Voltage-activated calcium and potassium currents in human pancreatic beta-cells

Physiology of pancreatic β-cells: Ion channels and molecular mechanisms implicated in stimulus-secretion coupling

The human and mouse islet of Langerhans is an endocrine organ composed of five different cells types; insulin-secreting β-cells, glucagon-producing α-cells, somatostatin-producing δ-cells, pancreatic polypeptide-secreting PP cells and ɛ-cells that secretes ghrelin. The most important cells are the pancreatic β-cells that comprise around 45-50% of human islets and 75-80% in the mouse. Pancreatic β-cells secrete insulin at high glucose concentration, thereby finely regulating glycaemia by the hypoglycaemic effects of this hormone. Different ion channels are implicated in the stimulus-secretion coupling of insulin. An increase in the intracellular ATP concentration leads to closure K_ATP channels, depolarizing the cell and opening voltage-gated calcium channels. The increase of intracellular calcium concentration induced by calcium entry through voltage-gated calcium channels promotes insulin secretion. Here, we briefly describe the diversity of ion channels present in pancreatic β-cells and the different mechanisms that are responsible to induce insulin secretion in human and mouse cells. Moreover, we described the pathophysiology due to alterations in the physiology of the main ion channels present in pancreatic β-cell and its implication to predispose metabolic disorders as type 2 diabetes mellitus.

Interaction between pancreatic β cell and electromagnetic fields: A systematic study toward finding the natural frequency spectrum of β cell system

Interaction between biological systems and environmental electric or magnetic fields has gained attention during the past few decades. Although there are a lot of studies that have been conducted for investigating such interaction, the reported results are considerably inconsistent. Besides the complexity of biological systems, the important reason for such inconsistent results may arise due to different excitation protocols that have been applied in different experiments. In order to investigate carefully the way that external electric or magnetic fields interact with a biological system, the parameters of excitation, such as intensity or frequency, should be selected purposefully due to the influence of these parameters on the system response. In this study, pancreatic β cell, the main player of blood glucose regulating system, is considered and the study is focused on finding the natural frequency spectrum of the system using modeling approach. Natural frequencies of a system are important characteristics of the system when external excitation is applied. The result of this study can help researchers to select proper frequency parameter for electrical excitation of β cell system. The results show that there are two distinct frequency ranges for natural frequency of β cell system, which consist of extremely low (or near zero) and 100-750 kHz frequency ranges. There are experimental works on β cell exposure to electromagnetic fields that support such finding.

Coordinate changes in histone modifications, mRNA levels, and metabolite profiles in clonal INS-1 832/13 β-cells accompany functional adaptations to lipotoxicity

Lipotoxicity is a presumed pathogenetic process whereby elevated circulating and stored lipids in type 2 diabetes cause pancreatic β-cell failure. To resolve the underlying molecular mechanisms, we exposed clonal INS-1 832/13 β-cells to palmitate for 48 h. We observed elevated basal insulin secretion but impaired glucose-stimulated insulin secretion in palmitate-exposed cells. Glucose utilization was unchanged, palmitate oxidation was increased, and oxygen consumption was impaired. Halting exposure of the clonal INS-1 832/13 β-cells to palmitate largely recovered all of the lipid-induced functional changes. Metabolite profiling revealed profound but reversible increases in cellular lipids. Glucose-induced increases in tricarboxylic acid cycle intermediates were attenuated by exposure to palmitate. Analysis of gene expression by microarray showed increased expression of 982 genes and decreased expression of 1032 genes after exposure to palmitate. Increases were seen in pathways for steroid biosynthesis, cell cycle, fatty acid metabolism, DNA replication, and biosynthesis of unsaturated fatty acids; decreases occurred in the aminoacyl-tRNA synthesis pathway. The activity of histone-modifying enzymes and histone modifications of differentially expressed genes were reversibly altered upon exposure to palmitate. Thus, Insig1, Lss, Peci, Idi1, Hmgcs1, and Casr were subject to epigenetic regulation. Our analyses demonstrate that coordinate changes in histone modifications, mRNA levels, and metabolite profiles accompanied functional adaptations of clonal β-cells to lipotoxicity. It is highly likely that these changes are pathogenetic, accounting for loss of glucose responsiveness and perturbed insulin secretion.

Triton X-100 inhibits L-type voltage-operated calcium channels

Triton X-100 (TX-100) is a nonionic detergent frequently used at millimolar concentrations to disrupt cell membranes and solubilize proteins. At low micromolar concentrations, TX-100 has been reported to inhibit the function of potassium channels. Here, we have used electrophysiological and functional techniques to examine the effects of TX-100 on another class of ion channels, L-type voltage-operated calcium channels (VOCCs). TX-100 (30 nmol·L(-1) to 3 μmol·L(-1)) caused reversible concentration-dependent inhibition of recombinant L-type VOCC (CaV 1.2) currents and of native L-type VOCC currents recorded from rat vascular smooth muscle cells and cardiac myocytes, and murine and human pancreatic β-cells. In functional studies, TX-100 (165 nmol·L(-1) to 3.4 μmol·L(-1)) caused concentration-dependent relaxation of rat isolated mesenteric resistance arteries prestimulated with phenylephrine or KCl. This effect was independent of the endothelium. TX-100 (1.6 μmol·L(-1)) inhibited depolarization-induced exocytosis in both murine and human isolated pancreatic β-cells. These data indicate that at concentrations within the nanomolar to low micromolar range, TX-100 significantly inhibits L-type VOCC activity in a number of cell types, an effect paralleled by inhibition of cell functions dependent upon activation of these channels. This inhibition occurs at concentrations below those used to solubilize proteins and may compromise the use of solutions containing TX-100 in bioassays.

Regulation of insulin secretion in human pancreatic islets

Pancreatic β cells secrete insulin, the body's only hormone capable of lowering plasma glucose levels. Impaired or insufficient insulin secretion results in diabetes mellitus. The β cell is electrically excitable; in response to an elevation of glucose, it depolarizes and starts generating action potentials. The electrophysiology of mouse β cells and the cell's role in insulin secretion have been extensively investigated. More recently, similar studies have been performed on human β cells. These studies have revealed numerous and important differences between human and rodent β cells. Here we discuss the properties of human pancreatic β cells: their glucose sensing, the ion channel complement underlying glucose-induced electrical activity that culminates in exocytotic release of insulin, the cellular control of exocytosis, and the modulation of insulin secretion by circulating hormones and locally released neurotransmitters. Finally, we consider the pathophysiology of insulin secretion and the interactions between genetics and environmental factors that may explain the current diabetes epidemic.

High passage MIN6 cells have impaired insulin secretion with impaired glucose and lipid oxidation

Type 2 diabetes is a metabolic disorder characterized by the inability of beta-cells to secrete enough insulin to maintain glucose homeostasis. MIN6 cells secrete insulin in response to glucose and other secretagogues, but high passage (HP) MIN6 cells lose their ability to secrete insulin in response to glucose. We hypothesized that metabolism of glucose and lipids were defective in HP MIN6 cells causing impaired glucose stimulated insulin secretion (GSIS). HP MIN6 cells had no first phase and impaired second phase GSIS indicative of global functional impairment. This was coupled with a markedly reduced ATP content at basal and glucose stimulated states. Glucose uptake and oxidation were higher at basal glucose but ATP content failed to increase with glucose. HP MIN6 cells had decreased basal lipid oxidation. This was accompanied by reduced expressions of Glut1, Gck, Pfk, Srebp1c, Ucp2, Sirt3, Nampt. MIN6 cells represent an important model of beta cells which, as passage numbers increased lost first phase but retained partial second phase GSIS, similar to patients early in type 2 diabetes onset. We believe a number of gene expression changes occurred to produce this defect, with emphasis on Sirt3 and Nampt, two genes that have been implicated in maintenance of glucose homeostasis.

Ionic mechanisms and Ca2+ dynamics underlying the glucose response of pancreatic β cells: a simulation study

To clarify the mechanisms underlying the pancreatic β-cell response to varying glucose concentrations ([G]), electrophysiological findings were integrated into a mathematical cell model. The Ca²⁺ dynamics of the endoplasmic reticulum (ER) were also improved. The model was validated by demonstrating quiescent potential, burst–interburst electrical events accompanied by Ca²⁺ transients, and continuous firing of action potentials over [G] ranges of 0–6, 7–18, and >19 mM, respectively. These responses to glucose were completely reversible. The action potential, input impedance, and Ca²⁺ transients were in good agreement with experimental measurements. The ionic mechanisms underlying the burst–interburst rhythm were investigated by lead potential analysis, which quantified the contributions of individual current components. This analysis demonstrated that slow potential changes during the interburst period were attributable to modifications of ion channels or transporters by intracellular ions and/or metabolites to different degrees depending on [G]. The predominant role of adenosine triphosphate–sensitive K⁺ current in switching on and off the repetitive firing of action potentials at 8 mM [G] was taken over at a higher [G] by Ca²⁺- or Na⁺-dependent currents, which were generated by the plasma membrane Ca²⁺ pump, Na⁺/K⁺ pump, Na⁺/Ca²⁺ exchanger, and TRPM channel. Accumulation and release of Ca²⁺ by the ER also had a strong influence on the slow electrical rhythm. We conclude that the present mathematical model is useful for quantifying the role of individual functional components in the whole cell responses based on experimental findings.

Analyzing electrical activities of pancreatic β cells using mathematical models

Bursts of repetitive action potentials are closely related to the regulation of glucose-induced insulin secretion in pancreatic β cells. Mathematical studies with simple β-cell models have established the central principle that the burst-interburst events are generated by the interaction between fast membrane excitation and slow cytosolic components. Recently, a number of detailed models have been developed to simulate more realistic β cell activity based on expanded findings on biophysical characteristics of cellular components. However, their complex structures hinder our intuitive understanding of the underlying mechanisms, and it is becoming more difficult to dissect the role of a specific component out of the complex network. We have recently developed a new detailed model by incorporating most of ion channels and transporters recorded experimentally (the Cha-Noma model), yet the model satisfies the charge conservation law and reversible responses to physiological stimuli. Here, we review the mechanisms underlying bursting activity by applying mathematical analysis tools to representative simple and detailed models. These analyses include time-based simulation, bifurcation analysis and lead potential analysis. In addition, we introduce a new steady-state I-V (ssI-V) curve analysis. We also discuss differences in electrical signals recorded from isolated single cells or from cells maintaining electrical connections within multi-cell preparations. Towards this end, we perform simulations with our detailed pancreatic β-cell model.

A biophysical model of electrical activity in human β-cells

Electrical activity in pancreatic β-cells plays a pivotal role in glucose-stimulated insulin secretion by coupling metabolism to calcium-triggered exocytosis. Mathematical models based on rodent data have helped in understanding the mechanisms underlying the electrophysiological patterns observed in laboratory animals. However, human β-cells differ in several aspects, and in particular in their electrophysiological characteristics, from rodent β-cells. Hence, from a clinical perspective and to obtain insight into the defects in insulin secretion relevant for diabetes mellitus, it is important to study human β-cells. This work presents the first mathematical model of electrical activity based entirely on published ion channel characteristics of human β-cells. The model reproduces satisfactorily a series of experimentally observed patterns in human β-cells, such as spiking and rapid bursting electrical activity, and their response to a range of ion channel antagonists. The possibility of Human Ether-a-Go-Go-related- and leak channels as drug targets for diabetes treatment is discussed based on model results.

Tight coupling between glucose and mitochondrial metabolism in clonal beta-cells is required for robust insulin secretion

The biochemical mechanisms underlying glucose-stimulated insulin secretion from pancreatic beta-cells are not completely understood. To identify metabolic disturbances in beta-cells that impair glucose-stimulated insulin secretion, we compared two INS-1-derived clonal beta-cell lines, which are glucose-responsive (832/13 cells) or glucose-unresponsive (832/2 cells). To this end, we analyzed a number of parameters in glycolytic and mitochondrial metabolism, including mRNA expression of genes involved in cellular energy metabolism. We found that despite a marked impairment of glucose-stimulated insulin secretion, 832/2 cells exhibited a higher rate of glycolysis. Still, no glucose-induced increases in respiratory rate, ATP production, or respiratory chain complex I, III, and IV activities were seen in the 832/2 cells. Instead, 832/2 cells, which expressed lactate dehydrogenase A, released lactate regardless of ambient glucose concentrations. In contrast, the glucose-responsive 832/13 line lacked lactate dehydrogenase and did not produce lactate. Accordingly, in 832/2 cells mRNA expression of genes for glycolytic enzymes were up-regulated, whereas mitochondria-related genes were down-regulated. This could account for a Warburg-like effect in the 832/2 cell clone, lacking in 832/13 cells as well as primary beta-cells. In human islets, mRNA expression of genes such as lactate dehydrogenase A and hexokinase I correlated positively with HbA(1c) levels, reflecting perturbed long term glucose homeostasis, whereas that of Slc2a2 (glucose transporter 2) correlated negatively with HbA(1c) and thus better metabolic control. We conclude that tight metabolic regulation enhancing mitochondrial metabolism and restricting glycolysis in 832/13 cells is required for clonal beta-cells to secrete insulin robustly in response to glucose. Moreover, a similar expression pattern of genes controlling glycolytic and mitochondrial metabolism in clonal beta-cells and human islets was observed, suggesting that a similar prioritization of mitochondrial metabolism is required in healthy human beta-cells. The 832 beta-cell lines may be helpful tools to resolve metabolic perturbations occurring in Type 2 diabetes.

A model of action potentials and fast Ca2+ dynamics in pancreatic beta-cells

We examined the ionic mechanisms mediating depolarization-induced spike activity in pancreatic beta-cells. We formulated a Hodgkin-Huxley-type ionic model for the action potential (AP) in these cells based on voltage- and current-clamp results together with measurements of Ca(2+) dynamics in wild-type and Kv2.1 null mouse islets. The model contains an L-type Ca(2+) current, a "rapid" delayed-rectifier K(+) current, a small slowly-activated K(+) current, a Ca(2+)-activated K(+) current, an ATP-sensitive K(+) current, a plasma membrane calcium-pump current and a Na(+) background current. This model, coupled with an equation describing intracellular Ca(2+) homeostasis, replicates beta-cell AP and Ca(2+) changes during one glucose-induced spontaneous spike, the effects of blocking K(+) currents with different inhibitors, and specific complex spike in mouse islets lacking Kv2.1 channels. The currents with voltage-independent gating variables can also be responsible for burst behavior. Original features of this model include new equations for L-type Ca(2+) current, assessment of the role of rapid delayed-rectifier K(+) current, and Ca(2+)-activated K(+) currents, demonstrating the important roles of the Ca(2+)-pump and background currents in the APs and bursts. This model provides acceptable fits to voltage-clamp, AP, and Ca(2+) concentration data based on in silico analysis.

Voltage-gated ion channels in human pancreatic beta-cells: electrophysiological characterization and role in insulin secretion

OBJECTIVE

To characterize the voltage-gated ion channels in human beta-cells from nondiabetic donors and their role in glucose-stimulated insulin release.

RESEARCH DESIGN AND METHODS

Insulin release was measured from intact islets. Whole-cell patch-clamp experiments and measurements of cell capacitance were performed on isolated beta-cells. The ion channel complement was determined by quantitative PCR.

RESULTS

Human beta-cells express two types of voltage-gated K(+) currents that flow through delayed rectifying (K(V)2.1/2.2) and large-conductance Ca(2+)-activated K(+) (BK) channels. Blockade of BK channels (using iberiotoxin) increased action potential amplitude and enhanced insulin secretion by 70%, whereas inhibition of K(V)2.1/2.2 (with stromatoxin) was without stimulatory effect on electrical activity and secretion. Voltage-gated tetrodotoxin (TTX)-sensitive Na(+) currents (Na(V)1.6/1.7) contribute to the upstroke of action potentials. Inhibition of Na(+) currents with TTX reduced glucose-stimulated (6-20 mmol/l) insulin secretion by 55-70%. Human beta-cells are equipped with L- (Ca(V)1.3), P/Q- (Ca(V)2.1), and T- (Ca(V)3.2), but not N- or R-type Ca(2+) channels. Blockade of L-type channels abolished glucose-stimulated insulin release, while inhibition of T- and P/Q-type Ca(2+) channels reduced glucose-induced (6 mmol/l) secretion by 60-70%. Membrane potential recordings suggest that L- and T-type Ca(2+) channels participate in action potential generation. Blockade of P/Q-type Ca(2+) channels suppressed exocytosis (measured as an increase in cell capacitance) by >80%, whereas inhibition of L-type Ca(2+) channels only had a minor effect.

CONCLUSIONS

Voltage-gated T-type and L-type Ca(2+) channels as well as Na(+) channels participate in glucose-stimulated electrical activity and insulin secretion. Ca(2+)-activated BK channels are required for rapid membrane repolarization. Exocytosis of insulin-containing granules is principally triggered by Ca(2+) influx through P/Q-type Ca(2+) channels.

The electrophysiology of the beta-cell based on single transmembrane protein characteristics

The electrophysiology of beta-cells is at the origin of insulin secretion. beta-Cells exhibit a complex behavior upon stimulation with glucose including repeated bursts and continuous spiking. Mathematical modeling is most suitable to improve knowledge about the function of various transmembrane currents provided the model is based on reliable data. This is the first attempt to build a mathematical model for the beta-cell electrophysiology in a bottom-up approach that relies on single protein conductance data. The results of previous whole-cell-based models are reconsidered. The full simulation including all prominent transmembrane proteins in beta-cells is used to provide a functional interpretation of their role in beta-cell bursting and an updated vantage point of beta-cell electrophysiology. As a result of a number of in silico knock-out and block experiments the novel model makes some unexpected predictions: single-channel conductance data imply that large-conductance calcium-gated potassium currents acquire the potential of driving oscillations at supralarge glucose levels. A more complex burst interruption model is presented. It also turns out that, depending on the species, sodium currents may be more relevant than considered so far. Experiments are proposed to verify these predictions.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Glucose-sensing mechanisms in pancreatic beta-cells

The appropriate secretion of insulin from pancreatic beta-cells is critically important to the maintenance of energy homeostasis. The beta-cells must sense and respond suitably to postprandial increases of blood glucose, and perturbation of glucose-sensing in these cells can lead to hypoglycaemia or hyperglycaemias and ultimately diabetes. Here, we review beta-cell glucose-sensing with a particular focus on the regulation of cellular excitability and exocytosis. We examine in turn: (i) the generation of metabolic signalling molecules; (ii) the regulation of beta-cell membrane potential; and (iii) insulin granule dynamics and exocytosis. We further discuss the role of well known and putative candidate metabolic signals as regulators of insulin secretion.

Delayed-rectifier (KV2.1) regulation of pancreatic beta-cell calcium responses to glucose: inhibitor specificity and modeling

The delayed-rectifier (voltage-activated) K(+) conductance (K(V)) in pancreatic islet beta-cells has been proposed to regulate plasma membrane repolarization during responses to glucose, thereby determining bursting and Ca(2+) oscillations. Here, we verified the expression of K(V)2.1 channel protein in mouse and human islets of Langerhans. We then probed the function of K(V)2.1 channels in islet glucose responses by comparing the effect of hanatoxin (HaTx), a specific blocker of K(V)2.1 channels, with a nonspecific K(+) channel blocker, tetraethylammonium (TEA). Application of HaTx (1 microM) blocked delayed-rectifier currents in mouse beta-cells, resulting in a 40-mV rightward shift in threshold of activation of the voltage-dependent outward current. In the presence of HaTx, there was negligible voltage-activated outward current below 0 mV, suggesting that K(V)2.1 channels form the predominant part of this current in the physiologically relevant range. We then employed HaTx to study the role of K(V)2.1 in the beta-cell Ca(2+) responses to elevated glucose in comparison with TEA. Only HaTx was able to induce slow intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in cells stimulated with 20 mM glucose, whereas TEA induced an immediate rise in [Ca(2+)](i) followed by rapid oscillations. In human islets, HaTx acted in a similar fashion. The data were analyzed using a detailed mathematical model of ionic flux and Ca(2+) regulation in beta-cells. The results can be explained by a specific HaTx effect on the K(V) current, whereas TEA affects multiple K(+) conductances. The results underscore the importance of K(V)2.1 channel in repolarization of the pancreatic beta-cell plasma membrane and its role in regulating insulin secretion.

Biophysical and pharmacological properties of the voltage-gated potassium current of human pancreatic beta-cells

Voltage-gated potassium (Kv) currents of human pancreatic islet cells were studied by whole-cell patch clamp recording. On average, 75% of the cells tested were identified as beta-cells by single cell, post-recording RT-PCR for insulin mRNA. In most cells, the dominant Kv current was a delayed rectifier. The delayed rectifier activated at potentials above -20 mV and had a V(1/2) for activation of -5.3 mV. Onset of inactivation was slow for a major component (tau = 3.2 s at +20 mV) observed in all cells; a smaller component (tau = 0.30 s) with an amplitude of approximately 25% was seen in some cells. Recovery from inactivation had a tau of 2.5 s at -80 mV and steady-state inactivation had a V(1/2) of -39 mV. In 12% of cells (21/182) a low-threshold, transient Kv current (A-current) was present. The A-current activated at membrane potentials above -40 mV, inactivated with a time constant of 18.5 ms at -20 mV, and had a V(1/2) for steady-state inactivation of -52 mV. TEA inhibited total Kv current with an IC50 = 0.54 mm and PAC, a disubstituted cyclohexyl Kv channel inhibitor, inhibited with an IC50 = 0.57 microm. The total Kv current was insensitive to margatoxin (100 nm), agitoxin-2 (50 nm), kaliotoxin (50 nm) and ShK (50 nm). Hanatoxin (100 nm) inhibited total Kv current by 65% at +20 mV. Taken together, these data provide evidence of at least two distinct types of Kv channels in human pancreatic beta-cells and suggest that more than one type of Kv channel may be involved in the regulation of glucose-dependent insulin secretion.

Beta-cell CaV channel regulation in physiology and pathophysiology

The beta-cell is equipped with at least six voltage-gated Ca2+ (CaV) channel alpha1-subunits designated CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1. These principal subunits, together with certain auxiliary subunits, assemble into different types of CaV channels conducting L-, P/Q-, N-, R-, and T-type Ca2+ currents, respectively. The beta-cell shares customary mechanisms of CaV channel regulation with other excitable cells, such as protein phosphorylation, Ca2+-dependent inactivation, and G protein modulation. However, the beta-cell displays some characteristic features to bring these mechanisms into play. In islet beta-cells, CaV channels can be highly phosphorylated under basal conditions and thus marginally respond to further phosphorylation. In beta-cell lines, CaV channels can be surrounded by tonically activated protein phosphatases dominating over protein kinases; thus their activity is dramatically enhanced by inhibition of protein phosphatases. During the last 10 years, we have revealed some novel mechanisms of beta-cell CaV channel regulation under physiological and pathophysiological conditions, including the involvement of exocytotic proteins, inositol hexakisphosphate, and type 1 diabetic serum. This minireview highlights characteristic features of customary mechanisms of CaV channel regulation in beta-cells and also reviews our studies on newly identified mechanisms of beta-cell CaV channel regulation.

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.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Hyperinsulinism of infancy: the regulated release of insulin by KATP channel-independent pathways

Hyperinsulinism of infancy (HI) is a congenital defect in the regulated release of insulin from pancreatic beta-cells. Here we describe stimulus-secretion coupling mechanisms in beta-cells and intact islets of Langerhans isolated from three patients with a novel SUR1 gene defect. 2154+3 A to G SUR1 (GenBank accession number L78207) is the first report of familial HI among nonconsanguineous Caucasians identified in the U.K. Using patch-clamp methodologies, we have shown that this mutation is associated with both a decrease in the number of operational ATP-sensitive K+ channels (KATP channels) in beta-cells and impaired ADP-dependent regulation. There were no apparent defects in the regulation of Ca2+- and voltage-gated K+ channels or delayed rectifier K+ channels. Intact HI beta-cells were spontaneously electrically active and generating Ca2+ action currents that were largely insensitive to diazoxide and somatostatin. As a consequence, when intact HI islets were challenged with glucose and tolbutamide, there was no rise in intracellular free calcium ion concentration ([Ca2+]i) over basal values. Capacitance measurements used to monitor exocytosis in control and HI beta-cells revealed that there were no defects in Ca2+-dependent exocytotic events. Finally, insulin release studies documented that whereas tolbutamide failed to cause insulin secretion as a consequence of impaired [Ca2+]i signaling, glucose readily promoted insulin release. Glucose was also found to augment the actions of protein kinase C- and protein kinase A-dependent agonists in the absence of extracellular Ca2+. These findings document the relationship between SUR1 gene defects and insulin secretion in vivo and in vitro and describe for the first time KATP channel-independent pathways of regulated insulin secretion in diseased human beta-cells.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

Glucose- and arginine-induced insulin secretion by human pancreatic beta-cells: the role of HERG K(+) channels in firing and release

The human ether-a-go-go-related genes (herg) are expressed in tissues other than heart and brain where the HERG K(+) channels are known to regulate the repolarization of the heart action potential and the neuronal spike-frequency accommodation. We provide evidence that herg1 transcripts are present in human pancreatic islets that were used to study both insulin secretion and electrical activity with radioimmunoassay and single cell perforated patch-clamp techniques, respectively. Glucose- and arginine-induced islets insulin secretion data suggested a net increase of release under perfusion with antiarrhythmic drugs known to selectively block HERG channels. Indeed we could routinely isolate a K(+) current that was recognized as biophysically and pharmacologically similar to the HERG current. An analysis of the glucose- and arginine-induced electrical activity (several applications during 30 min) in terms of firing frequency and putative insulin release was done in control and in the presence of selective blockers of HERG channels: the firing frequency and the release increased by 32% and 77%, respectively. It is concluded that HERG channels have a crucial role in regulating insulin secretion and firing of human beta-cells. This raises the possibility that some genetically characterized hyperinsulinemic diseases of unknown origin might involve mutations in the HERG channels.

Characterization of a Ca2+-activated K+ current in insulin-secreting murine betaTC-3 cells

1. The whole-cell perforated-patch recording mode was used to record a Ca2+-dependent K+ current (IK(Ca)) in mouse betaTC-3 insulin-secreting cells. 2. Depolarizing voltage steps (to potentials where Ca2+ currents are activated) evoked a slowly activating, outward current, which exhibited a slow deactivation (in seconds) upon subsequent hyperpolarization. 3. This current was shown to increase with progressively longer depolarizing voltage steps. It could be reversibly abolished by the removal of Ca2+ from the external medium or by application of Ca2+ channel blockers, such as Cd2+ and nifedipine. It was concluded that the depolarization-evoked current was activated by Ca2+. 4. Variations in external K+ concentration led to shifts in the reversal potential of the Ca2+-dependent current as predicted by the Nernst equation for a K+-selective current. 5. The Ca2+-activated K+ current was insensitive to external TEA (10 mM), a concentration sufficient to block the large-conductance Ca2+-dependent (maxi-KCa) channel in beta-cells. It was also insensitive to apamin, tubocurarine and scyllatoxin (leiurotoxin I), specific blockers of small-conductance KCa channels. 6. The current was blocked by quinine, a non-specific KCa channel blocker and, surprisingly, by charybdotoxin (ChTX; 100 nM) but not iberiotoxin, a charybdotoxin analogue, which blocks the maxi-KCa channel. It was sensitive to block by clotrimazole and could be potently and reversibly potentiated by micromolar concentrations of niflumic acid. Thus, the current exhibited unique pharmacological characteristics, not conforming to the known KCa channel classes. 7. The ChTX-sensitive KCa channel was permeable to Tl+, K+, Rb+ and NH4+ but not Cs+ ions. 8. The ChTX-sensitive IK(Ca) could be activated by the muscarinic agonists in the presence or absence of external Ca2+, presumably by releasing Ca2+ from internal stores. 9. Acutely isolated porcine islet cells also exhibited a slow IK(Ca) resembling that described in betaTC-3 cells in kinetic properties, insensitivity to TEA (5 mM) and sensitivity to quinidine, an analogue of quinine. The porcine IK(Ca), however, was not sensitive to block by 100-200 nM ChTX. It is likely, that species differences account for pharmacological differences between the mouse and porcine slow IK(Ca).

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.

Voltage-dependent Na+ and Ca2+ currents in human pancreatic islet beta-cells: evidence for roles in the generation of action potentials and insulin secretion

We describe three voltage-dependent inward currents in human pancreatic beta-cells. First, a rapidly inactivating Na+ current, blocked by tetrodotoxin (TTX) is seen upon brief depolarization to or beyond -40 mV. Second, a transient, low-voltage-activated (LVA), amiloride-blockable Ca2+ current is seen upon depolarization to or beyond -55 mV; it inactivates within less than 1s of sustained depolarization to -40 mV. Third, a more sustained, high-voltage-activated (HVA) Ca2+ current, which shows variable sensitivity to dihydropyridines is seen upon depolarization to or beyond -40 mV, and thereafter slowly inactivates over a time course of many seconds. Our pharmacological evidence suggests that all three currents contribute to action potential initiation and upstroke when the background membrane potential (Vm) is equal or negative to -45 to -40 mV, a situation often induced by glucose concentrations (5-6 mM) in the range of those seen post-prandially. Consistent with this, TTX drastically reduces both transient and sustained insulin secretion in the presence of 5-6 mM glucose, but has little effect in 10 mM glucose, at which concentration cells rapidly depolarize to approximately -35 mV, a Vm sufficient to rapidly inactivate Na+ and LVA Ca2+ currents.

Block of non-L-, non-N-type Ca2+ channels in rat insulinoma RINm5F cells by omega-agatoxin IVA and omega-conotoxin MVIIC

The high-voltage-activated (HVA) Ba2+ currents of rat insulinoma RINm5F cells insensitive to dihydropyridines (DHP) and omega-conotoxin GVIA (omega-CTx-GVIA) have been studied for their sensitivity to omega-agatoxin-IVA (omega-Aga-IVA) and omega-CTx-MVIIC. Blockade of HVA currents by omega-Aga-IVA was partial (mean 24%), reversible and saturated around 350 nM (half block approximately 60 nM). Blockade by omega-CTx-MVIIC was more potent (mean 45%), partly irreversible and saturated above 3 microM. The effects of both toxins were additive with that of nifedipine (5 microM) and were more pronounced at positive potentials. omega-Aga-IVA action was additive with that of omega-CTx-GVIA (3 microM) but was largely prevented by cell pre-treatment with omega-CTx-MVIIC (3 microM). In contrast, omega-CTx-MVIIC block was attenuated by omega-CTx-GVIA treatment (approximately 15%), suggesting that omega-CTx-MVIIC blocks the N-type (approximately 15%) and the non-L-, non-N-type channel sensitive to omega-Aga-IVA (approximately 30%). Consistent with this, cells deprived of most non-L-type channels by pre-incubation with omega-CTx-GVIA and omega-CTx-MVIIC exhibited predominant L-type currents that activated at more negative potentials than in normal cells (-30 mV in 5 mM Ba2+) and were effectively depressed by nifedipine (maximal block of 95% from -30 mV to +40 mV). Our results suggest that, besides L- and N-type channels, insulin-secreting RINm5F cells possess also a non-L-, non-N-type channel that contributes significantly to the total current (approximately 30%). Although the pharmacology of this channel is similar to Q-type and alpha 1 class A channels, its range of activation (> -20 mV) and its slow inactivation time course resemble more that of N- and P-type channels. The channel is therefore referred to as "Q-like".

Single-microelectrode voltage clamp measurements of pancreatic beta-cell membrane ionic currents in situ

A conventional patch clamp amplifier was used to test the feasibility of measuring whole-cell ionic currents under voltage clamp conditions from beta-cells in intact mouse islets of Langerhans perifused with bicarbonate Krebs buffer at 37 degrees C. Cells impaled with a high resistance microelectrode (ca. 0.150 G omega) were identified as beta-cells by the characteristic burst pattern of electrical activity induced by 11 mM glucose. Voltage-dependent outward K+ currents were enhanced by glucose both in the presence and absence of physiological bicarbonate buffer and also by bicarbonate regardless of the presence or absence of glucose. For comparison with the usual patch clamp protocol, similar measurements were made from single rat beta-cells at room temperature; glucose did not enhance the outward currents in these cells. Voltage-dependent inward currents were recorded in the presence of tetraethylammonium (TEA), an effective blocker of the K+ channels known to be present in the beta-cell membrane. Inward currents exhibited a fast component with activation-inactivation kinetics and a delayed component with a rather slow inactivation; inward currents were dependent on Ca2+ in the extracellular solution. These results suggest the presence of either two types of voltage-gated Ca2+ channels or a single type with fast and slow inactivation. We conclude that it is feasible to use a single intracellular microelectrode to measure voltage-gated membrane currents in the beta-cell within the intact islet at 37 degrees C, under conditions that support normal glucose-induced insulin secretion and that glucose enhances an as yet unidentified voltage-dependent outward K+ current.

Intracellular Ca2+ signals in human-derived pancreatic somatostatin-secreting cells (QGP-1N)

Single-cell microfluorimetry techniques have been used to examine the effects of acetylcholine (0.1-100 microM) on the intracellular free calcium ion concentration ([Ca2+]i) in a human-derived pancreatic somatostatin-secreting cell line, QGP-1N. When applied to the bath solution, acetylcholine was found to evoke a marked and rapid increase in [Ca2+]i at all concentrations tested. These responses were either sustained, or associated with the generation of complex patterns of [Ca2+]i transients. Overall, the pattern of response was concentration related. In general, 0.1-10 microM acetylcholine initiated a series of repetitive oscillations in cytoplasmic Ca2+, whilst at higher concentrations the responses consisted of a rapid rise in [Ca2+]i followed by a smaller more sustained increase. Without external Ca2+, 100 microM acetylcholine caused only a transient rise in [Ca2+]i, whereas lower concentrations of the agonist were able to initiate, but not maintain, [Ca2+]i oscillations. Acetylcholine-evoked Ca2+ signals were abolished by atropine (1-10 microM), verapamil (100 microM) and caffeine (20 mM). Nifedipine failed to have any significant effect upon agonist-evoked increases in [Ca2+]i, whilst 50 mM KCl, used to depolarise the cell membrane, only elicited a transient increase in [Ca2+]i. Ryanodine (50-500 nM) and caffeine (1-20 mM) did not increase basal Ca2+ levels, but the Ca(2+)-ATPase inhibitors 2,5-di(tert-butyl)-hydroquinone (TBQ) and thapsigargin both elevated [Ca2+]i levels. These data demonstrate for the first time cytosolic Ca2+ signals in single isolated somatostatin-secreting cells of the pancreas. We have demonstrated that acetylcholine will evoke both Ca2+ influx and Ca2+ mobilisation, and we have partially addressed the subcellular mechanism responsible for these events.

Inactivation of voltage-dependent calcium current in an insulinoma cell line

We have studied the mechanism of Ca current inactivation in the beta-cell line HIT-T15 by conventional and perforated patch recording techniques, using two pulse voltage protocols and a combination of current and tail current measurements. In 5 mM Ca, from a holding potential of -80 mV, the maximum current showed a complex time course of inactivation: a relatively fast, double exponential inactivation (tau h1 approximately 12 ms and tau h2 approximately 60 ms) and a very slowly inactivating component (tau > 1 s). The faster component (tau h1) was due to the voltage-dependent inactivation of a low-threshold-activated (LVA), T-type current, which deactivates more slowly (tau approximately 3-5 ms) than the other components (tau approximately 0.2-0.3 ms). The intermediate component (tau h2) was due to the Ca-dependent inactivation of a portion of the high-threshold-activated (HVA) current. A saturating dose of the dihydropyridine (DHP) nifedipine (10 microM) did not affect the LVA current, but inhibited by 68 +/- 5% the transient, Ca-sensitive portion of the HVA current and by 33 +/- 12% the long lasting component. We suggest that three components of the calcium current can be resolved in HIT cells and the main target of DHPs is a HVA current, which inactivates faster than the DHP-resistant HVA component and does so primarily through calcium influx.

Inactivation of HIT cell Ca2+ current by a simulated burst of Ca2+ action potentials

A novel voltage-clamp protocol was developed to test whether slow inactivation of Ca2+ current occurs during bursting in insulin-secreting cells. Single insulin-secreting HIT cells were patch-clamped and their Ca2+ currents were isolated pharmacologically. A computed beta-cell burst was used as a voltage-clamp command and the net Ca2+ current elicited was determined as a cadmium difference current. Ca2+ current rapidly activated during the computed plateau and spike depolarizations and then slowly decayed. Integration of this Ca2+ current yielded an estimate of total Ca influx. To further analyze Ca2+ current inactivation during a burst, repetitive test pulses to + 10 mV were added to the voltage command. Current elicited by these pulses was constant during the interburst, but then slowly and reversibly decreased during the depolarizing plateau. This inactivation was reduced by replacing external Ca2+ with Ba2+ as a charge carrier, and in some cells inactivation was slower in Ba2+. Experimental results were compared with the predictions of the Keizer-Smolen mathematical model of bursting, after subjecting model equations to identical voltage commands. In this model, bursting is driven by the slow, voltage-dependent inactivation of Ca current during the plateau active phase. The K-S model could account for the slope of the slow decay of spike-elicited Ca current, the waveform of individual Ca current spikes, and the suppression of test pulse-elicited Ca current during a burst command. However, the extent and rate of fast inactivation were underestimated by the model. Although there are significant differences between the data obtained and the predictions of the K-S model, the overall results show that as predicted by the model, Ca current slowly inactivates during a burst of imposed spikes, and inactivation is dependent on both Ca2+ influx and membrane depolarization. We thus show that clamping cells to their physiological voltage waveform can be readily accomplished and is a powerful approach for understanding the contribution of individual ion currents to bursting.

Sensitivity to dihydropyridines, omega-conotoxin and noradrenaline reveals multiple high-voltage-activated Ca2+ channels in rat insulinoma and human pancreatic beta-cells

High-voltage-activated (HVA) Ba2+ currents of rat insulinoma (RINm5F) and human pancreatic beta-cells were tested for their sensitivity to dihydropyridines (DHPs), omega-conotoxin (omega-CgTx) and noradrenaline. In RINm5F cells, block of HVA currents by nimodipine, nitrendipine and nifedipine was voltage- and dose-dependent (apparent KD < 37 nM) and largely incomplete even at saturating doses of DHPs (mean 53%, at 10 microM and 0 mV). Analysis of slow tail currents in Bay K 8644-treated cells indicated the existence of Bay K 8644-insensitive channels that turned on at slightly more positive voltages and deactivated more quickly than Bay K 8644-modified channels. DHP Ca2+ agonists and antagonists in human beta-cells had similar features to RINm5F cells except that DHP block was more pronounced (76%, at 10 microM and 0 mV) and Bay K 8644 action was more effective, suggesting a higher density of L-type Ca2+ channels in these cells. In RINm5F cells, but not in human beta-cells, DHP-resistant currents were sensitive to omega-CgTx. The toxin depressed 10-20% of the DHP-resistant currents sparing a "residual" current (25-35%) with similar voltage-dependent characteristics and Ca2+/Ba2+ permeability. Noradrenaline (10 microM) exhibited different actions on the various HVA current components: (1) it prolonged the activation kinetics of omega-CgTx-sensitive currents, (2) it depressed by about 20% the size of DHP-sensitive currents, and (3) it had little or no effects on the residual DHP- and omega-CgTx-resistant current although intracellularly applied guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) prolonged its activation time course.(ABSTRACT TRUNCATED AT 250 WORDS)