Biomedical subjects

Michael B Wheeler

Publications and source records attributed to Michael B Wheeler.

35 records · Page 2Linked to original sources

PubMed2004

Disruption of pancreatic beta-cell lipid rafts modifies Kv2.1 channel gating and insulin exocytosis.

In pancreatic beta-cells, the predominant voltage-gated Ca(2+) channel (Ca(V)1.2) and K(+) channel (K(V)2.1) are directly coupled to SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor) proteins. These SNARE proteins modulate channel expression and gating and closely associate these channels with the insulin secretory vesicles. We show that K(V)2.1 and Ca(V)1.2, but not K(V)1.4, SUR1, or Kir6.2, target to specialized cholesterol-rich lipid raft domains on beta-cell plasma membranes. Similarly, the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2, but not Munc-13-1 or n-Sec1, are associated with lipid rafts. Disruption of the lipid rafts by depleting membrane cholesterol with methyl-beta-cyclodextrin shunts K(V)2.1, Ca(V)1.2, and SNARE proteins out of lipid rafts. Furthermore, methyl-beta-cyclodextrin inhibits K(V)2.1 but not Ca(V)1.2 channel activity and enhances single-cell exocytic events and insulin secretion. Membrane compartmentalization of ion channels and SNARE proteins in lipid rafts may be critical for the temporal and spatial coordination of insulin release, forming what has been described as the excitosome complex.

Animals↗

PubMed2004

Gene and protein kinase expression profiling of reactive oxygen species-associated lipotoxicity in the pancreatic beta-cell line MIN6.

Oligonucleotide microarrays were used to define oleic acid (OA)-regulated gene expression and proteomic technology to screen protein kinases in MIN6 insulinoma cells. The effects of oxidative stress caused by OA and potential protective effects of N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species (ROS), on global gene expression and beta-cell function were investigated. Long-term exposure of MIN6 cells to OA led to a threefold increase in basal insulin secretion, a 50% decrease in insulin content, an inhibition of glucose-stimulated insulin secretion (GSIS), and a twofold increase in the level of ROS. The addition of NAC normalized both the OA-induced insulin content and ROS elevation, but it failed to restore GSIS. Microarray studies and subsequent quantitative PCR analysis showed that OA consistently regulated the expression of 45 genes involved in metabolism, cell growth, signal transduction, transcription, and protein processing. The addition of NAC largely normalized the expression of the OA-regulated genes involved in cell growth and differentiation but not other functions. A protein kinase screen showed that OA regulated the expression and/or phosphorylation levels of kinases involved in stress-response mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and cell cycle control pathways. Importantly, these findings indicate that chronic OA exposure can impair beta-cell function through ROS-dependent and -independent mechanisms.

Acetylcysteine↗

PubMed2004

Uncoupling protein 2 and islet function.

Stressors such as chronic hyperglycemia or hyperlipidemia may lead to insufficient insulin secretion in susceptible individuals, contributing to type 2 diabetes. The molecules mediating this effect are just beginning to be identified. Uncoupling protein (UCP)-2 may be one such negative modulator of insulin secretion. Accumulating evidence shows that beta-cell UCP2 expression is upregulated by glucolipotoxic conditions and that increased activity of UCP2 decreases insulin secretion. Mitochondrial superoxide has been identified as a posttranslational regulator of UCP2 activity in islets; thus, UCP2 may provide protection to beta-cells at one level while simultaneously having detrimental effects on insulin secretion. Interestingly, the latter appears to be the dominant outcome, because UCP2 knockout mice display an increased beta-cell mass and retained insulin secretion capacity in the face of glucolipotoxicity.

Animals↗

PubMed2003

Antagonism of rat beta-cell voltage-dependent K+ currents by exendin 4 requires dual activation of the cAMP/protein kinase A and phosphatidylinositol 3-kinase signaling pathways.

Antagonism of voltage-dependent K+ (Kv) currents in pancreatic beta-cells may contribute to the ability of glucagon-like peptide-1 (GLP-1) to stimulate insulin secretion. The mechanism and signaling pathway regulating these currents in rat beta-cells were investigated using the GLP-1 receptor agonist exendin 4. Inhibition of Kv currents resulted from a 20-mV leftward shift in the voltage dependence of steady-state inactivation. Blocking cAMP or protein kinase A (PKA) signaling (Rp-cAMP and H-89, respectively) prevented the inhibition of currents by exendin 4. However, direct activation of this pathway alone by intracellular dialysis of cAMP or the PKA catalytic subunit (cPKA) could not inhibit currents, implicating a role for alternative signaling pathways. A number of phosphorylation sites associated with phosphatidylinositol 3 (PI3)-kinase activation were up-regulated in GLP-1-treated MIN6 insulinoma cells, and the PI3 kinase inhibitor wortmannin could prevent antagonism of beta-cell currents by exendin 4. Antagonists of Src family kinases (PP1) and the epidermal growth factor (EGF) receptor (AG1478) also prevented current inhibition by exendin 4, demonstrating a role for Src kinase-mediated trans-activation of the EGF tyrosine kinase receptor. Accordingly, the EGF receptor agonist betacellulin could replicate the effects of exendin 4 in the presence of elevated intracellular cAMP. Downstream, the PKCzeta pseudosubstrate inhibitor could prevent current inhibition by exendin 4. Therefore, antagonism of beta-cell Kv currents by GLP-1 receptor activation requires both cAMP/PKA and PI3 kinase/PKCzeta signaling via trans-activation of the EGF receptor. This represents a novel dual pathway for the control of Kv currents by G protein-coupled receptors.

Androstadienes↗

PubMed2003

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids.

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.

Animals↗

PubMed2003

The phosphatidylinositol 3-kinase inhibitor LY294002 potently blocks K(V) currents via a direct mechanism.

Voltage-dependent K+ (Kv) channels negatively regulate Ca2+ entry into pancreatic beta-cells by repolarizing glucose-stimulated action potentials. A role for phosphatidylinositol 3-kinase (PI3K) modulation of Kv channel function was investigated using the PI3K inhibitors wortmannin and LY294002, and LY303511, a negative control compound with respect to PI3K activity. In MIN6 insulinoma cells, wortmannin (100 nM) had no effect on whole-cell outward K+ currents, but LY294002 and LY303511 reversibly blocked currents in a dose-dependent manner (IC50=9.0+/-0.7 microM and 64.6+/-9.1 microM, respectively). Western blotting confirmed the specific inhibitory effects of LY294002 and wortmannin on insulin-stimulated PI3K activity. Kv currents in rat beta-cells at near physiological temperatures were inhibited 92% by 25 microM LY294002. Kv2.1 and Kv1.4 are highly expressed in beta-cells, and in Kv2.1-transfected tsA201 cells, 50 microM LY294002 and 100 microM LY303511 reversibly inhibited currents by 99% and 41%, respectively. In Kv1.4-transfected tsA201 cells, 50 microM LY294002 reduced the inactivation time constant from 73 to 18 ms. The insulinotropic properties of LY294002 and its effects in other excitable cells may be caused by inhibition of Kv currents rather than PI3K antagonism. Furthermore, LY294002 may represent a novel structure from which future Kv channel blockers may be developed.

Androstadienes↗

PubMed2003

Temperature and redox state dependence of native Kv2.1 currents in rat pancreatic beta-cells.

In pancreatic beta-cells, voltage-dependent K(+) (Kv) channels repolarise glucose-stimulated action potentials. Kv channels are therefore negative regulators of Ca(2+) entry and insulin secretion. We have recently demonstrated that Kv2.1 mediates the majority of beta-cell voltage-dependent outward K(+) current and now investigate the function of native beta-cell Kv2.1 channels at near-physiological temperatures (32-35 degrees C). While beta-cell voltage-dependent outward K(+) currents inactivated little at room temperature, both fast-inactivation (111.5 +/- 14.3 ms) and slow-inactivation (1.21 +/- 0.12 s) was observed at 32-35 degrees C. Kv2.1 mediates the fast-inactivating current observed at 32-35 degrees C, since it could be selectively ablated by expression of a dominant-negative Kv2.1 construct (Kv2.1N). The surprising ability of Kv2.1N to selectively remove the fast-inactivating component, together with its sensitivity to tetraethylammonium (TEA), demonstrate that this component is not mediated by the classically fast-inactivating and TEA-resistant channels such as Kv1.4 and 4.2. Increasing the intracellular redox state by elevating the cytosolic NADPH/NADP(+) ratio from 1/10 to 10/1 increased the rates of both fast- and slow-inactivation. In addition, increasing the intracellular redox state also increased the relative contribution of the fast-inactivation component from 38.8 +/- 2.1 % to 55.9 +/- 1.8 %. The present study suggests that, in beta-cells, Kv2.1 channels mediate a fast-inactivating K(+) current at physiological temperatures and may be regulated by the metabolic generation of NADPH.

Animals↗

PubMed2003

Peroxisome proliferator-activated receptor-alpha agonist treatment in a transgenic model of type 2 diabetes reverses the lipotoxic state and improves glucose homeostasis.

Abnormalities in insulin action are the characteristics of type 2 diabetes. Dominant-negative muscle-specific IGF-I receptor (MKR) mice exhibit elevated lipid levels at an early age and eventually develop type 2 diabetes. To evaluate the role of elevated lipids in the progression of the diabetic state, MKR mice were treated with WY14,643, a peroxisome proliferator-activated receptor (PPAR)-alpha agonist. WY14,643 treatment markedly reduced serum fatty acid and triglyceride levels within a few days, as well as muscle triglyceride levels, and subsequently normalized glucose and insulin levels in MKR mice. Hyperinsulinemic-euglycemic clamp analysis showed that WY14,643 treatment enhanced muscle and adipose tissue glucose uptake by improving whole-body insulin sensitivity. Insulin suppression of endogenous glucose production by the liver of MKR mice was also improved. The expression of genes involved in fatty acid oxidation was increased in liver and skeletal muscle, whereas gene expression levels of hepatic gluconeogenic enzymes were decreased in WY14,643-treated MKR mice. WY14,643 treatment also improved the pattern of glucose-stimulated insulin secretion from the perfused pancreata of MKR mice and reduced the beta-cell mass. Taken together, these findings suggest that the reduction in circulating or intracellular lipids by activation of PPAR-alpha improved insulin sensitivity and the diabetic condition of MKR mice.

Animals↗

PubMed2002

Epac-selective cAMP analog 8-pCPT-2'-O-Me-cAMP as a stimulus for Ca2+-induced Ca2+ release and exocytosis in pancreatic beta-cells.

The second messenger cAMP exerts powerful stimulatory effects on Ca(2+) signaling and insulin secretion in pancreatic beta-cells. Previous studies of beta-cells focused on protein kinase A (PKA) as a downstream effector of cAMP action. However, it is now apparent that cAMP also exerts its effects by binding to cAMP-regulated guanine nucleotide exchange factors (Epac). Although one effector of Epac is the Ras-related G protein Rap1, it is not fully understood what the functional consequences of Epac-mediated signal transduction are at the cellular level. 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3'-5'-cyclic monophosphate (8-pCPT-2'-O-Me-cAMP) is a newly described cAMP analog, and it activates Epac but not PKA. Here we demonstrate that 8-pCPT-2'-O-Me-cAMP acts in human pancreatic beta-cells and INS-1 insulin-secreting cells to mobilize Ca(2+) from intracellular Ca(2+) stores via Epac-mediated Ca(2+)-induced Ca(2+) release (CICR). The cAMP-dependent increase of [Ca(2+)](i) that accompanies CICR is shown to be coupled to exocytosis. We propose that the interaction of cAMP and Epac to trigger CICR explains, at least in part, the blood glucose-lowering properties of an insulinotropic hormone (glucagon-like peptide-1, also known as GLP-1) now under investigation for use in the treatment of type-2 diabetes mellitus.

Bacterial Proteins↗

PubMed2002

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.

Animals↗

PubMed2002

Abnormal expression of pancreatic islet exocytotic soluble N-ethylmaleimide-sensitive factor attachment protein receptors in Goto-Kakizaki rats is partially restored by phlorizin treatment and accentuated by high glucose treatment.

The role of glucotoxicity in dysregulation of islet exocytotic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex proteins and insulin response was explored in the hyperglycemic Goto-Kakizaki (GK) rat. Syntaxin-1A and vesicle-associated membrane protein isoform 2, which drive insulin granule exocytotic fusion, and the associated nSec1, which modulates the SNARE complex assembly, were diminished in GK pancreatic islets to approximately 40% of the levels in control Wistar rat islets. Phlorizin treatment (12 d) induced normoglycemic control in GK rats, resulting in partial restoration of the insulin response to glucose. Furthermore, islet SNARE complex and nSec1 proteins increased by about 40%. Phlorizin treatment did not affect levels of islet SNARE proteins in controls or on the same SNARE complex proteins in GK rat brain. To examine the role of hyperglycemia per se, GK and control rat islets were exposed for 5 d in culture to 5.5 and 16.7 mM glucose. High glucose treatment greatly increased the levels of synaptosomal-associated membrane protein of 25 kDa and, less markedly, the levels of syntaxin-1A and nSec1 in control islets more than in GK rat islets, whereas levels were reduced in both. This was accompanied by sustained impairment of the insulin response to glucose in GK islets and a normal response in control islets. Thus, GK islets demonstrate dysregulation of SNARE protein expression, and their compensatory increase by high glucose exposure is abrogated. Conversely, normoglycemic control results in partial replenishment of these critical components of the insulin exocytotic machinery and improvement in the insulin response. We propose that dysregulation of SNARE proteins is an important mechanism behind glucotoxicity-mediated impairment of the insulin response to glucose.

Animals↗

PubMed2002

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.

Adenoviridae↗

PubMed2002

Glucagon-like peptide-1 inhibits pancreatic ATP-sensitive potassium channels via a protein kinase A- and ADP-dependent mechanism.

Glucagon-like peptide-1 (GLP-1) elicits a glucose-dependent insulin secretory effect via elevation of cAMP and activation of protein kinase A (PKA). GLP-1-mediated closure of ATP-sensitive potassium (K(ATP)) channels is involved in this process, although the mechanism of action of PKA on the K(ATP) channels is not fully understood. K(ATP) channel currents and membrane potentials were measured from insulin-secreting INS-1 cells and recombinant beta-cell K(ATP) channels. 20 nM GLP-1 depolarized INS-1 cells significantly by 6.68 +/- 1.29 mV. GLP-1 reduced recombinant K(ATP) channel currents by 54.1 +/- 6.9% in mammalian cells coexpressing SUR1, Kir6.2, and GLP-1 receptor clones. In the presence of 0.2 mM ATP, the catalytic subunit of PKA (cPKA, 20 nM) had no effect on SUR1/Kir6.2 activity in inside-out patches. However, the stimulatory effects of 0.2 mM ADP on SUR1/Kir6.2 currents were reduced by 26.7 +/- 2.9% (P < 0.05) in the presence of cPKA. cPKA increased SUR1/Kir6.2 currents by 201.2 +/- 20.8% (P < 0.05) with 0.5 mM ADP present. The point mutation S1448A in the ADP-sensing region of SUR1 removed the modulatory effects of cPKA. Our results indicate that PKA-mediated phosphorylation of S1448 in the SUR1 subunit leads to K(ATP) channel closure via an ADP-dependent mechanism. The marked alteration of the PKA-mediated effects at different ADP levels may provide a cellular mechanism for the glucose-sensitivity of GLP-1.

ATP-Binding Cassette Transporters↗

PubMed2002

Uncoupling protein 2 knockout mice have enhanced insulin secretory capacity after a high-fat diet.

Uncoupling protein 2 (UCP2) may act as an important regulator of insulin secretion. In this study, beta-cell function in UCP2-deficient mice was examined after a 45% high-fat diet (HFD) to assess its role during the development of diet-induced type 2 diabetes. HFD-fed UCP2 (-/-) mice have lower fasting blood glucose and elevated insulin levels when compared with wild-type (WT) mice. UCP2 (-/-) mice also have enhanced beta-cell glucose sensitivity compared with WT mice after HFD, a result that is due in part to the deterioration of glucose responsiveness in WT mice. HFD-fed UCP2 (-/-) mice have increased insulin secretory capacity as a result of increased pancreatic beta-cell mass and insulin content per islet. Islets from WT mice exposed to 0.5 mmol/l palmitate for 48 h have significantly reduced mitochondrial membrane potential, ATP concentrations, and glucose responsiveness compared with UCP2 (-/-) islets, suggesting that elevated UCP2 in WT mice increases proton leak and decreases mitochondrial ATP production. Highly increased carnitine palmitoyl transferase-1 gene expression in UCP2 (-/-) mice is suggestive of enhanced fatty acid oxidizing capacity, particularly after HFD stress. These results further establish UCP2 as a component in glucose sensing and suggest a possible new aspect of UCP2 function during the progression of type 2 diabetes.

Animals↗

PubMed2002

Exogenous nitric oxide and endogenous glucose-stimulated beta-cell nitric oxide augment insulin release.

The role nitric oxide (NO) plays in physiological insulin secretion has been controversial. Here we present evidence that exogenous NO stimulates insulin secretion, and that endogenous NO production occurs and is involved in the regulation of insulin release. Radioimmunoassay measurement of insulin release and a dynamic assay of exocytosis using the dye FM1-43 demonstrated that three different NO donors-hydroxylamine (HA), sodium nitroprusside, and 3-morpholinosydnonimine (SIN-1)-each stimulated a marked increase in insulin secretion from INS-1 cells. Pharmacological manipulation of the guanylate cyclase/guanosine 3',5'-cyclic monophosphate pathway indicated that this pathway was involved in mediating the effect of the intracellular NO donor, HA, which was used to simulate endogenous NO production. This effect was further characterized as involving membrane depolarization and intracellular Ca(2+) ([Ca(2+)](i)) elevation. SIN-1 application enhanced glucose-induced [Ca(2+)](i) responses in primary beta-cells and augmented insulin release from islets in a glucose-dependent manner. Real-time monitoring of NO using the NO-sensitive fluorescent dye, diaminofluorescein, was used to provide direct and dynamic imaging of NO generation within living beta-cells. This showed that endogenous NO production could be stimulated by elevation of [Ca(2+)](i) levels and by glucose in both INS-1 and primary rat beta-cells. Scavenging endogenously produced NO-attenuated glucose-stimulated insulin release from INS-1 cells and rat islets. Thus, the results indicated that applied NO is able to exert an insulinotropic effect, and implicated endogenously produced NO in the physiological regulation of insulin release.

Animals↗

PubMed2002

The multiple actions of GLP-1 on the process of glucose-stimulated insulin secretion.

The physiological effects of glucagon-like peptide-1 (GLP-1) are of immense interest because of the potential clinical relevance of this peptide. Produced in intestinal L-cells through posttranslational processing of the proglucagon gene, GLP-1 is released from the gut in response to nutrient ingestion. Peripherally, GLP-1 is known to affect gut motility, inhibit gastric acid secretion, and inhibit glucagon secretion. In the central nervous system, GLP-1 induces satiety, leading to reduced weight gain. In the pancreas, GLP-1 is now known to induce expansion of insulin-secreting beta-cell mass, in addition to its most well-characterized effect: the augmentation of glucose-stimulated insulin secretion. GLP-1 is believed to enhance insulin secretion through mechanisms involving the regulation of ion channels (including ATP-sensitive K(+) channels, voltage-dependent Ca(2+) channels, voltage-dependent K(+) channels, and nonselective cation channels) and by the regulation of intracellular energy homeostasis and exocytosis. The present article will focus principally on the mechanisms proposed to underlie the glucose dependence of GLP-1's insulinotropic effect.

Animals↗

PubMed2002

Glucagon-like peptide-1 receptor activation antagonizes voltage-dependent repolarizing K(+) currents in beta-cells: a possible glucose-dependent insulinotropic mechanism.

Glucagon-like peptide-1 (GLP-1) acts through its G-protein-coupled receptor to enhance glucose-stimulated insulin secretion from pancreatic beta-cells. This is believed to result from modulation of at least two ion channels: ATP-sensitive K(+) (K(ATP)) channels and voltage-dependent Ca(2+) channels. Here, we report that GLP-1 receptor signaling also regulates the activity of beta-cell voltage-dependent K(+) (K(V)) channels, themselves potent glucose-dependent regulators of insulin secretion. GLP-1 receptor activation with exendin 4 (10(-8) mol/l) in rat beta-cells antagonized K(V) currents by 43.3 +/- 6.3%, whereas the GLP-1 receptor antagonist exendin 9-39 had no effect. The effect of GLP-1 receptor activation on K(V) currents could be replicated (current reduction of 55.7 +/- 6.0%) by G-protein activation with GMP-PNP (10 nmol/l). The cAMP pathway antagonist Rp-cAMPS (100 micro mol/l) prevented current inhibition by exendin 4, implicating cAMP signaling in GLP-1 receptor modulation of beta-cell K(V) currents. Finally, exendin 4 (10(-8) mol/l) increased the amplitude (130 +/- 5.7%) and duration (285 +/- 15.9%) of the beta-cell depolarization response to current injection, independent of any effect on K(ATP) or Ca(2+) channels. The present results demonstrate that GLP-1 receptor signaling can antagonize beta-cell repolarization by reducing voltage-dependent K(+) currents, an effect likely to contribute to GLP-1's glucose-dependent insulinotropic effect.

Animals↗