Inhibition of purified nitric oxide synthase from rat cerebellum and macrophage by L-arginine analogs

Nitric Oxide Bioavailability in Obstructive Sleep Apnea: Interplay of Asymmetric Dimethylarginine and Free Radicals

Obstructive sleep apnea (OSA) occurs in 2% of middle-aged women and 4% of middle-aged men and is considered an independent risk factor for cerebrovascular and cardiovascular diseases. Nitric oxide (NO) is an important endothelium derived vasodilating substance that plays a critical role in maintaining vascular homeostasis. Low levels of NO are associated with impaired endothelial function. Asymmetric dimethylarginine (ADMA), an analogue of L-arginine, is a naturally occurring product of metabolism found in the human circulation. Elevated levels of ADMA inhibit NO synthesis while oxidative stress decreases its bioavailability, so impairing endothelial function and promoting atherosclerosis. Several clinical trials report increased oxidative stress and ADMA levels in patients with OSA. This review discusses the role of oxidative stress and increased ADMA levels in cardiovascular disease resulting from OSA.

Methylated N(ω)-hydroxy-L-arginine analogues as mechanistic probes for the second step of the nitric oxide synthase-catalyzed reaction

Nitric oxide synthase (NOS) catalyzes the conversion of L-arginine to L-citrulline through the intermediate N(ω)-hydroxy-L-arginine (NHA), producing nitric oxide, an important mammalian signaling molecule. Several disease states are associated with improper regulation of nitric oxide production, making NOS a therapeutic target. The first step of the NOS reaction has been well-characterized and is presumed to proceed through a compound I heme species, analogous to the cytochrome P450 mechanism. The second step, however, is enzymatically unprecedented and is thought to occur via a ferric peroxo heme species. To gain insight into the details of this unique second step, we report here the synthesis of NHA analogues bearing guanidinium methyl or ethyl substitutions and their investigation as either inhibitors of or alternate substrates for NOS. Radiolabeling studies reveal that N(ω)-methoxy-L-arginine, an alternative NOS substrate, produces citrulline, nitric oxide, and methanol. On the basis of these results, we propose a mechanism for the second step of NOS catalysis in which a methylated nitric oxide species is released and is further metabolized by NOS. Crystal structures of our NHA analogues bound to nNOS have been determined, revealing the presence of an active site water molecule only in the presence of singly methylated analogues. Bulkier analogues displace this active site water molecule; a different mechanism is proposed in the absence of the water molecule. Our results provide new insights into the steric and stereochemical tolerance of the NOS active site and substrate capabilities of NOS.

Arginine-based inhibitors of nitric oxide synthase: therapeutic potential and challenges

In the past three decades, nitric oxide has been well established as an important bioactive molecule implicated in regulation of cardiovascular, nervous, and immune systems. Therefore, it is not surprising that much effort has been made to find specific inhibitors of nitric oxide synthases (NOS), the enzymes responsible for production of nitric oxide. Among the many NOS inhibitors developed to date, inhibitors based on derivatives and analogues of arginine are of special interest, as this category includes a relatively high number of compounds with good potential for experimental as well as clinical application. Though this group of inhibitors covers early nonspecific compounds, modern drug design strategies such as biochemical screening and computer-aided drug design have provided NOS-isoform-specific inhibitors. With an emphasis on major advances in this field, a comprehensive list of inhibitors based on their structural characteristics is discussed in this paper. We provide a summary of their biochemical properties as well as their observed effects both in vitro and in vivo. Furthermore, we focus in particular on their pharmacology and use in recent clinical studies. The potential of newly designed specific NOS inhibitors developed by means of modern drug development strategies is highlighted.

N(G)-aminoguanidines from primary amines and the preparation of nitric oxide synthase inhibitors

A concise, general, and high-yielding method for the preparation of N(G)-aminoguanidines from primary amines is reported. Using available and readily prepared materials, primary amines are converted to protected N(G)-aminoguanidines in a one-pot procedure. The method has been successfully applied to a number of examples including the syntheses of four nitric oxide synthase (NOS) inhibitors. The inhibitors prepared were investigated as competitive inhibitors and as mechanistic inactivators of the inducible isoform of NOS (iNOS). In addition, one of the four inhibitors prepared, N(G)-amino-N(G)-2,2,2-trifluoroethyl-L-arginine 19, displays the unique ability to both inhibit NO formation and prevent NADPH consumption by iNOS without irreversible inactivation of the enzyme.

Design and Synthesis of C5 Methylated l-Arginine Analogues as Active Site Probes for Nitric Oxide Synthase

The role of nitric oxide (NO) as a biological signaling molecule is well established. NO is produced by the nitric oxide synthases (NOSs, EC 1.14.13.39), a class of heme proteins capable of converting l-arginine to NO and l-citrulline. Despite the large body of knowledge associated with the NOSs, mechanistic details relating to the unique oxidative chemistry performed by these enzymes remain to be fully elucidated. Furthermore, a number of disease states are associated with either the over- or underproduction of NO, making the NOS pathway an attractive target for the development of therapeutics. For these reasons, molecular tools capable of providing mechanistic insights into the production of NO and/or the inhibition of the NOSs remain of interest. We report here the stereospecific synthesis and testing of a number of new l-arginine analogues bearing a minimal substitution, methylation at position 5 of the amino acid side chain (such analogues have not been previously reported). The synthetic approach employed a modified photolysis procedure whereby irradiation of the appropriate diacylperoxide precursors at 254 nm gave access to the required unnatural amino acids in good yields. A heme domain construct of the inducible NOS isoform (iNOSheme) was used to assess the binding of each compound to the enzyme active site. The compounds were also investigated as either inhibitors of, or alternate substrates for, the inducible NOS isoform. The results obtained provide new insight into the steric and stereochemical tolerance of the enzyme active site. These findings also further support the role of a conserved active site water molecule previously proposed to be necessary for NOS catalysis.

Intracellular degradation of insulin and crinophagy are maintained by nitric oxide and cyclo-oxygenase 2 activity in isolated pancreatic islets

BACKGROUND INFORMATION

Pancreatic beta-cells require an optimal insulin content to allow instantaneous secretion of insulin. This is maintained by insulin biosynthesis and intracellular degradation of insulin. Degradation may be effected by crinophagy, i.e. the fusion of secretory granules with lysosomes. IL-1beta (interleukin 1beta) induces distinct changes of beta-cell lysosomes. To study the mechanisms for intracellular insulin degradation and crinophagy, isolated mouse pancreatic islets were exposed to IL-1beta and known pathways for IL-1beta actions were blocked. Intracellular insulin degradation was determined by following the fate of radioactively labelled insulin. Crinophagy was studied by ultrastructural analysis. The effects of blocking pathways for IL-1beta were monitored by measurements of nitrite and PGE(2) (prostaglandin E(2)).

RESULTS

IL-1beta caused an enhancement of islet intracellular insulin degradation and an increase in the lysosomal incorporation of beta-cell secretory granules. The effects of IL-1beta were abolished by aminoguanidine, a selective inhibitor of inducible NOS (nitric oxide synthase), or by rofecoxib, a specific inhibitor of COX-2 (cyclo-oxygenase 2). In the absence of IL-1beta, nitroarginine, which is a selective inhibitor of constitutive NOS, caused a decrease in intracellular degradation of insulin in parallel with a decreased production of NO and PGE(2) by the islets.

CONCLUSIONS

The correlation between the enhanced intracellular insulin degradation and lysosomal changes caused by IL-1beta suggests that insulin degradation may be effected by crinophagy. Under physiological conditions, significant beta-cell degradation of insulin may depend on the activity of COX-2, possibly stimulated by endogenous NO.

Agmatine treatment and vein graft reconstruction enhance recovery after experimental facial nerve injury

The rate of nerve regeneration is a critical determinant of the degree of functional recovery after injury. Here, we sought to determine whether treatment with the neuroprotective compound, agmatine, with or without nerve reconstruction utilizing a regional autogenous vein graft would accelerate the rate of facial nerve regeneration. Experiments compared the following seven groups of adult male rats: (A) Intact untreated controls. (B) Sham operation with interruption of the nerve blood supply (controls). (C) Transection of the mandibular branch of the facial nerve (generating a gap of 3 mm) followed by saline treatment. (D) Nerve transection with unsutured autogenous vein (external jugular) graft reconstruction plus saline treatment. (E) Nerve transection with sutured vein graft approximation (coaptation of the proximal and distal nerve stumps) plus saline. (F) Nerve transection with sutured vein graft followed by agmatine treatment (four daily intraperitoneal injections of 100 mg/kg agmatine sulfate). (G) Nerve transection with unsutured vein graft followed by agmatine treatment. Functional recovery, as assessed by grading vibrissae movements and by recording nerve conduction velocity and numbers of regenerated axons, indicated that either vein reconstruction or agmatine treatment resulted in accelerated and more complete recovery as compared with controls. But best results were observed in animals that underwent combined treatment, i.e., vein reconstruction plus agmatine injection. We conclude that agmatine treatment can accelerate facial nerve regeneration and that agmatine treatment together with autogenous vein graft offers an advantageous alternative to other facial nerve reconstruction procedures.

Partial nicotinic receptor blockade unmasks a modulatory role of nitric oxide on urethral striated neuromuscular transmission

The objective of this study was to investigate the possible modulatory role of endogenous nitric oxide (NO) production on the urethral striated muscle (USM) function in the sheep urethra. Significant NO synthase (NOS) activity was measured in both the particulate and cytosolic fractions of USM homogenates. NOS activity was calcium-dependent and showed greater inhibition by NOS inhibitors selective of the neural NOS isoform (nNOS). nNOS immunoreactivity was present in intramural nerves as well as in the sarcolemma of some striated fibers, being denser at the neuromuscular junction (NMJ). Double immunolabeling showed co-localization of nNOS with both alpha-bungarotoxin and choline acetyltransferase, at the USM endplates. For the first time, functional data support a role of NO on the USM contractility "in vitro," which became evident following partial nicotinic receptor inactivation with low concentrations of D-tubocurarine. Only under D-tubocurarine (0.25 microM) treatment, different NOS inhibitors, specially N(G)-propyl-L-arginine, as well as the guanylate cyclase inhibitor ODQ, all showed a significant enhancing effect on contractions induced by electrical field stimulation of intrinsic somatic nerves. These data suggest that local production of NO at the urethral NMJ may modulate release and/or action of acetylcholine on motor endplates by cyclic GMP-mediated effects. This modulatory action could be especially relevant when neuromuscular transmission at the USM is impaired.

Modulation of prostaglandin biosynthesis by nitric oxide and nitric oxide donors

The biosynthesis and release of nitric oxide (NO) and prostaglandins (PGs) share a number of similarities. Two major forms of nitric-oxide synthase (NOS) and cyclooxygenase (COX) enzymes have been identified to date. Under normal circumstances, the constitutive isoforms of these enzymes (constitutive NOS and COX-1) are found in virtually all organs. Their presence accounts for the regulation of several important physiological effects (e.g. antiplatelet activity, vasodilation, and cytoprotection). On the other hand, in inflammatory setting, the inducible isoforms of these enzymes (inducible NOS and COX-2) are detected in a variety of cells, resulting in the production of large amounts of proinflammatory and cytotoxic NO and PGs. The release of NO and PGs by the inducible isoforms of NOS and COX has been associated with the pathological roles of these mediators in disease states as evidenced by the use of selective inhibitors. An important link between the NOS and COX pathways was made in 1993 by Salvemini and coworkers when they demonstrated that the enhanced release of PGs, which follows inflammatory mechanisms, was nearly entirely driven by NO. Such studies raised the possibility that COX enzymes represent important endogenous "receptor" targets for modulating the multifaceted roles of NO. Since then, numerous papers have been published extending the observation across various cellular systems and animal models of disease. Furthermore, other studies have highlighted the importance of such interaction in physiology as well as in the mechanism of action of drugs such as organic nitrates. More importantly, mechanistic studies of how NO switches on/off the PG/COX pathway have been undertaken and additional pathways through which NO modulates prostaglandin production unraveled. On the other hand, NO donors conjugated with COX inhibitors have recently found new interest in the understanding of NO/COX reciprocal interaction and potential clinical use. The purpose of this article is to cover the advances which have occurred over the years, and in particular, to summarize experimental data that outline how the discovery that NO modulates prostaglandin production has impacted and extended our understanding of these two systems in physiopathological events.

Neurochemical Evidence for Agmatine Modulation of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine (MPTP) Neurotoxicity

Agmatine treatment is known to exert neuroprotective effects in several models of neurotoxic and ischemic brain and spinal cord injuries. Here we sought to find out whether agmatine treatment would also prove to be neuroprotective in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. Concomitant daily treatment (intraperitoneal injections) with agmatine (100 mg/kg for 5 days) and MPTP (40 mg/kg for 2 days) exacerbated MPTP-related toxicity as evidenced by a larger reduction in dopamine uptake into striatal synaptosomes (42.4% as compared to 58.3% of control, respectively). In contrast, agmatine treatment commencing after MPTP, produced partial protection (31%) against MPTP dopaminergic toxicity. The findings implicate agmatine in mechanisms regulating MPTP neurotoxicity, but underscore the characteristic neuroprotective efficacy of agmatine when applied after the insult.

Suppression of inducible nitric oxide generation by agmatine aldehyde: beneficial effects in sepsis

The induction of inducible nitric oxide synthase (iNOS) serves an important immuno-protective function in inflammatory states, but ungoverned nitric oxide (NO) generation can contribute to a number of pathologic consequences. Delineation of the mechanisms that can downregulate iNOS-generated NO in inflammation could have therapeutic relevance. Here we show that agmatine, a metabolite of arginine, inhibits iNOS mediated nitric oxide generation in cytokine stimulated cell culture preparations. This effect was not cell type specific. Increased diamine oxidase (DAO) and decreased aldehyde dehydrogenase (AldDH) activities are also representative of inflammatory settings. Increasing the conversion of agmatine to an aldehyde form by addition of purified DAO or suppression of aldehyde breakdown by inhibition of AldDH activity increases the inhibitory effects of agmatine in an additive fashion. Inhibitors of DAO, but not monoamine oxidase (MAO), decreased the inhibitory effects of agmatine, as did the addition of AldDH or reacting aldehydes with phenylhydrazine. We examined rats given lipopolysaccharide (LPS) to evaluate the potential effects of agmatine in vivo. Endotoxic rats administered agmatine prevented the decreases in blood pressure and renal function normally associated with sepsis. Agmatine treatment also increased the survival of LPS treated mice. Our data demonstrate the capacity of agmatine aldehyde to suppress iNOS mediated NO generation, and indicate a protective function of agmatine in a model of endotoxic shock. How agmatine may aid in coordinating the early NO phase and the later repair phase responses in models of inflammation is discussed.

Accelerated functional recovery and neuroprotection by agmatine after spinal cord ischemia in rats

Treatment with agmatine, decarboxylated arginine, proved to be non-toxic and to exert neuroprotective effects in several models of neurotoxic and ischemic brain and spinal cord injuries. Here we sought to find out whether agmatine treatment would also prove beneficial in a rat spinal cord ischemia model (balloon occlusion of the abdominal aorta bellow the branching point of the left subclavian artery for 5 min). Agmatine was injected (100 mg/kg, i.p. ) 5 min after beginning of re-perfusion and again once daily for the next 3 post-operative days. Motor performance ('combined motor score') was recorded for up to 17 days post-operative and motoneuron cell counts (in representative spinal cord sections) performed on the 17th post-operative day. Agmatine treatment was found to accelerate recovery of motor deficits and to prevent the loss of motoneurons in the spinal cord after transient ischemia. Together, the present and previous findings demonstrate that agmatine is an efficacious neuroprotective agent and that this naturally occurring non-toxic compound should be tried for therapeutic use after neurotrauma and in neurodegenerative diseases.

Modulation of polymorphonuclear leukocytes function by nitric oxide

Recognition of the endothelium-derived relaxation factor as nitric oxide (NO) gave rise to an impression that NO was synthesised only by the endothelial lining of the vessel wall. Later it was found that NO is synthesized constitutively by the enzyme nitric oxide synthase (NOS) in various cells. However, inflammatory cytokines can induce NOS (known as inducible NOS [iNOS]) activity in all the somatic cells. Blood cells, such as eosinophils, platelets, neutrophils, monocytes, and macrophages, also synthesize NO. Among them, polymorphonuclear leukocytes (PMNs) constitute an important proportion and are also the major participants in a number of pathological conditions with suggestive involvement of NO. PMNs can synthesize NO at rates similar to endothelial cells, thus suggesting the importance of PMN-derived NO in various physiological and pathological conditions. Most of the studies so far focus on the peripheral PMNs, while studies on PMNs after emigration are limited, thus warranting systematic studies on PMNs from both sources. The role of the endothelial NOS (eNOS) and functions of NO derived from the endothelial cells has been studied extensively. However, understanding of the PMNs NOS and its regulatory role in their function is unraveling. The present review summarizes the modulatory role of NO on PMNs functions and points out the discrepancies relating to presence of NOS in PMNs. This information will be helpful in understanding the importance of NO in physiological and pathological conditions associated with PMNs.

Recognition of alpha-amino acids bearing various C=NOH functions by nitric oxide synthase and arginase involves very different structural determinants

Several alpha-amino acids bearing a C=NOH function separated from the Calpha carbon by two to five atoms have been synthesized and tested as substrates or inhibitors of recombinant nitric oxide synthases (NOS) I and II and as inhibitors of rat liver arginase (RLA). These include four N-hydroxyguanidines, N(omega)-hydroxy-L-arginine (NOHA) and its analogues homo-NOHA, nor-NOHA, and dinor-NOHA, two amidoximes bearing the -NH-C(CH(3))=NOH group, and two amidoximes bearing the -CH(2)-C(NH(2))=NOH group. Their behavior toward NOS and RLA was compared to that of the corresponding compounds bearing a C=NH function instead of the C=NOH function. The results obtained clearly show that efficient recognition of these alpha-amino acids by NOS and RLA involves very different structural determinants. NOS favors molecules bearing a -NH-C(R)=NH motif separated from Calpha by three or four CH(2) groups, such as arginine itself, with the necessary presence of delta-NH and omega-NH groups and a more variable R substituent. The corresponding molecules with a C=NOH function exhibit a much lower affinity for NOS. On the contrary, RLA best recognizes molecules bearing a C=NOH function separated from Calpha by three or four atoms, the highest affinity being observed in the case of three atoms. The presence of two omega-nitrogen atoms is important for efficient recognition, as in the two best RLA inhibitors, N(omega)-hydroxynorarginine and N(omega)-hydroxynorindospicine, which exhibit IC(50) values at the micromolar level. However, contrary to what was observed in the case of NOS, the presence of a delta-NH group is not important. These different structural requirements of NOS and RLA may be directly linked to the position of crucial residues that have been identified from crystallographic data in the active sites of both enzymes. Thus, binding of arginine analogues to NOS particularly relies on strong interactions of their delta-NH and omega-NH(2) groups with glutamate 371 (of NOS II), whereas binding of C=NOH molecules to RLA is mainly based on interactions of their terminal OH group with the binuclear Mn(II).Mn(II) cluster of the enzyme and on possible additional bonds between their omega-NH(2) group with histidine 141, glutamate 277, and one Mn(II) ion. The different modes of interaction displayed by both enzymes depend on their different catalytic functions and give interesting opportunities to design useful molecules to selectively regulate NOS and arginase.

Inhibition of nitric oxide synthase as a potential therapeutic target

Nitric oxide (NO) regulates numerous physiological processes, including neurotransmission, smooth muscle contractility, platelet reactivity, and the cytotoxic activity of immune cells. Because of the ubiquitous nature of NO, inappropriate release of this mediator has been linked to the pathogenesis of a number of disease states. This provides the rationale for the design of therapies that modulate NO concentrations selectively. A well-characterized family of compounds are the inhibitors of NO synthase, the enzyme responsible for the generation of NO; such agents are potentially beneficial in the treatment of conditions associated with an overproduction of NO, including septic shock, neurodegenerative disorders, and inflammation. This article provides an overview of NO synthase inhibitors, focusing on agents that prevent binding of substrate L-arginine.

Selective inhibitors of neuronal nitric oxide synthase--is no NOS really good NOS for the nervous system?

It is now ten years since NO was shown to account for the biological activity of endothelium-derived relaxing factor (EDRF). It is also the tenth anniversary of the identification of L-NG monomethyl arginine (L-NMMA) as the very first inhibitor of NO biosynthesis. That EDRF and NO were one and the same sparked an explosion of interest in the biochemistry and pharmacology of NO which has yet to subside. In contrast, the first ever nitric oxide synthase (NOS) inhibitor slipped seamlessly into the literature virtually without comment at the time. Over the following decade, L-NMMA (and like NOS inhibitors) have proved invaluable as tools for probing the biological roles of NO in health and disease and, in particular, have increased our understanding of the function of NO in the nervous system. Further advances in this important area now require the development of inhibitors selective for the neuronal isoform of NOS (nNOS). Here, Philip Moore and Rachel Handy provide an up-to-date account of the literature regarding the biochemical and pharmacological characterization of NOS inhibitors with particular reference to compounds with greater selectivity for the nNOS isoform.

Inhibition of constitutive nitric oxide synthase by benexate

We evaluated the inhibitory action of benexate (benzyl 2-[trans-4 -(guanidinomethyl) cyclohexylcarbonyloxy] benzoate hydrochloride beta-cyclodextrin clathrate), an anti-ulcer agent, on the formation of nitric oxide by stomach and brain enzyme preparations and on the purified neuronal nitric oxide synthase (NOS). Benexate markedly inhibited NOS activities of both stomach and brain preparations, with IC50 values of 68 and 29 microM, respectively. The results of double-reciprocal analysis suggested that the inhibition was competitive with an arginine substrate. Experiments with purified NOS revealed that benexate suppressed not only citrulline formation but also the oxidation of NADPH and the production of hydrogen peroxide by the enzyme. Taken together, it is indicated that benexate is an inhibitor for NOS in spite of the fact the drug elicits an increase in blood flow in a gastric mucosa.

Characterization of nitric oxide synthase activity in sheep urinary tract: functional implications

1. To define further the role of nitric oxide (NO) in urinary tract function, we have measured the presence of nitric oxide synthase (NOS) activity, and its relationship with functional NO-mediated responses to electrical field stimulation (EFS) in the urethra, the detrusor and the ureter from sheep. NOS activity was assayed by the conversion of L-[14C]-arginine to L-[14C]-citrulline. Endogenous production of citrulline was confirmed by thin layer chromatography. 2. NOS enzymatic activity was detected in the cytosolic fraction from tissue homogenates with the following regional distribution (pmol citrulline mg-1 protein min-1): urethra (33 +/- 3.3), detrusor (13.1 +/- 1.1) and ureter (1.5 +/- 0.2). No activity was detected in the particulate fraction of any region. 3. NOS activity was dependent on Ca(2+)-calmodulin and required exogenously added NADPH and tetrahydrobyoptein (BH4) for maximal activity. Exclusion of calmodulin from the incubation mixture did not modify NOS activity, but it was significantly reduced in the presence of the calmodulin antagonist, calmidazolium, suggesting the presence of enough endogenous calmodulin to sustain the observed NOS activity. 4. NOS activity was inhibited to a greater extent by NG-nitro-L-arginine (L-NOARG) and its methyl ester (L-NAME) than by NG-monomethyl-L-arginine (L-NMMA), while 7-nitroindazole (7-NI) was a weak inhibitor and L-cannavine had no effect. 5. Citrulline formation could be inhibited by superoxide dismutase in an oxyhaemoglobin-sensitive manner, suggesting feedback inhibition of NOS by NO. 6. EFS induced prominent NO-mediated relaxations in the urethra while minor or no responses were observed in the detrusor and the ureter, respectively. Urethral relaxations to EFS were inhibited by NOS inhibitors with the rank order of potency: L-NOARG = L-NAME > 7-NI > L-NMMA. 7. In conclusion, we have demonstrated the presence of NO-synthesizing enzymatic activity in the sheep urinary tract which shows similar characteristics to the constitutive NOS isoform found in brain. We suggest that the enzymatic activity measured in the urethral muscle layer may account for the NO-mediated urethral relaxation during micturition whereas regulation of detrusor and ureteral motor function by NOS containing nerves is less likely.

VIP-induced relaxation of guinea-pig intestinal smooth muscle cells: sequential involvement of cyclic AMP and nitric oxide

1. A possible interaction between cyclic AMP and nitric oxide (NO) in mediating the relaxant effect of vasoactive intestinal polypeptide (VIP) on intestinal smooth muscle cells has been investigated. The effects of the inhibitor of NO synthesis, NG-nitro-L-arginine methyl ester (L-NAME), have been studied on VIP-, forskolin-, and 8 bromo-cyclic AMP- induced relaxation of cells, dispersed by enzymatic digestion of muscle strips from the circular layer of guinea-pig ileum. 2. VIP alone did not modify the length of isolated muscle cells. By contrast, when the cells were contracted by cholecystokinin octapeptide, CCK8 (10 nM), VIP inhibited this contraction, inducing a concentration-dependent relaxation of the cells. Maximal relaxation was induced by 1 microM VIP (EC50 = 408.2 +/- 16.7 pM). 3. N-ethylmaleimide, inhibitors of adenylate cyclase or somatostatin, abolished the relaxing effect of VIP. (R)-p-cAMPs, an antagonist of cyclic AMP on protein kinase A also inhibited the VIP-induced relaxation by 92.1 +/- 6.3%. Inhibitors of nitric oxide synthase (NOS), L-NAME and L-NMMA, partially inhibited VIP-induced relaxation. The effect of L-NAME was reversed by L-arginine but not by D-arginine. 4. (R)-p-cAMPS and L-NAME also inhibited the cell relaxation induced either by forskolin which directly stimulates adenylate cyclase activity or 8-bromo-cyclic AMP, an analogue of cyclic AMP. 5. When cells were incubated for 30 min with dexamethasone 10 microM, a glucocorticoid known to decrease the synthesis of iNOS, the relaxing effect of a maximal concentration of VIP was decreased by 52 +/- 4% and L-NMMA had no further effect on this residual VIP-induced relaxation. Milrinone, a phosphodiesterase type III inhibitor, potentiated the relaxant effect of VIP. 6. These data demonstrate that the intracellular pathway mediating the relaxant effect of VIP in intestinal smooth muscle cells includes the sequential activation of adenylate cyclase, protein kinase A, activation of NOS and finally production of NO and cyclic GMP. NO could in turn regulate the cyclic AMP-dependent pathway of cell relaxation.

Selective pharmacological inhibition of distinct nitric oxide synthase isoforms

Nitric oxide (NO) is produced in physiological and pathophysiological conditions by three distinct isoforms of NO synthase (NOS): endothelial NOS (ecNOS), inducible NOS (iNOS), and brain NOS (bNOS). Selective inhibition of iNOS may be beneficial in various forms of shock and inflammation, whereas inhibition of bNOS may protect against neuroinjury. This article surveys the enzymatic mechanism of NO production, lists the strategies and pharmacological tools for selective inhibition of distinct NOS isoforms, and considers the side-effects of the various approaches. Selective inhibition of NOS isoforms is achieved by: (a) targeting the differential co-factor (calmodulin or tetrahydrobiopterin) requirement of various NOS isoforms, and NOS; (b) targeting the differential substrate requirements of cells expressing various isoforms of NOS (L-arginine uptake blockers or arginase); (c) the use of pharmacological agents that are selectively taken up by cells expressing various isoforms of NOS (7-nitroindazole); or (d) developing pharmacological NOS inhibitors with isoform specificity. The amino acid-based NOS inhibitor, NG-nitro-L-arginine, shows a preference for ecNOS and bNOS over iNOS, whereas L-N6-(1-iminoethyl)lysine is selective for iNOS over bNOS. Certain non-amino acid-based small molecules, such as aminoguanidine and certain S-alkylated isothioureas, also express selectivity towards iNOS and have anti-inflammatory and anti-shock properties. 7-nitroindazole, a bNOS-selective inhibitor, protects in central nervous system injury. Clearly, there are a number of distinct approaches that are worthy of further research efforts in order to achieve even more selective targeting of various NOS isoforms