Regulation of glucagon release from pancreatic A-cells

Effects of Dietary Carbohydrate Content on Circulating Metabolic Fuel Availability in the Postprandial State

Context

According to the carbohydrate-insulin model of obesity, an elevated insulin-to-glucagon ratio in response to a high-carbohydrate diet directs metabolic fuels toward storage, resulting in lower circulating energy.

Objective

To determine differences in total circulating energy post-meal related to dietary carbohydrate.

Design

Ancillary study within the Framingham State Food Study.

Setting

University community.

Participants

29 adults (aged 20 to 65 years) with overweight or obesity (body mass index ≥25 kg/m²)

Intervention

After achieving 10% to 14% weight loss on a run-in diet, participants were randomized to weight-loss-maintenance test diets varying in carbohydrate content (high-carbohydrate, 60% of total energy, n = 11; moderate-carbohydrate, 40%, n = 8; low-carbohydrate, 20%, n = 10) and controlled for protein (20%). During 24-hour metabolic ward admissions between 10 and 15 weeks on the test diets, metabolic fuels and hormones were measured.

Main Outcome Measure

Energy availability (EA) based on energy content of blood glucose, beta-hydroxybutyrate, and free fatty acids, in the late postprandial period (180 to 300 minutes). Insulin at 30 minutes into the test meal (Meal Insulin-30) was measured as an effect modifier.

Results

Insulin-to-glucagon ratio was 7-fold higher in participants on the high- vs low-carbohydrate diet (2.5 and 0.36, respectively). Late postprandial EA was 0.58 kcal/L lower on the high- vs low-carbohydrate diet (P < 0.0001), primarily related to suppression of free fatty acids. Early postprandial EA (30 to 180 minutes) declined fastest in the high-carbohydrate group, and Meal Insulin-30 modified this diet effect.

Conclusions

During weight-loss maintenance on a high-carbohydrate diet, late postprandial EA is reduced, consistent with the carbohydrate-insulin model.

Tripartite cell networks for glucose homeostasis

Controlling the excess and shortage of energy is a fundamental task for living organisms. Diabetes is a representative metabolic disease caused by the malfunction of energy homeostasis. The islets of Langerhans in the pancreas release long-range messengers, hormones, into the blood to regulate the homeostasis of the primary energy fuel, glucose. The hormone and glucose levels in the blood show rhythmic oscillations with a characteristic period of 5-10 min, and the functional roles of the oscillations are not clear. Each islet has [Formula: see text] and [Formula: see text] cells that secrete glucagon and insulin, respectively. These two counter-regulatory hormones appear sufficient to increase and decrease glucose levels. However, pancreatic islets have a third cell type, [Formula: see text] cells, which secrete somatostatin. The three cell populations have a unique spatial organization in islets, and they interact to perturb their hormone secretions. The mini-organs of islets are scattered throughout the exocrine pancreas. Considering that the human pancreas contains approximately a million islets, the coordination of hormone secretion from the multiple sources of islets and cells within the islets should have a significant effect on human physiology. In this review, we introduce the hierarchical organization of tripartite cell networks, and recent biophysical modeling to systematically understand the oscillations and interactions of [Formula: see text], [Formula: see text], and [Formula: see text] cells. Furthermore, we discuss the functional roles and clinical implications of hormonal oscillations and their phase coordination for the diagnosis of type II diabetes.

Influence of two breakfast meals differing in glycemic load on satiety, hunger, and energy intake in preschool children

Background

Glycemic load (GL) is the product of glycemic index of a food and amount of available carbohydrate in that food divided by 100. GL represents quality and quantity of dietary carbohydrate. Little is known about the role of GL in hunger, satiety, and food intake in preschool children. The aim of this study was to investigate the effect of two breakfast meals differing in GL on hunger, satiety, and subsequent food intake at lunch in preschool children aged 4-6 y.

Methods

Twenty three subjects consumed low-GL (LGL) and high-GL (HGL) breakfast meals according to a randomized crossover design followed by an ad libitum lunch 4 h after consumption of breakfast. Children were asked to consume meals until they are full. Each treatment was repeated twice in non-consecutive days and data were averaged.

Results

Children in LGL group consumed significantly lower amounts of GL, total carbohydrate, energy, energy density, and dietary fiber and higher amounts of protein and fat at the breakfast compared to those in HGL group. Prior to lunch, children were hungrier in the HGL intervention group compared to the LGL intervention group (P < 0.03). However, no significant difference was observed between LGL and HGL intervention groups in the amount of food and energy consumed during lunch.

Conclusions

Decreased hunger in children prior to lunch in LGL group is likely due to higher protein and fat content of LGL breakfast. Diets that are low in GL can be recommended as part of healthy diet for preschool children.

Downstream regulatory element antagonistic modulator regulates islet prodynorphin expression

Calcium-binding proteins regulate transcription and secretion of pancreatic islet hormones. Here, we demonstrate neuroendocrine expression of the calcium-binding downstream regulatory element antagonistic modulator (DREAM) and its role in glucose-dependent regulation of prodynorphin (PDN) expression. DREAM is distributed throughout beta- and alpha-cells in both the nucleus and cytoplasm. As DREAM regulates neuronal dynorphin expression, we determined whether this pathway is affected in DREAM(-/-) islets. Under low glucose conditions, with intracellular calcium concentrations of <100 nM, DREAM(-/-) islets had an 80% increase in PDN message compared with controls. Accordingly, DREAM interacts with the PDN promoter downstream regulatory element (DRE) under low calcium (<100 nM) conditions, inhibiting PDN transcription in beta-cells. Furthermore, beta-cells treated with high glucose (20 mM) show increased cytoplasmic calcium (approximately 200 nM), which eliminates DREAM's interaction with the DRE, causing increased PDN promoter activity. As PDN is cleaved into dynorphin peptides, which stimulate kappa-opioid receptors expressed predominantly in alpha-cells of the islet, we determined the role of dynorphin A-(1-17) in glucagon secretion from the alpha-cell. Stimulation with dynorphin A-(1-17) caused alpha-cell calcium fluctuations and a significant increase in glucagon release. DREAM(-/-) islets also show elevated glucagon secretion in low glucose compared with controls. These results demonstrate that PDN transcription is regulated by DREAM in a calcium-dependent manner and suggest a role for dynorphin regulation of alpha-cell glucagon secretion. The data provide a molecular basis for opiate stimulation of glucagon secretion first observed over 25 years ago.

Molecular physiology of vesicular glutamate transporters in the digestive system

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Packaging and storage of glutamate into glutamatergic neuronal vesicles require ATP-dependent vesicular glutamate uptake systems, which utilize the electrochemical proton gradient as a driving force. Three vesicular glutamate transporters (VGLUT1-3) have been recently identified from neuronal tissue where they play a key role to maintain the vesicular glutamate level. Recently, it has been demonstrated that glutamate signaling is also functional in peripheral neuronal and non-neuronal tissues, and occurs in sites of pituitary, adrenal, pineal glands, bone, GI tract, pancreas, skin, and testis. The glutamate receptors and VGLUTs in digestive system have been found in both neuronal and endocrinal cells. The glutamate signaling in the digestive system may have significant relevance to diabetes and GI tract motility disorders. This review will focus on the most recent update of molecular physiology of digestive VGLUTs.

Pancreatic vasopressin V1b receptors: characterization in In-R1-G9 cells and localization in human pancreas

Vasopressin (AVP) receptors present in In-R1-G9 cells, a hamster glucagon-secreting alpha-pancreatic cell line, were characterized using SSR-149415, a selective nonpeptide V1b receptor antagonist, and reference AVP compounds. Binding experiments, using [3H]AVP as a ligand, identified a single population of high-affinity binding sites. SSR-149415 competitively inhibited this binding and exhibited nanomolar and stereospecific affinity for these sites. The affinity of various AVP/oxytocin ligands confirmed a V1b binding profile. In functional studies, AVP was a potent stimulant in inducing intracellular Ca2+ increase, glucagon secretion, and cell proliferation. These effects were fully antagonized by SSR-149415 with a nanomolar potency, whereas its diasteroisomer as well as two selective V1a and V2 receptor antagonists were much less potent. Additionally, the order of potency of AVP agonists and antagonists was in agreement with V1b-mediated effects. By RT-PCR, we confirmed the presence of V1b receptor mRNA in both In-R1-G9 cells and in human pancreas. The distribution pattern of V1b receptors investigated in human pancreas by immunohistochemistry showed strong labeling in islets of Langerhans, and colocalization studies indicated that this receptor was expressed in alpha-glucagon, beta-insulin, and somatostatin pancreatic cells. Thus, in In-R1-G9 cells, AVP mediates intracellular Ca2+ increase, glucagon secretion, and cell proliferation by activating V1b receptors, and these effects are potently antagonized by SSR-149415. Moreover, the presence of V1b receptors also found in human Langerhans islets could suggest hormonal control of AVP in human pancreas.

A reappraisal of the blood glucose homeostat which comprehensively explains the type 2 diabetes mellitus-syndrome X complex

Blood glucose concentrations are unaffected by exercise despite very high rates of glucose flux. The plasma ionised calcium levels are even more tightly controlled after meals and during lactation. This implies 'integral control'. However, pairs of integral counterregulatory controllers (e.g. insulin and glucagon, or calcitonin and parathyroid hormone) cannot operate on the same controlled variable, unless there is some form of mutual inhibition. Flip-flop functional coupling between pancreatic alpha- and beta-cells via gap junctions may provide such a mechanism. Secretion of a common inhibitory chromogranin by the parathyroids and the thyroidal C-cells provides another. Here we describe how the insulin:glucagon flip-flop controller can be complemented by growth hormone, despite both being integral controllers. Homeostatic conflict is prevented by somatostatin-28 secretion from both the hypothalamus and the pancreatic islets. Our synthesis of the information pertaining to the glucose homeostat that has accumulated in the literature predicts that disruption of the flip-flop mechanism by the accumulation of amyloid in the pancreatic islets in type 2 diabetes mellitus will lead to hyperglucagonaemia, hyperinsulinaemia, insulin resistance, glucose intolerance and impaired insulin responsiveness to elevated blood glucose levels. It explains syndrome X (or metabolic syndrome) as incipient type 2 diabetes in which the glucose control system, while impaired, can still maintain blood glucose at the desired level. It also explains why it is characterised by high plasma insulin levels and low plasma growth hormone levels, despite normoglycaemia, and how this leads to central obesity, dyslipidaemia and cardiovascular disease in both syndrome X and type 2 diabetes.

Islet amyloid polypeptide inhibits glucagon release and exerts a dual action on insulin release from isolated islets

We have studied the influence of a wide concentration range of islet amyloid polypeptide (IAPP) on both glucagon and insulin release stimulated by various types of secretagogues. In an islet incubation medium devoid of glucose, the rate of glucagon release being high, we observed a marked suppressive action by low concentrations of IAPP, 10(-10) and 10(-8) M, on glucagon release. Similarly, glucagon release stimulated by L-arginine, the cholinergic agonist carbachol, or the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX), an activator of the cyclic AMP system, was inhibited by IAPP in the 10(-10) and 10(-8) M concentration range. Moreover, basal glucagon release at 7 and 10 mM glucose was suppressed by IAPP. In contrast, IAPP exerted a dual action on insulin release. Hence, low concentrations of IAPP brought about a modest increase of basal insulin secretion at 7 mM glucose and also of insulin release stimulated by carbachol. High concentrations of IAPP, however, inhibited insulin release stimulated by glucose (10 and 16.7 mM), IBMX, carbachol and L-arginine. In conclusion, our data suggest that IAPP has complex effects on islet hormone secretion serving as an inhibitor of glucagon release and having a dual action on insulin secretion exerting mainly a negative feedback on stimulated and a positive feedback on basal insulin release.

Two generations of insulinotropic imidazoline compounds

The imidazoline RX871024 increased basal- and glucose-stimulated insulin release in vitro and in vivo. The compound inhibited activity of ATP-sensitive K(+) channels as well as voltage-gated K(+) channels, which led to membrane depolarization, an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)), and insulin release. Importantly, RX871024 also enhanced the insulinotropic effect of glucose in cells with clamped [Ca(2+)](i) but in the presence of high ATP and Ca(2+)concentration inside the cell. We believe that the latter effect on insulin exocytosis was at least in part mediated by a rise in diacylglycerol, which then activated protein kinase C (PKC) and increased the generation of arachidonic acid (AA) metabolites. Activation of both the PKC and AA pathways resulted in potentiation of glucose effects on insulin secretion. Unlike RX871024, the novel imidazoline BL11282 did not block ATP-dependent K(+) channels, but similarly to RX871024, it stimulated insulin secretion in depolarized or permeabilized islets. Accordingly, BL11282 did not influence glucose and insulin levels under basal conditions either in vitro or in vivo, but it markedly enhanced the insulinotropic effects of glucose. BL11282 restored the impaired insulin response to glucose in islets from spontaneously diabetic GK rats. We conclude that BL11282 belongs to a new class of insulinotropic compounds that demonstrate a strong glucose-dependent effect on insulin exocytosis.

Glucagon replacement via micro-osmotic pump corrects hypoglycemia and alpha-cell hyperplasia in prohormone convertase 2 knockout mice

Prohormone convertase 2 (PC2) plays an essential role in the processing of proglucagon to mature active glucagon in pancreatic alpha-cells (J Biol Chem 276:27197-27202, 2001). Mice lacking PC2 demonstrate multiple defects, including chronic mild hypoglycemia and dramatic hyperplasia of the pancreatic alpha-cells. To define the contribution of mature glucagon deficiency to the hypoglycemia and alpha-cell hyperplasia, we have attempted to correct the defects by delivery of exogenous glucagon by micro-osmotic pumps. Intraperitoneal delivery of 0.5 microg glucagon/h in PC2(-/-) mice resulted in the normalization of blood glucose concentrations. Islet remodeling through the loss of hyperplastic alpha-cells was evident by day 11 after pump implantation; by 25 days postimplantation, PC2(-/-) islets were indistinguishable from wild-type islets. These rapid changes were brought about by induction of apoptosis in the alpha-cell population. Morphological normalization of islets was also accompanied by marked downregulation of endogenous preproglucagon gene expression, but with little or no change in the level of preproinsulin gene expression. Exogenous glucagon delivery also normalized hepatic expression of the gluconeogenic enzyme PEPCK. These results demonstrate that the lack of mature glucagon in PC2(-/-) mice is responsible for the aberrant blood glucose levels, islet morphology, and gene expression, and they confirm the role of glucagon as a tonic insulin antagonist in regulating glycemia.

Chronic blockade of NO synthase paradoxically increases islet NO production and modulates islet hormone release

Islet production of nitric oxide (NO) and CO in relation to islet hormone secretion was investigated in mice given the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) in their drinking water. In these mice, the total islet NO production was paradoxically increased, reflecting induction of inducible NOS (iNOS) in background of reduced activity and immunoreactivity of constitutive NOS (cNOS). Unexpectedly, normal mice fasted for 24 h also displayed iNOS activity, which was further increased in L-NAME-drinking mice. Glucose-stimulated insulin secretion in vitro and in vivo was increased in fasted but unaffected in fed mice after L-NAME drinking. Glucagon secretion was increased in vitro. Control islets incubated with different NOS inhibitors at 20 mM glucose displayed increased insulin release and decreased cNOS activity. These NOS inhibitors potentiated glucose-stimulated insulin release also from islets of L-NAME-drinking mice. In contrast, glucagon release was suppressed. In islets from L-NAME-drinking mice, cyclic nucleotides were upregulated, and forskolin-stimulated hormone release, CO production, and heme oxygenase (HO)-2 expression increased. In conclusion, chronic NOS blockade evoked iNOS-derived NO production in pancreatic islets and elicited compensatory mechanisms against the inhibitory action of NO on glucose-stimulated insulin release by inducing upregulation of the islet cAMP and HO-CO systems.

Mechanisms of AVP-induced glucagon release in clonal alpha-cells in-R1-G9: involvement of Ca(2+)-dependent and -independent pathways

1. The mechanisms underlying AVP-induced increase in [Ca(2+)](i) and glucagon release in clonal alpha-cells In-R1-G9 were investigated. 2. AVP increased [Ca(2+)](i) and glucagon release in a concentration-dependent manner. After the administration of AVP, glucagon was released within 30 s, quickly reached the maximum within 2 min, and maintained a steady-state concentration for at least 15 min. 3. In Ca(2+)-containing medium, AVP increased [Ca(2+)](i) in a biphasic pattern; a peak followed by a sustained plateau. In Ca(2+)-free medium, the Ca(2+) response to AVP became monophasic with lower amplitude and no plateau. Both the basal and AVP-induced glucagon releases were lower in the absence than in the presence of extracellular Ca(2+). When [Ca(2+)](i) was stringently deprived by BAPTA, a Ca(2+) chelator, AVP still significantly increased glucagon release. 4. Pretreatment with thapsigargin, a microsomal Ca(2+) ATPase inhibitor, abolished both the Ca(2+) peak and sustained plateau. 5.AVP increased intracellular concentration of IP(3). 6. U-73122 (8 microM), a phospholipase C inhibitor, abolished AVP-induced increases in [Ca(2+)](i), but only reduced AVP-induced glucagon release by 39%. 7. Pretreatment with nimodipine, an L-type Ca(2+) channel blocker failed to alter AVP-induced glucagon release or increase in [Ca(2+)](i). 8. The results suggest that AVP causes glucagon release through both Ca(2+)-dependent and -independent pathways. For the Ca(2+)-dependent pathway, the G(q) protein activates phospholipase C, which catalyzes the formation of IP(3). IP(3) induces Ca(2+) release from the endoplasmic reticulum, which, in turn, triggers Ca(2+) influx. Both Ca(2+) release and Ca(2+) influx may contribute to AVP-induced glucagon release.

Secretion of the intestinotropic hormone glucagon-like peptide 2 is differentially regulated by nutrients in humans

BACKGROUND & AIMS

Glucagon-like peptide 2(1-33) (GLP-2(1-33)), an intestinally derived hormone, stimulates growth in rodent small and large bowel. To explore the physiology of GLP-2(1-33) secretion, we measured plasma GLP-2 levels in 6 healthy male volunteers, before and after test meals.

METHODS

Blood samples were collected over 24 hours with the subjects consuming a normal, solid mixed diet (2500 kcal) and for 4 hours after liquid test meals (400 kcal/300 mL) composed of carbohydrate, fat, or protein. All studies commenced at 9 AM. Plasma was extracted and analyzed in radioimmunoassays for N-terminal immunoreactive GLP-2 (N-IR-GLP-2; measures bioactive GLP-2(1-33)) as well as total IR-GLP-2 (T-IR-GLP-2), which includes GLP-2(1-33), GLP-2(3-33) (an inactive degradation product of GLP-2(1-33)), and the pancreatic major proglucagon fragment (an inactive precursor that contains GLP-2). Basal and nutrient-stimulated plasma samples were also analyzed by high-performance liquid chromatography to determine the levels of GLP-2(1-33) and GLP-2(3-33).

RESULTS

N-IR-GLP-2 levels were increased 2.0 +/- 0.2- to 2.8 +/- 0.5-fold 40 minutes after each mixed meal (P < 0.05-0.01) and returned to basal overnight, whereas T-IR-GLP-2 levels were increased 1.3 +/- 0.1-fold 40 minutes after breakfast only (P < 0.05). After ingestion of carbohydrate or fat alone, plasma N-IR-GLP-2 concentrations increased by 5.6 +/- 2.0- and 2.7 +/- 0.6-fold within 1 hour (P < 0.05). High-performance liquid chromatography analysis showed a relative increase in the levels of GLP-2(1-33) compared with GLP-2(3-33) (P < 0.05). Ingestion of the protein meal did not alter N-IR-GLP-2 levels, whereas T-IR-GLP-2 was increased by fat and protein (by 1.7 +/- 0. 2-fold for each, P < 0.01) but not by carbohydrate.

CONCLUSIONS

These results show that secretion of GLP-2(1-33) from the intestine is regulated in a nutrient-dependent manner in normal humans.

High glycemic index foods, overeating, and obesity

OBJECTIVE

The prevalence of obesity has increased dramatically in recent years. However, the role of dietary composition in body weight regulation remains unclear. The purpose of this work was to investigate the acute effects of dietary glycemic index (GI) on energy metabolism and voluntary food intake in obese subjects.

METHODS

Twelve obese teenage boys were evaluated on three separate occasions using a crossover study protocol. During each evaluation, subjects consumed identical test meals at breakfast and lunch that had a low, medium, or high GI. The high- and medium-GI meals were designed to have similar macronutrient composition, fiber content, and palatability, and all meals for each subject had equal energy content. After breakfast, plasma and serum concentrations of metabolic fuels and hormones were measured. Ad libitum food intake was determined in the 5-hour period after lunch.

RESULTS

Voluntary energy intake after the high-GI meal (5.8 megajoule [mJ]) was 53% greater than after the medium-GI meal (3.8 mJ), and 81% greater than after the low-GI meal (3.2 mJ). In addition, compared with the low-GI meal, the high-GI meal resulted in higher serum insulin levels, lower plasma glucagon levels, lower postabsorptive plasma glucose and serum fatty acids levels, and elevation in plasma epinephrine. The area under the glycemic response curve for each test meal accounted for 53% of the variance in food intake within subjects.

CONCLUSIONS

The rapid absorption of glucose after consumption of high-GI meals induces a sequence of hormonal and metabolic changes that promote excessive food intake in obese subjects. Additional studies are needed to examine the relationship between dietary GI and long-term body weight regulation.

Somatostatin induces hyperpolarization in pancreatic islet alpha cells by activating a G protein-gated K+ channel

Somatostatin inhibits glucagon-secretion from pancreatic alpha cells but its underlying mechanism is unknown. In mouse alpha cells, we found that somatostatin induced prominent hyperpolarization by activating a K+ channel, which was unaffected by tolbutamide but prevented by pre-treating the cells with pertussis toxin. The K+ channel was activated by intracellular GTP (with somatostatin), GTPgammaS or Gbetagamma subunits. It was thus identified as a G protein-gated K+ (K(G)) channel. RT-PCR and immunohistochemical analyses suggested the K(G) channel to be composed of Kir3.2c and Kir3.4. This study identified a novel ionic mechanism involved in somatostatin-inhibition of glucagon-secretion from pancreatic alpha cells.

Neurotransmitters and their receptors in the islets of Langerhans of the pancreas: what messages do acetylcholine, glutamate, and GABA transmit?

Although neurotransmitters are present in pancreatic islets of Langerhans and can be shown to alter hormone secretion, their precise physiological roles in islet function and their cellular mechanisms of action are unclear. Recent research has identified specific neurotransmitter receptor isoforms in islets that may be important physiologically, because selective receptor agonists activate islet ion channels, modify intracellular [Ca2+], and affect secretion. This article focuses on the putative roles of acetylcholine, glutamate, and GABA in islet function. It has been hypothesized that acetylcholine potentiates insulin secretion by either promoting Ca release from cellular stores, activating a store depletion-activated channel, or activating a novel Na channel. GABA and glutamate, in contrast, have been proposed to mediate a novel paracrine signaling pathway whereby alpha- and beta-cells communicate within the islet. The evidence supporting these hypotheses will be critically evaluated.

A high affinity glutamate/aspartate transport system in pancreatic islets of Langerhans modulates glucose-stimulated insulin secretion

To examine the role of glutamatergic signaling in the function of pancreatic islets, we have characterized a high affinity glutamate/aspartate uptake system in this tissue. The islet [3H]glutamate uptake activity was Na(+)-dependent, and it was blocked by L-trans-pyrrolidine-2,4-dicarboxylic acid, a blocker of neuronal and glial glutamate transporters. Islet glutamate transport activity exhibited a Vmax of 8.48 +/- 1.47 fmol/min/islet (n = 4), which corresponds to 102.2 +/- 17.7 pmol/min/mg islet protein. The apparent Km of islet glutamate transport activity depended on the glucose concentration used in the assay. In the presence of glucose concentrations that do not stimulate insulin secretion (2.8 mM), the apparent Km was 34.7 +/- 7.8 microM (n = 3). However, in high glucose (16.7 mM) the apparent Km increased to 112.7 +/- 16.5 microM (n = 3) with little or no change in Vmax. Like most known plasma membrane glutamate transporters, islet glutamate transporters also transported D-aspartate. Anti-D-aspartate immunoreactivity showed that the islet glutamate/aspartate transport activity was localized to the non-beta cell islet mantle. In perifusion experiments with isolated islets in the absence of exogenous amino acids, L-trans-pyrrolidine-2,4-dicarboxylic acid in the presence of 8.3 mM glucose potentiated insulin secretion 23.3 +/- 2.3% (n = 3) compared with 8.3 mM glucose alone. This effect was abolished in the presence of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione. Furthermore, 6-cyano-7-nitroquinoxaline-2,3-dione alone inhibited glucose-stimulated insulin secretion in isolated islets by 15.9 +/- 5.9% (n = 3). Taken together these data suggest that a high affinity glutamate transport system exists in pancreatic islets and that this system contributes to a glutamatergic signaling pathway that can modulate glucose-inducible insulin secretion.

Adrenaline stimulates glucagon secretion in pancreatic A-cells by increasing the Ca2+ current and the number of granules close to the L-type Ca2+ channels

We have monitored electrical activity, voltage-gated Ca2+ currents, and exocytosis in single rat glucagon-secreting pancreatic A-cells. The A-cells were electrically excitable and generated spontaneous Na+- and Ca2+-dependent action potentials. Under basal conditions, exocytosis was tightly linked to Ca2+ influx through omega-conotoxin-GVIA-sensitive (N-type) Ca2+ channels. Stimulation of the A-cells with adrenaline (via beta-adrenergic receptors) or forskolin produced a greater than fourfold PKA-dependent potentiation of depolarization-evoked exocytosis. This enhancement of exocytosis was due to a 50% enhancement of Ca2+ influx through L-type Ca2+ channels, an effect that accounted for <30% of the total stimulatory action. The remaining 70% of the stimulation was attributable to an acceleration of granule mobilization resulting in a fivefold increase in the number of readily releasable granules near the L-type Ca2+ channels.

Differential expression of glutamate receptor subtypes in rat pancreatic islets

Immunocytochemistry was carried out on sections of rat pancreas to localize the expression of glutamate receptor subunits and the major pancreatic peptide hormones. Glutamate receptor expression was concentrated in pancreatic islets, and each islet cell type expressed different neuronal glutamate receptors of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate classes. AMPA receptor subunits were expressed in alpha, beta, and pancreatic polypeptide cells, whereas kainate receptors were found predominantly in alpha and delta cells. Patch clamp electrophysiology was used to measure the functional properties of islet cell glutamate receptors. L-glutamate and other glutamate receptor agonists evoked currents in islet cells that were blocked by the selective AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione and potentiated by cyclothiazide in a manner indistinguishable from that of neuronal AMPA receptors. Activation of islet cell AMPA receptors produced steady-state cation currents that depolarized the cells an average of 20.7 +/- 5.4 mV (n = 6). Currents mediated by functional kainate receptors were also observed in a line of transformed pancreatic alpha cells. Thus, L-glutamate probably regulates islet physiology via actions at both AMPA and kainate receptor classes. The pattern of receptor expression suggests that glutamate receptor activation may have multiple, complex consequences for islet physiology.

Suppression of Ca2+ oscillations in glucagon-producing alpha 2-cells by insulin/glucose and amino acids

The cytoplasmic Ca2+ concentration ([Ca2+]i) was continuously monitored in single glucagon-producing alpha 2-cells isolated from the mouse pancreas and later identified by immunostaining. Up to 60% of the alpha 2-cells exhibited spontaneous [Ca2+]i oscillations (frequency 0.1-0.3/min) in a medium containing 3 mM glucose. In originating from a basal level of 60-100 nM, reaching peak values of 300-400 nM and promptly disappearing after blocking voltage-dependent Ca2+ channels with methoxyverapamil, the oscillations resembled those in insulin-releasing beta-cells stimulated by glucose. The oscillatory activity was suppressed when combining elevation of glucose to 20 mM with the addition of 2-2000 ng/ml insulin. Whereas 10 mM of L-arginine or l-glycine transformed the oscillations into sustained elevation of [Ca2+]i, there was no response to 1 mM tolbutamide or 0.1-1 mM gamma-aminobutyric acid. The observations that alpha 2-cells differ from islet cells secreting insulin and somatostatin in responding to adrenaline with mobilisation of intracellular calcium can be used for their rapid identification. It is suggested that the oscillations reflect periodic entry of Ca2+ due to variations of the membrane potential.

Chromium and other insulin sensitizers may enhance glucagon secretion: implications for hypoglycemia and weight control

Increased pancreatic beta-cell secretory activity usually is associated with decreased alpha-cell activity; stimulated beta-cells release gamma-aminobutyric acid, which hyperpolarizes alpha-cells, inhibiting glucagon release. Thus, insulin secretion and glucagon secretion are usually inversely coupled. This suggests that chromium and other insulin-sensitizing modalities, by down-regulating beta-cell activity, may increase glucagon secretion. Such an effect might play a role in the documented therapeutic activity of supplemental chromium and biguanides in reactive hypoglycemia, and might also be of benefit to dieters.

Atrial natriuretic peptide (ANP)-induced inhibition of glucagon secretion: mechanism of action in isolated rat pancreatic islets

ANP increases insulin levels in vivo. Because in vitro an ANP-induced increase in cGMP levels of islets of Langerhans was observed but no simultaneous increase in insulin release, secreted glucagon may be a candidate for this second messenger affected by ANP. The inhibitory effect of glucose on glucagon secretion was pronounced by 1.0 nM ANP at 3.0 mM glucose as well as at 5.6 and 8.3 mM glucose. Because in other tissues cGMP (the specific second messenger of ANP1 inhibits Ca2+ channels, the uptake of 45Ca2+ was investigated. ANP (1.0 nM) inhibited 45Ca2+ uptake, which was nearly completely abolished by a pertussis toxin (PT) pretreatment. The inhibition of 45Ca2+ uptake fits to inhibitory ANP effects on glucagon secretion but does not fit to insulin secretion. The glucagon secretion coupling cascade affected by ANP probably involves an increase in cGMP because 8-Br-cGMP (a membrane-permeable cGMP analogue) also decreased glucagon secretion. ANP(4-23), a truncated form of ANP, which is selective for the ANP clearance receptor, also inhibited glucagon secretion. HS-42-1, a guanylate cyclase receptor antagonist, tended to reverse the effect of ANP on glucagon release. The data indicate that in the presence of ANP, the in vivo homeostasis of glucose, though plasma insulin levels are increased, is not due to an ANP-mediated increase in glucagon secretion; ANP has a complex inhibitory effect on glucagon release. The data further indicate that the ANP-induced inhibition of glucagon secretion probably involves the cGMP system, an inhibition of Ca2+ uptake and the involvement of PT-sensitive G-proteins. Moreover, an involvement of the clearance receptor seems to be likely. ANP is a valuable tool for investigating glucagon secretion from pancreatic islets because paracrine effects of insulin can be excluded.

The severity of obstructive sleep apnoea is associated with insulin resistance

Oral glucose tolerance tests (OGTT) were performed on eighteen patients with suspected obstructive sleep apnoea who also completed a whole-night polygraphic recording with oximetry. Insulin resistance indices (IRI) were calculated as the product of areas under glucose and insulin curves. In the resulting multiple regression analysis the dependent variable was IRI and the independent variables were age, body mass index (BMI) and the number of nocturnal hypoxic episodes with over 4% desaturation per hour (ODI4). ODI4 was between 4.6 and 70 (median 22.3); IRI ranged from 2.20 to 33.55 (median 7.50). In the regression model the coefficient of determination (R2) for IRI was 0.441 (F-ratio = 3.681, P = 0.038). The strongest determinant of IRI was ODI4 and the regression coefficient of BMI was not significantly different from zero even when possible outliers were excluded. It was found that insulin resistance is related to the severity of sleep anoea. This may be due to a hypoxia-induced hormonal stress reaction which decreases tissue insulin sensitivity. Since upper body obesity is associated with both insulin resistance and sleep apnoea, the distribution of fat should be taken into account in future studies.