nNOS and Ca2+ influx in rat pancreatic acinar and submandibular salivary gland cells

Examination of protective and therapeutic effects of ruscogenin on cerulein-induced experimental acute pancreatitis in rats

Purpose

To determine the potential protective and therapeutic effects and action mechanism of ruscogenin on cerulein-induced acute pancreatitis (AP) model in rats.

Methods

Overall, 32 rats were attenuated to the sham (2-mL/kg/day isotonic solution for 4 weeks), control (20-µg/kg cerulein-induced AP for 12 hours), prophylaxis groups (cerulein-induced AP following 3-mL/kg/day ruscogenin for 4 weeks) and treatment (3-mL/kg/day ruscogenin following cerulein-induced AP for 12 hours). Blood samples were collected for biochemical analysis of nitric oxide synthase 1 (NOS1/neuronal NOS), malondialdehyde (MDA) and intercellular adhesion molecule 1 (ICAM-1). After sacrification, pancreas tissues were collected and prepared for light microscopic (hematoxylin and eosin), immunohistochemical (nuclear factor kappa B) and biochemical analysis (tumor necrosis factor-alpha [TNF-α], interleukin-6 and 1β [IL-6 and IL-1β], CRP, high-sensitivity CRP [hs-CRP] amylase, lipase, and ICAM-1). Ultrastructural analysis was performed by transmission electron microscopy.

Results

The protective and therapeutic actions of ruscogenin were accomplished by improvements in histopathology, by decreasing blood cytokine levels of CRP, hs-CRP levels, TNF-α, IL-6, IL-1β, ICAM-1, by reducing the pancreatic enzymes amylase and lipase in blood, and by suppressing the expression of nuclear factor kappa B, ICAM-1, and NOS-1, but not MDA in pancreatic tissues. Ruscogenin also improved cerulein-induced ultrastructural degenerations in endocrine and exocrine cells, especially in treatment group.

Conclusion

The present findings have demonstrated the beneficial protective and therapeutical effects of ruscogenin, nominating it as a highly promising supplementary agent to be considered in the treatment of AP, and even as a protective agent against the damages induced by disease.

Hydrogen sulfide: a novel gaseous signaling molecule and intracellular Ca2+ regulator in rat parotid acinar cells

In addition to nitric oxide (NO), hydrogen sulfide (H2S) is recognized as a crucial gaseous messenger that exerts many biological actions in various tissues. An attempt was made to assess the roles and underlying mechanisms of both gases in isolated rat parotid acinar cells. Ductal cells and some acinar cells were found to express NO and H2S synthases. Cevimeline, a muscarinic receptor agonist upregulated endothelial NO synthase in parotid tissue. NO and H2S donors increased the intracellular Ca2+ concentration ([Ca2+]i). This was not affected by inhibitors of phospholipase C and inositol 1,4,5-trisphosphate receptors, but was decreased by blockers of ryanodine receptors (RyRs), soluble guanylyl cyclase, and protein kinase G. The H2S donor evoked NO production, which was decreased by blockade of NO synthases or phosphoinositide 3-kinase or by hypotaurine, an H2S scavenger. The H2S donor-induced [Ca2+]i increase was diminished by a NO scavenger or the NO synthases blocker. These results suggest that NO and H2S play important roles in regulating [Ca2+]i via soluble guanylyl cyclase-cGMP-protein kinase G-RyRs, but not via inositol 1,4,5-trisphosphate receptors. The effect of H2S may be partially through NO produced via phosphoinositide 3-kinase-Akt-endothelial NO synthase. It was concluded that both gases regulate [Ca2+]i in a synergistic way, mainly via RyRs in rat parotid acinar cells.

From nitric oxide to hyperbaric oxygen: invisible and subtle but nonnegligible gaseous signaling molecules in acute pancreatitis

Nitric oxide (NO), carbon monoxide, and hydrogen sulfide in addition to hydrogen are well established as gaseous signal molecules throughout the body. Although the role of gasotransmitters in acute pancreatitis (AP) has been explored for many years, many details remain to be elucidated. The physiologic effect of NO in AP mainly relies on induced NO synthase, which stimulates the production of cytokines in the blood. Carbon monoxide inhibits nuclear factor-κB activation, which leads to amelioration of the inflammatory response. Hydrogen sulfide displays a dual role in the mechanism of AP according to its concentration in the system. Hydrogen is a newly discovered gaseous signaling molecule, and currently, there is little evidence that it has any function in alleviating inflammation. We discovered that hyperbaric oxygen is a novel gasotransmitter that has potential use in the treatment of AP. The correlation among hyperbaric oxygen, hypoxia inducible factor 1α, and other signaling pathways should be further studied. We also discuss some prospects and issues that remain to be resolved in this review. In summary, the discovery of gaseous signal molecules has established a new platform for deep investigation of the mechanism of AP, and our knowledge of the role of gasotransmitters in AP will increase with further research.

Arterialization of Venous Blood for Differentiation of Sickle Cell Subjects in Vaso-occlusive Crisis

These studies were designed as two experiments. Experiment 1 was performed to validate the hypothesis that oxygen saturation of the venous blood may be a marker for vaso-occlusive crisis (VOC) in sickle cell patients undergoing hydroxyurea (HU) treatments. Experiment 2 was performed to test the hypothesis that an acute increase in the blood nitric oxide (NO) concentration by administering HU modulates the perception of pain in sickle cell subjects in VOC. The percent saturations of oxyhemoglobin (%O<PRE>2</PRE>Hb), reduced hemoblogin (%RHb), carboxy-hemoglobin (%COHb), met-hemoglobin (%MHb), fetal hemoglobin (HbF), and nitric oxide metabolites were measured in venous blood samples collected from sickle cell disease (SCD) who were on and off HU and O<PRE>2</PRE> at steady state and during VOC. The results showed the ratio of %O<PRE>2</PRE>Hb/RHb in VOC+HU was significantly higher than patients in the steady state who were on and off of HU (p<0.05). The %COHb was higher in all SCD groups, %COHb values were significantly different in SCD at steady state who were on HU. HU and O<PRE>2</PRE> treatment did not play important role on venous blood %O<PRE>2</PRE>Hb and pain scores in SCD during VOC. A single oral dose of HU was associated with a significant increase in the venous concentration of nitric oxide metabolites (NOx), p<0.05. These findings suggest that the ratio %O<PRE>2</PRE>Hb/RHb in venous blood and pain scores differentiate HU-untreated and HU-treated at steady state subjects from HU-treated subjects in VOC; however, the acute increase in venous NOx produced by administering HU to HU-treated subjects in VOC does not explain this difference.

The role of nitric oxide in the physiology and pathophysiology of the exocrine pancreas

SIGNIFICANCE

Nitric oxide (NO), a ubiquitous gaseous signaling molecule, contributes to both pancreatic physiology and pathophysiology.

RECENT ADVANCES

The present review provides a general overview of NO synthesis, signaling, and function. Further, it specifically discusses NO metabolism and its effects in the exocrine pancreas and focuses on the role of NO in the pathogenesis of acute pancreatitis and pancreatic ischemia/reperfusion injury.

CRITICAL ISSUES

Unfortunately, the role of NO in pancreatic physiology and pathophysiology remains controversial in numerous areas. Many questions regarding the messenger molecule still remain unanswered.

FUTURE DIRECTIONS

Probably the least is known about the downstream targets of NO, which need to be identified, especially at the molecular level.

Inhibitors of Bcl-2 protein family deplete ER Ca2+ stores in pancreatic acinar cells

Physiological stimulation of pancreatic acinar cells by cholecystokinin and acetylcholine activate a spatial-temporal pattern of cytosolic [Ca⁺²] changes that are regulated by a coordinated response of inositol 1,4,5-trisphosphate receptors (IP₃Rs), ryanodine receptors (RyRs) and calcium-induced calcium release (CICR). For the present study, we designed experiments to determine the potential role of Bcl-2 proteins in these patterns of cytosolic [Ca⁺²] responses. We used small molecule inhibitors that disrupt the interactions between prosurvival Bcl-2 proteins (i.e. Bcl-2 and Bcl-xl) and proapoptotic Bcl-2 proteins (i.e. Bax) and fluorescence microfluorimetry techniques to measure both cytosolic [Ca⁺²] and endoplasmic reticulum [Ca⁺²]. We found that the inhibitors of Bcl-2 protein interactions caused a slow and complete release of intracellular agonist-sensitive stores of calcium. The release was attenuated by inhibitors of IP₃Rs and RyRs and substantially reduced by strong [Ca²⁺] buffering. Inhibition of IP₃Rs and RyRs also dramatically reduced activation of apoptosis by BH3I-2′. CICR induced by different doses of BH3I-2′ in Bcl-2 overexpressing cells was markedly decreased compared with control. The results suggest that Bcl-2 proteins regulate calcium release from the intracellular stores and suggest that the spatial-temporal patterns of agonist-stimulated cytosolic [Ca⁺²] changes are regulated by differential cellular distribution of interacting pairs of prosurvival and proapoptotic Bcl-2 proteins.

Different role of isoproterenol and NOS inhibitors on salivary ducts of rats

OBJECTIVES

Nitric oxide (NO) is a diffusible intracellular messenger that is present in saliva. Chronic treatment with isoproterenol, a beta receptor agonist, stimulates the release of NO from acinar cells and induces salivary gland hypertrophy. The aim of this study was to investigate the effect of NO synthesis inhibitors and isoproterenol on rat salivary glands. We analyzed salivary gland weight and the number of ducts per unit area (0.5mm(2)) by NADPH-diaphorase histochemistry (to identify the presence of the enzyme NO synthase-NOS) and haematoxylin-and-eosin (HE).

METHODS

For 8 days male Wistar rats received daily single intraperitoneal injections of saline or a NOS inhibitor (40mg/kg N(omega)-nitro-L-arginine L-NOARG or N(omega)-nitro-l-arginine methyl ester L-NAME). This was followed, 30min later, by subcutaneous injection of isoproterenol (2 or 5mg/kg) or saline.

RESULTS

Isoproterenol increased parotid and submandibular gland weights. Isoproterenol (2mg/kg) induced a decrease of ducts per unit area inversely correlated to the weight of the parotid gland. This effect was augmented by L-NAME. In the submandibular gland L-NAME attenuated isoproterenol (2mg/kg) weight increase. In the submandibular gland isoproterenol and NOS inhibitors induced an increase in ducts per unit area (HE and NADPH-diaphorase). No effect was observed in the sublingual gland.

CONCLUSION

To our knowledge this is the first description of isoproterenol and NOS inhibitors increasing duct density in the submandibular gland. Our results corroborate the hypothesis that NO plays different roles in parotid and submandibular glands.

Role of oxidative stress in pancreatic inflammation

Reactive oxygen and reactive nitrogen species (ROS/RNS) have been implicated in the pathogenesis of acute and chronic pancreatitis. Clinical and basic science studies have indicated that ROS/RNS formation processes are intimately linked to the development of the inflammatory disorders. The detrimental effects of highly reactive ROS/RNS are mediated by their direct actions on biomolecules (lipids, proteins, and nucleic acids) and activation of proinflammatory signal cascades, which subsequently lead to activation of immune responses. The present article summarizes the possible sources of ROS/RNS formation and the detailed signaling cascades implicated in the pathogenesis of pancreatic inflammation, as observed in acute and chronic pancreatitis. A therapeutic ROS/RNS-scavenging strategy has been advocated for decades; however, clinical studies examining such approaches have been inconsistent in their results. Emerging evidence indicates that pancreatitis-inducing ROS/RNS generation may be attenuated by targeting ROS/RNS-generating enzymes and upstream mediators.

Expression of nitric oxide synthase isoforms and nitric oxide production in acute pancreatitis and associated lung injury

BACKGROUND/AIMS

The role of nitric oxide (NO) has been increasingly implicated in the pathophysiology of acute pancreatitis (AP). Studies have shown increased NO production in AP although not all are agreeable on whether NO is beneficial or detrimental in AP. This study aims to profile NO production and NO synthase (NOS) expression in the pancreas and lungs in the progression of AP in mice to gain insights to the role played by different NOS isoforms.

METHODS

AP was induced in mice by hourly administration of cerulein. NO production was determined by measuring the total nitrite and nitrate (NOx) content while NOS expression was measured by Western blot.

RESULTS

Pancreatic NO production increased sharply and was sustained throughout AP. iNOS expression was greatly increased while eNOS was downregulated at the later stages. In the lungs, there was an unexpected early increase in the constitutive NOS expression; however iNOS was also significantly overexpressed at the later time point along with a significant increase in NO. Acinar cells were found to overproduce NO in response to cerulein hyperstimulation with iNOS again being the major contributor.

CONCLUSION

These data show that NO production and NOS expression are differentially regulated temporally and in magnitude in the pancreas and lungs in response to cerulein hyperstimulation which suggests differing roles for each NOS isoform. and IAP.

Atrial natriuretic factor intracellular signaling in the rat submandibular gland

We previously reported that intravenously administered atrial natriuretic factor (ANF) induced no salivation but enhanced agonist-evoked secretion in submandibular glands. The gene expression of ANF and natriuretic peptide receptors (NPR) was later reported in the glands. In the present study we sought to establish the intracellular signalling mechanisms underlying ANF modulation of salivary secretion. Fasted rats were prepared with submandibular duct and femoral cannulation. Dose-response curves to methacholine (MC) and norepinephrine (NE) were performed in the presence of cANP (4-23 amide) (selective NPR-C agonist) and ANF. Local injection of the agonist or ANF-induced no salivation, but enhanced MC and NE-evoked secretion. ANF and cANP (4-23 amide) enhanced phosphoinositide turnover being the effect abolished by U73122 (PLC inhibitor). Further ANF and cANP (4-23 amide) decreased basal cAMP content but failed to affect isoproterenol or forskolin-evoked cAMP. ANF response was inhibited by pertussis toxin and mimicked by cANP (4-23 amide) strongly supporting NPR-C activation. ANF-induced cAMP reduction was abolished by PLC and PKC inhibitors. The content of cGMP was dose dependently stimulated by ANF but not modified by cANP (4-23 amide). These findings support that ANF through NPR-C receptors coupled to PLC activation and adenylyl cyclase inhibition interacts with sialogogic agonists in the submandibular gland to potentiate salivation.

A potentiating effect of endogenous NO in the physiologic secretion from airway submucosal glands

It is known that several second messengers, such as Ca(2+) or cAMP, play important roles in the intracellular pathway of electrolyte secretion in tracheal submucosal gland. However, the participation of cGMP, and therefore nitric oxide (NO), is not well understood. To investigate the physiologic role of NO, we first examined whether tracheal glands can synthesize NO in response to acetylcholine (ACh), and then whether endogenous NO has some effects on the ACh-triggered ionic currents. From the experiments using the NO-specific fluorescent indicator 4,5-diaminofluorescein diacetate salt (DAF-2DA), we found that a physiologically relevant low dose of ACh (100 nM) stimulated the endogenous NO synthesis, and it was almost completely suppressed in the presence of the nonspecific NO synthase (NOS) inhibitor Nomega-Nitro-L-arginine Methyl Ester Hydrochloride (L-NAME) or the neuronal NOS (nNOS)-specific inhibitor 7-Nitroindazole (7-NI). Patch-clamp experiments revealed that both the NOS inhibitors (L-NAME or 7-NI) and cGK inhibitors (KT-5823 or Rp-8-Br-cGMP) partially decreased ionic currents induced by 30 nM of ACh, but not in the case of 300 nM of ACh. Our results indicate that NO can be synthesized through the activation of nNOS endogenously and has potentiating effects on the gland secretion, under a physiologically relevant ACh stimulation. When cells were stimulated by an inadequately potent dose of ACh, which caused an excess elevation in [Ca(2+)](i), the cells were desensitized. Therefore, due to NO, gland cells become more sensitive to calcium signaling and are able to maintain electrolyte secretion without desensitization.

Progestin facilitation of lordosis in rodents involves adenylyl cyclase activity in the ventral tegmental area

Increasing cAMP, or activating dopamine type 1 (D(1)) or GABA(A)/benzodiazepine receptor complexes (GBRs), in the ventral tegmental area (VTA) enhances lordosis of rodents. Whether D(1)- and/or GBR-mediated increases in progestin-facilitated lordosis involve the cAMP-synthesizing enzyme, adenylyl cyclase, in the VTA, was investigated. In Experiment 1, ovariectomized estradiol (E(2); 10 microg at h 0)+progesterone (P; 250 microg at h 45)-primed hamsters first received bilateral infusions of the adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine (DDA; 12 microM/side), or vehicle, and then were infused with the D(1) agonist, SKF38393 (100 ng/side), the GBR agonist, muscimol (100 ng/side), or vehicle, to the VTA. Lordosis was evaluated before and 30 min after each infusion. In Experiment 2, ovariectomized, E(2)-primed (10 microg at h 0) rats received VTA infusions of DDA (12 microM/side) or vehicle; SKF38393 (100 ng/side), muscimol (100 ng/side), or vehicle; and the neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP; 100 or 200 ng/side), or beta-cyclodextrin vehicle. Lordosis was assessed before the series of infusions, immediately after drug infusions and 10 or 60 min after 3alpha,5alpha-THP infusions. Progestin- or progestin plus SKF38393-or muscimol-mediated increases in lordosis were blocked by DDA pretreatment. Thus, in the VTA, progestins' membrane action may involve adenylyl cyclase.

Calcium-dependent release of NO from intracellular S-nitrosothiols

The paper describes a novel cellular mechanism for rapid calcium-dependent nitric oxide (NO) release. This release occurs due to NO liberation from S-nitrosothiols. We have analysed the changes of NO concentration in acutely isolated pancreatic acinar cells. Supramaximal acetylcholine (ACh) stimulation induced a Ca(2+)-dependent increase in the fluorescence in the majority of cells loaded with the NO probe DAF-FM via a patch pipette. The ACh-induced NO signals were insensitive to inhibitors of calmodulin and protein kinase C but were inhibited by calpain antagonists. The initial part of the NO signals induced by 10 muM ACh showed little sensitivity to inhibition of NO synthase (NOS); however, cell pretreatment with NO donors (increasing cellular S-nitrosothiol contents) substantially enhanced the initial component of NO responses. Pancreatic acinar cells were able to generate fast calcium-dependent NO responses when stimulated with physiological or supramaximal doses of secretagogues. Importantly, the source of this NO is the already available S-nitrosothiol store rather than de novo synthesis by NOS. A similar mechanism of NO release was found in dorsal root ganglia neurons.

Free radicals and the pancreatic acinar cells: role in physiology and pathology

Reactive oxygen and nitrogen species (ROS and RNS) play an important role in signal transduction and cell injury processes. Nitric oxide synthase (NOS)-the key enzyme producing nitric oxide (NO)-is found in neuronal structures, vascular endothelium and, possibly, in acinar and ductal epithelial cells in the pancreas. NO is known to regulate cell homeostasis, and its effects on the acinar cells are reviewed here. ROS are implicated in the early events within the acinar cells, leading to the development of acute pancreatitis. The available data on ROS/RNS involvement in the apoptotic and necrotic death of pancreatic acinar cells will be discussed.

cGMP modulation of ACh-stimulated exocytosis in guinea pig antral mucous cells

In guinea pig antral mucous cells, ACh stimulates the Ca(2+)-regulated exocytosis, which has a characteristics feature: an initial transient phase followed by a sustained phase. The effects of cGMP on ACh-stimulated exocytosis were studied in guinea pig antral mucous cells using video microscopy. cGMP enhanced the frequency of ACh-stimulated exocytotic events, whereas cGMP alone did not induce any exocytotic events under the ACh-unstimulated condition. cGMP did not stimulate either Ca(2+) mobilization or cAMP accumulation. The Ca(2+) dose-response studies demonstrated that cGMP shifted the dose-response curve upward with no shift to the lower concentration. This indicates that cGMP increased maximal responsiveness of the Ca(2+)-regulated exocytosis, but not the Ca(2+) sensitivity. Moreover, under a condition of ATP depletion by dinitrophenol (DNP) or anoxia (N(2) bubbling), ACh evoked only a sustained phase in exocytotic events with no initial transient phase. However, ACh evoked an initial transient phase followed by a sustained phase with addition of cGMP before ATP depletion, whereas only a sustained phase was evoked in a case of cGMP addition after ATP depletion. Thus cGMP-induced enhancement in ACh-stimulated exocytotic events requires ATP, suggesting that cGMP modulates ATP-dependent priming of Ca(2+)-regulated exocytosis in antral mucous cells. In conclusion, cGMP increases the number of primed granules via acceleration of the ATP-dependent priming, which enhances the Ca(2+)-regulated exocytosis stimulated by ACh.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of mice with null mutations in many of these genes has demonstrated the physiological roles of individual proteins. This review focuses on recent developments in determining the molecular identification of the proteins that regulate the fluid secretion process.

Neuronal nitric oxide synthase and splanchnic blood flow in anaesthetized rats

AIMS

To evaluate to what extent the neuronal form of constitutive nitric oxide synthase (nNOS) contributes to the blood perfusion of splanchnic organs, including the islets of Langerhans.

METHODS

The nNOS inhibitor 7-nitroindazole (300 mg kg(-1) i.p.) was administered to anaesthetized Sprague-Dawley rats, some of which were pre-treated with the ganglionic blocker hexamethonium (20 mg kg(-1) i.v.) The blood perfusion of the splanchnic organs, including the pancreatic islets was then measured with a microsphere technique.

RESULTS

Nitroindazole decreased total pancreatic, duodenal and renal blood flow, whereas pancreatic islet, colonic and adrenal blood flows were unchanged. A slight increase in mean arterial blood pressure was seen after nitroindazole treatment. Nitroindazole did not affect blood glucose or serum insulin concentrations. In separate experiments, hexamethonium affected none of the studied blood flow values, suggesting that the effects of nNOS-inhibition were not mediated from the nervous system.

CONCLUSION

Nitric oxide derived from the activity of nNOS contributes to the blood perfusion in the upper portions of the gastrointestinal tract, viz. the parts supplied by the cranial mesenteric artery, and the kidneys, whilst no effects are seen on colonic or adrenal blood flow. Pancreatic islet blood flow was unaffected by nNOS inhibition, thereby suggesting that NO derived from the other isoforms of NOS maintains the high basal islet blood perfusion.

Cholecystokinin-8-Induced Hypoplasia of the Rat Pancreas: Influence of Nitric Oxide on Cell Proliferation and Programmed Cell Death

The background of cholecystokinin-8 (CCK-8)-induced hypoplasia in the pancreas is not known. In order to increase our understanding we studied the roles of nitric oxide and NF-kappaB in rats. CCK-8 was injected for 4 days, in a mode known to cause hypoplasia, and the nitric oxide formation was either decreased by means of N(omega)-nitro-L-arginine (L-NNA) or increased by S-nitroso-N-acetylpencillamine (SNAP). The activation of NF-kappaB was quantified by ELISA detection, apoptosis with caspase-3 and histone-associated DNA-fragmentation and mitotic activity in the acinar, centroacinar and ductal cells were visualized by the incorporation of [(3)H]-thymidine. Pancreatic histology and weight as well as protein- and DNA contents were also studied. Intermittent CCK injections reduced pancreatic weight, protein and DNA contents and increased apoptosis, acinar cell proliferation and nuclear factor kappaB (NF-kappaB) activation. It also caused vacuolisation of acinar cells. The inhibition of endogenous nitric oxide formation by L-NNA further increased apoptosis and NF-kappaB activation but blocked the increased proliferation and vacuolisation of acinar cells. The DNA content was not further reduced. SNAP given together with CCK-8 increased apoptosis and other pathways of cell death, raised proliferation of acinar cells and strongly reduced the DNA content in the pancreas. Histological examination showed no inflammation in any group. We conclude that during CCK-8-induced pancreatic hypoplasia, endogenously formed nitric oxide suppresses apoptosis but increases cell death along non-apoptotic pathways and stimulates regeneration of acinar cells. Exogenous nitric oxide enhances the acinar cell turnover by increasing both apoptotic and non-apoptotic cell death and cell renewal. In this situation NF-kappaB activation seems not to inhibit apoptosis nor promote cell proliferation.

Ca2+, calmodulin and phospholipids regulate nitricoxide synthase activity in the rabbit submandibular gland

Nitric oxide (NO) plays an important role as an intra- and intercellular signaling molecule in mammalian tissues. In the submandibular gland, NO has been suggested to be involved in the regulation of secretion and in blood flow. NO is produced by activation of NO synthase (NOS). Here, we have investigated the regulation of NOS activity in the rabbit submandibular gland. NOS activity was detected in both the cytosolic and membrane fractions. Characteristics of NOS in the cytosolic and partially purified membrane fractions, such as Km values for l-arginine and EC(50) values for calmodulin and Ca(2+), were similar. A protein band that cross-reacted with anti-nNOS antibody was detected in both the cytosolic and membrane fractions. The membrane-fraction NOS activity increased 1.82-fold with treatment of Triton X-100, but the cytosolic-fraction NOS activity did not. The NOS activity was inhibited by phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PIP(2)). The inhibitory effects of phospholipids on the NOS activity were relieved by an increase in Ca(2+) concentrations. These results suggest that the Ca(2+)- and calmodulin-regulating enzyme nNOS occurs in cytosolic and membrane fractions, and PA and PIP(2) regulate the NOS activity in the membrane site by regulating the effect of Ca(2+) in the rabbit submandibular gland.

Arachidonic acid regulates two Ca2+ entry pathways via nitric oxide

Several regulated Ca2+ entry pathways have been identified, with capacitative Ca2+ entry (CCE) being the most characterized. In the present study, we examined Ca2+ entry pathways regulated by arachidonic acid (AA) in mouse parotid acini. AA induced Ca2+ release from intracellular stores, and increased Ca2+ entry. AA inhibited thapsigargin (Tg)-induced CCE, whereas AA activated Ca2+ entry when CCE was blocked by gadolinium (Gd3+). AA-induced Ca2+ entry was associated with depletion of calcium from ryanodine-sensitive stores; both AA-induced Ca2+ release and Ca2+ entry were inhibited by tetracaine and the nitric oxide synthase (NOS) inhibitor, 7-nitroindazole (7-NI). The nitric oxide (NO) donor, 1,2,3,4-ox-triazolium,5-amino-3-(3,4-dichlorophenyl)-chloride (GEA 3162), but not 8-bromo-cGMP, mimicked the effects of AA in inhibiting CCE. Results suggest that AA acts via nitric acid to inhibit the CCE pathway that is selective for Ca2+, and to activate a second Ca2+ entry pathway that is dependent on depletion of Ca2+ from ryanodine-sensitive stores.

Endothelial nitric oxide synthase is protective in the initiation of caerulein-induced acute pancreatitis in mice

The effect of inhibiting nitric oxide (NO) synthase (NOS) or enhancing NO on the course of acute pancreatitis (AP) is controversial, in part because three NOS isoforms exist: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). We investigated whether inhibition or selective gene deletion of NOS isoforms modified the initiation phase of caerulein-induced AP in mice and explored whether this affected pancreatic microvascular blood flow (PMBF). We investigated the effects of nonspecific NOS inhibition with N(omega)-nitro-l-arginine (l-NNA; 10 mg/kg ip) or targeted deletion of eNOS, nNOS, or iNOS genes on the initiation phase of caerulein-induced AP in mice using in vivo and in vitro models. Western blot analysis was performed to assess eNOS phosphorylation status, an indicator of enzyme activity, and microsphere studies were used to measure PMBF. l-NNA and eNOS deletion, but not nNOS or iNOS deletion, increased pancreatic trypsin activity and serum lipase during the initiation phase of in vivo caerulein-induced AP. l-NNA and eNOS did not affect trypsin activity in caerulein-hyperstimulated isolated acini, suggesting that nonacinar events mediate the effect of NOS blockade in vivo. The initiation phase of AP in wild-type mice was associated with eNOS Thr(495) residue dephosphorylation, which accompanies eNOS activation, and a 178% increase in PMBF; these effects were absent in eNOS-deleted mice. Thus eNOS is the main isoform influencing the initiation of caerulein-induced AP. eNOS-derived NO exerts a protective effect through actions on nonacinar cell types, most likely endothelial cells, to produce greater PMBF.

Secretagogue-stimulated pancreatic secretion is differentially regulated by constitutive NOS isoforms in mice

Nitric oxide (NO) and NO synthase (NOS) play controversial roles in pancreatic secretion. NOS inhibition reduces CCK-stimulated in vivo pancreatic secretion, but it is unclear which NOS isoform is responsible, because NOS inhibitors lack specificity and three NOS isoforms exist: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). Mice having individual NOS gene deletions were used to clarify the NOS species and cellular interactions influencing pancreatic secretion. In vivo secretion was performed in anesthetized mice by collecting extraduodenal pancreatic duct juice and measuring protein output. Nonselective NOS blockade was induced with N(omega)-nitro-L-arginine (L-NNA; 10 mg/kg). In vivo pancreatic secretion was maximal at 160 pmol.kg(-1).h(-1) CCK octapeptide (CCK-8) and was reduced by NOS blockade (45%) and eNOS deletion (44%). Secretion was unaffected by iNOS deletion but was increased by nNOS deletion (91%). To determine whether the influence of NOS on secretion involved nonacinar events, in vitro CCK-8-stimulated secretion of amylase from isolated acini was studied and found to be unaltered by NOS blockade and eNOS deletion. Influence of NOS on in vivo secretion was further examined with carbachol. Protein secretion, which was maximal at 100 nmol.kg(-1).h(-1) carbachol, was reduced by NOS blockade and eNOS deletion but unaffected by nNOS deletion. NOS blockade by L-NNA had no effect on carbachol-stimulated amylase secretion in vitro. Thus constitutive NOS isoforms can exert opposite effects on in vivo pancreatic secretion. eNOS likely plays a dominant role, because eNOS deletion mimics NOS blockade by inhibiting CCK-8 and carbachol-stimulated secretion, whereas nNOS deletion augments CCK-8 but not carbachol-stimulated secretion.

Luminal space enlargement by carbachol in rat parotid intralobular ducts

Carbachol (CCh) enlarges the luminal space in rat parotid intralobular ducts, but the mechanism of their enlargement remains obscure. We investigated the involvement of intracellular calcium ions in the enlargement of luminal space by monitoring the luminal space under optical sectioning in a confocal laser scanning microscope using sulforhodamine B. Carbachol increased the intracellular concentration of calcium ions ([Ca2+]i) and the inside diameter without any change in the outside diameter. Removal of extracellular calcium ions modulated CCh-induced changes in [Ca2+]i to transient, but did not markedly inhibit the CCh-induced increase in the inside diameter. Additional loading of BAPTA (1,2-bis (o-aminophenoxy-ethane-n,n,n',n'-tetraacetic acid) in the duct cells suppressed CCh-induced changes. Diphenylamine-2-carboxylate (DPC), but not cytochalasin D, calmodulin inhibitor or nitric oxide synthase inhibitor profoundly suppressed CCh-induced changes. These results suggest that CCh induces enlargement of the luminal space through the activation of DPC-sensitive channels by the release of calcium ions from the intracellular pool.

Effect of SNI-2011 on amylase secretion from parotid tissue in rats and in neuronal nitric oxide synthase knockout mice

The effect of (+/-)cis-2-methylspilo(1,3-oxathiolane-5,3')quinuclidine (SNI-2011) on the secretory pathway of amylase in parotid tissues was investigated. SNI-2011-induced exocytosis was inhibited by a cell-permeable Ca(2+) chelator or inhibitors of calmodulin kinase II, neuronal nitric oxide synthase (nNOS), soluble guanyl cyclase, cyclic GMP-dependent protein kinase (PKG), and myosin light chain kinase, suggesting that these enzymes were coupled with the exocytosis. Stimulation with SNI-2011 of isolated rat parotid acinar cells loaded with 4,5-diaminofluorescein/diacetate (DAF-2/DA) induced a fast increase in DAF fluorescence corresponding to an increase in the NO production. SNI-2011-induced amylase secretion from parotid tissues in nNOS knockout mice has not been observed yet in spite of the expression of muscarinic M(3) receptors and the maintenance of secretory response to isoproterenol in the tissues. These results indicate the implication of the activation of Ca(2+)- and calmodulin-dependent enzymes and NOS-PKG signaling pathway in SNI-2011-induced amylase secretion from parotid acinar cells.

Regulation of intracellular Ca2+ stores by multiple Ca2+-releasing messengers

Although glucose-elicited insulin secretion depends on Ca(2+) entry through voltage-gated Ca(2+) channels in the surface cell membrane of the pancreatic beta-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores, particularly in relation to hormone- or neurotransmitter-induced insulin secretion. There is now direct evidence for Ca(2+) entry-induced release of Ca(2+) from the endoplasmic reticulum in neurons, but with regard to glucose stimulation of beta-cells, there is conflicting evidence about the operation of such a process. This finding suggests that the sensitivity of the Ca(2+) release channels in the endoplasmic reticulum membrane varies under different conditions and therefore is regulated. Recent evidence from studies of pancreatic acinar cells has revealed combinatorial roles of multiple messengers in setting the sensitivity of the endoplasmic reticulum for Ca(2+) release. Here we focus on the possible combinatorial roles of inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in beta-cell function.

Nitric oxide signalling in salivary glands

Nitric oxide (NO) plays multiple roles in both intracellular and extracellular signalling mechanisms with implications for health and disease. This review focuses on the role of NO signalling in salivary secretion. Attention will be paid primarily to endogenous NO production in acinar cells resulting from specific receptor stimulation and to NO-regulated Ca2+ homeostasis. Due to the fact that NO readily crosses membranes by simple diffusion, endogenous NO may play a physiological role in processes as diverse as modifying the secretory output, controlling blood supply to the gland, modulating transmitter output from nerve endings, participating in the host defence barrier, and affecting growth and differentiation of surrounding tissue. Furthermore, the role of NO in the pathogenesis of human oral diseases will be considered.

Methacholine-induced cGMP production is regulated by nitric oxide generation in rabbit submandibular gland cells

Guanosine 3',5'-monophosphate (cGMP) is an intracellular messenger in various kinds of cell. We investigated the regulation of cGMP production by nitric oxide (NO) in rabbit submandibular gland cells. Methacholine, a muscarinic cholinergic agonist, stimulated cGMP production in a dose- and time-dependent manner, but the alpha-agonist phenylephrine, substance P and the beta-agonist isoproterenol failed to evoke cGMP production. In fura-2-loaded cells, methacholine induced an increase in intracellular Ca2+ ([Ca2+]i) in a concentration-dependent manner, which was similar to that for cGMP production. When the external Ca2+ was chelated with EGTA, methacholine failed to induce cGMP production. Ca2+ ionophore A23187 and thapsigargin, which induce the increase in [Ca2+]i without activation of Ca2+-mobilizing receptors, mimicked the effect of methacholine. cGMP production induced by methacholine, A23187 and thapsigargin was clearly inhibited by NG-nitro-L-arginine methylester (L-NAME), a specific inhibitor of nitric oxide synthase (NOS). S-Nitroso-N-acetyl-DL-penicillamine (SNAP), a NO donor, induced cGMP formation. In the lysate of rabbit submandibular gland cells, Ca2+-regulated nitric oxide synthase activity was detected. These findings suggest that cGMP production induced by the activation of muscarinic cholinergic receptors is regulated by NO generation via the increase in [Ca2+]i.

Effect of nitric oxide in ischemia/reperfusion of the pancreas

BACKGROUND

Ischemia/reperfusion injury, and thus graft pancreatitis, remains a major problem in pancreas transplantation. Contradictory results about the role of nitric oxide (NO) in pancreatic ischemia/reperfusion have been reported; however, in none of the reports has a detailed comparison between inhibition of NO synthase and NO supplementation been carried out.

METHODS

Vascular isolation of the pancreatic tail was performed in landrace pigs. After splenectomy catheters placed in the distal part of the splenic vessels allowed collection of the venous effluent and perfusion of the pancreatic tail. Three hours of complete warm ischemia was followed by 6 h of reperfusion. The effect of the NO donor sodium nitroprusside (SNP) and L-arginine was compared to a control group and NO synthase inhibition with L-NAME.

RESULTS

Lipase in the venous effluent of the pancreas was significantly decreased in the SNP and the L-arginine groups. Vascular resistance was markedly elevated in the L-NAME group and reduced in the NO donor groups. Tissue pO2 after reperfusion was only significantly elevated in the SNP group. Granulocyte infiltration and also overall histological tissue injury were most severe in the control group followed by the L-NAME group, the SNP group, and the L-ARG group.

CONCLUSION

The data show that supplementation of nitric oxide is clearly protective in pancreatic ischemia/reperfusion. However, inhibition of NO synthesis does not lead to an equally clear aggravation of tissue injury.

The influence of nitric oxide on basal and cholecystokinin-8-induced proliferation and apoptosis in the rat pancreas

Nitric oxide (NO) is formed by different cell types in the pancreas. In this study, inhibition of endogenous nitric oxide by N(omega)-nitro-L-arginine (L-NNA) reduced the urinary excretion of NO(2)/NO(3) and raised serum L-arginine and the NO donator S-nitroso-N-acetylpenicillamine (SNAP) increased the urinary excretion of NO(2)/NO(3). The peptide cholecystokinin-8 (CCK-8) has a strong influence on exocrine pancreatic proliferation. Rat pancreas was excised and studied with regard to tissue weight, protein and DNA contents after 3 days of treatment with saline, L-NNA or SNAP given separately or combined with CCK-8. Further, proliferation of different pancreatic cells was studied with [3H]-thymidine incorporation and apoptotic activity was studied by analysing caspase-3 activity and histone-associated DNA fragments. The effects of L-NNA indicate that endogenous nitric oxide formation has a tonic inhibition on apoptosis in the pancreas during both basal condition and growth stimulation by CCK-8. In CCK-induced hyperplasia, NO inhibits the proliferation of acinar cells but stimulates ductal cells. Endogenous NO may regulate the balance between proliferation and apoptosis and in a situation of growth stimulation by CCK-8, it has a tonic inhibition on both mitogenesis and apoptosis thus slowing down the acinar cell turnover in the pancreas.

The muscarinic acetylcholine receptor-stimulated increase in aquaporin-5 levels in the apical plasma membrane in rat parotid acinar cells is coupled with activation of nitric oxide/cGMP signal transduction

The present study investigated the role of nitric oxide (NO)/cGMP signal transduction in the M(3) muscarinic acetylcholine receptor (mAChR)-stimulated increase in aquaporin-5 (AQP5) levels in the apical plasma membrane (APM) of rat parotid glands. Pretreatment of rat parotid tissue with the NO scavenger 2-(4carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium inhibited both acetylcholine (ACh)- and pilocarpine-induced increases in AQP5 in the APM. NO donors [3-morpholinosydnonimine (SIN-1) and (S)-nitroso-N-acetylpenicillamine (SNAP)] mimicked the effects of mAChR agonists. A selective protein kinase G inhibitor [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo-[1,2,3-fg-3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT5823)] and an NO synthase inhibitor (N(6)-imminoethyl-L-lysine) blocked SIN-1- and SNAP-induced increases in AQP5 in the APM. A calmodulin kinase II inhibitor [(8)-5-isoquinolinesulfonic acid, 4-[2-(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-(4-phenyl-1-piperazinyl)-propyl]phenyl ester (KN-62)] decreased the pilocarpine-induced increase of AQP5 in the APM. Using diaminofluorescinein-2 diacetate, enhanced NO synthase activity was detected in isolated parotid acinar cells after ACh-treatment. Treatment with dibutyryl cGMP, but not dibutyryl cAMP, induced an increase in AQP5 levels in the APM. BAPTA-AM inhibited the cGMP-induced increase in AQP5 in the APM. Pretreatment of the tissues with a myosin light chain kinase inhibitor [(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9)] inhibited a mAChR-stimulated increase in AQP5 levels in the APM. Although there was a significant ACh-induced increase in AQP5 in the APM in the absence of extracellular Ca(2+), the maximal effect of ACh on the AQP5 levels in the APM occurred in the presence of extracellular Ca(2+). These results suggest that NO/cGMP signal transduction has a crucial role in Ca(2+) homeostasis in the mAChR-stimulated increase in AQP5 levels in the APM of rat parotid glands.

Calcium-independent and cAMP-dependent modulation of soluble guanylyl cyclase activity by G protein-coupled receptors in pituitary cells

It is well established that G protein-coupled receptors stimulate nitric oxide-sensitive soluble guanylyl cyclase by increasing intracellular Ca(2+) and activating Ca(2+)-dependent nitric-oxide synthases. In pituitary cells receptors that stimulated adenylyl cyclase, growth hormone-releasing hormone, corticotropin-releasing factor, and thyrotropin-releasing hormone also stimulated calcium signaling and increased cGMP levels, whereas receptors that inhibited adenylyl cyclase, endothelin-A, and dopamine-2 also inhibited spontaneous calcium transients and decreased cGMP levels. However, receptor-controlled up- and down-regulation of cyclic nucleotide accumulation was not blocked by abolition of Ca(2+) signaling, suggesting that cAMP production affects cGMP accumulation. Agonist-induced cGMP accumulation was observed in cells incubated in the presence of various phosphodiesterase and soluble guanylyl cyclase inhibitors, confirming that G(s)-coupled receptors stimulated de novo cGMP production. Furthermore, cholera toxin (an activator of G(s)), forskolin (an activator of adenylyl cyclase), and 8-Br-cAMP (a permeable cAMP analog) mimicked the stimulatory action of G(s)-coupled receptors on cGMP production. Basal, agonist-, cholera toxin-, and forskolin-stimulated cGMP production, but not cAMP production, was significantly reduced in cells treated with H89, a protein kinase A inhibitor. These results indicate that coupling seven plasma membrane-domain receptors to an adenylyl cyclase signaling pathway provides an additional calcium-independent and cAMP-dependent mechanism for modulating soluble guanylyl cyclase activity in pituitary cells.

Cytosolic Ca(2+) and Ca(2+)-activated Cl(-) current dynamics: insights from two functionally distinct mouse exocrine cells

The dynamics of Ca(2+) release and Ca(2+)-activated Cl(-) currents in two related, but functionally distinct exocrine cells, were studied to gain insight into how the molecular specialization of Ca(2+) signalling machinery are utilized to produce different physiological endpoints: in this case, fluid or exocytotic secretion. Digital imaging and patch-clamp methods were used to monitor the temporal and spatial properties of changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) and Cl(-) currents following the controlled photolytic release of caged-InsP(3) or caged-Ca(2+). In parotid and pancreatic acinar cells, changes in [Ca(2+)](c) and activation of a Ca(2+)-activated Cl(-) current occurred with close temporal coincidence. In parotid, a rapid global Ca(2+) signal was invariably induced, even with low-level photolytic release of threshold amounts of InsP(3). In pancreas, threshold stimulation generated an apically delimited [Ca(2+)](c) signal, while a stronger stimulus induced a global [Ca(2+)](c) signal which exhibited characteristics of a propagating wave. InsP(3) was more effective in parotid, where [Ca(2+)](c) signals initiated with shorter latency and exhibited a faster time-to-peak than in pancreas. The increased potency of InsP(3) in parotid probably results from a four-fold higher number of InsP(3) receptors as measured by radiolabelled InsP(3) binding and western blot analysis. The Ca(2+) sensitivity of the Cl(-) channels in parotid and pancreas was determined from the [Ca(2+)]-current relationship measured during a dynamic 'Ca(2+) ramp' produced by the continuous, low-level photolysis of caged-Ca(2+). In addition to a greater number of InsP(3) receptors, the Cl(-) current density of parotid acinar cells was more than four-fold greater than that of pancreatic cells. Whereas activation of the current was tightly coupled to increases in Ca(2+) in both cell types, local Ca(2+) clearance was found to contribute substantially to the deactivation of the current in parotid. These data reveal specializations of common modules of Ca(2+)-release machinery and subsequent effector activation that are specifically suited to the distinct functional roles of these two related cell types.

Activation of endogenous nitric oxide synthase coupled with methacholine-induced exocytosis in rat parotid acinar cells

Methacholine (MCh) interacted with M(3) muscarinic receptors in rat parotid tissue slices and induced amylase secretion. MCh- and calcimycin-induced exocytosis was completely inhibited by N-[2-(N-(4-chlorocinnamyl)-N-methylaminomethyl)phenyl]-N-[2-hydroxyethyl]-4-methoxybenzenesulfonamide, N(G)-nitro-L-arginine methylester (L-NAME), 1H-(1,2,4)-oxadiazolo[4,3-a]quinoxaline-1-one, and 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, suggesting that activations of calmodulin (CaM) kinase II, nitric oxide synthase (NOS), and cGMP-dependent protein kinase (PKG) were coupled with the exocytosis. These suggestions were supported by the results that exposure of the slices to MCh induced a rapid increase in these enzyme activities. Western blot analysis showed that neuronal NOS (nNOS) was expressed in isolated parotid acinar cells of rats. To measure nitric oxide (NO) production in response to the stimulation with MCh in real time, the isolated parotid acinar cells had been preloaded with 4,5-diaminofluorescein diacetate and incubated with the agonist. MCh (1 microM) induced a fast increase in 4,5-diaminofluorescein fluorescence, corresponding to an increase in the NO synthesis in the presence of extracellular Ca(2+) but not in the absence of it. When the isolated parotid acinar cells preloaded with L-NAME or 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethylester) were treated simultaneously with MCh, the increase in the fluorescence also was not observed. The MCh-induced increase in the fluorescence was not observed in the cells incubated in the absence of extracellular calcium, showing the importance of Ca(2+) entry from extracellular sites for MCh-induced NOS activation. These results indicate that nNOS is endogenously present in rat parotid acinar cells and that the rapid activation of this enzyme together with those of CaM kinase II and PKG contributes to MCh-induced amylase secretion.

Nitric oxide-induced signalling in rat lacrimal acinar cells

The aim of the present study was to investigate the physiological role of nitric oxide (NO) in mediating secretory processes in rat lacrimal acinar cells. In addition, we wanted to determine whether the acinar cells possess endogenous nitric oxide synthase (NOS) activity by measuring NO production using the fluorescent NO indicator 4,5-diaminofluorescein (DAF-2). We initiated investigations by adding NO from an external source by means of the NO-donor, S-nitroso-N-acetyl-penicillamine (SNAP). Cellular concentrations of cyclic guanosine 5'-phosphate (cGMP) ([cGMP]) were measured by radioimmunoassay (RIA), and we found that SNAP induced a fast increase in the [cGMP], amounting to 350% of the [cGMP] in resting cells. Moreover, addition of SNAP and elevating [cGMP] in fura-2 loaded lacrimal acinar cells, resulted in a cGMP-dependent protein kinase-mediated release of Ca2+ from intracellular stores, leading to a rise in the intracellular free Ca2+ concentration ([Ca2+]i). The Mn2+ quenching studies revealed that the Ca2+ release was not accompanied by Ca2+ influx. Finally, we demonstrate that lacrimal acinar cells possess endogenous NOS activity, which is activated by beta-adrenergic stimulation and not by a rise in [Ca2+]i alone. We show that in rat lacrimal acinar cells, NO and cGMP induce Ca2+ release from intracellular stores via G kinase activation. However, the changes in [Ca2+]i are relatively small, suggesting that this pathway plays a modulatory role in Ca2+ signalling, thus not by itself causing fast transient increases in [Ca2+]i. In addition, we suggest that endogenously produced NO activated by beta-adrenergic receptor stimulation, plays an important role in signalling to the surrounding tissue.

Polarized expression of G protein-coupled receptors and an all-or-none discharge of Ca2+ pools at initiation sites of [Ca2+]i waves in polarized exocrine cells

In the present work we examined localization and behavior of G protein-coupled receptors (GPCR) in polarized exocrine cells to address the questions of how luminal to basal Ca(2+) waves can be generated in a receptor-specific manner and whether quantal Ca(2+) release reflects partial release from a continuous pool or an all-or-none release from a compartmentalized pool. Immunolocalization revealed that expression of GPCRs in polarized cells is not uniform, with high levels of GPCR expression at or near the tight junctions. Measurement of phospholipase Cbeta activity and receptor-dependent recruitment and trapping of the box domain of RGS4 in GPCRs complexes indicated autonomous functioning of G(q)-coupled receptors in acinar cells. These findings explain the generation of receptor-specific Ca(2+) waves and why the waves are always initiated at the apical pole. The initiation site of Ca(2+) wave at the apical pole and the pattern of wave propagation were independent of inositol 1,4,5-trisphosphate concentration. Furthermore, a second Ca(2+) wave with the same initiation site and pattern was launched by inhibition of sarco/endoplasmic reticulum Ca(2+)-ATPase pumps of cells continuously stimulated with sub-maximal agonist concentration. By contrast, rapid sequential application of sub-maximal and maximal agonist concentrations to the same cell triggered Ca(2+) waves with different initiation sites. These findings indicate that signaling specificity in pancreatic acinar cells is aided by polarized expression and autonomous functioning of GPCRs and that quantal Ca(2+) release is not due to a partial Ca(2+) release from a continuous pool, but rather, it is due to an all-or-none Ca(2+) release from a compartmentalized Ca(2+) pool.

Ca(2+)-regulated nitric oxide generation in rabbit parotid acinar cells

In rabbit parotid acinar cells, the muscarinic cholinergic agonist methacholine induced an increase in the intracellular Ca(2+) concentration and provoked nitric oxide (NO) generation. Ca(2+)-mobilizing reagents such as thapsigargin and the Ca(2+) ionophore A23187 mimicked the effect of methacholine on NO generation. Methacholine-induced NO generation was inhibited by the removal of extracellular Ca(2+). Immunoblot analysis indicated that the antibody against the neuronal type of nitric oxide synthase (NOS) cross-reacted with NOS in the cytosol of rabbit parotid gland cells. Immunofluorescence testing showed that neuronal NOS is present in the cytosol of acinar cells but less in the ductal cells. NOS was purified approximately 8100-fold from the cytosolic fraction of rabbit parotid glands by chromatography on Sephacryl S-200, DEAE-Sephacel, and 29,59-ADP-Sepharose. The purified NOS was a NADPH- and tetrahydroxybiopterin-dependent enzyme and was activated by Ca(2+) within the physiological range in the presence of calmodulin. These results suggest that NO is generated by the activation of the neuronal type of NOS, which is regulated in rabbit parotid acinar cells by the increase in intracellular Ca(2+) levels induced by the activation of muscarinic receptors.

Parasympathetic modulation of amylase secretion by IFN gamma in murine submandibular glands

IFN gamma is a pleiotropic cytokine that exerts immunologic and non-immunologic functions. We show here that at low doses (10 U/ml), it stimulates amylase secretion in murine submandibular glands (SMG) "via" muscarinic receptor activation, comparable to that produced by the muscarinic agonist carbachol. Both effects are blocked by atropine. NG-monomethyl-L-arginine (L-NMMA) and EGTA inhibited the cytokine effect on amylase secretion, involving the participation of a calcium-dependent isoform of nitric oxide synthase (NOS). We confirm NOS activation because IFN gamma stimulates nitrite production and enzyme activity in SMG. Carbachol (10(-7) M) did not modify basal nitric oxide production. In addition, both IFN gamma and carbachol increase prostaglandin E2 production in SMG, but while indomethacin potentiates IFN gamma effect on amylase secretion, it blunted amylase secretion exerted by carbachol. Thus, IFN gamma and carbachol stimulate IFN gamma secretion on SMG in a dose-dependent manner. Our results are pointing to neuroregulatory functions of IFN gamma in murine SMG, because it regulates its own levels in oral cavity, perhaps to exert a local immuno-surveillance.

Nitric oxide and cGMP activate Ca2+-release processes in rat parotid acinar cells

We characterized the enzymic properties of ADP-ribosyl cyclase in rat parotid acinar cells by using a fluorescence technique. ADP-ribosyl cyclase is capable of synthesizing the Ca2+ -mobilizing nucleotide cADP-ribose (cADPR) from NAD(+) and has previously been shown to be regulated by cGMP via a cGMP-dependent protein kinase (G kinase). We therefore investigated whether NO/cGMP-activated pathways are present in rat parotid acinar cells and whether NO/cGMP signalling exerts control over cellular Ca2+ signalling processes. Our results showed that stimulation of acinar cells with adrenaline, isoproterenol, substance P and NO resulted in a rise in the [cGMP]. In addition, NO induced a release of Ca2+ from intracellular ryanodine-sensitive stores via a cGMP/G-kinase-mediated process. Thus our data reveal that a rise in [cGMP], caused by either neurotransmitter or NO activation, activates a G kinase, which in turn controls Ca2+ release from ryanodine-sensitive stores. Since parotid acinar cells possess ADP-ribosyl cyclase activity, we propose a model in which cADPR is the link between NO/cGMP signalling pathways and release of Ca2+ from ryanodine-sensitive stores.

Role of Ins(1,4,5)P3, cADP-ribose and nicotinic acid-adenine dinucleotide phosphate in Ca2+ signalling in mouse submandibular acinar cells

cADP-ribose (cADPr) and nicotinic acid-adenine dinucleotide phosphate (NAADP) are two putative second messengers; they were first shown to stimulate Ca(2+) mobilization in sea urchin eggs. We have used the patch-clamp whole-cell technique to determine the role of cADPr and NAADP in relation to that of Ins(1,4,5)P(3) in mouse submandibular acinar cells by measuring agonist-evoked and second-messenger-evoked changes in Ca(2+)-dependent K(+) and Cl(-) currents. Both Ins(1,4,5)P(3) and cADPr were capable of reproducing the full range of responses normally seen in response to stimulation with acetylcholine (ACh). Low concentrations of agonist (10-20 nM ACh) or second messenger [1-10 microM Ins(1,4,5)P(3) or cADPr] elicited a sporadic transient activation of the Ca(2+)-dependent currents; mid-range concentrations [50-500 nM ACh, 50 microM Ins(1,4,5)P(3) or 50-100 microM cADPr] elicited high-frequency (approx. 2 Hz) trains of current spikes; and high concentrations [more than 500 nM ACh, more than 50 microM Ins(1,4,5)P(3) or more than 100 microM cADPr] gave rise to a sustained current response. The response to ACh was inhibited by antagonists of both the Ins(1,4,5)P(3) receptor [Ins(1,4,5)P(3)R] and the ryanodine receptor (RyR) but could be completely blocked only by an Ins(1,4,5)P(3)R antagonist (heparin). NAADP (50 nM to 100 microM) did not itself activate the Ca(2+)-dependent ion currents, nor did it inhibit the activation of these currents by ACh. These results show that, in these cells, both Ins(1,4,5)P(3)R and RyR are involved in the propagation of the Ca(2+) signal stimulated by ACh and that cADPr can function as an endogenous regulator of RyR. Furthermore, although NAADP might have a role in hormone-stimulated secretion in pancreatic acinar cells, it does not contribute to ACh-evoked secretion in submandibular acinar cells.

Autocrine action and its underlying mechanism of nitric oxide on intracellular Ca2+ homeostasis in vascular endothelial cells

The rise in cytosolic Ca(2+) concentration (Ca(2+)(i)) in vascular endothelial cells (ECs) activates the production and release of nitric oxide (NO). NO modifies Ca(2+)(i) homeostasis in many types of nonendothelial cells. However, its effect on endothelial Ca(2+)(i) homeostasis at basal and excited states remains unclear. In the present study, to elucidate the effect of NO on basal Ca(2+)(i), inositol 1,4,5-trisphosphate-induced Ca(2+)(i) release (IICR) was blocked by expressing an antisense against type-1 inositol 1,4,5-trisphosphate receptors or by microinjecting heparin to individual ECs, and the effects of NO that was released by and diffused from adjacent IICR-intact ECs were recorded. After ATP or bradykinin stimulation, IICR-inhibited ECs showed a marked reduction of basal Ca(2+)(i), which was abolished by N(G)-monomethyl-l-arginine monoacetate pretreatment. The reduction disappeared in sparsely seeded ECs. Exogenous NO gas mimicked the effect of ATP or bradykinin to reduce basal Ca(2+)(i). Blocking plasma membrane Ca(2+)-ATPase (PMCA), but not Na(+)-Ca(2+) exchange or sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, suppressed the reduction, indicating that the reduction resulted from a NO-dependent potentiation of PMCA. To elucidate the effect of NO on elevated Ca(2+)(i), ATP-, bradykinin-, or thapsigargin-evoked Ca(2+)(i) response in the presence and absence of NO production was compared in adjacent IICR-intact ECs. NO was found to potentiate PMCA, which, in turn, greatly attenuated agonist-evoked Ca(2+)(i) elevation. NO also potentiated Ca(2+) influx, which markedly increased the sustained phase of Ca(2+)(i) elevation and possibly NO production. NO did not affect other Ca(2+)(i)-elevating and Ca(2+)(i)-sequestrating components. Thus, NO-dependent potentiation of PMCA is crucial for Ca(2+)(i) homeostasis over a wide Ca(2+)(i) range.

Nitric oxide stimulates tyrosine phosphorylation of p125(FAK) and paxillin in rat pancreatic acini

Some of the effects of several oncogenes, integrins, growth factors, and neuropeptides are mediated by tyrosine phosphorylation of the non-receptor tyrosine kinase p125(FAK) and the cytoskeletal protein paxillin. We have demonstrated that different stimuli cause tyrosine phosphorylation of p125(FAK) and paxillin in rat pancreatic acini. The aim of the present study was to determine whether exogenous NO activates this pathway. We demonstrate that in isolated rat pancreatic acini, a NO donor, sodium nitroprusside (SNP) stimulates, in a dose- and time-dependent way, tyrosine phosphorylation of p125(FAK) and paxillin. The same effects could be observed after incubating acini with 8-Br-cGMP. Moreover, the stimulation caused by SNP was completely abolished by two different guanylyl cyclase inhibitors, methylene blue, and LY-83583. These inhibitors also diminished unstimulated phosphorylation of p125(FAK) and paxillin. We conclude that in rat pancreatic acini exogenous NO causes p125(FAK) and paxillin tyrosine phosphorylation that is mediated by a guanylyl cyclase-dependent pathway.

Regulation of calcium in salivary gland secretion

Neurotransmitter-regulation of fluid secretion in the salivary glands is achieved by a coordinated sequence of intracellular signaling events, including the activation of membrane receptors, generation of the intracellular second messenger, inositol 1,4,5, trisphosphate, internal Ca2+ release, and Ca2+ influx. The resulting increase in cytosolic [Ca2+] ([Ca2+]i) regulates a number of ion transporters, e.g., Ca2+-activated K+ channel, Na+/K+/2Cl- co-transporter in the basolateral membrane, and the Ca2+-activated Cl- channel in the luminal membrane, which are intricately involved in fluid secretion. Thus, regulation of [Ca2+]i is central to the regulation of salivary acinar cell function and is achieved by the concerted activities of several ion channels and Ca2+-pumps localized in various cellular membranes. Ca2+ pumps, present in the endoplasmic reticulum and the plasma membrane, serve to remove Ca2+ from the cytosol. Ca2+ channels present in the endoplasmic reticulum and the plasma membrane facilitate rapid influx of Ca2+ into the cytosol from the internal Ca2+ stores and from the external medium, respectively. It is well-established that prolonged fluid secretion is regulated via a sustained elevation in [Ca2+]i that is primarily achieved by the influx of Ca2+ into the cell from the external medium. This Ca2+ influx occurs via a putative plasma-membrane-store-operated Ca2+ channel which has not yet been identified in any non-excitable cell type. Understanding the molecular nature of this Ca2+ influx mechanism is critical to our understanding of Ca2+ signaling in salivary gland cells. This review focuses on the various active and passive Ca2+ transport mechanisms in salivary gland cells--their localization, regulation, and role in neurotransmitter-regulation of fluid secretion. In addition to a historical perspective of Ca2+ signaling, recent findings and challenging problems facing this field are highlighted.

Endoplasmic reticulum Ca(2+)-ATPase inhibitors stimulate membrane guanylate cyclase in pancreatic acinar cells

In this study, we show that particulate guanylate cyclase (GC) is present in rat pancreatic acinar cells and is located both on plasma membrane and membranes of endoplasmic reticulum (ER). Western blot analysis indicates that the enzyme isoform GC-A is present in the acinar cell membranes. The specific inhibitors of ER Ca(2+)-ATPase thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ), and cyclopiazonic acid all activated particulate GC in pancreatic acini, both in membrane fractions and intact cells. These inhibitors also induced dephosphorylation of GC. Dose dependencies of Ca(2+)-ATPase inhibition and GC activation by BHQ are very similar, and those for thapsigargin partially overlap. ER Ca(2+)-ATPase and GC are coimmunoprecipitated both by antisera against membrane GC and by antisera against ER Ca(2+)-ATPase, suggesting a physical association between the two enzymes. The results suggest that thapsigargin and the other inhibitors act through ER Ca(2+)-ATPase to activate membrane GC in pancreatic acinar cells, although their direct effect on GC cannot be excluded.

Multiple functional P2X and P2Y receptors in the luminal and basolateral membranes of pancreatic duct cells

Purinergic receptors in the basolateral and luminal membranes of the pancreatic duct can act by a feedback mechanism to coordinate transport activity in the two membranes during ductal secretion. The goal of the present work was to identify and localize the functional P2 receptors (P2R) in the rat pancreatic duct. The lack of selective agonists and/or antagonists for any of the cloned P2R dictated the use of molecular and functional approaches to the characterization of ductal P2R. For the molecular studies, RNA was prepared from microdissected pancreatic intralobular ducts and was shown to be free of mRNA for amylase and endothelial nitric oxide synthase (markers for acinar and endothelial cells, respectively). A new procedure is described to obtain an enriched preparation of single duct cells suitable for electrophysiological studies. Localization of P2R was achieved by testing the effect of various P2R agonists on intracellular Ca(2+) concentration ([Ca(2+)](i)) of microperfused intralobular ducts. RT-PCR analysis suggested the expression of six subtypes of P2R in the pancreatic duct: three P2YR and three P2XR. Activation of Cl(-) current by various nucleotides and coupling of the receptors activated by these nucleotides to G proteins confirmed the expression of multiple P2R in duct cells. Measurement of [Ca(2+)](i) in microperfused intralobular ducts suggested the expression of P2X(1)R, P2X(4)R, probably P2X(7)R, and as yet unidentified P2YR, possibly P2Y(1)R, in the basolateral membrane. Expression of P2Y(2)R, P2Y(4)R, and P2X(7)R was found in the luminal membrane. The unprecedented expression of such a variety of P2R in one cell type, many capable of activating Cl(-) channels, suggests that these receptors may have an important role in pancreatic duct cell function.

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.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Nitric oxide modulates salivary amylase and fluid, but not epidermal growth factor secretion in conscious rats

The involvement of the L-arginine/NO pathway in the control of salivary fluid, amylase and epidermal growth factor (EGF) secretion was investigated in conscious rats. For the collection of saliva, an oesophageal cannula was implanted. To obtain steady secretion, submaximal carbachol background infusion was given. Different treatments included NO synthase inhibitor N(G)-nitro-L-arginine (NOLA; with or without phentolamine, propranolol), L-arginine, D-arginine and NO donor 3-morpholinosydnonimine (SIN-1) administration. Volume, amylase activity and EGF output in the secreted fluid were determined in 30 min mixed saliva samples. Carbachol infusion alone produced a modest, sustained salivary fluid and amylase secretion. NOLA (30 mg/kg) further increased both fluid (p<0.001) and amylase outputs (p<0.001). These latter effects were prevented by L-arginine but not by D-arginine or by phentolamine. Propranolol administration decreased both fluid and amylase secretion below the carbachol plateau, and NOLA did not modify this suppressed secretory rate. SIN-1 did not alter either volume or amylase secretion. Interestingly, NOLA given without carbachol did not modify salivation. Neither carbachol nor NOLA changed salivary EGF output. The present results suggest that the L-arginine/NO pathway has a modulatory role in the cholinergic control of salivary amylase secretion, but not in EGF output. The mechanisms of inhibitory action of NO on salivary fluid and amylase secretion remain to be identified.

Promiscuous coupling of receptors to Gq class alpha subunits and effector proteins in pancreatic and submandibular gland cells

Mice with deficiencies in one or more Gq class alpha subunit genes were used to examine the role of the alpha subunit in regulating Ca2+ signaling in pancreatic and submandibular gland cells. Western blot analysis showed that these cells express three of the four Gq class subunits, Galphaq, Galpha11, and Galpha14 but not Galpha15. Surprisingly, all parameters of Ca2+ signaling were identical in cells from wild type and four lines of mutant mice: 1) Galpha11-/-, 2) Galpha11-/-/Galpha14-/-, 3) Galpha14-/-/Galpha15-/-, and 4) Galphaq-/-/Galpha15-/-. These parameters included the Kapp for several Gq class coupled receptors, induction of [Ca2+]i oscillations by weak stimulation, and a biphasic [Ca2+]i response by strong stimulation. Furthermore, Ca2+ release from internal stores and Ca2+ entry were not affected in cells from any of the mutant mice. We conclude that Galphaq, Galpha11, and Galpha14 promiscuously couple several receptors (m3 muscarinic, bombesin, cholecystokinin, and alpha1 adrenergic) to effector proteins that activate both Ca2+ release from internal stores and Ca2+ entry.