Nitric oxide induces intracellular Ca2+ mobilization and increases secretion of incorporated 5-hydroxytryptamine in rat pancreatic beta-cells

Nitric Oxide Synthase Blockade Impairs Spontaneous Calcium Activity in Mouse Primary Hippocampal Culture Cells

Oscillation of intracellular calcium concentration is a stable phenomenon that affects cellular function throughout the lifetime of both electrically excitable and non-excitable cells. Nitric oxide, a gaseous secondary messenger and the product of nitric oxide synthase (NOS), affects intracellular calcium dynamics. Using mouse hippocampal primary cultures, we recorded the effect of NOS blockade on neuronal spontaneous calcium activity. There was a correlation between the amplitude of spontaneous calcium events and the number of action potentials (APs) (Spearman R = 0.94). There was a linear rise of DAF-FM fluorescent emission showing an increase in NO concentration with time in neurons (11.9 ± 1.0%). There is correlation between the integral of the signal from DAF-FM and the integral of the spontaneous calcium event signal from Oregon Green 488 (Spearman R = 0.58). Blockade of NOS affected the parameters of the spontaneous calcium events studied (amplitude, frequency, integral, rise slope and decay slope). NOS blockade by Nw-Nitro-L-arginine suppressed the amplitude and frequency of spontaneous calcium events. The NOS blocker 3-Bromo-7-Nitroindazole reduced the frequency but not the amplitude of spontaneous calcium activity. Blockade of the well-known regulator of NOS, calcineurin with cyclosporine A reduced the integral of calcium activity in neurons. The differences and similarities in the effects on the parameters of spontaneous calcium effects caused by different blockades of NO production help to improve understanding of how NO synthesis affects calcium dynamics in neurons.

The Effect of Cinnamaldehyde on iNOS Activity and NO-Induced Islet Insulin Secretion in High-Fat-Diet Rats

Introduction

Obesity and insulin resistance are associated with alterations in nitric oxide level and insulin secretion. Previous studies demonstrated that cinnamaldehyde (CNMA) improved islet insulin secretion and restored nitric oxide (NO) level, but its underlying mechanisms have not been investigated. This study aimed to investigate the effect of CNMA on inducible nitric oxide synthase (iNOS) activity and NO-induced islet insulin secretion in high-fat-diet (HFD) treated rats.

Materials and Methods

Forty male Wistar rats (12 weeks old) were randomly divided into four equal groups, namely, control, CNMA, HFD, and HFD + CNMA. Control and CNMA groups were treated with standard laboratory animals' diet, while HFD and HDF + CNMA groups were fed with an HFD diet enriched with 25% W/W tail fat for 16 weeks. CNMA was administrated orally (20 mg/kg body weight, daily) during the study period. Islet insulin secretion and the inducible NOS activity in the presence or absence of L-NAME (NO synthase inhibitor, 5 mmol/L) were evaluated.

Results

L-NAME-suppressed insulin secretion in control, HFD, and HFD + CNMA groups; however, in the CNMA group, it could not exhibit such effect (P < 0.01). Islets of HFD-treated animals showed significantly higher iNOS activity than controls. CNMA treatment significantly suppressed iNOS activities in CNMA and HFD + CNMA groups compared with control and HFD, respectively.

Conclusion

These results suggest that the beneficial effect of CNMA on insulin secretion might be due to its inhibitory effect on iNOS activity.

Insulin secretion: The nitric oxide controversy

Nitric oxide (NO) is a gas that serves as a ubiquitous signaling molecule participating in physiological activities of various organ systems. Nitric oxide is produced in the endocrine pancreas and contributes to synthesis and secretion of insulin. The potential role of NO in insulin secretion is disputable - both stimulatory and inhibitory effects have been reported. Available data indicate that effects of NO critically depend on its concentration. Different isoforms of NO synthase (NOS) control this and have the potential to decrease or increase insulin secretion. In this review, the role of NO in insulin secretion as well as the possible reasons for discrepant findings are discussed. A better understanding of the role of NO system in the regulation of insulin secretion may facilitate the development of new therapeutic strategies in the management of diabetes.

Role of Nitric Oxide in Insulin Secretion and Glucose Metabolism

Nitric oxide (NO) contributes to carbohydrate metabolism and decreased NO bioavailability is involved in the development of type 2 diabetes mellitus (T2DM). NO donors may improve insulin signaling and glucose homeostasis in T2DM and insulin resistance (IR), suggesting the potential clinical importance of NO-based interventions. In this review, site-specific roles of the NO synthase (NOS)-NO pathway in carbohydrate metabolism are discussed. In addition, the metabolic effects of physiological low levels of NO produced by constitutive NOS (cNOS) versus pathological high levels of NO produced by inducible NOS (iNOS) in pancreatic β-cells, adipocytes, hepatocytes, and skeletal muscle cells are summarized. A better understanding of the NOS-NO system in the regulation of glucose homeostasis can hopefully facilitate the development of new treatments for T2DM.

Critical Roles of Carbon Monoxide and Nitric Oxide in Ca2+ Signaling for Insulin Secretion in Pancreatic Islets

AIMS

Glucagon-like peptide-1 (GLP-1) increases intracellular Ca²⁺ concentrations, resulting in insulin secretion from pancreatic β-cells through the sequential production of Ca²⁺ mobilizing messengers nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR). We previously found that NAADP activates the neuronal type of nitric oxide (NO) synthase (nNOS), the product of which, NO, activates guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP), which, in turn, induces cADPR formation. Our aim was to explore the relationship between Ca²⁺ signals and gasotransmitters formation in insulin secretion in β-cells upon GLP-1 stimulation.

RESULTS

We show that NAADP-induced cGMP production by nNOS activation is dependent on carbon monoxide (CO) formation by heme oxygenase-2 (HO-2). Treatment with exogenous NO and CO amplifies cGMP formation, Ca²⁺ signal strength, and insulin secretion, whereas this signal is impeded when exposed to combined treatment with NO and CO. Furthermore, CO potentiates cGMP formation in a dose-dependent manner, but higher doses of CO inhibited cGMP formation. Our data with regard to zinc protoporphyrin, a HO inhibitor, and HO-2 knockdown, revealed that NO-induced cADPR formation and insulin secretion are dependent on HO-2. Consistent with this observation, the administration of NO or CO donors to type 2 diabetic mice improved glucose tolerance, but the same did not hold true when both were administered concurrently.

INNOVATION

Our research reveals the role of two gas transmitters, CO and NO, for Ca²⁺ second messengers formation in pancreatic β-cells.

CONCLUSION

These results demonstrate that CO, the downstream regulator of NO, plays a role in bridging the gap between the Ca²⁺ signaling messengers during insulin secretion in pancreatic β-cells.

Low Level Pro-inflammatory Cytokines Decrease Connexin36 Gap Junction Coupling in Mouse and Human Islets through Nitric Oxide-mediated Protein Kinase Cδ

Pro-inflammatory cytokines contribute to the decline in islet function during the development of diabetes. Cytokines can disrupt insulin secretion and calcium dynamics; however, the mechanisms underlying this are poorly understood. Connexin36 gap junctions coordinate glucose-induced calcium oscillations and pulsatile insulin secretion across the islet. Loss of gap junction coupling disrupts these dynamics, similar to that observed during the development of diabetes. This study investigates the mechanisms by which pro-inflammatory cytokines mediate gap junction coupling. Specifically, as cytokine-induced NO can activate PKCδ, we aimed to understand the role of PKCδ in modulating cytokine-induced changes in gap junction coupling. Isolated mouse and human islets were treated with varying levels of a cytokine mixture containing TNF-α, IL-1β, and IFN-γ. Islet dysfunction was measured by insulin secretion, calcium dynamics, and gap junction coupling. Modulators of PKCδ and NO were applied to determine their respective roles in modulating gap junction coupling. High levels of cytokines caused cell death and decreased insulin secretion. Low levels of cytokine treatment disrupted calcium dynamics and decreased gap junction coupling, in the absence of disruptions to insulin secretion. Decreases in gap junction coupling were dependent on NO-regulated PKCδ, and altered membrane organization of connexin36. This study defines several mechanisms underlying the disruption to gap junction coupling under conditions associated with the development of diabetes. These mechanisms will allow for greater understanding of islet dysfunction and suggest ways to ameliorate this dysfunction during the development of diabetes.

Arginine Thiazolidine Carboxylate Stimulates Insulin Secretion through Production of Ca2+-Mobilizing Second Messengers NAADP and cADPR in Pancreatic Islets

Oxothiazolidine carboxylic acid is a prodrug of cysteine that acts as an anti-diabetic agent via insulin secretion and the formation of the Ca2+-mobilizing second messenger, cyclic ADP-ribose (cADPR). Here we show that a hybrid compound, arginine thiazolidine carboxylate (ATC), increases cytoplasmic Ca2+ in pancreatic β-cells, and that the ATC-induced Ca2+ signals result from the sequential formation of two Ca2+-mobilizing second messengers: nicotinic acid adenine dinucleotide phosphate (NAADP) and cADPR. Our data demonstrate that ATC has potent insulin-releasing properties, due to the additive action of its two components; thiazolidine carboxylate (TC) and L-arginine. TC increases glutathione (GSH) levels, resulting in cAMP production, followed by a cascade pathway of NAADP/nitric oxide (NO)/cGMP/cADPR synthesis. L-arginine serves as the substrate for NO synthase (NOS), which results in cADPR synthesis via cGMP formation. Neuronal NOS is specifically activated in pancreatic β-cells upon ATC treatment. These results suggest that ATC is an ideal candidate as an anti-diabetic, capable of modulating the physiological Ca2+ signalling pathway to stimulate insulin secretion.

Glucose-Dependent Insulin Secretion in Pancreatic β-Cell Islets from Male Rats Requires Ca2+ Release via ROS-Stimulated Ryanodine Receptors

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells requires an increase in intracellular free Ca2+ concentration ([Ca2+]). Glucose uptake into β-cells promotes Ca2+ influx and reactive oxygen species (ROS) generation. In other cell types, Ca2+ and ROS jointly induce Ca2+ release mediated by ryanodine receptor (RyR) channels. Therefore, we explored here if RyR-mediated Ca2+ release contributes to GSIS in β-cell islets isolated from male rats. Stimulatory glucose increased islet insulin secretion, and promoted ROS generation in islets and dissociated β-cells. Conventional PCR assays and immunostaining confirmed that β-cells express RyR2, the cardiac RyR isoform. Extended incubation of β-cell islets with inhibitory ryanodine suppressed GSIS; so did the antioxidant N-acetyl cysteine (NAC), which also decreased insulin secretion induced by glucose plus caffeine. Inhibitory ryanodine or NAC did not affect insulin secretion induced by glucose plus carbachol, which engages inositol 1,4,5-trisphosphate receptors. Incubation of islets with H2O2 in basal glucose increased insulin secretion 2-fold. Inhibitory ryanodine significantly decreased H2O2-stimulated insulin secretion and prevented the 4.5-fold increase of cytoplasmic [Ca2+] produced by incubation of dissociated β-cells with H2O2. Addition of stimulatory glucose or H2O2 (in basal glucose) to β-cells disaggregated from islets increased RyR2 S-glutathionylation to similar levels, measured by a proximity ligation assay; in contrast, NAC significantly reduced the RyR2 S-glutathionylation increase produced by stimulatory glucose. We propose that RyR2-mediated Ca2+ release, induced by the concomitant increases in [Ca2+] and ROS produced by stimulatory glucose, is an essential step in GSIS.

Nitric oxide signals postovulatory aging-induced abortive spontaneous egg activation in rats

OBJECTIVE

The aim of this study was to determine whether an increase of intracellular nitric oxide (NO) level signals postovulatory aging-induced abortive spontaneous egg activation (SEA) in rats.

METHODS

Freshly ovulated eggs (arrested at metaphase-II stage; M-II) were cultured in vitro for 3 hours to induce postovulatory egg aging. The morphological changes, inducible nitric oxide synthase (iNOS) expression, NO, cytosolic free Ca(2+), 3',5' cyclic guanosine monophosphate (cGMP), cell division cycle 25B (Cdc25B) and Wee1 levels, specific phosphorylation (pThr-14/Tyr-15) as well as total cyclin-dependent kinases-1 (Cdk1) (PSTAIRE) levels were analyzed.

RESULTS

Postovulatory aging induced generation of NO possibly through an iNOS-mediated pathway. The increase in NO level was associated with augmented cytosolic free Ca(2+) as well as cGMP levels in aged eggs. A significant increase in Wee1 level and decrease of Cdc25B level were observed in aged eggs. An accumulation of phosphorylated Cdk1 (pThr-14/Tyr-15) level was observed in aged eggs, while total Cdk1 (PSTAIR) level remained unchanged.

CONCLUSION

Our study demonstrates that generation of NO through an iNOS-mediated pathway increases cytosolic free Ca2+and cGMP levels. High levels of these signal molecules trigger the accumulation of phosphorylated Cdk1 in aged eggs. Thus, NO signals the accumulation of phosphorylated Cdk1 and induces postovulatory aging-induced abortive SEA in the rat.

Islet Endothelial Cells Derived From Mouse Embryonic Stem Cells

The islet endothelium comprises a specialized population of islet endothelial cells (IECs) expressing unique markers such as nephrin and α-1 antitrypsin (AAT) that are not found in endothelial cells in surrounding tissues. However, due to difficulties in isolating and maintaining a pure population of these cells, the information on these islet-specific cells is currently very limited. Interestingly, we have identified a large subpopulation of endothelial cells exhibiting IEC phenotype, while deriving insulin-producing cells from mouse embryonic stem cells (mESCs). These cells were identified by the uptake of low-density lipoprotein (LDL) and were successfully isolated and subsequently expanded in endothelial cell culture medium. Further analysis demonstrated that the mouse embryonic stem cell-derived endothelial cells (mESC-ECs) not only express classical endothelial markers, such as platelet endothelial cell adhesion molecule (PECAM1), thrombomodulin, intercellular adhesion molecule-1 (ICAM-1), and endothelial nitric oxide synthase (eNOS) but also IEC-specific markers such as nephrin and AAT. Moreover, mESC-ECs secrete basement membrane proteins such as collagen type IV, laminin, and fibronectin in culture and form tubular networks on a layer of Matrigel, demonstrating angiogenic activity. Further, mESC-ECs not only express eNOS, but also its eNOS expression is glucose dependent, which is another characteristic phenotype of IECs. With the ability to obtain highly purified IECs derived from pluripotent stem cells, it is possible to closely examine the function of these cells and their interaction with pancreatic β-cells during development and maturation in vitro. Further characterization of tissue-specific endothelial cell properties may enhance our ability to formulate new therapeutic angiogenic approaches for diabetes.

Comparison of the post-mortem interval on the effect of vascular responses to the activation of ionotropic and metabotropic receptors

The contractibility of blood vessels depends on their normal structure and the availability of calcium ions; it changes under the influence of numerous contracting and relaxing factors, which control the activities of various pathways of intracellular and intercellular signaling. The main aim of the study was to investigate, by means of perfusion pressure in rat tail arteries, the role of Ca²⁺ in vascular response to α-1 adrenoceptor activation by phenylephrine (PHE) and Bay K8644 agonist of the L-type calcium channel and caffeine before and after a post-mortem interval (PMI) of 2, 4, 6 and 8 h. A phasic increase of perfusion pressure in rat tail arteries, as induced by PHE or caffeine, in Ca²⁺-free solutions was used as an indicator of intracellular Ca²⁺ release through the inositol 1,4,5-triphosphate and ryanodine receptor pathways, respectively. In Ca²⁺-free-ethylene glycol tetraacetic acid (EGTA)-poly(sodium styrenesulfonate) (PSS) and in Ca²⁺-EGTA-PSS, the PHE induced elevation of perfusion pressure significantly decreased. Vascular responses to caffeine (20 mmol/1) in Ca²⁺-free-EGTA-PSS, with an increase of PMI from 2-8 h, did not change significantly. A similar effect was observed with vascular responses to KCl 40 mmol/1 in Ca²⁺-EGTA-PSS. To confirm whether the inhibitory effect of 2, 4, 6 and 8 h PMI was mediated through the formation of NO, nitro-L-arginine (L-NNA), a potent NO synthase inhibitor, was used. Exposure to L-NNA (10^-5 M) blocked the inhibition induced by an increase of PMI. The blocked effects of L-NNA were reversed by L-arginine (10^-4 M). In conclusion, these patterns of change in artery responses provide insight into the post-mortem change in the receptor-mediated signaling components in epithelial and smooth muscle cells, and support the further study of post-mortem vascular responses triggered by G protein-coupled receptors (metabotropic) and channel-linked receptors (ionotropic) as potential markers for estimating short and long-term PMIs, respectively.

Dual role of nitric oxide in pancreatic β-cells

An involvement of inducible nitric oxide (NO) synthase (NOS) in pancreatic β-cell degeneration during the process of type 1 diabetes has been well discussed. Recently, there is growing evidence for pivotal roles of constitutive NOS (cNOS) in β-cells; the presence of NOS1 and NOS3 in pancreatic β-cells and the effects of low-concentration NO, which is assumed to be derived from cNOS, on β-cell functions have been reported. However, the roles of cNOS-derived NO in β-cells are still under debate. One of the reasons seems to be that NO has multiple biological activities, which are dependent on its concentration. In β-cells, NO has been shown to exert positive and negative regulation of insulin secretion and anti- and pro-apoptotic activities, which is likely to be dependent on concentrations. In this review article, we will describe the current understanding of the roles of NO in pancreatic β-cells, especially focusing on cNOS-derived NO and its differential roles depending on concentrations.

Inorganic nitrite stimulates pancreatic islet blood flow and insulin secretion

Reactive nitrogen and oxygen species have been proposed to be involved in control of insulin release from the pancreatic β cell. Recent evidence suggests that the supposedly inert anions nitrate and nitrite are metabolized in blood and tissues to form nitric oxide (NO) and other bioactive nitrogen oxides. Here we present evidence for a novel stimulatory role of nitrite in influencing pancreatic islet physiology via a dual mechanism, involving both indirect enhancement (through microcirculation redistribution) and direct insulinotropic effects on the β cell. In rats, intraperitoneal injection of sodium nitrite increased pancreatic islet blood flow by 50% and serum insulin concentrations by 30%, while whole pancreatic blood flow and glycemia remained unaffected. Nitrite also dose dependently enhanced insulin secretion from rat β cells in vitro under nonstimulatory glucose concentrations. This effect was not mimicked by nitrate and was abolished by the guanylyl cyclase (GC) inhibitor ODQ and the NO scavenger cPTIO. It was also mimicked by a cyclic GMP agonist (8-CPT-cGMP) and a classical NO donor (NONOate). Interestingly, a reactive oxygen species scavenger (vitamin E analog, Trolox) abolished the insulin secretion induced by nitrite. We conclude that nitrite exerts dual stimulatory effects on pancreatic islet function, including enhancement of islet blood flow and subsequent insulin secretion in vivo and direct stimulation of insulin release in vitro. The insulinotropic effect of nitrite is cGMP-dependent and involves formation of reactive nitrogen and oxygen species.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

Role of cADPR in sodium nitroprusside-induced opossum esophageal longitudinal smooth muscle contraction

Nitric oxide (NO) relaxes most smooth muscle, including the circular smooth muscle (CSM) of the esophagus, whereas in the adjacent longitudinal smooth muscle (LSM), it causes contraction. The second messenger pathways responsible for this NO-induced LSM contraction are unclear, given that these opposing effects of NO are both cGMP dependent. In intestinal LSM, but not CSM, cADP ribose (cADPR)-dependent pathways participate in Ca(2+) mobilization and muscle contraction; whether similar differences exist in the esophagus is unknown. The purpose of this study was to determine whether cADPR plays a role in the NO-mediated contraction of opossum esophageal LSM. Standard isometric tension recordings were performed using both LSM and CSM strips from opossum distal esophagus that were hung in 10-ml tissue baths perfused with oxygenated Krebs solution. cADPR produced concentration-dependent contraction of LSM strips with an EC(50) of 1 nM and peak contraction of 57 +/- 18% of the 60 mM KCl-induced contraction. cADPR had no effect on CSM strips at concentrations up to 10(-6) M. The EC(50) of cADPR caused contraction (18 +/- 2% from initial resting length) of isolated LSM cells. Sodium nitroprusside (SNP; 300 muM) induced contraction of LSM strips that averaged 67 +/- 5% of the KCl response. cADPR antagonists 8-bromo-cADPR and 8-amino-cADPR, as well as ryanodine receptor antagonists ryanodine and tetracaine, significantly inhibited the SNP-induced contraction. In conclusion, in the opossum esophagus, 1) cADPR induces contraction of LSM, but not CSM, and 2) NO-induced contraction of LSM appears to involve a cADPR-dependent pathway.

Glucose-induced release of nitric oxide from mouse pancreatic islets as detected with nitric oxide-selective glass microelectrodes

Nitric oxide (NO) is believed to play an important role in pancreatic islet physiology and pathophysiology. Research in this area has been hampered, however, by the use of indirect methods to measure islet NO. To investigate the role of NO in islet function, we positioned NO-sensitive, recessed-tip microelectrodes in close proximity to individual islets and monitored oxidation current to detect subnanomolar NO in the bath. NO release from islets consisted of a series of rapid bursts lasting several seconds and/or slow oscillations with a period of approximately 100-300 s. Average baseline NO near the islets in 2.8 mM glucose was 524+/-59 nM (n=12). Raising glucose from 2.8 to 11.1 mM augmented NO release by 429+/-133 nM (n=12, P<0.05), an effect blocked by the NO synthase inhibitor L-NAME (n=3). We also observed that glucose-stimulated increases in NO release were contemporaneous with changes in NAD(P)H and O2 but occurred well before increases in calcium associated with glucose-stimulated insulin secretion. In summary, we demonstrate that NO release from islets is oscillatory and rapidly augmented by glucose, suggesting that NO release occurs early following an increase in glucose metabolism and may contribute to the stimulated insulin secretion triggered by suprathreshold glucose.

Staphylococcal enterotoxin A modulates intracellular Ca2+ signal pathway in human intestinal epithelial cells

We demonstrate here that staphylococcal enterotoxin A (SEA) induces an increase in intracellular calcium ([Ca2+]i) in human intestinal epithelial cells and the [Ca2+]i is released from intracellular stores. SEA-induced increase of [Ca2+]i was clearly inhibited by treatment with a nitric oxide synthase (NOS) inhibitors, N(G)-monomethyl-L-arginine and guanidine. Intestinal epithelial cells express endothelial NOS in resting cell condition, and express inducible NOS after stimulating with tumor necrosis factor (TNF)-alpha. TNF-alpha-pretreated cells showed a significant increase in [Ca2+]i that was also inhibited by the NOS inhibitor. These results suggest that SEA modulated [Ca2+]i signal is dependent on NOS expression in human intestinal epithelial cells.

A calcium-induced calcium influx factor, nitric oxide, modulates the refilling of calcium stores in astrocytes

The roles of nitric oxide are primarily undefined in astrocytes, cells that are active partners in synaptic transmission. Because nitric oxide synthases are present in astrocytes, we imaged the formation of nitric oxide in cultured murine cortical astrocytes using DAF-FM (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate). We demonstrated that physiological concentrations of ATP induced a Ca2+-dependent production of nitric oxide. We then investigated the roles of nitric oxide in astrocytic Ca2+ signaling by exogenous application of a nitric oxide donor and found that nitric oxide induced an influx of external Ca2+. Because these observations raise the possibility that nitric oxide-dependent Ca2+ influx could lead to the refilling of internal stores with Ca2+, we directly monitored the Ca2+ levels of the cytosol and of internal stores while manipulating nitric oxide. Cultures were coloaded with mag-fluo-4 and X-rhod-1 to differentially load the internal stores and cytosol, respectively. ATP induced a cytosolic increase in Ca2+ that results from the IP3-dependent release of Ca2+ from internal stores, detected as a simultaneous reduction in mag-fluo-4 and an increase in X-rhod-1 fluorescence. To monitor store refilling, we measured the recovery of mag-fluo-4 fluorescence after removal of ATP. When nitric oxide signaling was blocked by the nitric oxide scavenger 2-phenyl-4,4,5,5-ketramethyl-imidazoline-1-oxyl-3-oxide or by the nitric oxide synthase inhibitor NG-monomethyl-l-arginine, fluorescence recovery was significantly reduced. These data suggest that transmitters that induce Ca2+ signaling in astrocytes lead to the Ca2+-dependent synthesis of nitric oxide. This in turn stimulates a Ca2+ influx pathway that is, in part, responsible for the refilling of internal Ca2+ stores.

Endocrinology: nitric oxide-mediated insulin secretion in response to citrulline in islet beta-cells

INTRODUCTION

Nitric oxide (NO) synthases (NOSs) are expressed in insulin secreting beta-cells. However, physiologic role of NO in insulin release is still controversial. We previously reported that argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL), together with NOS, constitute the citrulline-argininosuccinate-arginine (Cit-AS-Arg) cycle in beta-cells and that this cycle metabolizes citrulline to produce NO and increase cytosolic Ca2+ concentration ([Ca2+]i) in islet beta-cells.

AIMS

This study examined whether this cycle could be linked to insulin release.

METHODOLOGY

Islets of Langerhans were isolated from Wistar rats by collagenase digestion and further dispersed into single beta-cells. [Ca2+]i in beta-cells was measured by dual-wavelength fura-2 microfluorometry combined with digital imaging. NO production was assayed by DAF-2 microfluorometry. Insulin release was determined by ELISA.

RESULTS

Citrulline at a physiologic concentration (0.1 mM) increased insulin release from rat islets and increased [Ca2+]i in rat beta-cells in the presence of 8.3 mmol/l glucose, and they were inhibited by a NOS inhibitor, NG-monomethyl-L-arginine (NMMA). Citrulline induced NO production in rat beta-cells. A NO-donor increased insulin release and [Ca2+]i in rat islet beta-cells.

CONCLUSION

The metabolism of physiologic concentrations of citrulline by the Cit-AS-Arg cycle leads to NO production and resultant potentiation of glucose-induced insulin release, in which a Ca2+-mediated pathway could be involved.

Ca(2+)-induced Ca(2+) release in the pancreatic beta-cell: direct evidence of endoplasmic reticulum Ca(2+) release

The role of the Ca(2+)-induced Ca(2+) release channel (ryanodine receptor) in MIN6 pancreatic beta-cells was investigated. An endoplasmic reticulum (ER)-targeted "cameleon" was used to report lumenal free Ca(2+). Depolarization of MIN6 cells with KCl led to release of Ca(2+) from the ER. This ER Ca(2+) release was mimicked by treatment with the ryanodine receptor agonists caffeine and 4-chloro-m-cresol, reversed by voltage-gated Ca(2+) channel antagonists and blocked by treatment with antagonistic concentrations of ryanodine. The depolarization-induced rise in cytoplasmic Ca(2+) was also inhibited by ryanodine, which did not alter voltage-gated Ca(2+) channel activation. Both ER and cytoplasmic Ca(2+) changes induced by depolarization occurred in a dose-dependent manner. Glucose caused a delayed rise in cytoplasmic Ca(2+) but no detectable change in ER Ca(2+). Carbamyl choline caused ER Ca(2+) release, a response that was not altered by ryanodine. Taken together, these results provide strong evidence that Ca(2+)-induced Ca(2+) release augments cytoplasmic Ca(2+) signals in pancreatic beta-cells.

Dual effect of nitric oxide on cytosolic Ca2+ concentration and insulin secretion in rat pancreatic beta-cells

In isolated rat pancreatic beta-cells, the nitric oxide (NO) donor NOC-7 at 1 microM reduced the amplitude of the oscillations of cytosolic Ca(2+) concentration ([Ca(2+)](c)) induced by 11.1 mM glucose, and at 10 microM terminated them. In the presence of N(G)-nitro-l-arginine (l-NNA), however, NOC-7 at 0.5 and 1 microM increased the amplitude of the [Ca(2+)](c) oscillations, although the NO donor at 10 microM still suppressed them. Aqueous NO solution also had a dual effect on the [Ca(2+)](c) oscillations. The soluble guanylate cyclase inhibitor LY-83583 and the cGMP-dependent protein kinase inhibitor KT5823 inhibited the stimulatory effect of NO, and 8-bromo-cGMP increased the amplitude of the [Ca(2+)](c) oscillations. Patch-clamp analyses in the perforated configuration showed that 8-bromo-cGMP inhibited whole cell ATP-sensitive K(+) currents in the isolated rat pancreatic beta-cells, suggesting that the inhibition by cGMP of ATP-sensitive K(+) channels is, at least in part, responsible for the stimulatory effect of NO on the [Ca(2+)](c) oscillations. In the presence of l-NNA, the glucose-induced insulin secretion from isolated islets was facilitated by 0.5 microM NOC-7, whereas it was suppressed by 10 microM NOC-7. These results suggest that NO facilitates glucose-induced [Ca(2+)](c) oscillations of beta-cells and insulin secretion at low concentrations, which effects are mediated by cGMP, whereas NO inhibits them in a cGMP-independent manner at high concentrations.

Two distinct effects of cGMP on cytosolic Ca2+ concentration of rat pancreatic beta-cells

The present study investigated the effects of cGMP on cytosolic Ca(2+) concentration ([Ca(2+)](c)) of isolated rat pancreatic beta-cells. In the presence of 7.0 mM glucose, NOC 7, a nitric oxide (NO) donor, caused an increase in [Ca(2+)](c) of the beta-cells, which was abolished by the soluble guanylate cyclase inhibitor ODQ. Similar [Ca(2+)](c) elevation was evoked by 8-bromo-cGMP. The [Ca(2+)](c) elevating responses to NOC 7 and 8-bromo-cGMP were abolished by nicardipine or in a Ca(2+)-free medium, but were not affected by thapsigargin, suggesting that they are produced by the Ca(2+) influx through L-type voltage-operated Ca(2+) channels. In contrast, NOC 7 and 8-bromo-cGMP decreased the [Ca(2+)](c) when it was raised in advance by the elevation of external K(+) concentration to 30 mM or by 4-aminopyridine. The pretreatment with thapsigargin almost abolished the [Ca(2+)](c) reduction induced by the agents, suggesting that the action is likely to be primarily attributable to an acceleration of the Ca(2+) sequestration into the endoplasmic reticulum. These results suggest that cGMP has two distinct effects on the [Ca(2+)](c) of rat pancreatic beta-cells: a facilitation of the Ca(2+) influx through L-type voltage-operated Ca(2+) channels and an acceleration of the Ca(2+) sequestration in the endoplasmic reticulum.

Amplification of exocytosis by Ca2+-induced Ca2+ release in INS-1 pancreatic beta cells

Functional coupling between Ca(2+)-induced Ca(2+) release (CICR) and quantal exocytosis in 5-hydroxytryptamine-loaded INS-1 beta cells was assessed through the use of carbon fibre amperometry in combination with Fura-2. CICR was evoked by the glucagon-like-peptide-1 (GLP-1) receptor agonist exendin-4 (Ex-4) and was accompanied by quantal secretory events appearing as amperometric current spikes time-locked to the increase of [Ca(2+)](i). The action of Ex-4 was reproduced by treatment with caffeine, and the source of Ca(2+) serving as a stimulus for exocytosis originated from ryanodine and thapsigargin-sensitive Ca(2+) stores. Two distinct patterns of exocytosis occurred within 5 s following the initiation of CICR. Non-summating exocytosis (NS-type) was defined as multiple asynchronous current spikes, and the half-height duration of each spike was 12-48 ms. Summating exocytosis (S-type) was defined as a cluster of spikes. It generated a macroscopic current, the half-height duration of which was 243-682 ms. The release charge of S-type exocytosis was 3.2-fold greater than that of NS-type when measured 2 s following the initiation of secretion. NS-type exocytosis was observed frequently under conditions in which the basal Ca(2+) concentration ([Ca(2+)](B)) was low (75-150 nM), whereas S-type exocytosis predominated under conditions in which the [Ca(2+)](B) was elevated (200-275 nM). Depolarization-induced Ca(2+) influx triggered NS-type exocytosis in most cells tested, irrespective of [Ca(2+)](B). It is concluded that CICR is a highly effective stimulus for exocytosis in INS-1 cells. The increase of [Ca(2+)](i) that accompanies CICR stimulates the asynchronous release of a small number of secretory granules under conditions of low [Ca(2+)](B). When [Ca(2+)](B) is slightly elevated, CICR targets a much larger pool of secretory granules that undergo summating exocytosis. The transition from NS-type to S-type exocytosis may represent an amplification mechanism for Ca(2+)-dependent exocytosis.

Protection of rat pancreatic islets by potassium channel openers against alloxan, sodium nitroprusside and interleukin-1beta mediated suppression--possible involvement of the mitochondrial membrane potential

AIMS/HYPOTHESIS

We aimed to study the effects of two K(ATP) channel openers (KCO), diazoxide and the more potent compound NNC 55-0118, on beta-cell suppression and/or toxicity induced by alloxan, sodium nitroprusside and IL-1beta.

METHODS

Islets from rats were exposed to 0.3 mmol/l diazoxide or NNC 55-0118 for 30 min and either alloxan (0.5 mmol/l), sodium nitroprusside (0.5 mmol/l) or IL-1beta (12.5 or 25 U/ml) were added and the incubation continued for 30 min. Islets were then washed and incubated for 24 h before examination.

RESULTS

After exposure to alloxan, islets showed reduced glucose oxidation rate and impaired glucose-stimulated insulin release. NNC 55-0118 counteracted the effects of alloxan, while diazoxide was less effective. After treatment with sodium nitroprusside, islet glucose oxidation rates were reduced and this was prevented by pretreatment with NNC 55-0118. In short-term experiments the potassium channel openers (KCOs) did not influence the IL-1beta effect on insulin secretion. However, long-term addition (24 h) of NNC 55-0118 counteracted IL-1beta induced inhibition of the glucose oxidation rate. It was shown, using the fluorescent probe JC-1, that the mitochondrial membrane potential was reduced by the potassium channel openers (KCOs), most strongly by NNC 55-0118. Nevertheless culture with KCOs for 72 h did not cause irreversible damage to the islets.

CONCLUSION/INTERPRETATION

Potassium channel openers (KCOs), in particular NNC 55-0118, prevented the toxic effects of alloxan and sodium nitroprusside. IL-1beta mediated suppression was reduced by NNC 55-0118 provided the long-term addition of the potassium channel opener (KCO). The protective mechanism of potassium channel openers (KCOs) might involve a decrease of the mitochondrial membrane potential.

Exogenous nitric oxide and endogenous glucose-stimulated beta-cell nitric oxide augment insulin release

The role nitric oxide (NO) plays in physiological insulin secretion has been controversial. Here we present evidence that exogenous NO stimulates insulin secretion, and that endogenous NO production occurs and is involved in the regulation of insulin release. Radioimmunoassay measurement of insulin release and a dynamic assay of exocytosis using the dye FM1-43 demonstrated that three different NO donors-hydroxylamine (HA), sodium nitroprusside, and 3-morpholinosydnonimine (SIN-1)-each stimulated a marked increase in insulin secretion from INS-1 cells. Pharmacological manipulation of the guanylate cyclase/guanosine 3',5'-cyclic monophosphate pathway indicated that this pathway was involved in mediating the effect of the intracellular NO donor, HA, which was used to simulate endogenous NO production. This effect was further characterized as involving membrane depolarization and intracellular Ca(2+) ([Ca(2+)](i)) elevation. SIN-1 application enhanced glucose-induced [Ca(2+)](i) responses in primary beta-cells and augmented insulin release from islets in a glucose-dependent manner. Real-time monitoring of NO using the NO-sensitive fluorescent dye, diaminofluorescein, was used to provide direct and dynamic imaging of NO generation within living beta-cells. This showed that endogenous NO production could be stimulated by elevation of [Ca(2+)](i) levels and by glucose in both INS-1 and primary rat beta-cells. Scavenging endogenously produced NO-attenuated glucose-stimulated insulin release from INS-1 cells and rat islets. Thus, the results indicated that applied NO is able to exert an insulinotropic effect, and implicated endogenously produced NO in the physiological regulation of insulin release.

cGMP/protein kinase G-dependent inhibition of N-type Ca2+ channels induced by nitric oxide in human neuroblastoma IMR32 cells

Although data from our laboratory and others suggest that nitric oxide (NO) exerts an overall inhibitory action on high-voltage-activated Ca2+ channels, conflicting observations have been reported regarding its effects on N-type channels. We performed whole-cell and cell-attached patch-clamp recordings in IMR32 cells to clarify the functional role of NO in the modulation of N channels of human neuronal cells. During depolarizing steps to +10 mV from V(h) = -90 mV, the NO donor, sodium nitroprusside (SNP; 200 microm), reduced macroscopic N currents by 34% (p < 0.01). The magnitude of inhibition was similar at all voltages tested (range, -40 to +50 mV). No significant inhibition was observed when SNP was applied together with the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium salt (300 microm), or after cell treatment with the guanylate cyclase inhibitor, 1H-[1,2,4] oxadiazole [4,3-a] quinoxalin-1-one (10 microm). 8-bromoguanosine-cGMP (8-Br-cGMP) (400 microm) mimicked the effects of SNP, reducing Ba2+ currents by 37% (p < 0.001). Cell treatment with the protein kinase G (PKG) inhibitor KT5823 (1 microm) or guanosine 3',5'-cyclic monophosphorothioate, 8-(4-chloro-phenylthio)-Rp-isomer, triethylammonium salt (20 microm) virtually abolished the effects of 8-Br-cGMP. At the single-channel level, 8-Br-cGMP reduced the channel open probability by 59% and increased both the mean shut time and the null sweep probability, but it had no significant effects on channel conductance, mean open time, or latency of first openings. These data suggest that NO inhibits N-channel gating through cGMP and PKG. The consequent decrease in Ca2+ influx through these channels may affect different neuronal functions, including neurotransmitter release.

Role of nitric oxide synthase isoforms in glucose-stimulated insulin release

The role of islet constitutive nitric oxide synthase (cNOS) in insulin-releasing mechanisms is controversial. By measuring enzyme activities and protein expression of NOS isoforms [i.e., cNOS and inducible NOS (iNOS)] in islets of Langerhans cells in relation to insulin secretion, we show that glucose dose-dependently stimulates islet activities of both cNOS and iNOS, that cNOS-derived nitric oxide (NO) strongly inhibits glucose-stimulated insulin release, and that short-term hyperglycemia in mice induces islet iNOS activity. Moreover, addition of NO gas or an NO donor inhibited glucose-stimulated insulin release, and different NOS inhibitors effected a potentiation. These effects were evident also in K+-depolarized islets in the presence of the ATP-sensitive K+ channel opener diazoxide. Furthermore, our results emphasize the necessity of measuring islet NOS activity when using NOS inhibitors, because certain concentrations of certain NOS inhibitors might unexpectedly stimulate islet NO production. This is shown by the observation that 0.5 mmol/l of the NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) stimulated cNOS activity in parallel with an inhibition of the first phase of glucose-stimulated insulin release in perifused rats islets, whereas 5.0 mmol/l of L-NMMA markedly suppressed cNOS activity concomitant with a great potentiation of the insulin secretory response. The data strongly suggest, but do not definitely prove, that glucose indeed has the ability to stimulate both cNOS and iNOS in the islets and that NO might serve as a negative feedback inhibitor of glucose-stimulated insulin release. The results also suggest that hyperglycemia-evoked islet NOS activity might be one of multiple factors involved in the impairment of glucose-stimulated insulin release in type II diabetes mellitus.

Nitric oxide inhibits neuroendocrine Ca(V)1 L-channel gating via cGMP-dependent protein kinase in cell-attached patches of bovine chromaffin cells

Nitric oxide (NO) regulates the release of catecholamines from the adrenal medulla but the molecular targets of its action are not yet well identified. Here we show that the NO donor sodium nitroprusside (SNP, 200 microM) causes a marked depression of the single Ca(V)1 L-channel activity in cell-attached patches of bovine chromaffin cells. SNP action was complete within 3-5 min of cell superfusion. In multichannel patches the open probability (NP(o)) decreased by approximately 60 % between 0 and +20 mV. Averaged currents over a number of traces were proportionally reduced and showed no drastic changes to their time course. In single-channel patches the open probability (P(o)) at +10 mV decreased by the same amount as that of multichannel patches (approximately 61 %). Such a reduction was mainly associated with an increased probability of null sweeps and a prolongation of mean shut times, while first latency, mean open time and single-channel conductance were not significantly affected. Addition of the NO scavenger carboxy-PTIO or cell treatment with the guanylate cyclase inhibitor ODQ prevented the SNP-induced inhibition. 8-Bromo-cyclicGMP (8-Br-cGMP; 400 microM) mimicked the action of the NO donor and the protein kinase G blocker KT-5823 prevented this effect. The depressive action of SNP was preserved after blocking the cAMP-dependent up-regulatory pathway with the protein kinase A inhibitor H89. Similarly, the inhibitory action of 8-Br-cGMP proceeded regardless of the elevation of cAMP levels, suggesting that cGMP/PKG and cAMP/PKA act independently on L-channel gating. The inhibitory action of 8-Br-cGMP was also independent of the G protein-induced inhibition of L-channels mediated by purinergic and opiodergic autoreceptors. Since Ca(2+) channels contribute critically to both the local production of NO and catecholamine release, the NO/PKG-mediated inhibition of neuroendocrine L-channels described here may represent an important autocrine signalling mechanism for controlling the rate of neurotransmitter release from adrenal glands.

The ryanodine receptor calcium channel of beta-cells: molecular regulation and physiological significance

The list of Ca(2+) channels involved in stimulus-secretion coupling in beta-cells is increasing. In this respect the roles of the voltage-gated Ca(2+) channels and IP(3) receptors are well accepted. There is a lack of consensus about the significance of a third group of Ca(2+) channels called ryanodine (RY) receptors. These are large conduits located on Ca(2+) storage organelle. Ca(2+) gates these channels in a concentration- and time-dependent manner. Activation of these channels by Ca(2+) leads to fast release of Ca(2+) from the stores, a process called Ca(2+)-induced Ca(2+) release (CICR). A substantial body of evidence confirms that beta-cells have RY receptors. CICR by RY receptors amplifies Ca(2+) signals. Some properties of RY receptors ensure that this amplification process is engaged in a context-dependent manner. Several endogenous molecules and processes that modulate RY receptors determine the appropriate context. Among these are several glycolytic intermediates, long-chain acyl CoA, ATP, cAMP, cADPR, NO, and high luminal Ca(2+) concentration, and all of these have been shown to sensitize RY receptors to the trigger action of Ca(2+). RY receptors, thus, detect co-incident signals and integrate them. These Ca(2+) channels are targets for the action of cAMP-linked incretin hormones that stimulate glucose-dependent insulin secretion. In beta-cells some RY receptors are located on the secretory vesicles. Thus, despite their low abundance, RY receptors are emerging as distinct players in beta-cell function by virtue of their large conductance, strategic locations, and their ability to amplify Ca(2+) signals in a context-dependent manner.

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.

cAMP-regulated guanine nucleotide exchange factor II (Epac2) mediates Ca2+-induced Ca2+ release in INS-1 pancreatic beta-cells

1. The signal transduction pathway responsible for cAMP-dependent Ca2+-induced Ca2+ release (CICR) from endoplasmic reticulum Ca2+ stores was assessed in the insulin-secreting cell line INS-1. 2. CICR was triggered by the GLP-1 receptor agonist exendin-4, an effect mimicked by caffeine, Sp-cAMPS or forskolin. CICR required influx of Ca2+ through L-type voltage-dependent Ca2+ channels, and was blocked by treatment with nimodipine, thapsigargin, or ryanodine, but not by the IP3 receptor antagonist xestospongin C. 3. Treatment with the cAMP antagonist 8-Br-Rp-cAMPS blocked CICR in response to exendin-4, whereas the PKA inhibitor H-89 was ineffective when tested at a concentration demonstrated to inhibit PKA-dependent gene expression. 4. RT-PCR of INS-1 cells demonstrated expression of mRNA coding for the type-II isoform of cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF-II, Epac2). 5. CICR in response to forskolin was blocked by transient transfection and expression of a dominant negative mutant isoform of cAMP-GEF-II in which inactivating mutations were introduced into the exchange factor's two cAMP-binding domains. 6. It is concluded that CICR in INS-1 cells results from GLP-1 receptor-mediated sensitization of the intracellular Ca2+ release mechanism, a signal transduction pathway independent of PKA, but which requires cAMP-GEF-II.

Nitric oxide inhibition of cAMP synthesis in parotid acini: regulation of type 5/6 adenylyl cyclase

The nitric oxide (NO) donor, GEA 3162, inhibited isoproterenol-induced cyclic AMP (cAMP) accumulation in a concentration- and time-dependent manner in mouse parotid acini; SIN-1 mimicked these effects. Inhibition of stimulated cAMP accumulation was independent of phosphodiesterase activity. GEA 3162 also inhibited forskolin-induced cAMP accumulation. Removal of extracellular Ca(2+), addition of La(3+), or the calmodulin (CaM) inhibitor, calmidazolium, did not prevent the NO-mediated response, and addition of the soluble guanylyl inhibitor, ODQ, did not reverse GEA 3162-induced inhibition of cAMP accumulation. GEA 3162 also inhibited adenylyl cyclase in vitro independently of Ca(2+)/CaM. Further studies revealed that the NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), reduced significantly thapsigargin-induced Ca(2+) release and capacitative Ca(2+) entry and reversed thapsigargin inhibition of the AC Type 5/6 isoform (AC5/6). Data suggest that NO produced endogenously has dual effects on cAMP accumulation in mouse parotid acini, an inhibitory effect on AC activity and a modulatory effect on capacitative Ca(2+) entry resulting in AC5/6 inhibition.

A neuronal isoform of nitric oxide synthase expressed in pancreatic beta-cells controls insulin secretion

Evidence is presented showing that a neuronal isoform of nitric oxide synthase (NOS) is expressed in rat pancreatic islets and INS-1 cells. Sequencing of the coding region indicated a 99.8% homology with rat neuronal NOS (nNOS) with four mutations, three of them resulting in modifications of the amino acid sequence. Double-immunofluorescence studies demonstrated the presence of nNOS in insulin-secreting beta-cells. Electron microscopy studies showed that nNOS was mainly localized in insulin secretory granules and to a lesser extent in the mitochondria and the nucleus. We also studied the mechanism involved in the dysfunction of the beta-cell response to arginine and glucose after nNOS blockade with N(G)-nitro-L-arginine methyl ester. Our data show that miconazole, an inhibitor of nNOS cytochrome c reductase activity, either alone for the experiments with arginine or combined with sodium nitroprusside for glucose, is able to restore normal secretory patterns in response to the two secretagogues. Furthermore, these results were corroborated by the demonstration of a direct enzyme-substrate interaction between nNOS and cytochrome c, which is strongly reinforced in the presence of the NOS inhibitor. Thus, we provide immunochemical and pharmacological evidence that beta-cell nNOS exerts, like brain nNOS, two catalytic activities: a nitric oxide production and an NOS nonoxidating reductase activity, both of which are essential for normal beta-cell function. In conclusion, we suggest that an imbalance between these activities might be implicated in beta-cell dysregulation involved in certain pathological hyperinsulinic states.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Role of nitric oxide in the regulation of monoaminergic neurotransmission

Data accumulated in the last decade indicate that nitric oxide (NO) participates in the regulation of neurotransmission in the central nervous system. Due to its physicochemical properties, NO is an ideal mediator of nonsynaptic interactions. The importance of monoaminergic systems in the function of the brain is clearly shown by the number of severe neuropsychiatric diseases (e.g. depression, Parkinson's disease) caused by the impairment of monoaminergic neurotransmission. Because of their neuroanatomical characteristic, monoaminergic systems participate mainly in nonsynaptic interactions. Since NO is a potential nonsynaptic modulator, it may have an important role in the regulation of monoaminergic systems. The aim of the present review is to survey the literature on the effect of NO on dopaminergic, noradrenergic and serotonergic neurotransmission. The potential mechanisms of action are summarized. Since there is no agreement in the literature on the nature of the effect of NO exerted on monoaminergic neurotransmission, and there are contradictory data concerning the mechanisms involved, the possible reasons for this unusual inconsistency are also discussed.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

Dual effect of NO on K(+)(ATP) current of mouse pancreatic B-cells: stimulation by deenergizing mitochondria and inhibition by direct interaction with the channel

Nitric oxide (NO) is assumed to contribute to the impairment of B-cell function in type 1 diabetes mellitus (IDDM). In the present paper we show that in mouse B-cells with intact metabolism authentic NO (20 microM) led to a biphasic effect on the K(+)(ATP) current, namely a transient increase and a consecutive almost complete inhibition. This resembles closely the effect that we have observed previously with the NO donor S-nitrosocysteine (SNOC, 1 mM) suggesting that merely NO caused both phases of this effect. We now demonstrate that the rise in the current amplitude was accompanied by a depolarization of the mitochondrial membrane potential DeltaPsi and a concomitant reduction in the ATP/ADP ratio. Thus, it seems likely that the increase in current amplitude is due to the interference of NO with cell metabolism. The subsequent inhibition of the K(+)(ATP) current is assumed to be caused by a direct effect on the channel since K(+)(ATP) single channel current activity measured in excised patches was strongly reduced by authentic NO and SNOC. Our data reveal new insights into the mechanisms underlying the biphasic action of NO on K(+)(ATP) channels in pancreatic B-cells.

A fundamental role for the nitric oxide-G-kinase signaling pathway in mediating intercellular Ca(2+) waves in glia

In this study, we highlight a role for the nitric oxide-cGMP-dependent protein kinase (NO-G-kinase) signaling pathway in glial intercellular Ca(2+) wave initiation and propagation. Addition of the NO donor molsidomine (100-500 microM) or puffing aqueous NO onto primary glial cell cultures evoked an increase in [Ca(2+)](i) in individual cells and also local intercellular Ca(2+) waves, which persisted after removal of extracellular Ca(2+). High concentrations of ryanodine (100-200 microM) and antagonists of the NO-G-kinase signaling pathway essentially abrogated the NO-induced increase in [Ca(2+)](i), indicating that NO mobilizes Ca(2+) from a ryanodine receptor-linked store, via the NO-G-kinase signaling pathway. Addition of 10 microM nicardipine to cells resulted in a slowing of the molsidomine-induced rise in [Ca(2+)](i), and inhibition of Mn(2+) quench of cytosolic fura-2 fluorescence mediated by a bolus application of 2 microM aqueous NO to cells, indicating that NO also induces Ca(2+) influx in glia. Mechanical stress of individual glial cells resulted in an increase in intracellular NO in target and neighboring cells and intercellular Ca(2+) waves, which were NO, cGMP, and G-kinase dependent, because incubating cells with nitric oxide synthase, guanylate cyclase, and G-kinase inhibitors, or NO scavengers, reduced Delta[Ca(2+)](i) and the rate of Ca(2+) wave propagation in these cultures. Results from this study suggest that NO-G-kinase signaling is coupled to Ca(2+) mobilization and influx in glial cells and that this pathway plays a fundamental role in the generation and propagation of intercellular Ca(2+) waves in glia.

Effect of nitric oxide on endoplasmic reticulum calcium homeostasis, protein synthesis and energy metabolism

It has been suggested that nitric oxide (NO) may contribute to ischemia-induced cell injury. However, the mechanisms underlying NO toxicity have not yet been fully elucidated. In the present study, we investigated the effect of NO on the level of endoplasmic reticulum (ER) calcium stores, on ER Ca2+ pump activity, on protein synthesis, on concentrations of high-energy phosphates, and on gadd153 mRNA levels. Primary neuronal cells were exposed to the NO-donor (+/-)-S-Nitroso-N-acetylpenicillamine (SNAP) for 1 h, 2 h, 6 h or 24 h. The level of ER calcium stores was evaluated by measuring the increase in cytoplasmic calcium activity induced by exposing cells to thapsigargin, an irreversible inhibitor of ER Ca(2+)-ATPase; the activity of ER Ca(2+)-ATPase was determined by measuring a phosphorylated intermediate; SNAP-induced changes in gadd153 expression were evaluated by quantitative PCR; SNAP-induced changes in protein synthesis were investigated by measuring the incorporation of L-[4,5-3H]leucine into proteins, and changes in the levels of ATP, ADP, AMP were measured by HPLC. Exposing cells to SNAP for 1 h to 2 h induced a marked depletion of ER calcium stores through an inhibition of ER Ca(2+)-ATPase (to 58% of control), and a concentration-dependent suppression of protein synthesis which was reversed in the presence of hemoglobin, suggesting NO-related effects. ATP levels and adenylate energy charge were significantly decreased only when cells were exposed to the highest SNAP concentration for 6 h or 24 h, excluding significant effects of NO on the energy state of cells in the acute state, i.e. when ER calcium stores were already completely depleted and protein synthesis severely suppressed. In light of the regulatory role of ER calcium homeostasis in the control of protein synthesis, the results imply that the suppression of protein synthesis resulted from NO-induced inhibition of ER Ca(2+)-ATPase and depletion of ER calcium stores, and that NO-induced disturbances of energy metabolism are secondary to the effect of NO on ER calcium homeostasis. It is, therefore, concluded that ER calcium stores are a primary target of NO-toxicity.

Comparison of amperometric methods for detection of exocytosis from single pancreatic beta-cells of different species

Two methods for amperometric detection of exocytosis at single pancreatic beta-cells were compared. In the first, direct detection of insulin was accomplished using an insulin-sensitive chemically modified electrode. In the second, 5-hydroxytryptamine (5-HT) that had been allowed to accumulate within the beta-cell secretory vesicles was detected with a bare carbon electrode. The goal of the comparison was to determine whether 5-HT secretion was a valid marker of insulin secretion in single beta-cells. To aid in this comparison, some experiments involved simultaneous measurement of insulin and 5-HT at cells previously allowed to accumulate 5-HT. Upon application of common insulin secretagogues, current spikes resulting from detection of 5-HT, insulin, or both compounds were obtained indicative of secretion via exocytosis. The mean area of current spikes obtained from simultaneous measurements equaled the sum of the mean area of insulin and 5-HT measured independently. Additionally, analyses of the number of spikes obtained for detection of insulin, 5-HT, or both compounds were similar for several common secretagogues. These data support the hypothesis that accumulated 5-HT is released from insulin containing secretory vesicles, exclusively. In addition, measurement of insulin and 5-HT from beta-cells of different species was compared to determine whether a species dependence exists for the two methods compared here. Detection of 5-HT results in a similar number of spikes that are equivalent to insulin in frequency and amplitude in human, porcine, and canine beta-cells; however, in mouse and INS-1 beta-cells, 5-HT is more readily detected than insulin.

Nitric oxide induces apoptosis via Ca2+-dependent processes in the pancreatic beta-cell line MIN6

An excessive production of nitric oxide (NO) in response to cytokines has been shown to be the major cause of the destruction of islet beta-cells associated with type 1 (insulin-dependent) diabetes mellitus. The NO-induced beta-cell death is the typical apoptosis. In the present study, we show evidence that supports a tight link between NO, Ca2+, protease and apoptosis in beta-cells. Three different NO donors, SNAP, NOR3 and NOC7, induced apoptosis in a beta-cell line, MIN6 cells, in a concentration-dependent manner. SNAP at 200 microM increased cytosolic Ca2+ concentration ([Ca2+]i) and induced apoptosis. The SNAP-induced apoptosis was blocked by a Ca2+ chelator, BAPTA-AM, and by an inhibitor of a Ca2+-dependent protease, calpain. In conclusion, an excessive NO production induces apoptosis, wherein an increase in [Ca2+]i and resultant activation of calpain play a key role.

In vitro, nitric oxide (NO) stimulates LH secretion and partially prevents the inhibitory effect of dopamine on PRL release

In recent years nitric oxide (NO) has emerged as an important intra- and intercellular transmitter involved in the control of hypothalamic-pituitary axis. In order to discriminate the potential actions of NO at hypothalamic or pituitary level in the control of PRL and LH release, we have studied PRL and LH secretion by dispersed pituitary cells obtained from males, cycling and lactating females in the presence of 1) sodium nitroprusside (SNP), a NO donor; 2) cyclic guanosine monophosphate (cGMP), the second messenger for a wide range of NO actions; 3) Nw-nitro-L-arginine methyl ester (NAME), a competitive inhibitor of NO synthase (NOS) and 4) oxadialoquinoxalione (OQD) and LY 83,583, antagonists of guanylyl cyclases. We found that SNP (at doses of 100 and 500 micromol) stimulated LH and FSH release and partially blocked the inhibitory action of dopamine (50 and 100 nmol) on prolactin secretion. These effects were not mimicked by cGMP and remained in the presence of OQD and LY 83,583. NAME alone had no significant effect on hormone secretion. These results suggest that NO plays a role in the control of gonadotropins and prolactin secretion acting directly at the pituitary level and that these effects are mediated by mechanisms other than changes in cGMP levels.

Nitric oxide acts independently of cGMP to modulate capacitative Ca(2+) entry in mouse parotid acini

Carbachol- and thapsigargin-induced changes in cGMP accumulation were highly dependent on extracellular Ca(2+) in mouse parotid acini. Inhibition of nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC) resulted in complete inhibition of agonist-induced cGMP levels. NOS inhibitors reduced agonist-induced Ca(2+) release and capacitative Ca(2+) entry, whereas the inhibition of sGC had no effect. The effects of NOS inhibition were not reversed by 8-bromo-cGMP. The NO donor GEA-3162 increased cGMP levels blocked by the inhibition of sGC. GEA-3162-induced increases in Ca(2+) release from ryanodine-sensitive stores and enhanced capacitative Ca(2+) entry, both of which were unaffected by inhibitors of sGC but reduced by NOS inhibitors. Results support a role for NO, independent of cGMP, in agonist-mediated Ca(2+) release and Ca(2+) entry. Data suggest that agonist-induced Ca(2+) influx activates a Ca(2+)-dependent NOS, leading to the production of NO and the release of Ca(2+) from ryanodine-sensitive stores, providing a feedback loop by which store-depleted Ca(2+) channels are activated.

Regulation of ryanodine receptors by reactive nitrogen species

The ryanodine receptors (RyRs) are large intracellular calcium release channels that play an important role in the control of the calcium levels in excitable and non-excitable cells. Many endogenous modulators such as Mg2+, ATP, or calmodulin can affect the channel activities of the three known mammalian RyR isoforms. RyRs also are known to be redox-responsive. However, the molecular basis and the physiological relevance of redox modulation of RyRs are unclear. Recent evidence suggests that nitric oxide (NO) and related molecules may be endogenous regulators of the skeletal and cardiac muscle RyRs. The two tissues express nitric oxide synthases (NOSs), and NO or NO-related species have been shown to affect Ca2+ release channel activities directly via covalent modifications of thiol groups. Both an oxidative and a nitrosative modification of RyRs have been described, leading to either a reversible or irreversible alteration of RyR ion channel activity. Additional mechanisms of regulation may include cyclic GMP-dependent signaling pathways and NO modification of RyR regulatory proteins such as the surface membrane L-type Ca2+ channel. Modification of RyRs by NO may influence a variety of physiological functions such as insulin release, vasomotor control, and muscle contraction.

cAMP-dependent mobilization of intracellular Ca2+ stores by activation of ryanodine receptors in pancreatic beta-cells. A Ca2+ signaling system stimulated by the insulinotropic hormone glucagon-like peptide-1-(7-37)

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone currently under investigation for use as a novel therapeutic agent in the treatment of type 2 diabetes mellitus. In vitro studies of pancreatic islets of Langerhans demonstrated that GLP-1 interacts with specific beta-cell G protein-coupled receptors, thereby facilitating insulin exocytosis by raising intracellular levels of cAMP and Ca2+. Here we report that the stimulatory influence of GLP-1 on Ca2+ signaling results, in part, from cAMP-dependent mobilization of ryanodine-sensitive Ca2+ stores. Studies of human, rat, and mouse beta-cells demonstrate that the binding of a fluorescent derivative of ryanodine (BODIPY FL-X ryanodine) to its receptors is specific, reversible, and of high affinity. Rat islets and BTC3 insulinoma cells are shown by reverse transcriptase polymerase chain reaction analyses to express mRNA corresponding to the type 2 isoform of ryanodine receptor-intracellular Ca2+ release channel (RYR2). Single-cell measurements of [Ca2+]i using primary cultures of rat and human beta-cells indicate that GLP-1 facilitates Ca2+-induced Ca2+ release (CICR), whereby mobilization of Ca2+ stores is triggered by influx of Ca2+ through L-type Ca2+ channels. In these cells, GLP-1 is shown to interact with metabolism of D-glucose to produce a fast transient increase of [Ca2+]i. This effect is reproduced by 8-Br-cAMP, but is blocked by a GLP-1 receptor antagonist (exendin-(9-39)), a cAMP antagonist ((Rp)-cAMPS), an L-type Ca2+ channel antagonist (nimodipine), an antagonist of the sarco(endo)plasmic reticulum Ca2+ ATPase (thapsigargin), or by ryanodine. Characterization of the CICR mechanism by voltage clamp analysis also demonstrates a stimulation of Ca2+ release by caffeine. These findings provide new support for a model of beta-cell signal transduction whereby GLP-1 promotes CICR by sensitizing intracellular Ca2+ release channels to the stimulatory influence of cytosolic Ca2+.

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

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

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.

Nitric oxide does not initiate but potentiates glucose-induced insulin secretion in pancreatic beta-cells

The role of nitric oxide (NO) in glucose-induced insulin secretion was studied in pancreatic beta-cells, HIT-T15. A role for NO is suggested since glucose stimulated NO production in a concentration-dependent manner. NG-monomethyl-L-arginine, a potent inhibitor of nitric oxide synthase, significantly inhibited glucose-induced nitric oxide production as well as insulin release in HIT-T15. Furthermore, this inhibitory effect can be reversed by sodium nitroprusside (SNP), a well known NO donor. While SNP alone did not stimulate insulin release, it potentiated the secretory response of HIT-T15 cells to glucose by approximately two-fold. Potentiation by SNP appears to be mediated by NO, since (i) the potentiation was completely abolished by 10 microM hemoglobin, a scavenger of NO; and (ii) was not affected by rhodanese plus sodium thiosulphate. Neither hemoglobin alone nor the combination of rhodanese and sodium thiosulphate had any effect on glucose induced insulin release. These results are consistent with the hypothesis that glucose-induced formation of NO may potentiate the effect of glucose by a positive feedback mechanism.

Nitric oxide gates fertilization channels in ascidian oocytes through nicotinamide nucleotide metabolism

In this paper we use the nitric oxide (NO) donor sodium nitroprusside to examine the response of the unfertilised oocyte of the ascidian Ciona intestinalis to nitric oxide. We show that the release of NO triggers an inward current that displays similar properties to the ascidian fertilisation current. Furthermore, the production of NO causes the release of intracellular calcium through a ruthenium-red sensitive mechanism. Our data suggest that these effects are due to the stimulation of nicotinamide nucleotide metabolism, but the active second messenger is not cyclic adenosine diphosphate ribose (cADPr). Finally, we show that NO production increases at fertilisation. The results suggest that ascidian sperm trigger the release of NO and this second messenger causes the breakdown of nicotinamide nucleotides leading to the production of a second messenger which induces the fertilisation current and may assist in the production of the increase in calcium.

The CD38/cyclic ADP-ribose system: a topological paradox

CD38 was first identified as a lymphocyte differentiation antigen that showed typical properties of an orphan receptor involved in many programs of cell proliferation and activation. However, CD38 proved also to be a bifunctional ectoenzyme that catalyzes the transient formation of cyclic ADP-ribose (cADPR) in a variety of cell types. This property raises many intriguing and so far unanswered questions, since cADPR is a new second messenger molecule directly involved in the control of calcium homeostasis by means of receptor-mediated release of calcium from ryanodine-sensitive intracellular stores. The relationship between receptor-like and enzymatic properties of CD38 is still unknown. The apparent topological paradox of ectocellular synthesis and intracellular activity of cADPR might be explained by: (a) influx of cADPR across the plasma membrane to reach its target stores, as suggested by experiments on cerebellar granule cells; and (b) NAD(+)-induced internalization, following membrane oligomerization, of CD38 with consequent partial import of cADPR metabolism to an intracellular compartment, as recently observed in lymphoid B cells. These two distinct mechanisms and other potential ones (e.g. binding of ectocellularly formed cADPR to cell surface receptors and initiation of signal-transducing pathways across the plasmamembrane) seem to be paradigmatic of processes affecting different types of cells. Although in some biological systems, such as Aplysia and sea urchin egg, cADPR metabolism is restricted to the intracellular environment, in mammalian cells the CD38/cADPR system provides new challenges in terms of subcellular compartmentation and qualifies as an unusual example of "ectobiochemistry" with potential, still unrecognized, properties of cellular regulation.