Do nitric oxide and cGMP play a role in calcium cycling?

Dynamic remodeling of TRPC5 channel-caveolin-1-eNOS protein assembly potentiates the positive feedback interaction between Ca2+ and NO signals

The cell signaling molecules nitric oxide (NO) and Ca2+ regulate diverse biological processes through their closely coordinated activities directed by signaling protein complexes. However, it remains unclear how dynamically the multi-component protein assemblies behave within the signaling complexes upon the interplay between NO and Ca2+ signals. Here we demonstrate that TRPC5 channels activated by stimulation of G-protein-coupled ATP receptors mediate Ca2+ influx, that triggers NO production from endothelial NO synthase (eNOS), inducing secondary activation of TRPC5 via cysteine S-nitrosylation and eNOS in vascular endothelial cells. Mutations in the caveolin-1-binding domains of TRPC5 disrupt its association with caveolin-1 and impair Ca2+ influx and NO production, suggesting that caveolin-1 serves primarily as the scaffold for TRPC5 and eNOS to assemble into the signal complex. Interestingly, during ATP receptor activation, eNOS is dissociated from caveolin-1 and in turn directly associates with TRPC5, which accumulates at the plasma membrane dependently on Ca2+ influx and calmodulin (CaM). This protein reassembly likely results in a relief of eNOS from the inhibitory action of caveolin-1 and an enhanced TRPC5 S-nitrosylation by eNOS localized in the proximity, thereby facilitating the secondary activation of Ca2+ influx and NO production. In isolated rat aorta, vasodilation induced by acetylcholine was significantly suppressed by the TRPC5 inhibitor AC1903. Thus, our study provides evidence that dynamic remodeling of the protein assemblies among TRPC5, eNOS, caveolin-1, and CaM determines the ensemble of Ca2+ mobilization and NO production in vascular endothelial cells.

Inhibiting serotonin signaling through HTR2B in visceral adipose tissue improves obesity-related insulin resistance

Insulin resistance is a cornerstone of obesity-related complications such as type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease. A high rate of lipolysis is known to be associated with insulin resistance, and inhibiting adipose tissue lipolysis improves obesity-related insulin resistance. Here, we demonstrate that inhibition of serotonin (5-hydroxytryptamine [5-HT]) signaling through serotonin receptor 2B (HTR2B) in adipose tissues ameliorates insulin resistance by reducing lipolysis in visceral adipocytes. Chronic high-fat diet (HFD) feeding increased Htr2b expression in epididymal white adipose tissue, resulting in increased HTR2B signaling in visceral white adipose tissue. Moreover, HTR2B expression in white adipose tissue was increased in obese humans and positively correlated with metabolic parameters. We further found that adipocyte-specific Htr2b-knockout mice are resistant to HFD-induced insulin resistance, visceral adipose tissue inflammation, and hepatic steatosis. Enhanced 5-HT signaling through HTR2B directly activated lipolysis through phosphorylation of hormone-sensitive lipase in visceral adipocytes. Moreover, treatment with a selective HTR2B antagonist attenuated HFD-induced insulin resistance, visceral adipose tissue inflammation, and hepatic steatosis. Thus, adipose HTR2B signaling could be a potential therapeutic target for treatment of obesity-related insulin resistance.

Near-infrared light-triggered NO release for spinal cord injury repair

Traumatic spinal cord injury (SCI) is caused by external physical impacts and can induce complex cascade events, sometimes converging to paralysis. Existing clinical drugs to traumatic SCI have limited therapeutic efficacy because of either the poor blood-spinal cord barrier (BSCB) permeability or a single function. Here, we suggest a "pleiotropic messenger" strategy based on near-infrared (NIR)-triggered on-demand NO release at the lesion area for traumatic SCI recovery via the concurrent neuroregeneration and neuroprotection processing. This NO delivery system was constructed as upconversion nanoparticle (UCNP) core coated by zeolitic imidazolate framework-8 (ZIF-8) with NO donor (CysNO). This combined strategy substantial promotes the repair of SCI in vertebrates, ascribable to the pleiotropic effects of NO including the suppression of gliosis and inflammation, the promotion of neuroregeneration, and the protection of neurons from apoptosis, which opens intriguing perspectives not only in nerve repair but also in neurological research and tissue engineering.

Metabolomic analysis of differential changes in metabolites during ATP oscillations in chondrogenesis

Prechondrogenic condensation is a critical step for skeletal pattern formation. Recent studies reported that ATP oscillations play an essential role in prechondrogenic condensation. However, the molecular mechanism to underlie ATP oscillations remains poorly understood. In the present study, it was investigated how changes in metabolites are implicated in ATP oscillations during chondrogenesis by using capillary electrophoresis time-of-flight mass spectrometry (CE-TOF-MS). CE-TOF-MS detected 93 cationic and 109 anionic compounds derived from known metabolic pathways. 15 cationic and 18 anionic compounds revealed significant change between peak and trough of ATP oscillations. These results implicate that glycolysis, mitochondrial respiration and uronic acid pathway oscillate in phase with ATP oscillations, while PPRP and nucleotides synthesis pathways oscillate in antiphase with ATP oscillations. This suggests that the ATP-producing glycolysis and mitochondrial respiration oscillate in antiphase with the ATP-consuming PPRP/nucleotide synthesis pathway during chondrogenesis.

TRP channels: sensors and transducers of gasotransmitter signals

The transient receptor potential (trp) gene superfamily encodes cation channels that act as multimodal sensors for a wide variety of stimuli from outside and inside the cell. Upon sensing, they transduce electrical and Ca²⁺ signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of gaseous messenger molecules that control various cellular processes. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via cysteine (Cys) S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Recent studies have revealed that changes in the availability of molecular oxygen (O₂) also control the activation of TRP channels. Anoxia induced by O₂-glucose deprivation and severe hypoxia (1% O₂) activates TRPM7 and TRPC6, respectively, whereas TRPA1 has recently been identified as a novel sensor of hyperoxia and mild hypoxia (15% O₂) in vagal and sensory neurons. TRPA1 also detects other gaseous molecules such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂). In this review, we focus on how signaling by gaseous molecules is sensed and integrated by TRP channels.

Signalling pathways in the nitric oxide donor-induced dopamine release in the striatum of freely moving rats: evidence that exogenous nitric oxide promotes Ca2+ entry through store-operated channels

We showed previously, using in vitro microdialysis, that the activation of the soluble guanylate cyclase (sGC)/cyclic GMP pathway was the underlying mechanism of the extracellular Ca(2+)-dependent effects of exogenous NO on dopamine (DA) secretion from PC12 cells. In this study, the co-infusion of the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3] quinoxalin-1-one (ODQ) failed to affect the NO donor 3-morpholinosydnonimine (SIN-1, 5.0 mM)-induced DA increase (sevenfold baseline) in dialysates from the striatum of freely moving rats. Ca(2+) omission from the perfusion fluid abolished baseline DA release but did not affect SIN-1-induced DA increases. The reintroduction of Ca(2+) in the perfusion fluid restored the baseline dialysate DA; however, when Ca(2+) reintroduction was associated with the infusion of either SIN-1 or the NO-donor S-nitrosoglutathione (SNOG), a sustained DA overflow was observed. DA overflow was selectively inhibited by the co-infusion of the store-operated channel blocker 2-aminoethoxydiphenyl borate. The chelation of intracellular Ca(2+) by co-infusing 1,2-bis (o-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester (BAPTA-AM, 0.2 mM) greatly potentiated both SIN-1- and SNOG-induced increases in dialysate DA. BAPTA-AM-induced potentiation was inhibited by Ca(2+) omission. We conclude that the sGC/cyclic GMP pathway is not involved in the extracellular Ca(2+)-independent exogenous NO-induced striatal DA release; however, when intracellular Ca(2+) is either depleted (by Ca(2+) omission) or chelated (by BAPTA-AM co-infusion), exogenous NO does promote Ca(2+) entry, most likely through store-operated channels, with a consequent further increase in DA release.

Role of the nitric oxide/cyclic GMP pathway and ascorbic acid in 3-morpholinosydnonimine (SIN-1)-induced increases in dopamine secretion from PC12 cells. A microdialysis in vitro study

We showed previously, using in vitro microdialysis, that activation of the nitric oxide (NO)/cyclic GMP pathway was the underlying mechanism of exogenous NO-induced dopamine (DA) secretion from PC12 cells. In this study, infusion of the potential peroxynitrite generator 3-morpholinosydnonimine (SIN-1, 1.0 mM for 60 min) induced a long-lasting decrease in dialysate DA+3-methoxytyramine (3-MT) in dialysates from PC12 cell suspensions. Ascorbic acid (0.2 mM) co-infusion allowed SIN-1 to increase dialysate DA+3-MT. SIN-1+ascorbic acid effects were abolished by Ca(2+) omission. Infusion of high K(+) (75 mM) induced a 2.5-fold increase in dialysate DA+3-MT. The increase was inhibited by SIN-1 co-infusion. Conversely, co-infusion of ascorbic acid (0.2 mM) with SIN-1+high K(+) resulted in a 3.5 fold increase in dialysate DA+3-MT. The L-type Ca(2+) channel inhibitor nifedipine selectively inhibited the DA+3-MT increase pertaining to high K(+), while the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4]-oxadiazolo[4,3]quinoxalin-1-one selectively inhibited the increase pertaining to SIN-1 effects. These results suggest that activation of the NO/sGC/cyclic GMP pathway is the underlying mechanism of extracellular Ca(2+)-dependent effects of SIN-1 on DA secretion from PC12 cells. Extracellular Ca(2+) entry occurs through nifedipine-insensitive channels. Ascorbic acid is a key determinant in modulating the distinct profiles of SIN-1 effects.

Role of the nitric oxide/cyclic GMP pathway and extracellular environment in the nitric oxide donor-induced increase in dopamine secretion from PC12 cells: a microdialysis in vitro study

In vitro microdialysis was used to investigate the mechanism of nitric oxide (NO) donor-induced changes in dopamine (DA) secretion from PC12 cells. Infusion of the NO-donor S-nitroso-N-acetylpenicillamine (SNAP, 1.0 mm) induced a long-lasting increase in DA and 3-methoxytyramine (3-MT) dialysate concentrations. SNAP-induced increases were inhibited either by pre-infusion of the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4] oxadiazolo[4,3]quinoxalin-1-one (ODQ, 0.1 mm) or by Ca2+ omission. Ca2+ re-introduction restored SNAP effects. SNAP-induced increases in DA + 3-MT were unaffected by co-infusion of the l-type Ca2+ channel inhibitor nifedipine. The NO-donor (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR-3, 1.0 mm) induced a short-lasting decrease in dialysate DA + 3-MT. Ascorbic acid (0.2 mm) co-infusion allowed NOR-3 to increase dialysate DA + 3-MT. ODQ pre-infusion inhibited NOR-3 + ascorbic acid-induced DA + 3-MT increases. Infusion of high K+ (75 mm) induced a 2.5-fold increase in dialysate DA + 3-MT. The increase was abolished by NOR-3 co-infusion. Conversely, co-infusion of ascorbic acid (0.2 mm) with NOR-3 + high K+ restored high K+ effects. Co-infusion of nifedipine inhibited high K+-induced DA + 3-MT increases. These results suggest that activation of the NO/sGC/cyclic GMP pathway may be the underlying mechanism of extracellular Ca2+-dependent effects of exogenous NO on DA secretion from PC12 cells. Extracellular Ca2+ entry may occur through nifedipine-insensitive channels. NO effects and DA concentrations in dialysates largely depend on both the timing of NO generation and the extracellular environment in which NO is generated.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Atrial natriuretic peptide attenuates Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes

BACKGROUND & AIMS

Oscillations in cytosolic free Ca2+ concentration are a fundamental mechanism of intracellular signaling in hepatocytes. The aim of this study was to examine the effects of atrial natriuretic peptide (ANP) on cytosolic Ca2+ oscillations in rat hepatocytes.

METHODS

Cyclic guanosine monophosphate (cGMP) was measured by enzyme immunoassay. Cytosolic Ca2+ oscillations were recorded from single aequorin-injected hepatocytes. Ca2+ efflux from hepatocyte populations was measured by using extracellular fura-2. Ca2+ influx was estimated by Mn2+ quench of fluorescence of fura-2 dextran injected into single hepatocytes.

RESULTS

ANP attenuated cytosolic Ca2+ oscillations through a decrease in their frequency. In addition, ANP dramatically stimulated plasma membrane Ca2+ efflux and modestly inhibited basal Ca2+ influx. All of the observed effects of ANP were mimicked by the cGMP analogue 8-bromo-cGMP (8-Br-cGMP), and were prevented by inhibition of protein kinase G. In contrast, activation of cytosolic guanylyl cyclase by sodium nitroprusside had no effect on Ca2+ efflux, Ca2+ influx, or Ca2+ oscillations.

CONCLUSIONS

ANP decreases the frequency of Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes. Attenuation of oscillatory Ca2+ signaling in hepatocytes may represent a key role for ANP in vivo.

Inducible nitric-oxide synthase attenuates vasopressin-dependent Ca2+ signaling in rat hepatocytes

Increases in both Ca(2+) and nitric oxide levels are vital for a variety of cellular processes; however, the interaction between these two crucial messengers is not fully understood. Here, we demonstrate that expression of inducible nitric-oxide synthase in hepatocytes, in response to inflammatory mediators, dramatically attenuates Ca(2+) signaling by the inositol 1,4,5-trisphosphate-forming hormone, vasopressin. The inhibitory effects of induction were reversed by nitric oxide inhibitors and mimicked by prolonged cyclic GMP elevation. Induction was without effect on Ca(2+) signals in response to AlF(4)(-) or inositol 1,4,5-trisphosphate, indicating that phospholipase C activation and release of Ca(2+) from inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores were not targets for nitric oxide inhibition. Vasopressin receptor levels, however, were dramatically reduced in induced cultures. Our data provide a possible mechanism for hepatocyte dysfunction during chronic inflammation.

Nitric oxide reactivity and mechanisms involved in its biological effects

Nitric oxide (NO) is implicated in many different biological functions. This is due to its widespread distribution in tissue and to its ability to react with a range of molecules in the organism, of which haemoglobin (Hb), soluble guanylyl cyclase (GC), and superoxide anion are of particular note. In this review we describe the biological pathways of NO and their involvement in its physiological effects and toxicity. This endothelial factor rapidly diffuses into the vascular compartment, and the reaction with the Hb haem group is the main metabolic pathway for endogenous NO. Hb is, therefore, a scavenger for this mediator, which prevents it from reaching the tissue components. NO also reacts with the GC haem group, and this combination is fundamental to its acute vasorelaxing effect. Although molecular oxygen plays a very small part in the oxidization process of NO in biological systems, NO reacts with the superoxide anion to generate peroxynitrite at a rate that is limited only by its diffusion coefficient. This reaction is important in pathological conditions because the peroxynitrite thus formed is a selective oxidant and nitrating agent that interacts with numerous biological molecules, thereby damaging them. In addition, of particular note are the interactions of NO with thiol groups, which may mediate several relevant effects in the organism. NO may also activate endogenous ribosyltransferases, which facilitate the transfer of adenosine diphosphate-ribose groups from nicotine adenine dinucleotide to the G protein amino acid residues. These last two processes may also be involved in the control of arterial tone and more precisely so when chronic NO production takes place.

Endothelin-1 changes polymorphonuclear leukocytes' deformability and CD11b expression and promotes their retention in the lung

Endothelin (ET)1 influences polymorphonuclear leukocyte (PMN)- endothelial cell interactions. The aim of this study was to examine the effect of ET-1 on factors that influence PMN-endothelial interaction and retention in the lung both in vitro and in vivo. In vitro, high concentration of ET-1 (> or = 10(-8) M) rapidly increased PMN F-actin content (10(-7) M: 58 +/- 6% increase, P<0.01), whereas lower concentration of ET-1 (< or = 10(-9) M) caused a small but consistent decrease in F-actin content (10(-10) M: 6.9+/-1.5% decrease, P< 0.01). Preincubation of PMNs with the nitric oxide donor sodium nitroprusside (SNP) inhibited the F-actin content increase by 10(-7) M of ET-1 (P<0.01), and enhanced the F-actin content decrease by 10(-10) M of ET-1 (P<0.01). Preincubation of PMNs with Nomega-nitro-L-arginine methylester prevented the F-actin content decrease by 10(-10) M of ET-1. ET-1 (10(-7) M) reduced the deformability of PMNs (P<0.01), which was inhibited by preincubation of PMNs with SNP (P<0.05). ET-1 (10(-9) to 10(-7) M) increased CD11b expression of PMNs (P<0.01), which was inhibited by preincubation of PMNs with SNP. In vivo studies showed that the retention of PMNs treated with ET-1 increased from 45+/-8 to 70+/-5% compared with naive PMNs during their first pass through the lung (P<0.05). We conclude that ET-1 changes the F-actin content, the deformability, and the CD11b expression of PMNs in a dose-dependent fashion and that this leads to increased PMN sequestration in pulmonary microvessels.

Effects of Ca(2+) channel activity on renal hemodynamics during acute attenuation of NO synthesis in the rat

In cultured vascular muscle cells, nitric oxide (NO) has been shown to inhibit voltage-dependent Ca(2+) channels, which are involved in renal blood flow (RBF) autoregulation. Therefore, our purpose was to specify in vivo the effects of this interaction on RBF autoregulation. To do so, hemodynamics were investigated in anesthetized rats during Ca(2+) channel blockade before or after acute NO synthesis inhibition. Rats were treated intravenously with vehicle (n = 10), 0.3 mg/kg body wt N(G)-nitro-L-arginine-methyl ester (L-NAME; n = 7), 4.5 microg. kg body wt(-1). min(-1) nifedipine (n = 8) alone, or with nifedipine infused before (n = 8), after (n = 8), or coadministered with L-NAME (n = 10). Baseline renal vascular resistance (RVR) averaged 14.0 +/- 1.2 resistance units and did not change after vehicle. RVR increased or decreased significantly by 27 and 29% after L-NAME or nifedipine, respectively. Nifedipine reversed, but did not prevent, RVR increase after or coadministered with L-NAME. RBF autoregulation was maintained after L-NAME, but the autoregulatory pressure limit (P(A)) was significantly lowered by 15 mmHg. Nifedipine pretreatment or coadministration with L-NAME limited P(A) resetting or suppressed autoregulation at higher doses. Results were similar with verapamil. Intrarenal blockade of Ca(2+)-activated K(+) channels also prevented autoregulatory resetting by L-NAME (n = 8). These findings suggest NO inhibits voltage-dependent Ca(2+) channels and thereby modulates RBF autoregulatory efficiency.

Nitric oxide reduces the sequestration of polymorphonuclear leukocytes in lung by changing deformability and CD18 expression

Nitric oxide (NO) influences polymorphonuclear leukocytes (PMN)-endothelial cell interactions. The aim of this study was to evaluate this effect in the lung and investigate this mechanism. PMN sequestration in the lung was evaluated in vivo after the infusion of complement fragments. Rabbits (n = 9) that inhaled 40 ppm of NO were compared with control rabbits (n = 9) over a 2-h period following infusion of complement fragments. Circulating PMN counts immediately decreased after infusion of complement fragments in both groups followed by a recovery to baseline. This recovery was maintained in the NO-treated group compared with the control rabbits (p < 0.05). NO reduced PMN sequestration in the lung measured by both arteriovenous PMN difference across the lung (p < 0.01) and the myeloperoxidase (MPO) content of the lung tissue (p < 0.01). NO had no effect on the complement fragments-induced PMN release from the bone marrow. In vitro studies showed that NO partially inhibited F-actin assembly (p < 0.01) reduced the change in deformability (p < 0.05) and inhibited CD18 upregulation (p < 0.05) but had no effect on the L-selectin shedding of PMN stimulated by complement fragments. We conclude that NO reduces the sequestration of activated PMN by reducing deformability change via inhibition of F-actin assembly and inhibiting the upregulation of CD18.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).

Permeable analogues of cGMP promote hepatic calcium inflow induced by the synergistic action of glucagon and vasopressin but inhibit that induced by vasopressin alone

Treatment of perfused rat liver with the nitric oxide-generating reagent molsidomine led to substantial increases in cGMP without itself affecting basal Ca2+ fluxes. Under these conditions the ability of glucagon plus vasopressin to induce Ca2+ influx was greatly enhanced. The permeable analogue of cGMP (8-bromo-cGMP) enhanced glucagon plus vasopressin-induced Ca2+ influx to a similar extent as that with molsidomine. This suggests that the effect of the latter is attributable to the generation of cGMP which itself enhances the ability of the two hormones to induce synergistic Ca2+ influx. While 8-bromo-cGMP (or molsidomine) did not influence Ca2+ fluxes induced by glucagon, these agents strongly inhibited Ca2+ influx induced by vasopressin alone. These data show that while 8-bromo-cGMP has no effect on basal Ca2+ fluxes, it is able to modify the Ca2+ influx induced by glucagon and vasopressin action in hepatic tissue.

Functional importance of the dihydropyridine-sensitive, yet voltage-insensitive store-operated Ca2+ influx of U937 cells

The Ca2+ current activated by Ca2+ store depletion in non-excitable cells is classically regarded as being dihydropyridine-insensitive, suggesting that store-operated Ca2+ channels (SOCs) are dissimilar to voltage-gated Ca2+ channels (VGCs) of excitable-cells. Here, we demonstrate dihydropyridine-sensitivity for the store-operated Ca2+ influx induced by ATP and thapsigargin (Tg) in the non-excitable U937 cell-line. Ca2+ store depletion by prior treatment of cells with either Tg or ATP, stimulated a Ca2+ entry mechanism that was inhibited by nicardipine, nifedipine, and the specific L-type Ca2+ channel blocker, calciseptine. A functional requirement for this Ca2+ influx mechanism in agonist-induced mitogenesis seemed likely, since nicardipine, a particularly potent inhibitor of store-operated Ca2+ influx in these cells, suppressed ATP- and Tg-stimulated cell proliferation. Depolarisation of cells with KCl, or gramicidin failed to elicit an increase in cytosolic Ca2+, suggesting that while the store-operated Ca2+ influx channel of U937 cells shares pharmacologic properties with the L-type Ca2+ channel, it is voltage-insensitive and therefore may resemble an L-type Ca2+ channel lacking a voltage sensor.

Cyclic GMP induces oscillatory calcium signals in rat hepatocytes

The ability of guanosine-3',5'-cyclic monophosphate (cGMP) to induce increases in the intracellular free calcium ion concentration ([Ca2+]i) was studied at the single cell level in fura-2-loaded rat hepatocytes. Both 8-bromo-cGMP (Br-cGMP) and dibutyryl cGMP (db-cGMP) produced oscillatory [Ca2+]i increases in hepatocytes. In addition, Br-cGMP increased the frequency of agonist-induced spiking or converted [Ca2+]i oscillations into sustained nonoscillatory [Ca2+]i responses. Addition of the nitric oxide donor sodium nitroprusside also produced oscillatory [Ca2+]i increases similar to those generated by cGMP analogues. In the absence of extracellular Ca2+, cGMP-induced [Ca2+]i responses were significantly reduced and mainly appeared as single transient [Ca2+]i increases. The effects of cGMP analogues do not appear to be mediated by a secondary increase in cAMP or activation of cAMP-dependent protein kinase (PKA), since [Ca2+]i responses to cGMP analogues were inhibited by the G-kinase inhibitor 8-bromoguanosine-3',5'-cyclic monophosphorothioate (Rp-Br-cGMP[S]). Both Br-cGMP and db-cGMP also increased [Ca2+]i in the presence of the PKA inhibitor 8-bromoadenosine-3',5'-cyclic monophosphorothioate (Rp-Br-cAMP[S]) and when the cGMP-inhibitable cAMP phosphodiesterase activity was inhibited by pretreatment with siguazodan. Br-cGMP stimulated the Mn2+-induced quench of compartmentalized fura-2 in intact hepatocytes, indicating a site of action at the level of the Ca2+ stores. This locus was further supported by the finding that pretreatment of hepatocytes with Br-cGMP potentiated submaximal inositol 1,4,5-trisphosphate (InsP3)-induced Mn2+ quench in subsequently permeabilized hepatocytes. db-cGMP also decreased PKA-mediated back phosphorylation of the hepatic type-1 InsP3 receptor, indicating that G-kinase phosphorylates the InsP3 receptor at sites targeted by PKA. These data indicate that phosphorylation of the hepatic InsP3 receptor by G-kinase increases the sensitivity to InsP3 for [Ca2+]i release and is associated with the production of [Ca2+]i oscillations in single rat hepatocytes.

Calcium store depletion potentiates a phosphodiesterase inhibitor- and dibutyryl cGMP-evoked calcium influx in rat pituitary GH3 cells

A role for cGMP in the control of capacitative Ca2+ influx was identified in rat pituitary GH3 cells. Application of 50 microM - 1 mM of the non-specific phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), or the specific cGMP-phosphodiesterase inhibitor, zaprinast, induced a dose-dependent increase in the intracellular free Ca2+ concentration [Ca2+]i of the pituitary cell line, as assessed by video ratio imaging using fura-2. Response onset times were identical and response profiles were similar in all cells analysed. Application of 50 microM dibutyryl cGMP to GH3 cells resulted in heterogeneous Ca2+ responses, consisting of single or multiple transients with varying onset times. In all cases, increases in [Ca2+]i were predominantly due to Ca2+ influx, since no responses were detected in low Ca2+ medium, or following pre-incubation of cells with 1 microM verapamil, or nicardipine. Depleting intracellular Ca2+ stores by prior treatment of cells with 1 microM thapsigargin resulted in a dramatic potentiation in the Ca2+ influx mediated by both phosphodiesterase inhibitors and dibutyryl cGMP, suggesting that cGMP modulates a dihydropyridine-sensitive Ca2+ entry mechanism in GH3 cells which is possibly regulated by the state of filling of Ca2+ stores.

Intracellular ADP modulates the Ca2+ release-activated Ca2+ current in a temperature- and Ca2+-dependent Way

The rat basophilic cell line RBL-1 is known to express high levels of the Ca2+ current activated by store depletion, known as Ca2+ release-activated Ca2+ current (ICRAC), the main Ca2+ influx pathway so far identified in nonexcitable cells. We show here that, as reported in other cell types, metabolic drugs strongly inhibit the Ca2+ influx operated by store depletion in RBL-1 cells also. We have tested the hypothesis that intracellular adenine and/or guanine nucleotide levels act as coupling factors between ICRAC and cell metabolism. Using the whole cell configuration of the patch-clamp technique, we demonstrate that addition of ADP to the intracellular solution significantly reduces ICRAC induced by inositol 1,4,5-trisphosphate. This phenomenon differs from other regulatory pathways of ICRAC, since it is highly temperature-dependent, is observable only in the presence of low intracellular Ca2+ buffering capacity, and requires a cytosolic factor(s) which is rapidly lost during cell dialysis. Moreover, the inhibition is specific for ADP and is partially mimicked by ADPbetaS and AMP, but not by GDP or GTP.