Nitric oxide synthase regulatory sites. Phosphorylation by cyclic AMP-dependent protein kinase, protein kinase C, and calcium/calmodulin protein kinase; identification of flavin and calmodulin binding sites

Induction of nitric oxide synthase in cultured vascular smooth muscle cells: the role of cyclic AMP

1. Interleukin-1 beta (IL-1 beta) is a potent stimulant of inducible nitric oxide synthase (iNOS) mRNA and nitric oxide (NO) production in vascular smooth muscle (VSM) cells in culture. These studies investigate the role of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in this process. 2. Dibutyryl cyclic AMP (db cyclic AMP, 0.1-1 mM), forskolin (1-10 microM) and the phosphodiesterase inhibitor, Ro 20-1724 (1-10 microM), all of which increase intracellular cyclic AMP, had no effect on NO production when added alone but markedly enhanced NO production by IL-1 beta-stimulated VSM cells in a dose-dependent manner. Consistent with a cyclic AMP-mediated action, isoprenaline (1-10 microM) increased NO production from IL-1 beta-stimulated cells. Dibutyryl cyclic GMP (db cyclic GMP) had no effect at concentrations up to 1 mM. 3. Pursuing these observations, iNOS protein levels were examined by Western blot analysis and iNOS mRNA levels were measured by reverse transcription and amplification of the resultant cDNA using the polymerase chain reaction. In addition to enhancing NO production, db cyclic AMP increased iNOS protein and mRNA above that produced by IL-1 beta alone. 4. These data demonstrate a major effect of cyclic AMP on cytokine-induced NOS activity in VSM cells, mediated at least in part by regulating synthesis of iNOS, and has implications for the pathogenesis and management of septic shock.

Forskolin and 3-isobutyl-1-methylxanthine increase basal and sodium nitroprusside-elevated cyclic GMP levels in adult guinea-pig cerebellar slices

In this study, the interaction between 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) in [3H]adenine- or [3H]-guanine-prelabelled adult guinea-pig cerebellar slices was investigated. Basal levels of [3H]cGMP were enhanced by forskolin, although no plateau was reached over the concentration range tested (0.1-100 microM). However, forskolin elicited a concentration-dependent, saturable potentiation of sodium nitroprusside (SNP)-stimulated [3H]cGMP accumulation (forskolin EC50 value of 0.98 +/- 0.23 microM; 10 microM forskolin produced a 1.8 +/- 0.3-fold potentiation of the SNP response at 2.5 min). The forskolin potentiation was observed at all concentrations of SNP tested (0.001-10 mM). Forskolin also elicited a large stimulation of [3H]-cAMP in [3H]adenine-prelabelled guinea-pig cerebellar slices; however, 1,9-dideoxyforskolin failed to elicit either a [3H]cAMP response or a potentiation of the SNP-induced [3H]cGMP response at concentrations up to 100 microM. Pretreatment with oxyhaemoglobin (50 microM) inhibited the response to SNP (1 mM) and forskolin (10 microM), as well as the response evoked by the combination of SNP and forskolin. NG-Nitro-L-arginine (100 microM) inhibited the response to forskolin alone, but did not change the response to SNP or the potentiation induced by forskolin on SNP-induced [3H]cGMP levels. The protein kinase inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7; 100 microM), staurosporine (10 microM), polymyxin B (100 microM), and Ro 31-8220 (10 microM) had no effect on the [3H]cGMP response to either SNP or the combination of SNP plus forskolin. N6,2'-Dibutyryl cAMP, at concentrations up to 10 mM, was also without effect on [3H]cGMP levels induced by SNP.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide and the cardiovascular system

Nitric oxide is a simple diatomic molecule that functions as a cellular messenger in a number of organ systems, included among which is the cardiovascular system. Serving several physiologic roles in the cardiovascular system, nitric oxide is an important determinant of basal vascular tone, regulates myocardial contractility, and modulates platelet-vessel wall interactions. From the perspective of cardiovascular pathophysiology, nitric oxide has been implicated in the pathogenesis of essential hypertension, atherosclerosis, and the hypotension associated with shock states. This review will focus in detail on the chemistry, biology, and pathobiology of nitric oxide as it relates to cardiovascular function.

Non-adrenergic, non-cholinergic relaxation of the bovine retractor penis muscle: role of S-nitrosothiols

1. This study examined the possibility that an S-nitrosothiol, rather than nitric oxide, functions as the non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the bovine retractor penis (BRP) muscle. 2. Treatment of BRP muscle with either of two sulphydryl inactivating agents, diamide (1 mM) and N-ethylmaleimide (0.3 mM), inhibited NANC relaxation and this was prevented by pretreating tissues with L-cysteine (3 mM), L-glutathione (3 mM) or dithiothreitol (3 mM). Inhibition was not specific, however, since the inactivating agents also inhibited the relaxant actions of authentic nitric oxide (0.3 microM), glyceryl trinitrate (0.001-1 microM) and isoprenaline (0.01-1 microM). 3. Reacting nitric oxide with L-cysteine in nominally oxygen-free solution at pH 3, followed by purging to remove free nitric oxide and neutralisation, produced greater and more prolonged relaxant activity when assayed on rabbit aortic rings than could be attributed to nitric oxide alone. H.p.l.c. analysis of the mixture identified a new peak distinct from either L-cysteine or nitric oxide which was responsible for the relaxant activity. The spectral absorption of this new compound had two bands with peaks at 218 and 335 nm. 4. Using a series of structural analogues of L-cysteine (all at 15 mM) it was found that removal of the carboxyl group (L-cysteamine), replacement of the carboxyl with an ester function (L-cysteine methyl ester) or substitution at the amino group (N-acetyl-L-cysteine) had no effect on the ability to generate relaxant activity upon reaction with nitric oxide (0.1 mM). In contrast, substitution at the sulphydryl group (S-methyl-L-cysteine, L-cysteinesulfinic acid and L-cysteic acid), or formation of disulphides(L-cystine and L-cystamine) led to a complete loss of ability to generate relaxant activity. L-Glutathione was also able to react with nitric oxide to produce relaxant activity, and this too was blocked upon substitution of the free sulphydryl group (S-methyl-L-glutathione). A free sulphydryl group was therefore required to generate relaxant activity following reaction with nitric oxide.5. Reacting L-cysteine (10 mM) with nitric oxide (~ 3 mM) under more stringent oxygen-free conditions followed by purging to remove free nitric oxide resulted in the generation of low relaxant activity and small absorption peaks at 218 and 335 nm and these were unaffected upon exposure to the air. In contrast, admitting air to the reaction chamber before purging enhanced both relaxant activity and the absorption peaks at 218 and 335 nm by some 40 fold and the solution turned pink due to the appearance of another absorption peak at 543 nm. This enhanced relaxant activity was not due to nitrogen dioxide being the reactive species, since at 0.1 mM this gas failed to react with L-cysteine to generate relaxant activity, and at 1 mM generated less activity than the equivalent concentration of nitricoxide.6. The relaxant activity generated by reacting nitric oxide with L-cysteine or L-glutathione was abolished following treatment with haemoglobin (3 MicroM), methylene blue (10 MicroM) or Nmethylhydroxylamine(100 MicroM), but was unaffected by N0-nitro-L-arginine (30 MicroM). Furthermore, two agents that generate superoxide anion, pyrogallol (0.1 mM) and hydroquinone (0.1 mM), also inhibited this relaxant activity as well as that induced by authentic nitric oxide (0.3 MicroM) but as previously reported, had no effect on relaxation induced by NANC nerve stimulation. Superoxide dismutase(100 u ml1) reversed the actions of pyrogallol and hydroquinone but had no effect on NANC relaxation.7. In conclusion, the reaction of nitric oxide with L-cysteine or L-glutathione generates relaxant activity which exceeds that of nitric oxide alone and probably results from formation of S-nitrosocysteine and S-nitrosoglutathione, respectively. The effects of pyrogallol and hydroquinone suggest that the NANCneurotransmitter is a superoxide anion-resistant, nitric oxide-releasing molecule and that neither S-nitrocysteine nor S-nitrosoglutathione is a suitable candidate for this.

Phosphorylation by Ca2+/calmodulin-dependent protein kinase II and protein kinase C of sepiapterin reductase, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin

Sepiapterin reductase, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin, was stoichiometrically phosphorylated by Ca2+/calmodulin-dependent protein kinase II and protein kinase C (Ca2+/phospholipid-dependent protein kinase) in vitro. Maximal incorporation of phosphate into the enzyme subunit by these was 3.05 +/- 0.05 (n = 4) and 0.74 +/- 0.03 (n = 5) 32P mol per mol enzyme subunit, respectively. The enzyme was not phosphorylated by cyclic nucleotide-dependent protein kinase of either the cAMP-dependent or cGMP-dependent type in this study. Dihydropteridine reductase, another enzyme working in direct supply of tetrahydrobiopterin, was also a good substrate for Ca2+/calmodulin-dependent protein kinase II. Phosphorylation of sepiapterin reductase by these protein kinases modified the kinetic properties of the enzyme. It is likely that these multifunctional Ca(2+)-activated protein kinases may play a role in the regulation of the physiological function of the BH4-generating enzymes in vivo, as was previously found in the case of BH4-requiring enzymes.

Role of calcium and calmodulin in flow-induced nitric oxide production in endothelial cells

These experiments demonstrate that exposure of cultured endothelial cells (EC) to well-defined laminar fluid flow results in an elevated rate of NO production. NO production was monitored by release of NOx (NO2- + NO3(2-) and by cellular guanosine 3',5'-cyclic monophosphate (cGMP) concentration. NO synthase (NOS) inhibitor blocked the flow-mediated stimulation of both NOx and cGMP, indicating that both measurements reflect NO production. Exposure to laminar flow increased NO release in a biphasic manner, with an initial rapid production consequent to the onset of flow followed by a less rapid, sustained production. A similar rapid increase in NO production resulted from an increase in flow above a preexisting level. The rapid initial production of NO was not dependent on shear stress within a physiological range (6-25 dyn/cm2) but may be dependent on the rate of change in shear stress. The sustained release of NO was dependent on physiological levels of shear stress. The calcium (Ca2+) or calmodulin (CaM) dependence of the initial and sustained production of NO was compared with bradykinin (BK)-mediated NO production. Both BK and the initial production were inhibited by Ca2+ and CaM antagonists. In contrast, the sustained shear stress-mediated NO production was not affected, despite the continued functional presence of the antagonists. Dexamethasone had no effect on either the initial or the sustained shear stress-mediated NO production. An inducible NOS does not, therefore, explain the apparent Ca2+/CaM independence of the sustained shear stress-mediated NO production.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C modulates receptor-independent activation of endothelial nitric oxide synthase

The intracellular regulation of nitric oxide synthase has been the focus of intense investigation. Bioassay studies using vascular rings have suggested that protein kinase C inhibits endothelium-dependent vascular relaxation. However, information regarding the effects of protein kinase C on the synthesis of nitric oxide in endothelial cells is not available. Therefore, we investigated the effects of protein kinase C to regulate receptor-independent activation of nitric oxide synthase activity in cultured bovine pulmonary artery endothelial cells. Activation of protein kinase C by phorbol 12-myristate 13-acetate or 1,2-dioctanoyl-sn-glycerol inhibited receptor-dependent and receptor-independent nitric oxide synthase activity. The inhibition of nitric oxide synthase by protein kinase C was concentration dependent and markedly blunted by staurosporine. The inhibition of protein kinase C by staurosporine alone enhanced basal nitric oxide synthase activity. Furthermore, depletion of protein kinase C enhanced both basal and agonist-stimulated nitric oxide synthase activity. These studies indicate that protein kinase C modulates the activity of the constitutive Ca2+/calmodulin-dependent endothelial nitric oxide synthase in the basal state and following agonist stimulation through direct inhibition of the enzyme as well as receptor desensitization. These direct regulatory effects of protein kinase C on endothelial nitric oxide synthase activity may have important implications in the physiologic regulation of vascular tone.

Nitric oxide activation of poly(ADP-ribose) synthetase in neurotoxicity

Poly(adenosine 5'-diphosphoribose) synthetase (PARS) is a nuclear enzyme which, when activated by DNA strand breaks, adds up to 100 adenosine 5'-diphosphoribose (ADP-ribose) units to nuclear proteins such as histones and PARS itself. This activation can lead to cell death through depletion of beta-nicotinamide adenine dinucleotide (the source of ADP-ribose) and adenosine triphosphate. Nitric oxide (NO) stimulated ADP-ribosylation of PARS in rat brain. Benzamide and other derivatives, which inhibit PARS, blocked N-methyl-D-aspartate- and NO-mediated neurotoxicity with relative potencies paralleling their ability to inhibit PARS. Thus, NO appeared to elicit neurotoxicity by activating PARS.

Inhibition of inducible nitric oxide synthase in macrophages by oxidized low-density lipoproteins

Uptake of oxidized low-density lipoprotein (LDL) by monocyte/macrophages to form "foam" cells has been implicated in atherogenesis. Activated monocyte/macrophages synthesize nitric oxide (NO) from L-arginine. NO is cytotoxic, antiproliferative, and a vasodilator that inhibits platelet and monocyte adhesion. NO synthase mRNA, protein, and enzyme activity were induced in J774.A1 macrophages activated with lipopolysaccharide and gamma interferon. When macrophages were incubated with oxidized LDL for 24 hours and activated, there was a dose- and time-dependent inhibition of NO synthesis, assessed as nitrite accumulation in the media. When activated cells were incubated with nontoxic doses of lipoprotein (25 micrograms/mL), neither native LDL nor acetyl LDL inhibited NO production, whereas oxidized LDL produced 50% inhibition. Levels of enzyme protein were unchanged by Western blot. Inhibition was a function of the degree of oxidation of LDL but was independent of cholesteryl esterification by the cells. Incubations of oxidized LDL with cells that had been pretreated with dextran sulfate or cytochalasin B yielded no evidence that inhibition was dependent on the scavenger receptor or directed endocytosis. Kinetic studies of inducible NO synthase from J774.A1 cells that were incubated with increasing doses of oxidized LDL indicated a pattern of noncompetitive inhibition. Inhibition of the enzyme was produced by lipids extracted from oxidized LDL but not by lipids extracted from native LDL. Because phosphatidylcholine (PC) is converted to lysophosphatidylcholine (LPC) during the oxidation of LDL, the effects of LPC were investigated. PC vesicles containing LPC did not inhibit enzyme activity but produced modest reductions in nitrite accumulation from cells. In contrast, PC vesicles had no significant effect. The data indicate that oxidized LDL lipid inhibits the activity of inducible NO synthase in activated macrophages. NO production by this enzyme and its inhibition by oxidized LDL lipid may influence cell-to-cell interactions and vasomotor tone in atherosclerotic blood vessels.

Inhibition of neuronal nitric oxide synthase by antipsychotic drugs

There is rapidly accumulating evidence that generation of nitric oxide (NO) through a Ca2+ and calmodulin-dependent pathway plays various important roles in the central nervous system. In the present study, effects of several antipsychotics on the activity of NO synthase were investigated in rat cerebellum and neuroblastoma N1E-115 cells, due to the known ability of these agents to inhibit calmodulin. In cytosolic preparations of rat cerebellum, the antipsychotic drugs inhibited the conversion of [3H]L-arginine into [3H]L-citrulline by NO synthase in a concentration-dependent manner. This inhibition was noncompetitive in nature, and it exhibited an excellent correlation with blockade of calmodulin activity. Furthermore, these drugs attenuated cyclic GMP formation induced by a calcium ionophore in N1E-115 cells, a response which takes place as a consequence of NO generation. Taken together, our data demonstrate that antipsychotic drugs inhibit NO formation in vitro. It is unlikely, however, that these actions might contribute to their therapeutic and/or side effects, since they take place at relatively high concentrations.

Assembly and regulation of NADPH oxidase and nitric oxide synthase

Research over the past year has revealed several interesting advances in the biosynthesis of the superoxide anion and nitric oxide. Highlights include the demonstration that the G protein Rac 2 is required for NADPH oxidase activation, the finding that nitric oxide is a feedback inhibitor of nitric oxide synthase isoforms, and the discovery that the continuous catalytic activity of the immune/inflammatory nitric oxide synthase is due to strong calmodulin binding, which is independent of elevated calcium levels. Interferon-gamma primes neutrophils and macrophages for both O2- and nitric oxide synthesis. However, NADPH oxidase and immune/inflammatory nitric oxide synthase are differentially regulated such that their activities are not simultaneously induced.

The complex role of nitric oxide in the pathophysiology of focal cerebral ischemia

Nitrogen monoxide (NO) has recently emerged as an important mediator of cellular and molecular events which impacts the pathophysiology of cerebral ischemia. Although tempting to ask whether NO is "good or bad" for cerebral ischemia, the question underestimates the complexities of NO chemistry and physiology as well as oversimplifies the pathophysiology of focal cerebral ischemia. Important vascular and neuronal actions of NO have been defined which both enhance tissue survival and mediate cellular injury and death, and these will be reviewed. Strategies which modify NO synthesis and/or metabolism may someday assume therapeutic importance, but not until the tissue compartments generating NO, the activities of the enzymes that are inducibly and constitutively expressed, and the redox state of NO during the stages of ischemic injury, are defined with greater precision. Our knowledge of these processes is rudimentary. This review will summarize the evidence from animal models which supports an emerging role for NO in ischemic pathophysiology. Important aspects of NO synthesis and inhibitors of this process will also be discussed.

Inhibitory actions of diphenyleneiodonium on endothelium-dependent vasodilatations in vitro and in vivo

1. This study examined the in vitro and in vivo inhibitory effects of diphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synthase, on endothelium-dependent vasodilatations. 2. DPI (3 x 10(-8)-3 x 10(-6) M) concentration-dependently inhibited acetylcholine (ACh)-induced relaxation in preconstricted rat thoracic aortic rings, with an IC50 of 1.8 x 10(-7) M and a maximal inhibition of nearly 100%. DPI (3 x 10(-6) M) also completely inhibited the relaxation induced by the calcium ionophore, A23187 but not by sodium nitroprusside (SNP). The inhibitory effect of DPI (3 x 10(-7) M) on ACh-induced relaxation was prevented by pretreatment with NADPH (5 x 10(-3) M) and FAD (5 x 10(-4) M) but not L-arginine (L-Arg, 2 x 10(-3) M). Pretreatment with NADPH did not alter the inhibitory effect of NG-nitro-L-arginine on ACh-induced relaxation. 3. The inhibitory effect of DPI on ACh-induced relaxation in the aortae lasted > 4 h after washout. In contrast to pretreatment, post-treatment (1 h later) with NADPH (5 x 10(-3) M) reversed only slightly the inhibitory effect of DPI. 4. In conscious rats, DPI (10(-5) mol kg-1) inhibited the depressor response to i.v. infused ACh, but not SNP. However, it caused only a transient pressor response which was previously shown to be due completely to sympathetic activation. 5. Thus, DPI is an efficacious and 'irreversible' inhibitor of endothelium-dependent vasodilatation in vivo and in vitro. The mechanism of the inhibition may involve antagonism of the effects of FAD and NADPH, co-factors of NO synthase. However, unlike the N0-substituted arginine analogues (another class of NO synthase inhibitors), DPI-induced suppression of endothelium-dependent vasodilatation in vivo does not lead to a sustained rise in blood pressure.

Nitric oxide production during focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

Nitric oxide has been implicated as a mediator of glutamate excitotoxicity in primary neuronal cultures.

METHODS

A number of indicators of brain nitric oxide production (nitric and cyclic guanosine monophosphate [cGMP] concentrations and nitric oxide synthase activity) were examined after bilateral carotid ligation and right middle cerebral artery occlusion in adult rats.

RESULTS

Brain nitrite was significantly increased in the right versus left cortex 5, 10, and 20 minutes after middle cerebral artery occlusion (P < .05), with a return to baseline at 60 minutes. There were no significant changes in cerebellar concentrations. Cortical levels of cGMP were increased at 10, 20, and 60 minutes after occlusion, with significant right-to-left differences (P < .05). Cerebellar concentrations of cGMP were also increased but without significant side-to-side differences. Nitric oxide synthase activity increased approximately 10-fold from baseline 10 minutes after occlusion in the right cortex but decreased markedly by 60 minutes from its peak at 10 minutes. The right-to-left difference in nitric oxide synthase activity was significant at 20 minutes (P < .05). Pretreatment of rats with NG-nitro-L-arginine, a nitric oxide synthase inhibitor, abolished the rise in nitrite and cGMP.

CONCLUSIONS

These results suggest that a sharp transient increase in the activity of nitric oxide synthase occurs during the first hour of cerebral ischemia, which leads to a burst in nitric oxide production and activation of guanylate cyclase.

Nitric oxide: an ubiquitous messenger

During the last decade, a multitude of experimental arguments have led to the concept that EDRF is nitric oxide (NO), a messenger not only involved in the control of vasomotor tone but also in vascular homeostasis, neuronal and immunological functions. Regardless of its origin, endogenous NO is produced through the conversion of L-arginine to L-citrulline by NO-synthase (NOS) from which several isoforms have recently been isolated, purified and cloned. NOS-type I (isolated from brain) and type III (isolated from endothelial cells) are termed "constitutive-NOS" and produce picomolar levels of NO from which only a small fraction elicits physiological responses. These isoforms are regulated by Ca(2+)-calmodulin with NADPH, FAD/FMN and tetrahydrobiopterin as co-factors and reveal a high degree of homology with the amino-acid sequence of cytochrome P450 reductase within the C-terminal domain. Functionally, neuronal-NOS type I is important in neurotransmission (modulation of NMDA receptor), the central control of vascular homeostasis and possibly learning and memory. In the peripheral nervous system, NOS appears to be linked to nonadrenergic noncholinergic (NANC) neuronal pathways. Endothelial-NOS type III is essential for the control of vascular tone in response to the release of endogenous mediators, although shear stress is the major trigger of endothelial-NOS activity under physiological conditions. NOS-type III also contributes to the prevention of abnormal platelet aggregation. NOS-types II and IV (isolated from macrophages) are Ca(2+)-calmodulin independent and are termed "inducible-NOS" since their activation is only promoted under pathophysiological situations where macrophages exert cytotoxic effects in response to cytokines. In contrast with NOS-types I and III, activation of NOS-type II in these cells induces the formation of nanomolar levels of NO which act as a defense mechanism of the immune system. Dysfunctions of the L-arginine-NO pathway have been characterized in multiple diseases (atherosclerosis, hypertension, diabetes, sepsis, cerebral ischemia, etc) and the design of more selective activators/inhibitors of NOS isoforms is a new challenge for the understanding of their pathophysiology and treatment.

Protein kinases modulate the sensitivity of hippocampal neurons to nitric oxide toxicity and anoxia

Multiple processes lead to neuronal death after ischemia, but the generation of nitric oxide (NO) is a key component in this cascade of events. The mechanisms that regulate the extent of neuronal degeneration during anoxia and NO toxicity are multifactorial. Neuronal death may be modulated by the activity of signal transduction systems that influence the toxicity of NO or its metabolic products such as cGMP. The enzyme responsible for the production of NO, nitric oxide synthase (NOS), is phosphorylated by protein kinase C (PKC), the cAMP-dependent protein kinase (PKA), and the calcium/calmodulin-dependent protein kinase II (CaM-II). We examined in primary cultured hippocampal neurons whether the protein kinases PKC, PKA, CaM-II, and cGMP-dependent protein kinase modified the toxic effects of anoxia and NO. Down-regulation of PKC activity with PMA (1 microM) increased hippocampal neuronal survival during anoxia and NO exposure from approximately 22% to 88%. Inhibitors of PKC activity (H-7, H-8, sphingosine, and staurosporine) also were neuroprotective. Down-regulation of PKC activity increased survival during anoxia even in the presence of the NOS inhibitor, N omega-methyl-L-arginine. Thus, although down-regulation of PKC activity may increase neuronal survival by decreasing NOS activity, it also is likely that PKC contributes to ischemic neuronal death by mechanisms that are independent of NOS. Inhibition of the cGMP-dependent protein kinase activity, but not the activity of the CaM-II also was neuroprotective during NO administration. In contrast to the protective effects of inhibition of PKC and the cGMP-dependent protein kinase, activation rather than inhibition of PKA increased hippocampal neuronal survival during NO exposure. These results indicate that neuronal survival during anoxia and NO exposure is linked to the modulation of PKC, PKA, and cGMP-dependent protein kinase activity but is not dependent on the CaM-II pathway. Understanding the involvement of PKC, PKA, and the cGMP-dependent protein kinase in modulating the effect of neuronal death during ischemia and NO toxicity may help in directing future therapeutic modalities for cerebrovascular disease.

Role of intercellular and intracellular communication by nitric oxide in coupling of muscarinic receptors to activation of guanylate cyclase in neuronal cells

Muscarinic receptor-mediated cyclic GMP formation and release of nitric oxide (NO) (or a precursor thereof) were compared in mouse neuroblastoma N1E-115 cells. [3H]Cyclic GMP was assayed in cells prelabeled with [3H]guanine. Release of NO upon the addition of muscarinic agonists to unlabeled neuroblastoma cells (NO donor cells) was quantitated indirectly by its ability to increase the [3H]cyclic GMP level in labeled cells whose muscarinic receptors were inactivated by irreversible alkylation (NO detector cells). Carbachol increased NO release in a concentration-dependent manner, with half-maximal stimulation at 173 microM (compared to 96 microM for direct activation of cyclic GMP formation). The maximal effect of carbachol in stimulating release of NO when measured indirectly was lower than that in elevating [3H]cyclic GMP directly in donor cells. Hemoglobin was more effective in blocking the actions of released NO than in attenuating direct stimulation of [3H]cyclic GMP synthesis. There was a good correlation between the ability of a series of muscarinic agonists to release NO or to activate [3H]cyclic GMP formation directly, and the potency of pirenzepine in inhibiting the two responses. Furthermore, there was a similar magnitude of desensitization of both responses by prolonged receptor activation or stimulation of protein kinase C. NO release was also regulated in relation to the cellular growth phase. A model is proposed in which a fraction of NO generated upon receptor activation does not diffuse extracellularly and stimulates cyclic GMP synthesis within the same cell where it is formed (locally acting NO). The remainder of NO that is extruded extracellularly might travel to neighboring cells (neurotransmitter NO) or might be taken back into the cells of origin (homing NO).

Strategies of antioxidant defense

Cellular protection against the deleterious effects of reactive oxidants generated in aerobic metabolism, called oxidative stress, is organized at multiple levels. Defense strategies include three levels of protection; prevention, interception, and repair. Regulation of the antioxidant capacity includes the maintenance of adequate levels of antioxidant and the localization of antioxidant compounds and enzymes. Short-term and long-term adaptation and cell specialisation in these functions are new areas of interest. Control over the activity of prooxidant enzymes, such as NADPH oxidase and NO synthases, is crucial. Synthetic antioxidants mimic biological strategies.

Effect of ageing on tissue levels of amino acids involved in the nitric oxide pathway in rat brain

Nitric oxide (NO) and citrulline are produced from L-arginine by the action of NO synthase after activation of excitatory amino acid receptors. In addition to its role in neurodegeneration, there is convincing evidence that NO is also involved in long-term potentiation, a cellular analog of learning and memory in the mammalian nervous system. In the present study, concentrations of L-arginine, citrulline, aspartic acid and glutamic acid were determined in various brain regions of young and old rats. The aim was to examine whether changes in brain concentrations of these amino acids might be indicative of a possible decrease in NO production with ageing, in relation with the well-established decline of cognitive function. Brain aspartic acid, citrulline and L-arginine concentrations were found to be lower in old rats compared to young animals, although the decrease did not always reach statistical significance. In contrast, no change in glutamic acid levels was found. In all brain structures of young and old rats, concentrations of L-arginine were higher than the concentration for NO synthase to function at maximum velocity in the rat brain. Therefore, the decrease in citrulline concentrations found in some brain regions of old rats might be seen, at least partly, as a reflection of a lower production of NO with ageing, although further work is clearly needed to ascertain a decrease in rat brain NO synthase activity with age.

Cloned human brain nitric oxide synthase is highly expressed in skeletal muscle

Complementary DNA clones corresponding to human brain nitric oxide (NO) synthase have been isolated. The deduced amino acid sequence revealed an overall identity with rat brain NO synthase of about 93% and contained all suggested consensus sites for binding of the co-factors. The cDNA transfected COS-1 cells showed significant NO synthase activity with the typical co-factor requirements. Unexpectedly, messenger RNA levels of this isoform of NO synthase was more abundant in human skeletal muscle than human brain. Moreover, we detected high NO synthase activity and the expressed protein in human skeletal muscle by Western blot analysis, indicating a possible novel function of NO in skeletal muscle.

Nitric oxide as a signal in blood vessels

In the five years since the discovery that nitric oxide is produced as a signal in blood vessels, a great deal has been discovered about the processes involved. This article reviews current knowledge about the vascular cell synthesis, effects and subsequent destruction of this messenger molecule.

Nitric oxide and neurons

In the past 2 years powerful evidence has emerged to suggest that nitric oxide functions as a neurotransmitter in both the central and peripheral nervous systems. Recent evidence suggests that it may play a role in mediating forms of synaptic plasticity such as long-term potentiation in the CA1 region of the hippocampus, and long-term depression in the cerebellum. Abnormal secretion of nitric oxide may be responsible for the neurotoxicity mediated by NMDA receptors that results in the pathophysiology of strokes and neurodegenerative diseases.