Purinergic receptors on insulin-secreting cells

Adenosine protects pancreatic beta cells against apoptosis induced by endoplasmic reticulum stress

Chronic exposure to high glucose induces endoplasmic reticulum (ER) stress in pancreatic beta cells (PBCs). The previous evidence showed that adenosine modulate PBCs viability and insulin secretion. The aim of this study was to evaluate possible involvement of adenosine in protection of MIN6 β-cells from Tunicamycin (Tu)-induced ER stress. MIN6 cells were cotreated with Tu and different concentrations of adenosine. Cell viability, proliferation, and apoptosis were evaluated using 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT), 5-bromo-2'-deoxyuridine (Brdu), and colony formation assays. Caspase-12 activity was assayed using the fluorometric method. Thioflavin T (ThT) staining was used for the evaluation of protein aggregation. Insulin secretion was evaluated using specific an ELISA kit. Ca²⁺ mobilization assayed using Fura2/AM probe. BIP, CHOP, XBP-1, and XBP-1s expression in both messenger RNA (mRNA) and protein levels were evaluated using the reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. Bcl-2, p-eIF2α/eIF2α, and GADD34 levels also determined with Western blot analysis. Adenosine protected MIN6 cells against Tu-induced ER stress in a dose-dependent manner and increased their proliferation. Decreased caspase-12 activity and upregulated Bcl-2 protein may explain antiapoptotic effects of adenosine. ThT staining indicated an attenuated aggregation of misfolded proteins. Adenosine effectively increased insulin secretion in Tu-treated cells. BIP, CHOP, XBP1, and sXBP1 expression were decreased significantly in cotreated cells, indicating alleviation of ER stress. However, adenosine potentiated the expression of GADD34 and decreased p-eIF2α/eIF2α ratio. Adenosine increased cytosolic Ca ²⁺ levels, which may promote adenosine triphosphate (ATP) synthesis in mitochondria, helping ER to preserve protein hemostasis. Taken together, adenosine upregulated Bcl-2 and GADD34 to protect PBCs against Tu-induced apoptosis and increase Insulin secretion.

Possible neuroprotective role of P2X2 in the retina of diabetic rats

BACKGROUND

Purinergic receptors are expressed in different tissues including the retina. These receptors are involved in processes like cell growth, proliferation, activation and survival. ATP is the major activator of P2 receptors. In diabetes, there is a constant ATP production and this rise of ATP leads to a persistent activation of purinergic receptors. Antagonists of these receptors are used to evaluate their inhibition effects. Recently, the P2X2 has been reported to have a neuroprotective role.

METHODS

We carried out a study in groups of diabetic and non-diabetic rats (N = 5) treated with intraperitoneal injections of PPADS, at 9 and 24 weeks of diabetes. Control group received only the buffer. Animals were euthanized at 34 weeks of diabetes or at a matching age. Rat retinas were analyzed with immunohistochemistry and western blot using antibodies against GFAP, P2X2, P2Y2 and VEGF-A.

RESULTS

Diabetic animals treated with PPADS disclosed a much more extended staining of VEGF-A than diabetics without treatment. A lower protein expression of VEGF-A was found at the retina of diabetic animals without treatment of purinergic antagonists compared to diabetics with the antagonist treatment. Inhibition of P2X2 receptor by PPADS decreases cell death in the diabetic rat retina.

CONCLUSION

Results might be useful for better understanding the pathophysiology of diabetic retinopathy.

Unlocking the Potential of Purinergic Signaling in Transplantation

Purinergic signaling has been recognized as playing an important role in inflammation, angiogenesis, malignancy, diabetes and neural transmission. Activation of signaling pathways downstream from purinergic receptors may also be implicated in transplantation and related vascular injury. Following transplantation, the proinflammatory "danger signal" adenosine triphosphate (ATP) is released from damaged cells and promotes proliferation and activation of a variety of immune cells. Targeting purinergic signaling pathways may promote immunosuppression and ameliorate inflammation. Under pathophysiological conditions, nucleotide-scavenging ectonucleotidases CD39 and CD73 hydrolyze ATP, ultimately, to the anti-inflammatory mediator adenosine. Adenosine suppresses proinflammatory cytokine production and is associated with improved graft survival and decreased severity of graft-versus-host disease. Furthermore, purinergic signaling is involved both directly and indirectly in the mechanism of action of several existing immunosuppressive drugs, such as calcineurin inhibitors and mammalian target of rapamycin inhibitors. Targeting of purinergic receptor pathways, particularly in the setting of combination therapies, could become a valuable immunosuppressive strategy in transplantation. This review focuses on the role of the purinergic signaling pathway in transplantation and immunosuppression and explores possible future applications in clinical practice.

Pancreatic β-cell identity, glucose sensing and the control of insulin secretion

Insulin release from pancreatic β-cells is required to maintain normal glucose homoeostasis in man and many other animals. Defective insulin secretion underlies all forms of diabetes mellitus, a disease currently reaching epidemic proportions worldwide. Although the destruction of β-cells is responsible for Type 1 diabetes (T1D), both lowered β-cell mass and loss of secretory function are implicated in Type 2 diabetes (T2D). Emerging results suggest that a functional deficiency, involving de-differentiation of the mature β-cell towards a more progenitor-like state, may be an important driver for impaired secretion in T2D. Conversely, at least in rodents, reprogramming of islet non-β to β-cells appears to occur spontaneously in models of T1D, and may occur in man. In the present paper, we summarize the biochemical properties which define the 'identity' of the mature β-cell as a glucose sensor par excellence. In particular, we discuss the importance of suppressing a group of 11 'disallowed' housekeeping genes, including Ldha and the monocarboxylate transporter Mct1 (Slc16a1), for normal nutrient sensing. We then survey the changes in the expression and/or activity of β-cell-enriched transcription factors, including FOXO1, PDX1, NKX6.1, MAFA and RFX6, as well as non-coding RNAs, which may contribute to β-cell de-differentiation and functional impairment in T2D. The relevance of these observations for the development of new approaches to treat T1D and T2D is considered.

Mitochondrial and ER-targeted eCALWY probes reveal high levels of free Zn2+

Zinc (Zn2+) ions are increasingly recognized as playing an important role in cellular physiology. Whereas the free Zn2+ concentration in the cytosol has been established to be 0.1-1 nM, the free Zn2+ concentration in subcellular organelles is not well-established. Here, we extend the eCALWY family of genetically encoded Förster Resonance Energy Transfer (FRET) Zn2+ probes to permit measurements in the endo(sarco)plasmic reticulum (ER) and mitochondrial matrix. Deployed in a variety of mammalian cell types, these probes reveal resting mitochondrial free [Zn2+] values of ∼300 pM, somewhat lower than in the cytosol but 3 orders of magnitude higher than recently reported using an alternative FRET-based sensor. By contrast, free ER [Zn2+] was found to be ≥5 nM, which is >5000-fold higher than recently reported but consistent with the proposed role of the ER as a mobilizable Zn2+ store. Treatment of β-cells or cardiomyocytes with sarco(endo)plasmic reticulum Ca2+-ATPase inhibitors, mobilization of ER Ca2+ after purinergic stimulation with ATP, or manipulation of ER redox, exerted no detectable effects on [Zn2+]ER. These findings question the previously proposed role of Ca2+ in Zn2+ mobilization from the ER and suggest that high ER Zn2+ levels may be an important aspect of cellular homeostasis.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

ATP regulates sodium channel kinetics in pancreatic islet beta cells

Pancreatic beta cells act as glucose sensors, in which intracellular ATP ([ATP](i)) are altered with glucose concentration change. The characterization of voltage-gated sodium channels under different [ATP](i) remains unclear. Here, we demonstrated that increasing [ATP](i) within a certain range of concentrations (2-8 mM) significantly enhanced the voltage-gated sodium channel currents, compared with 2 mM cytosolic ATP. This enhancement was attenuated by even high intracellular ATP (12 mM). Furthermore, elevated ATP modulated the sodium channel kinetics in a dose-dependent manner. Increased [ATP](i) shifted both the current-voltage curve and the voltage-dependent inactivation curve of sodium channel to the right. Finally, the sodium channel recovery from inactivation was significantly faster when the intracellular ATP level was increased, especially in 8 mM [ATP](i), which is an attainable concentration by the high glucose stimulation. In summary, our data suggested that elevated cytosolic ATP enhanced the activity of Na(+) channels, which may play essential roles in modulating β cell excitability and insulin release when blood glucose concentration increases.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

Effects of adenosine A(1) receptor antagonism on insulin secretion from rat pancreatic islets

Adenosine is known to influence different kinds of cells, including beta-cells of the pancreas. However, the role of this nucleoside in the regulation of insulin secretion is not fully elucidated. In the present study, the effects of adenosine A(1) receptor antagonism on insulin secretion from isolated rat pancreatic islets were tested using DPCPX, a selective adenosine A(1) receptor antagonist. It was demonstrated that pancreatic islets stimulated with 6.7 and 16.7 mM glucose and exposed to DPCPX released significantly more insulin compared with islets incubated with glucose alone. The insulin-secretory response to glucose and low forskolin appeared to be substantially potentiated by DPCPX, but DPCPX was ineffective in the presence of glucose and high forskolin. Moreover, DPCPX failed to change insulin secretion stimulated by the combination of glucose and dibutyryl-cAMP, a non-hydrolysable cAMP analogue. Studies on pancreatic islets also revealed that the potentiating effect of DPCPX on glucose-induced insulin secretion was attenuated by H-89, a selective inhibitor of protein kinase A. It was also demonstrated that formazan formation, reflecting metabolic activity of cells, was enhanced in islets exposed to DPCPX. Moreover, DPCPX was found to increase islet cAMP content, whereas ATP was not significantly changed. These results indicate that adenosine A(1) receptor blockade in rat pancreatic islets potentiates insulin secretion induced by both physiological and supraphysiological glucose concentrations. This effect is proposed to be due to increased metabolic activity of cells and increased cAMP content.

Involvement of P2X receptors in the regulation of insulin secretion, proliferation and survival in mouse pancreatic β-cells

In order to clarify the functional role of ionotropic purinergic (P2X) receptors in pancreatic β-cells, we examined the effect of several P2 receptor agonists and antagonists on insulin secretion by mouse pancreatic islets, mouse Beta-TC6 cell proliferation and survival of dispersed islet cells in culture. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed the expression of mRNAs of P2X(4) receptor in mouse islets and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5) and P2X(7) receptors in Beta-TC6 cells. The presence of P2X(4) receptor proteins in islets and Beta-TC6 cells was confirmed by immunofluorescent staining and Western blot analysis. We have previously found that the functional P2Y(1) receptor but not P2Y(2) and P2Y(4) receptors was present in islets. In this study we found that a nonspecific P2 receptor agonist, ATP (1 μM) stimulated insulin secretion by islets in the presence of high glucose (20 mM) in culture. The effect of ATP was partially inhibited by a P2 receptor antagonist PPADS as well as a P2Y(1) receptor antagonist MRS2179. In addition, a P2X(4) receptor potentiator ivermectin per se augmented glucose-induced insulin secretion and slightly potentiated the effect of ATP. These results suggested the involvement of P2Y(1)and P2X receptors. We also found that ATP inhibited proliferation of Beta-TC6 cells in a concentration-dependent manner during 72 h culture. The inhibitory effect of ATP was completely reversed by PPADS and partially by treating cells with small interfering RNA targeted for P2X(4) receptor mRNA. Furthermore, we found that the phosphorylation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) was suppressed by treatment with ATP in Beta-TC6 cells. In addition, we found that ATP reduced the cell viability and DNA synthesis of islet cells in culture. The effect of ATP on the cell viability was blocked by PPADS or MRS2179. These results suggested that P2X receptors as well as the P2Y(1) receptor played a role in the modulation of insulin secretion, proliferation and cell viability in mouse pancreatic β-cells.

P2Y receptors in health and disease

The purine- and pyrimidine-sensitive P2Y receptors belong to the large group of G-protein-coupled receptors that are the target of approximately one-third of the pharmaceutical drugs used in the clinic today. It is therefore not unexpected that the P2Y receptors could be useful targets for drug development. This chapter will discuss P2Y receptor-based therapies currently used, in development and possible future developments. The platelet inhibitors blocking the ADP-receptor P2Y(12) reduce myocardial infarction, stroke, and mortality in patients with cardiovascular disease. Clopidogrel (Plavix) was for many years the second most selling drug in the world. The improved P2Y(12) inhibitors prasugrel, ticagrelor, and elinogrel are now entering the clinic with even more pronounced protective effects. The UTP-activated P2Y(2) receptor stimulates ciliary movement and secretion from epithelial cells. Cystic fibrosis is a monogenetic disease where reduced chloride ion secretion results in a severe lung disease and early death. No specific treatment has been available, but the P2Y(2) agonist Denufosol has been shown to improve lung function and is expected to be introduced as treatment for cystic fibrosis soon. In preclinical studies, there are indications that P2Y receptors can be important for diabetes, osteoporosis, cardiovascular, and atherosclerotic disease. In conclusion, P2Y receptors are important for the health of humans for many diseases, and we can expect even more beneficial drugs targeting P2Y receptors in the future.

ADP mediates inhibition of insulin secretion by activation of P2Y13 receptors in mice

AIMS/HYPOTHESES

To investigate the effects of extracellular purines on insulin secretion from mouse pancreatic islets.

METHODS

Mouse islets and beta cells were isolated and examined with mRNA real-time quantification, cAMP quantification and insulin and glucagon secretion. ATP release was measured in MIN6c4 cells. Insulin and glucagon secretion were measured in vivo after glucose injection.

RESULTS

Enzymatic removal of extracellular ATP at low glucose levels increased the secretion of both insulin and glucagon, while at high glucose levels insulin secretion was reduced and glucagon secretion was stimulated, indicating an autocrine effect of purines. In MIN6c4 cells it was shown that glucose does induce release of ATP into the extracellular space. Quantitative real-time PCR demonstrated the expression of the ADP receptors P2Y(1) and P2Y(13) in both intact mouse pancreatic islets and isolated beta cells. The stable ADP analogue 2-MeSADP had no effect on insulin secretion. However, co-incubation with the P2Y(1) antagonist MRS2179 inhibited insulin secretion, while co-incubation with the P2Y(13) antagonist MRS2211 stimulated insulin secretion, indicating that ADP acting via P2Y(1) stimulates insulin secretion, while signalling via P2Y(13) inhibits the secretion of insulin. P2Y(13) antagonism through MRS2211 per se increased the secretion of both insulin and glucagon at intermediate (8.3 mmol/l) and high (20 mmol/l) glucose levels, confirming an autocrine role for ADP. Administration of MRS2211 during glucose injection in vivo resulted in both increased secretion of insulin and reduced glucose levels.

CONCLUSIONS/INTERPRETATION

In conclusion, ADP acting on the P2Y(13) receptors inhibits insulin release. An antagonist to P2Y(13) increases insulin release and could be evaluated for the treatment of diabetes.

ADP receptor P2Y(13) induce apoptosis in pancreatic beta-cells

Pancreatic beta-cell loss represents a key factor in the pathogenesis of diabetes. Since the influence of purinergic signaling in beta-cell apoptosis has not been much investigated, we examined the role of the ADP receptor P2Y(13) using the pancreatic insulinoma-cell line MIN6c4 as a model system. Real time-PCR revealed high expression of the ADP receptors P2Y(1) and P2Y(13). Adding the ADP analogue, 2MeSADP, to MIN6c4 cells induced calcium influx/mobilization and inhibition of cAMP production by activation of P2Y(1) and P2Y(13), respectively. 2MeSADP reduced cell proliferation and increased Caspase-3 activity; both these effects could be fully reversed by the P2Y(13) receptor antagonist MRS2211. We further discovered that blocking the P2Y(13) receptor results in enhanced ERK1/2, Akt/PKB and CREB phosphorylation mechanisms involved in beta-cell survival. These results indicate that P2Y(13) is a proapoptotic receptor in beta-cells as the P2Y(13) receptor antagonist MRS2211 is able to protect the cells from ADP induced apoptosis.

TRPM2 functions as a lysosomal Ca2+-release channel in beta cells

TRPM2 is a Ca2+-permeable cation channel that is specifically activated by adenosine diphosphoribose (ADPR). Channel activation in the plasma membrane leads to Ca2+ influx and has been linked to apoptotic mechanisms. The primary agonist, ADPR, is produced both extra- and intracellularly and causes increases in intracellular calcium concentration ([Ca2+]i), but the mechanisms involved are not understood. Using short interfering RNA and a knockout mouse, we report that TRPM2, in addition to its role as a plasma membrane channel, also functions as a Ca2+-release channel activated by intracellular ADPR in a lysosomal compartment. We show that both functions of TRPM2 are critically linked to hydrogen peroxide-induced beta cell death. Additionally, extracellular ADPR production by the ectoenzyme CD38 from its substrates NAD+ (nicotinamide adenine dinucleotide) or cADPR causes IP3-dependent Ca2+ release via P2Y and adenosine receptors. Thus, ADPR and TRPM2 represent multimodal signaling elements regulating Ca2+ mobilization in beta cells through membrane depolarization, Ca2+ influx, and release of Ca2+ from intracellular stores.

Ecto-nucleoside triphosphate diphosphohydrolase 3 in the ventral and lateral hypothalamic area of female rats: morphological characterization and functional implications

BACKGROUND

Based on its distribution in the brain, ecto-nucleoside triphosphate diphosphohydrolase 3 (NTPDase3) may play a role in the hypothalamic regulation of homeostatic systems, including feeding, sleep-wake behavior and reproduction. To further characterize the morphological attributes of NTPDase3-immunoreactive (IR) hypothalamic structures in the rat brain, here we investigated: 1.) The cellular and subcellular localization of NTPDase3; 2.) The effects of 17beta-estradiol on the expression level of hypothalamic NTPDase3; and 3.) The effects of NTPDase inhibition in hypothalamic synaptosomal preparations.

METHODS

Combined light- and electron microscopic analyses were carried out to characterize the cellular and subcellular localization of NTPDase3-immunoreactivity. The effects of estrogen on hypothalamic NTPDase3 expression was studied by western blot technique. Finally, the effects of NTPDase inhibition on mitochondrial respiration were investigated using a Clark-type oxygen electrode.

RESULTS

Combined light- and electron microscopic analysis of immunostained hypothalamic slices revealed that NTPDase3-IR is linked to ribosomes and mitochondria, is predominantly present in excitatory axon terminals and in distinct segments of the perikaryal plasma membrane. Immunohistochemical labeling of NTPDase3 and glutamic acid decarboxylase (GAD) indicated that gamma-amino-butyric-acid- (GABA) ergic hypothalamic neurons do not express NTPDase3, further suggesting that in the hypothalamus, NTPDase3 is predominantly present in excitatory neurons. We also investigated whether estrogen influences the expression level of NTPDase3 in the ventrobasal and lateral hypothalamus. A single subcutaneous injection of estrogen differentially increased NTPDase3 expression in the medial and lateral parts of the hypothalamus, indicating that this enzyme likely plays region-specific roles in estrogen-dependent hypothalamic regulatory mechanisms. Determination of mitochondrial respiration rates with and without the inhibition of NTPDases confirmed the presence of NTPDases, including NTPDase3 in neuronal mitochondria and showed that blockade of mitochondrial NTPDase functions decreases state 3 mitochondrial respiration rate and total mitochondrial respiratory capacity.

CONCLUSION

Altogether, these results suggest the possibility that NTPDases, among them NTPDase3, may play an estrogen-dependent modulatory role in the regulation of intracellular availability of ATP needed for excitatory neuronal functions including neurotransmission.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Dual effect of ATP on glucose-induced insulin secretion in HIT-T15 cells

OBJECTIVES

: The study investigated the dual effect of purinergic nucleotides on the secretion of insulin from pancreatic beta cells.

METHODS

: The level of insulin secretion in HIT-T15 cells of static incubation was measured using a radioimmunoassay.

RESULTS

: The adenine nucleotides reduced the level of glucose-induced insulin secretion in a concentration-dependent manner, and the relative potency order (IC50; muM) was BzATP (6.9) > ATP (20.4) >/= alpha, beta-methylene ATP (23.3) >/= 2-methylthio-ATP (24.9). Suramin and PPADS (200 muM), which are blockers of the purinergic receptors, had a little influence on the activity of ATP. However, the inhibitory effect of ATP was reversed by preincubation with oxidized ATP (200 muM), which is a P2X7 antagonist. The level of insulin secretion in these preincubated cells exposed to the purinergic nucleotides increased in the following order: ATP > alpha, beta-methylene ATP >/= 2-methylthio-ATP. A pretreatment with foskolin and PDBu (100 nM) potentiated the increasing effect of ATP on insulin secretion. The Western blotting showed the expression of P2X7 and P2Y11 receptors.

CONCLUSIONS

: Purinergic stimulation has inhibitory activity on glucose-dependent insulin secretion through the activation of the P2X7 receptor, whereas it has enhancing effect through the activation of the P2Y11 receptor in HIT-T15 cells.

Polyphenols in cocoa and cocoa products: is there a link between antioxidant properties and health?

Cocoa and cocoa products have received much attention due to their significant polyphenol contents. Cocoa and cocoa products, namely cocoa liquor, cocoa powder and chocolates (milk and dark chocolates) may present varied polyphenol contents and possess different levels of antioxidant potentials. For the past ten years, at least 28 human studies have been conducted utilizing one of these cocoa products. However, questions arise on which of these products would deliver the best polyphenol contents and antioxidant effects. Moreover, the presence of methylxanthines, peptides, and minerals could synergistically enhance or reduce antioxidant properties of cocoa and cocoa products. To a greater extent, cocoa beans from different countries of origins and the methods of preparation (primary and secondary) could also partially influence the antioxidant polyphenols of cocoa products. Hence, comprehensive studies on the aforementioned factors could provide the understanding of health-promoting activities of cocoa or cocoa products components.

Electrophysiological and immunocytochemical evidence for P2X purinergic receptors in pancreatic beta cells

OBJECTIVES

Glucose-induced insulin secretion from pancreatic beta cells is modulated by several hormones and transmitters, namely adenosine triphosphate (ATP) via purinergic receptors. Although P2Y receptors are well documented in beta cells, the presence of P2X receptors remains elusive. We present the first electrophysiological evidence for the presence of P2X receptors in single beta cells of different species.

METHODS

Ionic currents were recorded from voltage-clamped beta cells near their resting potential using the perforated (nystatin) whole-cell patch-clamp configuration. Receptors were detected by immunocytochemistry.

RESULTS

When bathed in substimulatory (2 mM) glucose, mouse beta cells, isolated from islets displaying immunochemical colocalization of P2X1 or P2X3 receptors and insulin, developed large (approximately 250 pA/pF), rapidly activating, and then biexponentially decaying (tau1, approximately 20 milliseconds/tau2, approximately 1 second) inward currents on exposure to micromolar concentrations of ATP and alpha,beta-methylene ATP. The ATP also evoked inward currents (100-300 pA/pF) from porcine and human beta cells, albeit with a slower and more complex inactivation pattern.

CONCLUSIONS

The ATP-gated ion channels are present in pancreatic beta cells from different species. Specifically, mouse beta cells express rapidly desensitizing P2X1 and P2X3 receptors. Paracrine or neural activation of these receptors may contribute to the initial outburst of glucose- or acetylcholine-evoked insulin release, thus enhancing the islet secretory response.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

A1 receptor deficiency causes increased insulin and glucagon secretion in mice

Adenosine influences metabolism and the adenosine receptor antagonist caffeine decreases the risk of type 2 diabetes. In this study the metabolic role of one adenosine receptor subtype, the adenosine A(1)R, was evaluated in mice lacking this receptor [A(1)R (-/-)]. The HbA1c levels and body weight were not significantly different between wild type [A(1)R (+/+)] and A(1)R (-/-) mice (3-4 months) fed normal lab chow. At rest, plasma levels of glucose, insulin and glucagon were similar in both genotypes. Following glucose injection, glucose tolerance was not appreciably altered in A(1)R (-/-) mice. Glucose injection induced sustained increases in plasma insulin and glucagon levels in A(1)R (-/-) mice, whereas A(1)R (+/+) control mice reacted with the expected transient increase in insulin and decrease in glucagon levels. Pancreas perfusion experiments showed that A(1)R (-/-) mice had a slightly higher basal insulin secretion than A(1)R (+/+) mice. The first phase insulin secretion (initiated with 16.7 mM glucose) was of the same magnitude in both genotypes, but the second phase was significantly enhanced in the A(1)R (-/-) pancreata compared with A(1)R (+/+). Insulin- and contraction-mediated glucose uptake in skeletal muscle were not significantly different between in A(1)R (-/-) and A(1)R (+/+) mice. All adenosine receptors were expressed at mRNA level in skeletal muscle in A(1)R (+/+) mice and the mRNA A(2A)R, A(2B)R and A(3)R levels were similar in A(1)R (-/-) and A(1)R (+/+) mice. In conclusion, the A(1)R minimally affects muscle glucose uptake, but is important in regulating pancreatic islet function.

G-protein-coupled receptors and islet function-implications for treatment of type 2 diabetes

Islet function is regulated by a number of different signals. A main signal is generated by glucose, which stimulates insulin secretion and inhibits glucagon secretion. The glucose effects are modulated by many factors, including hormones, neurotransmitters and nutrients. Several of these factors signal through guanine nucleotide-binding protein (G protein)-coupled receptors (GPCR). Examples of islet GPCR are GPR40 and GPR119, which are GPCR with fatty acids as ligands, the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), the receptors for the islet hormones glucagon and somatostatin, the receptors for the classical neurotransmittors acetylcholine (ACh; M(3) muscarinic receptors) and noradrenaline (beta(2)- and alpha(2)-adrenoceptors) and for the neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP; PAC(1) and VPAC(2) receptors), cholecystokinin (CCK(A) receptors) and neuropeptide Y (NPY Y1 receptors). Other islet GPCR are the cannabinoid receptor (CB(1) receptors), the vasopressin receptors (V1(B) receptors) and the purinergic receptors (P(2Y) receptors). The islet GPCR couple mainly to adenylate cyclase and to phospholipase C (PLC). Since important pharmacological strategies for treatment of type 2 diabetes are stimulation of insulin secretion and inhibition of glucagon secretion, islet GPCR are potential drug targets. This review summarizes knowledge on islet GPCR.

The impact of adenosine and A(2B) receptors on glucose homoeostasis

Adenosine and adenosine receptor antagonists are involved in glucose homoeostasis. The participating receptors are not known, mainly due to a lack of specific agonists and antagonists, but are reasonable targets for anti-diabetic therapy. The stable, albeit nonselective, adenosine analogue NECA (5'-N-ethylcarboxamidoadenosine) (10 microM) reduced glucose-stimulated insulin release from INS-1 cells. This was mimicked by A(1)-(CHA), A(2A)-(CGS-21680) and A(3)-receptor agonists (Cl-IB-MECA). Two newly synthesized A(2B)-receptor antagonists, PSB-53 and PSB-1115, counteracted the inhibitory effect of NECA. These in-vitro effects were mirrored by in-vivo data with respect to CHA, CGS and Cl-IB-MECA. Distinct concentrations of either PSB-53 or PSB-1115 reversed the decrease in plasma insulin induced by NECA. This was not mimicked by a corresponding change in blood glucose. The effect of PSB-1115 was also obvious in diabetic GotoKakizaki rats: plasma insulin was increased whereas blood glucose was unchanged. During most experiments the effects on blood glucose were not impressive probably because of the physiologically necessary homoeostasis. The adenosine levels were not different in normal Wistar rats and in diabetic GotoKakzaki rats. Altogether the A(2B)-receptor antagonists showed an anti-diabetic potential mainly by increasing plasma insulin levels under conditions when the adenosine tonus was elevated in-vivo and increased insulin release in-vitro.

Involvement of stretch-activated cation channels in hypotonically induced insulin secretion in rat pancreatic β-cells

In isolated rat pancreatic β-cells, hypotonic stimulation elicited an increase in cytosolic Ca2+ concentration ([Ca2+]c) at 2.8 mM glucose. The hypotonically induced [Ca2+]c elevation was significantly suppressed by nicardipine, a voltage-dependent Ca2+ channel blocker, and by Gd3+, amiloride, 2-aminoethoxydiphenylborate, and ruthenium red, all cation channel blockers. In contrast, the [Ca2+]c elevation was not inhibited by suramin, a P2 purinoceptor antagonist. Whole cell patch-clamp analyses showed that hypotonic stimulation induced membrane depolarization of β-cells and produced outwardly rectifying cation currents; Gd3+ inhibited both responses. Hypotonic stimulation also increased insulin secretion from isolated rat islets, and Gd3+ significantly suppressed this secretion. Together, these results suggest that osmotic cell swelling activates cation channels in rat pancreatic β-cells, thereby causing membrane depolarization and subsequent activation of voltage-dependent Ca2+ channels and thus elevating insulin secretion.

Polyoxometalates--a new class of potent ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) inhibitors

Polyoxotungstates were identified as potent inhibitors of NTPDases1, 2, and 3. The most potent compound was K(6)H(2)[TiW(11)CoO(40)], exhibiting K(i) values of 0.140 microM (NTPDase1), 0.910 microM (NTPDase2), and 0.563 microM (NTPDase3). One of the compounds, (NH(4))(18)[NaSb(9)W(21)O(86)], was selective for NTPDases2 and 3 versus NTPDase1. NTPDase inhibition might contribute to the described biological effects of polyoxometalates, including their anti-cancer activity.

The transgenic expression of human CD39 on murine islets inhibits clotting of human blood

Platelet activation is believed to play an important role in the triggering of thrombosis of human blood by pig islets. We used a transgenic mouse model to investigate whether overexpression of CD39 (ecto nucleoside triphosphate diphosphohydrolase 1 [ENTPD1], EC 3.6.1.5), an ectonucleotidase that degrades the platelet agonists ATP, could interfere with this process. Islets isolated from CD39 transgenic mice showed 2.4-fold higher NTPDase activity than wild-type controls. When incubated with human blood, these islets significantly delayed clotting time compared to wild type islets (7.9 +/- 0.89 min versus 4.3 +/- 0.77 min, P = 0.007). Importantly, expression of human CD39 in the islets of transgenic mice had no deleterious effect on glucose metabolism. These results suggest that transgenic expression of human CD39 does not interfere with islet function and may be a useful strategy to inhibit thrombosis induced by intraportal administration of islet xenografts.

Extracellular ATP stimulates exocytosis via localized Ca(2+) release from acidic stores in rat pancreatic beta cells

Three different methods, membrane capacitance (C(m)) measurement, amperometry and FM dye labeling were used to investigate the role of extracellular ATP in insulin secretion from rat pancreatic beta cells. We found that extracellular application of ATP mobilized intracellular Ca(2+) stores and synchronously triggered vigorous exocytosis. No influence of ATP on the readily releasable pool of vesicles was observed, which argues against a direct modulation of the secretory machinery at a level downstream of Ca(2+) elevation. The stimulatory effects of ATP were greatly reduced by intracellular perfusion of BAPTA but not EGTA, suggesting a close spatial association of fusion sites with intracellular Ca(2+) releasing sites. ATP-induced Ca(2+) transients and exocytosis were not blocked by thapsigargin (TG), by a ryanodine receptor antagonist or by dissipation of pH in acidic stores by monensin alone, but they were greatly attenuated by IP(3) receptor inhibition as well as ionomycin plus monensin, suggesting involvement of IP(3)-sensitive acidic Ca(2+) stores. Taken together, our data suggest that extracellular ATP triggers exocytosis by mobilizing spatially limited acidic Ca(2+) stores through IP(3) receptors. This mechanism may explain how insulin secretion from the pancreas is coordinated through diffusible ATP that is co-released with insulin.

Inhibition of purinoceptors amplifies glucose-stimulated insulin release with removal of its pulsatility

External ATP has been proposed to be an autocrine regulator of glucose-stimulated insulin secretion and responsible for the synchronization of the Ca2+ rhythmicity in the beta-cells required for a pulsatile release of insulin from the pancreas. The importance of external ATP for glucose-stimulated insulin release was evaluated in rats with the aid of 2-deoxy-N-methyladenosine-3,5-bisphosphate (MRS 2179), an inhibitor of the purinoceptors known to affect the Ca2+ signaling in beta-cells. The concentration of cytoplasmic Ca2+ was measured in single beta-cells and small aggregates with ratiometric fura-2 technique and the release of insulin recorded from isolated islets and the perfused pancreas. Addition of 1 micromol/l ATP induced premature cytoplasmic Ca2+ concentration ([Ca2+]i) oscillations similar to those found in beta-cells exposed to 20 mmol/l glucose. In most experiments, the presence of 10 micromol/l MRS 2179 did not remove the glucose-induced [Ca2+]i rhythmicity in single beta-cells or the synchronization seen in coupled cells. Nevertheless, the same concentration of MRS 2179 promptly interrupted the pulsatility (frequency 0.22 +/- 0.01/min) of insulin secretion, raising the total amounts released from the pancreas. Prolonged exposure of islets to 1 and 10 micromol/l MRS 2179 enhanced insulin secretion at 20 mmol/l glucose 33% (P < 0.05) and 63% (P < 0.01), respectively, without affecting the release at 3 mmol/l glucose. The results support the idea that neural ATP signals entrain the islets into a common rhythm resulting in pulsatile release of insulin and that glucose stimulation of the secretory activity is counteracted by accumulation of inhibitory ATP around the beta-cells.

Pancreatic beta-cells communicate via intermittent release of ATP

The role of external ATP for intercellular communication was studied in glucose-stimulated pancreatic beta-cells isolated from ob/ob mice. Digital image analyses with fura-2 revealed spontaneous transients of cytoplasmic Ca2+ appearing in synchrony in the absence of cell contacts. After removal of slow oscillations with methoxyverapamil, addition of ATP (0.1-100 microM) resulted in prompt firing of a transient, followed by suppression of the generation and synchronization of spontaneously occurring transients. It was possible to trigger transients during the suppressive phase by raising the concentration of ATP. The dual action of ATP was mimicked by ADP or 2-methylthio-ATP but not by AMP or UTP. The number of spontaneous transients and their synchronization were reduced in the presence of the dephosphorylating agent apyrase. Additional evidence that intermittent release of ATP participates in the generation of spontaneous Ca2+ transients was obtained from the suppression observed from use of antagonists of the purinoceptors [suramin (0.3-30 microM), pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS; 10-30 microM) and 2-deoxy-N-methyladenosine (MRS 2179; 0.3-30 microM)] or from counteracting beta-cell release of ATP by inhibiting exocytosis with 100 nM epinephrine, 100 nM somatostatin, or lowering the temperature below 30 degrees C. The data indicate that ATP has time-dependent actions (prompt stimulation followed by inhibition) on the generation of Ca2+ transients mediated by P2Y receptors. It is proposed that beta-cells both receive a neural ATP signal with coordinating effects on their Ca2+ oscillations and propagate this message to adjacent cells via intermittent release of ATP combined with gap junction coupling.

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic beta-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca2+]c) in pancreatic beta-cells stimulated by extracellular ATP. Extracellular ATP (50 microM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse beta-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat beta-cells than in mouse. In response to extracellular ATP, rat beta-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse beta-cells. Rat and mouse beta-cells both responded to 9 mM glucose by increasing [Ca2+]c. This increase, however, was pronounced only in the rat beta-cells. In 9 mM glucose, extracellular ATP induced a pronounced calcium release above the increased level of [Ca2+]c in rat beta-cells. In mouse beta-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca2+]c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse beta-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse beta-cells but stimulates insulin secretion from rat beta-cells, we suggest that store-operated Ca2+ entry may be related to exocytosis in pancreatic rat beta-cells.

Regulation of glucagon-like peptide-1 receptor and calcium-sensing receptor signaling by L-histidine

Receptor-specific agonists of the extracellular calcium-sensing receptor (CaSR) potentiate glucose-induced insulin secretion, an effect similar to that of glucagon-like peptide-1 (GLP-1). We have sequenced the full open reading frame of the CaSR from rat insulinoma (INS-1) cells and find that the predicted amino acid sequence of the receptor is identical with that of the receptor from the parathyroid gland. This receptor couples to both Gq/11 and Gi/o, and this dual coupling may partly explain the varying effects of nonspecific agonists on secretion reported previously. L-Histidine (L-His) increases the sensitivity of the CaSR to extracellular Ca2+ and potentiates glucose-dependent insulin secretion from INS-1 cells. This potentiation is partially inhibited at low extracellular [Ca2+] where the CaSR is ineffective. Coexpression of the CaSR and GLP-1 receptor (GLP-1R) produces a pertussis toxin-sensitive inhibition of GLP-1-induced cAMP production in response to elevated extracellular [Ca2+]. However, l-His potentiates cAMP response element reporter activity in INS-1 cells and in human embryonic kidney-293 cells expressing either the GLP-1R alone or the CaSR and GLP-1R. INS-1 cells express the RNA for the CaSR at a lower level than that for the GLP-1R. This difference in expression level of the receptors may explain the potentiation of insulin secretion by L-His despite coupling of the CaSR to Gi/o. In conclusion, L-His can potentiate both GLP-1R- and CaSR-activated signaling pathways, and these effects may play a role in the potentiation of glucose-induced insulin secretion in response to meals containing protein in addition to carbohydrates and fat.

Physiological and pathophysiological functions of the ecto-nucleotide pyrophosphatase/phosphodiesterase family

The ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) multigene family contains five members. NPP1-3 are type II transmembrane metalloenzymes characterized by a similar modular structure composed of a short intracellular domain, a single transmembrane domain and an extracellular domain containing a conserved catalytic site. The short intracellular domain of NPP1 has a basolateral membrane-targeting signal while NPP3 is targeted to the apical surface of polarized cells. NPP4-5 detected by database searches have a predicted type I membrane orientation but have not yet been functionally characterized. E-NPPs have been detected in almost all tissues often confined to specific substructures or cell types. In some cell types, NPP1 expression is constitutive or can be induced by TGF-beta and glucocorticoids, but the signal transduction pathways that control expression are poorly documented. NPP1-3 have a broad substrate specificity which may reflect their role in a host of physiological and biochemical processes including bone mineralization, calcification of ligaments and joint capsules, modulation of purinergic receptor signalling, nucleotide recycling, and cell motility. Abnormal NPP expression is involved in pathological mineralization, crystal depositions in joints, invasion and metastasis of cancer cells, and type 2 diabetes. In this review we summarize the present knowledge on the structure and the physiological and biochemical functions of E-NPP and their contribution to the pathogenesis of diseases.

Pharmacological agents that directly modulate insulin secretion

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Glucose-induced islet blood flow increase in rats: interaction between nervous and metabolic mediators

This study investigated the mechanisms for glucose-induced islet blood flow increase in rats. The effects of adenosine, adenosine receptor antagonists, and vagotomy on islet blood flow were evaluated with a microsphere technique. Vagotomy prevented the islet blood flow increase expected 3, 10, and 20 min after injection of glucose, whereas theophylline (a nonspecific adenosine receptor antagonist) prevented the islet blood flow increase from occurring 10 and 20 min after glucose administration. Administration of selective adenosine receptor antagonists suggested that the response to theophylline was mediated by A1 receptors. Exogenous administration of adenosine did not affect islet blood flow, but local accumulation of adenosine, induced by the adenosine uptake inhibitor dipyridamole, caused a doubling of islet blood flow. In conclusion, the increased islet blood flow seen 3 min after induction of hyperglycemia is caused by the vagal nerve, whereas the increase in islet blood perfusion seen at 10 and 20 min after glucose administration is caused by both the vagal nerve and adenosine.

Effect of purinergic agonists and antagonists on insulin secretion from INS-1 cells (insulinoma cell line) and rat pancreatic islets

The effects of purinergic agonists on insulin release are controversial in the literature. In our studies (mainly using INS-1 cells, but also using rat pancreatic islets), ATP had a dual effect on insulin release depending on the ATP concentration: increasing insulin release (EC50 approximately/= 0.0032 microM) and inhibiting insulin release (EC50 approximately/= 0.32 microM) at both 5.6 and 8.3 mM glucose. This is compatible with the view that either two different receptors are involved, or the cells desensitize and (or) the effect of an inhibitory degradation product such as adenosine (ectonucleotidase effect) emerges. The same dual effects of ATP on insulin release were obtained using rat pancreatic islets instead of INS-1 cells. ADPbetaS, which is less degradable than ATP and rather specific for P2Y1 receptors, had a dual effect on insulin release at 8.3 mM glucose: stimulatory (EC50 approximately/= 0.02 microM) and inhibitory (EC50 approximately/= 0.32 microM). The effectiveness of this compound indicates the possible involvement of a P2Y1 receptor. 2-Methylthio-ATP exhibited an insulinotropic effect at very high concentrations (EC50 approximately/= 15 microM at 8.3 mM glucose). This indicated that distinct P2X or the P2Y1 receptor may be involved in these insulin-secreting cells. UTP increased insulin release (EC50 approximately/= 2 microM) very weakly, indicating that a P2U receptor (P2X3 or possibly a P2Y2 or P2Y4) are not likely to be involved. Suramin (50 microM) antagonized the insulinotropic effect of ATP (0.01 microM) and UTP (0.32 microM). Since suramin is not selective, the data indicated that various P2X and P2Y receptors may be involved. PPADS (100 microM), a P2X and P2Y1,4,6 receptor antagonist, was ineffective using either low or high concentrations of ATP and ADPbetaS, which combined with the suramin data hints at a P2Y receptor effect of the compounds. Adenosine inhibited insulin release in a concentration-dependent manner. DPCPX (100 microM), an adenosine (A1) receptor antagonist, inhibited the inhibitory effects of both adenosine and of high concentrations of ATP. Adenosine deaminase (1 U/mL) abolished the inhibitory effect of high ATP concentrations, indicating the involvement of the degradation product adenosine. Repetitive addition of ATP did not desensitize the stimulatory effect of ATP. U-73122 (2 microM), a PLC inhibitor, abolished the ATP effect at low concentrations. The data indicate that ATP at low concentrations is effective via P2Y receptors and the PLC-system and not via P2X receptors; it inhibits insulin release at high concentrations by being metabolized to adenosine.

Effect of 5-aminoimidazole-4-carboxamide riboside (AICA-r) on isolated thoracic aorta responses in streptozotocin-diabetic rats

Diabetes mellitus alters the vascular responsiveness to several vasoconstrictors and vasodilators. 5-amino-4-imidazole-carboxamide riboside (AICA-r), a nucleoside corresponding to AICA-ribotide and an intermediate of the de novo pathway of purine biosynthesis, was recently proposed as a new insulinotropic tool in non-insulin-dependent diabetes mellitus. The aim of the present study was to define whether AICA-r affects altered vascular responsiveness to vasoconstrictors and vasodilators in the thoracic aorta of neonatal streptozotocin (STZ)-diabetic rats. The results of this study indicate that a 1-month treatment with AICA-r significantly increases the body weight in diabetic rats; significantly decreases the blood glucose level of diabetic rats (from 302+/-47 to 135+/-11 mg/dL, p<0.001); does not significantly affect the fast, slow, and total components of responses to noradrenaline in all the experimental groups; reverses the increased Emax values of noradrenaline in diabetic rats to near-control values; reverses the completely abolished responses of acetylcholine (pD2 and percent relaxation) in diabetic rats to control values; and reverses the decreased pD2 values of sodium nitroprussiate in diabetic rats to control values. In conclusion, AICA-r treatment in neonatal STZ-diabetic rats improved increased blood glucose levels, accelerated weight gain, reversed endothelial dysfunction, and normalized vascular responses.

Localization of NTPDase1/CD39 in normal and transformed human pancreas

Elevated levels of extracellular ATP have been observed in many tumors. We have localized NTPDase1/CD39, one of the principal extracellular nucleotide-hydrolyzing enzymes, in normal and cancerous human pancreas. NTPDase/E-ATPDase activity was demonstrated with an enzyme histochemical technique on cryosections of human pancreas. Acinar and duct epithelial cells were devoid of E-ATPDase activity in both normal and transformed tissue. Endothelial cells and smooth muscle around blood vessels and larger ducts showed strong activity. Nerves, connective tissue, and the beta-cells of the islets were also stained. In cancerous tissue this activity was diminished in the smooth muscle around the ducts and was absent from newly formed connective tissue. Immunostaining for CD39 supported these results but revealed the presence of inactive CD39 in the duct epithelial cells. We hypothesize that the significantly diminished activity of NTPDase1 in the tissues surrounding the ducts may be associated with the processes that lead to tumor formation in human pancreas.

Dynamic imaging of endoplasmic reticulum Ca2+ concentration in insulin-secreting MIN6 Cells using recombinant targeted cameleons: roles of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2 and ryanodine receptors

The endoplasmic reticulum (ER) plays a pivotal role in the regulation of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt)) and hence in insulin secretion from pancreatic beta-cells. However, the molecular mechanisms involved in both the uptake and release of Ca(2+) from the ER are only partially defined in these cells, and the presence and regulation of ER ryanodine receptors are a matter of particular controversy. To monitor Ca(2+) fluxes across the ER membrane in single live MIN6 beta-cells, we have imaged changes in the ER intralumenal free Ca(2+) concentration ([Ca(2+)](ER)) using ER-targeted cameleons. Resting [Ca(2+)](ER) (approximately 250 micromol/l) was markedly reduced after suppression (by approximately 40%) of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b but not the SERCA3 isoform by microinjection of antisense oligonucleotides, implicating SERCA2b as the principle ER Ca(2+)-ATPase in this cell type. Nutrient secretagogues that elevated [Ca(2+)](cyt) also increased [Ca(2+)](ER), an effect most marked at the cell periphery, whereas inositol 1,4,5-trisphosphate-generating agents caused a marked and homogenous lowering of [Ca(2+)](ER). Demonstrating the likely presence of ryanodine receptors (RyRs), caffeine and 4-chloro-3-ethylphenol both caused an almost complete emptying of ER Ca(2+) and marked increases in [Ca(2+)](cyt). Furthermore, photolysis of caged cyclic ADP ribose increased [Ca(2+)](cyt), and this effect was largely abolished by emptying ER/Golgi stores with thapsigargin. Expression of RyR protein in living MIN6, INS-1, and primary mouse beta-cells was also confirmed by the specific binding of cell-permeate BODIPY TR-X ryanodine. RyR channels are likely to play an important part in the regulation of intracellular free Ca(2+) changes in the beta-cell and thus in the regulation of insulin secretion.

P2Y-purinoceptor mediated inhibition of L-type Ca2+ channels in rat pancreatic beta-cells

We used the patch-clamp technique to study the effects of extracellular ATP on the activity of ion channels recorded in rat pancreatic beta-cells. In cell-attached membrane patches, action currents induced by 8.3 mM glucose were inhibited by 0.1 mM ATP, 0.1 mM ADP or 15 microM ADPbetaS but not by 0.1 mM AMP or 0.1 mM adenosine. In perforated membrane patches, action potentials were measured in current clamp, induced by 8.3 mM glucose, and were also inhibited by 0.1 mM ATP with a modest hyperpolarization to -43 mV. In whole-cell clamp experiments, ATP dose-dependently decreased the amplitudes of L-type Ca2+ channel currents (ICa) to 56.7+/-4.0% (p<0.001) of the control, but did not influence ATP-sensitive K+ channel currents observed in the presence of 0.1 mM ATP and 0.1 mM ADP in the pipette. Agonists of P2Y purinoceptors, 2-methylthio ATP (0.1 mM) or ADPbetaS (15 microM) mimicked the inhibitory effect of ATP on ICa, but PPADS (0.1 mM) and suramin (0.2 mM), antagonists of P2 purinoceptors, counteracted this effect. When we used 0.1 mM GTPgammaS in the pipette solution, ATP irreversibly reduced ICa to 58.4+/-6.6% of the control (p<0.001). In contrast, no inhibitory effect of ATP was observed when 0.2 mM GDPbetaS was used in the pipette solution. The use of either 20 mM BAPTA instead of 10 mM EGTA, or 0.1 mM compound 48/80, a blocker of phospholipase C (PLC), in the pipette solution abolished the inhibitory effect of ATP on ICa, but 1 microM staurosporine, a blocker of protein kinase C (PKC), did not. When the beta-cells were pretreated with 0.4 microM thapsigargin, an inhibitor of the endoplasmic reticulum (ER) Ca2+ pump, ATP lost the inhibitory effect on ICa. These results suggest that extracellular ATP inhibits action potentials by Ca2+-induced ICa inhibition in which an increase in cytosolic Ca2+ released from thapsigargin-sensitive store sites was brought about by a P2Y purinoceptor-coupled G-protein, PI-PLC and IP3 pathway.

Purification, characterization, and localization of an ATP diphosphohydrolase in porcine kidney

Membranes of pig kidney cortex tissue were solubilized in the presence of Triton X-100. Partial purification of ATP diphosphohydrolase (ATPDase) was achieved by successive chromatography on concanavalin A-Sepharose, Q-Sepharose Fast Flow, and 5'-AMP-Sepharose 4B. Monoclonal antibodies against ATPDase were generated. Further purification of the ATPDase was obtained by immunoaffinity chromatography with these monoclonal antibodies. NH(2)-terminal amino acid sequencing of the 78-kDa protein showed a sequence very homologous to mammalian CD39. The protein is highly glycosylated, with a nominal molecular mass of approximately 57 kDa. The purified enzyme hydrolyzed di- and triphosphates of adenosine, guanosine, cytidine, uridine, inosine, and thymidine, but AMP and diadenosine polyphosphates could not serve as substrates. All enzyme activities were dependent on divalent cations and were partially inhibited by 10 mM sodium azide. The distribution of the enzyme in pig kidney cortex was examined immunohistochemically. The enzyme was found to be present in blood vessel walls of glomerular and peritubular capillaries.

Stimulatory effect of exogenous diadenosine tetraphosphate on insulin and glucagon secretion in the perfused rat pancreas

1. Diadenosine triphosphate (AP3A) and diadenosine tetraphosphate (AP4A) are released by various cells (e.g. platelets and chromaffin cells), and may act as extracellular messengers. In pancreatic B-cells, AP3A and AP4A are inhibitors of the ATP-regulated K+ channels, and glucose increases intracellular levels of both substances. 2. We have studied the effect of exogenous AP3A and AP4A on insulin and glucagon secretion by the perfused rat pancreas. 3. AP3A did not significantly modify insulin or glucagon release, whereas AP4A induced a prompt, short-lived insulin response ( approximately 4 fold higher than basal value; P<0.05) in pancreases perfused at different glucose concentrations (3.2, 5.5 or 9 mM). AP4A-induced insulin release was abolished by somatostatin and by diazoxide. These two substances share the capacity to activate ATP-dependent K+ channels, suggesting that these channels are a potential target for AP4A in the B-cell. 4. AP4A stimulated glucagon release at both 3.2 and 5.5 mM glucose. This effect was abolished by somatostatin. 5. The results suggest that extracellular AP4A may play a physiological role in the control of insulin and glucagon secretion.

Evidence for two different types of P2 receptors stimulating insulin secretion from pancreatic B cell

Adenine nucleotides have been shown to stimulate insulin secretion by acting on P2 receptors of the P2Y type. Since there have been some discrepancies in the insulin response of different analogues of ATP and ADP, we investigated whether two different types of P2 receptors exist on pancreatic B cells. The effects of alpha,beta-methylene ATP, which is more specific for the P2X subtype, were studied in vitro in pancreatic islets and isolated perfused pancreas from rats, in comparison with the potent P2Y receptor agonist ADPbetaS. In isolated islets, incubated with a slightly stimulating glucose concentration (8.3 mM), alpha,beta-me ATP (200 microM) and ADPbetaS (50 microM) similarly stimulated insulin secretion; by contrast, under a non stimulating glucose concentration (3 mM), alpha,beta-me ATP was still effective whereas ADPbetaS was not. In the same way, in islets perifused with 3 mM glucose, alpha,beta-me ATP but not ADPbetaS induced a partial but significant reduction in the peak 86Rb efflux induced by the ATP-dependent potassium channel opener diazoxide. In the isolated pancreas, perfused with a non stimulating glucose concentration (4.2 mM), ADPbetaS and alpha,beta-me ATP (5-50 microM), administered for 10 min, induced an immediate, transient and concentration-dependent increase in the insulin secretion; their relative potency was not significantly different. In contrast, with a slightly stimulating glucose concentration (8.3 mM), ADPbetaS was previously shown to be 100 fold more potent than alpha,beta-me ATP. Furthermore, at 4.2 mM glucose a second administration of alpha,beta-me ATP was ineffective. In the same way, ADPbetaS was also able to desensitize its own insulin response. At 3 mM glucose, alpha,beta-me ATP as well as ADPbetaS (50 microM) induced a transient stimulation of insulin secretion and down regulated the action of each other. These results give evidence that pancreatic B cells, in addition to P2Y receptors, which potentiate glucose-induced insulin secretion, are provided with P2X receptors, which transiently stimulate insulin release at low non-stimulating glucose concentration and slightly affect the potassium conductance of the membrane.

AMP-activated protein kinase is activated by low glucose in cell lines derived from pancreatic beta cells, and may regulate insulin release

The role of the AMP-activated protein kinase (AMPK) cascade in the glucose-sensitive pancreatic beta cell lines HIT-T15 and INS-1 was addressed. In both cell types, removal of glucose leads to a >5-fold activation of AMPK activity. Activation of AMPK was due to phosphorylation, since the effect was reversed by protein phosphatase treatment of the extracts, and was restored by re-addition of MgATP and the purified upstream kinase. When the effects of different concentrations of medium glucose were examined, insulin secretion and AMPK activity were inversely related, and varied over the same concentration range. The activation in response to glucose removal appeared to be due to changes in the concentration of the known regulators of the cascade, i.e. AMP and ATP, since AMPK activation was associated with a large increase in the cellular AMP/ATP ratio, and the two parameters varied over the same range of glucose concentrations. In late-passage HIT-T15 cells that had lost the glucose-dependent insulin secretion response, both AMPK activity and the AMP/ATP ratio also became insensitive to the extracellular glucose concentration. Treatment of INS-1 cells, but not HIT-T15 cells, with AICA riboside (5-aminoimidazole-4-carboxamide riboside) results in accumulation of the ribotide, ZMP (AICA riboside monophosphate), and activation of AMPK. AICA riboside treatment of INS-1 cells, and of isolated rat islets, had both inhibitory and stimulatory effects on insulin secretion. These results show that in beta cell lines the AMP-activated protein kinase, like its yeast homologue the SNF1 complex, can respond to the level of glucose in the medium, and may be involved in regulating insulin release.

Preservation of insulin secretory responses to P2 purinoceptor agonists in Zucker diabetic fatty rats

The role of P2 purinoceptor agonists in regulatory insulin secretion in Zucker diabetic fatty (ZDF) rats was studied using the isolated perfused pancreas and intracellular Ca2+ concentration ([Ca2+]i) microfluorimetry. The relative potency of different purinoceptor agonists to stimulate the insulin secretory process was consistent with the conclusion that responses in [Ca2+]i and insulin secretion are mediated by the P2y subtype of purinoceptors. Additional studies using specific antagonists of the Ca2+ signaling pathway indicated that activation of P2y purinoceptor releases Ca2+ from intracellular stores and promotes Ca2+ entry through voltage-independent rather than voltage-dependent Ca2+ channels on the beta-cell membrane. Perfused pancreas and isolated islets from ZDF rats demonstrated markedly reduced or absent insulin secretion and [Ca2+]i responses to glucose and KCl. In contrast, responses to P2y purinoceptor agonists were normal, indicating that the secretion coupling pathway activated by these agonists is preserved in glucose-unresponsive islets from diabetic animals. These observations raise the possibility that the purinoceptor pathway may play an important role in regulating insulin secretion in hyperinsulinemic non-insulin-dependent diabetes mellitus.