Carbachol induces sustained glucose-dependent oscillations of cytoplasmic Ca2+ in hyperpolarized pancreatic beta cells

Indicator-dependent differences in detection of local intracellular Ca2+ release events evoked by voltage-gated Ca2+ entry in pancreatic β-cells

Genetically encoded Ca²⁺ indicators have become widely used in cell signalling studies as they offer advantages over cell-loaded dye indicators in enabling specific cellular or subcellular targeting. Comparing responses from dye and protein-based indicators may provide information about indicator properties and cell physiology, but side-by-side recordings in cells are scarce. In this study, we compared cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) changes in insulin-secreting β-cells recorded with commonly used dyes and indicators based on circularly permuted fluorescent proteins. Total internal reflection fluorescence (TIRF) imaging of K⁺ depolarization-triggered submembrane [Ca²⁺]_i increases showed that the dyes Fluo-4 and Fluo-5F mainly reported stable [Ca²⁺]_i elevations, whereas the proteins R-GECO1 and GCaMP5G more often reported distinct [Ca²⁺]_i spikes from an elevated level. [Ca²⁺]_i spiking occurred also in glucose-stimulated cells. The spikes reflected Ca²⁺ release from the endoplasmic reticulum, triggered by autocrine activation of purinergic receptors after exocytotic release of ATP and/or ADP, and the spikes were consequently prevented by SERCA inhibition or P2Y₁-receptor antagonism. Widefield imaging, which monitors the entire cytoplasm, increased the spike detection by the Ca²⁺ dyes. The indicator-dependent response patterns were unrelated to Ca²⁺ binding affinity, buffering and mobility, and probably reflects the much slower dissociation kinetics of protein compared to dye indicators. Ca²⁺ dyes thus report signalling within the submembrane space excited by TIRF illumination, whereas the protein indicators also catch Ca²⁺ events originating outside this volume. The study highlights that voltage-dependent Ca²⁺ entry in β-cells is tightly linked to local intracellular Ca²⁺ release mediated via an autocrine route that may be more important than previously reported direct Ca²⁺ effects on phospholipase C or on intracellular channels mediating calcium-induced calcium release.

Glucose control of glucagon secretion-'There's a brand-new gimmick every year'

Glucagon from the pancreatic α-cells is a major blood glucose-regulating hormone whose most important role is to prevent hypoglycaemia that can be life-threatening due to the brain's strong dependence on glucose as energy source. Lack of blood glucose-lowering insulin after malfunction or autoimmune destruction of the pancreatic β-cells is the recognized cause of diabetes, but recent evidence indicates that diabetic hyperglycaemia would not develop unless lack of insulin was accompanied by hypersecretion of glucagon. Glucagon release has therefore become an increasingly important target in diabetes management. Despite decades of research, an understanding of how glucagon secretion is regulated remains elusive, and fundamentally different mechanisms continue to be proposed. The autonomous nervous system is an important determinant of glucagon release, but it is clear that secretion is also directly regulated within the pancreatic islets. The present review focuses on pancreatic islet mechanisms involved in glucose regulation of glucagon release. It will be argued that α-cell-intrinsic processes are most important for regulation of glucagon release during recovery from hypoglycaemia and that paracrine inhibition by somatostatin from the δ-cells shapes pulsatile glucagon release in hyperglycaemia. The electrically coupled β-cells ultimately determine islet hormone pulsatility by releasing synchronizing factors that affect the α- and δ-cells.

Muscarinic control of MIN6 pancreatic β cells is enhanced by impaired amino acid signaling

We have shown recently that the class C G protein-coupled receptor T1R1/T1R3 taste receptor complex is an early amino acid sensor in MIN6 pancreatic β cells. Amino acids are unable to activate ERK1/2 in β cells in which T1R3 has been depleted. The muscarinic receptor agonist carbachol activated ERK1/2 better in T1R3-depleted cells than in control cells. Ligands that activate certain G protein-coupled receptors in pancreatic β cells potentiate glucose-stimulated insulin secretion. Among these is the M3 muscarinic acetylcholine receptor, the major muscarinic receptor in β cells. We found that expression of M3 receptors increased in T1R3-depleted MIN6 cells and that calcium responses were altered. To determine whether these changes were related to impaired amino acid signaling, we compared responses in cells exposed to reduced amino acid concentrations. M3 receptor expression was increased, and some, but not all, changes in calcium signaling were mimicked. These findings suggest that M3 acetylcholine receptors are increased in β cells as a mechanism to compensate for amino acid deficiency.

Glucose regulation of glucagon secretion

Glucagon secreted by pancreatic α-cells is the major hyperglycemic hormone correcting acute hypoglycaemia (glucose counterregulation). In diabetes the glucagon response to hypoglycaemia becomes compromised and chronic hyperglucagonemia appears. There is increasing awareness that glucagon excess may underlie important manifestations of diabetes. However opinions differ widely how glucose controls glucagon secretion. The autonomous nervous system plays an important role in the glucagon response to hypoglycaemia. But it is clear that glucose controls glucagon secretion also by mechanisms involving direct effects on α-cells or indirect effects via paracrine factors released from non-α-cells within the pancreatic islets. The present review discusses these mechanisms and argues that different regulatory processes are involved in a glucose concentration-dependent manner. Direct glucose effects on the α-cell and autocrine mechanisms are probably most significant for the glucagon response to hypoglycaemia. During hyperglycaemia, when secretion from β- and δ-cells is stimulated, paracrine inhibitory factors generate pulsatile glucagon release in opposite phase to pulsatile release of insulin and somatostatin. High concentrations of glucose have also stimulatory effects on glucagon secretion that tend to balance and even exceed the inhibitory influence. The latter actions might underlie the paradoxical hyperglucagonemia that aggravates hyperglycaemia in persons with diabetes.

cAMP induces stromal interaction molecule 1 (STIM1) puncta but neither Orai1 protein clustering nor store-operated Ca2+ entry (SOCE) in islet cells

The events leading to the activation of store-operated Ca(2+) entry (SOCE) involve Ca(2+) depletion of the endoplasmic reticulum (ER) resulting in translocation of the transmembrane Ca(2+) sensor protein, stromal interaction molecule 1 (STIM1), to the junctions between ER and the plasma membrane where it binds to the Ca(2+) channel protein Orai1 to activate Ca(2+) influx. Using confocal and total internal reflection fluorescence microscopy, we studied redistribution kinetics of fluorescence-tagged STIM1 and Orai1 as well as SOCE in insulin-releasing β-cells and glucagon-secreting α-cells within intact mouse and human pancreatic islets. ER Ca(2+) depletion triggered accumulation of STIM1 puncta in the subplasmalemmal ER where they co-clustered with Orai1 in the plasma membrane and activated SOCE. Glucose, which promotes Ca(2+) store filling and inhibits SOCE, stimulated retranslocation of STIM1 to the bulk ER. This effect was evident at much lower glucose concentrations in α- than in β-cells consistent with involvement of SOCE in the regulation of glucagon secretion. Epinephrine stimulated subplasmalemmal translocation of STIM1 in α-cells and retranslocation in β-cells involving raising and lowering of cAMP, respectively. The cAMP effect was mediated both by protein kinase A and exchange protein directly activated by cAMP. However, the cAMP-induced STIM1 puncta did not co-cluster with Orai1, and there was no activation of SOCE. STIM1 translocation can consequently occur independently of Orai1 clustering and SOCE.

Roles of IP3R and RyR Ca2+ channels in endoplasmic reticulum stress and beta-cell death

OBJECTIVE

Endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of diabetes, but the roles of specific ER Ca(2+) release channels in the ER stress-associated apoptosis pathway remain unknown. Here, we examined the effects of stimulating or inhibiting the ER-resident inositol trisphosphate receptors (IP(3)Rs) and the ryanodine receptors (RyRs) on the induction of beta-cell ER stress and apoptosis.

RESEARCH DESIGN AND METHODS

Kinetics of beta-cell death were tracked by imaging propidium iodide incorporation and caspase-3 activity in real time. ER stress and apoptosis were assessed by Western blot. Mitochondrial membrane potential was monitored by flow cytometry. Cytosolic Ca(2+) was imaged using fura-2, and genetically encoded fluorescence resonance energy transfer (FRET)-based probes were used to measure Ca(2+) in ER and mitochondria.

RESULTS

Neither RyR nor IP(3)R inhibition, alone or in combination, caused robust death within 24 h. In contrast, blocking sarco/endoplasmic reticulum ATPase (SERCA) pumps depleted ER Ca(2+) and induced marked phosphorylation of PKR-like ER kinase (PERK) and eukaryotic initiation factor-2alpha (eIF2alpha), C/EBP homologous protein (CHOP)-associated ER stress, caspase-3 activation, and death. Notably, ER stress following SERCA inhibition was attenuated by blocking IP(3)Rs and RyRs. Conversely, stimulation of ER Ca(2+) release channels accelerated thapsigargin-induced ER depletion and apoptosis. SERCA block also activated caspase-9 and induced perturbations of the mitochondrial membrane potential, resulting eventually in the loss of mitochondrial polarization.

CONCLUSIONS

This study demonstrates that the activity of ER Ca(2+) channels regulates the susceptibility of beta-cells to ER stress resulting from impaired SERCA function. Our results also suggest the involvement of mitochondria in beta-cell apoptosis associated with dysfunctional beta-cell ER Ca(2+) homeostasis and ER stress.

Oscillatory control of insulin secretion

Pancreatic beta-cells possess an inherent ability to generate oscillatory signals that trigger insulin release. Coordination of the secretory activity among beta-cells results in pulsatile insulin secretion from the pancreas, which is considered important for the action of the hormone in the target tissues. This review focuses on the mechanisms underlying oscillatory control of insulin secretion at the level of the individual beta-cell. Recent studies have demonstrated that oscillations of the cytoplasmic Ca(2+) concentration are synchronized with oscillations in beta-cell metabolism, intracellular cAMP concentration, phospholipase C activity and plasma membrane phosphoinositide lipid concentrations. There are complex interdependencies between the different messengers and signalling pathways that contribute to amplitude regulation and shaping of the insulin secretory response to nutrient stimuli and neurohormonal modulators. Several of these pathways may be important pharmacological targets for improving pulsatile insulin secretion in type 2 diabetes.

Feedback activation of phospholipase C via intracellular mobilization and store-operated influx of Ca2+ in insulin-secreting β-cells

Phospholipase C (PLC) regulates various cellular processes by catalyzing the formation of inositol-1,4,5-trisphosphate (IP3) and diacylglycerol from phosphatidylinositol-4,5-bisphosphate (PIP2). Here, we have investigated the influence of Ca2+ on receptor-triggered PLC activity in individual insulin-secreting β-cells. Evanescent wave microscopy was used to record PLC activity using green fluorescent protein (GFP)-tagged PIP2/IP3-binding pleckstrin homology domain from PLCδ1, and the cytoplasmic Ca2+ concentration ([Ca2+]i) was simultaneously measured using the indicator Fura Red. Stimulation of MIN6 β-cells with the muscarinic-receptor agonist carbachol induced rapid and sustained PLC activation. By contrast, only transient activation was observed after stimulation in the absence of extracellular Ca2+ or in the presence of the non-selective Ca2+ channel inhibitor La3+. The Ca2+-dependent sustained phase of PLC activity did not require voltage-gated Ca2+ influx, as hyperpolarization with diazoxide or direct Ca2+ channel blockade with nifedipine had no effect. Instead, the sustained PLC activity was markedly suppressed by the store-operated channel inhibitors 2-APB and SKF96365. Depletion of intracellular Ca2+ stores with the sarco(endo)plasmic reticulum Ca2+-ATPase inhibitors thapsigargin or cyclopiazonic acid abolished Ca2+ mobilization in response to carbachol, and strongly suppressed the PLC activation in Ca2+-deficient medium. Analogous suppressions were observed after loading cells with the Ca2+ chelator BAPTA. Stimulation of primary mouse pancreatic β-cells with glucagon elicited pronounced [Ca2+]i spikes, reflecting protein kinase A-mediated activation of Ca2+-induced Ca2+ release via IP3 receptors. These [Ca2+]i spikes were found to evoke rapid and transient activation of PLC. Our data indicate that receptor-triggered PLC activity is enhanced by positive feedback from Ca2+ entering the cytoplasm from intracellular stores and via store-operated channels in the plasma membrane. Such amplification of receptor signalling should be important in the regulation of insulin secretion by hormones and neurotransmitters.

Ca2+-induced Ca2+ release by activation of inositol 1,4,5-trisphosphate receptors in primary pancreatic beta-cells

The effect of sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibition on the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was studied in primary insulin-releasing pancreatic beta-cells isolated from mice, rats and human subjects as well as in clonal rat insulinoma INS-1 cells. In Ca(2+)-deficient medium the individual primary beta-cells reacted to the SERCA inhibitor cyclopiazonic acid (CPA) with a slow rise of [Ca(2+)](i) followed by an explosive transient elevation. The [Ca(2+)](i) transients were preferentially observed at low intracellular concentrations of the Ca(2+) indicator fura-2 and were unaffected by pre-treatment with 100 microM ryanodine. Whereas 20mM caffeine had no effect on basal [Ca(2+)](i) or the slow rise in response to CPA, it completely prevented the CPA-induced [Ca(2+)](i) transients as well as inositol 1,4,5-trisphosphate-mediated [Ca(2+)](i) transients in response to carbachol. In striking contrast to the primary beta-cells, caffeine readily mobilized intracellular Ca(2+) in INS-1 cells under identical conditions, and such mobilization was prevented by ryanodine pre-treatment. The results indicate that leakage of Ca(2+) from the endoplasmic reticulum after SERCA inhibition is feedback-accelerated by Ca(2+)-induced Ca(2+) release (CICR). In primary pancreatic beta-cells this CICR is due to activation of inositol 1,4,5-trisphosphate receptors. CICR by ryanodine receptor activation may be restricted to clonal beta-cells.

Ca(2+)-induced Ca(2+) release via inositol 1,4,5-trisphosphate receptors is amplified by protein kinase A and triggers exocytosis in pancreatic beta-cells

Hormones, such as glucagon and glucagon-like peptide-1, potently amplify nutrient stimulated insulin secretion by raising cAMP. We have studied how cAMP affects Ca(2+)-induced Ca(2+) release (CICR) in pancreatic beta-cells from mice and rats and the role of CICR in secretion. CICR was observed as pronounced Ca(2+) spikes on top of glucose- or depolarization-dependent rise of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). cAMP-elevating agents strongly promoted CICR. This effect involved sensitization of the receptors underlying CICR, because many cells exhibited the characteristic Ca(2+) spiking at low or even in the absence of depolarization-dependent elevation of [Ca(2+)](i). The cAMP effect was mimicked by a specific activator of protein kinase A in cells unresponsive to activators of cAMP-regulated guanine nucleotide exchange factor. Ryanodine pretreatment, which abolishes CICR mediated by ryanodine receptors, did not prevent CICR. Moreover, a high concentration of caffeine, known to activate ryanodine receptors independently of Ca(2+), failed to mobilize intracellular Ca(2+). On the contrary, a high caffeine concentration abolished CICR by interfering with inositol 1,4,5-trisphosphate receptors (IP(3)Rs). Therefore, the cell-permeable IP(3)R antagonist 2-aminoethoxydiphenyl borate blocked the cAMP-promoted CICR. Individual CICR events in pancreatic beta-cells were followed by [Ca(2+)](i) spikes in neighboring human erythroleukemia cells, used to report secretory events in the beta-cells. The results indicate that protein kinase A-mediated promotion of CICR via IP(3)Rs is part of the mechanism by which cAMP amplifies insulin release.

A store-operated mechanism determines the activity of the electrically excitable glucagon-secreting pancreatic α-cell

The glucagon-releasing pancreatic alpha-cells are electrically excitable cells but the signal transduction leading to depolarization and secretion is not well understood. To clarify the mechanisms we studied [Ca(2+)](i) and membrane potential in individual mouse pancreatic alpha-cells using fluorescent indicators. The physiological secretagogue l-adrenaline increased [Ca(2+)](i) causing a peak, which was often followed by maintained oscillations or sustained elevation. The early effect was due to mobilization of Ca(2+) from the endoplasmic reticulum (ER) and the late one to activation of store-operated influx of the ion resulting in depolarization and Ca(2+) influx through voltage-dependent L-type channels. Consistent with such mechanisms, the effects of adrenaline on [Ca(2+)](i) and membrane potential were mimicked by inhibitors of the sarco(endo)plasmic reticulum Ca(2+) ATPase. The alpha-cells express ATP-regulated K(+) (K(ATP)) channels, whose activation by diazoxide leads to hyperpolarization. The resulting inhibition of the voltage-dependent [Ca(2+)](i) response to adrenaline was reversed when the K(ATP) channels were inhibited by tolbutamide. However, tolbutamide alone rarely affected [Ca(2+)](i), indicating that the K(ATP) channels are normally closed in mouse alpha-cells. Glucose, which is the major physiological inhibitor of glucagon secretion, hyperpolarized the alpha-cells and inhibited the late [Ca(2+)](i) response to adrenaline. At concentrations as low as 3mM, glucose had a pronounced stimulatory effect on Ca(2+) sequestration in the ER amplifying the early [Ca(2+)](i) response to adrenaline. We propose that adrenaline stimulation and glucose inhibition of the alpha-cell involve modulation of a store-operated current, which controls a depolarizing cascade leading to opening of L-type Ca(2+) channels. Such a control mechanism may be unique among excitable cells.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Store-operated influx of Ca2+ in pancreatic β-cells exhibits graded dependence on the filling of the endoplasmic reticulum

The store-operated pathway for Ca2+ entry was studied in individual mouse pancreatic β-cells by measuring the cytoplasmic concentrations of Ca2+ ([Ca2+]i) and Mn2+ ([Mn2+]i) with the fluorescent indicator fura-2. Influx through the store-operated pathway was initially shut off by pre-exposure to 20 mM glucose, which maximally stimulates intracellular Ca2+ sequestration. To avoid interference with voltage-dependent Ca2+ entry the cells were hyperpolarized with diazoxide and the channel blocker methoxyverapamil was present. Activation of the store-operated pathway in response to Ca2+ depletion of the endoplasmic reticulum was estimated from the sustained elevation of [Ca2+]i or from the rate of increase in [Mn2+]i due to influx of these extracellular ions. Increasing concentrations of the inositol 1,4,5-trisphosphate-generating agonist carbachol or the sarco(endo)plasmatic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) cause gradual activation of the store-operated pathway. In addition, the carbachol- and CPA-induced influx of Mn2+ depended on store filling in a graded manner. The store-operated influx of Ca2+/Mn2+ was inhibited by Gd3+ and 2-aminoethoxydiphenyl borate but neither of these agents discriminated between store-operated and voltage-dependent entry. The finely tuned regulation of the store-operated mechanisms in the β-cell has direct implications for the control of membrane potential and insulin secretion.

Direct cytoplasmic CA(2+) responses to gastrin-releasing peptide in single beta cells

The neuropeptide gastrin releasing peptide (GRP) stimulates insulin secretion and induces oscillations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)) in clonal insulinoma cells. It is not known whether GRP affects [Ca(2+)](cyt) in normal beta cells. We investigated, in single, normal, mouse islet beta cells, the effects of GRP on [Ca(2+)](cyt), by dual wavelength spectrophotofluorometry. Beta cells were identified by their typical response to glucose or tolbutamide. At 15 mM glucose, GRP (100 nM) evoked an immediate [Ca(2+)](cyt) transient to 423 +/- 48 nM compared to 126 +/- 18 nM before GRP (P < 0.001). After the initial transient, [Ca(2+)](cyt) exhibited either a sustained elevation or oscillations. At 3.3 mM glucose, in cells with a non-oscillating [Ca(2+)](cyt), GRP stimulated a prompt increase in [Ca(2+)](cyt) (from 60 +/- 6 to 285 +/- 30 nM, P = 0.024) followed by either a sustained increase in [Ca(2+)](cyt) or [Ca(2+)](cyt) oscillations. We conclude that GRP promptly elevates [Ca(2+)](cyt) by a direct action in normal mouse pancreatic beta cells.

Tolbutamide and diazoxide modulate phospholipase C-linked Ca(2+) signaling and insulin secretion in beta-cells

Arginine vasopressin (AVP), bombesin, and ACh increase cytosolic free Ca(2+) and potentiate glucose-induced insulin release by activating receptors linked to phospholipase C (PLC). We examined whether tolbutamide and diazoxide, which close or open ATP-sensitive K(+) channels (K(ATP) channels), respectively, interact with PLC-linked Ca(2+) signals in HIT-T15 and mouse beta-cells and with PLC-linked insulin secretion from HIT-T15 cells. In the presence of glucose, the PLC-linked Ca(2+) signals were enhanced by tolbutamide (3-300 microM) and inhibited by diazoxide (10-100 microM). The effects of tolbutamide and diazoxide on PLC-linked Ca(2+) signaling were mimicked by BAY K 8644 and nifedipine, an activator and inhibitor of L-type voltage-sensitive Ca(2+) channels, respectively. Neither tolbutamide nor diazoxide affected PLC-linked mobilization of internal Ca(2+) or store-operated Ca(2+) influx through non-L-type Ca(2+) channels. In the absence of glucose, PLC-linked Ca(2+) signals were diminished or abolished; this effect could be partly antagonized by tolbutamide. In the presence of glucose, tolbutamide potentiated and diazoxide inhibited AVP- or bombesin-induced insulin secretion from HIT-T15 cells. Nifedipine (10 microM) blocked both the potentiating and inhibitory actions of tolbutamide and diazoxide on AVP-induced insulin release, respectively. In glucose-free medium, AVP-induced insulin release was reduced but was again potentiated by tolbutamide, whereas diazoxide caused no further inhibition. Thus tolbutamide and diazoxide regulate both PLC-linked Ca(2+) signaling and insulin secretion from pancreatic beta-cells by modulating K(ATP) channels, thereby determining voltage-sensitive Ca(2+) influx.

Mobilization of Ca2+ stores in individual pancreatic beta-cells permeabilized or not with digitonin or alpha-toxin

The concentration of free Ca2+ in the cytoplasm and organelles of individual mouse pancreatic beta-cells was estimated with dual wavelength microfluorometry and the indicators Fura-2 and furaptra. Measuring the increase of cytoplasmic Ca2+ resulting from intracellular mobilization of the ion in ob/ob mouse beta-cells, most organelle calcium (92%) was found in acidic compartments released when combining the Ca2+ ionophore Br-A23187 with a protonophore. Only 3-4% of organelle calcium was recovered from a pool sensitive to the Ca(2+)-ATPase inhibitor thapsigargin. Organelle Ca2+ was also measured directly in furaptra-loaded beta-cells after controlled plasma membrane permeabilization. The permeabilizing agent alpha-toxin was superior to digitonin in preserving the integrity of intracellular membranes, but digitonin provided more reproducible access to intracellular sites. After permeabilization, the thapsigargin-sensitive fraction of Ca2+ detected by furaptra was as high as 90%, suggesting that the indicator essentially measures Ca2+ in endoplasmic reticulum (ER). Both alpha-toxin- and digitonin-permeabilized cells exhibited ATP-dependent uptake of Ca2+ into thapsigargin-sensitive stores with half-maximal and maximal filling at 6-11 microM and 1 mM ATP respectively. Most of the thapsigargin-sensitive Ca2+ was mobilized by inositol 1,4,5-trisphosphate (IP3), whereas caffeine, ryanodine, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate lacked effects both in beta-cells from ob/ob mice and normal NMRI mice. Mobilization of organelle Ca2+ by 4-chloro-3-methylphenol was attributed to interference with the integrity of the ER rather than to activation of ryanodine receptors. The observations emphasize the importance of IP3 for Ca2+ mobilization in pancreatic beta-cells, but question a role for ryanodine receptor agonists.

Glucose regulation of free Ca(2+) in the endoplasmic reticulum of mouse pancreatic beta cells

Free Ca(2+) was measured in organelles of individual mouse pancreatic beta cells loaded with the low affinity indicator furaptra. After removal of cytoplasmic indicator by controlled digitonin permeabilization the organelle Ca(2+) was located essentially in the endoplasmic reticulum (ER), >90% being sensitive to inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPases. The Ca(2+) accumulation in the ER of intact beta cells depended in a hyperbolic fashion on the glucose concentration with half-maximal and maximal filling at 5.5 and >20 mM, respectively. Also elevation of cytoplasmic Ca(2+) by K(+) depolarization significantly enhanced the Ca(2+) accumulation. In permeabilized beta cells 1-3 mM ATP caused rapid Ca(2+) filling of the ER reaching almost 500 microM. At 50 nM, Ca(2+) ER became half-maximally filled at 45 microM ATP, whereas only 3.5 microM ATP was required at 200 nM Ca(2+). Inositol 1,4,5-trisphosphate induced a rapid release of about 65% of the ER Ca(2+), and its precursor phosphatidylinositol 4,5-bisphosphate was found to slowly mobilize 75% by another mechanism. It is concluded that glucose is an efficient stimulator of Ca(2+) uptake in the ER of pancreatic beta cells both by increasing ATP and cytoplasmic Ca(2+). Because physiological concentrations of cytoplasmic ATP are in the mM range, Ca(2+) sequestration can be anticipated to be modulated by factors reducing its ATP sensitivity.

Ca2+/calmodulin inhibition and phospholipase C-linked Ca2+ Signaling in clonal beta-cells

Neurotransmitters and hormones, such as arginine vasopressin (AVP) and bombesin, evoke frequency-modulated repetitive Ca2+ transients in insulin-secreting HIT-T15 cells by binding to receptors linked to phospholipase C (PLC). The role of calmodulin (CaM)-dependent mechanisms in the generation of PLC-linked Ca2+ transients was investigated by use of the naphthalenesulfonamide CaM antagonists W-7 and W-13 and their dechlorinated control analogs W-5 and W-12. W-7 (10-30 microM) and W-13 (30-100 microM), but not W-5 (100 microM) and W-12 (300 microM), reversibly inhibited the AVP- and bombesin-induced Ca2+ transients. As the generation of PLC-linked Ca2+ transients requires mobilization of internal Ca2+ and Ca2+ influx through voltage-sensitive (VSCC) and -insensitive (VICC) Ca2+ channels, the effects of the W compounds on these processes were further investigated. First, W-7 dose dependently diminished K+ (45 mM)-induced Ca2+ signals (IC50, approximately 25 microM), and W-13 (100 microM) reduced the K+ (45 mM)-induced [Ca2+]i rise by about 40-60%, whereas W-5 (100 microM) and W-12 (300 microM) had no effect. In addition, W-7 (100 microM) inhibited whole cell Ca2+ currents in mouse beta-cells by about 60%. Second, pretreatment of cells (5 min) with W-7 (30 microM), but not W-5 (30 microM), inhibited agonist-induced internal Ca2+ mobilization by about 75% in Ca2+-free medium. Neither W-7 (30 microM) nor W-5 (30 microM) affected AVP (100 nM)-stimulated formation of IP3. Third, capacitative Ca2+ influx through VICC activated by thapsigargin (2 microM) in the presence of verapamil (50 microM) was inhibited by W-7 (30 microM) but not by W-5 (30 microM). As all of the W compound effects corresponded well to their reported anticalmodulin activity, a specific anticalmodulin action can be assumed. Thus, Ca2+ via activation of CaM-dependent processes could provide positive feedback on the generation of PLC-linked Ca2+ transients in HIT-T15 cells. This appears to involve CaM-dependent regulation of both mobilization of internal Ca2+ and Ca2+ influx through VSCC and VICC.

Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells

1. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets. 2. During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]i oscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min-1 and a less prominent one of 0.27 min-1. 3. In contrast, practically all clusters (2-50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]i was more elevated, [Ca2+]i oscillations were more regular and their major frequency was slightly higher (but reached a plateau at approximately 0.25 min-1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly. 4. Image analysis revealed that the regular [Ca2+]i oscillations were well synchronized between all cells of the clusters. Even when the Ca2+ response was irregular, slow and fast [Ca2+]i oscillations induced by glucose were also synchronous in all cells. 5. In contrast, [Ca2+]i oscillations resulting from mobilization of intracellular Ca2+ by acetylcholine were restricted to certain cells only and were not synchronized. 6. Heptanol and 18alpha-glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose-induced Ca2+ oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells. 7. The results support theoretical models predicting an increased regularity of glucose-dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co-ordination of [Ca2+]i oscillations driven by depolarization-dependent Ca2+ influx during glucose stimulation.

Secretagogues modulate the calcium concentration in the endoplasmic reticulum of insulin-secreting cells. Studies in aequorin-expressing intact and permeabilized ins-1 cells

The precise regulation of the Ca2+ concentration in the endoplasmic reticulum ([Ca2+]er) is important for protein processing and signal transduction. In the pancreatic beta-cell, dysregulation of [Ca2+]er may cause impaired insulin secretion. The Ca2+-sensitive photoprotein aequorin mutated to lower its Ca2+ affinity was stably expressed in the endoplasmic reticulum (ER) of rat insulinoma INS-1 cells. The steady state [Ca2+]er was 267 +/- 9 microM. Both the Ca2+-ATPase inhibitor cyclopiazonic acid and 4-chloro-m-cresol, an activator of ryanodine receptors, caused an almost complete emptying of ER Ca2+. The inositol 1,4,5-trisphosphate generating agonists, carbachol, and ATP, reduced [Ca2+]er by 20-25%. Insulin secretagogues that raise cytosolic [Ca2+] by membrane depolarization increased [Ca2+]er in the potency order K+ >> glucose > leucine, paralleling their actions in the cytosolic compartment. Glucose, which augmented [Ca2+]er by about 25%, potentiated the Ca2+-mobilizing effect of carbachol, explaining the corresponding observation in cytosolic [Ca2+]. The filling of ER Ca2+ by glucose is not directly mediated by ATP production as shown by the continuous monitoring of cytosolic ATP in luciferase expressing cells. Both glucose and K+ increase [Ca2+]er, but only the former generated whereas the latter consumed ATP. Nonetheless, drastic lowering of cellular ATP with a mitochondrial uncoupler resulted in a marked decrease in [Ca2+]er, emphasizing the requirement for mitochondrially derived ATP above a critical threshold concentration. Using alpha-toxin permeabilized cells in the presence of ATP, glucose 6-phosphate did not change [Ca2+]er, invalidating the hypothesis that glucose acts through this metabolite. Therefore, insulin secretagogues that primarily stimulate Ca2+ influx, elevate [Ca2+]er to ensure beta-cell homeostasis.

Cytosolic Ca2+ oscillations by gastrin releasing peptide in single HIT-T15 cells

In single, superfused, FURA-2AM loaded insulin producing HIT-T15 cells, gastrin releasing peptide (GRP) induced a peak in cytoplasmnic Cu2+ ([Ca2+]i) followed by a sustained (high GRP concentrations) or oscillatory (low GRP concentrations) [Ca2+]i pattern. The GRP (25-50 microM)-induced [Ca2+]i oscillations ceased upon removal of glucose or addition of thapsigargin (1 microM), EGTA (2 mM), or diazoxide (200 microM), whereas nifedipine (10 microM) reduced their amplitude (by 35%). Both protein kinase C (PKC)-activation or PKC-inhibition disrupted GRP induced [Ca2+]i oscillations. GRP induced [Ca2+]i oscillations in insulin producing cells therefore rely on intracellular Ca2+ mobilization, voltage-dependent and voltage-independent Ca2+ entry mechanisms and the integrity of protein kinase C.

Calcium signaling in cultured human and rat duodenal enterocytes

Vagal stimuli increase duodenal mucosal HCO-3 secretion and may provide anticipatory protection against acid injury, but duodenal enterocyte (duodenocyte) responses and cholinoceptor selectivity have not been defined. We therefore developed a stable primary culture model of duodenocytes from rats and humans. Brief digestion of scraped rat duodenal mucosa or human biopsies with collagenase/dispase yielded cells that attached to the extracellular matrix Matrigel within a few hours of plating. Columnar cells with villus enterocyte morphology that exhibited spontaneous active movement were evident between 1 and 3 days of culture. Rat duodenocytes loaded with fura 2 responded to carbachol with a transient increase in intracellular calcium concentration ([Ca2+]i), with an apparent EC50 of approximately 3 microM. In a first type of signaling pattern, [Ca2+]i returned to basal or near basal values within 3-5 min. In a second type, observed in cells with enlarged vacuoles characteristic of crypt cell morphology, the initial transient increase was followed by rhythmic oscillations. Human duodenocytes responded with a more sustained increase in [Ca2+]i, and oscillations were not observed. Rat as well as human duodenocytes also responded to CCK-octapeptide but not to vasoactive intestinal polypeptide. Equimolar concentrations (100 nM) of the subtype-independent muscarinic antagonist atropine and the M3 antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide prevented the response to 10 microM carbachol, whereas the M1 antagonist pirenzepine and the M2 antagonists methoctramine and AF-DX 116BS had no effect at similar concentrations. Responses in rat and human duodenocytes were similar. A new agonist-sensitive primary culture model for rat and human duodenocytes has thus been established and the presence of enterocyte CCK and muscarinic M3 receptors demonstrated.

Store-operated Ca2+ entry in insulin-releasing pancreatic beta-cells

The fluorescent indicator Fura-2 was used to characterize the store-operated Ca2+ entry in insulin-releasing pancreatic beta-cells. To avoid interference with voltage-dependent Ca2+ entry, the cells were hyperpolarized with 400 microM diazoxide and the channel blocker methoxyverapamil was also present in some experiments. The cytoplasmic Ca2+ concentration ([Ca2+]j) of hyperpolarized mouse beta-cells was strikingly resistant to changes in external Ca2+. In cells exposed to 20 mM glucose, stimulation with 100 microM carbachol induced an initial [Ca2+]j peak followed by a sustained increase due to store-operated influx of the cation. Store-operated influx was also induced by the intracellular Ca(2+)-ATPase inhibitor thapsigargin. In the presence of store-operated influx, [Ca2+]j became markedly sensitive to variations in external Ca2+, but this sensitivity was blocked by La3+. In beta-cells exposed to both Ca2+ and Mn2+ there was slow Mn2+ quenching of the Fura-2 fluorescence, which was accelerated upon stimulation of store-operated influx. This acceleration was reversed by glucose-stimulated filling of the internal Ca2+ stores. The store-operated Ca2+ entry increased markedly during culture of the beta-cells. Activation of protein kinase C by the phorbol ester 12-O-tetradecanoylphorbol-13 acetate, inhibition of serine/threonine phosphatase by okadaic acid and inhibition of tyrosine kinase by genistein had little effect on the store-operated influx of Ca2+. In beta-cells equilibrated in 5 mM Sr2+, carbachol exposure resulted in a pronounced cytoplasmic Sr2+ ([Sr2+]j) peak due to intracellular mobilization, but little or no sustained elevation. Moreover, after activating the store-operated pathway by exposure to thapsigargin, variations in extracellular Sr2+ between 0-2 mM had only marginal effects on [Sr2+]j. Although the store-operated influx apparently accounts for a minor fraction of the Ca2+ entry, its depolarizing influence may under certain conditions be up-regulated with resulting distortion of the beta-cell function.

Glucose-stimulated insulin secretion correlates with changes in mitochondrial and cytosolic Ca2+ in aequorin-expressing INS-1 cells

Nutrient-stimulated insulin secretion is dependent upon the generation of metabolic coupling factors in the mitochondria of the pancreatic B cell. To investigate the role of Ca2+ in mitochondrial function, insulin secretion from INS-1 cells stably expressing the Ca2+-sensitive photoprotein aequorin in the appropriate compartments was correlated with changes in cytosolic calcium ([Ca2+]c) and mitochondrial calcium ([Ca2+]m). Glucose and KCl, which depolarize the cell membrane, as well as the Ca2+-mobilizing agonist, carbachol (CCh), cause substantial increases in [Ca2+]m which are associated with smaller rises in [Ca2+]c. The L-type Ca2+-channel blocker, SR7037, abolished the effects of glucose and KCl while attenuating the CCh response. Glucose-induced increases in [Ca2+]m, [Ca2+]c, and insulin secretion all demonstrate a pronounced initial peak followed by a sustained plateau. All three parameters are increased synergistically when glucose and CCh are combined. Finally, [Ca2+]m, [Ca2+]c, and insulin secretion also display desensitization phenomena following repeated additions of the three stimuli. The high sensitivity of [Ca2+]m to Ca2+ influx and the desensitization-resensitization effects can be explained by a model in which the mitochondria of INS-1 cells are strategically located to sense Ca2+ influx through plasma membrane Ca2+ channels. In conclusion, the correlation of [Ca2+]m and [Ca2+]c with insulin secretion may indicate a fundamental role for Ca2+ in the adaptation of oxidative metabolism to the generation of metabolic coupling factors and the energy requirements of exocytosis.

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.

Generation of repetitive Ca2+ transients by bombesin requires intracellular release and influx of Ca2+ through voltage-dependent and voltage independent channels in single HIT cells

In the present study, the bombesin-induced changes in cytosolic free Ca2+ ([Ca2+]i) were investigated in single Fura-2 loaded SV-40 transformed hamster beta-cells (HIT). Bombesin (50-500 pM) caused frequency-modulated repetitive Ca2+ transients. The average frequency of the Ca2+ transients induced by bombesin (200 pM) was 0.58 +/- 0.02 min-1 (n = 121 cells). High concentrations of bombesin (> or = 2 nM) triggered a large initial Ca2+ transient followed by a sustained plateau or by a decrease to basal levels. In Ca(2+)- free medium, bombesin caused only one or two Ca2+ transients and withdrawal of extracellular Ca2+ abolished the Ca2+ transients. The voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (50 microM) and nifedipine (10 microM), reduced amplitude and frequency of the Ca2+ transients and stopped the Ca2+ transients in some cells. Thapsigargin caused a sustained rise in [Ca2+]i in the presence of extracellular Ca2+ while in its absence the rise in [Ca2+]i was transient. Verapamil (50 microM) inhibited the thapsigargin-induced increase in [Ca2+]i by about 50%. Depletion of intracellular Ca2+ stores by repetitive stimulation with increasing concentrations of bombesin or thapsigargin in Ca(2+)-free medium caused an agonist-independent increase in [Ca2+]i when extracellular Ca2+ was restored, which was larger than in control cells that had been incubated in Ca(2+)-free medium for the same period of time. This rise in [Ca2+]i and the thapsigargin-induced increase in [Ca2+]i were only partly inhibited by VDCC-blockers. Thus, depletion of the agonist-sensitive Ca2+ pool enhances Ca2+ influx through VDCC and voltage-independent Ca2+ channels (VICC). In conclusion, the bombesin-induced Ca2+ response in single HIT cells is periodic in nature with frequency-modulated repetitive Ca2+ transients. Intracellular Ca2+ is mobilized during each Ca2+ transient, but Ca2+ influx through VDCC and VICC is required for maintaining the sustained nature of the Ca2+ response. Ca2+ influx in whole or part is activated by a capacitative Ca2+ entry mechanism.

Two distinct modes of Ca2+ signalling by ACh in rat pancreatic beta-cells: concentration, glucose dependence and Ca2+ origin

1. Calcium signalling by acetylcholine (ACh) in single rat pancreatic beta-cells was studied. The cytosolic free Ca2+ concentration ([Ca2+]i) was measured by dual-wavelength fura-2 microfluorometry. 2. In the presence of basal glucose (2.8 mM), 10(-6) to 10(-4) M ACh (high ACh) transiently increased [Ca2+]i. The [Ca2+]i response to 10(-5) M ACh was little altered under Ca(2+)-free conditions. Brief pulses of 10(-5) M ACh evoked successive [Ca2+]i responses, which were progressively inhibited by 0.2-0.5 microM thapsigargin, a specific inhibitor of the endoplasmic reticulum (ER) Ca2+ pump. 3. Elevation of glucose to 8.3 mM, a concentration which stimulates insulin release, increased [Ca2+]i to an initial peak followed by a sustained, moderate elevation. Addition of 10(-8) to 10(-7) M ACh (low ACh) evoked a further increase in [Ca2+]i. The [Ca2+]i response to 10(-7) M ACh was completely inhibited under Ca(2+)-free conditions by 1 microM nitrendipine, a blocker of L-type Ca2+ channels, and by 100 microM diazoxide, an opener of ATP-sensitive K+ channels. 4. In the presence of 8.3 mM glucose, [Ca2+]i responses to 10(-5) M ACh were reduced but not abolished by Ca(2+)-free conditions, nitrendipine and diazoxide. Successive [Ca2+]i transients induced by 10(-5) M ACh pulses in the presence of nitrendipine were progressively inhibited by thapsigargin. 5. The results revealed two distinct modes of Ca2+ signalling: low ACh increases [Ca2+]i by stimulating Ca2+ influx through voltage-dependent L-type Ca2+ channels only in the beta-cells in which glucose has already elevated [Ca2+]i, while high ACh increases [Ca2+]i at basal as well as stimulatory glucose concentrations by releasing Ca2+ from the ER. The former mechanism is likely to relate to the potentiator action and the latter to the initiator action of ACh on insulin release. High ACh and elevated glucose provoke both modes of Ca2+ signalling.

Regulatory effect of 1,25-dihydroxyvitamin D3 on insulin release and calcium handling via the phospholipid pathway in islets from vitamin D-deficient rats

The effect of 10(-8) M 1,25-dihydroxyvitamin D3 [1,25 (OH)2D3] on the phosphoinositide pathway, was studied on [3H] inositol and 45Ca2+ efflux and on insulin release of islets from vitamin D-deficient rats, during an acetylcholine (Ach) stimulus in perifusion. The insulin release, which was low in vitamin D-deficient rats, was enhanced by this treatment. The 3H flux, reflecting phosphoinositide breakdown, was also increased. The 45Ca2+ flux was stimulated both during the first 14 min peak (mobilization of IP3-sensitive reticular Ca2+ stores) and during the following sustained small elevation of 45Ca2+ flux, reflecting protein kinase C (PKC) activation and consequently increased phosphorylation of Ca2+ channel proteins. These effects were larger during perifusions performed in the presence of glucose which is known to open Ca2+ channels, suggesting a synergistic influence of glucose and 1,25(OH)2D3. This positive influence of 1,25(OH)2D3 in Ca2+ entry by Ca2+ channels was confirmed by the use of nifedipine-a Ca2+ channel blocker-which suppressed the 45Ca2+ flux and lowered insulin secretion. Moreover, the sustained 45Ca2+ flux also disappeared in islets from vitamin D-deficient rats supplemented by 1,25(OH)2D3 but perifused without extracellular Ca2+ supporting the hypothesis of 1,25(OH)2D3-induced activation of PKC. Thus, 1,25(OH)2D3 may provide supplementary calcium to the B cell by regulating the intracellular signalling processes involving phospholipid metabolism, PKC induction, Ca2+ mobilization and Ca2+ entry by Ca2+ channels.

Muscarinic stimulation exerts both stimulatory and inhibitory effects on the concentration of cytoplasmic Ca2+ in the electrically excitable pancreatic B-cell

Mouse pancreatic islets were used to investigate how muscarinic stimulation influences the cytoplasmic Ca2+ concentration ([Ca2+]i) in insulin-secreting B-cells. In the absence of extracellular Ca2+, acetylcholine (ACh) triggered a transient, concentration-dependent and thapsigargin-inhibited increase in [Ca2+]i. In the presence of extracellular Ca2+ and 15 mM glucose, ACh induced a biphasic rise in [Ca2+]i. The initial, transient phase increased with the concentration of ACh, whereas the second, sustained, phase was higher at low (0.1-1 microM) than at high (> or = 10 microM) concentrations of ACh. Thapsigargin attenuated (did not suppress) the first phase of the [Ca2+]i rise and did not affect the sustained response. This sustained rise was inhibited by omission of extracellular Na+ (which prevents the depolarizing action of ACh) and by D600 or diazoxide (which prevent activation of voltage-dependent Ca2+ channels). During steady-state stimulation, the Ca2+ action potentials in B-cells were stimulated by 1 microM ACh but inhibited by 100 microM ACh. When B-cells were depolarized by 45 mM K+, ACh induced a concentration-dependent, biphasic change in [Ca2+]i, consisting of a first peak rapidly followed by a decrease. Thapsigargin suppressed the peak without affecting the drop in [Ca2+]i. Measurements of 45Ca2+ efflux under similar conditions indicated that ACh decreases Ca2+ influx and slightly increases the efflux. All effects of ACh were blocked by atropine. In conclusion, three mechanisms at least are involved in the biphasic change in [Ca2+]i that muscarinic stimulation exerts in excitable pancreatic B-cells. A mobilization of Ca2+ from the endoplasmic reticulum contributes significantly to the first peak, but little to the steady-state rise in [Ca2+]i. This second phase results from an influx of Ca2+ through voltage-dependent Ca2+ channels activated by a Na(+)-dependent depolarization. However, when high concentrations of ACh are used, Ca2+ influx is attenuated.

Glucose induces oscillations of cytoplasmic Ca2+, Sr2+ and Ba2+ in pancreatic beta-cells without participation of the thapsigargin-sensitive store

Individual pancreatic beta -cells were used to study the glucose effects on the handling of Ca2+, Sr2+ and Ba2+. In extracellular medium containing one of these ions, single beta -cells responded to 11 mM glucose with large amplitude oscillations in cytoplasmic Ca2+, Sr2+ or Ba2+ with indistinguishable average frequencies (0.30-0.33/min). The oscillations disappeared after hyperpolarization with 400 microM diazoxide. Under such hyperpolarization, glucose stimulated the sequestration of Ca2+ and Sr2+ but not of repetitively mobilized by consecutive exposures to 100 microM carbachol. A 2-3 min exposure to 100 nM of the intracellular Ca(2+)-ATPase inhibitor thapsigargin also mobilized Ca2+ and Sr2+ and irreversibly abolished subsequent release by carbachol. However, thapsigargin did not prevent the large amplitude oscillations in Ca2+, Sr2+ or Ba2+ under non-hyperpolarizing conditions although the frequency of the Ca2+ oscillations was almost doubled. The results indicate that the slow oscillatory behavior of glucose-stimulated individual beta -cells does not depend on inositol 1,4,5-trisphosphate mediated release of intracellular Ca2+.

Coincidence of early glucose-induced depolarization with lowering of cytoplasmic Ca2+ in mouse pancreatic beta-cells

1. The temporal relationship between the early glucose-induced changes of membrane potential and cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in insulin-releasing pancreatic beta-cells. 2. The mean resting membrane potential and [Ca2+]i were about -70 mV and 60 nM, respectively, in 3 mM glucose. 3. Elevating the glucose concentration to 8-23 mM typically elicited a slow depolarization, which was paralleled by a lowering of [Ca2+]i. When the slow depolarization had reached a threshold of -55 to -40 mV, there was rapid further depolarization to a plateau with superimposed action potentials, and [Ca2+]i increased dramatically. 4. Imposing hyperpolarizations and depolarizations of 10 mV from a holding potential of -70 mV had no detectable effect on [Ca2+]i. Furthermore, glucose elevation elicited a decrease in [Ca2+]i even at a holding potential of -70 mV. 5. Step depolarizations induced [Ca2+]i transients, which decayed with time courses well fitted by double exponentials. The slower component became faster by a factor of about 4 upon elevation of glucose, suggesting involvement of ATP-dependent Ca2+ sequestration or extrusion of [Ca2+]i. 6. Glucose stimulation increased the size and accelerated the recovery of carbachol-triggered [Ca2+]i transients, and thapsigargin, an intracellular Ca(2+)-ATPase inhibitor, counteracted the glucose-induced lowering of [Ca2+]i, indicating that calcium transport into intracellular stores is involved in glucose-induced lowering of [Ca2+]i. 7. The results support the notion that in beta-cells, nutrient-induced elevation of ATP leads initially to ATP-dependent removal of Ca2+ from the cytoplasm, paralleled by a slow depolarization due to inhibition of ATP-sensitive K+ channels. Only after depolarization has reached a threshold do action potentials occur, inducing a sharp elevation in [Ca2+]i.

Interplay of glucose-stimulated Ca2+ sequestration and acetylcholine-induced Ca2+ release at the endoplasmic reticulum in rat pancreatic beta-cells

It is known that the stimulation with high glucose initially decreases as well as subsequently increases the cytosolic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells. In the present study, we aimed at exploring the ionic mechanism and physiological role of the glucose-induced decrease in [Ca2+]i by measuring [Ca2+]i in single pancreatic beta-cells from normal rats. The glucose-induced decrease in [Ca2+]i in beta-cells was completely inhibited by thapsigargin (Tg), a specific inhibitor of the endoplasmic reticulum (ER) Ca2+ pump (Ca(2+)-ATPase). On the other hand, neither a Ca(2+)-free nor a low-Na+ condition significantly altered the glucose-induced decrease in [Ca2+]i. At basal glucose concentrations (1-4.5 mM), an insulin secretagogue acetylcholine (ACh) evoked a rather transient increase in [Ca2+]i in the presence and absence of extracellular Ca2+. A rise in glucose concentration from 1 to 4.5 mM produced a sustained decrease in [Ca2+]i and concomitantly augmented the ACh-evoked increase in [Ca2+]i. The resting [Ca2+]i level determined by glucose was tightly and reciprocally correlated with the peak of the [Ca2+]i response to ACh. Successive ACh pulses elicited repeated [Ca2+]i responses, which were progressively inhibited by Tg, suggesting that Ca2+ released by ACh was taken up by the ER Ca2+ pump and thus cycled. The results demonstrate that glucose decreases [Ca2+]i in pancreatic beta-cells mainly by activating the Ca2+ pump in ER from which ACh mobilizes Ca2+. Furthermore, the glucose-stimulated sequestration of Ca2+ by ER results in an augmented [Ca2+]i response to ACh, providing a mechanistic basis for the glucose-dependent action of ACh to initiate insulin secretion.

Thapsigargin inhibits the glucose-induced decrease of intracellular Ca2+ in mouse islets of Langerhans

Stimulation of pancreatic islets of Langerhans with glucose results in changes in intracellular Ca2+ concentration ([Ca2+]i). With the use of mouse islets loaded with fura 2, the earliest glucose-induced alteration of [Ca2+]i was a pronounced decline in [Ca2+]i. This effect (phase 0) was evident 1 min after increasing extracellular glucose from 2 to 12 mM and was sustained for 3-5 min. Phase 0 was also observed when glucose was increased from 5 to 12 mM, indicating that it was not an experimental artifact resulting from substrate depletion. The [Ca2+]i-lowering effect of glucose was mimicked by D-glyceraldehyde but not by 2-deoxyglucose, pyruvate, glyburide, or 30 mM extracellular KCl. Mannoheptulose inhibited phase 0, whereas diazoxide, sodium azide, calmidazolium, or increasing extracellular [Ca2+] to 10 mM were all without effect. After the elevation of islet [Ca2+]i with 5 microM glyburide, 12 mM glucose caused a considerable transient decrease in [Ca2+]i. Under similar conditions, 5 mM caffeine attenuated phase 0, whereas 1 microM thapsigargin, a specific inhibitor of the sarcoplasmic and endoplasmic reticulum family of Ca(2+)-adenosinetriphosphatases (SERCA), almost completely inhibited any glucose-induced reduction of [Ca2+]i. These observations suggest that glucose causes an elevation of beta-cell SERCA activity triggered by factors generated during the cytosolic stages of glycolysis.

Caffeine inhibits cytoplasmic Ca2+ oscillations induced by carbachol and guanosine 5'-O-(3-thiotriphosphate) in hyperpolarized pancreatic beta-cells

The effects of caffeine on cytoplasmic Ca2+ oscillations induced by carbachol and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) were studied in individual mouse pancreatic beta-cells clamped at a hyperpolarized potential. Addition of 10 mM caffeine did not affect the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells exposed to 20 mM glucose and hyperpolarized with diazoxide. Under similar conditions 100 microM carbachol induced a typical response with a marked [Ca2+]i peak followed by a lower sustained elevation. Irrespective of whether 10 mM caffeine was present, there were [Ca2+]i transients with frequencies of 1-5/min superimposed on the sustained phase in 50-60% of the cells. In previously non-exposed cells the introduction of 10 mM caffeine caused temporary lowering of the sustained phase with disappearance of the transients. Subsequent omission of caffeine in the continued presence of carbachol caused a marked [Ca2+]i peak followed by reappearance of the [Ca2+]i transients. However, in cells oscillating in the presence of caffeine its omission caused disappearance of the transients. In this case reintroduction of caffeine restored the transients. In cells kept at -70 mV by a patch pipette containing 100 microM GTP-gamma-S and 3 mM Mg-ATP there were [Ca2+]i transients with frequencies of 0.5-2.5/min. These transients were sufficiently pronounced to activate repetitively a K+ current. Addition of 10 mM caffeine caused disappearance of the [Ca2+]i transients or reduction of their amplitudes and frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoplasmic Ca2+ oscillations in pancreatic beta-cells

In the last 15 years it has been a growing interest in the cyclic variations of circulating insulin [46]. After the suggestion that this phenomenon may be due to oscillations of the beta-cell membrane potential [8,39], it was demonstrated that [Ca2+]i oscillates in the glucose-stimulated beta-cell with a similar frequency to that of pulsatile insulin release. The present review describes four types of [Ca2+]i oscillations in the pancreatic beta-cell. The slow sinusoidal oscillations, referred to as type-a, are those which most closely correspond to pulsatile insulin release. Although not affecting the properties of the type-a oscillations in individual beta-cells, the concentration of glucose is a determinant for their generation and further transformation into a sustained increase. Accordingly, cytoplasmic Ca2+ is regulated by sudden transitions between oscillatory and steady-state levels at threshold concentrations of glucose, which are characteristic for the individual beta-cell. This behaviour explains the observation of a gradual recruitment of previously non-secreting cells with increase of the extracellular glucose concentration [44]. However, it still remains to be elucidated how the sudden transitions between these three states translate into the co-ordinated slow oscillations of [Ca2+]i in the intact islet. Cyclic variations of circulating insulin require a synchronization of the [Ca2+]i cycles also among the islets in the pancreas. It is still an open question by which means the millions of islets communicate mutually to establish a pattern of pulsatile insulin release from the whole pancreas. The discovery that the beta-cell is not only the functional unit for insulin synthesis but also generates the [Ca2+]i oscillations required for pulsatile insulin release has both physiological and clinical implications. The fact that minor damage to the beta-cells prevents the type-a oscillations with maintenance of a glucose response in terms of raised [Ca2+]i reinforces previous arguments [54] that loss of insulin oscillations is an early indicator of type-2 diabetes. Further analyses of the [Ca2+]i oscillations in the beta-cells should include not only the mechanisms for their generation and subsequent propagation within or among the islets but also how modulation of their frequency affects the insulin sensitivity of various target cells. The latter approach may be important in the attempts to maintain normoglycemia under conditions minimizing the vascular effects of insulin supposed to precipitate hypertonia and atherosclerosis [70,71,77].

BAY K 8644 stimulates glucose-dependent rise of cytoplasmic Ca2+ in hyperpolarized pancreatic beta-cells

The effect of BAY K 8644 on the cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in pancreatic beta-cells hyperpolarized by the K+ channel-activating agent diazoxide. After 50-60 min preexposure to 0-20 mM glucose in the presence of 400 microM diazoxide [Ca2+]i was close to the level in unstimulated beta-cells. The addition of 5 microM BAY K 8644 then triggered a rise of [Ca2+]i dependent on Ca2+ influx. The magnitude of the BAY K 8644 effect increased with the glucose concentration and was almost 10-fold higher in 20 mM than in the absence of the sugar. It is concluded that glucose can modulate Ca2+ entry through the voltage-dependent channels by a mechanism additional to depolarization. This action may help to explain why previous exposure to the sugar results in an augmented insulin response to a second challenge.