Delayed rectifier K+ channel overexpression in transgenic islets and beta-cells associated with impaired glucose responsiveness

Beta-Cell Ion Channels and Their Role in Regulating Insulin Secretion

Beta cells of the pancreatic islet express many different types of ion channels. These channels reside in the β-cell plasma membrane as well as subcellular organelles and their coordinated activity and sensitivity to metabolism regulate glucose-dependent insulin secretion. Here, we review the molecular nature, expression patterns, and functional roles of many β-cell channels, with an eye toward explaining the ionic basis of glucose-induced insulin secretion. Our primary focus is on KATP and voltage-gated Ca2+ channels as these primarily regulate insulin secretion; other channels in our view primarily help to sculpt the electrical patterns generated by activated β-cells or indirectly regulate metabolism. Lastly, we discuss why understanding the physiological roles played by ion channels is important for understanding the secretory defects that occur in type 2 diabetes. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Ion Channels as Therapeutic Targets for Type 1 Diabetes Mellitus

Ion channels are integral proteins expressed in almost all living cells and are involved in muscle contraction and nutrient transport. They play a critical role in the normal functioning of the excitable tissues of the nervous system and regulate the action potential and contraction events. Dysfunction of genes encodes ion channel proteins, which disrupt the channel function and lead to a number of diseases, among which is type 1 diabetes mellitus (T1DM). Therefore, understanding the complex mechanism of ion channel receptors is necessary to facilitate the diagnosis and management of treatment. In this review, we summarize the mechanism of important ion channels and their potential role in the regulation of insulin secretion along with the limitations of ion channels as therapeutic targets. Furthermore, we discuss the recent investigations of the mechanism regulating the ion channels in pancreatic beta cells, which suggest that ion channels are active participants in the regulation of insulin secretion.

Propofol inhibits stromatoxin-1-sensitive voltage-dependent K+ channels in pancreatic β-cells and enhances insulin secretion

Background

Proper glycemic control is an important goal of critical care medicine, including perioperative patient care that can influence patients’ prognosis. Insulin secretion from pancreatic β-cells is generally assumed to play a critical role in glycemic control in response to an elevated blood glucose concentration. Many animal and human studies have demonstrated that perioperative drugs, including volatile anesthetics, have an impact on glucose-stimulated insulin secretion (GSIS). However, the effects of the intravenous anesthetic propofol on glucose metabolism and insulin sensitivity are largely unknown at present.

Methods

The effect of propofol on insulin secretion under low glucose or high glucose was examined in mouse MIN6 cells, rat INS-1 cells, and mouse pancreatic β-cells/islets. Cellular oxygen or energy metabolism was measured by Extracellular Flux Analyzer. Expression of glucose transporter 2 (GLUT2), potassium channels, and insulin mRNA was assessed by qRT-PCR. Protein expression of voltage-dependent potassium channels (Kv2) was also assessed by immunoblot. Propofol’s effects on potassium channels including stromatoxin-1-sensitive Kv channels and cellular oxygen and energy metabolisms were also examined.

Results

We showed that propofol, at clinically relevant doses, facilitates insulin secretion under low glucose conditions and GSIS in MIN6, INS-1 cells, and pancreatic β-cells/islets. Propofol did not affect intracellular ATP or ADP concentrations and cellular oxygen or energy metabolism. The mRNA expression of GLUT2 and channels including the voltage-dependent calcium channels Cav1.2, Kir6.2, and SUR1 subunit of K_ATP, and Kv2 were not affected by glucose or propofol. Finally, we demonstrated that propofol specifically blocks Kv currents in β-cells, resulting in insulin secretion in the presence of glucose.

Conclusions

Our data support the hypothesis that glucose induces membrane depolarization at the distal site, leading to K_ATP channel closure, and that the closure of Kv channels by propofol depolarization in β-cells enhances Ca²⁺ entry, leading to insulin secretion. Because its activity is dependent on GSIS, propofol and its derivatives are potential compounds that enhance and initiate β-cell electrical activity.

Cytokine-mediated changes in K+ channel activity promotes an adaptive Ca2+ response that sustains β-cell insulin secretion during inflammation

Cytokines present during low-grade inflammation contribute to β-cell dysfunction and diabetes. Cytokine signaling disrupts β-cell glucose-stimulated Ca2+ influx (GSCI) and endoplasmic reticulum (ER) Ca2+ ([Ca2+]ER) handling, leading to diminished glucose-stimulated insulin secretion (GSIS). However, cytokine-mediated changes in ion channel activity that alter β-cell Ca2+ handling remain unknown. Here we investigated the role of K+ currents in cytokine-mediated β-cell dysfunction. Kslow currents, which control the termination of intracellular Ca2+ ([Ca2+]i) oscillations, were reduced following cytokine exposure. As a consequence, [Ca2+]i and electrical oscillations were accelerated. Cytokine exposure also increased basal islet [Ca2+]i and decreased GSCI. The effect of cytokines on TALK-1 K+ currents were also examined as TALK-1 mediates Kslow by facilitating [Ca2+]ER release. Cytokine exposure decreased KCNK16 transcript abundance and associated TALK-1 protein expression, increasing [Ca2+]ER storage while maintaining 2nd phase GSCI and GSIS. This adaptive Ca2+ response was absent in TALK-1 KO islets, which exhibited decreased 2nd phase GSCI and diminished GSIS. These findings suggest that Kslow and TALK-1 currents play important roles in altered β-cell Ca2+ handling and electrical activity during low-grade inflammation. These results also reveal that a cytokine-mediated reduction in TALK-1 serves an acute protective role in β-cells by facilitating increased Ca2+ content to maintain GSIS.

Osteopontin activates the diabetes-associated potassium channel TALK-1 in pancreatic β-cells

Glucose-stimulated insulin secretion (GSIS) relies on β-cell Ca2+ influx, which is modulated by the two-pore-domain K+ (K2P) channel, TALK-1. A gain-of-function polymorphism in KCNK16, the gene encoding TALK-1, increases risk for developing type-2 diabetes. While TALK-1 serves an important role in modulating GSIS, the regulatory mechanism(s) that control β-cell TALK-1 channels are unknown. Therefore, we employed a membrane-specific yeast two-hybrid (MYTH) assay to identify TALK-1-interacting proteins in human islets, which will assist in determining signaling modalities that modulate TALK-1 function. Twenty-one proteins from a human islet cDNA library interacted with TALK-1. Some of these interactions increased TALK-1 activity, including intracellular osteopontin (iOPN). Intracellular OPN is highly expressed in β-cells and is upregulated under pre-diabetic conditions to help maintain normal β-cell function; however, the functional role of iOPN in β-cells is poorly understood. We found that iOPN colocalized with TALK-1 in pancreatic sections and coimmunoprecipitated with human islet TALK-1 channels. As human β-cells express two K+ channel-forming variants of TALK-1, regulation of these TALK-1 variants by iOPN was assessed. At physiological voltages iOPN activated TALK-1 transcript variant 3 channels but not TALK-1 transcript variant 2 channels. Activation of TALK-1 channels by iOPN also hyperpolarized resting membrane potential (Vm) in HEK293 cells and in primary mouse β-cells. Intracellular OPN was also knocked down in β-cells to test its effect on β-cell TALK-1 channel activity. Reducing β-cell iOPN significantly decreased TALK-1 K+ currents and increased glucose-stimulated Ca2+ influx. Importantly, iOPN did not affect the function of other K2P channels or alter Ca2+ influx into TALK-1 deficient β-cells. These results reveal the first protein interactions with the TALK-1 channel and found that an interaction with iOPN increased β-cell TALK-1 K+ currents. The TALK-1/iOPN complex caused Vm hyperpolarization and reduced β-cell glucose-stimulated Ca2+ influx, which is predicted to inhibit GSIS.

Type 2 Diabetes-Associated K+ Channel TALK-1 Modulates β-Cell Electrical Excitability, Second-Phase Insulin Secretion, and Glucose Homeostasis

Two-pore domain K+ (K2P) channels play an important role in tuning β-cell glucose-stimulated insulin secretion (GSIS). The K2P channel TWIK-related alkaline pH-activated K2P (TALK)-1 is linked to type 2 diabetes risk through a coding sequence polymorphism (rs1535500); however, its physiological function has remained elusive. Here, we show that TALK-1 channels are expressed in mouse and human β-cells, where they serve as key regulators of electrical excitability and GSIS. We find that the rs1535500 polymorphism, which results in an alanine-to-glutamate substitution in the C-terminus of human TALK-1, increases channel activity. Genetic ablation of TALK-1 results in β-cell membrane potential depolarization, increased islet Ca2+ influx, and enhanced second-phase GSIS. Moreover, mice lacking TALK-1 channels are resistant to high-fat diet-induced elevations in fasting glycemia. These findings reveal TALK-1 channels as important modulators of second-phase insulin secretion and suggest a clinically relevant mechanism for rs1535500, which may increase type 2 diabetes risk by limiting GSIS.

TASK-1 Potassium Channels Limit Pancreatic α-Cell Calcium Influx and Glucagon Secretion

Glucose regulation of pancreatic α-cell Ca(2+) entry through voltage-dependent Ca(2+) channels is essential for normal glucagon secretion and becomes defective during the pathogenesis of diabetes mellitus. The 2-pore domain K(+) channel, TWIK-related acid-sensitive K(+) channel 1 (TASK-1), is an important modulator of membrane voltage and Ca(2+) entry. However, its role in α-cells has not been determined. Therefore, we addressed how TASK-1 channels regulate α-cell electrical activity, Ca(2+) entry, and glucagon secretion. We find that TASK-1 channels expressed in human and rodent α-cells are blocked by the TASK-1 channel inhibitor A1899. Alpha-cell 2-pore domain K(+) currents were also significantly reduced after ablation of mouse α-cell TASK-1 channels. Inhibition of TASK-1 channels with A1899 caused plasma membrane potential depolarization in both human and mouse α-cells, which resulted in increased electrical excitability. Moreover, ablation of α-cell TASK-1 channels increased α-cell electrical excitability under elevated glucose (11 mM) conditions compared with control α-cells. This resulted in significantly elevated α-cell Ca(2+) influx when TASK-1 channels were inhibited in the presence of high glucose (14 mM). However, there was an insignificant change in α-cell Ca(2+) influx after TASK-1 inhibition in low glucose (1 mM). Glucagon secretion from mouse and human islets was also elevated specifically in high (11 mM) glucose after acute TASK-1 inhibition. Interestingly, mice deficient for α-cell TASK-1 showed improvements in both glucose inhibition of glucagon secretion and glucose tolerance, which resulted from the chronic loss of α-cell TASK-1 currents. Therefore, these data suggest an important role for TASK-1 channels in limiting α-cell excitability and glucagon secretion during glucose stimulation.

Pancreatic β-cell-specific ablation of TASK-1 channels augments glucose-stimulated calcium entry and insulin secretion, improving glucose tolerance

Calcium entry through voltage-dependent Ca(2+) channels (VDCCs) is required for pancreatic β-cell insulin secretion. The 2-pore-domain acid-sensitive potassium channel (TASK-1) regulates neuronal excitability and VDCC activation by hyperpolarizing the plasma membrane potential (Δψp); however, a role for pancreatic β-cell TASK-1 channels is unknown. Here we examined the influence of TASK-1 channel activity on the β-cell Δψp and insulin secretion during secretagogue stimulation. TASK-1 channels were found to be highly expressed in human and rodent islets and localized to the plasma membrane of β-cells. TASK-1-like currents of mouse and human β-cells were blocked by the potent TASK-1 channel inhibitor, A1899 (250nM). Although inhibition of TASK-1 currents did not influence the β-cell Δψp in the presence of low (2mM) glucose, A1899 significantly enhanced glucose-stimulated (14mM) Δψp depolarization of human and mouse β-cells. TASK-1 inhibition also resulted in greater secretagogue-stimulated Ca(2+) influx in both human and mouse islets. Moreover, conditional ablation of mouse β-cell TASK-1 channels reduced K2P currents, increased glucose-stimulated Δψp depolarization, and augmented secretagogue-stimulated Ca(2+) influx. The Δψp depolarization caused by TASK-1 inhibition resulted in a transient increase in glucose-stimulated mouse β-cell action potential (AP) firing frequency. However, secretagogue-stimulated β-cell AP duration eventually increased in the presence of A1899 as well as in β-cells without TASK-1, causing a decrease in AP firing frequency. Ablation or inhibition of mouse β-cell TASK-1 channels also significantly enhanced glucose-stimulated insulin secretion, which improved glucose tolerance. Conversely, TASK-1 ablation did not perturb β-cell Δψp, Ca(2+) influx, or insulin secretion under low-glucose conditions (2mM). These results reveal a glucose-dependent role for β-cell TASK-1 channels of limiting glucose-stimulated Δψp depolarization and insulin secretion, which modulates glucose homeostasis.

Direct activation of β-cell KATP channels with a novel xanthine derivative

ATP-regulated potassium (KATP) channel complexes of inward rectifier potassium channel (Kir) 6.2 and sulfonylurea receptor (SUR) 1 critically regulate pancreatic islet β-cell membrane potential, calcium influx, and insulin secretion, and consequently, represent important drug targets for metabolic disorders of glucose homeostasis. The KATP channel opener diazoxide is used clinically to treat intractable hypoglycemia caused by excessive insulin secretion, but its use is limited by off-target effects due to lack of potency and selectivity. Some progress has been made in developing improved Kir6.2/SUR1 agonists from existing chemical scaffolds and compound screening, but there are surprisingly few distinct chemotypes that are specific for SUR1-containing KATP channels. Here we report the serendipitous discovery in a high-throughput screen of a novel activator of Kir6.2/SUR1: VU0071063 [7-(4-(tert-butyl)benzyl)-1,3-dimethyl-1H-purine-2,6(3H,7H)-dione]. The xanthine derivative rapidly and dose-dependently activates Kir6.2/SUR1 with a half-effective concentration (EC50) of approximately 7 μM, is more efficacious than diazoxide at low micromolar concentrations, directly activates the channel in excised membrane patches, and is selective for SUR1- over SUR2A-containing Kir6.1 or Kir6.2 channels, as well as Kir2.1, Kir2.2, Kir2.3, Kir3.1/3.2, and voltage-gated potassium channel 2.1. Finally, we show that VU0071063 activates native Kir6.2/SUR1 channels, thereby inhibiting glucose-stimulated calcium entry in isolated mouse pancreatic β cells. VU0071063 represents a novel tool/compound for investigating β-cell physiology, KATP channel gating, and a new chemical scaffold for developing improved activators with medicinal chemistry.

Inhibition of pancreatic β-cell Ca2+/calmodulin-dependent protein kinase II reduces glucose-stimulated calcium influx and insulin secretion, impairing glucose tolerance

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is caused by Ca(2+) entry via voltage-dependent Ca(2+) channels. CaMKII is a key mediator and feedback regulator of Ca(2+) signaling in many tissues, but its role in β-cells is poorly understood, especially in vivo. Here, we report that mice with conditional inhibition of CaMKII in β-cells show significantly impaired glucose tolerance due to decreased GSIS. Moreover, β-cell CaMKII inhibition dramatically exacerbates glucose intolerance following exposure to a high fat diet. The impairment of islet GSIS by β-cell CaMKII inhibition is not accompanied by changes in either glucose metabolism or the activities of KATP and voltage-gated potassium channels. However, glucose-stimulated Ca(2+) entry via voltage-dependent Ca(2+) channels is reduced in islet β-cells with CaMKII inhibition, as well as in primary wild-type β-cells treated with a peptide inhibitor of CaMKII. The levels of basal β-cell cytoplasmic Ca(2+) and of endoplasmic reticulum Ca(2+) stores are also decreased by CaMKII inhibition. In addition, CaMKII inhibition suppresses glucose-stimulated action potential firing frequency. These results reveal that CaMKII is a Ca(2+) sensor with a key role as a feed-forward stimulator of β-cell Ca(2+) signals that enhance GSIS under physiological and pathological conditions.

Glucose-induced ERM protein activation and translocation regulates insulin secretion

A key step in regulating insulin secretion is insulin granule trafficking to the plasma membrane. Using live-cell time-lapse confocal microscopy, we observed a dynamic association of insulin granules with filamentous actin and PIP2-enriched structures. We found that the scaffolding protein family ERM, comprising ezrin, radixin, and moesin, are expressed in β-cells and target both F-actin and PIP2. Furthermore, ERM proteins are activated via phosphorylation in a glucose- and calcium-dependent manner. This activation leads to a translocation of the ERM proteins to sites on the cell periphery enriched in insulin granules, the exocyst complex docking protein Exo70, and lipid rafts. ERM scaffolding proteins also participate in insulin granule trafficking and docking to the plasma membrane. Overexpression of a truncated dominant-negative ezrin construct that lacks the ERM F-actin binding domain leads to a reduction in insulin granules near the plasma membrane and impaired secretion. Conversely, overexpression of a constitutively active ezrin results in more granules near the cell periphery and an enhancement of insulin secretion. Diabetic mouse islets contain less active ERM, suggestive of a novel mechanism whereby impairment of insulin granule trafficking to the membrane through a complex containing F-actin, PIP2, Exo70, and ERM proteins contributes to defective insulin secretion.

Calcium-activated and voltage-gated potassium channels of the pancreatic islet impart distinct and complementary roles during secretagogue induced electrical responses

Glucose-induced β-cell action potential (AP) repolarization is regulated by potassium efflux through voltage gated (Kv) and calcium activated (K(Ca)) potassium channels. Thus, ablation of the primary Kv channel of the β-cell, Kv2.1, causes increased AP duration. However, Kv2.1(-/-) islet electrical activity still remains sensitive to the potassium channel inhibitor tetraethylammonium. Therefore, we utilized Kv2.1(-/-) islets to characterize Kv and K(Ca) channels and their respective roles in modulating the β-cell AP. The remaining Kv current present in Kv2.1(-/-) β-cells is inhibited with 5 μM CP 339818. Inhibition of the remaining Kv current in Kv2.1(-/-) mouse β-cells increased AP firing frequency by 39.6% but did not significantly enhance glucose stimulated insulin secretion (GSIS). The modest regulation of islet AP frequency by CP 339818 implicates other K(+) channels, possibly K(Ca) channels, in regulating AP repolarization. Blockade of the K(Ca) channel BK with slotoxin increased β-cell AP amplitude by 28.2%, whereas activation of BK channels with isopimaric acid decreased β-cell AP amplitude by 30.6%. Interestingly, the K(Ca) channel SK significantly contributes to Kv2.1(-/-) mouse islet AP repolarization. Inhibition of SK channels decreased AP firing frequency by 66% and increased AP duration by 67% only when Kv2.1 is ablated or inhibited and enhanced GSIS by 2.7-fold. Human islets also express SK3 channels and their β-cell AP frequency is significantly accelerated by 4.8-fold with apamin. These results uncover important repolarizing roles for both Kv and K(Ca) channels and identify distinct roles for SK channel activity in regulating calcium- versus sodium-dependent AP firing.

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

Angiotensin II exerts glucose-dependent effects on Kv currents in mouse pancreatic beta-cells via angiotensin II type 2 receptors

Hyperglycemia-associated glucotoxicity induces beta-cell apoptosis but the underlying mechanisms are unknown. Interestingly, prolonged exposure to high glucose upregulates the expression and function of the renin-angiotensin system (RAS). We hypothesize that the voltage-gated outward potassium (K(v)) current, which governs beta-cell membrane potential and insulin secretion, has a role in glucotoxicity. In this study, we investigated the effects of prolonged exposure to high glucose on mouse pancreatic beta-cells and concurrent effects on the RAS by examining changes in expression of angiotensin II (ANG II) receptors and changes in the expression and activity of K(v) channels. beta-Cells were incubated in high glucose medium for 1-7 days and then were examined with electrophysiological and molecular biology techniques. Prolonged exposure to high glucose produced a marked increase in beta-cell primary K(v) channel subunit, K(v)2.1, expression and K(v) current amplitude. Enhanced expression of ANG II type 1 receptor (AT(1)R) was also observed under high glucose conditions, whereas blockade of AT(1)R by losartan did not alter K(v) channel expression. External application of ANG II reduced K(v) current amplitude under normal, but not high, glucose conditions. The effect of ANG II on K(v) channel gating was abolished by ANG II type 2 receptor (AT(2)R) antagonism. These data suggest that hyperglycemia alters beta-cell function through modification of the K(v) channel which may be associated with the RAS.

A nanoporous, transparent microcontainer for encapsulated islet therapy

Present-day islet encapsulation techniques such as polymer microcapsules and microelectromechanical system (MEMS)-based biocapsules have shown promise in insulin replacement therapy, but they each have limitations-the permeability characteristics of existing polymeric capsules cannot be strictly controlled because of tortuosity and the large size of present-day MEMS biocapsules leads to necrotic regions within the encapsulation volume. We report on a new microcontainer to encapsulate and immunoprotect islets/beta cells that may be used for allo- or xenotransplantation in cell-based therapy. The microcontainers have membranes containing nanoslots to permit the bidirectional transport of nutrients, secretagogues, and cellular products while immunoprotecting the encapsulated cells. The 300-microm microcontainers were fabricated from an epoxy-based polymer, SU-8, with 50-microm-thick walls. Arrays of 25-nm wide slots were created in the SU-8 microcontainer lid. Isolated mouse islets were encapsulated in the microcontainer, and their physiological response to glucose was studied with fluorescence and two-photon imaging over 48 hours. The physiological response of the encapsulated islets was indistinguishable from controls. An agarose-filled microcontainer was imaged with magnetic resonance imaging to demonstrate the feasibility of future noninvasive, in vivo imaging. The SU-8 microcontainers maintained mechanical integrity upon islet loading and mechanical manipulation. Islet encapsulation, as well as the ability to visualize islet function within these transparent microcontainers, was demonstrated.

Action potentials and insulin secretion: new insights into the role of Kv channels

Coordinated electrical activity allows pancreatic beta-cells to respond to secretagogues with calcium entry followed by insulin secretion. Metabolism of glucose affects multiple membrane proteins including ion channels, transporters and pumps that collaborate in a cascade of electrical activity resulting in insulin release. Glucose induces beta-cell depolarization resulting in the firing of action potentials (APs), which are the primary electrical signal of the beta-cell. They are shaped by orchestrated activation of ion channels. Here we give an overview of the voltage-gated potassium (Kv) channels of the beta-cell, which are responsible in part for the falling phase of the AP, and how their regulation affects insulin secretion. beta cells contain several Kv channels allowing dynamic integration of multiple signals on repolarization of glucose-stimulated APs. Recent studies on Kv channel regulation by cAMP and arachidonic acid and on the Kv2.1 null mouse have greatly increased our understanding of beta-cell excitation-secretion coupling.

Kv2.1 ablation alters glucose-induced islet electrical activity, enhancing insulin secretion

Voltage-gated potassium currents (Kv), primarily due to Kv2.1 channels, are activated by glucose-stimulated pancreatic beta cell depolarization, but the exact role (or roles) of this channel in regulating insulin secretion remains uncertain. Here we report that, compared with controls, Kv2.1 null mice have reduced fasting blood glucose levels and elevated serum insulin levels. Glucose tolerance is improved and insulin secretion is enhanced compared to control animals, with similar results in isolated islets in vitro. Isolated Kv2.1(-/-) beta cells have residual Kv currents, which are decreased by 83% at +50 mV compared with control cells. The glucose-induced action potential (AP) duration is increased while the firing frequency is diminished, similar to the effect of specific toxins on control cells but substantially different from the effect of the less specific blocker tetraethylammonium. These results reveal the specific role of Kv2.1 in modulating glucose-stimulated APs of beta cells, exposing additional important currents involved in regulating physiological insulin secretion.

Resveratrol enhances insulin secretion by blocking K(ATP) and K(V) channels of beta cells

The present study investigated the effect of resveratrol on the electrophysiology and insulin secretion of pancreatic beta cells, and examined resveratrol-induced alterations in insulin levels and plasma glucose of normal and streptozotocin-induced diabetic rats. Whole-cell voltage clamp study in the MIN6 cell, a mouse beta cell line, revealed that resveratrol significantly inhibited ATP-sensitive K(+) current at 3 micromol/l, and voltage-gated K(+) currents at 30 micromol/l. Ca(2+)-activated K(+) current was activated by resveratrol at 100 micromol/l. In MIN6 cells stained with membrane potential dye DiBAC(4)(5), resveratrol markedly depolarized membrane potential at the concentrations of 3-100 micromol/l. Insulin secretion was increased in the presence of resveratrol in MIN6, Hit-T15, and RIN-m5F cells. Resveratrol (3 mg/kg, i.p.) increased insulin secretion associated with a lowering in plasma glucose in normal rats, but not in streptozotocin-diabetic rats within the initial 60 min. In conclusion, resveratrol can act as an insulin-secretagogue through I(KATP) and I(KV) inhibition which can contribute to plasma glucose lowering effect in normal rats.

Modulation of the pancreatic islet beta-cell-delayed rectifier potassium channel Kv2.1 by the polyunsaturated fatty acid arachidonate

Glucose stimulates both insulin secretion and hydrolysis of arachidonic acid (AA) esterified in membrane phospholipids of pancreatic islet beta-cells, and these processes are amplified by muscarinic agonists. Here we demonstrate that nonesterified AA regulates the biophysical activity of the pancreatic islet beta-cell-delayed rectifier channel, Kv2.1. Recordings of Kv2.1 currents from INS-1 insulinoma cells incubated with AA (5 mum) and subjected to graded degrees of depolarization exhibit a significantly shorter time-to-peak current interval than do control cells. AA causes a rapid decay and reduced peak conductance of delayed rectifier currents from INS-1 cells and from primary beta-cells isolated from mouse, rat, and human pancreatic islets. Stimulating mouse islets with AA results in a significant increase in the frequency of glucose-induced [Ca(2+)] oscillations, which is an expected effect of Kv2.1 channel blockade. Stimulation with concentrations of glucose and carbachol that accelerate hydrolysis of endogenous AA from islet phosphoplipids also results in accelerated Kv2.1 inactivation and a shorter time-to-peak current interval. Group VIA phospholipase A(2) (iPLA(2)beta) hydrolyzes beta-cell membrane phospholipids to release nonesterified fatty acids, including AA, and inhibiting iPLA(2)beta prevents the muscarinic agonist-induced accelerated Kv2.1 inactivation. Furthermore, glucose and carbachol do not significantly affect Kv2.1 inactivation in beta-cells from iPLA(2)beta(-/-) mice. Stably transfected INS-1 cells that overexpress iPLA(2)beta hydrolyze phospholipids more rapidly than control INS-1 cells and also exhibit an increase in the inactivation rate of the delayed rectifier currents. These results suggest that Kv2.1 currents could be dynamically modulated in the pancreatic islet beta-cell by phospholipase-catalyzed hydrolysis of membrane phospholipids to yield non-esterified fatty acids, such as AA, that facilitate Ca(2+) entry and insulin secretion.

Reduction in voltage-gated K+ currents in primary cultured rat pancreatic beta-cells by linoleic acids

Free fatty acids (FFAs), in addition to glucose, have been shown to stimulate insulin release through the G protein-coupled receptor (GPCR)40 receptor in pancreatic beta-cells. Intracellular free calcium concentration ([Ca(2+)](i)) in beta-cells is elevated by FFAs, although the mechanism underlying the [Ca(2+)](i) increase is still unknown. In this study, we investigated the action of linoleic acid on voltage-gated K(+) currents. Nystatin-perforated recordings were performed on identified rat beta-cells. In the presence of nifedipine, tetrodotoxin, and tolbutamide, voltage-gated K(+) currents were observed. The transient current represents less than 5%, whereas the delayed rectifier current comprises more than 95%, of the total K(+) currents. A long-chain unsaturated FFA, linoleic acid (10 microm), reversibly decreased the amplitude of K(+) currents (to less than 10%). This reduction was abolished by the cAMP/protein kinase A system inhibitors H89 (1 microm) and Rp-cAMP (10 microm) but was not affected by protein kinase C inhibitor. In addition, forskolin and 8'-bromo-cAMP induced a similar reduction in the K(+) current as that evoked by linoleic acid. Insulin secretion and cAMP accumulation in beta-cells were also increased by linoleic acid. Methyl linoleate, which has a similar structure to linoleic acid but no binding affinity to GPR40, did not change K(+) currents. Treatment of cultured cells with GPR40-specific small interfering RNA significantly reduced the decrease in K(+) current induced by linoleic acid, whereas the cAMP-induced reduction of K(+) current was not affected. We conclude that linoleic acid reduces the voltage-gated K(+) current in rat beta-cells through GPR40 and the cAMP-protein kinase A system, leading to an increase in [Ca(2+)](i) and insulin secretion.

Effects of I(Ks) channel inhibitors in insulin-secreting INS-1 cells

Potassium channels regulate insulin secretion. The closure of K(ATP) channels leads to membrane depolarisation, which triggers Ca(2+) influx and stimulates insulin secretion. The subsequent activation of K(+) channels terminates secretion. We examined whether KCNQ1 channels are expressed in pancreatic beta-cells and analysed their functional role. Using RT/PCR cellular mRNA of KCNQ1 but not of KCNE1 channels was detected in INS-1 cells. Effects of two sulfonamide analogues, 293B and HMR1556, inhibitors of KCNQ1 channels, were examined on voltage-activated outwardly rectifying K(+) currents using the patch-clamp method. It was found that 293B inhibited 60% of whole-cell outward currents induced by voltage pulses from -70 to +50 mV with a concentration for half-maximal inhibition (IC(50)) of 37 microM. The other sulfonamide analogue HMR1556 inhibited 48% of the outward current with an IC(50) of 7 microM. The chromanol 293B had no effect on tolbutamide-sensitive K(ATP) channels. Action potentials induced by current injections were broadened and after-repolarisation was attenuated by 293B. Insulin secretion in the presence but not in the absence of tolbutamide was significantly increased by 293B. These results suggest that 293B- and HMR1556-sensitive channels, probably in concert with other voltage-activated K(+) channels, influence action potential duration and frequency and thus insulin secretion.

FRET-based voltage probes for confocal imaging: membrane potential oscillations throughout pancreatic islets

Insulin secretion is dependent on coordinated pancreatic islet physiology. In the present study, we found a way to overcome the limitations of cellular electrophysiology to optically determine cell membrane potential ( Vm) throughout an islet by using a fast voltage optical dye pair. Using laser scanning confocal microscopy (LSCM), we observed fluorescence (Förster) resonance energy transfer (FRET) with the fluorescent donor N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidyl-ethanolamine and the acceptor bis-(1,3-diethylthiobarbiturate) trimethine oxonol in the plasma membrane of essentially every cell within an islet. The FRET signal was approximately linear from Vm−70 to +50 mV with a 2.5-fold change in amplitude. We evaluated the responses of islet cells to glucose and tetraethylammonium. Essentially, every responding cell in a mouse islet displayed similar time-dependent changes in Vm. When Vmwas measured simultaneously with intracellular Ca2+, all active cells showed tight coupling of Vmto islet cell Ca2+changes. Our findings indicate that FRET-based, voltage-sensitive dyes used in conjunction with LSCM imaging could be extremely useful in studies of excitation-secretion coupling in intact islets of Langerhans.

Expression of voltage-gated potassium channels in human and rhesus pancreatic islets

Voltage-gated potassium channels (Kv channels) are involved in repolarization of excitable cells. In pancreatic beta-cells, prolongation of the action potential by block of delayed rectifier potassium channels would be expected to increase intracellular free calcium and to promote insulin release in a glucose-dependent manner. However, the specific Kv channel subtypes responsible for repolarization in beta-cells, most importantly in humans, are not completely resolved. In this study, we have investigated the expression of 26 subtypes from Kv subfamilies in human islet mRNA. The results of the RT-PCR analysis were extended by in situ hybridization and/or immunohistochemical analysis on sections from human or Rhesus pancreas. Cell-specific markers were used to show that Kv2.1, Kv3.2, Kv6.2, and Kv9.3 are expressed in beta-cells, that Kv3.1 and Kv6.1 are expressed in alpha-cells, and that Kv2.2 is expressed in delta-cells. This study suggests that more than one Kv channel subtype might contribute to the beta-cell delayed rectifier current and that this current could be formed by heterotetramers of active and silent subunits.

Synaptosome-associated protein of 25 kilodaltons modulates Kv2.1 voltage-dependent K(+) channels in neuroendocrine islet beta-cells through an interaction with the channel N terminus

Insulin secretion is initiated by ionic events involving membrane depolarization and Ca(2+) entry, whereas exocytic SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins mediate exocytosis itself. In the present study, we characterize the interaction of the SNARE protein SNAP-25 (synaptosome-associated protein of 25 kDa) with the beta-cell voltage-dependent K(+) channel Kv2.1. Expression of Kv2.1, SNAP-25, and syntaxin 1A was detected in human islet lysates by Western blot, and coimmunoprecipitation studies showed that heterologously expressed SNAP-25 and syntaxin 1A associate with Kv2.1. SNAP-25 reduced currents from recombinant Kv2.1 channels by approximately 70% without affecting channel localization. This inhibitory effect could be partially alleviated by codialysis of a Kv2.1N-terminal peptide that can bind in vitro SNAP-25, but not the Kv2.1C-terminal peptide. Similarly, SNAP-25 blocked voltage-dependent outward K(+) currents from rat beta-cells by approximately 40%, an effect that was completely reversed by codialysis of the Kv2.1N fragment. Finally, SNAP-25 had no effect on outward K(+) currents in beta-cells where Kv2.1 channels had been functionally knocked out using a dominant-negative approach, indicating that the interaction is specific to Kv2.1 channels as compared with other beta-cell Kv channels. This study demonstrates that SNAP-25 can regulate Kv2.1 through an interaction at the channel N terminus and supports the hypothesis that SNARE proteins modulate secretion through their involvement in regulation of membrane ion channels in addition to exocytic membrane fusion.

Inhibition of Kv2.1 voltage-dependent K+ channels in pancreatic beta-cells enhances glucose-dependent insulin secretion

Voltage-dependent (Kv) outward K(+) currents repolarize beta-cell action potentials during a glucose stimulus to limit Ca(2+) entry and insulin secretion. Dominant-negative "knockout" of Kv2 family channels enhances glucose-stimulated insulin secretion. Here we show that a putative Kv2.1 antagonist (C-1) stimulates insulin secretion from MIN6 insulinoma cells in a glucose- and dose-dependent manner while blocking voltage-dependent outward K(+) currents. C-1-blocked recombinant Kv2.1-mediated currents more specifically than currents mediated by Kv1, -3, and -4 family channels (Kv1.4, 3.1, 4.2). Additionally, C-1 had little effect on currents recorded from MIN6 cells expressing a dominant-negative Kv2.1 alpha-subunit. The insulinotropic effect of acute Kv2.1 inhibition resulted from enhanced membrane depolarization and augmented intracellular Ca(2+) responses to glucose. Immunohistochemical staining of mouse pancreas sections showed that expression of Kv2.1 correlated highly with insulin-containing beta-cells, consistent with the ability of C-1 to block voltage-dependent outward K(+) currents in isolated mouse beta-cells. Antagonism of Kv2.1 in an ex vivo perfused mouse pancreas model enhanced first- and second-phase insulin secretion, whereas glucagon secretion was unaffected. The present study demonstrates that Kv2.1 is an important component of beta-cell stimulus-secretion coupling, and a compound that enhances, but does not initiate, beta-cell electrical activity by acting on Kv2.1 would be a useful antidiabetic agent.

Diabetes and insulin secretion: whither KATP?

The critical involvement of ATP-sensitive potassium (KATP) channels in insulin secretion is confirmed both by the demonstration that mutations that reduce KATP channel activity underlie many if not most cases of persistent hyperinsulinemia, and by the ability of sulfonylureas, which inhibit KATP channels, to enhance insulin secretion in type II diabetics. By extrapolation, we contend that mutations that increase beta-cell KATP channel activity should inhibit glucose-dependent insulin secretion and underlie, or at least predispose to, a diabetic phenotype. In transgenic animal models, this prediction seems to be borne out. Although earlier genetic studies failed to demonstrate a linkage between KATP mutations and diabetes in humans, recent studies indicate significant association of KATP channel gene mutations or polymorphisms and type II diabetes. We suggest that further efforts to understand the involvement of KATP channels in diabetes are warranted.

Differential effects of unaggregated and aggregated amyloid beta protein (1-40) on K(+) channel currents in primary cultures of rat cerebellar granule and cortical neurones

The effects of amyloid beta protein on voltage-gated K(+) channel currents were studied using the whole-cell patch-clamp technique. The 1-40 amino acid form of amyloid beta protein was applied to primary cultures of rat cerebellar granule and cortical neurones for 24 h. Both the unaggregated and aggregated forms of the peptide, which have differing biological activities, were used. In cerebellar granule neurones, 24-h pre-incubation with 1 microM unaggregated amyloid beta protein resulted in a 60% increase in the 'A'-type component of K(+) current. Increased delayed rectifier activity was Cd(2+)-sensitive and was presumed to be secondary to an increase in voltage-gated Ca(2+) channel current activity. Unaggregated amyloid beta protein had no effect on any component of the K(+) channel current in cortical neurones. One micromolar of aggregated amyloid beta protein had no effect on K(+) channel current in either cell type but reduced cell survival within 24 h as measured using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assays. The unaggregated form of amyloid beta protein had no neurotoxic effects when applied to either neurone type for up to 72 h. These data indicate that the unaggregated, non-pathological form of amyloid beta protein causes changes in the ion channel function of neurones, possibly reflecting a physiological role for the peptide.

Effect of insulinotropic agent nateglinide on Kv and Ca(2+) channels in pancreatic beta-cell

Novel insulinotropic agent nateglinide stimulates insulin via binding to sulfonylurea receptor and closing the ATP-dependent K+ (K(ATP)) channels in pancreatic beta-cells, leading to an increase in [Ca(2+)](i) for exocytosis. The voltage-dependent Ca(2+) channel and the delayed rectifier K+ (Kv) channels are also present in beta-cells and their activities determine the configuration of action potential and hence contribute to the regulation of [Ca(2+)](i) and insulin secretion. This study, by using the patch-clamp method in whole cell configuration, comparatively characterized the direct effects of sulfonylurea receptor ligands including nateglinide, glyburide, and repaglinide on Kv and Ca(2+) channels. Each agent inhibited Kv currents in a concentration-dependent manner with effective concentration range two to three orders higher than that for blocking K(ATP) channels. A marginal stimulation of Ca(2+) current was observed with all drugs, while repaglinide at concentration greater than 300 nM inhibited Ca(2+) current. The direct effects of these antidiabetic agents on Kv and Ca(2+) channels may act concertedly with their primary action on K(ATP) channels in regulating [Ca(2+)](i) and the stimulus-secretion coupling.

Members of the Kv1 and Kv2 voltage-dependent K(+) channel families regulate insulin secretion

In pancreatic beta-cells, voltage-dependent K(+) (Kv) channels are potential mediators of repolarization, closure of Ca(2+) channels, and limitation of insulin secretion. The specific Kv channels expressed in beta-cells and their contribution to the delayed rectifier current and regulation of insulin secretion in these cells are unclear. High-level protein expression and mRNA transcripts for Kv1.4, 1.6, and 2.1 were detected in rat islets and insulinoma cells. Inhibition of these channels with tetraethylammonium decreased I(DR) by approximately 85% and enhanced glucose-stimulated insulin secretion by 2- to 4-fold. Adenovirus-mediated expression of a C-terminal truncated Kv2.1 subunit, specifically eliminating Kv2 family currents, reduced delayed rectifier currents in these cells by 60-70% and enhanced glucose-stimulated insulin secretion from rat islets by 60%. Expression of a C-terminal truncated Kv1.4 subunit, abolishing Kv1 channel family currents, reduced delayed rectifier currents by approximately 25% and enhanced glucose-stimulated insulin secretion from rat islets by 40%. This study establishes that Kv2 and 1 channel homologs mediate the majority of repolarizing delayed rectifier current in rat beta-cells and that antagonism of Kv2.1 may prove to be a novel glucose-dependent therapeutic treatment for type 2 diabetes.

Targeted overactivity of beta cell K(ATP) channels induces profound neonatal diabetes

A paradigm for control of insulin secretion is that glucose metabolism elevates cytoplasmic [ATP]/[ADP] in beta cells, closing K(ATP) channels and causing depolarization, Ca2+ entry, and insulin release. Decreased responsiveness of K(ATP) channels to elevated [ATP]/[ADP] should therefore lead to decreased insulin secretion and diabetes. To test this critical prediction, we generated transgenic mice expressing beta cell K(ATP) channels with reduced ATP sensitivity. Animals develop severe hyperglycemia, hypoinsulinemia, and ketoacidosis within 2 days and typically die within 5. Nevertheless, islet morphology, insulin localization, and alpha and beta cell distributions were normal (before day 3), pointing to reduced insulin secretion as causal. The data indicate that normal K(ATP) channel activity is critical for maintenance of euglycemia and that overactivity can cause diabetes by inhibiting insulin secretion.

Transgenic mice in the analysis of metabolic regulation

In normal animals, the extracellular concentration of glucose is maintained within a very narrow range by the matching of glucose flux into and out of the extracellular space through the tightly coordinated secretion of insulin and glucagon. Functional alterations in beta-cells, liver, or skeletal muscle and adipose tissue may disrupt glucose homeostasis and lead to the development of non-insulin-dependent diabetes mellitus (type 2 diabetes). This review outlines the contribution of these organs and tissues to the control of glucose homeostasis. We discuss new insights obtained through studies of transgenic mice that overexpress or show decreased expression of putative key genes in the regulation of pancreatic beta-cell function, in the control of hepatic glucose uptake and output, and in the regulation of glucose uptake and utilization by skeletal muscle and adipose tissue.

Characterization of a Ca2+ release-activated nonselective cation current regulating membrane potential and [Ca2+]i oscillations in transgenically derived beta-cells

Although stimulation of insulin secretion by glucose is regulated by coupled oscillations of membrane potential and intracellular Ca2+ ([Ca2+]i), the membrane events regulating these oscillations are incompletely understood. In the presence of glucose and tetraethylammonium, transgenically derived beta-cells (betaTC3-neo) exhibit coupled voltage and [Ca2+]i oscillations strikingly similar to those observed in normal islets in response to glucose. Using these cells as a model system, we investigated the membrane conductance underlying these oscillations. Alterations in delayed rectifier or Ca2+-activated K+ channels were excluded as a source of the conductance oscillations, as they are completely blocked by tetraethylammonium. ATP-sensitive K+ channels were also excluded, since the ATP-sensitive K+ channel blocker tolbutamide substituted for glucose in inducing [Ca2+]i oscillations. Thapsigargin, which depletes intracellular Ca2+ stores, and maitotoxin, an activator of nonselective cation channels, both converted the glucose-dependent [Ca2+]i oscillations into a sustained elevation. On the other hand, both SKF 96365, a blocker of Ca2+ store-operated channels, and external Na+ removal suppressed the glucose-stimulated [Ca2+]i oscillations. Maitotoxin activated a nonselective cation current in betaTC3 cells that was attenuated by removal of extracellular Na+ and by SKF 96365, in the same manner to a current activated in mouse beta-cells following depletion of intracellular Ca2+ stores. Currents similar to these are produced by the mammalian trp-related channels, a gene family that includes Ca2+ store-operated channels and inositol 1,4,5-trisphosphate-activated channels. We found several of the trp family genes were expressed in betaTC3 cells by reverse transcriptase polymerase chain reaction using specific primers, but by Northern blot analysis, mtrp-4 was the predominant message expressed. We conclude that a conductance underlying glucose-stimulated oscillations in beta-cells is provided by a Ca2+ store depletion-activated nonselective cation current, which is plausibly encoded by homologs of trp genes.

Genomic organization, chromosomal localization, tissue distribution, and biophysical characterization of a novel mammalian Shaker-related voltage-gated potassium channel, Kv1.7

We report the isolation of a novel mouse voltage-gated Shaker-related K+ channel gene, Kv1.7 (Kcna7/KCNA7). Unlike other known Kv1 family genes that have intronless coding regions, the protein-coding region of Kv1.7 is interrupted by a 1.9-kilobase pair intron. The Kv1.7 gene and the related Kv3.3 (Kcnc3/KCNC3) gene map to mouse chromosome 7 and human chromosome 19q13.3, a region that has been suggested to contain a diabetic susceptibility locus. The mouse Kv1.7 channel is voltage-dependent and rapidly inactivating, exhibits cumulative inactivation, and has a single channel conductance of 21 pS. It is potently blocked by noxiustoxin and stichodactylatoxin, and is insensitive to tetraethylammonium, kaliotoxin, and charybdotoxin. Northern blot analysis reveals approximately 3-kilobase pair Kv1.7 transcripts in mouse heart and skeletal muscle. In situ hybridization demonstrates the presence of Kv1.7 in mouse pancreatic islet cells. Kv1.7 was also isolated from mouse brain and hamster insulinoma cells by polymerase chain reaction.

Expression and function of pancreatic beta-cell delayed rectifier K+ channels. Role in stimulus-secretion coupling

Voltage-dependent delayed rectifier K+ channels regulate aspects of both stimulus-secretion and excitation-contraction coupling, but assigning specific roles to these channels has proved problematic. Using transgenically derived insulinoma cells (betaTC3-neo) and beta-cells purified from rodent pancreatic islets of Langerhans, we studied the expression and role of delayed rectifiers in glucose-stimulated insulin secretion. Using reverse-transcription polymerase chain reaction methods to amplify all known candidate delayed rectifier transcripts, the expression of the K+ channel gene Kv2.1 in betaTC3-neo insulinoma cells and purified rodent pancreatic beta-cells was detected and confirmed by immunoblotting in the insulinoma cells. betaTC3-neo cells were also found to express a related K+ channel, Kv3.2. Whole-cell patch clamp demonstrated the presence of delayed rectifier K+ currents inhibited by tetraethylammonium (TEA) and 4-aminopyridine, with similar Kd values to that of Kv2.1, correlating delayed rectifier gene expression with the K+ currents. The effect of these blockers on intracellular Ca2+ concentration ([Ca2+]i) was studied with fura-2 microspectrofluorimetry and imaging techniques. In the absence of glucose, exposure to TEA (1-20 mM) had minimal effects on betaTC3-neo or rodent islet [Ca2+]i, but in the presence of glucose, TEA activated large amplitude [Ca2+]i oscillations. In the insulinoma cells the TEA-induced [Ca2+]i oscillations were driven by synchronous oscillations in membrane potential, resulting in a 4-fold potentiation of insulin secretion. Activation of specific delayed rectifier K+ channels can therefore suppress stimulus-secretion coupling by damping oscillations in membrane potential and [Ca2+]i and thereby regulate secretion. These studies implicate previously uncharacterized beta-cell delayed rectifier K+ channels in the regulation of membrane repolarization, [Ca2+]i, and insulin secretion.

Cloning of human pancreatic islet large conductance Ca(2+)-activated K+ channel (hSlo) cDNAs: evidence for high levels of expression in pancreatic islets and identification of a flanking genetic marker

Insulin secretion from pancreatic beta cells is dependent on membrane potential changes that result from the concerted regulation of multiple ion channels. Among the distinct K+ channels known to be expressed in beta cells, large conductance Ca(2+)-activated K+ channels have been suggested to play an important role in stimulus-secretion coupling. In the course of a strategy to identify transcripts that are enriched in human pancreatic islet cells, we isolated a partial cDNA encoding a human large conductance Ca(2+)-activated K+ channel mRNA (hSlo). Northern analysis of mRNA showed that among a panel of human tissues hSlo is expressed at its highest levels in pancreatic islets. Screening of human insulinoma and islet cDNA libraries with the partial cDNA resulted in the isolation of 19 hSlo cDNAs. These comprised three splice variants: one shared the common underlying structure of previously reported Slo cDNAs, another variant encoded a novel 60-amino acid insertion in the putative Ca(2+)-sensing domain of hSlo, while the third group of clones had an alternate exon encoding eight amino acids in the predicted COOH-terminal end. Analysis of somatic-cell hybrids containing different portions of chromosome 10 indicated that hSlo maps to chromosome 10q22.2-q23.1. Furthermore, high resolution localization was obtained by analysis of genome-wide radiation hybrids and the CEPH "B" mega-YAC library, both of which identified for the first time a highly polymorphic genetic marker (D10S195) linked to hSlo. These studies provide tools with which to explore the physiological role of Ca(2+)-activated K+ channel proteins in pancreatic islets, and also to investigate the contribution of this locus to the inherited susceptibility to non-insulin-dependent diabetes mellitus.

Activation of stimulus-secretion coupling in pancreatic beta-cells by specific products of glucose metabolism. Evidence for privileged signaling by glycolysis

The energy requirements of most cells supplied with glucose are fulfilled by glycolytic and oxidative metabolism, yielding ATP. In pancreatic beta-cells, a rise in cytosolic ATP is also a critical signaling event, coupling closure of ATP-sensitive K+ channels (KATP) to insulin secretion via depolarization-driven increases in intracellular Ca2+ ([Ca2+]i). We report that glycolytic but not Krebs cycle metabolism of glucose is critically involved in this signaling process. While inhibitors of glycolysis suppressed glucose-stimulated insulin secretion, blockers of pyruvate transport or Krebs cycle enzymes were without effect. While pyruvate was metabolized in islets to the same extent as glucose, it produced no stimulation of insulin secretion and did not block KATP. A membrane-permeant analog, methyl pyruvate, however, produced a block of KATP, a sustained rise in [Ca2+]i, and an increase in insulin secretion 6-fold the magnitude of that induced by glucose. These results indicate that ATP derived from mitochondrial pyruvate metabolism does not substantially contribute to the regulation of KATP responses to a glucose challenge, supporting the notion of subcompartmentation of ATP within the beta-cell. Supranormal stimulation of the Krebs cycle by methyl pyruvate can, however, overwhelm intracellular partitioning of ATP and thereby drive insulin secretion.

Functional expression of an epitope-tagged G protein-coupled K+ channel (GIRK1)

An epitope-tagged form of an inwardly rectifying and G protein-coupled K+ channel (GIRK1-cp) was expressed at high levels in transfected mammalian cells. Immunoblot analysis of transfected human embryonic kidney cells (HEK293) and mouse insulinoma cells (beta TC3) revealed several GIRK1-cp polypeptides, including the major 59-kDa band, corresponding to the predicted mass of the GIRK1 polypeptide plus the epitope tag. Immunohistochemical staining using two anti-tag antibodies showed abundant immunoreactive material, which was predominantly concentrated in the perinuclear area in both transfected cell types. While functional GIRK1-cp message was present in poly(A)+ RNA prepared from HEK293 cells expressing GIRK1-cp protein, appropriate K+ currents could not be detected. In contrast, whole cell recordings made directly from transfected beta TC3 cells expressing GIRK1-cp revealed inwardly rectifying, pertussis toxin-sensitive currents activated by norepinephrine and galanin. Single channel recordings in excised patches of beta TC3 cells expressing GIRK1-cp showed rectifying K+ currents when activated by 50 microM guanosine 5'-O-(thiotriphosphate), with a slope conductance of 39.1 +/- 1.0 picosiemens. This is the first report of stable heterologous expression of a functional G protein-coupled K+ channel in mammalian cells. The activity of an epitope-tagged channel in insulinoma cells demonstrates the utility of this system for further biochemical and biophysical analyses of G protein-K+ channel interactions.