Nutrient secretagogues induce bimodal early changes in cytoplasmic calcium of insulin-releasing ob/ob mouse beta-cells

Glucose-dependent activation, activity, and deactivation of beta cell networks in acute mouse pancreas tissue slices

Many details of glucose-stimulated intracellular calcium changes in β cells during activation, activity, and deactivation, as well as their concentration-dependence, remain to be analyzed. Classical physiological experiments indicated that in islets, functional differences between individual cells are largely attenuated, but recent findings suggest considerable intercellular heterogeneity, with some cells possibly coordinating the collective responses. To address the above with an emphasis on heterogeneity and describing the relations between classical physiological and functional network properties, we performed functional multicellular calcium imaging in mouse pancreas tissue slices over a wide range of glucose concentrations. During activation, delays to activation of cells and any-cell-to-first-responder delays are shortened, and the sizes of simultaneously responding clusters increased with increasing glucose concentrations. Exactly the opposite characterized deactivation. The frequency of fast calcium oscillations during activity increased with increasing glucose up to 12 mM glucose concentration, beyond which oscillation duration became longer, resulting in a homogenous increase in active time. In terms of functional connectivity, islets progressed from a very segregated network to a single large functional unit with increasing glucose concentration. A comparison between classical physiological and network parameters revealed that the first-responders during activation had longer active times during plateau and the most active cells during the plateau tended to deactivate later. Cells with the most functional connections tended to activate sooner, have longer active times, and deactivate later. Our findings provide a common ground for recent differing views on β cell heterogeneity and an important baseline for future studies of stimulus-secretion and intercellular coupling.NEW & NOTEWORTHY We assessed concentration-dependence in coupled β cells, degree of functional heterogeneity, and uncovered possible specialized subpopulations during the different phases of the response to glucose at the level of many individual cells. To this aim, we combined acute mouse pancreas tissue slices with functional multicellular calcium imaging over a wide range from threshold (7 mM) and physiological (8 and 9 mM) to supraphysiological (12 and 16 mM) glucose concentrations, classical physiological, and advanced network analyses.

Glucose intolerance and pancreatic β-cell dysfunction in the anorectic anx/anx mouse

Inflammation and impaired mitochondrial oxidative phosphorylation are considered key players in the development of several metabolic disorders, including diabetes. We have previously shown inflammation and mitochondrial dysfunction in the hypothalamus of an animal model for anorexia, the anx/anx mouse. Moreover, increased incidence of eating disorders, e.g., anorexia nervosa, has been observed in diabetic individuals. In the present investigation we evaluated whether impaired mitochondrial phosphorylation and inflammation also occur in endocrine pancreas of anorectic mice, and if glucose homeostasis is disturbed. We show that anx/anx mice exhibit marked glucose intolerance associated with reduced insulin release following an intraperitoneal injection of glucose. In contrast, insulin release from isolated anx/anx islets is increased after stimulation with glucose or KCl. In isolated anx/anx islets there is a strong downregulation of the mitochondrial complex I (CI) assembly factor, NADH dehydrogenase (ubiquinone) 1α subcomplex, assembly factor 1 (Ndufaf1), and a reduced CI activity. In addition, we show elevated concentrations of free fatty acids (FFAs) in anx/anx serum and increased macrophage infiltration (indicative of inflammation) in anx/anx islets. However, isolated islets from anx/anx mice cultured in the absence of FFAs do not exhibit increased inflammation. We conclude that the phenotype of the endocrine pancreas of the anx/anx mouse is characterized by increased levels of circulating FFAs, as well as inflammation, which can inhibit insulin secretion in vivo. The anx/anx mouse may represent a useful tool for studying molecular mechanisms underlying the association between diabetes and eating disorders.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

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.

Metabolic amplification of insulin secretion by glucose is independent of β-cell microtubules

Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. After exclusion of the contribution of actin microfilaments, we here tested whether amplification implicates microtubule-dependent granule mobilization. Mouse islets were treated with nocodazole or taxol, which completely depolymerized and polymerized tubulin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing [Ca2+]c and IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca2+]c or IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca2+]c. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was unaffected by microtubule disruption or stabilization. Both phases of IS were larger in response to glucose than tolbutamide, although triggering [Ca2+]c was lower. This difference, due to amplification, persisted in nocodazole- or taxol-treated islets, even when IS was augmented fourfold by microfilament disruption with cytochalasin B or latrunculin B. In conclusion, metabolic amplification rapidly augments first and second phases of IS independently of insulin granule translocation along microtubules. We therefore extend our previous proposal that it does not implicate the cytoskeleton but corresponds to acceleration of the priming process conferring release competence to insulin granules.

Mechanism of hyperinsulinism in short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency involves activation of glutamate dehydrogenase

The mechanism of insulin dysregulation in children with hyperinsulinism associated with inactivating mutations of short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) was examined in mice with a knock-out of the hadh gene (hadh(-/-)). The hadh(-/-) mice had reduced levels of plasma glucose and elevated plasma insulin levels, similar to children with SCHAD deficiency. hadh(-/-) mice were hypersensitive to oral amino acid with decrease of glucose level and elevation of insulin. Hypersensitivity to oral amino acid in hadh(-/-) mice can be explained by abnormal insulin responses to a physiological mixture of amino acids and increased sensitivity to leucine stimulation in isolated perifused islets. Measurement of cytosolic calcium showed normal basal levels and abnormal responses to amino acids in hadh(-/-) islets. Leucine, glutamine, and alanine are responsible for amino acid hypersensitivity in islets. hadh(-/-) islets have lower intracellular glutamate and aspartate levels, and this decrease can be prevented by high glucose. hadh(-/-) islets also have increased [U-(14)C]glutamine oxidation. In contrast, hadh(-/-) mice have similar glucose tolerance and insulin sensitivity compared with controls. Perifused hadh(-/-) islets showed no differences from controls in response to glucose-stimulated insulin secretion, even with addition of either a medium-chain fatty acid (octanoate) or a long-chain fatty acid (palmitate). Pull-down experiments with SCHAD, anti-SCHAD, or anti-GDH antibodies showed protein-protein interactions between SCHAD and GDH. GDH enzyme kinetics of hadh(-/-) islets showed an increase in GDH affinity for its substrate, α-ketoglutarate. These studies indicate that SCHAD deficiency causes hyperinsulinism by activation of GDH via loss of inhibitory regulation of GDH by SCHAD.

Glucose-dependent modulation of insulin secretion and intracellular calcium ions by GKA50, a glucokinase activator

Because glucokinase is a metabolic sensor involved in the regulated release of insulin, we have investigated the acute actions of novel glucokinase activator compound 50 (GKA50) on islet function. Insulin secretion was determined by enzyme-linked immunosorbent assay, and microfluorimetry with fura-2 was used to examine intracellular Ca(2+) homeostasis ([Ca(2+)](i)) in isolated mouse, rat, and human islets of Langerhans and in the MIN6 insulin-secreting mouse cell line. In rodent islets and MIN6 cells, 1 micromol/l GKA50 was found to stimulate insulin secretion and raise [Ca(2+)](i) in the presence of glucose (2-10 mmol/l). Similar effects on insulin release were also seen in isolated human islets. GKA50 (1 micromol/l) caused a leftward shift in the glucose-concentration response profiles, and the half-maximal effective concentration (EC(50)) values for glucose were shifted by 3 mmol/l in rat islets and approximately 10 mmol/l in MIN6 cells. There was no significant effect of GKA50 on the maximal rates of glucose-stimulated insulin secretion. In the absence of glucose, GKA50 failed to elevate [Ca(2+)](i) (1 micromol/l GKA50) or to stimulate insulin release (30 nmol/l-10 micromol/l GKA50). At 5 mmol/l glucose, the EC(50) for GKA50 in MIN6 cells was approximately 0.3 micromol/l. Inhibition of glucokinase with mannoheptulose or 5-thioglucose selectively inhibited the action of GKA50 on insulin release but not the effects of tolbutamide. Similarly, 3-methoxyglucose prevented GKA50-induced rises in [Ca(2+)](i) but not the actions of tolbutamide. Finally, the ATP-sensitive K(+) channel agonist diazoxide (200 micromol/l) inhibited GKA50-induced insulin release and its elevation of [Ca(2+)](i.) We show that GKA50 is a glucose-like activator of beta-cell metabolism in rodent and human islets and a Ca(2+)-dependent modulator of insulin secretion.

Paradoxical stimulation of glucagon secretion by high glucose concentrations

Hypersecretion of glucagon contributes to the dysregulation of glucose homeostasis in diabetes. To clarify the underlying mechanism, glucose-regulated glucagon secretion was studied in mouse pancreatic islets and clonal hamster In-R1-G9 glucagon-releasing cells. Apart from the well-known inhibition of secretion with maximal effect around 7 mmol/l glucose, we discovered that mouse islets showed paradoxical stimulation of glucagon release at 25-30 mmol/l and In-R1-G9 cells at 12-20 mmol/l sugar. Whereas glucagon secretion in the absence of glucose was inhibited by hyperpolarization with diazoxide, this agent tended to further enhance secretion stimulated by high concentrations of the sugar. Because U-shaped dose-response relationships for glucose-regulated glucagon secretion were observed in normal islets and in clonal glucagon-releasing cells, both the inhibitory and stimulatory components probably reflect direct effects on the alpha-cells. Studies of isolated mouse alpha-cells indicated that glucose inhibited glucagon secretion by lowering the cytoplasmic Ca(2+) concentration. However, stimulation of glucagon release by high glucose concentrations did not require elevation of Ca(2+), indicating involvement of novel mechanisms in glucose regulation of glucagon secretion. A U-shaped dose-response relationship for glucose-regulated glucagon secretion may explain why diabetic patients with pronounced hyperglycemia display paradoxical hyperglucagonemia.

Ca2+ controls slow NAD(P)H oscillations in glucose-stimulated mouse pancreatic islets

Exposure of pancreatic islets of Langerhans to physiological concentrations of glucose leads to secretion of insulin in an oscillatory pattern. The oscillations in insulin secretion are associated with oscillations in cytosolic Ca(2+) concentration ([Ca(2+)](c)). Evidence suggests that the oscillations in [Ca(2+)](c) and secretion are driven by oscillations in metabolism, but it is unclear whether metabolic oscillations are intrinsic to metabolism or require Ca(2+) feedback. To address this question we explored the interaction of Ca(2+) concentration and islet metabolism using simultaneous recordings of NAD(P)H autofluorescence and [Ca(2+)](c), in parallel with measurements of mitochondrial membrane potential (DeltaPsi(m)). All three parameters responded to 10 mm glucose with multiphasic dynamics culminating in slow oscillations with a period of approximately 5 min. This was observed in approximately 90% of islets examined from various mouse strains. NAD(P)H oscillations preceded those of [Ca(2+)](c), but their upstroke was often accelerated during the increase in [Ca(2+)](c), and Ca(2+) influx was a prerequisite for their generation. Prolonged elevations of [Ca(2+)](c) augmented NAD(P)H autofluorescence of islets in the presence of 3 mm glucose, but often lowered NAD(P)H autofluorescence of islets exposed to 10 mm glucose. Comparable rises in [Ca(2+)](c) depolarized DeltaPsi(m). The NAD(P)H lowering effect of an elevation of [Ca(2+)](c) was reversed during inhibition of mitochondrial electron transport. These findings reveal the existence of slow oscillations in NAD(P)H autofluorescence in intact pancreatic islets, and suggest that they are shaped by Ca(2+) concentration in a dynamic balance between activation of NADH-generating mitochondrial dehydrogenases and a Ca(2+)-induced decrease in NADH. We propose that a component of the latter reflects mitochondrial depolarization by Ca(2+), which reduces respiratory control and consequently accelerates oxidation of NADH.

In vivo and in vitro glucose-induced biphasic insulin secretion in the mouse: pattern and role of cytoplasmic Ca2+ and amplification signals in beta-cells

The mechanisms underlying biphasic insulin secretion have not been completely elucidated. We compared the pattern of plasma insulin changes during hyperglycemic clamps in mice to that of glucose-induced insulin secretion and cytosolic calcium concentration ([Ca(2+)](c)) changes in perifused mouse islets. Anesthetized mice were infused with glucose to clamp blood glucose at 8.5 (baseline), 11.1, 16.7, or 30 mmol/l. A first-phase insulin response consistently peaked at 1 min, and a slowly ascending second phase occurred at 16.7 and 30 mmol/l glucose. Glucose-induced insulin secretion in vivo is thus biphasic, with a similarly increasing second phase in the mouse as in humans. In vitro, square-wave stimulation from a baseline of 3 mmol/l glucose induced similar biphasic insulin secretion and [Ca(2+)](c) increases, with sustained and flat second phases. The glucose dependency (3-30 mmol/l) of both changes was sigmoidal with, however, a shift to the right of the relation for insulin secretion compared with that for [Ca(2+)](c). The maximum [Ca(2+)](c) increase was achieved by glucose concentrations, causing half-maximum insulin secretion. Because this was true for both phases, we propose that contrary to current concepts, amplifying signals are also implicated in first-phase glucose-induced insulin secretion. To mimic in vivo conditions, islets were stimulated with high glucose after being initially perifused with 8.5 instead of 3.0 mmol/l glucose. First-phase insulin secretion induced by glucose at 11.1, 16.7, and 30 mmol/l was decreased by approximately 50%, an inhibition that could not be explained by commensurate decreases in [Ca(2+)](c) or in the pool of readily releasable granules. Also unexpected was the gradually ascending pattern of the second phase, now similar to that in vivo. These observations indicated that variations in prestimulatory glucose can secondarily affect the magnitude and pattern of subsequent glucose-induced insulin secretion.

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.

Prolonged culture in low glucose induces apoptosis of rat pancreatic beta-cells through induction of c-myc

Prolonged culture in low-glucose concentrations (</=5mM) induces apoptosis in pancreatic beta-cells by a poorly defined mechanism. We now show that, in both purified rat beta-cells and isolated rat islets, culture in the presence of 3 or 5mM (G3-G5) instead of 10mM glucose (G10) induces a large increase in c-myc expression before onset of a caspase-dependent apoptosis. These effects were prevented by addition of leucine and glutamine to G3 and G5, and were mimicked by addition of the mitochondrial poison azide to G10. In contrast, inhibition of Ca(2+) influx and insulin secretion with diazoxide under control conditions did not stimulate islet c-myc expression nor beta-cell apoptosis. In rat beta-cells, adenovirus-mediated c-myc overexpression increased their rate of apoptosis, whereas antisense-c-myc expression reduced low-glucose-induced apoptosis by approximately 50%. In the insulin producing MIN6 cell line, apoptosis induction by either low glucose or an activator of AMP-activated protein kinase (AMPK) was associated with c-myc mRNA and protein upregulation. In conclusion, stimulation of beta-cell apoptosis by prolonged culture at low glucose partly results from early and sustained induction of beta-cell c-myc expression. These effects may be due to sustained restriction in nutrient-derived metabolic signals.

Anomeric specificity of the stimulatory effect of D-glucose on D-fructose phosphorylation by human liver glucokinase

D-Glucose was recently reported to stimulate d-fructose phosphorylation by human B-cell glucokinase. The present study aims at investigating the anomeric specificity of such a positive cooperativity. The alpha-anomer of D-glucose was found to increase much more markedly than beta-D-glucose the phosphorylation of D-fructose by human liver glucokinase. Such an anomeric preference diminished at high concentrations of the D-glucose anomers, i.e. when the effect of the aldohexose upon d-fructose phosphorylation became progressively less marked. A comparison between the effects of the two anomers of D-glucose and those of equilibrated D-glucose upon D-fructose phosphorylation by human liver glucokinase indicated that the results obtained with the equilibrated aldohexose were not significantly different from those expected from the combined effects of each anomers of D-glucose. In isolated rat islets incubated for 60 min at 4 degrees C, alpha-D-glucose (5.6 mm), but not beta-D-glucose (also 5.6 mm), augmented significantly the conversion of D-[U-(14)C]fructose (5.0 mm) to acidic radioactive metabolites. Likewise, in islets prelabeled with (45)Ca and perifused at 37 degrees C, D-fructose (20.0 mm) augmented (45)Ca efflux and provoked a biphasic stimulation of insulin release from islets exposed to alpha-D-glucose (5.6 mm), while inhibiting (45)Ca efflux and causing only a sluggish and modest increase in insulin output from islets exposed to beta-D-glucose (also 5.6 mm). The enhancing action of D-glucose upon D-fructose phosphorylation by glucokinase thus displays an obvious anomeric preference for alpha-D-glucose, and such an anomeric specificity remains operative in intact pancreatic islets.

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.

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

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

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.

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.

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.

The ability of a new hypoglycaemic agent, A-4166, compared to sulphonylureas, to increase cytosolic Ca2+ in pancreatic beta-cells under metabolic inhibition

1. N-(trans-4-isopropylcyclohexanecarbonyl)-D-phenylalanine (A-4166) is a new non-sulphonylurea oral hypoglycaemic agent which stimulates insulin release by increasing cytosolic Ca2+ concentration ([Ca2+]i) in beta-cells. 2. We studied comparative effects of A-4166 and sulphonylureas on [Ca2+]i, measured by dual-wavelength fura-2 microfluorometry, in single rat pancreatic beta-cells under normal conditions and conditions where glucose metabolism was inhibited. 3. A glucokinase inhibitor, mannoheptulose (10 mM), a mitochondrial respiratory inhibitor, KCN (100 microM), and uncouplers, dinitrophenol (DNP, 50 microM) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP, 0.3 microM), were used to abolish glucose-induced increases in [Ca2+]i in a reversible manner. 4. Under control conditions, A-4166 was one order more potent than tolbutamide in increasing [Ca2+]i, and maximal responses were evoked by 30 microM A-4166 and 300 microM tolbutamide. These equipotent concentrations were employed for the comparative study where glucose metabolism was inhibited. 5. In the presence of mannoheptulose, [Ca2+]i responses to tolbutamide, but not those to A-4166, were attenuated in a reversible manner. 6. KCN, DNP and FCCP inhibited [Ca2+]i responses to tolbutamide to a much greater extent than those to A-4166. Responses to tolbutamide even at 3.3 times the equipotent concentration (1000 microM) were also markedly attenuated by these inhibitors. Responses evoked by another sulphonylurea, gliclazide, were inhibited by DNP to a larger extent than A-4166-induced responses. 7. The results indicate that A-4166 acts more effectively than sulphonylureas to increase [Ca2+]i in beta-cells during metabolic inhibition.

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.

Metabolic regulation of intracellular calcium concentration in mouse pancreatic islets of Langerhans

Intracellular Ca2+ concentration ([Ca2+]i) handling during K(+)-induced Ca2+ loads was studied in single islets of Langerhans. K(+)-induced depolarization caused a rapid and transient rise in [Ca2+]i. After K+ removal [Ca2+]i declined with a time course usually fitted by the sum of two exponential functions. Partial Na+ removal increased the resting [Ca2+]i level, indicating the existence of a Na+/Ca2+ exchange, but only slightly impaired the recovery from Ca2+ loads. Metabolic poisoning with CN- increased the resting Ca2+ level and slowed down the recovery from Ca2+ loads. Removal of external Na+ in islets poisoned with CN- strongly inhibited Ca2+ removal mechanisms. An increase in the glucose concentration from 0 to 16 mM (in the presence of diazoxide) resulted in a decrease in the resting [Ca2+]i and an acceleration of [Ca2+]i recovery from K+ loads. These results suggest that the main mechanism responsible for Ca2+ homeostasis is dependent on metabolic energy and that such energy can be provided by glucose metabolism.

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.

Mass spectrometric characterization of arachidonate-containing plasmalogens in human pancreatic islets and in rat islet beta-cells and subcellular membranes

Pancreatic islets, when stimulated with D-glucose, secrete insulin by processes requiring glycolytic metabolism and generation of ATP. Hydrolysis of membrane phospholipids also occurs in glucose-stimulated islets, resulting in accumulation of nonesterified arachidonate, which facilitates Ca2+ entry and the rise in beta-cell [Ca2+] that triggers insulin secretion. Glucose-induced hydrolysis of arachidonate from islet phospholipids is mediated in part by an ATP-stimulated, Ca(2+)-independent (ASCI) phospholipase A2 (PLA2) which prefers plasmenyl over diacyl phospholipid substrates. Here we characterize the endogenous plasmalogen content of islet cells and subcellular membranes. Fast atom bombardment mass spectrometric analyses demonstrated that three of the most abundant molecular species of ethanolamine phospholipids in rat pancreatic islets were plasmalogens with sn-2 arachidonate residues and palmitic, oleic, or stearic aldehyde residues, respectively, in the sn-1 position. Purified populations of beta-cells prepared by fluorescence-activated cell sorting were also found to contain these plasmenylethanolamine molecular species in abundance similar to that in intact islets and greater than that in islet alpha-cells. Both islet plasma membranes (PM) and endoplasmic reticulum (ER) also contained these plasmenylethanolamine species, which accounted for 42% (PM) to 64% (ER) of the ethanolamine phospholipid arachidonate content of these membranes, as measured by stable isotope dilution mass spectrometry. Plasmenylethanolamine species were also abundant constituents of human pancreatic islets (accounting for 58% of their ethanolamine phospholipid arachidonate content) and were hydrolyzed more rapidly than diacyl ethanolamine phospholipid by human islet cytosolic ASCI-PLA2. Both secretagogue-induced eicosanoid release and insulin secretion from human islets were attenuated by an ASCI-PLA2 suicide substrate which sterically resembles plasmalogens. These observations are consistent with the hypotheses that islet beta-cell ASCI-PLA2-catalyzed hydrolysis of arachidonate from endogenous plasmenylethanolamine substrates may occur in membrane compartments which participate in regulation of the beta-cell cytosolic [Ca2+] and that this may be an intermediary biochemical event in the induction of insulin secretion.

Glycine transformation of Ca2+ oscillations into a sustained increase parallels potentiation of insulin release

Increase of the glucose concentration from 3 to 11 mM resulted in a triphasic release of insulin from perifused ob/ob-mouse beta-cells. A slight inhibition was followed after 2 min by a marked peak and a less pronounced sustained response. At the lower glucose concentration glycine had only marginal effects. However, in the presence of 11 mM glucose, 1-10 mM glycine triggered an immediate and dose-dependent response with an initial peak of insulin release followed by sustained stimulation. In individual beta-cells, rise of the glucose concentration from 3 to 11 mM induced initial lowering of the cytoplasmic Ca2+ concentration ([Ca2+]i) followed by large amplitude oscillations from a level of 50-90 nM to peak values exceeding 300 nM. Already at a concentration of 1 mM, glycine transformed the oscillatory pattern into a sustained level with increase of time-average [Ca2+]i. This elevation became more pronounced in the presence of 10 mM glycine. The effects of glycine on insulin release and [Ca2+]i required extracellular Na+ and were reproduced with the N-methyl analogue sarcosine. It is suggested that glycine potentiation of secretion reflects the elevation of time-average [Ca2+]i both by increased entry and reduced elimination of the cation from the cytoplasm.

Arachidonic acid metabolism in isolated pancreatic islets. VI. Carbohydrate insulin secretagogues must be metabolized to induce eicosanoid release

Pancreatic islets stimulated with D-glucose are known to liberate arachidonic acid from membrane phospholipids and release prostaglandin E2 (PGE2). A component of the eicosanoid release induced by D-glucose has been demonstrated to occur without calcium influx and must be triggered by other coupling mechanisms. In this study, we have attempted to identify mechanisms other than calcium influx which might couple D-glucose stimulation to hydrolysis of arachidonate from membrane phospholipids in islet cells. We have found that occupancy of the beta cell plasma membrane D-glucose transporter is insufficient and that D-glucose metabolism is required to induce islet PGE2 release because 3-O-methylglucose fails to induce and mannoheptulose prevents PGE2 release otherwise induced by 17 mM D-glucose. The carbohydrate insulin secretagogues mannose and D-glyceraldehyde have also been found to induce islet PGE2 release, but the non-secretagogue carbohydrates L-glucose and lactate do not. Carbohydrate secretagogues are known to be metabolized to yield ATP and induce depolarization of the beta cell plasma membrane. We have found that depolarization by 40 mM KCl induces PGE2 release only in the presence and not in the absence of extracellular calcium, but exogenous ATP induces islet PGE2 release with or without extracellular calcium. Carbachol is demonstrated here to interact synergistically with increasing concentrations of glucose to amplify PGE2 release and insulin secretion. Pertussis toxin treatment is shown here not to prevent PGE2 release induced by glucose or carbachol but to increase the basal rate of PGE2 release and the islet cyclic AMP content. Theophylline (10 mM) exerts similar effects. Eicosanoid release in pancreatic islets can thus be activated by multiple pathways including muscarinic receptor occupancy, calcium influx, increasing cAMP content, and a metabolic signal derived from nutrient secretagogues, such as ATP.

Glucose sensing of individual pancreatic beta-cells involves transitions between steady-state and oscillatory cytoplasmic Ca2+

Glucose stimulation of individual pancreatic beta-cells is associated with a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i) manifested either as large amplitude oscillations (0.2-0.5/min) or as a sustained increase. Determinants for the transitions between the basal and the two stimulated states have now been studied using dual-wavelength fluorometric measurements on individual ob/ob mouse beta-cells loaded with the Ca2+ indicator Fura-2. The transition from the basal state to large amplitude oscillations was induced by raising the glucose concentration to 7 mM or above. The frequencies and shapes of the [Ca2+]i cycles remained largely unaffected when raising glucose as high as 40 mM. However, in some cells the oscillatory pattern was transformed into a sustained increase of [Ca2+]i at high glucose concentrations. Although the peak values for the oscillations exceeded the steady-state increase, the time average [Ca2+]i was higher during the latter phase. Both types of glucose-induced transitions were facilitated by the presence of 1-100 nM glucagon. Protein kinase C activation by 10 nM of the phorbol ester TPA resulted in a transformation of the glucose-induced oscillations into a sustained increase of [Ca2+]i but the levels reached were considerably lower than obtained with glucose alone. It is concluded that the glucose sensing of the individual beta-cell is based on sudden transitions between steady-state and oscillating cytoplasmic Ca2+. It is these transitions rather than alterations of the oscillatory characteristics which determine the average [Ca2+]i regulating insulin release.

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.

Single pancreatic beta-cells from normal rats exhibit an initial decrease and subsequent increase in cytosolic free Ca2+ in response to glucose

Since it was reported that glucose stimulation initially lowers as well as subsequently raises the cytosolic free calcium concentration [( Ca2+]i) in pancreatic islet cells from hyperglycemic ob/ob mice, it has been argued whether the lowering of [Ca2+]i is physiological or artifactual. In the present study, [Ca2+]i in single pancreatic beta-cells from normal rats was measured by Fura-2 microfluorometry. Following elevation of the glucose concentration from 2.8 mM (basal) to 16.7 mM, a bimodal change in [Ca2+]i, an initial decrease and subsequent increase, was demonstrated. When the basal glucose concentration was raised to 5.6 mM, the stimulation with 16.7 mM glucose also induced the decrease in [Ca2+]i in the majority of the cells, though the amplitude of the decrease was reduced. An elevation of the glucose concentration from 2.8 to 5.6 mM induced the decrease in [Ca2+]i but not usually the increase in [Ca2+]i. Removal of extracellular Ca2+ eliminated the increase in [Ca2+]i without affecting the decrease in [Ca2+]i. Thus, the decrease and increase in [Ca2+]i were clearly dissociated under certain conditions. In contrast, mannoheptulose (an inhibitor of glucose metabolism) inhibited both the decrease and increase in [Ca2+]i. These results demonstrate that the glucose-induced bimodal change in [Ca2+]i is a physiological response of islet beta-cells, and that the decrease and increase in [Ca2+]i are generated by mutually-independent mechanisms which are operated through glucose metabolism by islet beta-cells.

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

The effect of carbachol on the cytoplasmic Ca2+ concentration [( Ca2+]i) was studied in insulin-releasing mouse pancreatic beta cells hyperpolarized by the K(+)-channel-activating agent diazoxide. By mobilizing intracellular Ca2+, carbachol induced an initial [Ca2+]i transient, which was more than tenfold higher after preexposure to 20 mM glucose than in a medium lacking substrate. The transient was followed by a sustained but less pronounced elevation, probably due to activation of the potential-independent entry of Ca2+. In individual beta cells exposed to 20 mM glucose small oscillations with a frequency of 1-4/min were superimposed on the sustained phase. These oscillations were insensitive to methoxyverapamil, and their frequency increased in a Na(+)-deficient medium. However, the oscillations faded away after lowering glucose to 3 mM and reappeared when increasing the sugar concentration. The results indicate that the glucose concentration is an important permissive determinant for sustained oscillations of [Ca2+]i in response to agents stimulating the formation of inositol 1,4,5-trisphosphate.

Propagation of cytoplasmic Ca2+ oscillations in clusters of pancreatic beta-cells exposed to glucose

Digital image analysis was employed for resolving the temporal and spatial variations of the cytoplasmic Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with the Ca(2+)-indicator Fura-2. Glucose-stimulated individual beta-cells exhibited large amplitude oscillations of [Ca2+]i with a mean frequency of 0.33 min-1. When Ca2+ diffusion was restricted by increasing the Ca2+ buffering capacity, the sugar-induced rise of [Ca2+]i preferentially affected the peripheral cytoplasm. When glucagon was present glucose also caused less prominent oscillations with about a 10-fold higher frequency superimposed on an elevated [Ca2+]i. In small clusters of 6-14 cells the average frequency of the large amplitude oscillations increased to 0.60 min-1. The clusters were found to contain micro-domains of electrically coupled cells with synchronized oscillations. After increasing the glucose concentration, adjacent domains became functionally coupled. The oscillations originated from different cells in the cluster. Also the fast glucagon-dependent oscillations were synchronized between cells and had different origins. The results indicate that coupling of beta-cells leads to an increased frequency of the large amplitude oscillations, and that the oscillatory characteristics are determined collectively among electrically coupled beta-cells rather than by particular pacemaker cells. In the light of these data it is necessary to reconsider the previous ideas that glucose-induced oscillations of membrane potential and [Ca2+]i require coupling between many beta-cells, and that the peak [Ca2+]i values reached during oscillations should increase with the size of the coupled cluster.

Insulin secretagogues induce Ca(2+)-like changes in cytoplasmic Mg2+ in pancreatic beta-cells

The effects of insulin secretagogues on the cytoplasmic Mg2+ concentration ([Mg2+]i) of pancreatic beta-cells were studied in suspensions and in individual beta-cells using dual-wavelength fluorometry and the indicator mag-fura-2. Average [Mg2+]i was in the 800-900 microM range in a medium containing 3 mM glucose. When the sugar concentration was raised to 20 mM, the cells reacted with an initial lowering of [Mg2+]i followed by an increase. The sugar apparently also stimulated leakage of the Mg2+ indicator. Addition of 100 microM tolbutamide or raising the K+ concentration by 25 mM caused relatively rapid increases of [Mg2+]i. Methoxyverapamil prevented the [Mg2+]i-increasing actions of glucose, K+ and tolbutamide. The greatest change in [Mg2+]i was obtained when beta-cells were exposed to 100 microM carbachol. In this case there was a more than 10% lowering, which was reversed upon removal of the agonist. Measurements of [Mg2+]i are important not only for understanding fluctuations of this ion, but may also aid to elucidate the mechanisms involved in the regulation of cytoplasmic Ca2+.

Secretagogue-induced oscillations of cytoplasmic Ca2+ in single beta and alpha-cells obtained from pancreatic islets by fluorescence-activated cell sorting

Pancreatic islet cells from normal adult rats were purified by autofluorescence-activated cell sorting (FACS). A purification of islet beta-cells (greater than 90%) and alpha-cells (greater than 70%) was obtained after FACS separation. Dual-wavelength microfluorimetry was employed for measurements of cytoplasmic Ca2+ concentration (Ca2+i) in single beta- and alpha-cells. In FACS-purified beta-cells, 11.8 mM glucose induced oscillations of Ca2+i with a frequency of 1 cycle per 2-4 min, whereas 20 mM glucose induced a rapid rise in Ca2+i followed by a sustained elevation in Ca2+i. In contrast, alpha-cells exhibited a decrease in Ca2+i following exposure to 20 mM glucose, and subsequent exposure to 20 mM arginine induced a large increase in Ca2+i with periodic fluctuations. FACS purification of a heterogeneous islet-cell population provides a unique model for characterizing secretagogue-induced changes in Ca2+i in single beta-and alpha-cells.

Ca2+ oscillations in pancreatic beta-cells exposed to leucine and arginine

Dual-wavelength microfluorometry with the fura-2 indicator was employed for continuous recordings of cytoplasmic Ca2+ (Ca2+i) in individual pancreatic beta-cells isolated from ob/ob-mice. When added to a medium containing 3 mmol l-1 glucose, both 10 mmol l-1 leucine and 20 mmol l-1 arginine induced rises in Ca2+i with periodic fluctuations. In the case of leucine, this increase was preceded by initial lowering followed by high-amplitude oscillations with a periodicity of 2-6 min. In a glucose-free medium arginine had no effect, and leucine was unable to induce more than a single peak of Ca2+i increase. When present at a concentration of 1 mmol l-1, leucine sometimes induced a couple of high-amplitude oscillations at 3 mmol l-1 glucose but lowered Ca2+i permanently in a glucose-free medium. It is likely that the high-amplitude oscillations of Ca2+i are related to the electrical activity of the beta-cells. Provided that some glucose was present, leucine initiated a similar type of Ca2+i response as obtained during glucose-induced insulin release. The observed leucine effect is therefore compatible with a role of glycolysis in generating high-amplitude Ca2+ oscillations and pulsatile insulin release.

Glucose-induced buffering of cytoplasmic Ca2+ in the pancreatic beta-cell--an artifact or a physiological phenomenon?

The effects of stimulated metabolism on the cytoplasmic Ca2+ concentration (Ca2+i) of insulin-releasing pancreatic beta-cells were studied. When the glucose concentration was increased from 5 to 20 mM, some cell preparations responded with initial lowering of Ca2+i followed by a rise, whereas Ca2+i only increased in others. After prolonged exposure to 5 or 10 mM of the sugar, depolarization with high concentrations of sulfonylurea or K+ caused rapid increases of Ca2+i. However, when subsequently raising glucose to 20 mM there were pronounced temporary decreases of Ca2+i. Marked Ca2+i reducing effects were also obtained after prolonged exposure to 20 mM glucose, when metabolism was augmented further by exposure to leucine or beta-2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. The results indicate that buffering of Ca2+i is not an artifact but may have physiological significance.