A comparative study of the effects of three guanylyl cyclase inhibitors on the L-type Ca2+ and muscarinic K+ currents in frog cardiac myocytes

Investigating inherited arrhythmias using hiPSC-derived cardiomyocytes

Fundamental to the functional behavior of cardiac muscle is that the cardiomyocytes are integrated as a functional syncytium. Disrupted electrical activity in the cardiac tissue can lead to serious complications including cardiac arrhythmias. Therefore, it is important to study electrophysiological properties of the cardiac tissue. With advancements in stem cell research, protocols for the production of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been established, providing great potential in modelling cardiac arrhythmias and drug testing. The hiPSC-CM model can be used in conjunction with electrophysiology-based platforms to examine the electrical activity of the cardiac tissue. Techniques for determining the myocardial electrical activity include multielectrode arrays (MEAs), optical mapping (OM), and patch clamping. These techniques provide critical approaches to investigate cardiac electrical abnormalities that underlie arrhythmias.

Repurposing an established drug: an emerging role for methylene blue in L-DOPA-induced dyskinesia

The nitric oxide (NO) system has been proven to be a valuable modulator of L-DOPA-induced dyskinesia in Parkinsonian rodents. NO activates the enzyme soluble guanylyl cyclase and elicits the synthesis of the second-messenger cGMP. Although we have previously described the anti-dyskinetic potential of NO synthase inhibitors on L-DOPA-induced dyskinesia, the effect of soluble guanylyl cyclase inhibitors remains to be evaluated. The aim of this study was to analyze whether the clinically available non-selective inhibitor methylene blue, or the selective soluble guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), could mitigate L-DOPA-induced dyskinesia in 6-hydroxydopamine-lesioned rats. Here, we demonstrated that methylene blue was able to reduce L-DOPA-induced dyskinesia incidence when chronically co-administered with L-DOPA during 3 weeks. Methylene blue chronic (but not acute) administration (2 weeks) was effective in attenuating L-DOPA-induced dyskinesia in rats rendered dyskinetic by a previous course of L-DOPA chronic treatment. Furthermore, discontinuous methylene blue treatment (e.g., co-administration of methylene blue and L-DOPA for 2 consecutive days followed by vehicle and L-DOPA co-administration for 5 days) was effective in attenuating dyskinesia. Finally, we demonstrated that microinjection of methylene blue or ODQ into the lateral ventricle effectively attenuated L-DOPA-induced dyskinesia. Taken together, these results demonstrate an important role of NO-soluble guanylyl cyclase-cGMP signaling on L-DOPA-induced dyskinesia. The clinical implications of this discovery are expected to advance the treatment options for patients with Parkinson's disease.

Nitric oxide donors increase PVR/CD155 DNAM-1 ligand expression in multiple myeloma cells: role of DNA damage response activation

Background

DNAX accessory molecule-1 (DNAM-1) is an activating receptor constitutively expressed by macrophages/dendritic cells and by T lymphocytes and Natural Killer (NK) cells, having an important role in anticancer responses; in this regard, combination therapies able to enhance the expression of DNAM-1 ligands on tumor cells are of therapeutic interest. In this study, we investigated the effect of different nitric oxide (NO) donors on the expression of the DNAM-1 ligand Poliovirus Receptor/CD155 (PVR/CD155) in multiple myeloma (MM) cells.

Methods

Six MM cell lines, SKO-007(J3), U266, OPM-2, RPMI-8226, ARK and LP1 were used to investigate the activity of different nitric oxide donors [DETA-NO and the NO-releasing prodrugs NCX4040 (NO-aspirin) and JS-K] on the expression of PVR/CD155, using Flow Cytometry and Real-Time PCR. Western-blot and specific inhibitors were employed to investigate the role of soluble guanylyl cyclase/cGMP and activation of the DNA damage response (DDR).

Results

Our results indicate that increased levels of nitric oxide can upregulate PVR/CD155 cell surface and mRNA expression in MM cells; in addition, exposure to nitric oxide donors renders myeloma cells more efficient to activate NK cell degranulation and enhances their ability to trigger NK cell-mediated cytotoxicity. We found that activation of the soluble guanylyl cyclase and increased cGMP concentrations by nitric oxide is not involved in the up-regulation of ligand expression. On the contrary, treatment of MM cells with nitric oxide donors correlated with the activation of a DNA damage response pathway and inhibition of the ATM /ATR/Chk1/2 kinase activities by specific inhibitors significantly abrogates up-regulation.

Conclusions

The present study provides evidence that regulation of the PVR/CD155 DNAM-1 ligand expression by nitric oxide may represent an additional immune-mediated mechanism and supports the anti-myeloma activity of nitric oxide donors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1023-5) contains supplementary material, which is available to authorized users.

Nitric oxide-dependent bradycardia in mutant analbuminemic rats

Nagase analbuminemic rats (NAR) are natural mutant Sprague-Dawley rats which do not express albumin due to a single splice mutation in the albumin gene. We accidentally discovered that NAR have a significant bradycardia compared with wild type Sprague-Dawley rats, and the present study was carried out to investigate the mechanism of bradycardia in analbuminemic rats. In vitro studies showed that the basal spontaneous beating rate of isolated atria is similar in NAR compared with wild type animals. However, the chronotropic responsiveness of isolated atria to cholinergic stimulation was markedly increased in NAR, an effect which was prevented by incubation with a nitric oxide synthase (NOS) or guanylyl cyclase inhibitor. NAR had a significant increase in plasma nitrite/nitrate concentrations. Administration of a NOS inhibitor for 5 days normalized heart rate in NAR. The level of NOS isoforms, caveolin-1 and caveolin-3 expression in the atria was assessed by real time PCR. There was no significant difference in the expression of NOS isoforms or caveolin-3 in NAR compared with wild type controls. However, NAR exhibited a significant decrease in caveolin-1 expression in the atria. Since caveolin-1 is known to inhibit endothelial NOS activity in cardiomyocytes, we suggest that decreased caveolin-1 levels may have a role in increased nitric oxide production in NAR. Our data suggest that a NOS/cGMP-dependent mechanism might be involved in increased responsiveness to vagal stimulation and bradycardia in analbuminemic condition.

Antagonism of soluble guanylyl cyclase attenuates cutaneous vasodilation during whole body heat stress and local warming in humans

We hypothesized that nitric oxide activation of soluble guanylyl cyclase (sGC) participates in cutaneous vasodilation during whole body heat stress and local skin warming. We examined the effects of the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on reflex skin blood flow responses to whole body heat stress and on nonreflex responses to increased local skin temperature. Blood flow was monitored by laser-Doppler flowmetry, and blood pressure by Finapres to calculate cutaneous vascular conductance (CVC). Intradermal microdialysis was used to treat one site with 1 mM ODQ in 2% DMSO and Ringer, a second site with 2% DMSO in Ringer, and a third site received Ringer. In protocol 1, after a period of normothermia, whole body heat stress was induced. In protocol 2, local heating units warmed local skin temperature from 34 to 41°C to cause local vasodilation. In protocol 1, in normothermia, CVC did not differ among sites [ODQ, 15 ± 3% maximum CVC (CVC(max)); DMSO, 14 ± 3% CVC(max); Ringer, 17 ± 6% CVC(max); P > 0.05]. During heat stress, ODQ attenuated CVC increases (ODQ, 54 ± 4% CVC(max); DMSO, 64 ± 4% CVC(max); Ringer, 63 ± 4% CVC(max); P < 0.05, ODQ vs. DMSO or Ringer). In protocol 2, at 34°C local temperature, CVC did not differ among sites (ODQ, 17 ± 2% CVC(max); DMSO, 18 ± 4% CVC(max); Ringer, 18 ± 3% CVC(max); P > 0.05). ODQ attenuated CVC increases at 41°C local temperature (ODQ, 54 ± 5% CVC(max); DMSO, 86 ± 4% CVC(max); Ringer, 90 ± 2% CVC(max); P < 0.05 ODQ vs. DMSO or Ringer). sGC participates in neurogenic active vasodilation during heat stress and in the local response to direct skin warming.

Cardiac dysrhythmia produced by Mesobuthus tamulus venom involves NO-dependent G-Cyclase signaling pathway

Role of G-protein coupled pathways in modulating the cardiotoxic effects produced by Indian red scorpion (Mesobuthus tamulus) venom were examined. The isometric contractions of spontaneously beating or paced (3.5 Hz) rat right atrial preparations in vitro were recorded. The cumulative concentration (0.01-3.0 microg/ml)-response of venom on spontaneously beating atria exhibited a marked decrease in rate (by 55%) and an increase in force (by 92%) only at a higher concentration (3.0 microg/ml). The venom-induced decrease in rate and increase in force were sensitive to atropine, N-omega-nitro-L-arginine methylester (NO synthase inhibitor) and methylene blue (guanylyl cyclase inhibitor). Further, nifedipine, a Ca(2+) channel antagonist, blocked the force changes but not the rate changes induced by venom. In the paced atrium, on the other hand, a concentration-dependent decrease in force was observed, and at 3 microg/ml, the decrease was 50%. Pretreatment with nifedipine, but not with methylene blue, significantly attenuated the venom-induced force changes in paced atrium. The observations of this study demonstrate that the venom-induced atrial dysrhythmia is mediated through the muscarinic receptor-dependent NO-G-cyclase cell-signaling pathways.

Species- and tissue-dependent effects of NO and cyclic GMP on cardiac ion channels

Biochemical studies have established the presence of a NO pathway in the heart, including sources of NO and various effectors. Several cardiac ion channels have been shown to be modified by NO, such as L-type Ca(2+), ATP-sensitive K(+), and pacemaker f-channels. Some of these effects are mediated by cGMP, through the activity of three main proteins: the cGMP-dependent protein kinase (PKG), the cGMP-stimulated phosphodiesterase (PDE2) and the cGMP-inhibited PDE (PDE3). Other effects appear independent of cGMP, as for instance the NO modulation of the ryanodine receptor-Ca(2+) channel. In the case of the cardiac L-type Ca(2+) channel current (I(Ca,L)), both cGMP-dependent and cGMP-independent effects have been reported, with important tissue and species specificity. For instance, in rabbit sinoatrial myocytes, NO inhibits the beta-adrenergic stimulation of I(Ca,L) through activation of PDE2. In cat and human atrial myocytes, NO potentiates the cAMP-dependent stimulation of I(Ca,L) through inhibition of PDE3. In rabbit atrial myocytes, NO enhances I(Ca,L) in a cAMP-independent manner through the activation of PKG. In ventricular myocytes, NO exerts opposite effects on I(Ca,L): an inhibition mediated by PKG in mammalian myocytes but by PDE2 in frog myocytes; a stimulation attributed to PDE3 inhibition in frog ventricular myocytes but to a direct effect of NO in ferret ventricular myocytes. Finally, NO can also regulate cardiac ion channels by a direct action on G-proteins and adenylyl cyclase.

Nitric oxide-sensitive guanylyl cyclase: structure and regulation

By the formation of the second messenger cGMP, NO-sensitive guanylyl cyclase (GC) plays a key role within the NO/cGMP signaling cascade which participates in vascular regulation and neurotransmission. The enzyme contains a prosthetic heme group that acts as the acceptor site for NO. High affinity binding of NO to the heme moiety leads to an up to 200-fold activation of the enzyme. Unexpectedly, NO dissociates with a half-life of a few seconds which appears fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogs act as NO sensitizers and led to the discovery of a novel pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. The two isoforms of the heterodimeric enzyme (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. The alpha2beta1-isoform mainly occurs in brain whereas the alpha1beta1-enzyme shows a broader distribution and represents the predominantly expressed form of NO-sensitive GC. Until recently, the enzyme has been thought to occur in the cytosol. However, latest evidence suggests that the alpha2-subunit mediates the membrane association of the alpha2beta1-isoform via interaction with a PDZ domain of the post-synaptic scaffold protein PSD-95. Binding to PSD-95 locates this isoform in close proximity to the NO-generating synthases thereby enabling the NO sensor to respond to locally elevated NO concentrations. In sum, the two known isoforms may stand for the neuronal and vascular form of NO-sensitive GC reflecting a possible association to the neuronal and endothelial NO-synthase, respectively.

NMDA receptors in nucleus tractus solitarii are linked to soluble guanylate cyclase

We sought to test the hypothesis that cardiovascular responses to activation of ionotropic, but not metabotropic, glutamate receptors in the nucleus tractus solitarii (NTS) depend on soluble guanylate cyclase (sGC) and that inhibition of sGC would attenuate baroreflex responses to changes in arterial pressure. In adult male Sprague-Dawley rats anesthetized with chloralose, the ionotropic receptor agonists N-methyl-d-aspartate (NMDA) and dl-α-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA) and the metabotropic receptor agonist trans-dl-amino-1,3-cyclopentane-dicarboxylic acid (ACPD) were microinjected into the NTS before and after microinjection of sGC inhibitors at the same site. Inhibition of sGC produced significant dose-dependent attenuation of cardiovascular responses to NMDA but did not alter responses produced by injection of AMPA or ACPD. Bilateral inhibition of sGC did not alter arterial pressure, nor did it attenuate baroreflex responses to pharmacologically induced changes in arterial pressure. This study links sGC with NMDA, but not AMPA or metabotropic, receptors in cardiovascular signal transduction through NTS.

Regulation of nitric oxide-sensitive guanylyl cyclase

In this review, we outline the current knowledge on the regulation of nitric oxide (NO)-sensitive guanylyl cyclase (GC). Besides NO, the physiological activator that binds to the prosthetic heme group of the enzyme, two novel classes of GC activators have been identified that may have broad pharmacological implications. YC-1 and YC-1-like substances act as NO sensitizers, whereas the substance BAY 58-2667 stimulates NO-sensitive GC NO-independently and preferentially activates the heme-free form of the enzyme. Sensitization and desensitization of NO/cGMP signaling have been reported to occur on the level of NO-sensitive GC; in the present study, an alternative mechanism is introduced explaining the adaptation of the NO-induced cGMP response by a long-term activation of the cGMP-degrading phosphodiesterase 5 (PDE5). Finally, regulation of GC expression and a possible modulation of GC activity by other factors are discussed.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Isoforms of NO-sensitive guanylyl cyclase

By the formation of cGMP the NO-sensitive guanylyl cyclase plays a key role within the NO/cGMP signaling cascade involved in vascular regulation and neurotransmission. The prosthetic heme group of the enzyme acts as the NO sensor, and binding of NO induces conformational changes leading to an up to 200-fold activation of the enzyme. The unexpected fast dissociation half-life of NO of a few seconds is fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogues acting as NO sensitizers uncovered a new pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. Two existing isoforms of the heterodimeric guanylyl cyclase (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. Up to now, the NO-sensitive guanylyl cyclase has been considered as a soluble enzyme. However, recent evidence about the alpha2beta1 isoform interacting with a PDZ domain of the postsynaptic scaffold protein PSD-95 suggests that the alpha2 subunit directs a membrane association of this isoform. The interaction with PSD-95 locates the alpha2beta1 isoform in close proximity to the NO-generating NO synthase thereby enabling the NO sensor to respond to locally raised NO concentrations.

Isoforms of NO-sensitive guanylyl cyclase

By the formation of cGMP the NO-sensitive guanylyl cyclase plays a key role within the NO/cGMP signaling cascade involved in vascular regulation and neurotransmission. The prosthetic heme group of the enzyme acts as the NO sensor, and binding of NO induces conformational changes leading to an up to 200-fold activation of the enzyme. The unexpected fast dissociation half-life of NO of a few seconds is fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogues acting as NO sensitizers uncovered a new pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. Two existing isoforms of the heterodimeric guanylyl cyclase (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. Up to now, the NO-sensitive guanylyl cyclase has been considered as a soluble enzyme. However, recent evidence about the alpha2beta1 isoform interacting with a PDZ domain of the postsynaptic scaffold protein PSD-95 suggests that the alpha2 subunit directs a membrane association of this isoform. The interaction with PSD-95 locates the alpha2beta1 isoform in close proximity to the NO-generating NO synthase thereby enabling the NO sensor to respond to locally raised NO concentrations.

Prostaglandins and nitric oxide mediate superoxide-induced myocardial contractile dysfunction in isolated rat hearts

Oxygen-derived free radicals have been implicated in the pathogenesis of myocardial injury. We therefore investigated the pathophysiology of myocardial injury induced in isolated rat hearts by perfusion with superoxide radical generated by reacting 2.5 mmol/l purine, 0.03 U/ml xanthine oxidase and 300 U/ml catalase. Perfusion with superoxide significantly (P<0.05) increased left ventricular end-diastolic pressure within 15 to 20 min. During the same time period, heart rate and left-ventricular developed pressure significantly declined to 44.6+/-8.2% and 31.0+/-4.9% of control, respectively. Superoxide perfusion also significantly increased production of prostaglandins, nitric oxide (detected as nitrites) and peroxynitrite (detected immunohistochemically as nitrotyrosine). N(G)-nitro-l-arginine (100 micromol/l), a nitric oxide synthase inhibitor, attenuated superoxide-induced generation of peroxynitrite, increased synthesis of prostacyclin, and partially blocked myocardial dysfunction, as did 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (30 micromol/l), a selective inhibitor of soluble guanylate cyclase, and ONO-3708 (10 micromol/l), a selective thromboxane A(2)receptor antagonist. In contrast, nitroglycerin (4 micromol/l) and sodium nitroprusside (1 micromol/l) each exacerbated the superoxide-induced myocardial dysfunction. These results suggest that nitric oxide and related reactive species contribute to myocardial injury induced by superoxide. Moreover, they suggest that oxidative stress can be delayed or inhibited by reducing levels of nitric oxide, by inhibiting soluble guanylate cyclase, and by blocking thromboxane/prostaglandin receptors.

Nitroxidergic influences on cardiovascular control by NTS: a link with glutamate

Glutamate (GLU) receptor activation, which is important in cardiovascular reflex transmission through the nucleus tractus solitarii (NTS), leads to release of nitric oxide (NO.) from central nitroxidergic neurons. Therefore, we hypothesized that GLU and NO. are linked in cardiovascular control by NTS. We first sought to determine if NO. released into NTS led to cardiovascular changes like those produced by GLU and found that the nitrosothiol S-nitrosocysteine, but not NO. itself or other NO. donors, elicited such responses in anesthetized rats. The responses were dependent on activation of soluble guanylate cyclase but, not being affected by a scavenger of NO., likely did not depend on release of NO. into the extracellular space. Responses to ionotropic GLU agonists in NTS, like those to S-nitrosocysteine, were inhibited by inhibition of soluble guanylate cyclase. Inhibition of neuronal NO. synthase (nNOS) also inhibited responses to ionotropic GLU agonists. The apparent physiologic link between GLU and NO. mechanisms in NTS was further supported by anatomical studies that demonstrated frequent association between GLU-containing nerve terminals and neurons containing nNOS. Furthermore, GLU receptors were often found on NTS neurons that were immunoreactive for nNOS. The anatomical relationships between GLU and nNOS and GLU receptors and nNOS were more pronounced in some subnuclei of NTS than in others. While seen in subnuclei that are known to receive cardiovascular afferents, the association was even more prominent in subnuclei that receive gastrointestinal afferents. These studies support a role for nitroxidergic neurons in mediating cardiovascular and other visceral reflex responses that result from release of GLU into the NTS.

G protein-mediated inhibitory effect of a nitric oxide donor on the L-type Ca2+ current in rat ventricular myocytes

1. The role of the cGMP pathway in the modulation of the cardiac L-type Ca2+ current (ICa,L) by nitric oxide (NO) was examined in rat ventricular myocytes. 2. The NO donors DEANO, SIN-1, SNP, SNAP and GSNO had no significant effects on basal ICa,L. However, DEANO (100 microM) inhibited ICa,L after the current had been previously stimulated by either isoprenaline (Iso, 1-10 nM), a beta-adrenergic agonist, or isobutylmethyl-xanthine (IBMX, 10-80 microM), a wide spectrum phosphodiesterase (PDE) inhibitor. 3. The anti-adrenergic effect of DEANO on ICa,L was not mimicked by other NO donors (SIN-1, SNAP and SPNO). 4. The NO-sensitive guanylyl cyclase inhibitor ODQ (10 microM), antagonized the inhibitory effect of DEANO on ICa,L. Likewise, inhibitors of the cGMP-dependent protein kinase (cG-PK), Rp-8-chloro-phenylthio-cGMP (10 microM) and KT5823 (0.1 and 0.3 microM), also abolished the inhibitory effect of DEANO on Iso (1-10 nM)-stimulated ICa,L. 5. Intracellular dialysis with exogenous cAMP (10-100 microM) blunted the inhibitory effect of DEANO (10 and 100 microM) on ICa,L. SNAP and SNP also had no effect on the cAMP-stimulated ICa,L. 6. Pre-treatment of the myocytes with pertussis toxin (0.5 microg ml-1, 4-6 h at 37 degrees C) eliminated the inhibitory effect of DEANO (100 microM) on ICa,L, in the presence of either Iso (0.01 and 1 nM) or IBMX (10-80 microM). 7. These results demonstrate that DEANO produces anti-adrenergic effects in rat ventricular myocytes. This effect of DEANO occurs in a cGMP-dependent manner, and involves activation of cG-PK and regulation of a pertussis toxin-sensitive G protein.

Differences in the nitric oxide/soluble guanylyl cyclase signalling pathway in the myocardium of neonatal and adult rats

The effects of a nitric oxide-donor, S-nitroso-N-acetylpenicillamine, and a direct activator of soluble guanylyl cyclase, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), on force of contraction (F(c)) and L-type Ca(2+) currents (I(Ca(L))) were investigated in myocardial preparations from neonatal and adult rats. Since hearts from adult and neonatal animals contained 160 and 47 mg/100 g wet weight myoglobin, respectively, its possible interaction with both drugs was also investigated. Both S-nitroso-N-acetylpenicillamine (100 microM) and YC-1 (30 microM) were ineffective in myocardial preparations from adult rats but reduced the magnitude of I(Ca(L)) and F(c) in preparations from neonatal rats. The latter effects were antagonised by 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 50 microM) and attenuated by myoglobin (30-300 microM), which also attenuated the effects of both drugs on pre-contracted aortic rings. The differential effects of S-nitroso-N-acetylpenicillamine and YC-1 in the myocardium from adult and neonatal rats may result from developmental changes in the content of myoglobin and/or in the NO/soluble guanylyl cyclase signal pathway.

Antagonism of the positive dromotropic effect of isoproterenol by adenosine: role of nitric oxide, cGMP-dependent cAMP-phosphodiesterase and protein kinase G

We hypothesized that nitric oxide (NO) plays an important role in mediating the anti-adrenergic effect of adenosine on atrioventricular (AV) nodal conduction. In guinea-pig hearts instrumented for measurement of AV nodal conduction time (atrium-to-His bundle, A-H, interval), the NO synthase (NOS) inhibitor, l-NMMA (100 microm), reversibly inhibited 80% (P=0.009, n=6) of adenosine's anti-adrenergic action on the positive dromotropic effect of isoproterenol (0.01 microm). In parallel studies carried out in rabbit AV nodal myocytes, intracellular mechanisms whereby NO mediates the inhibitory effect of adenosine on isoproterenol-induced A-H interval shortening were studied. Adenosine (3 microm) inhibited isoproterenol-stimulated (0.1 microm) I(Ca,L)(beta -I(Ca,L)) by 46+/-6% (P<0.001, n=17). Consistent with isolated heart data, the NOS inhibitors, l -NMMA (100 microm) and L-NNA (500 microm) attenuated the effect of adenosine on beta -I(Ca,L)by 69+/-8% (P<0.001, n=16) and 69+/-7% (P<0.001, n=10), respectively. An inhibitor of NO-stimulated guanylyl cyclase LY83538 (40 microm) reduced the inhibitory effect of adenosine on beta -I(Ca,L)by 97+/-6% (P=0.004, n=15). Similarly, the non-specific inhibitor of cAMP-phosphodiesterases IBMX (50 microm) decreased the anti-adrenergic effect of adenosine by 60% (P=0.02, n=6), whereas the extracellular application of the non-hydrolyzeable cAMP analog 8-Br-cAMP (500 microm) prevented this action of adenosine. Activation of cGMP-dependent protein kinase (PKG) by CPT-cGMP (300 microm) diminished beta -I(Ca,L), but to a significantly smaller degree (16+/-4%, P=0.025, n=12) than that caused by adenosine. NO mediates the anti-adrenergic effect of adenosine on AV nodal conduction by a mechanism predominately involving activation of cGMP-dependent cAMP-phosphodiesterase and to a lesser extent activation of PKG.

A protein phosphatase is involved in the cholinergic suppression of the Ca(2+)-activated K(+) current sI(AHP) in hippocampal pyramidal neurons

The slow calcium-activated potassium current sI(AHP) underlies spike-frequency adaptation and has a substantial impact on the excitability of hippocampal CA1 pyramidal neurons. Among other neuromodulatory substances, sI(AHP) is modulated by acetylcholine acting via muscarinic receptors. The second-messenger systems mediating the suppression of sI(AHP) by muscarinic agonists are largely unknown. Both protein kinase C and A do not seem to be involved, whereas calcium calmodulin kinase II has been shown to take part in the muscarinic action on sI(AHP). We re-examined the mechanism of action of muscarinic agonists on sI(AHP) combining whole-cell recordings with the use of specific inhibitors or activators of putative constituents of the muscarinic pathway. Our results suggest that activation of muscarinic receptors reduces sI(AHP) in a G-protein-mediated and phospholipase C-independent manner. Furthermore, we obtained evidence for the involvement of the cGMP-cGK pathway and of a protein phosphatase in the cholinergic suppression of sI(AHP), whereas release of Ca(2+) from IP(3)-sensitive stores seems to be relevant neither for maintenance nor for modulation of sI(AHP).

Peroxynitrite is a positive inotropic agent in atrial and ventricular fibres of the frog heart

1. We report opposite inotropic effects of NO donors in frog cardiac fibres. The negative effect, elicited by either 3-morpholino-sydnonimine (SIN-1) or S-nitroso-N-acetyl-penicillamine (SNAP), involved cyclic GMP (cGMP) production. However, SIN-1, unlike SNAP, could elicit a positive effect, in a superoxide dismutase (SOD)-sensitive manner. SIN-1, unlike SNAP, can release both NO and superoxide anion, the precursors of peroxynitrite (OONO-). The role of these messengers was examined. 2. Catalase did not reduce the positive inotropic effect of SIN-1. Thus, a conversion of superoxide anion into hydrogen peroxide was not involved in this effect. In addition, catalase did not modify the negative effects of SIN-1 plus SOD, or SNAP plus SOD. 3. LY 83583, a superoxide anion generator, elicited a positive inotropic effect, like SIN-1. The effect of LY 83583 was additive to the negative effects of SIN-1 or SNAP, and to the positive effect of SIN-1. Thus, superoxide anion generation, per se, did not account for the positive effect of SIN-1. 4. Authentic peroxynitrite (OONO-), but not mock-OONO- (negative control plus decomposed OONO-), exerted a dramatic positive inotropic effect in cardiac fibres. The effect of OONO- was larger in atrial fibres, as compared with ventricular fibres. 5. The positive effect of OONO- was not additive with that of SIN-1, suggesting a common mechanism of action. In contrast, the effects of either OONO- or SIN-1 were additive with the negative inotropic effect of SNAP. Furthermore, the effect of OONO-, like that of SIN-1, was not antagonized by 1H-[1,2,4]xidiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM), the guanylyl cyclase inhibitor. 6. The positive inotropic effects of SIN-1 and OONO- were not modified by hydroxyl radical scavengers, such as dimethyl-thio-urea (DMTU; 10 mM). 7. The positive inotropic effect of SIN-1 (100 microM) was abolished in sodium-free solutions, a treatment that eliminates the activity of the sodium-calcium exchanger. In contrast, the effect of SIN-1 was unchanged by a potassium channel inhibitor (tetraethyl-ammonium, 20 mM), or a sodium-potassium pump inhibitor (ouabain 10 microM). 8. We conclude that OONO- is a positive inotropic agent in frog cardiac fibres. The generation of OONO- accounts for the positive inotropic effect of SIN-1. OONO- itself was responsible for the positive inotropic effect, and appeared to modulate the activity of the sodium-calcium exchanger.

Studying the structure and regulation of soluble guanylyl cyclase

Soluble guanylyl cyclase acts as the receptor for the signaling molecule nitric oxide. The enzyme consists of two different subunits. Each subunit shows the cyclase catalytic domain, which is also conserved in the membrane-bound guanylyl cyclases and the adenylyl cyclases. The N-terminal regions of the subunits are responsible for binding of the prosthetic heme group of the enzyme, which is required for the stimulatory effect of nitric oxide (NO). The five-coordinated ferrous heme displays a histidine as the axial ligand; activation of soluble guanylyl cyclase by NO is initiated by binding of NO to the heme iron and proceeds via breaking of the histidine-to-iron bond. Recently, a novel pharmacological and possibly physiological principle of guanylyl cyclase sensitization was demonstrated. The substance YC-1 has been shown to activate the enzyme independent of NO, to potentiate the effect of submaximally effective NO concentrations, and to turn carbon monoxide into an effective activator of soluble guanylyl cyclase.

Elevated cAMP suppresses muscarinic inhibition of L-type calcium current in guinea pig ventricular myocytes

We investigated the effect of carbachol (CCh) on L-type Ca2+ current (ICa(L)) enhanced by dialyzed adenosine 3',5'-cyclic monophosphate (cAMP) and/or bath-applied 3-isobutyl-1-methylxanthine (IBMX) in guinea pig isolated ventricular myocytes. At pipette concentrations ([cAMP]pip) from 30 microM to 1 mM, cAMP increased ICa(L) to 25.8 +/- 0.9 microA/cm2 (682 +/- 24.8% increase above control). CCh (100 microM) did not inhibit ICa(L) at any [cAMP]pip. IBMX, a nonselective phosphodiesterase (PDE) inhibitor, increased ICa(L) maximally at 300 microM IBMX (17.9 +/- 0.7 microA/cm2; 449 +/- 20% increase). CCh (100 microM) inhibited ICa(L) by 92 +/- 9.5% at 30 microM IBMX and 78 +/- 4.6% at 100 microM IBMX; this effect was reduced or absent at higher IBMX concentrations (300 and 1,000 microM). Coadministration of cAMP and IBMX also progressively suppressed inhibition by CCh. CCh had a negligible effect on ICa(L) at 750 microM IBMX in the absence of pipette cAMP and at 50 microM IBMX in the presence of 100 microM [cAMP]pip. ACh-activated K+ current (IK(ACh)) was unchanged in atrial myocytes dialyzed with 100 microM cAMP; this excludes a phosphorylation-dependent desensitization of the muscarinic receptor (mAChR) or Gi by cAMP. LY83583 (100 microM), an inhibitor of cyclic guanosine monophosphate (cGMP) production, attenuated inhibition of ICa(L) by CCh in the presence of IBMX. 8-Bromo-cGMP (8-Br-cGMP), an activator of cGMP-dependent protein kinase (PKG), mimicked CCh in its actions on ICa(L) raised by both cAMP (no significant change) and IBMX (49 +/- 5.1% inhibition). Okadaic acid, an inhibitor of type 1 and 2A phosphatases, blocked inhibition of IBMX-stimulated ICa(L) by either CCh or 8-Br-cGMP. Thus the ability of CCh to inhibit ICa(L) appears caused by cGMP/PKG activation of an okadaic acid-sensitive protein phosphatase, and elevated levels of cAMP protect against this action.

Positive and negative inotropic effects of NO donors in atrial and ventricular fibres of the frog heart

1. The cardiac effects of the NO donors sodium nitroprusside (SNP), S-nitroso-N-acetyl-penicillamine (SNAP) and 3-morpholino-sydnonimine (SIN-1) were studied in frog fibres to evaluate the contribution of cyclic GMP-dependent mechanisms. 2. SNP and SNAP (0.1-100 microM) reduced the force of contraction in a concentration-dependent manner in atrial and ventricular fibres. This effect was associated with a reduction in the time to peak (TTP) and the time for half-relaxation of contraction (T). 3. SIN-1 (100 microM) also reduced the force of contraction in two-thirds of the atrial fibres. However, it exerted a positive inotropic effect in the remaining atrial fibres, as well as in most ventricular fibres. 4. The guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM) antagonized the negative inotropic effects of SIN-1 (50 microM) and SNAP (25 microM) but had no effect on the positive inotropic response to SIN-1 (100 microM). 5. In the presence of SIN-1, superoxide dismutase (SOD, 50-200 U ml-1) either potentiated the negative inotropic effect or turned the positive inotropic effect of the drug into a negative effect. SOD had no effects when applied alone or in the presence of SNAP. 6. 6-Anilino-5,8-quinolinedione (LY 83583, 3-30 microM), a superoxide anion generator also known as a cyclic GMP-lowering agent, exerted a positive inotropic effect, which was antagonized by SOD (200-370 U ml-1) but not by ODQ (10 microM). 7. We conclude that SNP, SNAP and SIN-1 exert cyclic GMP-dependent negative inotropic effects, which are attributed to the generation of NO. In addition, SIN-1 and LY 83583 exert cyclic GMP-independent positive inotropic effects, which require the generation of superoxide anion.

Failure of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) to inhibit soluble guanylyl cyclase in rat ventricular cardiomyocytes

1. The effects of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase (sGC), were investigated in aortic rings and ventricular cardiomyocytes from rats. The production of cyclic GMP was stimulated by NO.-donors or carbachol. Additionally, the effects of ODQ were studied in cytosolic extracts from both tissues in which the cyclic GMP production was stimulated by S-nitroso-N-acetylpenicillamine (SNAP). 2. In endothelium-intact aortic rings, SNAP (100 microM), 2,2'-(hydroxynitrosohydrazino)bis-ethana-mine (DETA NONOate; 100 microM), or carbachol (10 microM) increased cyclic GMP levels about 4 fold. These effects were abolished by ODQ (50 microM). 3. In cardiomyocytes, SNAP (100 microM), DETA NONOate (100 microM), or carbachol (10 microM) increased cyclic GMP levels about 2 fold. These effects were not affected by ODQ (50 microM). 4. In cytosolic extracts from aortic rings and cardiomyocytes, SNAP (100 microM) induced about 50 fold increases in cyclic GMP levels. ODQ (50 microM) reduced these effects by about 50%. 5. In extracts from cardiomyocytes, increases by SNAP (100 microM) of cyclic GMP levels were attenuated by myoglobin dependent on concentration: at 300 microM myoglobin, SNAP (100 microM) increased cyclic GMP levels only 3 fold. Inhibitory effects of ODQ (50 microM) were abolished by 300 microM myoglobin. 6. It is suggested that both NO. and ODQ can bind to myoglobin which, at high concentrations. can diminish their effects on sGC. Such a scavenger function of myoglobin could explain why NO. and ODQ exert only minor effects in cardiomyocytes (with high myoglobin content) but strong effects in aortic tissue (virtually devoid of myoglobin).

Nitric oxide-related cyclic GMP-independent relaxing effect of N-acetylcysteine in lipopolysaccharide-treated rat aorta

1. We have recently demonstrated the formation of protein-bound dinitrosyl-iron complexes (DNIC) in rat aortic rings exposed to lipopolysaccharide (LPS) and shown that N-acetylcysteine (NAC) can promote vasorelaxation in these arteries, possibly via the release of nitric oxide (NO) as low molecular weight DNIC from these storage sites. The aim of the present study was to investigate further the mechanism of the relaxation induced by NAC in LPS-treated vessels. 2. In rings incubated with LPS (10 microg ml(-1) for 18 h) and precontracted with noradrenaline (NA, 3 microM) plus N(omega)-nitro-L-arginine methylester (L-NAME, 3 mM), the relaxation evoked by NAC (0.1 to 10 mM) was abolished by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM, a selective inhibitor of soluble guanylyl cyclase) but not affected by Rp-8-bromoguanosine 3'5'-cyclic monophosphorothioate (Rp-8BrcGMPS, 60 microM a selective inhibitor of cyclic GMP-dependent protein kinase). Tetrabutylammonium (TBA, 3 mM, as a non selective K+ channels blocker) or elevated concentration of external KCl (25 or 50 mM) significantly attenuated the NAC-induced relaxation. Selective K+ channels blockers (10 microM glibenclamide, 0.1 microM charybdotoxin, 0.5 microM apamin or 3 mM 4-aminopyridine) did not affect the NAC-induced relaxation. The relaxing effect of NAC (10 mM) was not associated with an elevation of guanosine 3':5' cyclic monophosphate (cyclic GMP) in LPS-treated rings. 3. In aortic rings precontracted with NA (0.1 microM), low molecular weight DNIC (with thiosulphate as ligand, 1 nM to 10 microM) evoked a concentration-dependent relaxation which was antagonized by ODQ (1 microM) and Rp-8BrcGMPS (150 microM) but not significantly affected by TBA (3 mM) or by the use of KCl (50 mM) as preconstricting agent. The relaxation produced by DNIC (0.1 microM) was associated with an 11 fold increase in aortic cyclic GMP content, which was completely abolished by ODQ (1 microM). 4. Taken together with our previous data, the main finding of the present study is that the vascular relaxation induced by NAC in LPS-treated aorta, although probably related to NO through an interaction via preformed NO stores, was not mediated by activation of the cyclic GMP pathway. It may involve the activation of TBA-sensitive K+ channels. The differences in the mechanism of relaxation induced by NAC and by exogenous DNIC suggest that the generation of low molecular weight DNIC from protein-bound species does not play a major role in the NAC-induced relaxation observed in LPS-treated rat aorta. In addition, it is suggested that ODQ may display other properties than the inhibition of soluble guanylyl cyclase.

Role of the NO-cGMP pathway in the muscarinic regulation of the L-type Ca2+ current in human atrial myocytes

1. The whole-cell patch-clamp technique was used to examine the participation of nitric oxide synthase (NOS) and soluble guanylyl cyclase in the muscarinic regulation of the L-type Ca2+ current (ICa) in freshly isolated human atrial myocytes. 2. Acetylcholine (ACh, 1 microM) decreased basal ICa by 39.1 +/- 5.5% (n = 8) under control conditions, and by 38.0 +/- 6.1% (n = 6) in the presence of 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxaline-1-one (ODQ, 10 microM), a potent guanylyl cyclase inhibitor, and NG-monomethyl-L-arginine (L-NMMA, 1 mM), a competitive NOS inhibitor. L-NMMA alone had no effect on ICa, whilst ODQ increased ICa in 50% of the cells. 3. The accentuated antagonism of ACh on ICa, i.e. its ability to antagonize the stimulatory effect of beta-adrenergic agonists and, by extension, of other cAMP-elevating agents, was examined after the current was stimulated by either the beta-adrenergic agonist isoprenaline (Iso) or serotonin (5-HT). ACh (100 nM or 1 microM) completely blocked the stimulatory effects of 10 nM Iso or 10 nM 5-HT on ICa. 4. Extracellular application of Methylene Blue (MBlue, 10 microM), a guanylyl cyclase inhibitor, antagonized the inhibitory effect of 1 microM ACh on Iso- or 5-HT-stimulated ICa. However, this effect was overcome by a 100-fold higher ACh concentration and was not mimicked by an intracellular application of MBlue. 5. Inhibition of NOS and soluble guanylyl cyclase activities by addition of ODQ (10 microM) and L-NMMA (1 mM) to both extracellular and intracellular solutions, or by a 2 h pre-incubation of the cells with these inhibitors, modified neither the Iso (10 nM) response nor the inhibitory effect of ACh (100 nM or 1 microM) on Iso-stimulated ICa. 6. Extracellular application of the NO donor SNAP (S-nitroso-N-acetyl-D,L-penicillamine) at 100 nM produced a stimulatory effect on ICa in control conditions. This stimulatory effect was abolished by intracellular MBlue (20 microM) or by intracellular and extracellular application of ODQ (10 microM) in combination with L-NMMA (1 mM). 7. We conclude that the NO-cGMP pathway does not contribute significantly to the muscarinic regulation of ICa in human atrial myocytes.

Methylene blue is a muscarinic antagonist in cardiac myocytes

We studied the mechanism of action of methylene blue (Mblue), a putative guanylyl cyclase inhibitor, on the L-type calcium current (ICa) and the muscarinic activated K+ current (IK,ACh) in rat ventricular and atrial myocytes, respectively, and on the binding of [3H]quinuclidinyl benzylate in rat ventricular membranes. Superfusion, but not internal dialysis, with 30 microM Mblue antagonized the inhibitory effect of acetylcholine (ACh, 1 microM) on beta-adrenergic stimulation of ICa with isoprenaline (Iso, 10 nM or 1 microM). However, Mblue had no effect on the basal ICa or on the stimulation of ICa by Iso in the absence of ACh. The activation of IK,ACh by 3 microM ACh was also antagonized by Mblue in a dose-dependent manner. In contrast, Mblue had no effect on the activation of IK,ACh by either guanosine-5'-O-(3-thio)triphosphate or guanosine-5'-(beta,gamma-imido)triphosphate. Chlorpromazine (CPZ), a piperazine derivative like Mblue, also inhibited the muscarinic activation of IK,ACh in a dose-dependent manner. The specific binding of [3H]QNB, a muscarinic ligand, to rat ventricular membranes was displaced in a dose-dependent manner by Mblue and CPZ. The piperazine derivatives behaved like competitive antagonists of [3H]QNB binding, exhibiting equilibrium dissociation constant (Ki) values of 187 nM for Mblue and 366 nM for CPZ. In conclusion, Mblue exerts antimuscarinic effects on ICa and IK,ACh in rat cardiac myocytes that are best explained by the binding of Mblue to the M2 subtype of muscarinic receptors. This property probably contributes to the antimuscarinic effect of the putative guanylyl cyclase inhibitor reported in previous studies.