Bursting electrical activity in pancreatic beta-cells: evidence that the channel underlying the burst is sensitive to Ca2+ influx through L-type Ca2+ channels

Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance

Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.

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.

Electrophysiology of islet cells

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

Regulation by glucose of oscillatory electrical activity and 5-HT/insulin release from single mouse pancreatic islets in absence of functional K(ATP) channels

The glucose sensitivity of bursting electrical activity and pulsatile insulin release from pancreatic islets was determined in absence of functional K(ATP) channels. Membrane potential, [Ca(2+)](i) and 5-HT/insulin release were measured by intracellular recording, fura-2 fluorescence and 5-HT amperometry, respectively. Single mouse islets, bathed in tolbutamide or glibenclamide and high extracellular Ca(2+) (Ca(2+)(o)), displayed bursting activity and concomitant fast [Ca(2+)](i) and 5-HT/insulin oscillations. Sulphonylurea block of K(ATP) channel current was unaffected by raising Ca(2+)(o). Raising glucose or alpha-ketoisocaproic acid (KIC) concentration from 3 to 30 mM increased spiking activity and burst plateau duration. Staurosporine did not impair glucose potentiation of electrical activity, ruling out the involvement of serine/threonine kinases. Glucose enhanced both [Ca(2+)](i) and 5-HT/insulin oscillatory activity, causing a approximately 3-fold increase in overall 5-HT release rate. Cells lacking bursting activity in high Ca(2+)(o) and low glucose (or KIC) developed a pattern of intensified spiking in response to 11 mM glucose. It is concluded that beta-cells exhibit graded oscillatory electrical and secretory responses to glucose in absence of functional K(ATP) channels. This suggests that, under physiological conditions, early glucose sensing may involve other channels besides the K(ATP) channel.

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

AIMS/HYPOTHESIS

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

METHODS

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

RESULTS

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

CONCLUSIONS/INTERPRETATION

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

Impaired insulin secretion and glucose tolerance in beta cell-selective Ca(v)1.2 Ca2+ channel null mice

Insulin is secreted from pancreatic beta cells in response to an elevation of cytoplasmic Ca(2+) resulting from enhanced Ca(2+) influx through voltage-gated Ca(2+) channels. Mouse beta cells express several types of Ca(2+) channel (L-, R- and possibly P/Q-type). beta cell-selective ablation of the gene encoding the L-type Ca(2+) channel subtype Ca(v)1.2 (betaCa(v)1.2(-/-) mouse) decreased the whole-cell Ca(2+) current by only approximately 45%, but almost abolished first-phase insulin secretion and resulted in systemic glucose intolerance. These effects did not correlate with any major effects on intracellular Ca(2+) handling and glucose-induced electrical activity. However, high-resolution capacitance measurements of exocytosis in single beta cells revealed that the loss of first-phase insulin secretion in the betaCa(v)1.2(-/-) mouse was associated with the disappearance of a rapid component of exocytosis reflecting fusion of secretory granules physically attached to the Ca(v)1.2 channel. Thus, the conduit of Ca(2+) entry determines the ability of the cation to elicit secretion.

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.

Calcium-activated K+ channels of mouse beta-cells are controlled by both store and cytoplasmic Ca2+: experimental and theoretical studies

A novel calcium-dependent potassium current (K(slow)) that slowly activates in response to a simulated islet burst was identified recently in mouse pancreatic beta-cells (Göpel, S.O., T. Kanno, S. Barg, L. Eliasson, J. Galvanovskis, E. Renström, and P. Rorsman. 1999. J. Gen. Physiol. 114:759-769). K(slow) activation may help terminate the cyclic bursts of Ca(2+)-dependent action potentials that drive Ca(2+) influx and insulin secretion in beta-cells. Here, we report that when [Ca(2+)](i) handling was disrupted by blocking Ca(2+) uptake into the ER with two separate agents reported to block the sarco/endoplasmic calcium ATPase (SERCA), thapsigargin (1-5 microM) or insulin (200 nM), K(slow) was transiently potentiated and then inhibited. K(slow) amplitude could also be inhibited by increasing extracellular glucose concentration from 5 to 10 mM. The biphasic modulation of K(slow) by SERCA blockers could not be explained by a minimal mathematical model in which [Ca(2+)](i) is divided between two compartments, the cytosol and the ER, and K(slow) activation mirrors changes in cytosolic calcium induced by the burst protocol. However, the experimental findings were reproduced by a model in which K(slow) activation is mediated by a localized pool of [Ca(2+)] in a subspace located between the ER and the plasma membrane. In this model, the subspace [Ca(2+)] follows changes in cytosolic [Ca(2+)] but with a gradient that reflects Ca(2+) efflux from the ER. Slow modulation of this gradient as the ER empties and fills may enhance the role of K(slow) and [Ca(2+)] handling in influencing beta-cell electrical activity and insulin secretion.

Modulation of Ca(2+) signaling by K(+) channels in a hypothalamic neuronal cell line (GT1-1)

The pulsatile release of gonadotropin releasing hormone (GnRH) is driven by the intrinsic activity of GnRH neurons, which is characterized by bursts of action potentials correlated with oscillatory increases in intracellular Ca(2+). The role of K(+) channels in this spontaneous activity was studied by examining the effects of commonly used K(+) channel blockers on K(+) currents, spontaneous action currents, and spontaneous Ca(2+) signaling. Whole-cell recordings of voltage-gated outward K(+) currents in GT1-1 neurons revealed at least two different components of the current. These included a rapidly activating transient component and a more slowly activating, sustained component. The transient component could be eliminated by a depolarizing prepulse or by bath application of 1.5 mM 4-aminopyridine (4-AP). The sustained component was partially blocked by 2 mM tetraethylammonium (TEA). GT1-1 cells also express inwardly rectifying K(+) currents (I(K(IR))) that were activated by hyperpolarization in the presence of elevated extracellular K(+). These currents were blocked by 100 microM Ba(2+) and unaffected by 2 mM TEA or 1.5 mM 4-AP. TEA and Ba(2+) had distinct effects on the pattern of action current bursts and the resulting Ca(2+) oscillations. TEA increased action current burst duration and increased the amplitude of Ca(2+) oscillations. Ba(2+) caused an increase in the frequency of action current bursts and Ca(2+) oscillations. These results indicate that specific subtypes of K(+) channels in GT1-1 cells can have distinct roles in the amplitude modulation or frequency modulation of Ca(2+) signaling. K(+) current modulation of electrical activity and Ca(2+) signaling may be important in the generation of the patterns of cellular activity responsible for the pulsatile release of GnRH.

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.

The phantom burster model for pancreatic beta-cells

Pancreatic beta-cells exhibit bursting oscillations with a wide range of periods. Whereas periods in isolated cells are generally either a few seconds or a few minutes, in intact islets of Langerhans they are intermediate (10-60 s). We develop a mathematical model for beta-cell electrical activity capable of generating this wide range of bursting oscillations. Unlike previous models, bursting is driven by the interaction of two slow processes, one with a relatively small time constant (1-5 s) and the other with a much larger time constant (1-2 min). Bursting on the intermediate time scale is generated without need for a slow process having an intermediate time constant, hence phantom bursting. The model suggests that isolated cells exhibiting a fast pattern may nonetheless possess slower processes that can be brought out by injecting suitable exogenous currents. Guided by this, we devise an experimental protocol using the dynamic clamp technique that reliably elicits islet-like, medium period oscillations from isolated cells. Finally, we show that strong electrical coupling between a fast burster and a slow burster can produce synchronized medium bursting, suggesting that islets may be composed of cells that are intrinsically either fast or slow, with few or none that are intrinsically medium.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Control of pulsatile 5-HT/insulin secretion from single mouse pancreatic islets by intracellular calcium dynamics

1. Glucose-induced insulin release from single islets of Langerhans is pulsatile. We have investigated the correlation between changes in cytosolic free calcium concentration ([Ca2+]i) and oscillatory insulin secretion from single mouse islets, in particular examining the basis for differences in secretory responses to intermediate and high glucose concentrations. Insulin release was monitored in real time through the amperometric detection of the surrogate insulin marker 5-hydroxytryptamine (5-HT) via carbon fibre microelectrodes. The [Ca2+]i was simultaneously recorded by whole-islet fura-2 microfluorometry. 2. In 82 % of the experiments, exposure to 11 mM glucose evoked regular high-frequency (average, 3.4 min-1) synchronous oscillations in amperometric current and [Ca2+]i. In the remaining experiments (18 %), 11 mM glucose induced an oscillatory pattern consisting of high-frequency [Ca2+]i oscillations that were superimposed on low-frequency (average, 0.32 min-1) [Ca2+]i waves. Intermittent high-frequency [Ca2+]i oscillations gave rise to a similar pattern of pulsatile 5-HT release. 3. Raising the glucose concentration from 11 to 20 mM increased the duration of the steady-state [Ca2+]i oscillations without increasing their amplitude. In contrast, both the duration and amplitude of the associated 5-HT transients were increased by glucose stimulation. The amount of 5-HT released per secretion cycle was linearly related to the duration of the underlying [Ca2+]i oscillations in both 11 and 20 mM glucose. The slopes of the straight lines were identical, indicating that there is no significant difference between the ability of calcium oscillations to elicit 5-HT/insulin release in 11 and 20 mM glucose. 4. In situ 5-HT microamperometry has the potential to resolve the high-frequency oscillatory component of the second phase of glucose-induced insulin secretion. This component appears to reflect primarily the duration of the underlying [Ca2+]i oscillations, suggesting that glucose metabolism and/or access to glucose metabolites is not rate limiting to fast pulsatile insulin release.

A minimum of fuel is necessary for tolbutamide to mimic the effects of glucose on electrical activity in pancreatic beta-cells

Glucose stimulation of pancreatic beta-cells triggers electrical activity (slow waves of membrane potential with superimposed spikes) that is best monitored with intracellular microelectrodes. Closure of ATP-sensitive K+ channels underlies the depolarization to the threshold potential and participates in the increase in electrical activity produced by suprathreshold (>7 mM) concentrations of glucose, but it is still unclear whether this is the sole mechanism of control. This was investigated by testing whether blockade of ATP-sensitive K+ channels by low concentrations of tolbutamide is able to mimic the effects of glucose on mouse beta-cell electrical activity even in the absence of the sugar. The response to tolbutamide was influenced by the duration of the perifusion with the low glucose medium. Tolbutamide (25 microM) caused a rapid and sustained depolarization with continuous activity after 6 min of perifusion of the islet with 3 mM glucose, and a progressive depolarization with slow waves of the membrane potential after 20 min. In the absence of glucose, the beta-cell response to tolbutamide was a transient phase of depolarization with rare slow waves (6 min) or a silent, small, but sustained, depolarization (20 min). Readministration of 3 mM glucose was sufficient to restore slow waves, whereas an increase in the glucose concentration to 5 and 7 mM was followed by a lengthening of the slow waves and a shortening of the intervals. In contrast, induction of slow waves by tolbutamide proved very difficult in the absence of glucose, because the beta-cell membrane tended to depolarize from a silent level to the plateau level, at which electrical activity is continuous. Azide, a mitochondrial poison, abrogated the electrical activity induced by tolbutamide in the absence of glucose, which demonstrates the influence of the metabolism of endogenous fuels on the response to the sulfonylurea. The partial repolarization that azide also produced was reversed by increasing the concentration of tolbutamide, but reappearance of the spikes required the addition of glucose. It is concluded that inhibition of ATP-sensitive K+ channels is not the only mechanism by which glucose controls electrical activity in beta-cells.

Effects of extracellular calcium on electrical bursting and intracellular and luminal calcium oscillations in insulin secreting pancreatic beta-cells

The extracellular calcium concentration has interesting effects on bursting of pancreatic beta-cells. The mechanism underlying the extracellular Ca2+ effect is not well understood. By incorporating a low-threshold transient inward current to the store-operated bursting model of Chay, this paper elucidates the role of the extracellular Ca2+ concentration in influencing electrical activity, intracellular Ca2+ concentration, and the luminal Ca2+ concentration in the intracellular Ca2+ store. The possibility that this inward current is a carbachol-sensitive and TTX-insensitive Na+ current discovered by others is discussed. In addition, this paper explains how these three variables respond when various pharmacological agents are applied to the store-operated model.

The role of plasma membrane K+ and Ca2+ permeabilities for glucose induction of slow Ca2+ oscillations in pancreatic beta-cells

In individual pancreatic beta-cells the rise of the cytoplasmic Ca2+ concentration ([Ca2+]i), induced by 11 mM glucose, is manifested either as oscillations (0.2-0.5 min-1) or as a sustained elevation. The significance of the plasma membrane permeability of Ca2+ and K- for the establishment of these slow oscillations was investigated by dual wavelength microfluorometric measurements of [Ca2+]i in individual ob/ob mouse beta-cells loaded with fura-2. Increasing the extracellular Ca2+ to 10 mM or the addition of Ca2+ channel agonist BAY K 8644 (1 microM) or K+ channel blocker tetraethylammonium+ (TEA: 10-20 mM) caused steeper rises and higher peaks of the glucose-induced oscillations. However, when extracellular Ca2+ was lowered to 0.5 mM the oscillations were transformed into a sustained suprabasal level. When the beta-cells exhibited glucose-stimulated sustained elevation of [Ca2+]i in the presence of a physiological Ca2+ concentration (1.3 mM), it was possible to induce slow oscillations by promoting the entry of Ca2+ either by raising the extracellular Ca2+ concentration to 10 mM or adding TEA or BAY K 8644. The results indicate that glucose-induced slow oscillations of [Ca2+]i depend on the closure of ATP-regulated K+ channels and require that the rate of Ca2+ influx exceeds a critical level. Apart from an inherent periodicity in ATP production it is proposed that Ca(2+)-induced ATP consumption in the submembrane space contributes to the cyclic changes of the membrane potential determining periodic entry of Ca2+.

Contributions of modeling to understanding stimulus-secretion coupling in pancreatic beta-cells

Mechanisms of ionic control of insulin secretion in beta-cells of the pancreatic islets of Langerhans are reviewed. The focus is on aspects that have been treated by mathematical models, especially those related to bursting electrical activity. The study of these mechanisms is difficult because of the need to consider ionic fluxes, calcium handling, metabolism, and electrical coupling with other cells in the islet. The data come either from islets, where experimental maneuvers tend to have multiple effects, or from isolated cells, which have degraded electrical activity and secretory sensitivity. Modeling aids in the process by integrating data on individual components such as channels and calcium handling and testing hypotheses for coherence and quantitative plausibility. The study of a variety of models has led to some general mathematical results that have yielded qualitative model-independent insights.

Oscillations of cytosolic free calcium in bombesin-stimulated HIT-T15 cells

The mechanism underlying the generation of cytosolic free Ca2+ ([Ca2+]i) oscillations by bombesin, a receptor agonist activating phospholipase C, in insulin secreting HIT-T15 cells was investigated. At 25 microM, 61% of cells displayed [Ca2+]i oscillations with variable patterns. The bombesin-induced [Ca2+]i oscillations could last more than 1 h and glucose was required for maintaining these [Ca2+]i fluctuations. Bombesin-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+ entry and were attenuated by membrane hyperpolarization or by L-type Ca2+ channel blockers. These [Ca2+]i oscillations were apparently not associated with fluctuations in plasma membrane Ca2+ permeability as monitored by the Mn2+ quenching technique. 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) and 4-chloro-m-cresol, which interfere with intracellular Ca2+ stores, respectively, by inhibiting Ca(2+)-ATPase of endoplasmic reticulum and by affecting Ca(2+)-induced Ca2+ release, disrupted bombesin-induced [Ca2+]i oscillations. 4-chloro-m-cresol raised [Ca2+]i by mobilizing an intracellular Ca2+ pool, an effect not altered by ryanodine. Caffeine exerted complex actions on [Ca2+]i. It raised [Ca2+]i by promoting Ca2+ entry while inhibiting bombesin-elicited [Ca2+]i oscillations. Our results suggest that in bombesin-elicited [Ca2+]i oscillations in HIT-T15 cells: (i) the oscillations originate primarily from intracellular Ca2+ stores; and (ii) the Ca2+ influx required for maintaining the oscillations is in part membrane potential-sensitive and not coordinated with [Ca2+]i oscillations. The interplay between intracellular Ca2+ stores and voltage-sensitive and voltage-insensitive extracellular Ca2+ entry determines the [Ca2+]i oscillations evoked by bombesin.

A possible role of the ATP-sensitive potassium ion channel in determining the duration of spike-bursts in mouse pancreatic beta-cells

The pancreatic beta-cell displays an electrical activity consisting of spike bursts and silent phases at glucose concentrations of about 10 mM. The mechanism of initial depolarization induced by glucose is well defined. However, the mechanism inducing the silent phase has not been fully elucidated. In the present study, the possibility of involvement of ATP-sensitive K+ channels in repolarization was examined using the patch-clamp technique in the cell-attached recording configuration. Ouabain (0.1 mM), an inhibitor of Na+/K+-ATPase, caused a complete suppression of ATP-sensitive K+ channel activity followed by typical biphasic current deflections, which were due to action potentials. The channel activity was also inhibited by removal of K+ from a perifusion solution. Furthermore, the activity of ATP-sensitive K+ channels was markedly inhibited either by replacement of external NaCl with LiCl or by addition of amiloride (0.2 mM), a blocker of Na+/H+ antiport. Addition of L-type Ca2+ channel blockers such as Nifedipine for Mn2+ induced the complete suppression of K+ channel activity. These findings strongly suggest that a fall in ATP consumption results in sustained depolarization, and that the repolarizations interposed between spike-bursts under normal ionic conditions are due to the periodical fall of ATP concentration brought about by periodical acceleration of ATP consumption at Na+/K+-pumps. It is concluded that the elevation of intracellular Na+ concentration as a consequence of accelerated Na+/Ca2+-countertransport during the period of spike-burst enhances ATP consumption, leading to a fall in ATP concentration which is responsible for termination of spike-burst and initiation of repolarization.

Single-cell fura-2 microfluorometry reveals different purinoceptor subtypes coupled to Ca2+ influx and intracellular Ca2+ release in bovine adrenal chromaffin and endothelial cells

ATP and adenosine(5')tetraphospho(5')adenosine (Ap4A), released from adrenal chromaffin cells, are potent stimulators of endothelial cell function. Using single-cell fura-2 fluorescence recording techniques to measure free cytosolic Ca2+ concentration ([Ca2+]i), we have investigated the role of purinoceptor subtypes in the activation of cocultured chromaffin and endothelial cells. ATP evoked concentration-dependent [Ca2+]i rises (EC50 = 3.8 microM) in a subpopulation of chromaffin cells. Both ATP-sensitive and -insensitive cells were potently activated by nicotine, bradykinin and muscarine. Reducing extracellular free Ca2+ concentration to around 100 nM suppressed the [Ca2+]i transient evoked by ATP but not the [Ca2+]i response to bradykinin. ATP-sensitive chromaffin cells were also potently stimulated by 2-methylthioadenosine triphosphate (2MeSATP; EC50 = 12.5 microM) and UTP, but did not respond to either adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), a P2Y receptor agonist, adenosine 5'-[alpha,beta-methylene]triphosphate (pp-[CH2]pA), a P2X agonist or AMP. Adrenal endothelial cells displayed concentration-dependent [Ca2+]i responses when stimulated with ATP (EC50 = 0.86 microM), UTP (EC50 = 1.6 microM) and 2MeSATP (EC50 = 0.38 microM). 2MeSATP behaved as a partial agonist. Ap4A and ADP[beta S] also raised the [Ca2+]i in endothelial cells, whereas AMP and pp[CH2]pA were ineffective. Lowering extracellular free Ca2+ to around 100 nM did not affect the peak ATP-evoked [Ca2+]i rise in these cells. It is concluded that different purinoceptor subtypes are heterogeneously distributed among the major cell types of the adrenal medulla. An intracellular Ca(2+)-releasing P2U-type purinoceptor is specifically localized to adrenal endothelial cells, while a subpopulation of chromaffin cells expresses a non-P2X, non-P2Y subtype exclusively coupled to Ca2+ influx.