Tolbutamide and diazoxide influence insulin secretion by changing the concentration but not the action of cytoplasmic Ca2+ in beta-cells

End-stage Renal Disease in Which Diazoxide Was Effective in Treating Hypoglycemia Caused by Late Dumping Syndrome after Gastrectomy

We herein report a case in which diazoxide was effective in treating reactive hypoglycemia caused by late dumping syndrome in a patient with end-stage renal disease (ESRD). A 50-year-old man with ESRD and a history of gastrectomy underwent hemodialysis. Although he was administered voglibose to treat recurrent reactive hypoglycemia caused by late dumping syndrome, he had difficulty continuing treatment because of gastrointestinal side effects. When he began diazoxide treatment, the reactive hypoglycemia improved. The dose was gradually increased with no apparent side effects, and the hypoglycemic attacks disappeared one year after the start of treatment.

The impact of ATP-sensitive potassium channel modulation on mitochondria in a Parkinson's disease model using SH-SY5Y cells depends on their differentiation state

Inward rectifying potassium channels sensitive to ATP levels (KATP) have been the subject of investigation for several decades. Modulators of KATP channels are well-established treatments for metabolic as well as cardiovascular diseases. Experimental studies have also shown the potential of KATP modulation in neurodegenerative disorders. However, to date, data regarding the effects of KATP antagonists/agonists in experiments related to neurodegeneration remain inconsistent. The main source of confusion in evaluating available data seems to be the choice of experimental models. The present study aims to provide a comprehensive understanding of the effects of both opening and blocking KATP channels in two forms of SH-SY5Y cells. Our results offer valuable insights into the significance of metabolic differences between differentiated and non-differentiated SH-SY5Y cells, particularly in the context of glibenclamide and diazoxide effects under normal conditions and during the initiation of pathological events simulating Parkinson's disease in vitro. We emphasize the analysis of mitochondrial functions and changes in mitochondrial network morphology. The heightened protein expression of KATP channels identified in non-differentiated SH-SY5Y cells seems to be a platform for a more significant impact of KATP modulators in this cell type. The efficiency of rotenone treatment in inducing morphological changes in the mitochondrial network depends on the differentiation status of SH-SY5Y cells.

Effects of a bitter substance, denatonium benzoate, on pancreatic hormone secretion

There is increasing evidence linking bitter taste receptor (BTR) signaling to gut hormone secretion and glucose homeostasis. However, its effect on islet hormone secretion has been poorly characterized. This study investigated the effect of the bitter substance, denatonium benzoate (DB), on hormone secretion from mouse pancreatic islets and INS-1 832/13 cells. DB (0.5-1 mM) augmented insulin secretion at both 2.8 mM and 16.7 mM glucose. This effect was no longer present at 5 mM DB likely due to the greater levels of cellular apoptosis. DB-stimulated insulin secretion involved closure of the KATP channel, activation of T2R signaling in beta-cells, and intraislet glucagon-like peptide-1 (GLP-1) release. DB also enhanced glucagon and somatostatin secretion, but the underlying mechanism was less clear. Together, this study demonstrates that the bitter substance, DB, is a strong potentiator of islet hormone secretion independent of glucose. This observation highlights the potential for widespread off-target effects associated with the clinical use of bitter-tasting substances.NEW & NOTEWORTHY We show that the bitter substance, denatonium benzoate (DB), stimulates insulin, glucagon, somatostatin, and GLP-1 secretion from pancreatic islets, independent of glucose, and that DB augments insulin release via the KATP channel, bitter taste receptor signaling, and intraislet GLP-1 secretion. Exposure to a high dose of DB (5 mM) induces cellular apoptosis in pancreatic islets. Therefore, clinical use of bitter substances to improve glucose homeostasis may have unintended negative impacts beyond the gut.

Drug-induced hyperglycemia and diabetes

BACKGROUND

Drug-induced hyperglycemia and diabetes have negative and potentially serious health consequences but can often be unnoticed.

METHODS

We reviewed the literature searching Medline database for articles addressing drug-induced hyperglycemia and diabetes up to January 31, 2023. We also selected drugs that could induce hyperglycemia or diabetes according official data from drug information databases Thériaque and Micromedex. For each selected drug or pharmacotherapeutic class, the mechanisms of action potentially involved were investigated. For drugs considered to be at risk of hyperglycemia or diabetes, disproportionality analyses were performed using data from the international pharmacovigilance database VigiBase. In order to detect new pharmacovigilance signals, additional disproportionality analyses were carried out for drug classes with more than 100 cases reported in VigiBase, but not found in the literature or official documents.

RESULTS

The main drug classes found to cause hyperglycemia are glucocorticoids, HMG-coA reductase inhibitors, thiazide diuretics, beta-blockers, antipsychotics, fluoroquinolones, antiretrovirals, antineoplastic agents and immunosuppressants. The main mechanisms involved are alterations in insulin secretion and sensitivity, direct cytotoxic effects on pancreatic cells and increases in glucose production. Pharmacovigilance signal were found for a majority of drugs or pharmacological classes identified as being at risk of diabetes or hyperglycemia. We identified new pharmacovigilance signals with drugs not known to be at risk according to the literature or official data: phosphodiesterase type 5 inhibitors, endothelin receptor antagonists, sodium oxybate, biphosphonates including alendronic acid, digoxin, sartans, linosipril, diltiazem, verapamil, and darbepoetin alpha. Further studies will be needed to confirm these signals.

CONCLUSIONS

The risks of induced hyperglycemia vary from one drug to another, and the underlying mechanisms are multiple and potentially complex. Clinicians need to be vigilant when using at-risk drugs in order to detect and manage these adverse drug reactions. However, it is to emphasize that the benefits of appropriately prescribed treatments most often outweigh their metabolic risks.

Isoform-specific Roles of Prolyl Hydroxylases in the Regulation of Pancreatic β-Cell Function

Pancreatic β-cells can secrete insulin via 2 pathways characterized as KATP channel -dependent and -independent. The KATP channel-independent pathway is characterized by a rise in several potential metabolic signaling molecules, including the NADPH/NADP+ ratio and α-ketoglutarate (αKG). Prolyl hydroxylases (PHDs), which belong to the αKG-dependent dioxygenase superfamily, are known to regulate the stability of hypoxia-inducible factor α. In the current study, we assess the role of PHDs in vivo using the pharmacological inhibitor dimethyloxalylglycine (DMOG) and generated β-cell-specific knockout (KO) mice for all 3 isoforms of PHD (β-PHD1 KO, β-PHD2 KO, and β-PHD3 KO mice). DMOG inhibited in vivo insulin secretion in response to glucose challenge and inhibited the first phase of insulin secretion but enhanced the second phase of insulin secretion in isolated islets. None of the β-PHD KO mice showed any significant in vivo defects associated with glucose tolerance and insulin resistance except for β-PHD2 KO mice which had significantly increased plasma insulin during a glucose challenge. Islets from both β-PHD1 KO and β-PHD3 KO had elevated β-cell apoptosis and reduced β-cell mass. Isolated islets from β-PHD1 KO and β-PHD3 KO had impaired glucose-stimulated insulin secretion and glucose-stimulated increases in the ATP/ADP and NADPH/NADP+ ratio. All 3 PHD isoforms are expressed in β-cells, with PHD3 showing the most distinct expression pattern. The lack of each PHD protein did not significantly impair in vivo glucose homeostasis. However, β-PHD1 KO and β-PHD3 KO mice had defective β-cell mass and islet insulin secretion, suggesting that these mice may be predisposed to developing diabetes.

A High-Content Screen for the Identification of Plant Extracts with Insulin Secretion-Modulating Activity

Bioactive plant compounds and extracts are of special interest for the development of pharmaceuticals. Here, we describe the screening of more than 1100 aqueous plant extracts and synthetic reference compounds for their ability to stimulate or inhibit insulin secretion. To quantify insulin secretion in living MIN6 β cells, an insulin–Gaussia luciferase (Ins-GLuc) biosensor was used. Positive hits included extracts from Quillaja saponaria, Anagallis arvensis, Sapindus mukorossi, Gleditsia sinensis and Albizia julibrissin, which were identified as insulin secretion stimulators, whereas extracts of Acacia catechu, Myrtus communis, Actaea spicata L., Vaccinium vitis-idaea and Calendula officinalis were found to exhibit insulin secretion inhibitory properties. Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) were used to characterize several bioactive compounds in the selected plant extracts, and these bioactives were retested for their insulin-modulating properties. Overall, we identified several plant extracts and some of their bioactive compounds that may be used to manipulate pancreatic insulin secretion.

Effects of inhibition of phosphodiesterase 3B in pancreatic islets on insulin secretion: a potential link with some stimulatory pathways

OBJECTIVE

Elevated intracellular cAMP concentrations potentiate insulin secretion from pancreatic β cells. Phosphodiesterase 3B (PDE3B) is highly expressed in these cells and plays a role in the regulation of insulin secretion.

MATERIALS AND METHODS

In this study, effects of amrinone, an inhibitor of PDE3B on insulin release from isolated pancreatic islets, were determined.

RESULTS

Exposure of islets to amrinone for 15, 30 and 90 min markedly increased secretion induced by 6.7 mM glucose. Amrinone enhanced also secretion stimulated by 6.7 mM glucose and DB-cAMP, an activator of PKA. It was also demonstrated that amrinone potentiated insulin secretion induced by 6.7 mM glucose in the combination with PMA (activator of PKC) or acetylcholine. However, the insulin-secretory response to glucose and glibenclamide was unchanged by amrinone.

CONCLUSIONS

These results indicate that amrinone is capable of increasing insulin secretion; however, its action is restricted.

Biosynthetic Activity Differs Between Islet Cell Types and in Beta Cells Is Modulated by Glucose and Not by Secretion

A correct biosynthetic activity is thought to be essential for the long-term function and survival of islet cells in culture and possibly also after islet transplantation. Compared to the secretory activity, biosynthetic activity has been poorly studied in pancreatic islet cells. Here we aimed to assess biosynthetic activity at the single cell level to investigate if protein synthesis is dependent on secretagogues and increased as a consequence of hormonal secretion. Biosynthetic activity in rat islet cells was studied at the single cell level using O-propargyl-puromycin (OPP) that incorporates into newly translated proteins and chemically ligates to a fluorescent dye by "click" reaction. Heterogeneous biosynthetic activity was observed between the four islet cell types, with delta cells showing the higher relative protein biosynthesis. Beta cells protein biosynthesis was increased in response to glucose while 3-isobutyl-1-methylxanthine and phorbol-12-myristate-13-acetate, 2 drugs known to stimulate insulin secretion, had no similar effect on protein biosynthesis. However, after several hours of secretion, protein biosynthesis remained high even when cells were challenged to basal conditions. These results suggest that mechanisms regulating secretion and biosynthesis in islet cells are different, with glucose directly triggering beta cells protein biosynthesis, independently of insulin secretion. Furthermore, this OPP labeling approach is a promising method to identify newly synthesized proteins under various physiological and pathological conditions.

The role of α-ketoglutarate and the hypoxia sensing pathway in the regulation of pancreatic β-cell function

Anaplerosis and the associated mitochondrial metabolite transporters generate unique cytosolic metabolic signaling molecules that can regulate insulin release from pancreatic β-cells. It has been shown that mitochondrial metabolites, transported by the citrate carrier (CIC), dicarboxylate carrier (DIC), oxoglutarate carrier (OGC), and mitochondrial pyruvate carrier (MPC) play a vital role in the regulation of glucose-stimulated insulin secretion (GSIS). Metabolomic studies on static and biphasic insulin secretion, suggests that several anaplerotic derived metabolites, including α-ketoglutarate (αKG), are strongly associated with nutrient regulated insulin secretion. Support for a role of αKG in the regulation of insulin secretion comes from studies looking at αKG dependent enzymes, including hypoxia-inducible factor-prolyl hydroxylases (PHDs) in clonal β-cells, and rodent and human islets. This review will focus on the possible link between defective anaplerotic-derived αKG, PHDs, and the development of type 2 diabetes (T2D).

A randomized pilot efficacy and safety trial of diazoxide choline controlled-release in patients with Prader-Willi syndrome

Introduction

Prader-Willi syndrome (PWS) is a complex genetic condition characterized by hyperphagia, hypotonia, low muscle mass, excess body fat, developmental delays, intellectual disability, behavioral problems, and growth hormone deficiency. This study evaluated the safety and efficacy of orally administered Diazoxide Choline Controlled-Release Tablets (DCCR) in subjects with PWS.

Method

This was a single-center, Phase II study and included a 10-week Open-Label Treatment Period during which subjects were dose escalated, followed by a 4-week Double-Blind, Placebo-Controlled Treatment Period.

Results

Five female and eight male overweight or obese, adolescent and adult subjects with genetically-confirmed PWS with an average age of 15.5±2.9 years were enrolled in the study. There was a statistically significant reduction in hyperphagia at the end of the Open-Label Treatment Period (-4.32, n = 11, p = 0.006). The onset of effect on hyperphagia was rapid and greater reductions in hyperphagia were seen in subjects with moderate to severe Baseline hyperphagia (-5.50, n = 6, p = 0.03), in subjects treated with the highest dose (-6.25, n = 4, p = 0.08), and in subjects with moderate to severe Baseline hyperphagia treated with the highest dose (-7.83, n = 3, p = 0.09). DCCR treatment resulted in a reduction in the number of subjects displaying aggressive behaviors (-57.1%, n = 10, p = 0.01), clinically-relevant reductions in fat mass (-1.58 kg, n = 11, p = 0.02) and increases in lean body mass (2.26 kg, n = 11, p = 0.003). There was a corresponding decrease in waist circumference, and trends for improvements in lipids and insulin resistance. The most common adverse events were peripheral edema and transient increases in glucose. Many of the adverse events were common medical complications of PWS and diazoxide.

Conclusion

DCCR treatment appears to address various unmet needs associated with PWS, including hyperphagia and aggressive behaviors in this proof-of-concept study. If the results were replicated in a larger scale study, DCCR may be a preferred therapeutic option for patients with PWS.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

A Ratiometric Sensor for Imaging Insulin Secretion in Single β Cells

Despite the urgent need for assays to visualize insulin secretion there is to date no reliable method available for measuring insulin release from single cells. To address this need, we developed a genetically encoded reporter termed RINS1 based on proinsulin superfolder GFP (sfGFP) and mCherry fusions for monitoring insulin secretion. RINS1 expression in MIN6 β cells resulted in proper processing yielding single-labeled insulin species. Unexpectedly, glucose or drug stimulation of insulin secretion in β cells led to the preferential release of the insulin-sfGFP construct, while the mCherry-fused C-peptide remained trapped in exocytic granules. This physical separation was used to monitor glucose-stimulated insulin secretion ratiometrically by total internal reflection fluorescence microscopy in single MIN6 and primary mouse β cells. Further, RINS1 enabled parallel monitoring of pulsatile insulin release in tolbutamide-treated β cells, demonstrating the potential of RINS1 for investigations of antidiabetic drug candidates at the single-cell level.

Coupling of Insulin Secretion and Display of a Granule-resident Zinc Transporter ZnT8 on the Surface of Pancreatic Beta Cells

The islet-specific zinc transporter ZnT8 mediates zinc enrichment in the insulin secretory granules of the pancreatic beta cell. This granular zinc transporter is also a major self-antigen found in type 1 diabetes patients. It is not clear whether ZnT8 can be displayed on the cell surface and how insulin secretion may regulate the level of ZnT8 exposure to extracellular immune surveillance. Here we report specific antibody binding to the extracellular surface of rat insulinoma INS-1E cells that stably expressed a tagged human zinc transporter ZnT8. Flow cytometry analysis after fluorescent antibody labeling revealed strong correlations among the levels of ZnT8 expression, its display on the cell surface, and glucose-stimulated insulin secretion (GSIS). Glucose stimulation increased the surface display of endogenous ZnT8 from a basal level to 32.5% of the housekeeping Na⁺/K⁺ ATPase on the cell surface, thereby providing direct evidence for a GSIS-dependent surface exposure of the ZnT8 self-antigen. Moreover, the variation in tagged-ZnT8 expression and surface labeling enabled sorting of heterogeneous beta cells to subpopulations that exhibited marked differences in GSIS with parallel changes in endogenous ZnT8 expression. The abundant surface display of endogenous ZnT8 and its coupling to GSIS demonstrated the potential of ZnT8 as a surface biomarker for tracking and isolating functional beta cells in mixed cell populations.

Drug-Induced Hyperglycaemia and Diabetes

Drug-induced hyperglycaemia and diabetes is a global issue. It may be a serious problem, as it increases the risk of microvascular and macrovascular complications, infections, metabolic coma and even death. Drugs may induce hyperglycaemia through a variety of mechanisms, including alterations in insulin secretion and sensitivity, direct cytotoxic effects on pancreatic cells and increases in glucose production. Antihypertensive drugs are not equally implicated in increasing serum glucose levels. Glycaemic adverse events occur more frequently with thiazide diuretics and with certain beta-blocking agents than with calcium-channel blockers and inhibitors of the renin-angiotensin system. Lipid-modifying agents may also induce hyperglycaemia, and the diabetogenic effect seems to differ between the different types and daily doses of statins. Nicotinic acid may also alter glycaemic control. Among the anti-infectives, severe life-threatening events have been reported with fluoroquinolones, especially when high doses are used. Protease inhibitors and, to a lesser extent, nucleoside reverse transcriptase inhibitors have been reported to induce alterations in glucose metabolism. Pentamidine-induced hyperglycaemia seems to be related to direct dysfunction in pancreatic cells. Phenytoin and valproic acid may also induce hyperglycaemia. The mechanisms of second-generation antipsychotic-associated hyperglycaemia, diabetes mellitus and ketoacidosis are complex and are mainly due to insulin resistance. Antidepressant agents with high daily doses seem to be more frequently associated with an increased risk of diabetes. Ketoacidosis may occur in patients receiving beta-adrenergic stimulants, and theophylline may also induce hyperglycaemia. Steroid diabetes is more frequently associated with high doses of glucocorticoids. Some chemotherapeutic agents carry a higher risk of hyperglycaemia, and calcineurin inhibitor-induced hyperglycaemia is mainly due to a decrease in insulin secretion. Hyperglycaemia has been associated with oral contraceptives containing high doses of oestrogen. Growth hormone therapy and somatostatin analogues may also induce hyperglycaemia. Clinicians should be aware of medications that may alter glycaemia. Efforts should be made to identify and closely monitor patients receiving drugs that are known to induce hyperglycaemia.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl(-) and HCO3 (-) are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl(-) efflux from β-cells is mediated by the Ca(2+)-activated Cl(-) channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase-polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca(++): at 1 μM, the Cl(-) current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO3 (-) (1.83 ± 0.10) > Br(-) (1.42 ± 0.07) > Cl(-) (1.0). A linear single-channel current-voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl(-) and HCO3 (-) media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl⁻ and HCO₃⁻ are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl⁻ efflux from β-cells is mediated by the Ca²⁺-activated Cl⁻ channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase–polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca⁺⁺: at 1 μM, the Cl⁻ current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO₃⁻ (1.83 ± 0.10) > Br⁻ (1.42 ± 0.07) > Cl⁻ (1.0). A linear single-channel current–voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl⁻ and HCO₃⁻ media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Short term exposure of beta cells to low concentrations of interleukin-1β improves insulin secretion through focal adhesion and actin remodeling and regulation of gene expression

Type 2 diabetes involves defective insulin secretion with islet inflammation governed in part by IL-1β. Prolonged exposure of islets to high concentrations of IL-1β (>24 h, 20 ng/ml) impairs beta cell function and survival. Conversely, exposure to lower concentrations of IL-1β for >24 h improves these same parameters. The impact on insulin secretion of shorter exposure times to IL-1β and the underlying molecular mechanisms are poorly understood and were the focus of this study. Treatment of rat primary beta cells, as well as rat or human whole islets, with 0.1 ng/ml IL-1β for 2 h increased glucose-stimulated (but not basal) insulin secretion, whereas 20 ng/ml was without effect. Similar differential effects of IL-1β depending on concentration were observed after 15 min of KCl stimulation but were prevented by diazoxide. Studies on sorted rat beta cells indicated that the enhancement of stimulated secretion by 0.1 ng/ml IL-1β was mediated by the NF-κB pathway and c-JUN/JNK pathway acting in parallel to elicit focal adhesion remodeling and the phosphorylation of paxillin independently of upstream regulation by focal adhesion kinase. Because the beneficial effect of IL-1β was dependent in part upon transcription, gene expression was analyzed by RNAseq. There were 18 genes regulated uniquely by 0.1 but not 20 ng/ml IL-1β, which are mostly involved in transcription and apoptosis. These results indicate that 2 h of exposure of beta cells to a low but not a high concentration of IL-1β enhances glucose-stimulated insulin secretion through focal adhesion and actin remodeling, as well as modulation of gene expression.

Thrombocytopenia induced by diazoxide in a patient with an insulinoma

A 24-year-old healthy woman presented at our hospital due to hypoglycemia. A fasting test provoked spontaneous hypoglycemia, and contrast-enhanced abdominal computed tomography revealed a pancreatic tumor. We diagnosed her with insulinoma and initiated diazoxide treatment to prevent hypoglycemia. After 13 days of treatment, she experienced nasal bleeding, and her platelet count decreased from 186,000 /μL to 28,000 /μL. The thrombocytopenia was ameliorated five days after diazoxide was discontinued. Although diazoxide has hyperglycemic effects associated with decreased insulin secretion, diazoxide-induced thrombocytopenia is rare. A complete blood count should be obtained periodically in patients receiving treatment with diazoxide.

K(ATP) channels and islet hormone secretion: new insights and controversies

ATP-sensitive potassium channels (K(ATP) channels) link cell metabolism to electrical activity by controlling the cell membrane potential. They participate in many physiological processes but have a particularly important role in systemic glucose homeostasis by regulating hormone secretion from pancreatic islet cells. Glucose-induced closure of K(ATP) channels is crucial for insulin secretion. Emerging data suggest that K(ATP) channels also play a key part in glucagon secretion, although precisely how they do so remains controversial. This Review highlights the role of K(ATP) channels in insulin and glucagon secretion. We discuss how K(ATP) channels might contribute not only to the initiation of insulin release but also to the graded stimulation of insulin secretion that occurs with increasing glucose concentrations. The various hypotheses concerning the role of K(ATP) channels in glucagon release are also reviewed. Furthermore, we illustrate how mutations in K(ATP) channel genes can cause hyposecretion or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, and how defective metabolic regulation of the channel may underlie the hypoinsulinaemia and the hyperglucagonaemia that characterize type 2 diabetes mellitus. Finally, we outline how sulphonylureas, which inhibit K(ATP) channels, stimulate insulin secretion in patients with neonatal diabetes mellitus or type 2 diabetes mellitus, and suggest their potential use to target the glucagon secretory defects found in diabetes mellitus.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

Ficus deltoidea: A Potential Alternative Medicine for Diabetes Mellitus

Ficus deltoidea from the Moraceae family has been scientifically proven to reduce hyperglycemia at different prandial states. In this study, we evaluate the mechanisms that underlie antihyperglycemic action of Ficus deltoidea. The results had shown that hot aqueous extract of Ficus deltoidea stimulated insulin secretion significantly with the highest magnitude of stimulation was 7.31-fold (P < 0.001). The insulin secretory actions of the hot aqueous extract involved K⁺  _ATP channel-dependent and K⁺  _ATP-channel-independent pathway. The extract also has the ability to induce the usage of intracellular Ca²⁺ to trigger insulin release. The ethanolic and methanolic extracts enhanced basal and insulin-mediated glucose uptake into adipocytes cells. The extracts possess either insulin-mimetic or insulin-sensitizing property or combination of both properties during enhancing glucose uptake into such cells. Meanwhile, the hot aqueous and methanolic extracts augmented basal and insulin-stimulated adiponectin secretion from adipocytes cells. From this study, it is suggested that Ficus deltoidea has the potential to be developed as future oral antidiabetic agent.

Triphenyltin impairs insulin secretion by decreasing glucose-induced NADP(H) and ATP production in hamster pancreatic β-cells

Oral administration of triphenyltin chloride (TPT) (6 mg/100g body weight) inhibits insulin secretion by decreasing glucose-induced cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in pancreatic β-cells of the hamster. To test the possibility that the abnormal level of the [Ca(2+)](i) induced by TPT administration could be due to a defect in the metabolic signal of glucose in the β-cells, we tested the effects of TPT administration on the glucose-induced NAD(P)H and ATP production, and on the changes of membrane potential and [Ca(2+)](i) by glucose and high K(+) in the β-cells. The [Ca(2+)](i) was measured in islet cells loaded with fura-2. TPT administration significantly reduced the NAD(P)H and ATP production, the depolarization of plasma membrane, and insulin secretion by 15 mM glucose in islet cells. TPT administration also reduced the insulin secretion by 10mM dihydroxyacetone and glyceraldehyde. However, TPT administration did not affect the increase of [Ca(2+)](i) and the insulin secretion by 30 mMK(+) or 100 μM tolbutamide, and the membrane potential by 30 mMK(+), and the insulin secretion by 10mM α-ketoisocaproic acid and 0.5mM formycin A, an analog of ATP in the presence of 15 mM glucose. These results suggested that the pathogenesis of TPT-induced hyperglycemia in hamster involves the reduction of [Ca(2+)](i) and insulin secretion in response to K(ATP) channel-dependent depolarization, which is related to the decrease of NAD(P)H and ATP production in pancreatic islet cells after glucose metabolism.

Hypoglycemic activity of alkaloidal fraction of Tinospora cordifolia

The stem of Tinospora cordifolia (TC) is widely used in the therapy of diabetes in traditional folk medicine of India. In the present study, isoquinoline alkaloid rich fraction (AFTC) derived from stem of TC and three alkaloids viz., palmatine, jatrorrhizine and magnoflorine were evaluated for insulin-mimicking and insulin-releasing effect in vitro and in vivo. Their effect on hepatic gluconeogenesis was examined in rat hepatocytes. Insulin releasing effect was detected in vitro using rat pancreatic β-cell line, RINm5F. Furthermore, effects of AFTC and isolated alkaloids on serum glucose and insulin level were studied in fasted and glucose challenged normal rats. AFTC significantly decreased gluconeogenesis in rat hepatocytes as insulin did and it increases insulin secretion in RINm5F cells similar to tolbutamide. In acute 30 min test in vitro, AFTC, palmatine, jatrorrhizine and magnoflorine stimulated insulin secretion from the RINm5F cell line. As in vivo results, administration of AFTC (50, 100, and 200 mg/kg), palmatine, jatrorrhizine and magnoflorine (10, 20 and 40 mg/kg each) orally significantly decreased fasting serum glucose, and suppressed the increase of blood glucose levels after 2 g/kg glucose loading in normal rats. In vivo study further justified their insulin secreting potential by raising the serum insulin level in glucose fed rats. These results demonstrate the alkaloid present in TC contributed for antihyperglycemic activity. AFTC may have hypoglycemic effects via mechanisms of insulin releasing and insulin-mimicking activity and thus improves postprandial hyperglycemia.

Morphological localisation of sulfonylurea receptor 1 in endocrine cells of human, mouse and rat pancreas

AIMS/HYPOTHESIS

Sulfonylurea receptor 1 (SUR1) is the regulatory subunit of ATP-sensitive K channels in beta cells. Morphological methods (immunohistochemistry and sulfonylurea binding) were used to establish the cellular and subcellular location of SUR1 in human and rodent islets.

RESULTS

In the human, mouse and rat pancreas, all endocrine cells of the islets were immunolabelled with an anti-SUR1 antibody, whereas tissues containing SUR2 were consistently negative, as were those from Sur1 (also known as Abcc8)(-/-) mice. In beta cells of the three species, the plasma membrane was distinctly stained, but SUR1 was mainly present over the cytoplasm, with an intensity that varied between cells. Electron microscopy showed that SUR1 was immunolocalised in insulin, glucagon and somatostatin granules. In rat beta cells degranulated by in vivo treatment with glibenclamide (known as glyburide in the USA and Canada), the insulin and SUR1 staining intensity was similarly decreased by approximately 45%, whereas SUR1 staining was not changed in non-beta cells. In all islet cells, binding of glibenclamide labelled with fluorescent dipyrromethane boron difluoride (BODIPY-FL) was punctate over the cytoplasm, compatible with the labelling of endocrine granules. A faint labelling persisted in Sur1 (-/-) mice, but it was not different from that obtained with BODIPY-FL alone used as negative control.

CONCLUSIONS/INTERPRETATION

Our study immunolocalised SUR1 in alpha, beta and delta cells of human, mouse and rat islets, and for the first time visualised it in the plasma membrane. We also show that SUR1 is abundant in endocrine granules, where its function remains to be established. No specific sulfonylurea-binding sites other than SUR1 are identified in islet cells by the glibenclamide-BODIPY-FL technique.

Probing of specific binding of synthetic sulfonylurea with the insulinoma cell line MIN6

To overcome the limitation of conventional sulfonylurea (SU) for investigation of biological mechanisms related to KATP channels, a hypoglycemic sulfonylurea (SU) was conjugated with a non-reducing glucose bearing polystyrene (PS) derivative to provide enhanced interaction with an insulinoma cell line (MIN6). The specific interaction between the SU (K+ channel closer)-conjugated copolymer and MIN6 cells was confirmed by confocal laser microscopic images using rhodamine B isothiocyanate (RITC)-labeled SU-conjugated polymer, which revealed the specific interaction between SU-conjugated polymer and MIN6 cells. Moreover, the location of labeled polymer and the site of Ca2+ ion mobilization obtained from the same MIN6 cells were identical. Based on the specificity and insulinotropic activity, the SU-conjugated polymer is expected to be useful tool for the study of biological mechanisms of KATP channels.

Tolbutamide potentiates the volume-regulated anion channel current in rat pancreatic beta cells

AIMS/HYPOTHESIS

Hypoglycaemic sulphonylureas are thought to stimulate insulin release by binding to a sulphonylurea receptor, closing K(ATP) channels and inducing electrical activity. However, the fact that these drugs stimulate insulin release at high glucose concentrations where K(ATP) channels are closed suggests additional ionic actions. The aim of this study was to test the hypothesis that sulphonylureas influence the current of the glucose- and volume-regulated anion channel.

METHODS

Electrical and ion-channel activity were recorded in isolated rat beta cells using the patch-clamp technique. (86)Rb(+) efflux was measured using intact islets. Beta cell volume was measured using a video-imaging technique.

RESULTS

In the absence of glucose, tolbutamide (100 micromol/l) transiently depolarised the cells. In the presence of glucose (5 mmol/l), tolbutamide evoked a sustained period of electrical activity, whilst at 10 mmol/l glucose, the drug evoked a pronounced 'silent' depolarisation. In the absence of glucose, tolbutamide inhibited (86)Rb(+) efflux. However, at 10 mmol/l glucose, tolbutamide induced a transient stimulation of efflux. Tolbutamide potentiated the whole-cell volume-regulated anion conductance in a glucose-dependent manner with an EC(50) of 85 micromol/l. In single channel recordings, tolbutamide increased the channel-open probability. Tolbutamide caused beta cell swelling in the presence of glucose, but not in its absence.

CONCLUSIONS/INTERPRETATION

Tolbutamide can induce beta cell electrical activity by potentiating the glucose- and volume-regulated anion channel current. This effect is probably not due to a direct effect of the drug on the channel, but could be secondary to a metabolic action in the beta cell.

Restitution of defective glucose-stimulated insulin release of sulfonylurea type 1 receptor knockout mice by acetylcholine

Inhibition of ATP-sensitive K+ (K(ATP)) channels by an increase in the ATP/ADP ratio and the resultant membrane depolarization are considered essential in the process leading to insulin release (IR) from pancreatic beta-cells stimulated by glucose. It is therefore surprising that mice lacking the sulfonylurea type 1 receptor (SUR1-/-) in beta-cells remain euglycemic even though the knockout is expected to cause hypoglycemia. To complicate matters, isolated islets of SUR1-/- mice secrete little insulin in response to high glucose, which extrapolates to hyperglycemia in the intact animal. It remains thus unexplained how euglycemia is maintained. In recognition of the essential role of neural and endocrine regulation of IR, we evaluated the effects of acetylcholine (ACh) and glucagon-like peptide-1 (GLP-1) on IR and free intracellular Ca2+ concentration ([Ca2+]i) of freshly isolated or cultured islets of SUR1-/- mice and B6D2F1 controls (SUR1+/+). IBMX, a phosphodiesterase inhibitor, was also used to explore cAMP-dependent signaling in IR. Most striking, and in contrast to controls, SUR1-/-) islets are hypersensitive to ACh and IBMX, as demonstrated by a marked increase of IR even in the absence of glucose. The hypersensitivity to ACh was reproduced in control islets by depolarization with the SUR1 inhibitor glyburide. Pretreatment of perifused SUR1-/- islets with ACh or IBMX restored glucose stimulation of IR, an effect expectedly insensitive to diazoxide. The calcium channel blocker verapamil reduced but did not abolish ACh-stimulated IR, supporting a role for intracellular Ca2+ stores in stimulus-secretion coupling. The effect of ACh on IR was greatly potentiated by GLP-1 (10 nM). ACh caused a dose-dependent increase in [Ca2+]i at 0.1-1 microM or biphasic changes (an initial sharp increase in [Ca2+]i followed by a sustained phase of low [Ca2+]i) at 1-100 microM. The latter effects were observed in substrate-free medium or in the presence of 16.7 mM glucose. We conclude that SUR1 deletion depolarizes the beta-cells and markedly elevates basal [Ca2+]i. Elevated [Ca2+]i in turn sensitizes the beta-cells to the secretory effects of ACh and IBMX. Priming by the combination of high [Ca2+]i, ACh, and GLP-1 restores the defective glucose responsiveness, precluding the development of diabetes but not effectively enough to cause hyperinsulinemic hypoglycemia.

Capacitance measurements of exocytosis in mouse pancreatic alpha-, beta- and delta-cells within intact islets of Langerhans

Capacitance measurements of exocytosis were applied to functionally identified alpha-, beta- and delta-cells in intact mouse pancreatic islets. The maximum rate of capacitance increase in beta-cells during a depolarization to 0 mV was equivalent to 14 granules s(-1), <5% of that observed in isolated beta-cells. Beta-cell secretion exhibited bell-shaped voltage dependence and peaked at +20 mV. At physiological membrane potentials (up to approximately -20 mV) the maximum rate of release was approximately 4 granules s(-1). Both exocytosis (measured by capacitance measurements) and insulin release (detected by radioimmunoassay) were strongly inhibited by the L-type Ca(2+) channel blocker nifedipine (25 microm) but only marginally (<20%) affected by the R-type Ca(2+) channel blocker SNX482 (100 nm). Exocytosis in the glucagon-producing alpha-cells peaked at +20 mV. The capacitance increases elicited by pulses to 0 mV exhibited biphasic kinetics and consisted of an initial transient (150 granules s(-1)) and a sustained late component (30 granules s(-1)). Whereas addition of the N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microm) inhibited glucagon secretion measured in the presence of 1 mm glucose to the same extent as an elevation of glucose to 20 mm, the L-type Ca(2+) channel blocker nifedipine (25 microm) had no effect. Thus, glucagon release during hyperglycaemic conditions depends principally on Ca(2+)-influx through N-type rather than L-type Ca(2+) channels. Exocytosis in the somatostatin-secreting delta-cells likewise exhibited two kinetically separable phases of capacitance increase and consisted of an early rapid (600 granules s(-1)) component followed by a sustained slower (60 granules s(-1)) component. We conclude that (1) capacitance measurements in intact pancreatic islets are feasible; (2) exocytosis measured in beta-cells in situ is significantly slower than that of isolated cells; and (3) the different types of islet cells exhibit distinct exocytotic features.

Sulfonylurea-mediated stimulation of insulin exocytosis via an ATP-sensitive K+ channel-independent action

Several reports indicate that hypoglycemic sulfonylureas augment Ca(2+)-dependent insulin secretion via mechanisms other than inhibition of the ATP-sensitive K(+) channel. The effect involves a 65-kd protein in the granule membrane and culminates in intragranular acidification. Lowering of granule pH is necessary for the insulin granule to gain release competence. Proton pumping into the granule is driven by a v-type H(+)-ATPase, but requires simultaneous Cl(-) uptake into the granule via metabolically regulated ClC-3 Cl(-) channels to maintain electroneutrality. Here we discuss the possibility that modulation of granule ClC-3 channels represents the mechanism whereby sulfonylureas directly potentiate the beta-cell exocytotic machinery.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats

In addition to suppressing appetite, leptin may also modulate insulin secretion and action. Leptin was administered here to insulin-resistant rats to determine its effects on secretagogue-stimulated insulin release, whole body glucose disposal, and insulin-stimulated skeletal muscle glucose uptake and transport. Male Wistar rats were fed either a normal (Con) or a high-fat (HF) diet for 3 or 6 mo. HF rats were then treated with either vehicle (HF), leptin (HF-Lep, 10 mg. kg(-1). day(-1) sc), or food restriction (HF-FR) for 12-15 days. Glucose tolerance and skeletal muscle glucose uptake and transport were significantly impaired in HF compared with Con. Whole body glucose tolerance and rates of insulin-stimulated skeletal muscle glucose uptake and transport in HF-Lep were similar to those of Con and greater than those of HF and HF-FR. The insulin secretory response to either glucose or tolbutamide (a pancreatic beta-cell secretagogue) was not significantly diminished in HF-Lep. Total and plasma membrane skeletal muscle GLUT-4 protein concentrations were similar in Con and HF-Lep and greater than those in HF and HF-FR. The findings suggest that chronic leptin administration reversed a high-fat diet-induced insulin-resistant state, without compromising insulin secretion.

Tolbutamide and diazoxide modulate phospholipase C-linked Ca(2+) signaling and insulin secretion in beta-cells

Arginine vasopressin (AVP), bombesin, and ACh increase cytosolic free Ca(2+) and potentiate glucose-induced insulin release by activating receptors linked to phospholipase C (PLC). We examined whether tolbutamide and diazoxide, which close or open ATP-sensitive K(+) channels (K(ATP) channels), respectively, interact with PLC-linked Ca(2+) signals in HIT-T15 and mouse beta-cells and with PLC-linked insulin secretion from HIT-T15 cells. In the presence of glucose, the PLC-linked Ca(2+) signals were enhanced by tolbutamide (3-300 microM) and inhibited by diazoxide (10-100 microM). The effects of tolbutamide and diazoxide on PLC-linked Ca(2+) signaling were mimicked by BAY K 8644 and nifedipine, an activator and inhibitor of L-type voltage-sensitive Ca(2+) channels, respectively. Neither tolbutamide nor diazoxide affected PLC-linked mobilization of internal Ca(2+) or store-operated Ca(2+) influx through non-L-type Ca(2+) channels. In the absence of glucose, PLC-linked Ca(2+) signals were diminished or abolished; this effect could be partly antagonized by tolbutamide. In the presence of glucose, tolbutamide potentiated and diazoxide inhibited AVP- or bombesin-induced insulin secretion from HIT-T15 cells. Nifedipine (10 microM) blocked both the potentiating and inhibitory actions of tolbutamide and diazoxide on AVP-induced insulin release, respectively. In glucose-free medium, AVP-induced insulin release was reduced but was again potentiated by tolbutamide, whereas diazoxide caused no further inhibition. Thus tolbutamide and diazoxide regulate both PLC-linked Ca(2+) signaling and insulin secretion from pancreatic beta-cells by modulating K(ATP) channels, thereby determining voltage-sensitive Ca(2+) influx.

Diazoxide down-regulates leptin and lipid metabolizing enzymes in adipose tissue of Zucker rats

We have previously reported that attenuation of hyperinsulinemia by diazoxide (DZ), an inhibitor of glucose-mediated insulin secretion, increased insulin sensitivity and reduced body weight in obese Zucker rats. These findings prompted us to investigate the effects of DZ on key insulin-sensitive enzymes regulating adipose tissue metabolism, fatty acid synthase (FAS), and lipoprotein lipase (LPL), as well as on circulating levels of leptin. We also determined the direct effects of diazoxide on FAS in 3T3-L1 adipocytes. Seven-week-old female obese and lean Zucker rats were treated with DZ (150 mg/kg/d) or vehicle (C, control) for a period of 6 wk. Changes in plasma parameters by DZ include significant decreases in triglycerides, free fatty acids, glucose, and insulin, consistent with our previous reports. DZ obese rats exhibited lower plasma leptin levels (P<0.03) compared to their C animals. DZ significantly reduced adipose tissue FAS activity in both lean (P<0.0001) and obese (P<0.01) animals. LPL mRNA content was also decreased significantly in DZ-treated obese animals (P<0.009) as compared to their respective controls without a significant effect on lean animals. The possibility that DZ exerted a direct effect on adipocytes was further tested in cultured 3T3-L1 adipocytes. Although diazoxide (5 microM) alone did not change FAS activity in cultured 3T3-L1 adipocytes, it significantly attenuated insulin's effect on FAS activity (P<0.001). We demonstrate that DZ regulates key insulin-sensitive enzymes involved in regulation of adipose tissue metabolism. These findings suggest that modification of insulin-sensitive pathways can be therapeutically beneficial in obesity management.

The stimulatory action of tolbutamide on Ca2+-dependent exocytosis in pancreatic beta cells is mediated by a 65-kDa mdr-like P-glycoprotein

Intracellular application of the sulfonylurea tolbutamide during whole-cell patch-clamp recordings stimulated exocytosis >5-fold when applied at a cytoplasmic Ca2+ concentration of 0.17 microM. This effect was not detectable in the complete absence of cytoplasmic Ca2+ and when exocytosis was elicited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The stimulatory action could be antagonized by the sulfonamide diazoxide, by the Cl--channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), by intracellular application of the antibody JSB1 [originally raised against a 170-kDa multidrug resistance (mdr) protein], and by tamoxifen (an inhibitor of the mdr- and volume-regulated Cl- channels). Immunocytochemistry and Western blot analyses revealed that JSB1 recognizes a 65-kDa protein in the secretory granules. This protein exhibited no detectable binding of sulfonylureas and is distinct from the 140-kDa sulfonylurea high-affinity sulfonylurea receptors also present in the granules. We conclude that (i) tolbutamide stimulates Ca2+-dependent exocytosis secondary to its binding to a 140-kDa high-affinity sulfonylurea receptor in the secretory granules; and (ii) a granular 65-kDa mdr-like protein mediates the action. The processes thus initiated culminate in the activation of a granular Cl- conductance. We speculate that the activation of granular Cl- fluxes promotes exocytosis (possibly by providing the energy required for membrane fusion) by inducing water uptake and an increased intragranular hydrostatic pressure.

Glucose regulation of insulin secretion independent of the opening or closure of adenosine triphosphate-sensitive K+ channels in beta cells

Two major pathways are implicated in the stimulation of insulin secretion by glucose. The K+-ATP channel-dependent pathway involves closure of these channels, depolarization of the beta-cell membrane, acceleration of Ca2+ influx, and a rise in cytosolic free Ca2+ ([Ca2+]i). The K+-ATP channel-independent pathway potentiates the stimulation of exocytosis by high [Ca2+]i. To determine whether this second pathway is influenced by the configuration of the channel, we compared the effects of glucose on [Ca2+]i and insulin secretion in mouse islets under three conditions. First, in the presence of 20, 25, and 30 mM K+, i.e. without pharmacological action on K+-ATP channels, [Ca2+]i and insulin secretion were already elevated at 3 mM glucose. High glucose (20 mM) caused a transient decrease in [Ca2+]i followed by an ascent to slightly above control levels, and rapidly stimulated insulin secretion. Second, opening of K+-ATP channels with diazoxide did not influence [Ca2+]i and insulin secretion at 3 mM glucose and high K+. However, high glucose now caused a sustained lowering of [Ca2+]i accompanied by a slow increase in secretion that augmented with the K+ concentration. Third, when K+-ATP channels were blocked and beta-cells depolarized by high concentrations of tolbutamide or glibenclamide, [Ca2+]i and insulin secretion were elevated even in low glucose. High glucose transiently lowered [Ca2+]i, which then increased to or slightly above control levels, while insulin secretion was rapidly stimulated. Under all conditions the correlation between [Ca2+]i and insulin secretion was excellent at low and high glucose levels, and high glucose increased release at all [Ca2+]i. The potentiation of Ca2+-induced exocytosis by glucose is thus independent of the closed or open state of K+-ATP channels. It is only when the channels are opened by diazoxide that the increase in release is a strict amplification of the action of Ca2+. When the channels are closed (sulfonylureas) or still closable (high K+ alone), the effect of glucose on secretion also comprises a slight increase in [Ca2+]i and, in the latter case, is not strictly K+-ATP channel independent.