Origin of slow and fast oscillations of Ca2+ in mouse pancreatic islets

Glucose-induced cyclic AMP oscillations regulate pulsatile insulin secretion

Cyclic AMP (cAMP) and Ca(2+) are key regulators of exocytosis in many cells, including insulin-secreting beta cells. Glucose-stimulated insulin secretion from beta cells is pulsatile and involves oscillations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), but little is known about the detailed kinetics of cAMP signaling. Using evanescent-wave fluorescence imaging we found that glucose induces pronounced oscillations of cAMP in the submembrane space of single MIN6 cells and primary mouse beta cells. These oscillations were preceded and enhanced by elevations of [Ca(2+)](i). However, conditions raising cytoplasmic ATP could trigger cAMP elevations without accompanying [Ca(2+)](i) rise, indicating that adenylyl cyclase activity may be controlled also by the substrate concentration. The cAMP oscillations correlated with pulsatile insulin release. Whereas elevation of cAMP enhanced secretion, inhibition of adenylyl cyclases suppressed both cAMP oscillations and pulsatile insulin release. We conclude that cell metabolism directly controls cAMP and that glucose-induced cAMP oscillations regulate the magnitude and kinetics of insulin exocytosis.

RyR channels and glucose-regulated pancreatic beta-cells

Ryanodine receptor channel model is introduced to a dynamical model of pancreatic beta-cells to discuss the effects of RyR channels and glucose concentration on membrane potential. The results show Ca2+ concentration changes responding to enhance of glucose concentration is more quickly than that of activating RyR channels, and both methods can induce bursting action potential and increase free cytosolic Ca2+ concentration. An interesting finding is that moderate stimulation to RyR channels will result in a kind of "complex bursting", which is more effective in enhancing average Ca2+ concentration and insulin section.

Phantom bursting is highly sensitive to noise and unlikely to account for slow bursting in beta-cells: considerations in favor of metabolically driven oscillations

Pancreatic beta-cells show bursting electrical activity with a wide range of burst periods ranging from a few seconds, often seen in isolated cells, over tens of seconds (medium bursting), usually observed in intact islets, to several minutes. The phantom burster model [Bertram, R., Previte, J., Sherman, A., Kinard, T.A., Satin, L.S., 2000. The phantom burster model for pancreatic beta-cells. Biophys. J. 79, 2880-2892] provided a framework, which covered this span, and gave an explanation of how to obtain medium bursting combining two processes operating on different time scales. However, single cells are subjected to stochastic fluctuations in plasma membrane currents, which are likely to disturb the bursting mechanism and transform medium bursters into spikers or very fast bursters. We present a polynomial, minimal, phantom burster model and show that noise modifies the plateau fraction and lowers the burst period dramatically in phantom bursters. It is therefore unlikely that slow bursting in single cells is driven by the slow phantom bursting mechanism, but could instead be driven by oscillations in glycolysis, which we show are stable to random ion channel fluctuations. Moreover, so-called compound bursting can be converted to apparent slow bursting by noise, which could explain why compound bursting and mixed Ca(2+) oscillations are seen mainly in intact islets.

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca2+concentration ([Ca2+]i) and proliferation in pancreatic β-cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca2+]iinvolves Ca2+-induced Ca2+release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca2+]iand insulin release in BRIN-BD11 β-cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca2+]iand insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K+concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca2+]ielevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K+. Ovine prolactin increased [Ca2+]iand insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca2+]ibut did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca2+]i. Our study indicates that hGH stimulates rise in [Ca2+]iand insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells.

Glucose modulates [Ca2+]i oscillations in pancreatic islets via ionic and glycolytic mechanisms

Pancreatic islets of Langerhans display complex intracellular calcium changes in response to glucose that include fast (seconds), slow ( approximately 5 min), and mixed fast/slow oscillations; the slow and mixed oscillations are likely responsible for the pulses of plasma insulin observed in vivo. To better understand the mechanisms underlying these diverse patterns, we systematically analyzed the effects of glucose on period, amplitude, and plateau fraction (the fraction of time spent in the active phase) of the various regimes of calcium oscillations. We found that in both fast and slow islets, increasing glucose had limited effects on amplitude and period, but increased plateau fraction. In some islets, however, glucose caused a major shift in the amplitude and period of oscillations, which we attribute to a conversion between ionic and glycolytic modes (i.e., regime change). Raising glucose increased the plateau fraction equally in fast, slow, and regime-changing islets. A mathematical model of the pancreatic islet consisting of an ionic subsystem interacting with a slower metabolic oscillatory subsystem can account for these complex islet calcium oscillations by modifying the relative contributions of oscillatory metabolism and oscillatory ionic mechanisms to electrical activity, with coupling occurring via K(ATP) channels.

Contributions of Intracellular Compartments to Calcium Dynamics: Implicating an Acidic Store

Many cells show a plateau of elevated cytosolic Ca(2+) after a long depolarization, suggesting delayed Ca(2+) release from intracellular compartments such as mitochondria and endoplasmic reticulum (ER). Mouse pancreatic beta-cells show a thapsigargin-sensitive plateau ('hump') of Ca(2+) after a 30 s depolarization but not after a 10 s depolarization. Surprisingly, this hump depends primarily on compartments other than the mitochondria or ER. It is reduced by only 22% upon blocking mitochondrial Na(+)-Ca(2+) exchange and by only 18% upon blocking ryanodine or IP(3) receptors together. Further, the time course of ER Ca(2+) measured by a targeted cameleon does not depend on the duration of depolarizations. Instead, the hump is reduced 35% by treatments with the dipeptide glycylphenylalanine beta-napthylamide, a tool often used to lyse lysosomes. We show that this dipeptide does not disturb ER functions, but it lyses acidic compartments and releases Ca(2+) into the cytosol. Moreover, it induces leaks in and possibly lyses insulin granules and stops mobilization of secretory granules to the readily releasable pool in beta-cells. We conclude that the dipeptide compromises dense-core secretory granules and that these granules comprise an acidic calcium store in beta-cells whose loading and/or release is sensitive to thapsigargin and which releases Ca(2+) after cytosolic Ca(2+) elevation.

Glucose-induced mixed [Ca2+]c oscillations in mouse beta-cells are controlled by the membrane potential and the SERCA3 Ca2+-ATPase of the endoplasmic reticulum

Stimulatory concentrations of glucose induce two patterns of cytosolic Ca2+ concentration ([Ca2+]c) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [Ca2+]c and insulin release from single islets revealed that mixed [Ca2+]c oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single beta-cell within an islet and of [Ca2+]c in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most beta-cells of the islet. Each slow [Ca2+]c increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [Ca2+]c oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA3+/+ beta-cells. We conclude that glucose-induced mixed [Ca2+]c oscillations result from compound bursting in all beta-cells of the islet. Disruption of SERCA3 abolishes mixed [Ca2+]c oscillations and augments beta-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the beta-cell membrane potential during glucose stimulation.

Effect of intracellular delivery of energy metabolites on intracellular Ca2+ in mouse islets of Langerhans

Regulation of glucose-induced oscillations in intracellular Ca2+ concentration ([Ca2+]i) was investigated by using a novel technique, electroporation from an electrolyte-filled capillary, to deliver energy metabolites to the intracellular compartment of mouse islets. Intracellular application of ATP resulted in a nifedipine-sensitive increase in [Ca2+]i, consistent with a KATP-channel dependent mechanism of Ca2+ influx. [Ca2+]i in islets exposed to 10 mM glucose oscillated with a period of approximately 3 min, often superimposed with faster oscillations. Electroporation of ATP blocked all types of oscillations and elevated [Ca2+]i while delivery of ADP had no effect on oscillations. Intracellular delivery of glucose-6-phosphate or fructose-1,6-bisphosphate tended to transform slow oscillations to fast oscillations. These results demonstrate that modulation of ATP concentrations and glycolytic flux are important in development of slow oscillations.

Glucose metabolism and oscillatory behavior of pancreatic islets

A variety of oscillations are observed in pancreatic islets. We establish a model incorporating two oscillatory systems of different time scales: One is the well-known bursting model in pancreatic cells and the other is the glucose-insulin feedback model which considers direct and indirect feedback of secreted insulin. These two are coupled to interact with each other in the combined model, and two basic assumptions are made on the basis of biological observations: The conductance gK(ATP) for the ATP-dependent potassium current is a decreasing function of the glucose concentration whereas the insulin secretion rate is given by a function of the intracellular calcium concentration. Obtained via extensive numerical simulations are complex oscillations including clusters of bursts, slow and fast calcium oscillations, and so on. We also consider how the intracellular glucose concentration depends upon the extracellular glucose concentration, and examine the inhibitory effects of insulin.

Inositol (1,4,5)-trisphosphate dynamics and intracellular calcium oscillations in pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with transients of intracellular calcium concentration ([Ca2+]i) in the pancreatic beta-cell. We tested the hypothesis that inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] [Ca2+]i release is incorporated in glucose-induced [Ca2+]i oscillations in mouse islets and MIN6 cells. We found that depletion of intracellular Ca2+ stores with thapsigargin increased the oscillation frequency by twofold and inhibited the slow recovery phase of [Ca2+]i oscillations. We employed a pleckstrin homology domain-containing fluorescent biosensor, phospholipase C partial differential pleckstrin homology domain-enhanced green fluorescent protein, to visualize Ins(1,4,5)P3 dynamics in insulin-secreting MIN6 cells and mouse islets in real time using a video-rate confocal system. In both types of cells, stimulation with carbamoylcholine (CCh) and depolarization with KCl results in an increase in Ins(1,4,5)P3 accumulation in the cytoplasm. When stimulated with glucose, the Ins(1,4,5)P3 concentration in the cytoplasm oscillates in parallel with oscillations of [Ca2+]i. Maximal accumulation of Ins(1,4,5)P3 in these oscillations coincides with the peak of [Ca2+]i and tracks changes in frequencies induced by the voltage-gated K+ channel blockade. We show that Ins(1,4,5)P3 release in insulin-secreting cells can be stimulated by depolarization-induced Ca2+ flux. We conclude that Ins(1,4,5)P3 concentration oscillates in parallel with [Ca2+]i in response to glucose stimulation, but it is not the driving force for [Ca2+]i oscillations.

FRET-based voltage probes for confocal imaging: membrane potential oscillations throughout pancreatic islets

Insulin secretion is dependent on coordinated pancreatic islet physiology. In the present study, we found a way to overcome the limitations of cellular electrophysiology to optically determine cell membrane potential ( Vm) throughout an islet by using a fast voltage optical dye pair. Using laser scanning confocal microscopy (LSCM), we observed fluorescence (Förster) resonance energy transfer (FRET) with the fluorescent donor N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidyl-ethanolamine and the acceptor bis-(1,3-diethylthiobarbiturate) trimethine oxonol in the plasma membrane of essentially every cell within an islet. The FRET signal was approximately linear from Vm−70 to +50 mV with a 2.5-fold change in amplitude. We evaluated the responses of islet cells to glucose and tetraethylammonium. Essentially, every responding cell in a mouse islet displayed similar time-dependent changes in Vm. When Vmwas measured simultaneously with intracellular Ca2+, all active cells showed tight coupling of Vmto islet cell Ca2+changes. Our findings indicate that FRET-based, voltage-sensitive dyes used in conjunction with LSCM imaging could be extremely useful in studies of excitation-secretion coupling in intact islets of Langerhans.

Substrate effects on oscillations in metabolism, calcium and secretion in single mouse islets of Langerhans

Glucose induces complex patterns of oscillations in intracellular Ca2+ concentration ([Ca2+]i), metabolism and secretion in islets of Langerhans including "slow" and "fast" pulses with period of 2-5 min and 10-20 s respectively. In an effort to elucidate the origin of slow oscillations, individual mouse islets were exposed to different fuels including glyceraldehyde, pyruvate, methyl pyruvate and alpha-ketoisocaproate (KIC), all of which bypass key steps of glycolytic metabolism, while monitoring [Ca2+]i, oxygen consumption and secretion. Glyceraldehyde gave rise to slow oscillations only when substimulatory glucose was also added to the media. Glucosamine, an inhibitor of glucokinase, blocked these slow oscillations. KIC, pyruvate, and methyl pyruvate did not give rise to slow oscillations alone or with glucose present. The addition of glucose to islets bathed in nutrient-rich cell culture media accelerated metabolism and initiated slow oscillations while glyceraldehyde did not. It is concluded that glucose has a special role in accelerating metabolism and generating slow oscillations in isolated islets of Langerhans from mice. Combined with previous observations of Ca2+ dependency for all oscillations in islets, we propose that interactions between Ca2+ influx and glycolysis are responsible for the slow oscillations. In contrast, fast oscillations can occur independent of glycolytic flux.

Three roads to islet bursting: emergent oscillations in coupled phantom bursters

Glucose-induced membrane potential and Ca(2+) oscillations in isolated pancreatic beta-cells occur over a wide range of frequencies, from >6/min (fast) to <1/min (slow). However, cells within intact islets generally oscillate with periods of 10-60 s (medium). The phantom bursting concept addresses how beta-cells can generate such a wide range of frequencies. Here, we explore an updated phantom bursting model to determine how heterogeneity in a single parameter can explain both the broad frequency range observed in single cells and the rarity of medium oscillations. We then incorporate the single-cell model into an islet model with parameter heterogeneity. We show that strongly coupled islets must be composed of predominantly medium oscillating single cells or a mixture of fast and slow cells to robustly produce medium oscillations. Surprisingly, we find that this constraint does not hold for moderate coupling, and that robustly medium oscillating islets can arise from populations of single cells that are essentially all slow or all fast. Thus, with coupled phantom bursters, medium oscillating islets can be constructed out of cells that are either all fast, all slow, or a combination of the two.

Glucose dependence of imidazoline-induced insulin secretion: different characteristics of two ATP-Sensitive K+ channel-blocking compounds

The glucose dependence of the insulinotropic action of KATP channel-blocking imidazoline compounds was investigated. Administration of 100 micromol/l phentolamine, but not 100 micromol/l efaroxan, markedly increased insulin secretion of freshly isolated mouse islets when the perifusion medium contained 5 mmol/l glucose. When the glucose concentration was raised to 10 mmol/l in the continued presence of either imidazoline, a clear potentiation of secretion occurred as compared with 10 mmol/l glucose alone. In the presence of efaroxan, a brisk first-phase-like increase was followed by a sustained phase, whereas a more gradual increase resulted in the presence of phentolamine. Administration of 100 micromol/l phentolamine was somewhat more effective than 100 micromol/l efaroxan to inhibit KATP channel activity in intact cultured beta-cells (reduction by 96 vs. 83%). Both compounds were similarly effective to depolarize the beta-cells. When measured by the perforated patch-technique, the depolarization by efaroxan was often oscillatory, whereas that by phentolamine was sustained. In perifused cultured islets, both compounds increased the cytosolic calcium concentration ([Ca2+]c) in the presence of 5 and 10 mmol/l glucose. Efaroxan induced large amplitude oscillations of [Ca2+]c, whereas phentolamine induced a sustained increase. It appears that a KATP channel block by imidazolines is not incompatible with a glucose-selective enhancement of insulin secretion. The glucose selectivity of efaroxan may involve an inhibitory effect distal to [Ca2+]c increase and/or the generation of [Ca2+]c oscillations.

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.

Analysis of the noise-induced bursting-spiking transition in a pancreatic beta-cell model

A stochastic model of the electrophysiological behavior of the pancreatic beta cell is studied, as a paradigmatic example of a bursting biological cell embedded in a noisy environment. The analysis is focused on the distortion that a growing noise causes to the basic properties of the membrane potential signals, such as their periodic or chaotic nature, and their bursting or spiking behavior. We present effective computational tools to obtain as much information as possible from these signals, and we suggest that the methods could be applied to real time series. Finally, a universal dependence of the main characteristics of the membrane potential on the size of the considered cell cluster is presented.

Oscillations of phospholipase C activity triggered by depolarization and Ca2+ influx in insulin-secreting cells

Phospholipase C (PLC) is a ubiquitous enzyme involved in the regulation of a variety of cellular processes. Its dependence on Ca2+ is well recognized, but it is not known how PLC activity is affected by physiological variations of the cytoplasmic Ca2+ concentration ([Ca2+](i)). Here, we applied evanescent wave microscopy to monitor PLC activity in parallel with [Ca2+](i) in individual insulin-secreting INS-1 cells using the phosphatidylinositol 4,5-bisphosphate- and inositol 1,4,5-trisphosphate-binding pleckstrin homology domain from PLCdelta(1) fused to green fluorescent protein (PH(PLCdelta1)-GFP) and the Ca2+ indicator fura red. In resting cells, PH(PLCdelta1)-GFP was located predominantly at the plasma membrane. Activation of PLC by muscarinic or purinergic receptor stimulation resulted in PH(PLCdelta1)-GFP translocation from the plasma membrane to the cytoplasm, detected as a decrease in evanescent wave-excited PH(PLCdelta1)-GFP fluorescence. Using this translocation as a measure of PLC activity, we found that depolarization by raising extracellular [K+] triggered activation of the enzyme. This effect could be attributed both to a rise of [Ca2+](i) and to depolarization per se, because some translocation persisted during depolarization in a Ca2+-deficient medium containing the Ca2+ chelator EGTA. Moreover, oscillations of [Ca2+](i) resulting from depolarization with Ca2+ influx evoked concentration-dependent periodic activation of PLC. We conclude that PLC activity is under tight dynamic control of [Ca2+](i). In insulin-secreting beta-cells, this mechanism provides a link between Ca2+ influx and release from intracellular stores that may be important in the regulation of insulin secretion.

Crucial role of type 2 inositol 1,4,5-trisphosphate receptors for acetylcholine-induced Ca2+ oscillations in vascular myocytes

OBJECTIVE

The aim of this study was to correlate the expression of InsP3R subtypes in native vascular and visceral myocytes with specific Ca2+-signaling patterns.

METHODS AND RESULTS

By Western blot and immunostaining, we showed that rat portal vein expressed InsP3R1 and InsP3R2 but not InsP3R3, whereas rat ureter expressed InsP3R1 and InsP3R3 but not InsP3R2. Acetylcholine induced single Ca2+ responses in all ureteric myocytes but only in 50% of vascular myocytes. In the remaining vascular myocytes, the first transient peak was followed by Ca2+ oscillations. By correlating Ca2+ signals and immunostaining, we revealed that oscillating vascular cells expressed both InsP3R1 and InsP3R2 whereas nonoscillating vascular cells expressed only InsP3R1. Acetylcholine-induced oscillations were not affected by inhibitors of ryanodine receptors, Ca2+-ATPases, Ca2+ influx, and mitochondrial Ca2+ uniporter but were inhibited by intracellular infusion of heparin. Using specific antibodies against InsP3R subtypes, we showed that acetylcholine-induced Ca2+ oscillations were specifically blocked by the anti-InsP3R antibody. These data were supported by antisense oligonucleotides targeting InsP3R2, which selectively inhibited Ca2+ oscillations.

CONCLUSIONS

Our results suggest that in native smooth muscle cells, a differential expression of InsP3R subtypes encodes specific InsP3-mediated Ca2+ responses and that the presence of the InsP3R2 subtype is required for acetylcholine-induced Ca2+ oscillations in vascular myocytes.

A model of phosphofructokinase and glycolytic oscillations in the pancreatic beta-cell

We have constructed a model of the upper part of the glycolysis in the pancreatic beta-cell. The model comprises the enzymatic reactions from glucokinase to glyceraldehyde-3-phosphate dehydrogenase (GAPD). Our results show, for a substantial part of the parameter space, an oscillatory behavior of the glycolysis for a large range of glucose concentrations. We show how the occurrence of oscillations depends on glucokinase, aldolase and/or GAPD activities, and how the oscillation period depends on the phosphofructokinase activity. We propose that the ratio of glucokinase and aldolase and/or GAPD activities are adequate as characteristics of the glucose responsiveness, rather than only the glucokinase activity. We also propose that the rapid equilibrium between different oligomeric forms of phosphofructokinase may reduce the oscillation period sensitivity to phosphofructokinase activity. Methodologically, we show that a satisfying description of phosphofructokinase kinetics can be achieved using the irreversible Hill equation with allosteric modifiers. We emphasize the use of parameter ranges rather than fixed values, and the use of operationally well-defined parameters in order for this methodology to be feasible. The theoretical results presented in this study apply to the study of insulin secretion mechanisms, since glycolytic oscillations have been proposed as a cause of oscillations in the ATP/ADP ratio which is linked to insulin secretion.

Cx36 involvement in insulin secretion: characteristics and mechanism

Gap junctions connect the pancreatic beta-cells which produce insulin. To investigate their function, we have first determined that these junctions are made of Cx36. We have then tested the effect of changing the expression of Cx36, and other connexin isoforms, and have found that Cx36 modulates insulin secretion. In view of the prominent role of cytosolic Ca(2+) in this secretion, we have monitored this cation, and have found that its handling is altered in populations of insulin-producing cells lacking Cx36. The data identify a first molecular link between Cx36 and the stimulus-secretion pathway leading to insulin secretion.

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

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

The Ca2+ dynamics of isolated mouse beta-cells and islets: implications for mathematical models

[Ca(2+)](i) and electrical activity were compared in isolated beta-cells and islets using standard techniques. In islets, raising glucose caused a decrease in [Ca(2+)](i) followed by a plateau and then fast (2-3 min(-1)), slow (0.2-0.8 min(-1)), or a mixture of fast and slow [Ca(2+)](i) oscillations. In beta-cells, glucose transiently decreased and then increased [Ca(2+)](i), but no islet-like oscillations occurred. Simultaneous recordings of [Ca(2+)](i) and electrical activity suggested that differences in [Ca(2+)](i) signaling are due to differences in islet versus beta-cell electrical activity. Whereas islets exhibited bursts of spikes on medium/slow plateaus, isolated beta-cells were depolarized and exhibited spiking, fast-bursting, or spikeless plateaus. These electrical patterns in turn produced distinct [Ca(2+)](i) patterns. Thus, although isolated beta-cells display several key features of islets, their oscillations were faster and more irregular. beta-cells could display islet-like [Ca(2+)](i) oscillations if their electrical activity was converted to a slower islet-like pattern using dynamic clamp. Islet and beta-cell [Ca(2+)](i) changes followed membrane potential, suggesting that electrical activity is mainly responsible for the [Ca(2+)] dynamics of beta-cells and islets. A recent model consisting of two slow feedback processes and passive endoplasmic reticulum Ca(2+) release was able to account for islet [Ca(2+)](i) responses to glucose, islet oscillations, and conversion of single cell to islet-like [Ca(2+)](i) oscillations. With minimal parameter variation, the model could also account for the diverse behaviors of isolated beta-cells, suggesting that these behaviors reflect natural cell heterogeneity. These results support our recent model and point to the important role of beta-cell electrical events in controlling [Ca(2+)](i) over diverse time scales in islets.

Contribution of the endoplasmic reticulum to the glucose-induced [Ca(2+)](c) response in mouse pancreatic islets

Thapsigargin (TG), a blocker of Ca(2+) uptake by the endoplasmic reticulum (ER), was used to evaluate the contribution of the organelle to the oscillations of cytosolic Ca(2+) concentration ([Ca(2+)](c)) induced by repetitive Ca(2+) influx in mouse pancreatic beta-cells. Because TG depolarized the plasma membrane in the presence of glucose alone, extracellular K(+) was alternated between 10 and 30 mM in the presence of diazoxide to impose membrane potential (MP) oscillations. In control islets, pulses of K(+), mimicking regular MP oscillations elicited by 10 mM glucose, induced [Ca(2+)](c) oscillations whose nadir remained higher than basal [Ca(2+)](c). Increasing the depolarization phase of the pulses while keeping their frequency constant (to mimic the effects of a further rise of the glucose concentration on MP) caused an upward shift of the nadir of [Ca(2+)](c) oscillations that was reproduced by raising extracellular Ca(2+) (to increase Ca(2+) influx) without changing the pulse protocol. In TG-pretreated islets, the imposed [Ca(2+)](c) oscillations were of much larger amplitude than in control islets and occurred on basal levels. During intermittent trains of depolarizations, control islets displayed mixed [Ca(2+)](c) oscillations characterized by a summation of fast oscillations on top of slow ones, whereas no progressive summation of the fast oscillations was observed in TG-pretreated islets. In conclusion, the buffering capacity of the ER in pancreatic beta-cells limits the amplitude of [Ca(2+)](c) oscillations and may explain how the nadir between oscillations remains above baseline during regular oscillations or gradually increases during mixed [Ca(2+)](c) oscillations, two types of response observed during glucose stimulation.

Membrane potential dependent modulations of calcium oscillations in insulin-secreting INS-1 cells

This study was undertaken to examine the role of K(+) channels on cytosolic Ca(2+) ([Ca(2+)](i)) in insulin secreting cells. [Ca(2+)](i) was measured in single glucose-responsive INS-1 cells using the fluorescent Ca(2+) indicator Fura-2. Glucose, tolbutamide and forskolin elevated [Ca(2+)](i) and induced [Ca(2+)] oscillations. Whereas the glucose effect was delayed and observed in 60% and 93% of the cells, in a poorly and a highly glucose-responsive INS-1 cell clone, respectively, tolbutamide and forskolin increased [Ca(2+)](i) in all cells tested. In the latter clone, glucose induced [Ca(2+)](i) oscillations in 77% of the cells. In 16% of the cells a sustained rise of [Ca(2+)](i) was observed. The increase in [Ca(2+)](i) was reversed by verapamil, an L-type Ca(2+) channel inhibitor. Adrenaline decreased [Ca(2+)](i) in oscillating cells in the presence of low glucose and in cells stimulated by glucose alone or in combination with tolbutamide and forskolin. Adrenaline did not lower [Ca(2+)](i) in the presence of 30mM extracellular K(+), indicating that adrenaline does not exert a direct effect on Ca(2+) channels but increases K(+) channel activity. As for primary b-cells, [Ca(2+)](i) oscillations persisted in the presence of closed K(ATP) channels; these also persisted in the presence of thapsigargin, which blocks Ca(2+) uptake into Ca(2+) stores. In contrast, in voltage-clamped cells and in the presence of diazoxide (50mM), which hyperpolarizes the cells by opening K(ATP) channels, [Ca(2+)](i) oscillations were abolished. These results support the hypothesis that [Ca(2+)](i) oscillations depend on functional voltage-dependent Ca(2+) and K(+) channels and are interrupted by a hyperpolarization in insulin-secreting cells.

Glucose-dependent promotion of insulin release from mouse pancreatic islets by the insulin-mimetic compound L-783,281

An insulin-mimetic compound (L-783,281) was used in an attempt to determine the role of the beta-cell insulin receptor (IR) on insulin release. Islets were isolated from C57Bl/6j mice and cultured for 1 to 4 days. Insulin release from individual islets perifused in the presence of 3 mmol/l glucose was 10.5 plus minus 1.4 pg/min. Addition of 10 micromol/l L-783,281 had no effect on the rate of insulin secretion. When L-783,281 was added to perifusion medium containing 11 mmol/l glucose, the insulin-mimetic compound significantly increased insulin release. Insulin release from the isolated islet is pulsatile. In the presence of 3 mmol/l glucose, addition of L-783,281 significantly decreased the frequency of the oscillations from 0.35 plus minus 0.03 to 0.22 plus minus 0.04 oscillations/min. Addition of L-783,281 to perifusion medium containing 11 mmol/l glucose had no effect on the frequency of the insulin pulses (0.30 plus minus 0.05 oscillations/min). These results indicate that activation of the beta-cell IR by L-783,281 augments insulin release in the presence of a stimulatory glucose concentration. At nonstimulatory glucose concentrations, activation of the beta-cell IR may affect mechanisms related to the frequency of the insulin pulses.

Cellular function in multicellular system for hormone-secretion: electrophysiological aspect of studies on alpha-, beta- and delta-cells of the pancreatic islet

We review a new method to explore the cellular functions in multicellular system by application of the perforated patch-clamp technique to intact pancreatic islet of Langerhans. Using this approach, the integrity of the islet is preserved and intercellular communication via gap junctions and paracrine processes are maintained. By using low-resistance patch electrodes, rapid current responses can be monitored under voltage-clamp control. We have applied this methodology to answer questions not resolved by patch-clamp experiments on isolated single insulin-secreting beta-cells. First, the role of a K(+)-current dependent on Ca(2+)-influx for the termination of burst of action potentials in beta-cells could be documented. Neither the current, nor the bursting pattern of electrical activity is preserved in isolated beta-cells. Second, the conductance of gap junctions (approximately 1 nS) between beta-cells was determined. Third, electrical properties of glucagon-producing alpha- and somatostatin-secreting delta-cells and the different mechanisms for glucose-sensing in these cells could be explored. The findings emanating from these experiments may have implications for neuroscience research such as the mechanism of oscillatory electrical activity in general and processes involved in the glucose-sensing in some neurons, which response to changes of blood glucose concentration.

Control mechanisms of the oscillations of insulin secretion in vitro and in vivo

The mechanisms driving the pulsatility of insulin secretion in vivo and in vitro are still unclear. Because glucose metabolism and changes in cytosolic free Ca(2+) ([Ca(2+)](c)) in beta-cells play a key role in the control of insulin secretion, and because oscillations of these two factors have been observed in single isolated islets and beta-cells, pulsatile insulin secretion could theoretically result from [Ca(2+)](c) or metabolism oscillations. We could not detect metabolic oscillations independent from [Ca(2+)](c) changes in beta-cells, and imposed metabolic oscillations were poorly effective in inducing oscillations of secretion when [Ca(2+)](c) was kept stable, which suggests that metabolic oscillations are not the direct regulator of the oscillations of secretion. By contrast, tight temporal and quantitative correlations between the changes in [Ca(2+)](c) and insulin release strongly suggest that [Ca(2+)](c) oscillations are the direct drivers of insulin secretion oscillations. Metabolism may play a dual role, inducing [Ca(2+)](c) oscillations (via changes in ATP-sensitive K(+) channel activity and membrane potential) and amplifying the secretory response by increasing the efficiency of Ca(2+) on exocytosis. The mechanisms underlying the oscillations of insulin secretion by the isolated pancreas and those observed in vivo remain elusive. It is not known how the functioning of distinct islets is synchronized, and the possible role of intrapancreatic ganglia in this synchronization requires confirmation. That pulsatile insulin secretion is beneficial in vivo, by preventing insulin resistance, is suggested by the greater hypoglycemic effect of exogenous insulin when it is infused in a pulsatile rather than continuous manner. The observation that type 2 diabetic patients have impaired pulsatile insulin secretion has prompted the suggestion that such dysregulation contributes to the disease and justifies the efforts toward understanding of the mechanism underlying the pulsatility of insulin secretion both in vitro and in vivo.

Role of oscillations in membrane potential, cytoplasmic Ca2+, and metabolism for plasma insulin oscillations

A model for the relationship between ionic and metabolic oscillations and plasma insulin oscillations is presented. It is argued that the pancreatic beta-cell in vivo displays two intrinsic frequencies that are important for the regulation of plasma insulin oscillations. The rapid oscillatory activity (2--7 oscillations [osc] per minute), which is evident in both ionic and metabolic events, causes the required elevation in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) for the exocytosis of insulin granules. This activity is important for regulation of the amplitude of plasma insulin oscillations. The frequency of the rapid oscillatory ionic activities is regulated by glucose and allows the beta-cell to respond in an analogous way, with gradual changes in [Ca(2+)](i) and insulin release in response to the alterations in glucose concentration. The slower oscillatory activity (0.2--0.4 osc/min), which is evident in the metabolism of the beta-cell, has a frequency corresponding to the frequency observed in plasma insulin oscillations. The frequency is not affected by changes in the glucose concentration. This activity is suggested to generate energy in a pulsatile fashion, which sets the frequency of the plasma insulin oscillations. It is proposed that the slow oscillations in [Ca(2+)](i) observed in vitro are a manifestation of the metabolic oscillations and do not represent an in vivo phenomenon.

Do oscillations of insulin secretion occur in the absence of cytoplasmic Ca2+ oscillations in beta-cells?

That oscillations of the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) in beta-cells induce oscillations of insulin secretion is not disputed, but whether metabolism-driven oscillations of secretion can occur in the absence of [Ca(2+)](i) oscillations is still debated. Because this possibility is based partly on the results of experiments using islets from aged, hyperglycemic, hyperinsulinemic ob/ob mice, we compared [Ca(2+)](i) and insulin secretion patterns of single islets from 4- and 10-month-old, normal NMRI mice to those of islets from 7- and 10-month-old ob/ob mice (Swedish colony) and their lean littermates. The responses were subjected to cluster analysis to identify significant peaks. Control experiments without islets and with a constant insulin concentration were run to detect false peaks. Both ob/ob and NMRI islets displayed large synchronous oscillations of [Ca(2+)](i) and insulin secretion in response to repetitive depolarizations with 30 mmol/l K(+) in the presence of 0.1 mmol/l diazoxide and 12 mmol/l glucose. Continuous depolarization with high K(+) steadily elevated [Ca(2+)](i) in all types of islets, with no significant oscillation, and caused a biphasic insulin response. In islets from young (4-month-old) NMRI mice and 7-month-old lean mice, the insulin profile did not show significant peaks when [Ca(2+)](i) was stable. In contrast, two or more peaks were detected over 20 min in the response of most ob/ob islets. Similar insulin peaks appeared in the insulin response of 10-month-old lean and NMRI mice. However, the size of the insulin peaks detected in the presence of stable [Ca(2+)](i) was small, so that no more than 10-13% of total insulin secretion occurred in a pulsatile manner. In conclusion, insulin secretion does not oscillate when [Ca(2+)](i) is stably elevated in beta-cells from young normal mice. Some oscillations are observed in aged mice and are seen more often in ob/ob islets. These fluctuations of the insulin secretion rate at stably elevated [Ca(2+)](i), however, are small compared with the large oscillations induced by [Ca(2+)](i) oscillations in beta-cells.

Nitric oxide induces synchronous Ca2+ transients in pancreatic beta cells lacking contact

AIMS

To evaluate the role of nitric oxide (NO) in the coordination of the Ca2+ signals generating pulsatile insulin release in pancreatic beta cells isolated from ob/ob mice.

METHODOLOGY

Using ratiometric fura-2 technique for recording glucose-induced cytoplasmic Ca2+ transients, it was possible to demonstrate a synchronization of beta cells lacking contact.

RESULTS

The frequency of the transients increased 10-fold in the presence of 20 n M glucagon. Additional increase in frequency with maintenance of synchronization was observed when the beta cells were exposed to 100 microM of the NO donors sodium nitroprusside and hydroxylamine. Bolus additions of 0.1-10 microM gaseous NO resulted in prompt appearance of cytoplasmic Ca2+ transients. An activator of soluble guanylate cyclase (mesoporphyrin) increased the frequency of the transients, and inhibition of this enzyme with 1H-(1,2,4) oxadiazolo [4,3-a] quinoxalin-1-one had the opposite effect.

CONCLUSION

The results support the idea that nitrergic nerves generate beta-cell transients of Ca2+ synchronizing the activity of the numerous islets in the pancreas.

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.

Inositol 1,4,5-trisphosphate receptor isoform expression in mouse pancreatic islets: effects of carbachol

The inositol 1,4,5-trisphosphate receptors (IP3Rs) are ligand-gated Ca2+ channels that regulate intracellular Ca2+ mobilization. Among the IP3R mRNA isoforms I, II, and III, IP3R-I mRNA was expressed in mouse islets and the beta-cell line betaTC3, and was quantitatively the most abundant isoform as determined by reverse transcriptase-polymerase chain reaction. IP3R-II and -III mRNAs were expressed at similar levels in mouse islets, but neither isoform was detected in betaTC3 cells. Culture of mouse islets for 30 min and 2 hr at 20 mM glucose, or for 7 days at 11 mM glucose did not affect IP3R-I mRNA expression compared with islets cultured in 5.5 mM glucose. Culture of islets or betaTC3 cells with carbachol (0.5 mM) reduced IP3R-I mRNA expression levels below control. Mouse islet alpha- and beta-cells expressed IP3R-I and -III proteins, but IP3R-II protein was not detected by immunoblot or double-label immunohistochemistry. Culture of islets for up to 6 hr with carbachol reduced IP3R-I and -III protein expression in a time-dependent manner with a half-maximal effect on type I at 1 hr. Glucose (20 mM) stimulation for 2 hr did not affect IP3R-1 levels. The carbachol-induced decrease in IP3R-I and -III protein expression was reversed by carbobenzoxyl-leucinyl-leucinyl-leucinyl-H (MG-132), a proteasome inhibitor. Thus, glucose failed to regulate mouse islet IP3R mRNA expression, whereas carbachol stimulation down-regulated IP3R mRNA and protein. A proteasomal protein degradative pathway appeared to mediate the muscarinic receptor-induced effects on IP3R-I and -III.

The endoplasmic reticulum is a glucose-modulated high-affinity sink for Ca2+ in mouse pancreatic beta-cells

The regulation of organelle free Ca2+ was analysed in individual mouse pancreatic beta-cells loaded with the fluorescent low-affinity indicator furaptra. Removal of the cytoplasmic indicator by controlled digitonin permeabilization of the plasma membrane resulted in a sudden increase of the 340 nm/380 nm fluorescence excitation ratio followed by a gradual decay, reflecting the emptying of Ca2+ from organelle pools. Subsequent introduction of 3 mM ATP caused rapid refilling of a Ca2+ pool, which represented the endoplasmic reticulum (ER) in being mobilized with inositol 1,4,5-trisphosphate (IP3) and the sarco(endo)plasmic reticulum Ca2+-ATPase inhibitor thapsigargin. The concentration of Ca2+ in the ER observed immediately after permeabilization depended on the glucose concentration in a hyperbolic fashion with half-maximal filling at about 6 mM of the sugar. Glucose promotion of Ca2+ sequestration in the ER involved a high-affinity mechanism not requiring but accelerated by a rise of the cytoplasmic Ca2+ concentration. Glucose also exerted a long-term action on the ER storage of Ca2+, maintaining the set-point for its maximal concentration and preserving the response to IP3. The results indicate that the ER has an important role in the glucose-stimulated beta-cell by serving as a high-affinity sink for Ca2+, irrespective of the prevailing concentration of cytoplasmic Ca2+.

The phantom burster model for pancreatic beta-cells

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

Differential regulation of [Ca2+]i oscillations in mouse pancreatic islets by glucose, alpha-ketoisocaproic acid, glyceraldehyde and glycolytic intermediates

Glucose induces slow oscillations of the cytoplasmic Ca2+ concentration in pancreatic beta-cells. In order to elucidate the mechanisms responsible for the slow [Ca2+]i oscillations the effects of various nutrient insulin secretagogues on glucose-induced [Ca2+]i oscillations in intact mouse pancreatic islets and single beta-cells were studied. These were the glycolytic intermediates, glyceraldehyde and pyruvate, and the mitochondrial substrate, alpha-ketoisocaproic acid (KIC). Glucose, at a 10 or 15 mM concentration, induced the typical slow oscillations of [Ca2+]i (0.4 min(-1)). At higher glucose concentrations the frequency of these oscillations decreased further (0.2 min(-1)). Glyceraldehyde, an insulin secretagogue like glucose, did not cause slow oscillations of [Ca2+]i in the absence of glucose. However, it exhibited a synergistic action with glucose. Glyceraldehyde, at 3 or 10 mM concentration, induced slow [Ca2+]i oscillations at a substimulatory concentration of 5 mM glucose (0.3-0.4 min(-1)) and reduced the frequency of the glucose-induced [Ca2+]i oscillations at stimulatory concentrations of 10 or 15 mM glucose (0.2 min(-1)). KIC (5 or 10 mM) as well as pyruvate (10 mM), the end product of glycolysis, and its ester methyl pyruvate (10 mM), did not cause slow oscillations of [Ca2+]i in the absence of glucose. In contrast to glyceraldehyde, however, all three compounds were capable of preventing the slow [Ca2+]i oscillations induced by glucose. Mannoheptulose (2 mM), an inhibitor of glucokinase and glucose-induced insulin secretion, reversibly blocked any kind of [Ca2+]i oscillation and returned the [Ca2+]i to a basal level through its ability to inhibit glycolytic flux. It can be concluded therefore that only substrates which generate a glucokinase-mediated metabolic flux through glycolysis and produce glycolytic ATP can induce slow [Ca2+]i oscillations in pancreatic beta-cells.

Tetracaine stimulates insulin secretion through the mobilization of Ca2+from thapsigargin- and IP3-insensitive Ca2+reservoir in pancreatic β-cells

The effect of tetracaine on45Ca efflux, cytoplasmic Ca2+concentration [Ca2+]i, and insulin secretion in isolated pancreatic islets and β-cells was studied. In the absence of external Ca2+, tetracaine (0.1-2.0 mM) increased the45Ca efflux from isolated islets in a dose-dependant manner. Tetracaine did not affect the increase in45Ca efflux caused by 50 mM K+or by the association of carbachol (0.2 mM) and 50 mM K+. Tetracaine permanently increased the [Ca2+]iin isolated β-cells in Ca2+-free medium enriched with 2.8 mM glucose and 25 µM D-600 (methoxiverapamil). This effect was also observed in the presence of 10 mM caffeine or 1 µM thapsigargin. In the presence of 16.7 mM glucose, tetracaine transiently increased the insulin secretion from islets perfused in the absence and presence of external Ca2+. These data indicate that tetracaine mobilises Ca2+from a thapsigargin-insensitive store and stimulates insulin secretion in the absence of extracellular Ca2+. The increase in45Ca efflux caused by high concentrations of K+and by carbachol indicates that tetracaine did not interfere with a cation or inositol triphosphate sensitive Ca2+pool in β-cells.

Dynamical complexity and temporal plasticity in pancreatic beta-cells

We discuss some of the biological and mathematical issues involved in understanding and modelling the bursting electrical activity in pancreatic beta-cells. These issues include single-cell versus islet behaviour, parameter heterogeneity, channel noise, the effects of hormones, neurotransmitters, and ions, and multiple slow biophysical processes. Some of the key experimental and modelling studies are described, and some of the major open questions are discussed.

Regulation of inositol trisphosphate receptor isoform expression in glucose-desensitized rat pancreatic islets: role of cyclic adenosine 3',5'-monophosphate and calcium

The regulation of inositol 1,4,5-trisphosphate receptor (IP3R) messenger RNA (mRNA) and protein expression was investigated in glucose-desensitized rat isolated pancreatic islets. Islets were cultured for 4 days with glucose (11 mM; G-treated) to induce desensitization; IP3R-I mRNA levels were similar to basal (5.5 mM glucose) values, whereas IP3R-II mRNA levels were increased and IP3R-III levels were reduced compared with basal levels. Somatostatin increased the expression of IP3R-II mRNA and reduced the expression of IP3R-III mRNA compared with basal values, but did not significantly affect G-treated islet IP3R expression. When forskolin (FSK), 8-bromo-cAMP, and glucagon-like peptide 1-(7-36) amide were added to G-treated islets after 4 days of culture, IP3R-II mRNA levels were reduced, whereas IP3R-III mRNA levels increased, to levels observed in control islets, within 3 h. The levels of IP3R-I mRNA were unaffected by either somatostatin or FSK. The protein kinase A inhibitor. H-89, and actinomycin D prevented the effects of FSK. A Ca2+ ionophore mimicked the effects of FSK on IP3R mRNA expression, whereas blockade of voltage-dependent Ca2+ channels or chelation of intracellular Ca2+ inhibited the actions of FSK. cAMP also increased IP3R-III mRNA in insulinoma cells. In G-treated islets, FSK slowed IP3R-III mRNA degradation. FSK, but not glucose, stimulated protein kinase A activation in G-treated islets. Thus, cAMP mediates changes in IP3R-II and -III mRNA transcription and stability in glucose-desensitized islets. The regulated expression of IP3R-II and -III mRNA is mediated in part by intracellular Ca2+ availability.

Norepinephrine acts on the KATP channel and produces different effects on [Ca2+]i in oscillating and non-oscillating HIT-T15 cells

Norepinephrine (NE) is an inhibitor of insulin secretion that acts, in part, by decreasing intracellular free calcium ([Ca2+]i). We examined the effects of NE on [Ca2+]i in individual HIT-T15 cells loaded with indo 1. Cells were categorized as oscillators or non-oscillators on the basis of the pattern of the calcium response to glucose and the effect of NE on [Ca2+]i was subsequently measured in each cell. NE caused a simple decrease in [Ca2+]i in nonoscillators. In oscillators, NE decreased the amplitude and frequency of the oscillations. Furthermore, the duration of the NE effect in oscillators was longer than in non-oscillators. NE did not affect the rise in [Ca2+]i elicited by depolarizing concentrations of 20 mM or 35 mM KCl alone, or in the presence of 20 mM KCl, 100 microM diazoxide, and 10 mM glucose. In other experiments, NE had no effect on [Ca2+]i when the KATP channels were fully clamped with diazoxide or tolbutamide. We conclude that the action of NE to decrease [Ca2+]i in both oscillators and non-oscillators is mediated via activation of the KATP channel. Despite this common mechanism, NE exerts different effects on oscillating and non-oscillating cells.

Rhythmic bursts of calcium transients in acute anterior pituitary slices

Endocrine cells isolated from the anterior pituitary fire intracellular Ca2+ ([Ca2+]i) transients due to voltage-gated Ca2+ entry. However, the patterns of [Ca2+]i transients within the glandular parenchyma of the anterior pituitary are unknown. Here we describe, using real-time confocal laser microscopy, several spontaneous patterns of calcium signaling in acute pituitary slices prepared from male as well as cycling and lactating female rats. Forty percent of the cells demonstrated a spontaneous bursting mode, consisting of an active period of [Ca2+]i transients firing at a constant frequency, followed by a rest period during which cells were either silent or randomly active. The remaining recordings from endocrine cells either demonstrated random [Ca2+]i transients or were silent. These rhythmic bursts of [Ca2+]i transients, which required extracellular calcium, were detected in lactotrophs, somatotrophs, and corticotrophs within the acute slices. Of significance was the finding that the bursting mode could be adjusted by hypothalamic factors. In slices prepared from lactating rats, TRH recruited more bursting cells and finely adjusted the average duty cycle of [Ca2+]i bursts such that cells fired patterned bursts for approximately 70% of the recording period. Eighty-six percent of these cells were lactotrophs. Thus, the rhythmic [Ca2+]i bursts and their tuning by secretagogues may provide timing information that could encode for one or more cellular functions (e.g. exocytosis and/or gene expression) critical for the release of hormones by endocrine cells in the intact gland.

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.

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.

Detection of multiple patterns of oscillatory oxygen consumption in single mouse islets of Langerhans

A novel oxygen microsensor was used to measure oxygen levels in single mouse islets as a function of glucose concentration. Oxygen consumption of individual islets was 5.99 +/- 1.17, 9.21 +/- 2.15, and 12.22 +/- 2.16 pmol/min at 3, 10, and 20 mM glucose, respectively (mean +/- SEM, n = 10). Consumption of oxygen was islet-size dependent as larger islets consumed more oxygen than smaller islets but smaller islets consumed more oxygen per unit volume than larger islets. Elevating glucose levels from 3 to 10 mM induced pronounced fast oscillations in oxygen level (period of 12.1 +/- 1.7 s, n = 6) superimposed on top of large slow oscillations (period of 3.3 +/- 0.6 min, n = 6). The fast oscillations could be completely abolished by treatment with the L-type Ca2+-channel blocker nifedipine (40 microM) with a lesser effect on slow oscillations. Slow oscillations were almost completely dependent upon extracellular Ca2+. The oxygen patterns closely mimic those that have previously been reported for intracellular Ca2+ levels and are suggestive of an important role for Ca2+ in amplifying metabolic oscillations.

Voltage-dependent entry and generation of slow Ca2+ oscillations in glucose-stimulated pancreatic beta-cells

The role of voltage-dependent Ca2+ entry for glucose generation of slow oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i) was evaluated in individual mouse pancreatic beta-cells. Like depolarization with K+, a rise of the glucose concentration resulted in an enhanced influx of Mn2+, which was inhibited by nifedipine. This antagonist of L-type Ca2+ channels also blocked the slow oscillations of [Ca2+]i induced by glucose. The slow oscillations occurred in synchrony with variations in Mn2+ influx and bursts of action currents, with the elevation of [Ca2+]i being proportional to the frequency of the action currents. A similar relationship was obtained when Ca2+ was replaced with Sr2+. Occasionally, the slow [Ca2+]i oscillations were superimposed with pronounced spikes temporarily arresting the action currents. It is concluded that the glucose-induced slow oscillations of [Ca2+]i are caused by periodic depolarization with Ca2+ influx through L-type channels. Ca2+ spiking, due to intracellular mobilization, may be important for chopping the slow oscillations of [Ca2+]i into shorter ones characterizing beta-cells situated in pancreatic islets.

Synchronization of glucose-induced Ca2+ transients in pancreatic beta-cells by a diffusible factor

Glucose is known to induce transients of cytoplasmic Ca2+ by mobilizing intracellular stores when pancreatic beta-cells are exposed to glucagon. Dual wavelength microfluorometry with fura-2 was used to study such transients in individual beta-cells isolated from ob/ob-mice. The Ca2+ transients were often synchronized in beta-cells situated up to 80 microm apart. The messenger might be nitric oxide, as indicated from a decreased number of synchronized transients in the presence of 500 micromol/l oxyhemoglobin or 10 mmol/l Nomega-nitro-L-arginine methyl ester. The discovery that Ca2+ transients are synchronized in the absence of cell contact indicates the involvement of a diffusible factor in coordinating the activity of the insulin-releasing beta-cells.