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

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

γ-Hydroxybutyrate does not mediate glucose inhibition of glucagon secretion

Hypersecretion of glucagon from pancreatic α-cells strongly contributes to diabetic hyperglycemia. Moreover, failure of α-cells to increase glucagon secretion in response to falling blood glucose concentrations compromises the defense against hypoglycemia, a common complication in diabetes therapy. However, the mechanisms underlying glucose regulation of glucagon secretion are poorly understood and likely involve both α-cell-intrinsic and intraislet paracrine signaling. Among paracrine factors, glucose-stimulated release of the GABA metabolite γ-hydroxybutyric acid (GHB) from pancreatic β-cells might mediate glucose suppression of glucagon release via GHB receptors on α-cells. However, the direct effects of GHB on α-cell signaling and glucagon release have not been investigated. Here, we found that GHB (4-10 μm) lacked effects on the cytoplasmic concentrations of the secretion-regulating messengers Ca2+ and cAMP in mouse α-cells. Glucagon secretion from perifused mouse islets was also unaffected by GHB at both 1 and 7 mm glucose. The GHB receptor agonist 3-chloropropanoic acid and the antagonist NCS-382 had no effects on glucagon secretion and did not affect stimulation of secretion induced by a drop in glucose from 7 to 1 mm Inhibition of endogenous GHB formation with the GABA transaminase inhibitor vigabatrin also failed to influence glucagon secretion at 1 mm glucose and did not prevent the suppressive effect of 7 mm glucose. In human islets, GHB tended to stimulate glucagon secretion at 1 mm glucose, an effect mimicked by 3-chloropropanoic acid. We conclude that GHB does not mediate the inhibitory effect of glucose on glucagon secretion.

Glucose controls glucagon secretion by directly modulating cAMP in alpha cells

AIMS/HYPOTHESIS

Glucagon is critical for normal glucose homeostasis and aberrant secretion of the hormone aggravates dysregulated glucose control in diabetes. However, the mechanisms by which glucose controls glucagon secretion from pancreatic alpha cells remain elusive. The aim of this study was to investigate the role of the intracellular messenger cAMP in alpha-cell-intrinsic glucose regulation of glucagon release.

METHODS

Subplasmalemmal cAMP and Ca²⁺ concentrations were recorded in isolated and islet-located alpha cells using fluorescent reporters and total internal reflection microscopy. Glucagon secretion from mouse islets was measured using ELISA.

RESULTS

Glucose induced Ca²⁺-independent alterations of the subplasmalemmal cAMP concentration in alpha cells that correlated with changes in glucagon release. Glucose-lowering-induced stimulation of glucagon secretion thus corresponded to an elevation in cAMP that was independent of paracrine signalling from insulin or somatostatin. Imposed cAMP elevations stimulated glucagon secretion and abolished inhibition by glucose elevation, while protein kinase A inhibition mimicked glucose suppression of glucagon release.

CONCLUSIONS/INTERPRETATION

Glucose concentrations in the hypoglycaemic range control glucagon secretion by directly modulating the cAMP concentration in alpha cells independently of paracrine influences. These findings define a novel mechanism for glucose regulation of glucagon release that underlies recovery from hypoglycaemia and may be disturbed in diabetes.

cAMP signalling in insulin and glucagon secretion

The "second messenger" archetype cAMP is one of the most important cellular signalling molecules with central functions including the regulation of insulin and glucagon secretion from the pancreatic β- and α-cells, respectively. cAMP is generally considered as an amplifier of insulin secretion triggered by Ca²⁺ elevation in the β-cells. Both messengers are also positive modulators of glucagon release from α-cells, but in this case cAMP may be the important regulator and Ca²⁺ have a more permissive role. The actions of cAMP are mediated by protein kinase A (PKA) and the guanine nucleotide exchange factor Epac. The present review focuses on how cAMP is regulated by nutrients, hormones and neural factors in β- and α-cells via adenylyl cyclase-catalysed generation and phosphodiesterase-mediated degradation. We will also discuss how PKA and Epac affect ion fluxes and the secretory machinery to transduce the stimulatory effects on insulin and glucagon secretion. Finally, we will briefly describe disturbances of the cAMP system associated with diabetes and how cAMP signalling can be targeted to normalize hypo- and hypersecretion of insulin and glucagon, respectively, in diabetic patients.

Fluorescent protein vectors for pancreatic islet cell identification in live-cell imaging

The islets of Langerhans contain different types of endocrine cells, which are crucial for glucose homeostasis. β- and α-cells that release insulin and glucagon, respectively, are most abundant, whereas somatostatin-producing δ-cells and particularly pancreatic polypeptide-releasing PP-cells are more scarce. Studies of islet cell function are hampered by difficulties to identify the different cell types, especially in live-cell imaging experiments when immunostaining is unsuitable. The aim of the present study was to create a set of vectors for fluorescent protein expression with cell-type-specific promoters and evaluate their applicability in functional islet imaging. We constructed six adenoviral vectors for expression of red and green fluorescent proteins controlled by the insulin, preproglucagon, somatostatin, or pancreatic polypeptide promoters. After transduction of mouse and human islets or dispersed islet cells, a majority of the fluorescent cells also immunostained for the appropriate hormone. Recordings of the sub-plasma membrane Ca(2+) and cAMP concentrations with a fluorescent indicator and a protein biosensor, respectively, showed that labeled cells respond to glucose and other modulators of secretion and revealed a striking variability in Ca(2+) signaling among α-cells. The measurements allowed comparison of the phase relationship of Ca(2+) oscillations between different types of cells within intact islets. We conclude that the fluorescent protein vectors allow easy identification of specific islet cell types and can be used in live-cell imaging together with organic dyes and genetically encoded biosensors. This approach will facilitate studies of normal islet physiology and help to clarify molecular defects and disturbed cell interactions in diabetic islets.

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

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

Submembrane ATP and Ca2+ kinetics in α-cells: unexpected signaling for glucagon secretion

Cytoplasmic ATP and Ca(2+) are implicated in current models of glucose's control of glucagon and insulin secretion from pancreatic α- and β-cells, respectively, but little is known about ATP and its relation to Ca(2+) in α-cells. We therefore expressed the fluorescent ATP biosensor Perceval in mouse pancreatic islets and loaded them with a Ca(2+) indicator. With total internal reflection fluorescence microscopy, we recorded subplasma membrane concentrations of Ca(2+) and ATP ([Ca(2+)]pm; [ATP]pm) in superficial α- and β-cells of intact islets and related signaling to glucagon and insulin secretion by immunoassay. Consistent with ATP's controlling glucagon and insulin secretion during hypo- and hyperglycemia, respectively, the dose-response relationship for glucose-induced [ATP]pm generation was left shifted in α-cells compared to β-cells. Both cell types showed [Ca(2+)]pm and [ATP]pm oscillations in opposite phase, probably reflecting energy-consuming Ca(2+) transport. Although pulsatile insulin and glucagon release are in opposite phase, [Ca(2+)]pm synchronized in the same phase between α- and β-cells. This paradox can be explained by the overriding of Ca(2+) stimulation by paracrine inhibition, because somatostatin receptor blockade potently stimulated glucagon release with little effect on Ca(2+). The data indicate that an α-cell-intrinsic mechanism controls glucagon in hypoglycemia and that paracrine factors shape pulsatile secretion in hyperglycemia.

Neurotransmitter control of islet hormone pulsatility

Pulsatile secretion is an inherent property of hormone-releasing pancreatic islet cells. This secretory pattern is physiologically important and compromised in diabetes. Neurotransmitters released from islet cells may shape the pulses in auto/paracrine feedback loops. Within islets, glucose-stimulated β-cells couple via gap junctions to generate synchronized insulin pulses. In contrast, α- and δ-cells lack gap junctions, and glucagon release from islets stimulated by lack of glucose is non-pulsatile. Increasing glucose concentrations gradually inhibit glucagon secretion by α-cell-intrinsic mechanism/s. Further glucose elevation will stimulate pulsatile insulin release and co-secretion of neurotransmitters. Excitatory ATP may synchronize β-cells with δ-cells to generate coinciding pulses of insulin and somatostatin. Inhibitory neurotransmitters from β- and δ-cells can then generate antiphase pulses of glucagon release. Neurotransmitters released from intrapancreatic ganglia are required to synchronize β-cells between islets to coordinate insulin pulsatility from the entire pancreas, whereas paracrine intra-islet effects still suffice to explain coordinated pulsatile release of glucagon and somatostatin. The present review discusses how neurotransmitters contribute to the pulsatility at different levels of integration.

Glucose regulation of glucagon secretion

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

Oscillations of sub-membrane ATP in glucose-stimulated beta cells depend on negative feedback from Ca(2+)

AIMS/HYPOTHESIS

ATP links changes in glucose metabolism to electrical activity, Ca(2+) signalling and insulin secretion in pancreatic beta cells. There is evidence that beta cell metabolism oscillates, but little is known about ATP dynamics at the plasma membrane, where regulation of ion channels and exocytosis occur.

METHODS

The sub-plasma-membrane ATP concentration ([ATP]pm) was recorded in beta cells in intact mouse and human islets using total internal reflection microscopy and the fluorescent reporter Perceval.

RESULTS

Glucose dose-dependently increased [ATP]pm with half-maximal and maximal effects at 5.2 and 9 mmol/l, respectively. Additional elevations of glucose to 11 to 20 mmol/l promoted pronounced [ATP]pm oscillations that were synchronised between neighbouring beta cells. [ATP]pm increased further and the oscillations disappeared when voltage-dependent Ca(2+) influx was prevented. In contrast, K(+)-depolarisation induced prompt lowering of [ATP]pm. Simultaneous recordings of [ATP]pm and the sub-plasma-membrane Ca(2+) concentration ([Ca(2+)]pm) during the early glucose-induced response revealed that the initial [ATP]pm elevation preceded, and was temporarily interrupted by the rise of [Ca(2+)]pm. During subsequent glucose-induced oscillations, the increases of [Ca(2+)]pm correlated with lowering of [ATP]pm.

CONCLUSIONS/INTERPRETATION

In beta cells, glucose promotes pronounced oscillations of [ATP]pm, which depend on negative feedback from Ca(2+) . The bidirectional interplay between these messengers in the sub-membrane space generates the metabolic and ionic oscillations that underlie pulsatile insulin secretion.

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.

Isolated mouse islets respond to glucose with an initial peak of glucagon release followed by pulses of insulin and somatostatin in antisynchrony with glucagon

Recent studies of isolated human islets have shown that glucose induces hormone release with repetitive pulses of insulin and somatostatin in antisynchrony with those of glucagon. Since the mouse is the most important animal model we studied the temporal relation between hormones released from mouse islets. Batches of 5-10 islets were perifused and the hormones measured with radioimmunoassay in 30s fractions. At 3mM glucose, hormone secretion was stable with no detectable pulses of glucagon, insulin or somatostatin. Increase of glucose to 20mM resulted in an early secretory phase with a glucagon peak followed by peaks of insulin and somatostatin. Subsequent hormone secretion was pulsatile with a periodicity of 5min. Cross-correlation analyses showed that the glucagon pulses were antisynchronous to those of insulin and somatostatin. In contrast to the marked stimulation of insulin and somatostatin secretion, the pulsatility resulted in inhibition of overall glucagon release. The cytoarchitecture of mouse islets differs from that of human islets, which may affect the interactions between the hormone-producing cells. Although indicating that paracrine regulation is important for the characteristic patterns of pulsatile hormone secretion, the mouse data mimic those of human islets with more than 20-fold variations of the insulin/glucagon ratio. The data indicate that the mouse serves as an appropriate animal model for studying the temporal relation between the islet hormones controlling glucose production in the liver.

Ghrelin attenuates cAMP-PKA signaling to evoke insulinostatic cascade in islet β-cells

OBJECTIVE

Ghrelin reportedly restricts insulin release in islet β-cells via the Gα(i2) subtype of G-proteins and thereby regulates glucose homeostasis. This study explored whether ghrelin regulates cAMP signaling and whether this regulation induces insulinostatic cascade in islet β-cells.

RESEARCH DESIGN AND METHODS

Insulin release was measured in rat perfused pancreas and isolated islets and cAMP production in isolated islets. Cytosolic cAMP concentrations ([cAMP](i)) were monitored in mouse MIN6 cells using evanescent-wave fluorescence imaging. In rat single β-cells, cytosolic protein kinase-A activity ([PKA](i)) and Ca(2+) concentration ([Ca(2+)](i)) were measured by DR-II and fura-2 microfluorometry, respectively, and whole cell currents by patch-clamp technique.

RESULTS

Ghrelin suppressed glucose (8.3 mmol/L)-induced insulin release in rat perfused pancreas and isolated islets, and these effects of ghrelin were blunted in the presence of cAMP analogs or adenylate cyclase inhibitor. Glucose-induced cAMP production in isolated islets was attenuated by ghrelin and enhanced by ghrelin receptor antagonist and anti-ghrelin antiserum, which counteract endogenous islet-derived ghrelin. Ghrelin inhibited the glucose-induced [cAMP](i) elevation and [PKA](i) activation in MIN6 and rat β-cells, respectively. Furthermore, ghrelin potentiated voltage-dependent K(+) (Kv) channel currents without altering Ca(2+) channel currents and attenuated glucose-induced [Ca(2+)](i) increases in rat β-cells in a PKA-dependent manner.

CONCLUSIONS

Ghrelin directly interacts with islet β-cells to attenuate glucose-induced cAMP production and PKA activation, which lead to activation of Kv channels and suppression of glucose-induced [Ca(2+)](i) increase and insulin release.

Glucose- and hormone-induced cAMP oscillations in α- and β-cells within intact pancreatic islets

OBJECTIVE

cAMP is a critical messenger for insulin and glucagon secretion from pancreatic β- and α-cells, respectively. Dispersed β-cells show cAMP oscillations, but the signaling kinetics in cells within intact islets of Langerhans is unknown.

RESEARCH DESIGN AND METHODS

The subplasma-membrane cAMP concentration ([cAMP](pm)) was recorded in α- and β-cells in the mantle of intact mouse pancreatic islets using total internal reflection microscopy and a fluorescent translocation biosensor. Cell identification was based on the opposite effects of adrenaline on cAMP in α- and β-cells.

RESULTS

In islets exposed to 3 mmol/L glucose, [cAMP](pm) was low and stable. Glucagon and glucagon-like peptide-1(7-36)-amide (GLP-1) induced dose-dependent elevation of [cAMP](pm), often with oscillations synchronized among β-cells. Whereas glucagon also induced [cAMP](pm) oscillations in most α-cells, <20% of the α-cells responded to GLP-1. Elevation of the glucose concentration to 11-30 mmol/L in the absence of hormones induced slow [cAMP](pm) oscillations in both α- and β-cells. These cAMP oscillations were coordinated with those of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in the β-cells but not caused by the changes in [Ca(2+)](i). The transmembrane adenylyl cyclase (AC) inhibitor 2'5'-dideoxyadenosine suppressed the glucose- and hormone-induced [cAMP](pm) elevations, whereas the preferential inhibitors of soluble AC, KH7, and 1,3,5(10)-estratrien-2,3,17-β-triol perturbed cell metabolism and lacked effect, respectively.

CONCLUSIONS

Oscillatory [cAMP](pm) signaling in secretagogue-stimulated β-cells is maintained within intact islets and depends on transmembrane AC activity. The discovery of glucose- and glucagon-induced [cAMP](pm) oscillations in α-cells indicates the involvement of cAMP in the regulation of pulsatile glucagon secretion.

cAMP mediators of pulsatile insulin secretion from glucose-stimulated single beta-cells

Pulsatile insulin release from glucose-stimulated beta-cells is driven by oscillations of the Ca(2+) and cAMP concentrations in the subplasma membrane space ([Ca(2+)](pm) and [cAMP](pm)). To clarify mechanisms by which cAMP regulates insulin secretion, we performed parallel evanescent wave fluorescence imaging of [cAMP](pm), [Ca(2+)](pm), and phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) in the plasma membrane. This lipid is formed by autocrine insulin receptor activation and was used to monitor insulin release kinetics from single MIN6 beta-cells. Elevation of the glucose concentration from 3 to 11 mm induced, after a 2.7-min delay, coordinated oscillations of [Ca(2+)](pm), [cAMP](pm), and PIP(3). Inhibitors of protein kinase A (PKA) markedly diminished the PIP(3) response when applied before glucose stimulation, but did not affect already manifested PIP(3) oscillations. The reduced PIP(3) response could be attributed to accelerated depolarization causing early rise of [Ca(2+)](pm) that preceded the elevation of [cAMP](pm). However, the amplitude of the PIP(3) response after PKA inhibition was restored by a specific agonist to the cAMP-dependent guanine nucleotide exchange factor Epac. Suppression of cAMP formation with adenylyl cyclase inhibitors reduced already established PIP(3) oscillations in glucose-stimulated cells, and this effect was almost completely counteracted by the Epac agonist. In cells treated with small interfering RNA targeting Epac2, the amplitudes of the glucose-induced PIP(3) oscillations were reduced, and the Epac agonist was without effect. The data indicate that temporal coordination of the triggering [Ca(2+)](pm) and amplifying [cAMP](pm) signals is important for glucose-induced pulsatile insulin release. Although both PKA and Epac2 partake in initiating insulin secretion, the cAMP dependence of established pulsatility is mediated by Epac2.

Glucose generates coincident insulin and somatostatin pulses and antisynchronous glucagon pulses from human pancreatic islets

The kinetics of insulin, glucagon and somatostatin release was studied in human pancreatic islets. Batches of 10-15 islets were perifused and the hormones measured with RIA in 30-sec fractions. Increase of glucose from 3 to 20 mm resulted in a brief pulse of glucagon coinciding with suppression of basal insulin and somatostatin release. There was a subsequent drop of glucagon release concomitant with the appearance of a pronounced pulse of insulin and a slightly delayed pulse of somatostatin. Continued exposure to 20 mm glucose generated pulsatile release of the three hormones with 7- to 8-min periods accounting for 60-70% of the secreted amounts. Glucose caused pronounced stimulation of average insulin and somatostatin release. However, the nadirs between the glucagon pulses were lower than the secretion at 3 mm glucose, resulting in 18% suppression of average release. The repetitive glucagon pulses were antisynchronous to coincident pulses of insulin and somatostatin. The resulting greater than 20-fold variations of the insulin to glucagon ratio might be essential for minute-to-minute regulation of the hepatic glucose production.

Oscillatory control of insulin secretion

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

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.

Glucose inhibits glucagon secretion by a direct effect on mouse pancreatic alpha cells

AIMS/HYPOTHESIS

The mechanisms by which glucose regulates glucagon release are poorly understood. The present study aimed to clarify the direct effects of glucose on the glucagon-releasing alpha cells and those effects mediated by paracrine islet factors.

MATERIALS AND METHODS

Glucagon, insulin and somatostatin release were measured from incubated mouse pancreatic islets and the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) recorded in isolated mouse alpha cells.

RESULTS

Glucose inhibited glucagon release with maximal effect at 7 mmol/l. Since this concentration corresponded to threshold stimulation of insulin secretion, it is unlikely that inhibition of glucagon secretion is mediated by beta cell factors. Although somatostatin secretion data seemed consistent with a role of this hormone in glucose-inhibited glucagon release, a somatostatin receptor type 2 antagonist stimulated glucagon release without diminishing the inhibitory effect of glucose. In islets exposed to tolbutamide plus 8 mmol/l K(+), glucose inhibited glucagon secretion without stimulating the release of insulin and somatostatin, indicating a direct inhibitory effect on the alpha cells that was independent of ATP-sensitive K(+) channels. Glucose lowered [Ca(2+)](i) of individual alpha cells independently of somatostatin and beta cell factors (insulin, Zn(2+) and gamma-aminobutyric acid). Glucose suppression of glucagon release was prevented by inhibitors of the sarco(endo)plasmic reticulum Ca(2+)-ATPase, which abolished the [Ca(2+)](i)-lowering effect of glucose on isolated alpha cells.

CONCLUSIONS/INTERPRETATION

Beta cell factors or somatostatin do not seem to mediate glucose inhibition of glucagon secretion. We instead propose that glucose has a direct inhibitory effect on mouse alpha cells by suppressing a depolarising Ca(2+) store-operated current.

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.

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

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

Protein Kinase C Modulates Agonist-sensitive Release of Ca2+ from Internal Stores in HEK293 Cells Overexpressing the Calcium Sensing Receptor

This study examined the mechanism of Ca2+ entry and the role of protein kinase C (PKC) in Ca2+ signaling induced by activation of the calcium sensing receptor (CaR) in HEK293 cells stably expressing the CaR. We demonstrate that influx of Ca2+ following CaR activation exhibits store-operated characteristics in being associated with Ca2+ store depletion and inhibited by 2-aminoethoxydiphenyl borate. Inhibition of PKC with GF109203X, Go6983, or Go6976 and down-regulation of PKC activity enhanced the release of Ca2+ from internal stores in response to the polyvalent cationic CaR agonist neomycin, whereas activation of PKC with acute 12-O-tetradecanoylphorbol-13-acetate treatment decreased the release. In contrast, overexpression of wild type PKC-alpha or -epsilon augmented the neomycin-induced release of Ca2+ from internal stores, whereas dominant negative PKC-epsilon strongly decreased the release, but dominant negative PKC-alpha had little effect. Prolonged treatment of cells with 12-O-tetradecanoylphorbol-13-acetate effectively down-regulated immunoreactive PKC-alpha but had little effect on the expression of PKC-epsilon. Together these results indicate that diacylglycerol-responsive PKC isoforms differentially influence CaR agonist-induced release of Ca2+ from internal stores. The fundamentally different results obtained when overexpressing or functionally down-regulating specific PKC isoforms as compared with pharmacological manipulation of PKC activity indicate the need for caution when interpreting data obtained with the latter approach.

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

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

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

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

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

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

Involvement of ?1 and ?-adrenoceptors in adrenaline stimulation of the glucagon-secreting mouse ?-cell

Stimulation of glucagon release and inhibition of insulin secretion from the islets of Langerhans are important for the blood-glucose-elevating effect of adrenaline. The mechanisms by which adrenaline accomplishes these actions may involve direct effects and indirect ones mediated by altered release of other islet hormones. In the present study we investigated how adrenaline affects the cytoplasmic Ca2+ concentration, which controls glucagon secretion from the pancreatic alpha-cell. The studies were performed on isolated mouse alpha-cells, which were identified by immunocytochemistry. The adrenaline effects consisted of initial mobilisation of intracellular Ca2+, accompanied by voltage-dependent influx of the ion. Part of the effect could be attributed to beta-adrenoceptor activation, as it was mimicked by the rise in cAMP and inhibited by the antagonist propranolol as well as the protein kinase A inhibitor adenosine 3',5'-cyclic monophosphorothioate Rp-isomer. alpha1-Adrenoceptors were also involved, since the antagonists phentolamine and prazosin completely abolished the effects of adrenaline. Experiments with clonidine and yohimbine gave little evidence of a role of alpha2-adrenoceptors. The results indicate that alpha1- and beta-adrenoceptors on the alpha-cells mediate adrenaline-stimulated glucagon secretion. The complete inhibition of the adrenaline response after blocking alpha1-adrenoceptors indicates an interaction with the beta-adrenergic pathway.

The hepatitis C virus core protein modulates T cell responses by inducing spontaneous and altering T-cell receptor-triggered Ca2+ oscillations

Alterations of cytokine responses are thought to favor the establishment of persistent hepatitis C virus (HCV) infection, enhancing the risk of liver cirrhosis and hepatocellular carcinoma. Expression of the HCV core (C) protein modulates transcription of the IL-2 promoter in T lymphocytes by activating the nuclear factor of activated T lymphocyte (NFAT) pathway. Here we report on the effect of HCV C on Ca2+ signaling, which is essential for activation of NFAT. Expression of HCV C correlated with increased levels of cytosolic Ca2+ and spontaneous Ca2+ oscillations in transfected Jurkat cells. Triggering of the T-cell receptor induced a prolonged Ca2+ response characterized by vigorous high frequent oscillations in a high proportion of the responding cells. This was associated with decreased sizes and accelerated emptying of the intracellular calcium stores. The effect of HCV C on calcium mobilization was not dependent on phospholipase C-gamma 1 (PLC-gamma) activity or increased inositol 1,4,5-trisphosphate (IP3) production and did not require functional IP3 receptors, suggesting that insertion of the viral protein in the endoplasmic reticulum membrane may be sufficient to promote Ca2+ leakage with dramatic downstream consequences on the magnitude and duration of the response. Our data suggest that expression of HCV C in infected T lymphocytes may contribute to the establishment of persistent infections by inducing Ca2+ oscillations that regulate both the efficacy and information content of Ca2+ signals and are ultimately responsible for induction of gene expression and functional differentiation.

Stretch activation of Ca2+ transients in pancreatic beta cells by mobilization of intracellular stores

INTRODUCTION

Nonadrenergic, noncholinergic neurons have been proposed to synchronize pulsatile insulin release from the islets in the pancreas by triggering transient increases of the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells via an inositol trisphoshate-dependent mechanism.

AIMS

To test whether pancreatic beta-cells respond to stretch activation with similar types of transients and whether these Ca signals propagate to other beta-cells in the presence and absence of cell contacts.

METHODOLOGY

Single cells and small aggregates were prepared from beta-cell-rich islets from mice. After 2-5 days of culture, [Ca2+]i was measured with digital imaging and the indicator fura-2 during superfusion with a medium containing 20 mmol/L glucose and 50 micromol/L methoxyverapamil. Membrane stretch was induced by osmotic swelling or focal touch stimulation.

RESULTS

Lowering the medium osmolarity with 100-102 mOSM/L by removal of sucrose or by dilution resulted in a 2-3-fold increase in the number of transients during an initial 5-minute period. Sucrose omission was stimulatory also after isosmolar replacement with readily penetrating urea. The intracellular Ca2+-ATPase inhibitor thapsigargin suppressed both the spontaneously occurring transients and those initiated by volume expansion. Touch stimuli induced [Ca2+]i transients, which rapidly propagated to cells within the same aggregate or lacking contact.

CONCLUSION

The observations support the idea that beta-cells both receive and regenerate extracellular signals triggering [Ca2+]i transients. Touch stimulation is a useful tool for investigating the propagation of [Ca2+]i signals between pancreatic beta-cells lacking physical contact.

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.

Defective glucose-dependent cytosolic Ca2+ handling in islets of GK and nSTZ rat models of type 2 diabetes

We examined to what extent the abnormal glucose-dependent insulin secretion observed in NIDDM (non-insulin-dependent diabetes mellitus) is related to alterations in the handling of cytosolic Ca2+ of islets of Langerhans. Using two recognized rat models of NIDDM, the GK (Goto-Kakizaki) spontaneous model and the nSTZ (neonatal streptozotocin) induced model, we could detect several common alterations in the glucose-induced [Ca2+]i cytosolic responses. First, the initial reduction of [Ca2+]i following high glucose (16.7 mM) observed routinely in islets obtained from non-diabetic Wistar rats could not be detected in GK and nSTZ islets. Second, a delayed response for glucose to induce a subsequent 3% increase of [Ca2+]i over basal level was observed in both GK (321+/-40 s, n=11) and nSTZ (326+/-38 s, n=13) islets as compared with Wistar islets (198+/-20 s, n=11), values representing means+/-s.e.m. Third, the rate of increase in [Ca2+]i in response to a high glucose challenge was 25% and 40% lower in GK and nSTZ respectively, as compared with Wistar islets. Fourth, the maximal [Ca2+](i) level reached after 10 min of perifusion with 16.7 mM glucose was lower with GK and nSTZ islets and represented respectively 60% and 90% of that of Wistar islets. Further, thapsigargin, a blocker of Ca2+/ATPases (SERCA), abolished the initial reduction in [Ca2+]i observed in response to high glucose and induced fast [Ca2+]i oscillations with high amplitude in Wistar islets. The latter effect was not seen in GK and nSTZ islets. In these two NIDDM models, several common alterations in glucose-induced Ca2+ handling were revealed which may contribute to their poor glucose-induced insulin secretion.

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+.

Oxytocin-induced oscillations of cytoplasmic Ca2+ in human myometrial cells

BACKGROUND

To investigate the mechanisms of oxytocin (OT) induced oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i) in cultured human myometrial cells.

METHODS

[Ca2+]i was measured in individual myometrial cells by dual wavelength spectrophotofluorometry using the fluorescent indicator fura-2. Myometrium was obtained at abdominal hysterectomy (n=8) and during cesarean section (n=7).

RESULTS

OT (10-300 nM) typically induced [Ca2+]i oscillations with frequencies in the 0.6-0.8/min range. There were no obvious differences in the responses of cells taken from non-pregnant and term pregnant women. The frequency and amplitude of the oscillations were not significantly affected by OT concentrations up to 300 nM. The amplitude of the oscillations decreased in the presence of the voltage-dependent Ca2+ channel antagonist verapamil and gradually disappeared in Ca2+-free medium. The oscillations were further blocked by the inorganic Ca2+ antagonist La3+ and by the intracellular Ca2+-ATPase inhibitor 2.5-di-tert-butylhydroquinone (DTBHQ). Caffeine inhibited the OT-induced oscillations in a concentration-dependent manner. DTBHQ and high concentrations of OT made [Ca2+]i remarkably sensitive to changes in the external Ca2+ concentration.

CONCLUSIONS

The results indicate that OT-induced [Ca2+]i oscillations in human myometrial cells are due to inositol 1,4,5-trisphosphate-mediated release of intracellular Ca2+ combined with capacitative as well as voltage-dependent influx of the ion.

Caffeine inhibits a low affinity but not a high affinity mechanism for cholecystokinin-evoked Ca2+ signalling and amylase release from guinea pig pancreatic acini

Caffeine has been found to inhibit the formation and action of Ca2+-mobilizing inositol 1,4,5-trisphosphate (IP3) in pancreatic acinar cells. The aim of the present study was to investigate the effects of caffeine on cytoplasmic Ca2+ concentrations ([Ca2+]i) and amylase release in response to different agonists. [Ca2+]i was determined by cytofluorometry using fura-2 as indicator and amylase release with a substrate reagent. Stimulation with low concentrations of carbachol or cholecystokinin octapeptide (CCK-8) induces [Ca2+]i oscillations whereas higher concentrations cause sustained elevation of [Ca2+]i. The less efficacious agonists pilocarpine and CCK-JMV-180 evoke oscillations only. Caffeine inhibited carbachol-induced elevation of [Ca2+]i and amylase responses in a competitive manner, abolishing the responses to low and incompletely inhibiting the responses to high concentrations of the agonist. Also, the [Ca2+]i elevations by pilocarpine were abolished by caffeine. The effects on CCK-8-induced elevation of [Ca2+]i and amylase secretion were paradoxical, the caffeine inhibition being more pronounced at high than at low concentrations of CCK-8. This enigma was further emphasized by moderate effects of caffeine on the responses to CCK-JMV-180. The results indicate that carbachol, pilocarpine and high concentrations of CCK-8 elicit IP3-mediated responses and that CCK-JMV-180 and low concentrations of CCK-8 elevate [Ca2+]i and stimulate amylase release by another signal transduction mechanism.

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

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

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

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

Ca2+ signaling in mouse pancreatic polypeptide cells

Ca2+ signaling was studied in pancreatic polypeptide (PP)-secreting cells isolated from mouse islets of Langerhans. After measuring the cytoplasmic Ca2+ concentration ([Ca2+]i), the cells were identified by immunocytochemistry. Most PP-cells reacted to carbachol and epinephrine with prompt and reversible elevation of [Ca2+]i, often manifested as slow oscillations. The carbachol effect was muscarinic, because it was inhibited by atropine. Beta-adrenergic elevation of cAMP explains the epinephrine stimulation, which was mimicked by an activator of adenylate cyclase and blocked by an inhibitor of protein kinase A. The responses to carbachol and epinephrine apparently involve depolarization with opening of voltage-dependent Ca2+ channels, because the effects were prevented by the Ca2+ channel antagonist methoxyverapamil and by diazoxide, which activates ATP-dependent K+ (K(ATP)) channels. Being equipped with K(ATP) channels, the PP-cells often responded to tolbutamide or high concentrations of glucose with elevation of [Ca2+]i. Somatostatin reversed the [Ca2+]i elevation obtained by carbachol, epinephrine, tolbutamide, and glucose. These preliminary studies support the idea that glucose has a direct stimulatory effect on the PP-cells, which can be masked by locally released somatostatin. Expressing both K(ATP) channels and voltage-dependent Ca2+ channels, the PP-cells share fundamental regulatory mechanisms with other types of islet cells.

Requirement of the Src homology 2 domain protein Shb for T cell receptor-dependent activation of the interleukin-2 gene nuclear factor for activation of T cells element in Jurkat T cells

Stimulation of the T cell antigen receptor (TCR) induces tyrosine phosphorylation of numerous intracellular proteins. We have recently investigated the role of the adaptor protein Shb in the early events of T cell signaling and observed that Shb associates with Grb2, linker for activation of T cells (LAT) and the TCR zeta-chain in Jurkat cells. We now report that Shb also associates with phospholipase C-gamma1 (PLC-gamma1) in these cells. Overexpression of Src homology 2 domain defective Shb caused diminished phosphorylation of LAT and consequently the activation of mitogen-activated protein kinases was decreased upon TCR stimulation. In addition, the Shb mutant also blocked phosphorylation of PLC-gamma1 and the increase in cytoplasmic Ca(2+) following TCR stimulation. Nuclear factor for activation of T cells is a major target for Ras and calcium signaling pathways in T cells following TCR stimulation, and the overexpression of the mutant Shb prevented TCR-dependent activation of the nuclear factor for activation of T cells. Consequently, endogenous interleukin-2 production was decreased under these conditions. The results indicate a role for Shb as a link between the TCR and downstream signaling events involving LAT and PLC-gamma1 and resulting in the activation of transcription of the interleukin-2 gene.

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.

Cell interactions with collagen matrices in vivo and in vitro depend on phosphatidylinositol 3-kinase and free cytoplasmic calcium

We have investigated the role of phosphatidylinositol 3-kinase (PI3-kinase) in cellular interactions with collagenous matrices. Platelet-derived growth factor-BB (PDGF-BB) elicited a mobilization of intracellular Ca2+ in pig aortic endothelial (PAE) cells transfected with wild type PDGF beta-receptor. This response was greatly reduced in PAE cells transfected with PDGF beta-receptors mutated at positions Y740 and Y751 to prevent PI3-kinase binding. The experimental drug 1D-myo-inositol 1,2,6-trisphosphate (alpha-trinositol) induced a rapid increase and subsequent oscillations of the cytoplasmic Ca2+ concentration in cultured fibroblasts. This response was not due to an effect of alpha-trinositol on inositol 1,4,5-trisphosphate (IP3) receptors. alpha-Trinositol did not influence PDGF-BB elicited chemotaxis through collagen-coated membranes of PAE cells transfected with the wild-type PDGF beta-receptor, but restored PDGF-BB elicited chemotaxis of PAE cells transfected with the PI3-kinase binding-site mutated PDGF beta-receptor. Collagen gel contraction has been suggested to serve as a model for cellular control of interstitial fluid pressure (PIF) in dermis. The PI3-kinase inhibitors wortmannin (50 nM) and LY294002 (5 microM) inhibited the stimulation of fibroblast-mediated collagen gel contraction by 0.4 nM PDGF-BB. Injection of wortmannin in rat paw skin induced a lowering of PIF, and this effect was abolished in animals pre-treated with alpha-trinositol. Pretreatment of rats with alpha-trinositol abolished the decrease in PIF induced by injecting monoclonal anti-rat alpha 2 beta 1 integrin IgG in rat paw skin. Taken together our data indicate that cell-collagen interactions in vivo and in vitro depend on PI3-kinase, and that this dependence can be bypassed by a drug eliciting intracellular Ca2+ mobilization.

In situ characterization of nonmitochondrial Ca2+ stores in individual pancreatic beta-cells

Free Ca2+ was measured in intracellular stores of individual mouse pancreatic beta-cells using dual-wavelength microfluorometry and the low-affinity Ca2+ indicator furaptra. Controlled permeabilization of the plasma membrane with 4 micromol/l digitonin revealed that 22% of the furaptra was trapped in intracellular nonnuclear compartments. When 3 mmol/l ATP and 200 nmol/l Ca2+ were simultaneously present, this cation rapidly accumulated in the organelle pool, reaching an average concentration of 200-500 micromol/l. Whereas agents affecting the mitochondrial function (5 mmol/l succinate, 2 micromol/l ruthenium red, or 10 micromol/l antimycin A + 2 microg/ml oligomycin) had little effects, the Ca2+-ATPase inhibitor thapsigargin released 92% of the Ca2+ mobilizable with the ionophore Br-A23187. Digital imaging revealed regional differences in the organelle Ca2+. The regions with the highest Ca2+ concentration were particularly responsive to inositol 1,4,5-trisphosphate (IP3). IP3 mobilized Ca2+ in a dose-dependent way with half-maximal and maximal effects at about 1 and 5 micromol/l, respectively. High concentrations of IP3 released about half of the thapsigargin-sensitive Ca2+, but there were no responses to agents known to activate ryanodine receptors, such as 10 mmol/l caffeine, 0.1-1 micromol/l ryanodine, or 1-5 micromol/l cyclic ADP ribose. The results reinforce the concept that mobilization of intracellular Ca2+ in the pancreatic beta-cell is mediated by IP3 receptors rather than ryanodine receptors.

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

1. Pancreatic islets exposed to 11 mM glucose exhibited complex variations of cytoplasmic Ca2+ concentration ([Ca2+]i) with slow (0.3-0.9 min-1) or fast (2-7 min-1) oscillations or with a mixed pattern. 2. Using digital imaging and confocal microscopy we demonstrated that the mixed pattern with slow and superimposed fast oscillations was due to separate cell populations with the respective responses. 3. In islets with mixed [Ca2+]i oscillations, exposure to the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase inhibitors thapsigargin or 2,5-di-tert-butylhydroquinone (DTBHQ) resulted in a selective disappearance of the fast pattern and amplification of the slow pattern. 4. In addition, the protein kinase A inhibitor RP-cyclic adenosine 3',5'-monophosphorothioate sodium salt transformed the mixed [Ca2+]i oscillations into slow oscillations with larger amplitude. 5. Islets exhibiting only slow oscillations reacted to low concentrations of glucagon with induction of the fast or the mixed pattern. In this case the fast oscillations were also counteracted by DTBHQ. 6. The spontaneously occurring fast oscillations seemed to require the presence of cAMP-elevating glucagon, since they were more common in large islets and suppressed during culture. 7. Image analysis revealed [Ca2+]i spikes occurring irregularly in time and space within an islet. These spikes were preferentially observed together with fast [Ca2+]i oscillations, and they became more common after exposure to glucagon. 8. Both the slow and fast oscillations of [Ca2+]i in pancreatic islets rely on periodic entry of Ca2+. However, the fast oscillations also depend in some way on paracrine factors promoting mobilization of Ca2+ from intracellular stores. It is proposed that such a mobilization in different cells within a tightly coupled islet syncytium generates spikes which co-ordinate the regular bursts of action potentials underlying the fast oscillations.

Generation of glucose-dependent slow oscillations of cytoplasmic Ca2+ in individual pancreatic beta cells

Individual pancreatic beta cells respond to glucose stimulation with large amplitude (300-500 nM) oscillations in the cytoplasmic Ca2+ concentration ([Ca2+]i). These oscillations (frequency 0.05-0.5/min) depend on rhythmical depolarization of the plasma membrane, with influx of Ca2+ through voltage-operated channels, but do not require intracellular mobilization of Ca2+. Patch clamp analyses of the activity of ATP-sensitive K+ channels indicate that oscillations in beta-cell metabolism underlie the rhythmical depolarizations, causing the large amplitude oscillations of [Ca2+]. The oscillatory responses of adjacent beta cells are synchronized by gap-junctional coupling in cellular microdomains. With increasing glucose concentration, previously unresponsive domains are activated, and their oscillations entrained with those of other active domains. In pancreatic islets, glucose-induced large amplitude oscillations occur in parallel with insulin release pulses, the amplitudes of which are determined by the number of beta cells recruited into the secretory state.

Pulsatile insulin release from pancreatic islets with nonoscillatory elevation of cytoplasmic Ca2+

The relationship between insulin release and cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in isolated pancreatic islets from ob/ob mice. Although [Ca2+]i was low and stable in the presence of 3 mM glucose, basal insulin release exhibited low amplitude pulsatility, with a frequency of 0.32 +/- 0.04 min-1. Depolarization by raising K+ from 5.9 to 30.9 mM or by the addition of 1 mM tolbutamide caused a pronounced initial insulin pulse followed by declining pulses, but there was no change in frequency. This decline in amplitude of the insulin pulses was prevented in similar experiments performed in the presence of 11 mM glucose. Corresponding measurements of [Ca2+]i in islets exposed to tolbutamide or the high K+ concentration revealed stable elevations without oscillations. Although the [Ca2+]i level is an important determinant for the rate of secretion, the results indicate that pulsatile insulin release does not always depend on [Ca2+]i oscillations. It is suggested that cyclic generation of ATP may fuel pulsatile release under conditions when [Ca2+]i remains stable.