Significance of Na/Ca exchange for Ca2+ buffering and electrical activity in mouse pancreatic beta-cells

How Merkel cells transduce mechanical stimuli: A biophysical model of Merkel cells

Merkel cells combine with Aβ afferents, producing slowly adapting type 1(SA1) responses to mechanical stimuli. However, how Merkel cells transduce mechanical stimuli into neural signals to Aβ afferents is still unclear. Here we develop a biophysical model of Merkel cells for mechanical transduction by incorporating main ingredients such as Ca2+ and K+ voltage-gated channels, Piezo2 channels, internal Ca2+ stores, neurotransmitters release, and cell deformation. We first validate our model with several experiments. Then we reveal that Ca2+ and K+ channels on the plasma membrane shape the depolarization of membrane potentials, further regulating the Ca2+ transients in the cells. We also show that Ca2+ channels on the plasma membrane mainly inspire the Ca2+ transients, while internal Ca2+ stores mainly maintain the Ca2+ transients. Moreover, we show that though Piezo2 channels are rapidly adapting mechanical-sensitive channels, they are sufficient to inspire sustained Ca2+ transients in Merkel cells, which further induce the release of neurotransmitters for tens of seconds. Thus our work provides a model that captures the membrane potentials and Ca2+ transients features of Merkel cells and partly explains how Merkel cells transduce the mechanical stimuli by Piezo2 channels.

Impact of β-Amyloids Induced Disruption of Ca2+ Homeostasis in a Simple Model of Neuronal Activity

Alzheimer’s disease is characterized by a marked dysregulation of intracellular Ca²⁺ homeostasis. In particular, toxic β-amyloids (Aβ) perturb the activities of numerous Ca²⁺ transporters or channels. Because of the tight coupling between Ca²⁺ dynamics and the membrane electrical activity, such perturbations are also expected to affect neuronal excitability. We used mathematical modeling to systematically investigate the effects of changing the activities of the various targets of Aβ peptides reported in the literature on calcium dynamics and neuronal excitability. We found that the evolution of Ca²⁺ concentration just below the plasma membrane is regulated by the exchanges with the extracellular medium, and is practically independent from the Ca²⁺ exchanges with the endoplasmic reticulum. Thus, disruptions of Ca²⁺ homeostasis interfering with signaling do not affect the electrical properties of the neurons at the single cell level. In contrast, the model predicts that by affecting the activities of L-type Ca²⁺ channels or Ca²⁺-activated K⁺ channels, Aβ peptides promote neuronal hyperexcitability. On the contrary, they induce hypo-excitability when acting on the plasma membrane Ca²⁺ ATPases. Finally, the presence of pores of amyloids in the plasma membrane can induce hypo- or hyperexcitability, depending on the conditions. These modeling conclusions should help with analyzing experimental observations in which Aβ peptides interfere at several levels with Ca²⁺ signaling and neuronal activity.

Mechanisms of Supralinear Calcium Integration in Dendrites of Hippocampal CA1 Fast-Spiking Cells

In fast-spiking (FS), parvalbumin-expressing interneurons of the CA1 hippocampus, activation of the GluA2-lacking Ca²⁺-permeable AMPA receptors (CP-AMPARs) in basal dendrites is coupled to Ca²⁺-induced Ca²⁺-release (CICR), and can result in a supralinear summation of postsynaptic Ca²⁺-transients (post-CaTs). While this mechanism is important in controlling the direction of long-term plasticity, it is still unknown whether it can operate at all excitatory synapses converging onto FS cells or at a set of synapses receiving a particular input. Using a combination of patch-clamp recordings and two-photon Ca²⁺ imaging in acute mouse hippocampal slices with computational simulations, here we compared the generation of supralinear post-CaTs between apical and basal dendrites of FS cells. We found that, similar to basal dendrites, apical post-CaTs summated supralinearly and relied mainly on the activation of the CP-AMPARs, with a variable contribution of other Ca²⁺ sources, such as NMDA receptors, L-type voltage-gated Ca²⁺-channels and Ca²⁺ release. In addition, supralinear post-CaTs generated in apical dendrites had a slower decay time and a larger cumulative charge than those in basal, and were associated with a stronger level of somatic depolarization. The model predicted that modulation of ryanodine receptors and of the Ca²⁺ extrusion mechanisms, such as the Na⁺/Ca²⁺-exchanger and SERCA pump, had a major impact on the magnitude of supralinear post-CaTs. These data reveal that supralinear Ca²⁺ summation is a common mechanism of Ca²⁺ signaling at CP-AMPAR-containing synapses. Shaped in a location-specific manner through modulation of ryanodine receptors and Ca²⁺ extrusion mechanisms, CP-AMPAR/CICR signaling is suitable for synapse-specific bidirectional modification of incoming inputs in the absence of active dendritic conductances.

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

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

Control of βAR- and N-methyl-D-aspartate (NMDA) Receptor-Dependent cAMP Dynamics in Hippocampal Neurons

Norepinephrine, a neuromodulator that activates β-adrenergic receptors (βARs), facilitates learning and memory as well as the induction of synaptic plasticity in the hippocampus. Several forms of long-term potentiation (LTP) at the Schaffer collateral CA1 synapse require stimulation of both βARs and N-methyl-D-aspartate receptors (NMDARs). To understand the mechanisms mediating the interactions between βAR and NMDAR signaling pathways, we combined FRET imaging of cAMP in hippocampal neuron cultures with spatial mechanistic modeling of signaling pathways in the CA1 pyramidal neuron. Previous work implied that cAMP is synergistically produced in the presence of the βAR agonist isoproterenol and intracellular calcium. In contrast, we show that when application of isoproterenol precedes application of NMDA by several minutes, as is typical of βAR-facilitated LTP experiments, the average amplitude of the cAMP response to NMDA is attenuated compared with the response to NMDA alone. Models simulations suggest that, although the negative feedback loop formed by cAMP, cAMP-dependent protein kinase (PKA), and type 4 phosphodiesterase may be involved in attenuating the cAMP response to NMDA, it is insufficient to explain the range of experimental observations. Instead, attenuation of the cAMP response requires mechanisms upstream of adenylyl cyclase. Our model demonstrates that Gs-to-Gi switching due to PKA phosphorylation of βARs as well as Gi inhibition of type 1 adenylyl cyclase may underlie the experimental observations. This suggests that signaling by β-adrenergic receptors depends on temporal pattern of stimulation, and that switching may represent a novel mechanism for recruiting kinases involved in synaptic plasticity and memory.

Noradrenaline is a stress related molecule that facilitates learning and memory when released in the hippocampus. The facilitation of memory is related to modulation of synaptic plasticity, but the mechanisms underlying this modulation are not well understood. We utilize a combination of live cell imaging and computational modeling to discover how noradrenergic receptor stimulation interacts with other molecules, such as calcium, required for synaptic plasticity and memory storage. Though prior work has shown that noradrenergic receptors and calcium interact synergistically to elevate intracellular second messengers when combined simultaneously, our results demonstrate that prior stimulation of noradrenergic receptors inhibits the elevation of intracellular second messengers. Our results further demonstrate that the inhibition may be caused by the noradrenergic receptor switching signaling pathways, thereby recruiting a different set of memory kinases. This switching represents a novel mechanism for recruiting molecules involved in synaptic plasticity and memory.

Fractal Behavior of the Pancreatic β-Cell Near the Percolation Threshold: Effect of the KATP Channel On the Electrical Response

The molecular system built with true chemical bonds or strong molecular interaction can be described using conceptual mathematical tools. Modeling of the natural generated ionic currents on the human pancreatic β-cell activity had been already studied using complicated analytical models. In our present contribution, we prove the same using our simple electrical model. The ionic currents are associated with different proteins membrane channels (K-Ca, K(v), K(ATP), Ca(v)-L) and Na/Ca Exchanger (NCX). The proteins are Ohmic conductors and are modeled by conductance randomly distributed. Switches are placed in series with conductances in order to highlight the channel activity. However, the KATP channel activity is stimulated by glucose, and the NCX's conductance change according to the intracellular calcium concentration. The percolation threshold of the system is calculated by the fractal nature of the infinite cluster using the Tarjan's depth-first-search algorithm. It is shown that the behavior of the internal concentration of Ca(2+) and the membrane potential are modulated by glucose. The results confirm that the inhibition of KATP channels depolarizes the membrane and increases the influx of [Ca(2+)]i through NCX and Ca(v)-L channel for high glucose concentrations.

GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation

Strategies aimed at mimicking or enhancing the action of the incretin hormone glucagon-like peptide 1 (GLP-1) therapeutically improve glucose-stimulated insulin secretion (GSIS); however, it is not clear whether GLP-1 directly drives insulin secretion in pancreatic islets. Here, we examined the mechanisms by which GLP-1 stimulates insulin secretion in mouse and human islets. We found that GLP-1 enhances GSIS at a half-maximal effective concentration of 0.4 pM. Moreover, we determined that GLP-1 activates PLC, which increases submembrane diacylglycerol and thereby activates PKC, resulting in membrane depolarization and increased action potential firing and subsequent stimulation of insulin secretion. The depolarizing effect of GLP-1 on electrical activity was mimicked by the PKC activator PMA, occurred without activation of PKA, and persisted in the presence of PKA inhibitors, the KATP channel blocker tolbutamide, and the L-type Ca(2+) channel blocker isradipine; however, depolarization was abolished by lowering extracellular Na(+). The PKC-dependent effect of GLP-1 on membrane potential and electrical activity was mediated by activation of Na(+)-permeable TRPM4 and TRPM5 channels by mobilization of intracellular Ca(2+) from thapsigargin-sensitive Ca(2+) stores. Concordantly, GLP-1 effects were negligible in Trpm4 or Trpm5 KO islets. These data provide important insight into the therapeutic action of GLP-1 and suggest that circulating levels of this hormone directly stimulate insulin secretion by β cells.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

A computational systems analysis of factors regulating α cell glucagon secretion

Glucagon, a peptide hormone secreted from the α-cells of the pancreatic islets, is critical for blood glucose homeostasis. We reviewed the literature and employed a computational systems analysis of intracellular metabolic and electrical regulation of glucagon secretion to better understand these processes. The mathematical model of α-cell metabolic parameters is based on our previous model for pancreatic β-cells. We also formulated an ionic model for action potentials that incorporates Ca ( 2+) , K (+) , Na (+) and Cl (-) currents. Metabolic and ionic models are coupled to the equations describing Ca ( 2+) homeostasis and glucagon secretion that depends on activation of specific voltage-gated Ca ( 2+) channels. Paracrine and endocrine regulations were analyzed with an emphasis on their effects on a hyperpolarization of membrane potential. This general model simulates and gives insight into the mechanisms of regulation of glucagon secretion under a wide range of experimental conditions. We also reviewed and analyzed dysfunctional mechanisms in α-cells to determine key pharmacological targets for modulating glucagon secretion in type 1 and 2 diabetes.

Regulation of ATP production by mitochondrial Ca(2+)

Stimulation of mitochondrial oxidative metabolism by Ca(2+) is now generally recognised as important for the control of cellular ATP homeostasis. Here, we review the mechanisms through which Ca(2+) regulates mitochondrial ATP synthesis. We focus on cardiac myocytes and pancreatic β-cells, where tight control of this process is likely to play an important role in the response to rapid changes in workload and to nutrient stimulation, respectively. We also describe a novel approach for imaging the Ca(2+)-dependent regulation of ATP levels dynamically in single cells.

Subcellular location of PKA controls striatal plasticity: stochastic simulations in spiny dendrites

Dopamine release in the striatum has been implicated in various forms of reward dependent learning. Dopamine leads to production of cAMP and activation of protein kinase A (PKA), which are involved in striatal synaptic plasticity and learning. PKA and its protein targets are not diffusely located throughout the neuron, but are confined to various subcellular compartments by anchoring molecules such as A-Kinase Anchoring Proteins (AKAPs). Experiments have shown that blocking the interaction of PKA with AKAPs disrupts its subcellular location and prevents LTP in the hippocampus and striatum; however, these experiments have not revealed whether the critical function of anchoring is to locate PKA near the cAMP that activates it or near its targets, such as AMPA receptors located in the post-synaptic density. We have developed a large scale stochastic reaction-diffusion model of signaling pathways in a medium spiny projection neuron dendrite with spines, based on published biochemical measurements, to investigate this question and to evaluate whether dopamine signaling exhibits spatial specificity post-synaptically. The model was stimulated with dopamine pulses mimicking those recorded in response to reward. Simulations show that PKA colocalization with adenylate cyclase, either in the spine head or in the dendrite, leads to greater phosphorylation of DARPP-32 Thr34 and AMPA receptor GluA1 Ser845 than when PKA is anchored away from adenylate cyclase. Simulations further demonstrate that though cAMP exhibits a strong spatial gradient, diffusible DARPP-32 facilitates the spread of PKA activity, suggesting that additional inactivation mechanisms are required to produce spatial specificity of PKA activity.

The striatum is a part of the basal ganglia which plays a role in addiction and reward learning. Its importance is underscored by pathologies such as Parkinson's disease and Huntington's disease in which degeneration of the dopamine inputs to the striatum or degeneration of neurons in the striatum, respectively, produces motor dysfunction. Dopamine in the striatum activates cascades of signaling molecules, ultimately producing an activity dependent change in the strength of connections between neurons. However, the dispersive movement of signaling molecules seems incompatible with the strengthening of specific subsets of connections, which is required for formation of distinct memories. Anchoring proteins, which restrict molecules to particular compartments within the neuron, are proposed to achieve specificity. We develop a reaction-diffusion model of dopamine activated signaling pathways to explore mechanisms whereby anchoring proteins can produce specificity. We use an efficient Monte-Carlo simulator to implement the cascades of signaling molecules in a neuronal dendrite with multiple dendritic spines. Simulations demonstrate that spatial specificity requires both anchoring proteins and inactivation mechanisms that limit the diffusion of signaling molecules.

Electrophysiology of pancreatic β-cells in intact mouse islets of Langerhans

When exposed to intermediate glucose concentrations (6-16 mol/l), pancreatic β-cells in intact islets generate bursts of action potentials (superimposed on depolarised plateaux) separated by repolarised electrically silent intervals. First described more than 40 years ago, these oscillations have continued to intrigue β-cell electrophysiologists. To date, most studies of β-cell ion channels have been performed on isolated cells maintained in tissue culture (that do not burst). Here we will review the electrophysiological properties of β-cells in intact, freshly isolated, mouse pancreatic islets. We will consider the role of ATP-regulated K⁺-channels (K(ATP)-channels), small-conductance Ca²⁺-activated K⁺-channels and voltage-gated Ca²⁺-channels in the generation of the bursts. Our data indicate that K(ATP)-channels not only constitute the glucose-regulated resting conductance in the β-cell but also provide a variable K⁺-conductance that influence the duration of the bursts of action potentials and the silent intervals. We show that inactivation of the voltage-gated Ca²⁺-current is negligible at voltages corresponding to the plateau potential and consequently unlikely to play a major role in the termination of the burst. Finally, we propose a model for glucose-induced β-cell electrical activity based on observations made in intact pancreatic islets.

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.

Heterozygous inactivation of the Na/Ca exchanger increases glucose-induced insulin release, β-cell proliferation, and mass

OBJECTIVE

We have previously shown that overexpression of the Na-Ca exchanger (NCX1), a protein responsible for Ca(2+) extrusion from cells, increases β-cell programmed cell death (apoptosis) and reduces β-cell proliferation. To further characterize the role of NCX1 in β-cells under in vivo conditions, we developed and characterized mice deficient for NCX1.

RESEARCH DESIGN AND METHODS

Biologic and morphologic methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release, and point counting morphometry) were used to assess β-cell function in vitro. Blood glucose and insulin levels were measured to assess glucose metabolism and insulin sensitivity in vivo. Islets were transplanted under the kidney capsule to assess their performance to revert diabetes in alloxan-diabetic mice.

RESULTS

Heterozygous inactivation of Ncx1 in mice induced an increase in glucose-induced insulin release, with a major enhancement of its first and second phase. This was paralleled by an increase in β-cell proliferation and mass. The mutation also increased β-cell insulin content, proinsulin immunostaining, glucose-induced Ca(2+) uptake, and β-cell resistance to hypoxia. In addition, Ncx1(+/-) islets showed a two- to four-times higher rate of diabetes cure than Ncx1(+/+) islets when transplanted into diabetic animals.

CONCLUSIONS

Downregulation of the Na/Ca exchanger leads to an increase in β-cell function, proliferation, mass, and resistance to physiologic stress, namely to various changes in β-cell function that are opposite to the major abnormalities seen in type 2 diabetes. This provides a unique model for the prevention and treatment of β-cell dysfunction in type 2 diabetes and after islet transplantation.

Bursting and calcium oscillations in pancreatic beta-cells: specific pacemakers for specific mechanisms

Oscillatory phenomenon in electrical activity and cytoplasmic calcium concentration in response to glucose are intimately connected to multiple key aspects of pancreatic β-cell physiology. However, there is no single model for oscillatory mechanisms in these cells. We set out to identify possible pacemaker candidates for burst activity and cytoplasmic Ca(2+) oscillations in these cells by analyzing published hypotheses, their corresponding mathematical models, and relevant experimental data. We found that although no single pacemaker can account for the variety of oscillatory phenomena in β-cells, at least several separate mechanisms can underlie specific kinds of oscillations. According to our analysis, slowly activating Ca(2+)-sensitive K(+) channels can be responsible for very fast Ca(2+) oscillations; changes in the ATP/ADP ratio and in the endoplasmic reticulum calcium concentration can be pacemakers for both fast bursts and cytoplasmic calcium oscillations, and cyclical cytoplasmic Na(+) changes may underlie patterning of slow calcium oscillations. However, these mechanisms still lack direct confirmation, and their potential interactions raises new issues. Further studies supported by improved mathematical models are necessary to understand oscillatory phenomena in β-cell physiology.

Computational model of the insect pheromone transduction cascade

A biophysical model of receptor potential generation in the male moth olfactory receptor neuron is presented. It takes into account all pre-effector processes—the translocation of pheromone molecules from air to sensillum lymph, their deactivation and interaction with the receptors, and the G-protein and effector enzyme activation—and focuses on the main post-effector processes. These processes involve the production and degradation of second messengers (IP₃ and DAG), the opening and closing of a series of ionic channels (IP₃-gated Ca²⁺ channel, DAG-gated cationic channel, Ca²⁺-gated Cl⁻ channel, and Ca²⁺- and voltage-gated K⁺ channel), and Ca²⁺ extrusion mechanisms. The whole network is regulated by modulators (protein kinase C and Ca²⁺-calmodulin) that exert feedback inhibition on the effector and channels. The evolution in time of these linked chemical species and currents and the resulting membrane potentials in response to single pulse stimulation of various intensities were simulated. The unknown parameter values were fitted by comparison to the amplitude and temporal characteristics (rising and falling times) of the experimentally measured receptor potential at various pheromone doses. The model obtained captures the main features of the dose–response curves: the wide dynamic range of six decades with the same amplitudes as the experimental data, the short rising time, and the long falling time. It also reproduces the second messenger kinetics. It suggests that the two main types of depolarizing ionic channels play different roles at low and high pheromone concentrations; the DAG-gated cationic channel plays the major role for depolarization at low concentrations, and the Ca²⁺-gated Cl⁻ channel plays the major role for depolarization at middle and high concentrations. Several testable predictions are proposed, and future developments are discussed.

All sensory neurons transduce their natural stimulus, whether a molecule, a photon, or a mechanical force, in an electrical current flowing through their sensory membrane via similar molecular and ionic mechanisms. Olfactory receptor neurons (ORNs), whose stimuli are volatile molecules, are no exception, including one of the best known: the exquisitely sensitive ORNs of male moths that detect the sexual pheromone released by conspecific females. We provide a detailed computational model of the intracellular molecular mechanisms at work in this ORN type. We describe qualitatively and quantitatively how the initial event, the interaction of pheromone molecules with specialized receptors at the ORN surface, is amplified through a sequence of linked biochemical and electrical events into a whole cell response, the receptor potential. We detail the respective roles of the upward activating reactions involving a cascade of ionic channels permeable to cations, chloride and potassium, their control by feedback inactivating mechanisms, and the central regulatory role of calcium. This computational model contributes to an integrated understanding of this signalling pathway, provides testable hypotheses, and suggests new experimental approaches.

Splice variant-dependent regulation of beta-cell sodium-calcium exchange by acyl-coenzyme As

The sodium-calcium exchanger isoform 1 (NCX1) is intimately involved in the regulation of calcium (Ca(2+)) homeostasis in many tissues including excitation-secretion coupling in pancreatic beta-cells. Our group has previously found that intracellular long-chain acyl-coenzyme As (acyl CoAs) are potent regulators of the cardiac NCX1.1 splice variant. Despite this, little is known about the biophysical properties of beta-cell NCX1 splice variants and the effects of intracellular modulators on their important physiological function in health and disease. Here, we show that the forward-mode activity of beta-cell NCX1 splice variants is differentially modulated by acyl-CoAs and is dependent both upon the intrinsic biophysical properties of the particular NCX1 splice variant as well as the side chain length and degree of saturation of the acyl-CoA moiety. Notably, saturated long-chain acyl-CoAs increased both peak and total NCX1 activity, whereas polyunsaturated long-chain acyl-CoAs did not show this effect. Furthermore, we have identified the exon within the alternative splicing region that bestows sensitivity to acyl-CoAs. We conclude that the physiologically relevant forward-mode activity of NCX1 splice variants expressed in the pancreatic beta-cell are sensitive to acyl-CoAs of different saturation and alterations in intracellular acyl-CoA levels may ultimately lead to defects in Ca(2+)-mediated exocytosis and insulin secretion.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

A mathematical model of plasma membrane electrophysiology and calcium dynamics in vascular endothelial cells

Vascular endothelial cells (ECs) modulate smooth muscle cell (SMC) contractility, assisting in vascular tone regulation. Cytosolic Ca2+ concentration ([Ca2+]i) and membrane potential ( Vm) play important roles in this process by controlling EC-dependent vasoactive signals and intercellular communication. The present mathematical model integrates plasmalemma electrophysiology and Ca2+ dynamics to investigate EC responses to different stimuli and the controversial relationship between [Ca2+]i and Vm. The model contains descriptions for the intracellular balance of major ionic species and the release of Ca2+ from intracellular stores. It also expands previous formulations by including more detailed transmembrane current descriptions. The model reproduces Vm responses to volume-regulated anion channel (VRAC) blockers and extracellular K+ concentration ([K+]o) challenges, predicting 1) that Vm changes upon VRAC blockade are [K+]o dependent and 2) a biphasic response of Vm to increasing [K+]o. Simulations of agonist-induced Ca2+ mobilization replicate experiments under control and Vm hyperpolarization blockade conditions. They show that peak [Ca2+]i is governed by store Ca2+ release while Ca2+ influx (and consequently Vm) impacts more the resting and plateau [Ca2+]i. The Vm sensitivity of rest and plateau [Ca2+]i is dictated by a [Ca2+]i “buffering” system capable of masking the Vm-dependent transmembrane Ca2+ influx. The model predicts plasma membrane Ca2+-ATPase and Ca2+ permeability as main players in this process. The heterogeneous Vm impact on [Ca2+]i may elucidate conflicting reports on how Vm influences EC Ca2+. The present study forms the basis for the development of multicellular EC-SMC models that can assist in understanding vascular autoregulation in health and disease.

Dominant role of sarcoendoplasmic reticulum Ca2+-ATPase pump in Ca2+ homeostasis and exocytosis in rat pancreatic beta-cells

The exocytosis of insulin-containing granules from pancreatic beta-cells is tightly regulated by changes in cytosolic Ca2+ concentration ([Ca2+]i). We investigated the role of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pump, Na+/Ca2+ exchanger, and plasma membrane Ca2+-ATPase pump in the Ca2+ dynamics of single rat pancreatic beta-cells. When the membrane potential was voltage clamped at -70 mV (in 3 mm glucose at approximately 22 or 35 C), SERCA pump inhibition dramatically slowed (approximately 4-fold) cytosolic Ca2+ clearance and caused a sustained rise in basal [Ca2+]i via the activation of capacitative Ca2+ entry. SERCA pump inhibition increased ( approximately 1.8-fold) the amplitude of the depolarization-triggered Ca2+ transient at approximately 22 C. Inhibition of the Na+/Ca2+ exchanger or plasma membrane Ca2+-ATPase pump had only minor effects on Ca2+ dynamics. Simultaneous measurement of [Ca2+]i and exocytosis (with capacitance measurement) revealed that SERCA pump inhibition increased the magnitude of depolarization-triggered exocytosis. This enhancement in exocytosis was not due to the slowing of the cytosolic Ca2+ clearance but was closely correlated to the increase in the peak of the depolarization-triggered Ca2+ transient. When compared at similar [Ca2+]i with controls, the rise in basal [Ca2+]i during SERCA pump inhibition did not cause any enhancement in the magnitude of the ensuing depolarization-triggered exocytosis. Therefore, we conclude that in rat pancreatic beta-cells, the rapid uptake of Ca2+ by SERCA pump limits the peak amplitude of depolarization-triggered [Ca2+]i rise and thus controls the amount of insulin secretion.

Role of calcium binding proteins in the control of cerebellar granule cell neuronal excitability: experimental and modeling studies

Calcium binding proteins, such as calretinin, are abundantly expressed in distinctive patterns in the central nervous system but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin deficient mice exhibit dramatic alterations in motor coordination and in Purkinje cell firing recorded in vivo through unknown mechanisms. In the present paper, we review the results obtained with the patch clamp recording techniques in acute slice preparation. This data allow us to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mM of the exogenous fast calcium buffer BAPTA is infused in the cytosol to restore the calcium buffering capacity. Furthermore, we propose a mathematical model demonstrating that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium buffering capacity due to the absence of calretinin. We suggest that calcium binding proteins modulate intrinsic neuronal excitability and may therefore play a role in the information processing in the central nervous system.

Time-correlation between membrane depolarization and intracellular calcium in insulin secreting BRIN-BD11 cells: studies using FLIPR

Cytoplasmic Ca(2+) ([Ca(2+)](i)) and membrane potential changes were measured in clonal pancreatic beta cells using a fluorimetric imaging plate reader (FLIPR). KCl (30 mM) produced a fast membrane depolarization immediately followed by increase of [Ca(2+)](i) in BRIN-BD11 cells. l-Alanine (10 mM) but not l-arginine (10 mM) mimicked the KCl profile and also produced a fast membrane depolarization and elevation of [Ca(2+)](i). Conversely, a rise in glucose from 5.6 mM to 11.1 or 16.7 mM induced rapid membrane depolarization, followed by a slower and delayed increase of [Ca(2+)](i). GLP-1 (20 nM) did not affect membrane potential or [Ca(2+)](i). In contrast, acetylcholine (ACh, 100 microM) induced fast membrane depolarization immediately followed by a modest [Ca(2+)](i) increase. When extracellular Ca(2+) was buffered with EGTA, ACh mobilized intracellular calcium stores and the [Ca(2+)](i) increase was reduced by 2-aminoethoxydiphenyl borate but not by dantrolene, indicating the involvement of inositol triphosphate receptors (InsP(3)R). It is concluded that membrane depolarization of beta cells by glucose stimulation is not immediately followed by elevation of [Ca(2+)](i) and other metabolic events are involved in glucose induced stimulus-secretion coupling. It is also suggested that ACh mobilizes intracellular Ca(2+) through store operated InsP(3)R.

Paired turbulence and light do not produce a supralinear calcium increase in Hermissenda

The sea slug Hermissenda learns to associate light and hair cell stimulation, but not when the stimuli are temporally uncorrelated. Memory storage, which requires an elevation in calcium, occurs in the photoreceptors, which receive monosynaptic input from hair cells that sense acceleration stimuli such as turbulence. Both light and hair cell activity increase calcium concentration in the photoreceptor, but it is unknown whether paired calcium signals combine supralinearly to initiate memory storage. A correlate of memory storage is an enhancement of the long lasting depolarization (LLD) after light offset, which is attributed to a reduction in voltage dependent potassium currents; however, it is unclear what causes the LLD in the untrained animal. These issues were addressed using a multi-compartmental computer model of phototransduction, calcium dynamics, and ionic currents of the Hermissenda photoreceptor. Simulations of the interaction between light and hair cell activity show that paired stimuli do not produce a greater calcium increase than unpaired stimuli. This suggests that hair cell activity is acting via some other pathway to initiate memory storage. In addition, simulations show that a potassium leak channel, which closes with an increase in calcium, is required to produce both the untrained LLD and the enhanced LLD due to the decrease in voltage dependent potassium currents. Thus, the expression of this correlate of classical conditioning may depend on a leak potassium current.

Altered neuronal excitability in cerebellar granule cells of mice lacking calretinin

Calcium-binding proteins such as calretinin are abundantly expressed in distinctive patterns in the CNS, but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells, which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin-deficient mice exhibit dramatic alterations in motor coordination and Purkinje cell firing recorded in vivo through unknown mechanisms. In the present study, we used patch-clamp recording techniques in acute slice preparation to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin-deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mm of the exogenous fast-calcium buffer BAPTA is infused in the cytosol to restore the calcium-buffering capacity. A proposed mathematical model demonstrates that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium-buffering capacity resulting from the absence of calretinin. This result suggests that calcium-binding proteins modulate intrinsic neuronal excitability and may therefore play a role in information processing in the CNS.

Therapeutic significance of Y-27632, a Rho-kinase inhibitor, on the established liver fibrosis

BACKGROUND

Recently, we demonstrated that Y-27632, a Rho-kinase inhibitor, inhibited hepatic stellate cells (HSCs) activation in terms of cellular morphology, improved the progression of carbon tetrachloride (CCl(4))-induced rat liver fibrosis. The objective of the present study was to investigate the effects of Y-27632 on the established liver fibrosis.

METHODS AND METHODS

Liver cirrhosis was induced by intragastric administration of CCl(4) once a week for 12 weeks. After the first 6 weeks of CCl(4) injection, Y-27632 (30 mg/kg body weight) or saline was continuously administered to the rats via an intraperitoneally implanted osmotic pump during the final 6 weeks of CCl(4) injection. Two days after the last CCl(4) injection, 70% hepatectomy was performed.

RESULTS

Y-27632 prevented the development of CCl(4)-induced liver fibrosis and improved the fibrotic changes, hydroxyproline content, and serum hyaluronic acid level in the liver. Moreover, Y-27632 reduced the number of smooth muscle alpha-actin- and transforming growth factor beta1-positive cells, and inhibited the expression of Na(+)/Ca(2+) exchanger mRNA which was reported to be an indicator of HSCs activation and liver fibrosis. Further, the Y-27632-treated group showed markedly increased survival rate after hepatectomy.

CONCLUSIONS

These findings indicated that Y-27632 may be useful therapeutically in liver cirrhosis.

Altered neuronal excitability in cerebellar granule cells of mice lacking calretinin

Calcium-binding proteins such as calretinin are abundantly expressed in distinctive patterns in the CNS, but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells, which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin-deficient mice exhibit dramatic alterations in motor coordination and Purkinje cell firing recorded in vivo through unknown mechanisms. In the present study, we used patch-clamp recording techniques in acute slice preparation to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin-deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mm of the exogenous fast-calcium buffer BAPTA is infused in the cytosol to restore the calcium-buffering capacity. A proposed mathematical model demonstrates that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium-buffering capacity resulting from the absence of calretinin. This result suggests that calcium-binding proteins modulate intrinsic neuronal excitability and may therefore play a role in information processing in the CNS.

Dynamics of calcium clearance in mouse pancreatic beta-cells

Pancreatic beta-cells maintain glucose homeostasis by their regulated Ca(2+)-dependent secretion of insulin. Several cellular mechanisms control intracellular Ca(2+) levels, but their relative significance in mouse beta-cells is not fully known. We used photometry to measure the dynamics of cytosolic Ca(2+) ([Ca(2+)](i)) clearance after brief, depolarization-induced Ca(2+) entry. Treatment with thapsigargin or cyclopiazonic acid, inhibitors of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pumps, nearly doubled the peak and slowed the decay of the depolarization-induced Ca(2+) transients. The remaining thapsigargin-insensitive decay was slowed further by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) and plasma membrane Na(+)/Ca(2+) exchanger (NCX) via alkalization of the bath solution, by adding lanthanum, or by substitution of Na(+) with Li(+). Mitochondrial Ca(2+) uptake contributed little to clearance in thapsigargin-pretreated cells. Together, the SERCA, PMCA, and NCX transport mechanisms accounted for 89 to 97% of clearance in normal solutions. We developed a quantitative model for the dynamic role of removal mechanisms over a wide range of [Ca(2+)](i). According to our model, 50 to 64% of initial Ca(2+) removal is via the SERCA pump, whereas the NCX contributes 21-30% of the extrusion at high [Ca(2+)](i), and the PMCA contributes 21-27% at low [Ca(2+)](i).

Modeling of Ca2+ flux in pancreatic beta-cells: role of the plasma membrane and intracellular stores

We have developed a detailed mathematical model of ionic flux in beta-cells that includes the most essential channels and pumps in the plasma membrane. This model is coupled to equations describing Ca2+, inositol 1,4,5-trisphosphate (IP3), ATP, and Na+ homeostasis, including the uptake and release of Ca2+ by the endoplasmic reticulum (ER). In our model, metabolically derived ATP activates inward Ca2+ flux by regulation of ATP-sensitive K+ channels and depolarization of the plasma membrane. Results from the simulations support the hypothesis that intracellular Na+ and Ca2+ in the ER can be the main variables driving both fast (2-7 osc/min) and slow intracellular Ca2+ concentration oscillations (0.3-0.9 osc/min) and that the effect of IP3 on Ca2+ leak from the ER contributes to the pattern of slow calcium oscillations. Simulations also show that filling the ER Ca2+ stores leads to faster electrical bursting and Ca2+ oscillations. Specific Ca2+ oscillations in isolated beta-cell lines can also be simulated.

Glucose-dependent regulation of rhythmic action potential firing in pancreatic beta-cells by K(ATP)-channel modulation

The regulation of a K(+) current activating during oscillatory electrical activity (I(K,slow)) in an insulin-releasing beta-cell was studied by applying the perforated patch whole-cell technique to intact mouse pancreatic islets. The resting whole-cell conductance in the presence of 10 mM glucose amounted to 1.3 nS, which rose by 50 % during a series of 26 simulated action potentials. Application of the K(ATP)-channel blocker tolbutamide produced uninterrupted action potential firing and reduced I(K,slow) by approximately 50 %. Increasing glucose from 15 to 30 mM, which likewise converted oscillatory electrical activity into continuous action potential firing, reduced I(K,slow) by approximately 30 % whilst not affecting the resting conductance. Action potential firing may culminate in opening of K(ATP) channels by activation of ATP-dependent Ca(2+) pumping as suggested by the observation that the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor thapsigargin (4 microM) inhibited I(K,slow) by 25 % and abolished bursting electrical activity. We conclude that oscillatory glucose-induced electrical activity in the beta-cell involves the opening of K(ATP)-channel activity and that these channels, in addition to constituting the glucose-regulated K(+) conductance, also play a role in the graded response to supra-threshold glucose concentrations.

Na(+)-Ca(2+) exchanger controls the gain of the Ca(2+) amplifier in the dendrites of amacrine cells

We have previously shown that disabling forward-mode Na(+)-Ca(2+) exchange in amacrine cells greatly prolongs the depolarization-induced release of transmitter. To investigate the mechanism for this, we imaged [Ca(2+)](i) in segments of dendrites during depolarization. Removal of [Na(+)](o) produced no immediate effect on resting [Ca(2+)](i) but did prolong [Ca(2+)](i) transients induced by brief depolarization in both voltage-clamped and unclamped cells. In some cells, depolarization gave rise to stable patterns of higher and lower [Ca(2+)] over micrometer-length scales that collapsed once [Na(+)](o) was restored. Prolongation of [Ca(2+)](i) transients by removal of [Na(+)](o) is not due to reverse mode operation of Na(+)-Ca(2+) exchange but is instead a consequence of Ca(2+) release from endoplasmic reticulum (ER) stores over which Na(+)-Ca(2+) exchange normally exercises control. Even in normal [Na(+)](o), hotspots for [Ca(2+)] could be seen following depolarization, that are attributable to local Ca(2+)-induced Ca(2+) release. Hotspots were seen to be labile, probably reflecting the state of local stores or their Ca(2+) release channels. When ER stores were emptied of Ca(2+) by thapsigargin, [Ca(2+)] transients in dendrites were greatly reduced and unaffected by the removal of [Na(+)](o) implying that even when Na(+)-Ca(2+) exchange is working normally, the majority of the [Ca(2+)](i) increase by depolarization is due to internal release rather than influx across the plasma membrane. Na(+)-Ca(2+) exchange has an important role in controlling [Ca(2+)] dynamics in amacrine cell dendrites chiefly by moderating the positive feedback of the Ca(2+) amplifier.

Na/Ca exchange in function, growth, and demise of beta-cells

Recent knowledge concerning the Na/Ca exchanger (NCX) in the pancreatic beta-cell is reviewed. The beta-cell expresses various NCX1 splice variants in a species-specific pattern (NCX1.3 and 1.7 in the rat; NCX1.2, 1.3, and 1.7 in the mouse) and in variable and different proportions. In the rat beta-cell, the exchanger displays a high capacity, accounts for about 70% of Ca(2+) extrusion, and participates in Ca(2+) inflow during membrane depolarization. In the mouse, however, the contribution of the exchanger to Ca(2+) extrusion is more modest, and to Ca(2+) inflow, less evident. The exchanger has a stoichiometry of 3 Na(+) for 1 Ca(2+), is electrogenic, and displays a reversal potential at -20 mV. Although being of low magnitude, the current generated by the exchanger shapes glucose-induced beta-cell electrical activity and intracellular Ca(2+) oscillations. Intracellular Ca(2+) may also trigger apoptosis. For instance, overexpression of the exchanger increases Ca(2+)-dependent and Ca(2+)-independent beta-cell death by apoptosis, a phenomenon resulting from the depletion of ER Ca(2+) stores with subsequent activation of caspase-12. Na/Ca exchange overexpression also reduces beta-cell growth. Hence, the Na/Ca exchanger is a versatile system that appears to play an important role in the function, growth, and demise of the beta-cell.

Overexpression of the Na/Ca exchanger shapes stimulus-induced cytosolic Ca(2+) oscillations in insulin-producing BRIN-BD11 cells

In response to glucose, mouse beta-cells display slow oscillations of the membrane potential and cytosolic free Ca(2+) concentration ([Ca(2+)](i)), whereas rat beta-cells display a staircase increase in these parameters. Mouse and rat islet cells differ also by their level of Na/Ca exchanger (NCX) activity. The view that the inward current generated by Na/Ca exchange shapes stimulus-induced electrical activity and [Ca(2+)](i) oscillations in pancreatic beta-cells was examined in insulin-producing BRIN-BD11 cells overexpressing the Na/Ca exchanger. BRIN-BD11 cells were stably transfected with NCX1.7, one of the exchanger isoforms identified in the beta-cell. Overexpression could be assessed at the mRNA and protein level. Appropriate targeting to the plasma membrane could be assessed by microfluorescence and the increase in Na/Ca exchange activity. In response to K(+), overexpressing cells showed a more rapid increase in [Ca(2+)](i) on membrane depolarization as well as a more rapid decrease of [Ca(2+)](i) on membrane repolarization. In response to glucose and tolbutamide, control BRIN cells showed large amplitude [Ca(2+)](i) oscillations. In contrast, overexpressing cells showed a staircase increase in [Ca(2+)](i) without such large oscillations. Diazoxide-induced membrane hyperpolarization restored large amplitude [Ca(2+)](i) oscillations in overexpressing cells. The present data confirm that Na/Ca exchange plays a significant role in the rat beta-cell [Ca(2+)](i) homeostasis, the exchanger being a versatile system allowing both Ca(2+) entry and outflow. Our data suggest that the current generated by the exchanger shapes stimulus-induced membrane potential and [Ca(2+)](i) oscillations in insulin-secreting cells, with the difference in electrical activity and [Ca(2+)](i) behavior seen in mouse and rat beta-cells resulting in part from a difference in Na/Ca exchange activity between these two cells.

Fast exocytosis with few Ca(2+) channels in insulin-secreting mouse pancreatic B cells

The association of L-type Ca(2+) channels to the secretory granules and its functional significance to secretion was investigated in mouse pancreatic B cells. Nonstationary fluctuation analysis showed that the B cell is equipped with <500 alpha1(C) L-type Ca(2+) channels, corresponding to a Ca(2+) channel density of 0.9 channels per microm(2). Analysis of the kinetics of exocytosis during voltage-clamp depolarizations revealed an early component that reached a peak rate of 1.1 pFs(-1) (approximately 650 granules/s) 25 ms after onset of the pulse and is completed within approximately 100 ms. This component represents a subset of approximately 60 granules situated in the immediate vicinity of the L-type Ca(2+) channels, corresponding to approximately 10% of the readily releasable pool of granules. Experiments involving photorelease of caged Ca(2+) revealed that the rate of exocytosis was half-maximal at a cytoplasmic Ca(2+) concentration of 17 microM, and concentrations >25 microM are required to attain the rate of exocytosis observed during voltage-clamp depolarizations. The rapid component of exocytosis was not affected by inclusion of millimolar concentrations of the Ca(2+) buffer EGTA but abolished by addition of exogenous L(C753-893), the 140 amino acids of the cytoplasmic loop connecting the 2(nd) and 3(rd) transmembrane region of the alpha1(C) L-type Ca(2+) channel, which has been proposed to tether the Ca(2+) channels to the secretory granules. In keeping with the idea that secretion is determined by Ca(2+) influx through individual Ca(2+) channels, exocytosis triggered by brief (15 ms) depolarizations was enhanced 2.5-fold by the Ca(2+) channel agonist BayK8644 and 3.5-fold by elevating extracellular Ca(2+) from 2.6 to 10 mM. Recordings of single Ca(2+) channel activity revealed that patches predominantly contained no channels or many active channels. We propose that several Ca(2+) channels associate with a single granule thus forming a functional unit. This arrangement is important in a cell with few Ca(2+) channels as it ensures maximum usage of the Ca(2+) entering the cell while minimizing the influence of stochastic variations of the Ca(2+) channel activity.

Metabolic, cationic and secretory response to D-glucose in depolarized and Ca(2+)-deprived rat islets exposed to diazoxide

D-glucose stimulates insulin release from islets exposed to both diazoxide, to activate ATP-responsive K+ channels, and a high concentration of K+, to cause depolarization of the B-cell plasma membrane. Under these conditions, the insulinotropic action of D-glucose is claimed to occur despite unaltered cytosolic Ca2+ concentration, but no information is so far available on the changes in Ca2+ fluxes possibly caused by the hexose. In the present experiments, we investigated the effect of D-glucose upon 45Ca efflux from islets exposed to both diazoxide and high K+ concentrations. In the presence of diazoxide and at normal extracellular Ca2+ concentration, D-glucose (16.7 mmol/l) inhibited insulin release at 5 mmol/l K+, but stimulated insulin release of 90 mmol/l K+. In both cases, the hexose inhibited 45Ca outflow. In the presence of diazoxide, but absence of Ca2+, D-glucose (8.3 to 25.0 mmol/l) first caused a rapid decrease in insulin output followed by a progressive increase in secretory rate. This phenomenon was observed both at 5 mmol/l or higher concentrations (30, 60 and 90 mmol/l) of extracellular K+. It coincided with a monophasic decrease in 45Ca efflux and either a transient (at 5 mmol/l K+) or sustained (at 90 mmol/l K+) decrease in overall cytosolic Ca2+ concentration. The decrease in 45Ca efflux could be due to inhibition of Na(+)-Ca2+ countertransport with resulting localized Ca2+ accumulation in the cell web of insulin-producing cells. A comparable process may be involved in the secretory response to D-glucose in islets exposed to diazoxide and a high concentration of K+ in the presence of extracellular Ca2+.

Effect of Na/Ca exchange on plateau fraction and [Ca]i in models for bursting in pancreatic beta-cells

In the presence of an insulinotropic glucose concentration, beta-cells, in intact pancreatic islets, exhibit periodic bursting electrical activity consisting of an alternation of active and silent phases. The fraction of time spent in the active phase over a period is called the plateau fraction and is correlated with the rate of insulin release. However, the mechanisms that regulate the plateau fraction remain unclear. In this paper we investigate the possible role of the plasma membrane Na+/Ca2+ exchange of the beta-cell in controlling the plateau fraction. We have extended different single-cell models to incorporate this Ca2+-activated electrogenic Ca2+ transporter. We find that the Na+/Ca2+ exchange can provide a physiological mechanism to increase the plateau fraction as the glucose concentration is raised. In addition, we show theoretically that the Na+/Ca2+ exchanger is a key regulator of the cytoplasmic calcium concentration in clusters of heterogeneous cells with gap-junctional electrical coupling.