Epinephrine-induced hyperpolarization of islet cells without KATP channels

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Gi/o protein-coupled receptor inhibition of beta-cell electrical excitability and insulin secretion depends on Na+/K+ ATPase activation

Gi/o-coupled somatostatin or α2-adrenergic receptor activation stimulated β-cell NKA activity, resulting in islet Ca2+ fluctuations. Furthermore, intra-islet paracrine activation of β-cell Gi/o-GPCRs and NKAs by δ-cell somatostatin secretion slowed Ca2+ oscillations, which decreased insulin secretion. β-cell membrane potential hyperpolarization resulting from Gi/o-GPCR activation was dependent on NKA phosphorylation by Src tyrosine kinases. Whereas, β-cell NKA function was inhibited by cAMP-dependent PKA activity. These data reveal that NKA-mediated β-cell membrane potential hyperpolarization is the primary and conserved mechanism for Gi/o-GPCR control of electrical excitability, Ca2+ handling, and insulin secretion.

Effects of adrenergic-stimulated lipolysis and cytokine production on in vitro mouse adipose tissue-islet interactions

Inflammatory cytokines and non-esterified fatty acids (NEFAs) are obesity-linked factors that disturb insulin secretion. The aim of this study was to investigate whether pancreatic adipose tissue (pWAT) is able to generate a NEFA/cytokine overload within the pancreatic environment and as consequence to impact on insulin secretion. Pancreatic fat is a minor fat depot, therefore we used high-fat diet (HFD) feeding to induce pancreatic steatosis in mice. Relative Adipoq and Lep mRNA levels were higher in pWAT of HFD compared to chow diet mice. Regardless of HFD, Adipoq and Lep mRNA levels of pWAT were at least 10-times lower than those of epididymal fat (eWAT). Lipolysis stimulating receptors Adrb3 and Npr1 were expressed in pWAT and eWAT, and HFD reduced their expression in eWAT only. In accordance, HFD impaired lipolysis in eWAT but not in pWAT. Despite expression of Npr mRNA, lipolysis was stimulated solely by the adrenergic agonists, isoproterenol and adrenaline. Short term co-incubation of islets with CD/HFD pWAT did not alter insulin secretion. In the presence of CD/HFD eWAT, glucose stimulated insulin secretion only upon isoproterenol-induced lipolysis, i.e. in the presence of elevated NEFA. Isoproterenol augmented Il1b and Il6 mRNA levels both in pWAT and eWAT. These results suggest that an increased sympathetic activity enhances NEFA and cytokine load of the adipose microenvironment, including that of pancreatic fat, and by doing so it may alter beta-cell function.

Celebrities in the heart, strangers in the pancreatic beta cell: Voltage-gated potassium channels Kv 7.1 and Kv 11.1 bridge long QT syndrome with hyperinsulinaemia as well as type 2 diabetes

Voltage‐gated potassium (K_v) channels play an important role in the repolarization of a variety of excitable tissues, including in the cardiomyocyte and the pancreatic beta cell. Recently, individuals carrying loss‐of‐function (LoF) mutations in KCNQ1, encoding K_v7.1, and KCNH2 (hERG), encoding K_v11.1, were found to exhibit post‐prandial hyperinsulinaemia and episodes of hypoglycaemia. These LoF mutations also cause the cardiac disorder long QT syndrome (LQTS), which can be aggravated by hypoglycaemia. Interestingly, patients with LQTS also have a higher burden of diabetes compared to the background population, an apparent paradox in relation to the hyperinsulinaemic phenotype, and KCNQ1 has been identified as a type 2 diabetes risk gene. This review article summarizes the involvement of delayed rectifier K⁺ channels in pancreatic beta cell function, with emphasis on K_v7.1 and K_v11.1, using the cardiomyocyte for context. The functional and clinical consequences of LoF mutations and polymorphisms in these channels on blood glucose homeostasis are explored using evidence from pre‐clinical, clinical and genome‐wide association studies, thereby evaluating the link between LQTS, hyperinsulinaemia and type 2 diabetes.

GRK2 contributes to glucose mediated calcium responses and insulin secretion in pancreatic islet cells

Diabetes is a metabolic syndrome rooted in impaired insulin and/or glucagon secretory responses within the pancreatic islets of Langerhans (islets). Insulin secretion is primarily regulated by two key factors: glucose-mediated ATP production and G-protein coupled receptors (GPCRs) signaling. GPCR kinase 2 (GRK2), a key regulator of GPCRs, is reported to be downregulated in the pancreas of spontaneously obesogenic and diabetogenic mice (ob/ob). Moreover, recent studies have shown that GRK2 non-canonically localizes to the cardiac mitochondrion, where it can contribute to glucose metabolism. Thus, islet GRK2 may impact insulin secretion through either mechanism. Utilizing Min6 cells, a pancreatic ß-cell model, we knocked down GRK2 and measured glucose-mediated intracellular calcium responses and insulin secretion. Silencing of GRK2 attenuated calcium responses, which were rescued by pertussis toxin pre-treatment, suggesting a Gαi/o-dependent mechanism. Pancreatic deletion of GRK2 in mice resulted in glucose intolerance with diminished insulin secretion. These differences were due to diminished insulin release rather than decreased insulin content or gross differences in islet architecture. Furthermore, a high fat diet feeding regimen exacerbated the metabolic phenotype in this model. These results suggest a new role for pancreatic islet GRK2 in glucose-mediated insulin responses that is relevant to type 2 diabetes disease progression.

α2-Adrenergic Disruption of β Cell BDNF-TrkB Receptor Tyrosine Kinase Signaling

Adrenergic signaling is a well-known input into pancreatic islet function. Specifically, the insulin-secreting islet β cell expresses the Gi/o-linked α2-adrenergic receptor, which upon activation suppresses insulin secretion. The use of the adrenergic agonist epinephrine at micromolar doses may have supraphysiological effects. We found that pretreating β cells with micromolar concentrations of epinephrine differentially inhibited activation of receptor tyrosine kinases. We chose TrkB as an example because of its relative sensitivity to the effects of epinephrine and due to its potential regulatory role in the β cell. Our characterization of brain-derived neurotrophic factor (BDNF)-TrkB signaling in MIN6 β cells showed that TrkB is activated by BDNF as expected, leading to canonical TrkB autophosphorylation and subsequent downstream signaling, as well as chronic effects on β cell growth. Micromolar, but not nanomolar, concentrations of epinephrine blocked BDNF-induced TrkB autophosphorylation and downstream mitogen-activated protein kinase pathway activation, suggesting an inhibitory phenomenon at the receptor level. We determined epinephrine-mediated inhibition of TrkB activation to be Gi/o-dependent using pertussis toxin, arguing against an off-target effect of high-dose epinephrine. Published data suggested that inhibition of potassium channels or phosphoinositide-3-kinase signaling may abrogate the negative effects of epinephrine; however, these did not rescue TrkB signaling in our experiments. Taken together, these results show that (1) TrkB kinase signaling occurs in β cells and (2) use of epinephrine in studies of insulin secretion requires careful consideration of concentration-dependent effects. BDNF-TrkB signaling in β cells may underlie pro-survival or growth signaling and warrants further study.

What role do fat cells play in pancreatic tissue?

Background

It is now generally accepted that obesity is a major risk factor for type 2 diabetes mellitus (T2DM). Hepatic steatosis in particular, as well as visceral and ectopic fat accumulation within tissues, is associated with the development of the disease. We recently presented the first study on isolated human pancreatic adipocytes and their interaction with islets [Gerst, F., Wagner, R., Kaiser, G., Panse, M., Heni, M., Machann, J., et al., 2017. Metabolic crosstalk between fatty pancreas and fatty liver: effects on local inflammation and insulin secretion. Diabetologia 60(11):2240–2251.]. The results indicate that the function of adipocytes depends on the overall metabolic status in humans which, in turn, differentially affects islet hormone release.

Scope of Review

This review summarizes former and recent studies on factors derived from adipocytes and their effects on insulin-secreting β-cells, with particular emphasis on the human pancreas. The adipocyte secretome is discussed with a special focus on its influence on insulin secretion, β-cell survival and apoptotic β-cell death.

Major Conclusions

Human pancreatic adipocytes store lipids and release adipokines, metabolites, and pro-inflammatory molecules in response to the overall metabolic, humoral, and neuronal status. The differentially regulated adipocyte secretome impacts on endocrine function, i.e., insulin secretion, β-cell survival and death which interferes with glycemic control. This review attempts to explain why the extent of pancreatic steatosis is associated with reduced insulin secretion in some studies but not in others.

Differences in the actions of adrenaline and noradrenaline with regard to glucose intolerance in patients with pheochromocytoma

Glucose intolerance is often observed in patients with pheochromocytoma. However, it remains controversial issue that glucose intolerance on pheochromocytoma is caused by impaired insulin secretion and/or by increased insulin resistance. We aimed to reveal the mechanism of glucose intolerance on pheochromocytoma with regard to the type and amount of catecholamines released. We evaluated 12 individuals diagnosed with pheochromocytoma and who underwent surgery to remove it. We examined glycemic parameters before and after surgery and investigated the association between the change of parameters of insulin secretion (homeostasis model assessment of β-cell function (HOMA-β)), insulin resistance (homeostasis model assessment of insulin resistance (HOMA-IR)) and that of urinary levels of metanephrine/normetanephrine before and after surgery. Overall, fasting plasma glucose, glycated hemoglobin (HbA1c), HOMA-β, and HOMA-IR were improved significantly after surgery. Regression analysis showed that the improvement in HOMA-β from before to after surgery was significantly positively associated with an improvement in urinary levels of metanephrine from before to after surgery and showed a significantly negative association with improvement in urinary levels of normetanephrine from before to after surgery. The improvement in HOMA-IR from before to after surgery was significantly positively associated with an improvement in urinary levels of normetanephrine from before to after surgery. Our results showed that pheochromocytoma extirpation improved glycemic parameters. Furthermore, the different effects elicited by excess amounts of adrenaline and noradrenaline on glucose intolerance were demonstrated.

Sulfonylurea Blockade of KATP Channels Unmasks a Distinct Type of Glucose-Induced Ca2+ Decrease in Pancreatic β-Cells

OBJECTIVES

This study aimed to explore how sulfonylurea blockade of KATP channels affects the early Ca signals for glucose generation of insulin release.

METHODS

Cytoplasmic Ca was measured with ratiometric microfluorometry in isolated mouse islets loaded with Fura-PE3.

RESULTS

After sulfonylurea blockade of the KATP channels (50 μM-1 mM tolbutamide or 1 μM-1 mM gliclazide), increase of glucose from 3 to 20 mM resulted in suppression of elevated Ca during a 3- to 5-minute period. The Ca decrease was shorter after inhibition of the Na/K pump with ouabain (10 and 100 μM) but prolonged when the α2A adrenoceptors were activated with clonidine (1 and 10 nM) or epinephrine (10 nM). Inhibition of the sarco/endoplasmic reticulum Ca-ATPase pump with 10 μM cyclopiazonic acid counteracted the action of 10 nM clonidine, making the Ca decrease shorter than in controls. Extended superfusion of islets with a medium containing 20 mM glucose and 1 mM tolbutamide sometimes resulted in delayed appearance of Ca oscillations mediated by periodic interruption of elevated Ca.

CONCLUSIONS

Increase of glucose generates prompt suppression of cytoplasmic Ca in β-cells lacking functional KATP channels. Activation of α2A adrenoceptors markedly prolongs the period of glucose-induced Ca decrease, an effect counteracted by cyclopiazonic acid.

Endogenous α2A-Adrenoceptor-Operated Sympathoadrenergic Tones Attenuate Insulin Secretion via cAMP/TRPM2 Signaling

In pancreatic β-cells, pharmacological concentrations of catecholamines, including adrenaline, have been used to inhibit insulin release and explore the multiple mechanisms involved. However, the significance of these signaling pathways for physiological adrenergic functions in β-cells is largely unknown. In the process of glucose-induced insulin secretion, opening of background current through nonselective cation channels (NSCCs) might facilitate membrane depolarization by closure of the ATP-sensitive K⁺ channels. Here, we examined whether physiological insulinostatic adrenaline action is mediated via the transient receptor potential melastatin 2 (TRPM2) channel, a type of NSCC, in β-cells. Results showed that physiological concentrations of adrenaline strongly suppressed glucose-induced and incretin-potentiated cAMP production and insulin secretion and inhibited NSCCs current and membrane excitability via the α2A-adrenoceptor in wild-type mice; however, insulin secretion was not attenuated in TRPM2-knockout (KO) mice. Administration of yohimbine, an α2-adrenoceptor antagonist, failed to affect glucose tolerance in TRPM2-KO mice, in contrast to an improved glucose tolerance in wild-type mice receiving the antagonist. The current study demonstrated that a physiological concentration of adrenaline attenuates insulin release via coupling of α2A-adrenoceptor to cAMP/TRPM2 signaling, thereby providing a potential therapeutic tool to treat patients with type 2 diabetes.

Prostaglandin E1 inhibits endocytosis in the β-cell endocytosis

Prostaglandins inhibit insulin secretion in a manner similar to that of norepinephrine (NE) and somatostatin. As NE inhibits endocytosis as well as exocytosis, we have now examined the modulation of endocytosis by prostaglandin E1 (PGE1). Endocytosis following exocytosis was recorded by whole-cell patch clamp capacitance measurements in INS-832/13 cells. Prolonged depolarizing pulses producing a high level of Ca(2+) influx were used to stimulate maximal exocytosis and to deplete the readily releasable pool (RRP) of granules. This high Ca(2+) influx eliminates the inhibitory effect of PGE1 on exocytosis and allows specific characterization of the inhibitory effect of PGE1 on the subsequent compensatory endocytosis. After stimulating exocytosis, endocytosis was apparent under control conditions but was inhibited by PGE1 in a Pertussis toxin-sensitive (PTX)-insensitive manner. Dialyzing a synthetic peptide mimicking the C-terminus of the α-subunit of the heterotrimeric G-protein Gz into the cells blocked the inhibition of endocytosis by PGE1, whereas a control-randomized peptide was without effect. These results demonstrate that PGE1 inhibits endocytosis and Gz mediates the inhibition.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Inhibition of insulin secretion from rat pancreatic islets by dexmedetomidine and medetomidine, two sedatives frequently used in clinical settings

The aim of this study was to determine whether dexmedetomidine (DEX) and medetomidine (MED), α2-adrenergic agonists clinically used as sedatives, influence insulin secretion from rat pancreatic islets. Islets were isolated from adult male Wistar rats after collagenase digestion. Static incubation was used to determine effects of DEX or MED on insulin secretion and ionic-channel currents of β-cells. Results indicate that both drugs dose-dependently inhibit insulin secretion, DEX more potently than MED. The inhibitory effects were attenuated by addition of yohimbine or by pretreatment of rats with pertussis toxin (PTX). 10 nM DEX decreased the current amplitude of voltage-dependent Ca2+ channels, but this did not occur when the N-type Ca2+ channel blocker ω-conotoxin was added. In the presence of tetraethylammonium, a classical voltage-gated K+ channel (Kv channel) blocker, the magnitude of inhibition of insulin secretion by MED was reduced. However, when tolbutamide, a specific blocker of the ATP-sensitive K+ channel (KATP channel), was present, the magnitude of MED inhibition of insulin secretion was not influenced, suggesting that Kv-channel activity alteration, but not that of KATP channels, is involved in MED-associated insulin secretory inhibition. The Kv-channel currents were increased during 1 nM MED exposure at membrane potentials ranging from -30 mV to -10 mV, where action potentials were generated in response to glucose stimulation. These results indicate that DEX and MED inhibit insulin secretion through an α2-adrenoceptor and PTX-sensitive GTP-binding protein pathway that eventually involves Kv channel activation and Ca2+ channel inhibition.

Evolving insights regarding mechanisms for the inhibition of insulin release by norepinephrine and heterotrimeric G proteins

Norepinephrine has for many years been known to have three major effects on the pancreatic β-cell which lead to the inhibition of insulin release. These are activation of K(+) channels to hyperpolarize the cell and prevent the gating of voltage-dependent Ca(2+) channels that increase intracellular Ca(2+) concentration ([Ca(2+)](i)) and trigger release; inhibition of adenylyl cyclases, thus preventing the augmentation of stimulated insulin release by cyclic AMP; and a "distal" effect that occurs downstream of increased [Ca(2+)](i) to inhibit exocytosis. All three are mediated by the pertussis toxin (PTX)-sensitive heterotrimeric Gi and Go proteins. The distal inhibitory effect on exocytosis is now known to be due to the binding of G protein βγ subunits to the synaptosomal-associated protein of 25 kDa (SNAP-25) on the soluble NSF attachment protein receptor (SNARE) complex. Recent studies have uncovered two more actions of norepinephrine on the β-cell: 1) retardation of the refilling of the readily releasable granule pool after it has been discharged, an action that is mediated by Gαi(1) and/or Gαi(2); and 2) inhibition of endocytosis that is mediated by Gz. Of importance also are new findings that Gαo regulates the number of docked granules in the β-cell, and that Gαo(2) maintains a tonic inhibitory influence on secretion. The latter provides another explanation as to why PTX, which blocks the effect of Gαo(2), was initially called "islet activating protein." Finally, there is clear evidence that overexpression of α(2A)-adrenergic receptors in β-cells can cause type 2 diabetes.

Impaired sympathoadrenal axis function contributes to enhanced insulin secretion in prediabetic obese rats

The involvement of sympathoadrenal axis activity in obesity onset was investigated using the experimental model of treating neonatal rats with monosodium L-glutamate. To access general sympathetic nervous system activity, we recorded the firing rates of sympathetic superior cervical ganglion nerves in animals. Catecholamine content and secretion from isolated adrenal medulla were measured. Intravenous glucose tolerance test was performed, and isolated pancreatic islets were stimulated with glucose and adrenergic agonists. The nerve firing rate of obese rats was decreased compared to the rate for lean rats. Basal catecholamine secretion decreased whereas catecholamine secretion induced by carbachol, elevated extracellular potassium, and caffeine in the isolated adrenal medulla were all increased in obese rats compared to control. Both glucose intolerance and hyperinsulinaemia were observed in obese rats. Adrenaline strongly inhibited glucose-induced insulin secretion in obese animals. These findings suggest that low sympathoadrenal activity contributes to impaired glycaemic control in prediabetic obese rats.

Hormonal inhibition of endocytosis: novel roles for noradrenaline and G protein G(z)

The modulation of endocytosis following exocytosis by noradrenaline (NA), a physiological inhibitor of insulin secretion, was investigated in INS 832/13 cells using patch-clamp capacitance measurements. Endocytosis was inhibited by NA in a pertussis toxin-insensitive manner. Dialysing a synthetic peptide mimicking the C-terminus of the α-subunit of G(z) into the cells blocked the inhibition of endocytosis by NA. Cell-attached capacitance measurements indicated that inhibition by NA was due to a decreased number of endocytotic events without a change in vesicle size. Analysis of fission pore closure kinetics revealed two distinct fission modes, with NA selectively inhibiting the rapid fission pore closure events. Comparison of the actions of NA and deltamethrin, a calcineurin antagonist and potent inhibitor of endocytosis, demonstrated that they inhibit endocytosis by different mechanisms. These findings establish novel actions for NA and G(z) in insulin-secreting cells and possibly other cell types.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Pharmacological stimulation and inhibition of insulin secretion in mouse islets lacking ATP-sensitive K+ channels

BACKGROUND AND PURPOSE

ATP-sensitive potassium channels (K(ATP) channels) in beta cells are a major target for insulinotropic drugs. Here, we studied the effects of selected stimulatory and inhibitory pharmacological agents in islets lacking K(ATP) channels.

EXPERIMENTAL APPROACH

We compared insulin secretion (IS) and cytosolic calcium ([Ca(2+)](c)) changes in islets isolated from control mice and mice lacking sulphonylurea receptor1 (SUR1), and thus K(ATP) channels in their beta cells (Sur1KO).

KEY RESULTS

While similarly increasing [Ca(2+)](c) and IS in controls, agents binding to site A (tolbutamide) or site B (meglitinide) of SUR1 were ineffective in Sur1KO islets. Of two non-selective blockers of potassium channels, quinine was inactive, whereas tetraethylammonium was more active in Sur1KO compared with control islets. Phentolamine, efaroxan and alinidine, three imidazolines binding to K(IR)6.2 (pore of K(ATP) channels), stimulated control islets, but only phentolamine retained weaker stimulatory effects on [Ca(2+)](c) and IS in Sur1KO islets. Neither K(ATP) channel opener (diazoxide, pinacidil) inhibited Sur1KO islets. Calcium channel blockers (nimodipine, verapamil) or diphenylhydantoin decreased [Ca(2+)](c) and IS in both types of islets, verapamil and diphenylhydantoin being more efficient in Sur1KO islets. Activation of alpha(2)-adrenoceptors or dopamine receptors strongly inhibited IS while partially (clonidine > dopamine) lowering [Ca(2+)](c) (control > Sur1KO islets).

CONCLUSIONS AND IMPLICATIONS

Those drugs retaining effects on IS in islets lacking K(ATP) channels, also affected [Ca(2+)](c), indicating actions on other ionic channels. The greater effects of some inhibitors in Sur1KO than in control islets might be relevant to medical treatment of congenital hyperinsulinism caused by inactivating mutations of K(ATP) channels.

Glucose controls cytosolic Ca2+ and insulin secretion in mouse islets lacking adenosine triphosphate-sensitive K+ channels owing to a knockout of the pore-forming subunit Kir6.2

Glucose-induced insulin secretion is classically attributed to the cooperation of an ATP-sensitive potassium (K ATP) channel-dependent Ca2+ influx with a subsequent increase of the cytosolic free Ca2+ concentration ([Ca2+]c) (triggering pathway) and a K ATP channel-independent augmentation of secretion without further increase of [Ca2+]c (amplifying pathway). Here, we characterized the effects of glucose in beta-cells lacking K ATP channels because of a knockout (KO) of the pore-forming subunit Kir6.2. Islets from 1-yr and 2-wk-old Kir6.2KO mice were used freshly after isolation and after 18 h culture to measure glucose effects on [Ca2+]c and insulin secretion. Kir6.2KO islets were insensitive to diazoxide and tolbutamide. In fresh adult Kir6.2KO islets, basal [Ca2+]c and insulin secretion were marginally elevated, and high glucose increased [Ca2+]c only transiently, so that the secretory response was minimal (10% of controls) despite a functioning amplifying pathway (evidenced in 30 mm KCl). Culture in 10 mm glucose increased basal secretion and considerably improved glucose-induced insulin secretion (200% of controls), unexpectedly because of an increase in [Ca2+]c with modulation of [Ca2+]c oscillations. Similar results were obtained in 2-wk-old Kir6.2KO islets. Under selected conditions, high glucose evoked biphasic increases in [Ca2+]c and insulin secretion, by inducing K ATP channel-independent depolarization and Ca2+ influx via voltage-dependent Ca2+ channels. In conclusion, Kir6.2KO beta-cells down-regulate insulin secretion by maintaining low [Ca2+]c, but culture reveals a glucose-responsive phenotype mainly by increasing [Ca2+]c. The results support models implicating a K ATP channel-independent amplifying pathway in glucose-induced insulin secretion, and show that K ATP channels are not the only possible transducers of metabolic effects on the triggering Ca2+ signal.

Long lasting synchronization of calcium oscillations by cholinergic stimulation in isolated pancreatic islets

Individual mouse pancreatic islets exhibit oscillations in [Ca(2+)](i) and insulin secretion in response to glucose in vitro, but how the oscillations of a million islets are coordinated within the human pancreas in vivo is unclear. Islet to islet synchronization is necessary, however, for the pancreas to produce regular pulses of insulin. To determine whether neurohormone release within the pancreas might play a role in coordinating islet activity, [Ca(2+)](i) changes in 4-6 isolated mouse islets were simultaneously monitored before and after a transient pulse of a putative synchronizing agent. The degree of synchronicity was quantified using a novel analytical approach that yields a parameter that we call the "Synchronization Index". Individual islets exhibited [Ca(2+)](i) oscillations with periods of 3-6 min, but were not synchronized under control conditions. However, raising islet [Ca(2+)](i) with a brief application of the cholinergic agonist carbachol (25 microM) or elevated KCl in glucose-containing saline rapidly synchronized islet [Ca(2+)](i) oscillations for >/=30 min, long after the synchronizing agent was removed. In contrast, the adrenergic agonists clonidine or norepinephrine, and the K(ATP) channel inhibitor tolbutamide, failed to synchronize islets. Partial synchronization was observed, however, with the K(ATP) channel opener diazoxide. The synchronizing action of carbachol depended on the glucose concentration used, suggesting that glucose metabolism was necessary for synchronization to occur. To understand how transiently perturbing islet [Ca(2+)](i) produced sustained synchronization, we used a mathematical model of islet oscillations in which complex oscillatory behavior results from the interaction between a fast electrical subsystem and a slower metabolic oscillator. Transient synchronization simulated by the model was mediated by resetting of the islet oscillators to a similar initial phase followed by transient "ringing" behavior, during which the model islets oscillated with a similar frequency. These results suggest that neurohormone release from intrapancreatic neurons could help synchronize islets in situ. Defects in this coordinating mechanism could contribute to the disrupted insulin secretion observed in Type 2 diabetes.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

Both G i and G o heterotrimeric G proteins are required to exert the full effect of norepinephrine on the beta-cell K ATP channel

The effects of norepinephrine (NE), an inhibitor of insulin secretion, were examined on membrane potential and the ATP-sensitive K+ channel (K ATP) in INS 832/13 cells. Membrane potential was monitored under the whole cell current clamp mode. NE hyperpolarized the cell membrane, an effect that was abolished by tolbutamide. The effect of NE on K ATP channels was investigated in parallel using outside-out single channel recording. This revealed that NE enhanced the open activities of the K ATP channels approximately 2-fold without changing the single channel conductance, demonstrating that NE-induced hyperpolarization was mediated by activation of the K ATP channels. The NE effect was abolished in cells preincubated with pertussis toxin, indicating coupling to heterotrimeric G i/G o proteins. To identify the G proteins involved, antisera raised against alpha and beta subunits (anti-G alpha common, anti-G beta, anti-G alpha i1/2/3, and anti-G alpha o) were used. Anti-G alpha common totally blocked the effects of NE on membrane potential and K ATP channels. Individually, anti-G alpha i1/2/3 and anti-G alpha o only partially inhibited the action of NE on K ATP channels. However, the combination of both completely eliminated the action. Antibodies against G beta had no effects. To confirm these results and to further identify the G protein subunits involved, the blocking effects of peptides containing the sequence of 11 amino acids at the C termini of the alpha subunits were used. The data obtained were similar to those derived from the antibody work with the additional information that G alpha i3 and G alpha o1 were not involved. In conclusion, both G i and G o proteins are required for the full effect of norepinephrine to activate the K ATP channel.

17beta-Estradiol inhibits keratinocyte-derived chemokine production following trauma-hemorrhage

Neutrophil infiltration is a key step in the development of organ dysfunction following trauma-hemorrhage (T-H). Although we have previously shown that 17beta-estradiol (E2) prevents neutrophil infiltration and organ damage following T-H, the mechanism by which E2 inhibits neutrophil transmigration remains unknown. We hypothesized that E2 prevents neutrophil infiltration via modulation of keratinocyte-derived chemokine (KC), a major attractant for neutrophils. To examine this, male C3H/HeN mice were subjected to T-H or sham operation and thereafter resuscitated with Ringer lactate and E2 (1 mg/kg body wt) or vehicle. Animals were killed 2 h after resuscitation, and Kupffer cells were isolated. Plasma levels and Kupffer cell production capacities of KC, TNF-alpha, and IL-6 were determined by BD Cytometric Bead Arrays; lung mRNA expression of KC was measured with real-time PCR; myeloperoxidase activity assays were performed to determine neutrophil infiltration, and organ damage was assessed by edema formation. Treatment with E2 decreased systemic levels and restored Kupffer cell production of KC, TNF-alpha, and IL-6, as well as KC gene expression and protein in the lung. This was accompanied with a decrease in neutrophil infiltration and edema formation in the lung. These results suggest that E2 prevents lung neutrophil infiltration and organ damage in part by decreasing KC during posttraumatic immune response.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

The sulfonylurea receptors (SURs) ABCC8/SUR1 and ABCC9/SUR2 are members of the C-branch of the transport adenosine triphosphatase superfamily. Unlike their brethren, the SURs have no identified transport function; instead, evolution has matched these molecules with K(+) selective pores, either K(IR)6.1/KCNJ8 or K(IR)6.2/KCNJ11, to assemble adenosine triphosphate (ATP)-sensitive K(+) channels found in endocrine cells, neurons, and both smooth and striated muscle. Adenine nucleotides, the major regulators of ATP-sensitive K(+) (K(ATP)) channel activity, exert a dual action. Nucleotide binding to the pore reduces the activity or channel open probability, whereas Mg-nucleotide binding and/or hydrolysis in the nucleotide-binding domains of SUR antagonize this inhibitory action to stimulate channel openings. Mutations in either subunit can alter this balance and, in the case of the SUR1/KIR6.2 channels found in neurons and insulin-secreting pancreatic beta cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of K(ATP) channel activity by a K(IR)6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on K(ATP) channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.

Glucose and age-related changes in memory

Epinephrine, released from the adrenal medulla, enhances memory in young rats and mice and apparently does so, at least in part, by increasing blood glucose levels. Like epinephrine, administration of glucose enhances cognitive functions in humans and rodents, including reversing age-related impairments in learning and memory. Epinephrine responses to training are increased in aged rats but the subsequent increase in blood glucose levels is severely blunted. The absence of increases in blood glucose levels during training might contribute to age-related deficits in learning and memory. Also, extracellular glucose levels in the hippocampus are depleted during spontaneous alternation testing to a far greater extent in aged than in young rats. Importantly, systemic injections of glucose block the depletion in the hippocampus and also enhance performance on the alternation task. Thus, the extensive depletion of extracellular glucose during training in aged rats may be associated with age-related memory impairments, an effect that might be related to - or may exacerbate - the effects on learning and memory of an absence of the increases in blood glucose levels to training as seen in young rats. Together, these findings suggest that age-related changes in both peripheral and central glucose physiology contribute to age-related impairments in memory.

Impaired glucagon secretory responses in mice lacking the type 1 sulfonylurea receptor

Pancreatic alpha-cells, like beta-cells, express ATP-sensitive K(+) (K(ATP)) channels. To determine the physiological role of K(ATP) channels in alpha-cells, we examined glucagon secretion in mice lacking the type 1 sulfonylurea receptor (Sur1). Plasma glucagon levels, which were increased in wild-type mice after an overnight fast, did not change in Sur1 null mice. Pancreas perfusion studies showed that Sur1 null pancreata lacked glucagon secretory responses to hypoglycemia and to synergistic stimulation by arginine. Pancreatic alpha-cells isolated from wild-type animals exhibited oscillations of intracellular free Ca(2+) concentration ([Ca(2+)](i)) in the absence of glucose that became quiescent when the glucose concentration was increased. In contrast, Sur1 null alpha-cells showed continuous oscillations in [Ca(2+)](i) regardless of the glucose concentration. These findings indicate that K(ATP) channels in alpha-cells play a key role in regulating glucagon secretion, thereby adding to the paradox of how mice that lack K(ATP) channels maintain euglycemia.

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.

Dopamine D2-like receptors are expressed in pancreatic beta cells and mediate inhibition of insulin secretion

Dopamine signaling is mediated by five cloned receptors, grouped into D1-like (D1 and D5) and D2-like (D2, D3 and D4) families. We identified by reverse transcription-PCR the presence of dopamine receptors from both families in INS-1E insulin-secreting cells as well as in rodent and human isolated islets. D2 receptor expression was confirmed by immunodetection revealing localization on insulin secretory granules of INS-1E and primary rodent and human beta cells. We then tested potential effects mediated by the identified receptors on beta cell function. Dopamine (10 microM) and the D2-like receptor agonist quinpirole (5 microM) inhibited glucose-stimulated insulin secretion tested in several models, i.e. INS-1E beta cells, fluorescence-activated cell-sorted primary rat beta cells, and pancreatic islets of rat, mouse, and human origin. Insulin exocytosis is controlled by metabolism coupled to cytosolic calcium changes. Measurements of glucose-induced mitochondrial hyperpolarization and ATP generation showed that dopamine and D2-like agonists did not inhibit glucose metabolism. On the other hand, dopamine decreased cell membrane depolarization as well as cytosolic calcium increases evoked by glucose stimulation in INS-1E beta cells. These results show for the first time that dopamine receptors are expressed in pancreatic beta cells. Dopamine inhibited glucose-stimulated insulin secretion, an effect that could be ascribed to D2-like receptors. Regarding the molecular mechanisms implicated in dopamine-mediated inhibition of insulin release, our results point to distal steps in metabolism-secretion coupling. Thus, the role played by dopamine in glucose homeostasis might involve dopamine receptors, expressed in pancreatic beta cells, modulating insulin release.

Crosstalk between membrane potential and cytosolic Ca2+ concentration in beta cells from Sur1-/- mice

AIMS/HYPOTHESIS

Islets or beta cells from Sur1(-/-) mice were used to determine whether changes in plasma membrane potential (V(m)) remain coupled to changes in cytosolic Ca(2+) ([Ca(2+)](i)) in the absence of K(ATP) channels and thus provide a triggering signal for insulin secretion. The study also sought to elucidate whether [Ca(2+)](i) influences oscillations in V(m) in sur1(-/-) beta cells.

METHODS

Plasma membrane potential and ion currents were measured with microelectrodes and the patch-clamp technique. [Ca(2+)](i) was monitored with the fluorescent dye fura-2. Insulin secretion from isolated islets was determined by static incubations.

RESULTS

Membrane depolarisation of Sur1(-/-) islets by arginine or increased extracellular K(+), elevated [Ca(2+)](i) and augmented insulin secretion. Oligomycin completely abolished glucose-stimulated insulin release from Sur1(-/-) islets. Oscillations in V(m) were influenced by [Ca(2+)](i) as follows: (1) elevation of extracellular Ca(2+) lengthened phases of membrane hyperpolarisation; (2) simulating a burst of action potentials induced a Ca(2+)-dependent outward current that was augmented by increased Ca(2+) influx through L-type Ca(2+) channels; (3) Ca(2+) depletion of intracellular stores by cyclopiazonic acid increased the burst frequency in Sur1(-/-) islets, elevating [Ca(2+)](i) and insulin secretion; (4) store depletion activated a Ca(2+) influx that was not inhibitable by the L-type Ca(2+) channel blocker D600.

CONCLUSIONS/INTERPRETATION

Although V(m) is largely uncoupled from glucose metabolism in the absence of K(ATP) channels, increased electrical activity leads to elevations of [Ca(2+)](i) that are sufficient to stimulate insulin secretion. In Sur1(-/-) beta cells, [Ca(2+)](i) exerts feedback mechanisms on V(m) by activating a hyperpolarising outward current and by depolarising V(m) via store-operated ion channels.