves. In dogs with VAGX only, hypoglycemia increased plasma epinephrine (Epi), pancreatic norepinephrine (NE) output (+320 +/- 140 pg/min, P < 0.05), arterial plasma glucagon (+28 +/- 12 pg/ml, P < 0.01), and pancreatic glucagon output (+1,470 +/- 370 pg/min, P < 0.01). The addition of ADX eliminated the increase of Epi but did not increase pancreatic NE output (+370 +/- 190 pg/min, P < 0.025), arterial plasma glucagon (+20 +/- 5 pg/ml, P < 0.01), or pancreatic glucagon output (+810 +/- 200 pg/min, P < 0.01). In contrast, the addition of pancreatic denervation eliminated the increase of pancreatic NE output (-20 +/- 40 pg/min, P < 0.05 vs. VAGX), the arterial glucagon (+1 +/- 2 pg/ml, P < 0.01 vs. VAGX), and pancreatic glucagon output responses (+210 +/- 280 pg/min, P < 0.025 vs. VAGX) to hypoglycemia. Thus activation of pancreatic sympathetic nerves can contribute to the increased glucagon secretion during severe insulin-induced hypoglycemia in dogs.

Mechanism of impaired glucose-potentiated insulin secretion in diabetic 90% pancreatectomy rats. Study using glucagonlike peptide-1 (7-37)

Chronic hyperglycemia causes a near-total disappearance of glucose-induced insulin secretion. To determine if glucose potentiation of nonglucose secretagogues is impaired, insulin responses to 10(-9) M glucagonlike peptide-1 (GLP-1) (7-37) were measured at 2.8, 8.3, and 16.7 mM glucose with the in vitro perfused pancreas in rats 4-6 wk after 90% pancreatectomy (Px) and sham-operated controls. In the controls, insulin output to GLP-1 was > 100-fold greater at 16.7 mM glucose versus 2.8 mM glucose. In contrast, the increase was less than threefold in Px, reaching an insulin response at 16.7 mM glucose that was 10 +/- 2% of the controls, well below the predicted 35-40% fractional beta-cell mass in these rats. Px and control rats then underwent a 40-h fast followed by pancreas perfusion using a protocol of 20 min at 16.7 mM glucose followed by 15 min at 16.7 mM glucose/10(-9) M GLP-1. In control rats, fasting suppressed insulin release to high glucose (by 90%) and to GLP-1 (by 60%) without changing the pancreatic insulin content. In contrast, in Px the insulin response to GLP-1 tripled in association with a threefold increase of the insulin content, both now being twice normal when stratified for the fractional beta-cell mass. The mechanism of the increased pancreas insulin content was investigated by assessing islet glucose metabolism and proinsulin biosynthesis. In controls with fasting, both fell 30-50%. In Px, the degree of suppression with fasting was similar, but the attained levels both exceeded those of the controls because of higher baseline (nonfasted) values. In summary, chronic hyperglycemia is associated with a fasting-induced paradoxical increase in glucose-potentiated insulin secretion. In Px rats, the mechanism is an increase in the beta-cell insulin stores, which suggests a causative role for a lowered beta-cell insulin content in the impaired glucose-potentiation of insulin secretion.

Exogenous insulin dose-dependently suppresses glucopenia-induced glucagon secretion from perfused rat pancreas

To clarify the role of insulin in modulating the glucagon response to glucose concentration changes, we investigated the effects of exogenous insulin (10 mU/mL, 100 mU/mL, and 3.3 U/mL) on responses to high glucose (5.6-->16.7 mmol/L), low glucose (5.6-->1.4 mmol/L), and arginine (10 mmol/L) stimulation using the perfused rat pancreas. Although glucagon levels were slightly suppressed by all of the exogenous insulin concentrations tested for the initial few minutes at 5.6 mmol/L glucose, baseline levels were maintained thereafter. Glucagon responses to high or normal glucose concentrations were not altered, but glucopenia-induced glucagon secretion was significantly suppressed as compared with that of controls (0.77 +/- 0.14 ng/min [10 mU/mL, n = 5], 0.55 +/- 0.14 ng/min [100 mU/mL, n = 5], 0.27 +/- 0.13 ng/min [3.3 U/mL, n = 5] v 1.38 +/- 0.20 ng/min [controls, n = 9], P < 0.05, respectively). The first phase of the glucagon response to arginine was potentiated (2.03 +/- 0.24 v 1.17 +/- 0.22 ng/min, P < .05) by 10 mU/mL exogenous insulin. The second phase of the glucagon response to arginine was significantly suppressed in the presence of higher concentrations of exogenous insulin (1.16 +/- 0.23 ng/min [100 mU/mL], 0.96 +/- 0.08 ng/min [3.3 U/mL] v 1.57 +/- 0.17 ng/min, P < .05, respectively). These results suggest that glucagon secretion is modified by the combined suppressive effects of glucose and insulin, although it is mainly glucose that mediates glucagon secretion in the physiological glucose range. Glucopenia- or arginine-induced glucagon secretion is suppressed by insulin.

Redundant parasympathetic and sympathoadrenal mediation of increased glucagon secretion during insulin-induced hypoglycemia in conscious rats

Both the parasympathetic and sympathoadrenal inputs to the pancreas can stimulate glucagon release and are activated during hypoglycemia. However, blockade of only one branch of the autonomic nervous system may not reduce hypoglycemia-induced glucagon secretion, because the unblocked neural input is sufficient to mediate the glucagon response, ie, the neural inputs are redundant. Therefore, to determine if parasympathetic and sympathoadrenal activation redundantly mediate increased glucagon secretion during hypoglycemia, insulin was administered to conscious rats pretreated with a muscarinic antagonist (methylatropine, n = 7), combined alpha- and beta-adrenergic receptor blockade (tolazoline + propranolol, n = 5) or adrenergic blockade + methylatropine (n = 7). Insulin administration produced similar hypoglycemia in control and antagonist-treated rats (25 to 32 mg/dL). In control rats (n = 9), plasma immunoreactive glucagon (IRG) increased from a baseline level of 125 +/- 11 to 1,102 +/- 102 pg/mL during hypoglycemia (delta IRG = +977 +/- 98 pg/mL, P < .0005). The plasma IRG response was not significantly altered either by methylatropine (delta IRG = +677 +/- 141 pg/mL) or by adrenergic blockade (delta IRG = +1,374 +/- 314 pg/mL). However, the IRG response to hypoglycemia was reduced to 25% of the control value by the combination of adrenergic blockade + methylatropine (delta IRG = +250 +/- 83 pg/mL, P < .01 v control rats). These results suggest that the plasma glucagon response to hypoglycemia in conscious rats is predominantly the result of autonomic neural activation, and is redundantly mediated by the parasympathetic and sympathoadrenal divisions of the autonomic nervous system.

Beta-cell hypersensitivity for glucose precedes loss of glucose-induced insulin secretion in 90% pancreatectomized rats

Glucose-induced insulin secretion is impaired in the presence of chronic hyperglycaemia. Insulin secretion was studied in a diabetic rat model prior to the beta cells becoming non-responsive to glucose in order to map out the sequence of changes that accompany chronic hyperglycaemia. In vitro pancreas perfusions were carried out 1 and 2 weeks after a 90% pancreatectomy; controls underwent a sham pancreatectomy. One week post 90% pancreatectomy: (i) non-fasting plasma glucose values were 2-3 mmol/l above normal, (ii) the in vitro insulin response to 16.7 mmol/l glucose was 20 +/- 4% of shams, a response that was appropriate for the surgical reduction in beta-cell mass, (iii) the beta-cell sensitivity for glucose was increased as reflected by left-shifted dose-response curves for glucose-induced insulin secretion (half maximal insulin output 5.7 mmol/l glucose vs 16.5 mmol/l glucose in shams) and glucose potentiation of arginine-induced insulin secretion (half maximal insulin output 3.5 mmol/l glucose vs 14.8 mmol/l glucose in shams). This heightened beta-cell sensitivity for glucose was not a result of the hyperglycaemia, because similarly reduced half-maximal insulin responses were found after a 60% pancreatectomy, a surgical procedure in which plasma glucose values remained normal. In summary, a rise in beta-cell sensitivity for glucose precedes the loss of glucose-induced insulin secretion in diabetic rats.

Effect of insulin-like growth factor-1 on the responses to and recognition of hypoglycemia in humans. A comparison with insulin

Recombinant human insulin-like growth factor-1 (rhIGF-1) lowers blood glucose in humans but its effect on counterregulatory responses has not been established. We therefore compared infusions of rhIGF-1 (0.7 micrograms/kg per min) and insulin (0.8 mU/kg.min) for 120 min in 10 healthy volunteers (glucose allowed to fall freely). With both, glucose fell rapidly because of stimulation of glucose uptake and suppression of hepatic glucose production. Despite similar plasma glucose nadirs (2.6 +/- 0.1 vs. 2.7 +/- 0.1 mM), the glucagon response was absent (P < 0.005), growth hormone release was attenuated (P < 0.03), and norepinephrine levels were increased (P < 0.05) by rhIGF-1 compared with insulin. Absent glucagon responses were associated with a blunting of the rebound increase in glucose production (P < 0.05 vs. insulin). After stopping the infusions, glucose recovery was delayed with rhIGF-1 (P < 0.001 vs. insulin). To further evaluate the effects of rhIGF-1 during a standard hypoglycemic stimulus, eight additional healthy subjects received rhIGF-1 or insulin while glucose was clamped at 2.8 mM. Again the rise in glucagon during insulin-induced hypoglycemia was totally abolished by rhIGF-1. Growth hormone responses were delayed, whereas increases in norepinephrine, heart rate, and symptomatic awareness of hypoglycemia were greater with rhIGF-1 compared with insulin (P < 0.05). It was concluded that rhIGF-1 suppression of glucagon release during hypoglycemia impairs glucose recovery. Paradoxically, awareness of hypoglycemia is enhanced with rhIGF-1 in part due to stimulation of the sympathetic activity.

Diabetes and adrenal disease

Disorders of the adrenal cortex and medulla can result in glucose intolerance or overt diabetes mellitus. Cushing's syndrome, characterized by excessive secretion of glucocorticoids, impairs glucose tolerance primarily by causing insulin resistance at the post-receptor level. On the other hand, phaeochromocytoma and hyperaldosteronism, via the respective actions of catecholamines and hypokalaemia on the pancreatic beta-cell, impair glucose tolerance primarily by inhibiting insulin release. The glucose intolerance associated with these adrenal disorders is usually only mild to moderate in severity. Marked hyperglycaemia, glycosuria, and polyuria are uncommon and ketosis is rare. Moreover, the late complications of diabetes mellitus are distinctly uncommon in patients with these disorders, and the prognosis for morbidity and death is usually that of the underlying disease and not that of diabetes mellitus. The impaired glucose tolerance induced by all three of these adrenal disorders usually returns to normal once the underlying aetiology has been cured. These factors must guide the clinician in treatment of these secondary forms of diabetes, and suggest that tight (near normal) blood glucose control may not be an appropriate goal in patients with these disorders. The relationship between adrenal androgens and glucose tolerance is more uncertain. Several studies in humans have demonstrated an acute decline in serum concentrations of the adrenal steroids DHEA and DHEA-sulfate in response to experimentally-induced hyperinsulinaemia, but the regulatory role of insulin on adrenal androgen metabolism in normal physiology or disease remains speculative. In several animal models DHEA appears to exert potent anti-obesity and anti-diabetogenic actions, but such effects have yet to be demonstrated in humans. Human studies of DHEA are limited, and more research needs to be conducted to determine whether the observations made in animal models will prove applicable to man.

The loss of GLUT2 expression by glucose-unresponsive beta cells of db/db mice is reversible and is induced by the diabetic environment

Glucose-induced insulin secretion by beta cells of diabetic db/db mice was studied by a pancreas perfusion technique, and the levels of GLUT2 protein in pancreatic islets were assessed by immunofluorescence microscopy and protein blot analysis. Beta cells from diabetic mice had a high basal rate of insulin secretion; they did not respond to glucose stimulation but displayed a normal secretory response to arginine. At the same time, GLUT2 expression by db/db islets was lost whereas beta cells from nondiabetic db/+ mice expressed high levels of this transporter. GLUT2 levels in liver or kidney of diabetic mice were, however, mostly unaltered. Transplanting islets from db/db mice under the kidney capsule of db/+ mice restored normal GLUT2 levels. Conversely, transplantation of db/+ islets into db/db mice induced the disappearance of GLUT2 expression. When islets from db/+ mice were transplanted under the kidney capsule of streptozocin-diabetic mice, the immunodetection of GLUT2 also disappeared. We conclude that: (a) GLUT2 expression is decreased in glucose-unresponsive beta cells from db/db mice; (b) the decreased expression of GLUT2 is reversible; (c) the loss of GLUT2 expression is induced by the diabetic environment of db/db and streptozocin-induced diabetic mice. These observations together with previously published data suggest that a factor different from glucose or insulin regulates the beta cell expression of GLUT2.

[Val12] HRAS downregulates GLUT2 in beta cells of transgenic mice without affecting glucose homeostasis

Glucose-induced insulin release from pancreatic beta cells depends on the beta-cell metabolism of glucose, which generates intracellular signals for secretion. The beta-cell glucose transporter isotype GLUT2 and the glucose phosphorylating enzyme glucokinase have both been implicated in coupling insulin secretion to extracellular glucose levels. Here we present evidence that a pronounced decrease in beta-cell GLUT2 has no immediate effect on glucose homeostasis. Analysis of transgenic mice overexpressing human [Val12]HRAS oncoprotein under control of the insulin promoter reveals a great reduction in plasma-membrane GLUT2 levels. These mice are nonetheless able to maintain normal fed and fasting plasma glucose and insulin levels for a period of several months. Insulin secretion studied in isolated islets and the perfused pancreas is characterized by a normal incremental response to increasing glucose concentrations. Glucose metabolism, as measured by glucose phosphorylation and oxidation in isolated islets, shows a normal dose dependence on extracellular glucose concentrations. These findings suggest that normal GLUT2 expression in beta cells is not essential for glucose sensing. The transgenic mice provide an experimental system for studying the role of glucose phosphorylation in regulation of insulin release in the absence of GLUT2.

Intra-islet insulin permits glucose to directly suppress pancreatic A cell function

Inhibition of pancreatic glucagon secretion during hyperglycemia could be mediated by (a) glucose, (b) insulin, (c) somatostatin, or (d) glucose in conjunction with insulin. To determine the role of these factors in the mediation of glucagon suppression, we injected alloxan while clamping the arterial supply of the pancreatic splenic lobe of dogs, thus inducing insulin deficiency localized to the ventral lobe and avoiding hyperglycemia. Ventral lobe insulin, glucagon, and somatostatin outputs were then measured in response to a stepped IV glucose infusion. In control dogs glucagon suppression occurred at a glucose level of 150 mg/dl and somatostatin output increased at glucose greater than 250 mg/dl. In alloxan-treated dogs glucagon output was not suppressed nor did somatostatin output increase. We concluded that insulin was required in the mediation of glucagon suppression and somatostatin stimulation. Subsequently, we infused insulin at high rates directly into the artery that supplied the beta cell-deficient lobe in six alloxan-treated dogs. Insulin infusion alone did not cause suppression of glucagon or stimulation of somatostatin; however, insulin repletion during glucose infusions did restore the ability of hyperglycemia to suppress glucagon and stimulate somatostatin. We conclude that intra-islet insulin permits glucose to suppress glucagon secretion and stimulate somatostatin during hyperglycemia.

Beta-cell dysfunction in hyperglycaemic rat models: recovery of glucose-induced insulin secretion with lowering of the ambient glucose level

Glucose-induced insulin secretion is lost in the face of chronic hyperglycaemia. The same defect is present when normal rats are made hyperglycaemic by 48-h glucose infusions. Insulin secretory responses were mapped out during the post-infusion period in order to determine how long it takes for normal Beta-cell function to recover, and to identify factors which influence the rate of recovery. Male Sprague Dawley rats weighing 200-250 g were infused with 50% glucose or 77 mmol/l NaCl for 48 h. The glucose-infused rats were mildly hypoglycaemic for 14 h after the infusion ceased. Glucose-induced insulin secretion, absent at the end of the glucose infusion, was normal 6 h post-infusion. Such rapid recovery was not because of the short duration of hyperglycaemia; mild hypoglycaemia from a 5-h insulin infusion in 90% pancreatectomized rats resulted in a four-fold rise in glucose-induced insulin secretion. Under in vitro conditions, extreme glucose deprivation caused by perfusing the pancreas of glucose-infused rats with buffer devoid of glucose restored glucose-induced insulin secretion in just 37 min. Therefore, the suppression of glucose-induced insulin release by chronic hyperglycaemia is a dynamic situation that requires ongoing hyperglycaemia to prevent the reappearance of glucose responsiveness. This study shows recovery of glucose-induced insulin secretion after just 6 h of mild hypoglycaemia in vivo and even faster recovery with more severe glucose deprivation in vitro. Our results suggest that there is an inverse relationship between the rate of return of Beta-cell glucose responsiveness and the ambient glucose concentration.

Adrenergic control of the glucagon response to ammonia in the perfused rat pancreas

The isolated perfused rat pancreas was used to investigate how adrenergic influences within the pancreas might mediate ammonia-induced glucagon secretion. The addition of 2 mM ammonia to the perfusate increased norepinephrine release and glucagon secretion in the effluent. Upon cessation of ammonia addition, a pronounced burst of glucagon release was observed. Alpha-adrenergic blockade with phentolamine (10 microM) blocked the glucagon response to ammonia. Beta-adrenergic blockade with propranolol (10 microM) had no significant effect on the amount of glucagon release induced by ammonia. Depletion of norepinephrine from sympathetic nerve terminals by pretreatment with 6-hydroxydopamine lowered the pancreatic norepinephrine content to less than 16% of the control value and diminished the glucagon and norepinephrine response to ammonia almost completely. The burst of glucagon release after the removal of ammonia was inhibited to 2% of the control value by phentolamine and to 57% by propranolol. Pretreatment with 6-hydroxydopamine reduced the burst of glucagon secretion to 28% of the control value. Neither phentolamine nor propranolol reduced the magnitude of the ammonia-induced suppression of insulin secretion. We conclude that the effect of ammonia on glucagon release from the isolated rat pancreas is mediated by intrapancreatic adrenergic control.

Minimal chronic hyperglycemia is a critical determinant of impaired insulin secretion after an incomplete pancreatectomy

We now describe experiments that allow one to determine the consequences of B cell reduction alone vs. those that result from superimposed mild hyperglycemia. Male CD rats underwent a 60% pancreatectomy (Px); controls were sham operated. 1 wk later, either 10% sucrose (SUC) was substituted as fluid supply or tap water was continued (WAT). Plasma glucose and insulin values in Px-WAT remained equal to the sham groups; in Px-SUC the values were euglycemic for 25 d, but then nonfasting plasma glucose rose 15 mg/dl. After 6 wk, B cell mass in Px-WAT was reduced by 45% and non-B cell mass by 57%. In contrast, in Px-SUC both masses were comparable to the sham groups. The insulin response to 27.7 mM glucose was measured using the in vitro perfused pancreas. The responses were reduced in Px-WAT but in proportion to their reduced B cell mass; in contrast, it was 75% less than expected in Px-SUC. Also, the response to arginine given at 16.7 mM glucose was reduced only in Px-SUC. These results show that a lowering of B cell mass that does not result in hyperglycemia has no adverse effect on the remaining B cells. On the other hand, if even mild hyperglycemia develops, B cell function becomes impaired and results in inappropriately reduced insulin stores and insulin responses to marked stimuli.

Inhibition of adenosine 3',5'-cyclic monophosphate phosphodiesterase by colchicine: implications for glucagon and corticosteroid secretion

In the study reported, colchicine, often regarded as a specific inhibitor of microtubular function, was shown to exert a concentration-dependent inhibition of the low Km cyclic AMP phosphodiesterases of the pancreatic islet, adrenal cortex and various other tissues of the rat. The results indicated that colchicine is only slightly less active as an inhibitor of the enzyme than theophylline on a molar basis and kinetic analysis revealed that both inhibitors acted competitively in the case of the liver enzyme. Our results show that the inhibitory effect of colchicine upon cyclic AMP phosphodiesterase is a general property of the alkaloid at concentrations of 5 x 10(-5)M and above in both endocrine and non-endocrine tissues. Thus, results obtained employing colchicine at concentrations significantly greater than those which are known to lead to microtubular disaggregation must be viewed with great caution if incorrect implication of microtubular participation in biological processes is to be avoided. For example, we propose that the previously reported paradoxical stimulatory effects of colchicine on the secretion of glucagon from the rat pancreatic islet and on steroidogenesis in the rat adrenal may be due to cyclic AMP accumulation consequent upon phosphodiesterase inhibition in these endocrine tissues and not to microtubular disaggregation as has hitherto been assumed.

The effect of dexamethasone on B cell mass and function in partial pancreatctomized rats

We have now examined islet mass and B-cell secretory function 19 days following a 40–50% pancreatectomy (Px). Plasma glucose and insulin values in Px were equal to those of sham-operated control rats both in the fed state and following an in traperitoneal glucose tolerance test. Glucose potentiation of arginine-induced insulin secretion remained fully preserved when assessed with the invitro perfused pancreas. On the other hand, 5 days of dexamethasone caused mild hyperglycemia in Px, but not in the control group. Moreover, in vitro insulin secretion from the dexamethasone-treated Px rats tended to be modestly lower than the dexamethasone-treated controls. Islet mass returned to a value not significantly lower than that of shams, with a further 30% increase noted in both dexamethasone-treated groups. In contrast, pancreatic insulin content in both Px groups was only about 40% of comparable controls. These data show almost complete islet regeneration within 3 weeks of the 40–50% pancreatectomy. Islet function, on the other hands, was characterized by limited reserve capacity which coexisted with and may have contributed to the development of mild hyperglycemia in the presence of dexamethasone-induced insulin resistance.

Insulinotropin: glucagon-like peptide I (7-37) co-encoded in the glucagon gene is a potent stimulator of insulin release in the perfused rat pancreas

Insulin secretion is controlled by a complex set of factors that include not only glucose but amino acids, catecholamines, and intestinal hormones. We report that a novel glucagon-like peptide, co-encoded with glucagon in the glucagon gene is a potent insulinotropic factor. The glucagon gene encodes a proglucagon that contains in its sequence glucagon and additional glucagon-like peptides (GLPs). These GLPs are liberated from proglucagon in both the pancreas and intestines. GLP-I exists in at least two forms: 37 amino acids GLP-I(1-37), and 31 amino acids, GLP-I(7-37). We studied the effects of synthetic GLP-Is on insulin secretion in the isolated perfused rat pancreas. In the presence of 6.6 mM glucose, GLP-I(7-37) is a potent stimulator of insulin secretion at concentrations as low as 5 X 10(-11) M (3- to 10-fold increases over basal). GLP-I(1-37) had no effect on insulin secretion even at concentrations as high as 5 X 10(-7) M. The earlier demonstration of specific liberation of GLP-I(7-37) in the intestine and pancreas, and the magnitude of the insulinotropic effect at such low concentrations, suggest that GLP-I(7-37) participates in the physiological regulation of insulin secretion.

Chronic hyperglycemia is associated with impaired glucose influence on insulin secretion. A study in normal rats using chronic in vivo glucose infusions

We have proposed that chronic hyperglycemia alters the ability of glucose to modulate insulin secretion, and have now examined the effects of different levels of hyperglycemia on B cell function in normal rats using chronic glucose infusions. Rats weighing 220-300 g were infused with 0.45% NaCl or 20, 30, 35, or 50% glucose at 2 ml/h for 48 h, which raised the plasma glucose by 18 mg/dl in the 30% rats, 37 mg/dl in the 35% rats, and 224 mg/dl in the 50% group. Insulin secretion was then examined using the in vitro isolated perfused pancreas. Glucose-induced insulin secretion remained intact in the normoglycemic 20% glucose rats and it was potentiated in the mildly hyperglycemic 30% glucose rats. However, with even greater hyperglycemia in the 35% glucose group the insulin response to a high glucose perfusate was severely blunted, and it was totally lost in the most hyperglycemic 50% glucose rats. In a second protocol that examined glucose potentiation of arginine-stimulated insulin release, a similar impairment in the ability of glucose to modulate the insulin response to arginine was found with increasing levels of chronic hyperglycemia. On the other hand, the ability of a high glucose concentration to inhibit arginine-stimulated glucagon release was preserved in all glucose-infused rats, but the glucagon levels attained in response to the arginine at 2.8 mM glucose were much less in the 50% glucose rats than in all the other groups. These data clearly show that after 48 h of marked hyperglycemia, glucose influence upon insulin secretion in the rat is severely impaired. This model provides a relatively easy and reproducible method to study the effects of long-term hyperglycemia on B cell function.

Adrenergic modulation of pancreatic glucagon and insulin secretion in goats

Five goats were used to investigate adrenergic influences on the secretion of both glucagon and insulin. The secretion of glucagon was augmented via alpha-adrenergic stimulation. The secretion of insulin was enhanced by stimulation of beta-adrenergic receptors and inhibited by alpha-adrenergic stimulation.

Pancreatic islets of variable size--insulin secretion and glucose utilization

Glucose metabolism and insulin secretion were studied in isolated rat pancreatic islets of different sizes and the amount of tissue was quantitated by the measurement of DNA. It was found that larger islets (140-210 ng DNA/islet) utilized more glucose (based on the conversion of 3H-5-glucose to [3H]20) per ng of DNA than islets containing less DNA (60-120 ng/islet). However, the insulin secreted per ng of DNA in response to a given glucose concentration was the same in islets of all sizes. Also, the islet insulin and glucagon content when expressed in terms of DNA did not depend upon islet size. Thus, although glucose utilization rates expressed as a function of islet DNA content were greater in larger islets, no such relationship was found for glucose-induced insulin release or insulin and glucagon content.

Adrenergically mediated intrapancreatic control of the glucagon response to glucopenia in the isolated rat pancreas

Alpha adrenergic blockade with phentolamine (10 microM) reduces the glucagon response to severe glucopenia (from 150 to 25 mg/dl) to 22% of the control values in the isolated perfused rat pancreas. Propranolol (10 microM) had no significant effect. Neither alpha nor beta adrenergic blockade reduced the magnitude of glucopenic suppression of insulin secretion, but phentolamine increased insulin levels before and during glucopenia. The pattern of somatostatin secretion in these experiments resembled that of insulin. Depletion of norepinephrine from sympathetic nerve endings by pretreatment with 6-hydroxydopamine lowered the pancreatic norepinephrine content to less than 20% of control values and reduced the glucagon response to glucopenia to 69% of the controls. Combined alpha and beta adrenergic blockade during less severe glucopenia (from 120 to 60 mg/dl) reduced the glucagon response to 21% of controls. However, slight glucopenia (from 100 to 80 mg/dl), which elicited only 11% increase in glucagon in the control experiments, was not altered significantly by combined alpha and beta adrenergic blockade. Morphologic studies of adrenergic nerve terminals labeled with [3H]norepinephrine revealed associations with alpha cells. It is concluded that in the isolated rat pancreas adrenergic mediation accounts for most of the glucagon but not insulin response to glucopenia. It is controlled within the pancreas itself, possibly through a direct enhancement by glucopenia of norepinephrine release from nerve endings.

Partial pancreatectomy in the rat and subsequent defect in glucose-induced insulin release

To define the consequences of a known reduction of B cell mass in rats, 90% partial pancreatectomies were performed. For the 6 wk following surgery moderate hyperglycemia was maintained in the fed state but there were no differences in body weight nor plasma insulin concentrations compared with sham-pancreatectomized controls. 8-10 wk following surgery regeneration of the remnant was evident with remnant weight being 26%, B cell mass being 42%, and non-B cell mass being 47% of values found for control whole pancreas. There were comparable increases in the remnant content of insulin, glucagon, and somatostatin. Following meal challenges, intraperitoneal and intravenous glucose tolerance tests and intravenous arginine challenge given 6-7 wk after surgery, the insulin responses to glucose were blunted or absent but the responses following the meals or arginine were intact. Similarly, when the pancreatic remnant was perfused in vitro, insulin release after challenge with 300 mg/dl glucose was markedly reduced whereas intact responsiveness to 10 mM arginine was retained. These data suggest that the chronic stimulation of a reduced B cell mass can lead to a selective loss of glucose-induced insulin secretion.

Role of glucose and insulin in the dynamic regulation of glucagon release by the perfused rat pancreas

The effect of glucose upon the release of glucagon and insulin from the perfused rat pancreas in vitro was studied by varying both the concentration of glucose (from 3.3 to 4.6, 8.5, or 11.1 mmol/l) and the time of exposure to an elevated concentration of the sugar (5, 10 or 23 min). The results suggest that the amount of insulin released during the early period of stimulation could contribute to both the speed and extent of the inhibition in glucagon release. The rate of recovery from inhibition in the A cell, however, appeared to be independent of insulin and was related, in a dose-dependent and time-dependent manner, only to the glucose stimulus. It is suggested that a direct effect of glucose upon the A cell is involved in the physiological regulation of glucagon secretion. An indirect effect of glucose, as mediated via insulin release, may contribute to the rapidity and magnitude of inhibition in A cell secretory activity.

Dynamics of insulin and glucagon secretion by the isolated perfused cat pancreas

1. The dynamics of insulin and glucagon secretion by the isolated perfused cat pancreas was studied in response to glucose and amino acid stimuli. 2. Insulin secretion is biphasic in response to either glucose or amino acid stimuli. 3. Glucose caused a much more pronounced first phase release than did a complete amino acid mixture; whereas glucose and the amino acid mixture stimulated late second phase insulin secretion equipotently. 4. The same amino acid mixture produced a biphasic secretion of glucagon characterized by a rapid and transient spike-release which was followed by a substantial, relatively steady-state, second secretory phase. 5. Various combinations of glucose and amino acids resulted in insulin secretion which suggests a simple additive stimuli phenomenon; whereas no such additive effect was observed with respect to amino acid stimulation of glucagon secretion. 6. Instead, various of the amino acids appear to stimulate maximal glucagon secretion independently. 7. Glucose failed to cause a suppression of amino acid induced glucagon release. 8. The quantitative aspects of this study suggest that the cat pancreas is very responsive to amino acid modulation of hormone release, a fact which may well reflect the high amino acid content of the cat's diet.

Metabolic effects of theophylline in preterm infants

A loading dose of theophylline produces significant metabolic changes, including increase of plasma glucose concentration and an early rise in serum insulin in some infants. The chronic effects of theophylline on plasma glucose, serum insulin, and glucagon are not known at this time. The potential for development of metabolic derangements must be kept in mind when theophylline is used for protracted periods in premature infants.

The biological activity of catfish pancreatic somatostatin

Catfish pancreatic somatostatin, which contains eight additional amino acids on the amino terminus of a tetradecapeptide with considerable homology to tetradecapeptide somatostatin (SRIF), is a naturally occurring homolog of the hypothalamic peptide. The purpose of these studies was to determine the biological activity of this somatostatin homolog. Inhibition of 125I-labelled tyr1-SRIF binding to bovine pituitary plasma membranes by catfish pancreatic somatostatin was approximately 33% that of SRIF. Pancreatic somatostatin had full biological activity measured by inhibition of growth hormone release from isolated rat pituitary cells, but 0.01-0.1% the potency of SRIF. Pancreatic somatostatin at 100 ng/ml produced a 50-60% inhibition of insulin and glucagon secretion from perfused rat pancreas, while SRIF produced comparable inhibition at 10 ng/ml. This report demonstrates that a larger molecular form and natural homolog of SRIF, isolated from fish pancreas, has the same (but reduced) biological activities in rat assay systems as somatostatin originally isolated from sheep hypothalamus.

Acute hypoglycemia in man: neural control of pancreatic islet cell function

Acute hypoglycemia is associated with stimulation of the pancreatic alpha cells and a concurrent, prolonged suppression of insulin secretion by the beta cells. The islets receive a rich autonomic innervation and may therefore be subject to control by adrenergic and cholinergic mechanisms. The role of such neuroregulation of pancreatic islets in response to hypoglycemia has been examined in normal subjects, in subjects with a preganglionic sympathectomy due to traumatic tetraplegia, and in tetraplegic subjects given atropine to induce effective dual autonomic denervation. The normal rise of plasma norepinephrine following hypoglycemia was absent in both groups of tetraplegic subjects, providing evidence of a complete sympathectomy. Blood glucose recovery was significantly impaired only in the group of tetraplegic patients given atropine. Changes in plasma C-terminal glucagon-like immunoreactivity (C-GLI) and C-peptide immunoreactivity (CPR) following hypoglycemia were commensurate with blood glucose levels and were not significantly influenced by islet denervation. These observations suggest that neuroregulation of human islet cell function in response to hypoglycemia may be of limited importance and that stimulation of glucagon secretion may occur independent of cholinergic vagal control.

Effects of exogenous insulin, glucagon, and somatostatin on islet hormone secretion in the perfused chicken pancreas

The effects of exogenous insulin were examined in the isolated perfused chicken pancreas with the duodenum excluded. At low background glucose (50 mg/dl), exogenous insulin infused at a concentration of 20,000 microU/ml elicited clear stimulation of somatostatin secretion while simultaneously inhibiting glucagon release. When the background glucose concentration was elevated to 750 mg/dl, exogenous insulin, had no effect on either somatostatin or glucagon release. When graded doses of exogenous insulin were infused into the chicken pancreas at low background glucose, low concentrations (200 microU/ml) had little effect on somatostatin or glucagon release, but higher concentrations (2000 and 20,000 microU/ml) had clear effects on both somatostatin and glucagon secretion. Glucagon infused at 100 ng/ml stimulated both insulin and somatostatin release. When somatostatin was infused at 25 ng/ml, clear inhibition of glucagon was seen with insulin inhibited to a lesser extent. This study supports the notion of a negative feedback relation between B and D-cells of the pancreatic islets and suggests a paracrine mediation.

Continuous, clonal, insulin- and somatostatin-secreting cell lines established from a transplantable rat islet cell tumor

Continuous cell lines that secrete both insulin and somatostatin were established by two cooperating groups of investiagtors from a serially transplantable, radiation-induced, rat islet cell tumor. The cell lines, named RIN-r and RIN-m, were initiated from tumors maintained in inbred rats or in athymic nude mice, respectively. The cultured cells are epithelioid, free of fibroblast contamination, and can be cloned. They have a hypodiploid chromosome number, are tumorigenic, and posses amine-handling properties, including high levels of L-dopa decarboxylase and formaldehyde-induced fluorescence. Preliminary analysis of clones revealed a spectrum of insulin secretion from undetectable to relatively high. These clonal cell lines provide important systems to study the biology of insulin and somatostatin.

Insulin, glucagon, and somatostatin secretion from isolated perfused rat and chicken pancreas-duodenum

Insulin, glucagon, and somatostatin secretion were evaluated in the following isolated perfused models: rat pancreas-duodenum (both normal and streptozotocin-diabetic animals) and the chicken pancreas with and without duodenum. Insulin secretion in response to glucose or arginine was greater from the normal rat than either the diabetic rat or the chicken. Glucagon release from both species was suppressed by glucose and stimulated by arginine except that poor inhibition by glucose was found in the diabetic rat. Somatostatin could be measured in the effluent from both normal and diabetic rats, but the responses to glucose and arginine were variable and modest. Clear increases of secretion in the rat were only observed in response to a combination of glucose, arginine, theophylline, and isoproterenol. In contrast, the chicken somatostatin secretion was markedly stimulated by glucose and by arginine. In conclusion, the perfused chicken pancreas-duodenum has been shown to secrete large amounts of somatostatin in comparison to the rat and should prove to be a useful system for the study of D-cell regulation.

Inhibition of glucagon and insulin secretion from the perfused rat pancreas by a B-cell-selective somatostatin analog

The isolated perfused rat pancreas was used to study the effects of somatostatin and the analog des-Asn5-[D-Trps, D-Ser13]-somatostatin on arginine-stimulated insulin and glucagon secretion. Even though the analog was found to inhibit glucagon secretion at concentrations of 10 and 100 ng/ml, it had a relatively more inhibitory effect on the B cell than on the A cell than did somatostatin itself. These data suggest that the A- and B-cell receptors for these two peptides differ.

Effect of dihydroergocristine infusion on tolbutamide-induced insulin secretion in man

The insulinemic response to 1 g of tolbutamide i.v. is greatly enhanced (+ 145%) after a 60-min infusion of the alpha-lytic dihydrogenated ergot alkaloid, dihydroergocristine (83.3 micrograms/min, corresponding to a total dose of 5 mg) in 7 healthy subjects. No differences are observed in the glycemic responses.

Glucose memory of pancreatic B and A2 cells: evidence for common time-dependent actions of glucose on insulin and glucagon secretion in the perfused rat pancreas

The influence of previous exposure to glucose on the subsequent B- and A(2)-cell secretory responses to arginine was investigated in the perfused pancreas of the rat. Arginine (8 mM) was administered in two brief (9 min) pulses separated by a period of 66 min. In pancreata from 18-h-fasted animals the two pulses of arginine elicited biphasic glucagon secretory responses, while stimulation of insulin release was barely detectable. When 27.7 mM glucose was administered for 30 min during the intervening period up to 20 min before the second pulse of arginine, the glucagon response to arginine was diminished by 55% while the insulin release was markedly increased in comparison with the first pulse. 8.3 mM glucose, when administered before the second pulse of arginine, exerted effects that were smaller but otherwise similar to those of 27.7 mM glucose.The inclusion of 3.9 mM glucose during the stimulation periods with arginine decreased the glucagon and greatly increased the insulin secretory response. Under these conditions, previous exposure to 27.7 mM glucose inhibited the glucagon and enhanced the insulin response to the second stimulatory pulse of arginine to the same relative degree as when arginine was administered alone. Diazoxide (2 mM), when administered together with 27.7 mM glucose, almost completely inhibited insulin release induced by the presence of glucose, yet did not influence the modulation exerted by glucose on the subsequent insulin and glucagon secretory response to arginine. Conversely, these effects of the glucose pulse could not be reproduced by 1 mug/ml of porcine insulin. Previous exposure to glyceraldehyde (10 mM) mimicked the glucose effects.Also, in pancreata from fed rats, previous exposure to 27.7 mM glucose markedly inhibited subsequent arginine-induced glucagon secretion while the concomittant insulin response was enhanced.IT IS CONCLUDED THAT: (a) both A(2)- and B-cell responsiveness is modulated by a previous exposure to glucose which produces opposite effects in the two cell types, (b) this action of glucose does not depend on its insulin-releasing capacity, and (c) instead, a "memory" of glucose is induced as a consequence of the metabolism of the sugar in the A(2) and B cells.