Carbon monoxide modulates the response of human basophils to FcepsilonRI stimulation through the heme oxygenase pathway

We report the effects of exogenous and endogenous carbon monoxide (CO) on the immunological activation of human basophils. Hemin (1-100 microM), a heme oxygenase substrate analogue, significantly increased the formation of bilirubin from partially purified human basophils, thus indicating that these cells express heme oxygenase. This effect was reversed by preincubating the cells for 30 min with Zn-protoporphyrin IX (100 microM), a heme oxygenase inhibitor. Hemin (100 microM) also decreased immunoglobulin G anti-Fcepsilon (anti-IgE)-induced activation of basophils, measured by the expression of a membrane granule-associated protein, identified as cluster differentiation protein 63 (CD63), and by histamine release. These effects were reversed by Zn-protoporphyrin IX (100 microM), by oxyhemoglobin (HbO(2)), a CO scavenger (100 microM), and by 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ), an inhibitor of the soluble guanylyl cyclase (100 microM). Exposure of basophils to exogenous CO (10 microM for 30 min) also decreased their activation, while nitrogen (N(2)) was ineffective. HbO(2) and ODQ reversed the inhibition, reversing both membrane protein CD63 expression and histamine release to basal values. Both hemin and exogenous CO significantly raised cGMP levels in basophils and blunted the rise of calcium levels caused by immunological activation. This study suggests that CO increases cGMP formation, which in turn induces a fall in intracellular Ca(2+) concentration, thereby resulting in the inhibition of human basophil activation.

Nebivolol prevents vascular NOS III uncoupling in experimental hyperlipidemia and inhibits NADPH oxidase activity in inflammatory cells

OBJECTIVE

Nebivolol, in contrast to other selective beta1-adrenergic receptor antagonists like atenolol, improves endothelial function in patients with oxidative stress within vascular tissue. With the present studies we sought to determine whether beta receptor blockade with nebivolol may improve endothelial function in hyperlipidemia and whether this is attributable to reductions in vascular oxidative stress.

METHODS AND RESULTS

Watanabe heritable hyperlipidemic rabbits (WHHL) were treated with nebivolol (10 mg/kg per day for 8 weeks). New Zealand white rabbits (NZWR) served as controls. Nebivolol improved endothelial function, reduced vascular superoxide and vascular macrophage infiltration, and prevented NO synthase uncoupling in WHHL. Nebivolol treatment did not modify the expression of sGC or cGK-I but improved cGK-I activity (assessed by the phosphorylation state of the VAsodilator Stimulated Phosphoprotein at serine239, P-VASP). NAD(P)H oxidase activity in whole blood and isolated neutrophils was dose-dependently inhibited by nebivolol, whereas atenolol, metoprolol, and carvedilol were markedly less effective.

CONCLUSIONS

Nebivolol therapy effectively prevents NO synthase III uncoupling and prevents activation of the neutrophil NAD(P)H oxidase and infiltration of inflammatory cells. These novel antioxidative stress actions of this compound may explain partly the beneficial effects on endothelial function in patients with enhanced vascular oxidative stress.

On the role of nitric oxide in hippocampal long-term potentiation

Nitric oxide (NO) functions in several types of synaptic plasticity, including hippocampal long-term potentiation (LTP), in which it may serve as a retrograde messenger after postsynaptic NMDA receptor activation. In accordance with a prediction of this hypothesis, and with previous findings using guinea pig tissue, exogenous NO, when paired with a short tetanus (ST) to afferent fibers, generated a stable NMDA receptor-independent potentiation of rat CA1 hippocampal synaptic transmission that occluded LTP. Contrary to predictions, however, the pairing-induced potentiation was abolished in the presence of NO synthase inhibitors, indicating that endogenous NO is required for exogenous NO to facilitate LTP. Periodic application of NO while endogenous NO synthesis was blocked indicated that a tonic low level is necessary on both sides of the NO-ST pairing for the plasticity to occur. A similar dependence on tonic NO seems to extend to LTP, because application of an NO synthase inhibitor 5 min after tetanic stimulation blocked LTP as effectively as adding it beforehand. The posttetanus time window during which NO operated was restricted to <15 min. Inhibition of the guanylyl cyclase-coupled NO receptor indicated that the potentiation resulting from NO-ST pairing and the NO signal transduction pathway during early LTP are both through cGMP. We conclude that NO does not function simply as an acute signaling molecule in LTP induction but has an equally important role outside this phase. The results resonate with observations concerning the role of the hippocampal NO-cGMP pathway in certain types of learning behavior.

NOVELNEURALMODULATORS

The discovery that nitric oxide (NO) is produced by neurons and regulates synaptic activity has challenged the definition of a neurotransmitter. NO is not stored in synaptic vesicles and does not act at conventional receptors on the surface of adjacent neurons. The toxic gases carbon monoxide (CO) and hydrogen sulfide (H2S) are also produced by neurons and modulate synaptic activity. D-serine synthesis and release by astrocytes as an endogenous ligand for the "glycine" site of N-methyl D-aspartate (NMDA) receptors defy the concept that a neurotransmitter must be synthesized by neurons. We review the properties of these "atypical" neural modulators.

MsGC-beta3 forms active homodimers and inactive heterodimers with NO-sensitive soluble guanylyl cyclase subunits

Soluble guanylyl cyclases are typically obligate heterodimers, composed of a single alpha and a single beta subunit. MsGC-beta3, identified in the tobacco hornworm Manduca sexta, was the first example of a soluble guanylyl cyclase that exhibited enzyme activity without the need for coexpression with additional subunits. Subsequent studies have revealed that the mammalian beta2 subunit also shares this property. Using a combination of gel filtration chromatography, coprecipitation and site-directed mutagenesis we show that, as predicted, MsGC-beta3 forms active homodimers. We also demonstrate that MsGC-beta3 is capable of forming heterodimers with the nitric oxide (NO)-sensitive guanylyl cyclase subunits MsGC-alpha1 and MsGC-beta1. These heterodimers, however, show no enzyme activity and, like mammalian beta2 subunits, act in a dominant negative manner when combined with the NO-sensitive subunits to disrupt their activation by NO. In addition, we show that the unique C-terminal domain of MsGC-beta3 is not necessary for enzyme activity and might act as an auto-inhibitory domain.

Nitric oxide enhances angiogenesis via the synthesis of vascular endothelial growth factor and cGMP after stroke in the rat

We investigated the effects of NO on angiogenesis and the synthesis of vascular endothelial growth factor (VEGF) in a model of focal embolic cerebral ischemia in the rat. Compared with control rats, systemic administration of an NO donor, DETANONOate, to rats 24 hours after stroke significantly enlarged vascular perimeters and increased the number of proliferated cerebral endothelial cells and the numbers of newly generated vessels in the ischemic boundary regions, as evaluated by 3-dimensional laser scanning confocal microscopy. Treatment with DETANONOate significantly increased VEGF levels in the ischemic boundary regions as measured by ELISA. A capillary-like tube formation assay was used to investigate whether DETANONOate increases angiogenesis in ischemic brain via activation of soluble guanylate cyclase. DETANONOate-induced capillary-like tube formation was completely inhibited by a soluble guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ). Blocking VEGF activity by a neutralized antibody against VEGF receptor 2 significantly attenuated DETANONOate-induced capillary-like tube formation. Moreover, systemic administration of a phosphodiesterase type 5 inhibitor (Sildenafil) to rats 24 hours after stroke significantly increased angiogenesis in the ischemic boundary regions. Sildenafil and an analog of cyclic guanosine monophosphate (cGMP) also induced capillary-like tube formation. These findings suggest that exogenous NO enhances angiogenesis in ischemic brain, which is mediated by the NO/cGMP pathway. Furthermore, our data suggest that NO, in part via VEGF, may enhance angiogenesis in ischemic brain.

Cardiorenal and humoral properties of a novel direct soluble guanylate cyclase stimulator BAY 41-2272 in experimental congestive heart failure

BACKGROUND

BAY 41-2272 is a recently introduced novel orally available agent that directly stimulates soluble guanylate cyclase (sGC) and sensitizes it to its physiological stimulator, nitric oxide. To date, its therapeutic actions in congestive heart failure (CHF) remain undefined. We characterized the cardiorenal actions of intravenous BAY 41-2272 in a canine model of CHF and compared it to nitroglycerin (NTG).

METHODS AND RESULTS

CHF was induced by rapid ventricular pacing for 10 days. Cardiorenal and humoral function were assessed at baseline and with administration of 2 doses of BAY 41-2272 (2 and 10 micro g x kg(-1) x min(-1); n=8) or NTG (1 and 5 micro g x kg(-1) x min(-1); n=6). Administration of 10 micro g x kg(-1) x min(-1) BAY 41-2272 reduced mean arterial pressure (113+/-8 to 94+/-6 mm Hg; P<0.05), pulmonary artery pressure (29+/-2 to 25+/-2 mm Hg; P<0.05), and pulmonary capillary wedge pressure (25+/-2 to 20+/-2 mm Hg; P<0.05). Cardiac output (2.1+/-0.2 to 2.3+/-0.2 L/min; P<0.05) and renal blood flow (131+/-17 to 162+/-18 mL/min; P<0.05) increased. Glomerular filtration rate was maintained. There were no changes in plasma renin activity, angiotensin II, or aldosterone. NTG mediated similar hemodynamic changes and additionally decreased right atrial pressure and pulmonary vascular resistance.

CONCLUSION

The new sGC stimulator BAY 41-2272 potently unloaded the heart, increased cardiac output, and preserved glomerular filtration rate without activation of the renin-angiotensin-aldosterone system in experimental CHF. These beneficial properties make direct sGC stimulation with BAY 41-2272 a promising new strategy for the treatment of cardiovascular diseases such as CHF.

Level of haem oxygenase does not obligatorily reflect the sensitivity of PC12 cells to an oxidative shock induced by glutathione depletion

In order to investigate the function of haem oxygenase in neuronal cell death or survival, we have determined in PC12 cells whether induction of haem oxygenase mRNA and protein or inhibition of haem oxygenase activity may be able to modulate the cell response to an oxidative stress. Inhibition of glutathione biosynthesis by buthionine sulfoximine (BSO) has indeed been demonstrated, in this cell line, to decrease the intracellular content of glutathione and to trigger a gradual and programmed cell death. Inhibition of haem oxygenase by zinc protoporphyrin IX, a potent inhibitor of this enzyme, or by a recently described peptidic inhibitor, induced a significant decrease in the toxicity of BSO. This protective action was not due to an alteration in the metabolism of glutathione and was still observed when the protecting agent was added several hours after BSO treatment. Induction of haem oxygenase-1 mRNA and protein by either haemin or pyrrolidine dithiocarbamate was associated with no protection or a significant reduction in the toxicity of BSO respectively. Our results indicate that induction of haem oxygenase-1 is not obligatorily associated with an improved resistance towards oxidative stress and suggest that a byproduct of haem degradation may also become detrimental.

Post-transcriptional regulation of soluble guanylyl cyclase expression in rat aorta

We investigated the molecular mechanism of cyclic GMP-induced down-regulation of soluble guanylyl cyclase expression in rat aorta. 3-(5'-Hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), an allosteric activator of this enzyme, decreased the expression of soluble guanylyl cyclase alpha(1) subunit mRNA and protein. This effect was blocked by the enzyme inhibitor 4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b-1,4)oxazin-1-one (NS2028) and by actinomycin D. Guanylyl cyclase alpha(1) mRNA-degrading activity was increased in protein extracts from YC-1-exposed aorta and was attenuated by pretreatment with actinomycin D and NS2028. Gelshift and supershift analyses using an adenylate-uridylate-rich ribonucleotide from the 3'-untranslated region of the alpha(1) mRNA and a monoclonal antibody directed against the mRNA-stabilizing protein HuR revealed HuR mRNA binding activity in aortic extracts, which was absent in extracts from YC-1-stimulated aortas. YC-1 decreased the expression of HuR, and this decrease was prevented by NS2028. Similarly, down-regulation of HuR by RNA interference in cultured rat aortic smooth muscle cells decreased alpha(1) mRNA and protein expression. We conclude that HuR protects the guanylyl cyclase alpha(1) mRNA by binding to the 3'-untranslated region. Activation of guanylyl cyclase decreases HuR expression, inducing a rapid degradation of guanylyl cyclase alpha(1) mRNA and lowering alpha(1) subunit expression as a negative feedback response.

Persistent changes in spontaneous firing of Purkinje neurons triggered by the nitric oxide signaling cascade

Many types of neurons fire spontaneously because of the activity of pacemaking ion channels. Although endogenous firing can serve as a persistent signal to downstream targets, little attention has been paid to factors that might modulate such intrinsic electrical activity. We tested for modulation of spontaneous firing of Purkinje neurons in cerebellar slices under conditions in which principal synaptic inputs were blocked. Loose-patch recordings from single neurons show that sustained (>40 min) increases in the spontaneous firing rate can be triggered by activation of the nitric oxide-cGMP signaling pathway. Inhibitors of soluble guanylate cyclase and protein kinase G block this modulation. Increases in firing rate are also observed after stimulation of parallel fibers but not in response to basket cell activity. These findings elucidate a novel role for the nitric oxide-cGMP signaling cascade in the brain. This mechanism could permit long-term adjustments in the baseline firing rate of endogenously active neurons in response to changes in afferent activity.

The Rho-kinase inhibitor Y-27632 and the soluble guanylyl cyclase activator BAY41-2272 relax rabbit vaginal wall and clitoral corpus cavernosum

The effects of Y-27632, a Rho-kinase inhibitor and BAY41-2272, a soluble guanylyl cyclase activator, on the tone and nitrergic responses of rabbit vaginal wall and clitoral corpus cavernosum were investigated. Y-27632 and BAY41-2272 (10 nM-10 micro M) elicited concentration-dependent relaxation of phenylephrine-induced tone in both tissues. IC(50) values of Y-27632 for vaginal and clitoral tissues were 370+/-30 nM, and 467+/-14 nM, respectively. BAY41-2272 had IC(50) values of 478+/-54 nM and 304+/-38 nM respectively. The effect of the Y-27632 on the tissue tone was not affected by an inhibitor of nitric oxide synthase (L-NAME; 500 micro M). However, L-NAME reduced the potency of BAY41-2272 in the clitoral corpus cavernosum but not in the vaginal wall. BAY41-2272 enhanced nitrergic relaxation responses only in the clitoral corpus cavernosum. Y-27632 had no effect on nitrergic relaxations in either tissue. These results demonstrate that Y-27632 and BAY41-2272 elicit relaxation of the rabbit vaginal wall and clitoral corpus cavernosum.

Long-term potentiation in hippocampus involves sequential activation of soluble guanylate cyclase, cGMP-dependent protein kinase, and cGMP-degrading phosphodiesterase

Previous studies indicate that cGMP is involved in long-term potentiation (LTP). However, the effects of application of tetanus to induce LTP on cGMP content and the mechanisms by which cGMP may modulate LTP have not been reported. The aim of this work was to study the time course of the changes in cGMP content and of the activity of soluble guanylate cyclase (sGC) (the enzyme that synthesizes cGMP) during LTP. Moreover, we also studied how the changes in cGMP affect cGMP-dependent protein kinase (PKG) and cGMP-degrading phosphodiesterase and the possible role of these changes in LTP. Application of tetanus induced a rise in cGMP, reaching a maximum 10 sec after tetanus. cGMP content decreased below basal levels 5 min after tetanus and remained decreased after 60 min. Activity of sGC increased 5 min after tetanus and returned to basal at 60 min. Tetanus increased the activity of cGMP-degrading phosphodiesterase at 5 and 60 min. GMP, the product of degradation, was increased at 5 and 60 min. Activation of phosphodiesterase and a decrease in cGMP were prevented by inhibiting PKG with Rp-8-bromoguanosine-cGMPS (Rp-8-Br-cGMPS). Inhibition of sGC [with ODQ (oxadiazolo quinoxalin-1-one) or NS 2028 (4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one)], of PKG (with Rp-8-Br-cGMPS), or of cGMP-degrading phosphodiesterase [with zaprinast or MBAM (4-[[3',4'-(methylenedioxy)benzyl]amino]-6-methoxyquinazoline) ] impairs LTP. The results indicate that induction of LTP involves transient activation of sGC and an increase in cGMP, followed by activation of cGMP-dependent protein kinase, which, in turn, activates cGMP-degrading phosphodiesterase, resulting in long-lasting reduction of cGMP content.

Altered vascular function in fetal programming of hypertension

BACKGROUND AND PURPOSE

Reduced endothelium-dependent vasorelaxation partly due to loss of nitric oxide (NO) bioavailability occurs in most cases of chronic hypertension. Intrauterine nutritional deprivation has been associated with increased risk for hypertension and stroke, associated with relaxant dysfunction and decreased vascular compliance, but the underlying mechanisms are not known. The present studies were undertaken to investigate whether endothelial dysfunction associated with altered NO-dependent vasodilatation pathways is also observed in a model of in utero programming of hypertension.

METHODS

Pregnant Wistar rats were fed a normal (18%), low (9%), or very low (6%) protein isocaloric diet during gestation. Vasomotor response of resistance cerebral microvessels (<50 micro m) was studied in adult offspring of dams fed the 18% and 9% protein diets by a video imaging technique. Endothelial NOS (eNOS), soluble guanylate cyclase (sGC), and K(Ca) channel expression were measured by Western blot. NO synthase (NOS) activity was measured enzymatically as well as in situ by NADPH diaphorase staining.

RESULTS

Litter size and survival to adulthood were not affected by the diets. Birth weights of offspring of dams fed the 6% diet were markedly lower than those of dams fed the 9% diet, which were marginally lower than those of controls. Systolic blood pressures of adult offspring of mothers in the 6% and 9% groups were comparably greater (156+/-2 and 155+/-1 mm Hg, respectively) than that of control offspring (137+/-1 mm Hg); we therefore focused on the 9% and 18% groups. Cerebral microvessel constriction to thromboxane A(2) mimetic and dilation to carba-prostaglandin I(2) did not differ between diet groups. In contrast, vasorelaxation to the NO-dependent agents substance P and acetylcholine was diminished by 50% in low protein-exposed offspring, but eNOS expression and activity were similar between the 2 diet groups. Vasorelaxant response to the NO donor sodium nitroprusside was also decreased and was associated with reduced (by 50% to 65%) cGMP levels and sGC expression. cGMP analogues caused comparable vasorelaxation in the 2 groups. Expression of K(Ca) (another important mediator of NO action) and relaxation to the K(Ca) opener NS1619 were unchanged by antenatal diet.

CONCLUSIONS

Maternal protein deprivation, which leads to hypertension in the offspring, is associated with diminished NO-dependent relaxation of major organ (cerebral) microvasculature, which seems to be largely attributed to decreased sGC expression and cGMP levels. The study provides an additional explanation for abnormal vasorelaxation in nutrient-deprived subjects in utero.

Characterization of nitric oxide consumption pathways by normal, chronic granulomatous disease and myeloperoxidase-deficient human neutrophils

The detailed mechanisms by which acutely activated leukocytes metabolize NO and regulate its bioactivity are unknown. Therefore, healthy, chronic granulomatous disease (CGD) or myeloperoxidase (MPO)-deficient human neutrophils were examined for their ability to consume NO and attenuate its signaling. fMLP or PMA activation of healthy neutrophils caused NO consumption that was fully blocked by NADPH oxidase inhibition, and was absent in CGD neutrophils. Studies using MPO-deficient neutrophils, enzyme inhibitors, and reconstituted NADPH oxidase ruled out additional potential NO-consuming pathways, including Fenton chemistry, PGH synthase, lipoxygenase, or MPO. In particular, the inability of MPO to consume NO resulted from lack of H(2)O(2) substrate since all superoxide (O(2)(-.) reacted to form peroxynitrite. For healthy or MPO-deficient cells, NO consumption rates were 2- to 4-fold greater than O(2)(-.) generation, significantly faster than expected from 1:1 termination of NO with O(2)(-.). Finally, fMLP or PMA-stimulated NO consumption fully blocked NO-dependent neutrophil cGMP synthesis. These data reveal NADPH oxidase as the central regulator of NO signaling in human leukocytes. In addition, they demonstrate an important functional difference between CGD and either normal or MPO-deficient human neutrophils, namely their inability to metabolize NO which will alter their ability to adhere and migrate in vivo.

Region selective alterations of soluble guanylate cyclase content and modulation in brain of cirrhotic patients

Modulation of soluble guanylate cyclase (sGC) by nitric oxide (NO) is altered in brain from experimental animals with hyperammonemia with or without liver failure. The aim of this work was to assess the content and modulation of sGC in brain in chronic liver failure in humans. Expression of the alpha-1, alpha-2, and beta-1 subunits of sGC was measured by immunoblotting in autopsied frontal cortex and cerebellum from cirrhotic patients and controls. The contents of alpha-1 and alpha-2 subunits of guanylate cyclase was increased both in cortex and cerebellum, whereas the beta-1 subunit was not affected. Addition of the NO-generating agent S-nitroso-N-acetyl-penicillamine (SNAP) to homogenates of frontal cortex from controls increased the activity of sGC 87-fold, whereas, in homogenates from cirrhotic patients, the increase was significantly higher (183-fold). In contrast, in cerebellum, activation of guanylate cyclase by NO was significantly lower in patients (156-fold) than in controls (248-fold). A similar regional difference was found in rats with portacaval anastomosis. In conclusion, these findings show that the NO-guanylate cyclase signal transduction pathway is strongly altered in brain in patients with chronic liver failure and that the effects are different in different brain areas. Given that activation of sGC by NO in brain is involved in the modulation of important cerebral processes such as intercellular communication, learning and memory, and the sleep-wake cycle, these changes could be implicated in the pathogenesis of hepatic encephalopathy in these patients.

Current oral treatments for erectile dysfunction

Erectile dysfunction (ED) is defined as the inability to achieve and maintain a penile erection adequate for satisfactory sexual intercourse. It is a significant male health problem of global dimensions affecting approximately 150 million men worldwide. A broad range of options are currently available for the management of ED. They include oral agents (phosphodiesterase 5 inhibitors, dopamine agonists and alpha-receptor blocking drugs), intracavernosal injection (papaverine, phentolamine, prostaglandin E1, vasoactive intestinal peptide), transurethral vasoactive agents (prostaglandin E1), vacuum erection devices, vascular surgery and penile prostheses. Here we review the physiology of penile erection and the currently available oral preparations. In addition, novel therapeutic strategies to improve erectile function are discussed.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

Patch cramming reveals the mechanism of long-term suppression of cyclic nucleotides in intact neurons

To understand cyclic nucleotide dynamics in intact cells, we used the patch-cramming method with cyclic nucleotide-gated channels as real-time biosensors for cGMP. In neuroblastoma and sympathetic neurons, both muscarinic agonists and nitric oxide (NO) rapidly elevate cGMP. However, muscarinic agonists also elicit a long-term (2 hr) suppression (LTS) of subsequent cGMP responses. Muscarinic agonists elevate cGMP by triggering Ca2+ mobilization, which activates NO synthase to produce NO, leading to the activation of soluble guanylate cyclase (sGC). Here we examine the mechanism of LTS. Experiments using direct intracellular cGMP injection demonstrate that enhancement of phosphodiesterase (PDE) activity, rather than depression of sGC activity, is responsible for LTS. Biochemical measurements show that both cGMP and cAMP content is suppressed, consistent with the involvement of a nonselective PDE. Application of pharmacological agents that alter Ca2+ mobilization from intracellular stores and experiments involving injection of the Ca2+ chelator BAPTA show that Ca2+ mobilization is necessary and sufficient for LTS induction but also show that LTS maintenance is Ca2+-independent. Protein phosphatase injection reverses LTS, and specific inhibitors of Ca2+/calmodulin kinase II (CaMKII) prevent induction and inhibit maintenance. The switch between the Ca2+ dependence of LTS induction to the Ca2+ independence of LTS maintenance is consistent with CaMKII autophosphorylation, similar to proposed mechanisms of hippocampal long-term potentiation. Because the molecular machinery underlying LTS is common to many cells, LTS may be a widespread mechanism for long-term silencing of cyclic nucleotide signaling.

Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.

The expression pattern of nitric oxide-sensitive guanylyl cyclase in the rat heart changes during postnatal development

Nitric oxide (NO)-releasing drugs such as glyceryl trinitrate have been used in the treatment of ischemic heart disease for more than a century. Nevertheless, a detailed analysis of the expression of the NO target enzyme soluble guanylyl cyclase (sGC) in the heart is missing. The aim of the current study was to elucidate the expression, cell distribution, and activity of sGC in the rat heart during postnatal development. Using a novel antibody raised against a C-terminal peptide of the rat beta(1)-subunit of sGC, the enzyme was demonstrated in early postnatal and adult hearts by Western blotting analyses, showing maximal expression in 10-day-old animals. Measurements of basal, NO-, and NO/YC-1-stimulated sGC activity revealed an increase of sGC activity in hearts from neonatal to 10-day-old rats, followed by a subsequent decrease in adult animals. As shown by immunohistochemical analysis, sGC expression was present in vascular endothelium and smooth muscle cells in neonatal heart but expression shifted to endothelial cells in adult animals. In isolated cardiomyocytes, sGC activity was not detectable under basal conditions but significant sGC activity could be detected in the presence of NO. An increase in expression during the perinatal period and changes in the cell types expressing sGC at different phases of development suggest dynamic regulation rather than constitutive expression of the NO receptor in the heart.

Nicotinic-acetylcholine receptors are functionally coupled to the nitric oxide/cGMP-pathway in insect neurons

In addition to their ionotropic role, neuronal nicotinic acetylcholine receptors (nAChRs) can influence second messenger levels, transmitter release and gene transcription. In this study, we show that nAChRs in an insect CNS control cGMP levels by coupling to NO production. In conditions that inhibit spiking, nicotine induced cGMP synthesis. This increase in cGMP was blocked by nicotinic antagonists, and by inhibitors of both nitric oxide synthase and soluble guanylyl cyclase. The nicotinic-evoked increase in cGMP was localized to specific NO-sensitive neurons in the CNS, several of which are identified motoneurons. Because NO production requires Ca2+, we investigated the effect of nicotinic stimulation on [Ca2+]i in cultured neurons. We found that activation of nAChRs increased [Ca2+]i, which was blocked by nAChR antagonists. Nicotinic stimulation of neurons in the isolated CNS in low-Na+, also evoked increases in [Ca2+]i independent of fast changes in voltage. In addition, approximately 10% of the nicotinic-evoked [Ca2+]i increase in cultured neurons persisted when voltage-gated Ca2+ channels were blocked by Ni2+. Under the same conditions, nicotinic stimulation of cGMP in the CNS was unaffected. These combined results suggest that nicotinic stimulation is coupled to NOS potentially by directly gating Ca2+.

Regional distribution of protein and activity of the nitric oxide receptor, soluble guanylyl cyclase, in rat brain suggests multiple mechanisms of regulation

Nitric oxide (NO) is an unconventional neuromodulator that signals by intercellular diffusion. Its effects are often mediated by activation of its cytosolic receptor, the hemoprotein soluble guanylyl cyclase (sGC). Regional distribution of heterodimeric (alpha/beta) sGC at both the activity and protein level and its regulation are still unclear. Here, sGC was analyzed in rat brain by Western blot and NO donor-stimulated cyclic GMP accumulation. sGCalpha(1) and sGCbeta(1) immunoreactive protein signals strongly correlated with each other. However, V(max) values depended on the type of NO donor used. Sodium nitroprusside, the most widely used compound and formally an NO(+) donor, was up to 20-fold less effective in stimulating sGC activity than the NO donor diethylamine NONOate. In contrast to the rather even distribution of sGC proteins and SNP-stimulated cGMP accumulation in various regions of rat brain, diethylamine NONOate-stimulated sGC activity varied up to 8-fold between the different brain regions tested. In conclusion, we show that expression of both sGCalpha(1) and sGCbeta(1) subunits is tightly coregulated in rat brain, while yet unknown additional mechanisms affect the V(max) of sGC.

Endothelin-1 and -3 diminish neuronal NE release through an NO mechanism in rat anterior hypothalamus

The existence of endothelin binding sites on the catecholaminergic neurons of the hypothalamus suggests that endothelins (ETs) participate in the regulation of noradrenergic transmission modulating various hypothalamic-controlled processes such as blood pressure, cardiovascular activity, etc. The effects of ET-1 and ET-3 on the neuronal release of norepinephrine (NE) as well as the receptors and intracellular pathway involved were studied in the rat anterior hypothalamus. ET-1 (10 nM) and ET-3 (10 nM) diminished neuronal NE release and the effect blocked by the selective ET type B receptor antagonist BQ-788 (100 nM). N(omega)-nitro-L-arginine methyl ester (10 microM), methylene blue (10 microM), and KT5823 (2 microM), inhibitors of nitric oxide synthase activity, guanylate cyclase, and protein kinase G, respectively, prevented the inhibitory effects of both ETs on neuronal NE release. In addition, both ETs increased nitric oxide synthase activity. Furthermore, 100 microM picrotoxin, a GABA(A)-receptor antagonist, inhibited ET-1 and ET-3 response. Our results show that ET-1 as well as ET-3 has an inhibitory neuromodulatory effect on NE release in the anterior hypothalamus mediated by the ET type B receptor and the involvement of a nitric oxide-dependent pathway and GABA(A) receptors. ET-1 and ET-3 may thus diminish available NE in the synaptic gap leading to decreased noradrenergic activity.

Activation and up-regulation of spinal cord nitric oxide receptor, soluble guanylate cyclase, after formalin injection into the rat hind paw

Nitric oxide synthase is expressed abundantly in the spinal cord, and nitric oxide (NO) has been shown to play important roles in the central mechanism of inflammatory hyperalgesia. However, the expression and function of the NO receptor, soluble guanylate cyclase, is not fully understood in this processing at the spinal cord level. In the present study, we report that the soluble guanylate cyclase alpha(1) subunit but not the beta(1) subunit was expressed in rat spinal cord, particularly in the dorsal horn. We showed that intrathecal administration of a selective inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, produced a significant anti-nociception demonstrated by the decrease in the number of flinches and shakes in the formalin-induced inflammatory pain model. This was accompanied by a marked reduction in formalin-induced c-fos expression in the spinal cord. During formalin-induced long-lasting inflammation, we found that the expression of the alpha(1) subunit of soluble guanylate cyclase was dramatically increased in the lumbar spinal cord on the second and fourth days after formalin injection into the dorsal side of a hind paw. Intraperitoneal pretreatment with an N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and a neuronal NO synthase inhibitor, 7-nitroindazole, not only significantly blocked formalin-induced secondary thermal hyperalgesia but also suppressed formalin-produced increase in the alpha(1) subunit of soluble guanylate cyclase in the spinal cord. The present results indicate that peripheral inflammation not only initially activates but also later up-regulates soluble guanylate cyclase expression via the NMDA receptor-NO signaling pathway, suggesting that soluble guanylate cyclase might be involved in the central mechanism of formalin-induced inflammatory hyperalgesia in the spinal cord.

Differential sensitivity of guanylyl cyclase and mitochondrial respiration to nitric oxide measured using clamped concentrations

Nitric oxide (NO) signal transduction may involve at least two targets: the guanylyl cyclase-coupled NO receptor (NO(GC)R), which catalyzes cGMP formation, and cytochrome c oxidase, which is responsible for mitochondrial O(2) consumption and which is inhibited by NO in competition with O(2). Current evidence indicates that the two targets may be similarly sensitive to NO, but quantitative comparison has been difficult because of an inability to administer NO in known, constant concentrations. We addressed this deficiency and found that purified NO(GC)R was about 100-fold more sensitive to NO than reported previously, 50% of maximal activity requiring only 4 nm NO. Conversely, at physiological O(2) concentrations (20-30 microM), mitochondrial respiration was 2-10-fold less sensitive to NO than estimated beforehand. The two concentration-response curves showed minimal overlap. Accordingly, an NO concentration maximally active on the NO(GC)R (20 nm) inhibited respiration only when the O(2) concentration was pathologically low (50% inhibition at 5 microM O(2)). Studies on brain slices under conditions of maximal stimulation of endogenous NO synthesis suggested that the local NO concentration did not rise above 4 nm. It is concluded that under physiological conditions, at least in brain, NO is constrained to target the NO(GC)R without inhibiting mitochondrial respiration.

Aspirin protected the nitric oxide/cyclic GMP generating system in human peritoneum

OBJECTIVE

Changes in the expression of endothelial nitric oxide synthase (eNOS) in the peritoneum could be involved in the peritoneal dysfunction associated with peritoneal inflammation. The aim of the present study was to analyze the effect of Escherichia coli lipopolysaccharide (LPS) on eNOS expression in samples of human peritoneum. The effect of aspirin, a drug with anti-inflammatory properties, was also determined.

RESULTS

The eNOS protein expressed in human peritoneal tissue was reduced by LPS (10 microg/mL) in a time-dependent manner. The eNOS was expressed mainly in capillary endothelial cells and mesothelial cells. Anti-inflammatory doses of aspirin (1-10 mmol/L) restored eNOS expression in LPS-stimulated human peritoneal tissue samples. The main intracellular receptor of NO, soluble guanylate cyclase (sGC), was also downregulated by LPS. This effect was prevented by aspirin (5 mmol/L).

CONCLUSION

Protein expression of the eNOS-sGC system in the peritoneal tissue was downregulated by LPS. High doses of aspirin protected both eNOS protein expression and sGC in human peritoneum. These findings suggest a new mechanism of action of aspirin that could be involved in the prevention of peritoneal dysfunction during inflammation.

Altered guanylyl-cyclase activity in vitro of pulmonary arteries from fetal lambs with congenital diaphragmatic hernia

Nitric oxide (NO) plays a major role in the modulation of perinatal pulmonary vascular tone. Congenital diaphragmatic hernia (CDH), a major cause of severe persistent pulmonary hypertension of the newborn (PPHN), is often refractory to inhaled NO. Alterations in NO/cyclic guanosine 3',5' monophosphate (cGMP)-mediated pulmonary vasodilatation may contribute to PPHN in CDH. We assessed NO/cGMP-mediated pulmonary vasorelaxation in vitro in 140-d gestational lamb fetuses with surgically created left CDH (term = 147 d) to age-matched controls. Relaxation of fourth generation intralobar pulmonary artery rings in response to the endothelium-dependent vasodilator, acetylcholine (ACh), and to the specific inhibitor of cGMP-phosphodiesterase (PDE), zaprinast, did not differ between the two groups. By contrast, relaxation in response to the calcium ionophore A23187 was impaired in CDH as compared with control animals. Relaxation in response to the NO donor sodium nitroprusside (SNP) (a direct activator of soluble guanylyl cyclase [sGC]) was also impaired in CDH animals as compared with controls. Repeating the challenge increased vasorelaxation in response to SNP in CDH as compared with control animals. Immunohistochemistry revealed the presence of endothelial NO-synthase in the endothelium of pulmonary arteries from both control and CDH animals. We conclude that endothelium-dependent vasodilatation in response to ACh and A23187 was differently affected in the fetal surgical CDH-lamb model. Furthermore, activity of sGC but not that of PDE was impaired in CDH animals. PPHN and decreased inhaled NO responsiveness in CDH may involve decreased sGC activity.

P2Y(1) and P2Y(2) receptors are coupled to the NO/cGMP pathway to vasodilate the rat arterial mesenteric bed

1. To assess the role of nucleotide receptors in endothelial-smooth muscle signalling, changes in perfusion pressure of the rat arterial mesenteric bed, the luminal output of nitric oxide (NO) and guanosine 3',5' cyclic monophosphate (cGMP) accumulation were measured after the perfusion of nucleotides. 2. The rank order of potency of ATP and analogues in causing relaxation of precontracted mesenteries was: 2-MeSADP=2-MeSATP>ADP>ATP=UDP=UTP>adenosine. The vasodilatation was coupled to a concentration-dependent rise in NO and cGMP production. MRS 2179 selectively blocked the 2-MeSATP-induced vasodilatation, the NO surge and the cGMP accumulation, but not the UTP or ATP vasorelaxation. 3. mRNA encoding for P2Y(1), P2Y(2) and P2Y(6) receptors, but not the P2Y(4) receptor, was detected in intact mesenteries by RT-PCR. After endothelium removal, only P2Y(6) mRNA was found. 4. Endothelium removal or blockade of NO synthase obliterated the nucleotides-induced dilatation, the NO rise and cGMP accumulation. Furthermore, 2-MeSATP, ATP, UTP and UDP contracted endothelium-denuded mesenteries, revealing additional muscular P2Y and P2X receptors. 5. Blockade of soluble guanylyl cyclase reduced the 2-MeSATP and UTP-induced vasodilatation and the accumulation of cGMP without interfering with NO production. 6. Blockade of phosphodiesterases with IBMX increased 15-20 fold the 2-MeSATP and UTP-induced rise in cGMP; sildenafil only doubled the cGMP accumulation. A linear correlation between the rise in NO and cGMP was found. 7. Endothelial P2Y(1) and P2Y(2) receptors coupled to the NO/cGMP cascade suggest that extracellular nucleotides are involved in endothelial-smooth muscle signalling. Additional muscular P2Y and P2X receptors highlight the physiology of nucleotides in vascular regulation.

NO- and haem-independent activation of soluble guanylyl cyclase: molecular basis and cardiovascular implications of a new pharmacological principle

1. Soluble guanylyl cyclase (sGC) is the only proven receptor for the ubiquitous biological messenger nitric oxide (NO) and is intimately involved in many signal transduction pathways, most notably in regulating vascular tone and platelet function. sGC is a heterodimeric (alpha/ss) protein that converts GTP to cyclic GMP; NO binds to its prosthetic haem group. Here, we report the discovery of a novel sGC activating compound, its interaction with a previously unrecognized regulatory site and its therapeutic implications. 2. Through a high-throughput screen we identified BAY 58-2667, an amino dicarboxylic acid which potently activates sGC in an NO-independent manner. In contrast to NO, YC-1 and BAY 41-2272, the sGC stimulators described recently, BAY 58-2667 activates the enzyme even after it has been oxidized by the sGC inhibitor ODQ or rendered haem deficient. 3. Binding studies with radiolabelled BAY 58-2667 show a high affinity site on the enzyme. 4. Using photoaffinity labelling studies we identified the amino acids 371 (alpha-subunit) and 231 - 310 (ss-subunit) as target regions for BAY 58-2667. 5. sGC activation by BAY 58-2667 results in an antiplatelet activity both in vitro and in vivo and a potent vasorelaxation which is not influenced by nitrate tolerance. 6. BAY 58-2667 shows a potent antihypertensive effect in conscious spontaneously hypertensive rats. In anaesthetized dogs the hemodynamic effects of BAY 58-2667 and GTN are very similar on the arterial and venous system. 7. This novel type of sGC activator is a valuable research tool and may offer a new approach for treating cardiovascular diseases.

Beta-amyloid peptides decrease soluble guanylyl cyclase expression in astroglial cells

In astroglial cells beta-amyloid peptides (betaA) induce a reactive phenotype and increase expression of NO synthase. Here we show that treatment of rat brain astrocytes with betaA decreases their capacity to accumulate cyclic GMP (cGMP) in response to NO as a result of a decreased expression of soluble guanylyl cyclase (sGC) at the protein and mRNA levels. Potentiation of betaA-induced NO formation by interferon-gamma did not result in a larger decrease in cGMP formation and inhibition of NO synthase failed to reverse down-regulation of sGC, indicating that NO is not involved. The betaA effect was prevented by the protein synthesis inhibitor cycloheximide. Intracerebral betaA injection also decreased sGC beta1 subunit mRNA levels in adult rat hippocampus and cerebellum. A loss of sGC in reactive astrocytes surrounding beta-amyloid plaques could be a mechanism to prevent excess signalling via cGMP at sites of high NO production.

Chronic hypoxia attenuates cGMP-dependent pulmonary vasodilation

Chronic hypoxia (CH) augments endothelium-derived nitric oxide (NO)-dependent pulmonary vasodilation; however, responses to exogenous NO are reduced following CH in female rats. We hypothesized that CH-induced attenuation of NO-dependent pulmonary vasodilation is mediated by downregulation of vascular smooth muscle (VSM) soluble guanylyl cyclase (sGC) expression and/or activity, increased cGMP degradation by phosphodiesterase type 5 (PDE5), or decreased VSM sensitivity to cGMP. Experiments demonstrated attenuated vasodilatory responsiveness to the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate and to arterial boluses of dissolved NO solutions in isolated, saline-perfused lungs from CH vs. normoxic female rats. In additional experiments, the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, blocked vasodilation to NO donors in lungs from each group. However, CH was not associated with decreased pulmonary sGC expression or activity as assessed by Western blotting and cGMP radioimmunoassay, respectively. Consistent with our hypothesis, the selective PDE5 inhibitors dipyridamole and T-1032 augmented NO-dependent reactivity in lungs from CH rats, while having little effect in lungs from normoxic rats. However, the attenuated vasodilatory response to NO in CH lungs persisted after PDE5 inhibition. Furthermore, CH similarly inhibited vasodilatory responses to 8-bromoguanosine 3'5'-cyclic monophosphate. We conclude that attenuated NO-dependent pulmonary vasodilation after CH is not likely mediated by decreased sGC expression, but rather by increased cGMP degradation by PDE5 and decreased pulmonary VSM reactivity to cGMP.

Effects of nitroglycerin on soluble guanylate cyclase: implications for nitrate tolerance

Soluble guanylate cyclase (sGC) is a heterodimeric hemoprotein that catalyzes the conversion of GTP to cGMP. Upon binding NO to its heme cofactor, purified sGC was activated 300-fold. sGC was only activated 67-fold by nitroglycerin (GTN) and Cys; and in the absence of Cys, GTN did not activate sGC. Electronic absorption spectroscopy studies showed that upon NO binding, the Soret of ferrous sGC shifted from 431 to 399 nm. The data also revealed that activation of sGC by GTN/Cys was not via the expected ferrous heme-NO species as indicated by the absence of the 399 nm heme Soret. Furthermore, EPR studies of the reaction of GTN/Cys with sGC confirmed that no ferrous heme-NO species was formed but that there was heme oxidation. Potassium ferricyanide is known to oxidize ferrous sGC to the ferric oxidation state. Spectroscopic and activity data for the reactions of sGC with GTN alone or with K(3)Fe(CN)(6) were indistinguishable. These data suggest the following: 1) GTN/Cys do not activate sGC via GTN biotransformation to NO in vitro, and 2) in the absence of added thiol, GTN oxidizes sGC.

Novel arene receptors as nitric oxide (NO) sensors

Nitric oxide sensors are based on the reversible, specific, and efficient binding to various types of arene receptors via intermolecular complexes, which are characterized by complete electron delocalization and strong interaction between the donor and acceptor moieties.

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.

Pharmacology of the nitric oxide receptor, soluble guanylyl cyclase, in cerebellar cells

The nitric oxide (NO) receptor, soluble guanylyl cyclase (sGC), is commonly manipulated pharmacologically in two ways. Inhibition of activity is achieved using 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-l-one (ODQ) which oxidizes the haem prosthetic group to which NO binds, while the compound 3-(5-hydroxymethyl-2-furyl)-1-benzylindazole (YC-1) is considered an 'allosteric' activator. Knowledge of how these agents function and interact in a normal cellular environment is limited. These issues were addressed using rat cerebellar cells. Inhibition by ODQ was not simply competitive with NO. The rate of onset was ODQ concentration-dependent and developed in two kinetic phases. Recovery from inhibition occurred with a half-time of approximately 5 min. YC-1 slowed the rate at which sGC deactivated on removal of NO by 45 fold, consistent with YC-1 increasing the potency of NO for sGC. YC-1 also enhanced the maximal response to NO by 2 fold. Furthermore, when added to cells in which sGC was 90% desensitized, YC-1 abruptly enhanced sGC activity to a degree that indicated partial reversal of desensitization. After pre-exposure to YC-1, sGC became resistant to inhibition by ODQ. In addition, YC-1 rapidly reversed inhibition by ODQ in cells and for purified sGC, suggesting that YC-1 either increases the NO affinity of the oxidized sGC haem or reverses haem oxidation. It is concluded that the actions of ODQ and YC-1 on sGC are broadly similar in cells and purified preparations. Additionally, YC-1 transiently reverses sGC desensitization in cells. It is hypothesized that YC-1 has multiple actions on sGC, and thereby both modifies the NO binding site and enhances agonist efficacy.

BAY 41-2272 activates two isoforms of nitric oxide-sensitive guanylyl cyclase

Soluble guanylyl cyclase is an important target for endogenous nitric oxide and the guanylyl cyclase modulator, YC-1. Recently BAY 41-2272 was identified as a similar but more potent and more specific substance. While YC-1 also acts as non-specific phosphodiesterase inhibitor, BAY 41-2272 is devoid of an effect on phosphodiesterases. BAY 41-2272 has so far only been tested on the alpha(1)/beta(1) heterodimeric isoform of soluble guanylyl cyclase and its binding site has been mapped to a region in the alpha(1) subunit amino-terminal sequence. Although this region is poorly conserved in the alpha(2) subunit, we show in the current study that the alpha(2)/beta(1) heterodimeric enzyme isoform is activated by BAY 41-2272. Deletion analysis of the alpha(2) subunit and co-expression with the beta(1) subunit in the baculovirus/Sf9 system is consistent with the amino-terminal amino acids 104 to 401 of the alpha(2) subunit as binding site for BAY 41-2272.

The role of guanylyl cyclases in the permeability response to inflammatory mediators in pial venular capillaries in the rat

Inflammatory mediators have a role in the formation of cerebral oedema and there is evidence that cGMP is an important signal in vascular permeability increase. We have investigated the role and the source of cGMP in mediating the permeability response to acutely applied bradykinin and the histamine H(2) agonist dimaprit on single cerebral venular capillaries, by using the single vessel occlusion technique. We found that 8-bromo-cGMP applied acutely resulted in a small and reversible permeability increase with a log EC(50) -7.2 +/- 0.15 M. KT 5823, the inhibitor of cGMP-dependent protein kinase, abolished the permeability responses to both bradykinin and dimaprit, while zaprinast, an inhibitor of type 5 phosphodiesterase, potentiated the response to bradykinin. On the other hand, L-NMMA blocked the response to dimaprit, but not that to bradykinin. Inhibitors of soluble guanylyl cyclase, LY 85353 and methylene blue, also inhibited the permeability response to dimaprit, but not bradykinin. The permeability responses to the natriuretic peptides ANP and CNP were of similar magnitude to that of bradykinin with log EC(50) -10.0 +/- 0.33 M and -8.7 +/- 0.23 M, respectively. The natriuretic peptide receptor antagonist HS-142-1 blocked permeability responses to bradykinin as well as to ANP, and leukotriene D(4) blocked the responses to CNP and bradykinin, but not to dimaprit. In conclusion, the histamine H(2) receptor appears to signal via cGMP that is generated by a NO and soluble guanylyl cyclase, while bradykinin B(2) receptor also signals via cGMP but through particulate guanylyl cyclase.

The bradykinin/soluble guanylate cyclase signaling pathway is impaired in androgen-independent prostate cancer cells

The activation of soluble guanylate cyclase by bradykinin and sodium nitroprusside (SNP), a direct activator of soluble guanylate cyclase, was evaluated in androgen-sensitive LNCaP and androgen-independent PC3 and DU145 prostate cancer cells. Bradykinin and SNP activated soluble guanylate cyclase in LNCaP cells, but not in PC3 and DU145 cells. Western blot analysis revealed that the bradykinin B2 receptor, Gqalpha, phospholipase Cgamma and endothelial nitric oxide synthase were expressed in LNCaP, PC3 and DU145 cells. However, both Western blotting and reverse transcriptase--polymerase chain reaction indicated that soluble guanylate cyclase was only expressed in LNCaP cells. These results demonstrate that the impaired bradykinin-soluble guanylate cyclase pathway in PC3 and DU145 cells is likely due to lack of expression of soluble guanylate cyclase.

Functional and biochemical analysis of endothelial (dys)function and NO/cGMP signaling in human blood vessels with and without nitroglycerin pretreatment

BACKGROUND

In experimental animal models, long-term in vivo treatment with nitroglycerin (NTG) induces both endothelial dysfunction and tolerance to nitrates. However, it is still controversial whether nitrate tolerance in humans is associated with both endothelial dysfunction and impaired vascular response to nitrovasodilator-derived NO.

METHODS AND RESULTS

Patients undergoing elective bypass surgery were randomized to receive 48 hours of continuous NTG infusion (NTG group) or no nitrate therapy (control group). Segments of surgically removed arteria mammaria, vena saphena, and arteria radialis not required for the bypass procedure were used to examine (1) the vascular responsiveness to NTG and the endothelium-dependent vasodilator acetylcholine; (2) the expression of the NO target, the soluble guanylyl cyclase; (3) the expression of the soluble guanylyl cyclase/cGMP effector target, the cGMP-dependent protein kinase (cGK); and (4) the cGK activity as assessed by the phosphorylation state of its vascular substrate, the vasodilator-stimulated phosphoprotein at serine(239) (P-VASP). NTG treatment caused a marked degree of nitrate tolerance in all 3 vessel types studied and a significant cross-tolerance to the endothelium-dependent vasodilator acetylcholine in A. mammaria and A. radialis. Although soluble guanylyl cyclase, cGK-I, and VASP expression levels were not modified by NTG treatment, a marked decrease of P-VASP, a surrogate parameter for in-vivo cGK-I activity, was observed.

CONCLUSIONS

We conclude that long-term NTG treatment induces endothelial dysfunction and impaired vascular NO/cGMP signaling in humans, which can be monitored by measuring P-VASP levels.

Sp1 transcription factor as a molecular target for nitric oxide-- and cyclic nucleotide--mediated suppression of cGMP-dependent protein kinase-Ialpha expression in vascular smooth muscle cells

cGMP-dependent protein kinase (PKG) expression is highly variable and decreases in cultured vascular smooth muscle cells (VSMCs), exposure of cells to nitric oxide (NO), or in response to balloon catheter injury in vivo. In this study, the mechanisms of human type I PKG-alpha (PKG-Ialpha) gene expression were examined. Three structurally unrelated NO donors decreased PKG-Ialpha promoter activity after transfection of a promoter/luciferase construct in VSMCs. Promoter deletion analysis demonstrated that (1) a 120-bp promoter containing tandem Sp1 sites was sufficient to drive basal PKG-Ialpha promoter activity, and (2) NO was inhibitory at this site. Cyclic nucleotide analogues also suppressed PKG-Ialpha promoter activity with cAMP being more potent than cGMP. The effects of cyclic nucleotides to suppress PKG-Ialpha promoter activity were attenuated by a specific cAMP-dependent protein kinase (PKA) inhibitor. Single or double mutation of Sp1 binding sites abolished PKG-Ialpha expression. Moreover, Sp1 binding activity on the PKG-Ialpha promoter was detected in A7r5 cells, and this binding was inhibited by NO and cyclic nucleotides. These results indicate that PKG-Ialpha gene expression is driven by an Sp1 transcription mechanism, and that NO and cAMP inhibit Sp1-mediated PKG-Ialpha gene expression through separate mechanisms.

Effects of angiotensin II infusion on the expression and function of NAD(P)H oxidase and components of nitric oxide/cGMP signaling

Angiotensin II infusion causes endothelial dysfunction by increasing NAD(P)H oxidase-mediated vascular superoxide production. However, it remains to be elucidated how in vivo angiotensin II treatment may alter the expression of the gp91(phox) isoforms and the endothelial nitric oxide synthase (NOS III) and subsequent signaling events and whether, in addition to the NAD(P)H oxidase, NOS III contributes to vascular superoxide formation. We therefore studied the influence of in vivo angiotensin II treatment (7 days) in rats on endothelial function and on the expression of the NAD(P)H oxidase subunits p22(phox), nox1, nox4, and gp91(phox) and NOS III. Further analysis included the expression of NO-downstream targets, the soluble guanylyl cyclase (sGC), the cGMP-dependent protein kinase I (cGK-I), and the expression and phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) at Ser239 (P-VASP). Angiotensin II caused endothelial dysfunction and increased vascular superoxide. Likewise, we found an increase in vascular protein kinase C (PKC) activity, in the expression of nox1 (6- to 7-fold), gp91(phox) (3-fold), p22(phox) (3-fold), NOS III mRNA, and protein. NOS-inhibition with N(G)-nitro-L-arginine decreased superoxide in vessels from angiotensin II-treated animals, compatible with NOS-uncoupling. Vascular NO assessed with electron paramagnetic resonance was markedly reduced. Likewise, a decrease in sGC-expression and P-VASP levels was found. In vivo PKC-inhibition with chelerythrine reduced angiotensin II-induced superoxide production and markedly inhibited upregulation of NAD(P)H oxidase subunits. We therefore conclude that angiotensin II-induced increases in the activity and the expression of NAD(P)H oxidase are at least in part PKC-dependent. NADPH oxidase-induced superoxide production may trigger NOS III uncoupling, leading to impaired NO/cGMP signaling and to endothelial dysfunction in this animal model. The full text of this article is available at http://www.circresaha.org.

Effect of hypercholesterolemia on expression and function of vascular soluble guanylyl cyclase

BACKGROUND

Vasorelaxation to endothelial NO is mediated by activation of soluble guanylyl cyclase (sGC) and impaired by hypercholesterolemia in animals and humans. We investigated whether hypercholesterolemia impacts expression and function of sGC.

METHODS AND RESULTS

White New Zealand rabbits (n=10 per group) received a standard diet for 16 weeks (SD16) (n=20) or 32 weeks (SD32) and a cholesterol diet (7.5 g/kg) for 16 weeks (CD16) (n=20) or 32 weeks (CD32), respectively. Another group received cholesterol diet for 16 weeks followed by standard diet for 16 weeks (CD/SD). Aortic expression of the alpha1-subunit of sGC (sGC-alpha1) and beta1-subunit of sGC (sGC-beta1) was assessed by Western blot. Function was measured by aortic relaxation to S-Nitroso-N-acetyl-D, L-penicillamine (SNAP) and sGC activity in aortic cytosols. Hypercholesterolemia induced an upregulation of sGC-beta1 in CD16 (3.5+/-0.4-fold, P<0.001 versus SD16) and CD32 (4.0+/-0.4-fold, P<0.001 versus SD32). A similar increase was found for sGC-alpha1. In striking contrast, basal and NO-stimulated sGC activities in aortic cytosols of CD16 were only slightly enhanced (1.4-fold, P<0.05). Furthermore, the vasodilator potency of SNAP (EC50 in -logM) was 10-fold lower in CD16 (6.76a+/-0.09) than in SD16 (7.66+/-0.14, P<0.01). The increase of sGC expression was completely reversible, as indicated by comparable sGC-beta1 amounts in SD32 and CD/SD (1.2+/-0.1-fold, P>0.05). Immunohistochemical analysis suggests that a great portion of the overexpressed sGC is located in intimal lesions. Additional experiments showed that increased vascular superoxide production induced by 6-anilino-5,8-quinolinedione (LY85385) reduces sGC-activity but increases sGC-expression.

CONCLUSIONS

These results suggest that hypercholesterolemia induces a reversible overexpression of a dysfunctional vascular sGC, which may contribute to the pathogenesis of atherosclerosis.

Critical time-window for NO-cGMP-dependent long-term memory formation after one-trial appetitive conditioning

The nitric oxide (NO)-cGMP signaling pathway is implicated in an increasing number of experimental models of plasticity. Here, in a behavioral analysis using one-trial appetitive associative conditioning, we show that there is an obligatory requirement for this pathway in the formation of long-term memory (LTM). Moreover, we demonstrate that this requirement lasts for a critical period of approximately 5 hr after training. Specifically, we trained intact specimens of the snail Lymnaea stagnalis in a single conditioning trial using a conditioned stimulus, amyl-acetate, paired with a salient unconditioned stimulus, sucrose, for feeding. Long-term associative memory induced by a single associative trial was demonstrated at 24 hr and shown to last at least 14 d after training. Tests for LTM and its dependence on NO were performed routinely 24 hr after training. The critical period when NO was needed for memory formation was established by transiently depleting it from the animals at a series of time points after training by the injection of the NO-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide (PTIO). By blocking the activity of NO synthase and soluble guanylyl cyclase enzymes after training, we provided further evidence that LTM formation depends on an intact NO-cGMP pathway. An electrophysiological correlate of LTM was also blocked by PTIO, showing that the dependence of LTM on NO is amenable to analysis at the cellular level in vitro. This represents the first demonstration that associative memory formation after single-trial appetitive classical conditioning is dependent on an intact NO-cGMP signaling pathway.

Activation of protein kinase A by nitric oxide in cultured dorsal root ganglion neurites of the rat, examined by a fluorescence probe, ARII

To study the roles of nitric oxide (NO) in growth of nerve fibers, (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamine (NOR3), an NO-donor, was applied to cultured dorsal root ganglion (DRG) neurites from a micropipette. Ejection of a small volume of 1 mM NOR3 solution (not more than 1 pl/s) from a micropipette to terminal branches of neurites caused enlargement of the neurites, and often, elongation of their growth cones. This neurite enlargement was blocked by inhibitors for soluble guanylate cyclase. The neurite enlargement did not occur when protein kinase A (PKA) was inhibited. To prove that NOR3 activated PKA, we introduced a fluorescence peptide probe, ARII that reduces its fluorescence by activated PKA, to monitor PKA activity in DRG neurites. ARII fluorescence was reduced by NOR3, which was not observed when PKA was inhibited by its specific inhibitors. These indicated that PKA was indeed activated by NO. To examine whether the PKA activation is due to inhibition of phosphodiesterase III (PDE III) by cyclic GMP, we applied PDE III-specific inhibitors and found that the inhibitions activated PKA. Since PKA regulates various neuronal functions, our finding that NO activates PKA is important to understand roles of NO in nerve fibers.

On the activation of soluble guanylyl cyclase by nitric oxide

Soluble guanylyl cyclase (sGC) is the major cellular receptor for the intercellular messenger nitric oxide (NO) and mediates a wide range of physiological effects through elevation of intracellular cGMP levels. Critical to our understanding of how NO signals are decoded by receptive cells and translated into a useful physiological response is an appreciation of the molecular and kinetic details of the mechanism by which NO activates sGC. It is known that NO binds to a haem prosthetic group on the receptor and triggers a conformational change that increases the catalysis of cGMP synthesis by several hundred-fold. The haem is covalently attached to sGC at His-105 of the beta1 subunit, and it was thought previously that activation of sGC by NO occurs in two steps: binding of NO to the haem to form a biliganded state and then rupture of the bond to His-105 triggering an increase in catalytic activity. A recent investigation of the kinetics of sGC activation [Zhao, Y., Brandish, P. E., Ballou, D. P. & Marletta, M. A. (1999) Proc. Natl. Acad. Sci. USA, 96, 14753-14758], however, proposed an additional mechanism by which NO regulates sGC activity, namely, by influencing the rate of cleavage of the His-105 bond. The existence of a second (unidentified) NO-binding site on the enzyme was hypothesized and suggested to be fundamental to cellular NO-signal transduction. Here, we show that it is unnecessary to postulate any such additional mechanism because the results obtained are predicted by the simpler model of sGC activation with a single NO-binding event.

Multiple mechanisms of vascular smooth muscle relaxation by the activation of proteinase-activated receptor 2 in mouse mesenteric arterioles

1. Activation of PAR2 in second-order mesenteric arteriole (MA) rings from C57BL/6J, NOS3 (-/-) and PAR2 (-/-) mice was assessed for the contributions of NO, cyclo-oxygenases, guanylyl cyclase, adenylyl cyclase, and of K(+) channel activation to vascular smooth muscle relaxation. 2. PAR2 agonist, SLIGRL-NH(2) (0.1 to 30 microM), induced relaxation of cirazoline-precontracted MA from C57BL/6J and NOS3 (-/-), but not PAR2 (-/-) mice. Maximal relaxation (E(max)) was partially reduced by a combination of L-(G)N-nitroarginine methyl ester (L-NAME), 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and indomethacin. An ODQ/L-NAME/indomethacin resistant relaxation was also caused by trypsin (30 nM) in PAR2 (+/+), but not in PAR2 (-/-) mice. Relaxation was endothelium-dependent and inhibited by either 30 mM KCl-precontraction, or pretreatment with apamin, charybdotoxin, and their combination; iberiotoxin did not substitute for charybdotoxin nor did scyllatoxin substitute fully for apamin. 3. Tetraethylammonium (TEA), glibenclamide, tetrodotoxin, 17-octadecynoic acid, carboxy-2-phenyl-4,4,5,5,-tetramethyl-imidazoline-1-oxyl-3-oxide, SQ22536, carbenoxolone, arachidonyl trifluoromethyl ketone, 7-nitroindazole, N-(3-(aminomethyl)benzyl)acetamidine (1400W), N-(2-cyclohexyloxy-4-nitrophenyl)-methanesulfonamide (NS-398) and propanolol did not inhibit relaxation. 4-aminopyridine significantly increased the potency of SLIGRL-NH(2). A combination of 30 microM BaCl(2) and 10 microM ouabain significantly reduced the potency for relaxation, and in the presence of L-NAME, ODQ and indomethacin, E(max) was reduced. 4. We conclude PAR2-mediated relaxation of mouse MA utilizes multiple mechanisms that are both NO-cGMP-dependent, and -independent. The data are also consistent with a role for endothelium-dependent hyperpolarization of vascular smooth muscle that involves the activation of an apamin/charybdotoxin-sensitive K(+) channel(s) and, in part, may be mediated by K(+).

Nitric oxide and cyclic GMP are involved in angiotensin II AT(2) receptor effects on neurite outgrowth in NG108-15 cells

In their undifferentiated state, NG108-15 cells express only the angiotensin II (Ang II) type 2 receptor (AT(2)). We have previously shown that Ang II induced neurite outgrowth of NG108-15 cells, a process involving sustained activation of p42/p44(mapk) activity. We have also shown that Ang II stimulates nitric oxide (NO) production. The aim of the present study was to investigate the role of the NO/cyclic GMP (cGMP) cascade in the signal transduction of the AT(2) receptor-stimulated neurite outgrowth. Three-day treatment of cells with dbcGMP induced neurite outgrowth as did Ang II. Preincubation with an inhibitor of cGMP-dependent protein kinase, KT5823, resulted in the formation of short neurites, while in the presence of LY83583 or methylene blue, two inhibitors of guanylyl cyclase, cells resembled control cells with only one or two thin processes. Western blot analyses indicated that nNOS was present in NG108-15 cells. Immunoprecipitation with antiphosphotyrosine antibodies showed that Ang II induced NOS activity and increased cGMP production through a Gi-dependent pathway. However, neither L-NAME, KT5823, nor LY83583 affected the activation of p42/p44(mapk) induced by Ang II, indicating that the pathway NO/guanylyl cyclase/cGMP was not involved in Ang II-induced activation of MAPK. The present results suggest that the neurite outgrowth induced by Ang II results from at least parallel but complementary pathways, one involved in neurite elongation (through the cooperation of MAPK and PKG) and the other involved in sprouting (through cGMP).

The receptor-like properties of nitric oxide-activated soluble guanylyl cyclase in intact cells

Soluble guanylyl cyclase (sGC) is the main receptor for nitric oxide (NO), and so mediates a wide range of effects (e.g. vasodilatation, platelet disaggregation and neural signalling) through the accumulation of cGMP and the engagement of various downstream targets, such as protein kinases and ion channels. Until recently, our understanding of sGC functioning has been derived exclusively from studies of the enzyme in tissue homogenates or in its purified form. Here, NO binds to the haem prosthetic group of sGC, triggering a conformational change and a large increase in catalytic activity. The potency (EC50) of NO appears to be about 100-200 nM. The rate of activation of sGC by NO is rapid (milliseconds) and, in the presence of excess substrate, cGMP is formed at a constant rate; on removal of NO, sGC deactivates slowly (seconds-minutes). Recent investigation of the way that sGC behaves in its natural environment, within cells, has revealed several key differences. For example, the enzyme exhibits a rapidly desensitizing profile of activity; the potency of NO is 45 nM for the minimally-desensitized enzyme but becomes higher with time, deactivation of sGC on removal of NO is 25-fold faster than the fastest estimate for purified sGC. Overall, within cells, sGC behaves in a way that is analogous to the way that classical neurotransmitter receptors operate. The properties of cellular sGC have important implications for the understanding of NO-cGMP signalling. For example, the dynamics of the enzyme means that fluctuations in the rate of NO formation, even on subsecond time scale, will result in closely synchronized sGC activity in neighbouring cells; desensitization of sGC provides an economical way of generating a cellular cGMP signal and, in concert with phosphodiesterases, provides the basis for cGMP signal diversity, allowing different targets (outputs) to be selected from a common input (NO). Thus, despite exhibiting only limited molecular heterogeneity, cellular sGC functions in a way that introduces speed, complexity, and versatility into NO-cGMP signalling pathways.

Soluble guanylate cyclases in the retina

Soluble guanylate cyclase catalyzes the formation of cyclic GMP using GTP as substrate. It is now well established that soluble guanylate cyclase is highly activated by nitric oxide, and that many of the effects of nitric oxide on various cells and tissues are mediated through increased production of cyclic GMP. This review discusses the evidence for the presence of soluble guanylate cyclases in different classes of cells in vertebrate retina and the role of these enzymes in retinal physiology. It is concluded that the enzyme is present in nearly every class of cells in the retina and that it may be involved in signal transmission between some cells and in the modulation of signal transmission between others.