N-monomethyl arginine, an inhibitor of nitric oxide synthase, suppresses the development of adjuvant arthritis in rats

OBJECTIVE

To test the hypothesis that nitric oxide (NO) is involved in the pathophysiology of arthritis.

METHODS

Arthritis was induced in male Lewis rats by the injection of adjuvant into the base of the tail. The NO synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMA), was administered daily by the oral route for 19 days. Paw swelling, plasma fibrinogen levels, and urinary NO2/NO3 levels were measured to assess the effect of L-NMA on the arthritic response and whole-body NO production, respectively. On day 20, the ankle joints were processed for histopathologic evaluation.

RESULTS

The onset of clinical symptoms was preceded by elevated biosynthesis of NO. In a dose-dependent manner, L-NMA inhibited both NO biosynthesis and paw swelling; histopathologic changes in the ankle joints were also prevented. D-NMA had no effect on the development of arthritis, while L-arginine reversed the effects of L-NMA. Fibrinogen levels in rats with arthritis were unaffected by L-NMA.

CONCLUSION

NO is critical to the development of both the inflammatory and erosive components of adjuvant arthritis in rats. There may be a future clinical role for suitable inhibitors of NO production or activity.

Induction of nitric oxide synthase protects against malaria in mice exposed to irradiated Plasmodium berghei infected mosquitoes: involvement of interferon gamma and CD8+ T cells

Exposure of BALB/c mice to mosquitoes infected with irradiated Plasmodium berghei confers protective immunity against subsequent sporozoite challenge. Immunized mice challenged with viable sporozoites develop parasitemia when treated orally with substrate inhibitors of nitric oxide synthase (NOS). This suggests that the production of nitric oxide (NO) prevents the development of exoerythrocytic stages of malaria in liver. Liver tissue from immunized mice expressed maximal levels of mRNA for inducible NOS (iNOS) between 12 and 24 h after challenge with sporozoites. Intraperitoneal injection of neutralizing monoclonal antibody against interferon gamma (IFN-gamma) or in vivo depletion of CD8+ T cells, but not CD4+ T cells, at the time of challenge blocked expression of iNOS mRNA and ablated protection in immunized mice. These results show that both CD8+ T cells and IFN-gamma are important components in the regulation of iNOS in liver which contributes to the protective response of mice immunized with irradiated malaria sporozoites. IFN-gamma, likely provided by malaria-specific CD8+ T cells, induces liver cells, hepatocytes and/or Kupffer cells, to produce NO for the destruction of infected hepatocytes or the parasite within these cells.

SIN-1 reverses attenuation of hypercapnic cerebrovasodilation by nitric oxide synthase inhibitors

We sought to determine whether the attenuation of the hypercapnic cerebrovasodilation associated with inhibition of nitric oxide synthase (NOS) can be reversed by exogenous NO. Rats were anesthetized (halothane) and ventilated. Neocortical cerebral blood flow (CBF) was monitored by a laser-Doppler probe. The NOS inhibitor N omega-nitro-L-arginine methyl ester (L-NAME; 40 mg/kg iv) reduced resting CBF [-36 +/- 5% (SE); P < 0.01, analysis of variance] and attenuated the increase in CBF elicited by hypercapnia (partial pressure of CO2 = 50-60 mmHg) by 66% (P < 0.01). L-NAME reduced forebrain NOS catalytic activity by 64 +/- 3% (n = 10; P < 0.001). After L-NAME, intracarotid infusion of the NO donor 3-morpholinosydnonimine (SIN-1; n = 6) increased resting CBF and reestablished the CBF increase elicited by hypercapnia (P > 0.05 from before L-NAME). Similarly, infusion of the guanosine 3',5'-cyclic monophosphate (cGMP) analogue 8-bromo-cGMP (n = 6) reversed the L-NAME-induced attenuation of the hypercapnic cerebrovasodilation. The NO-independent vasodilator papaverine (n = 6) increased resting CBF but did not reverse the attenuation of the CO2 response. SIN-1 did not affect the attenuation of the CO2 response induced by indomethacin (n = 6). The observation that NO donors reverse the L-NAME-induced attenuation of the CO2 response suggests that a basal level of NO is required for the vasodilation to occur. The findings are consistent with the hypothesis that NO is not the final mediator of smooth muscle relaxation in hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of vascular nitric oxide synthase in endotoxin shock of Propionibacterium acnes-sensitized rats

The sensitivity of animals to endotoxin differs significantly between species. Thus, factors that determine the susceptibility to endotoxin may play important roles in the pathogenesis of septic shock. In order to determine the mechanism responsible for susceptibility to endotoxin, the effect of lipopolysaccharide (LPS) on the circulatory status of Propionibacterium acnes (PA)-sensitized rats was studied. Following the intravenous administration of a low dose of LPS, the arterial blood pressure of PA-treated rats, but not of normal animals, progressively decreased; the PA-sensitized animals died of circulatory shock within 7 h of LPS administration. N omega-nitro-L-arginine (NA) reduced the depressor effect of LPS by an L-arginine-inhibitable mechanism. Administration of LPS markedly increased the level of the inducible type of nitric oxide (NO) synthase in various tissues, including the aorta, of PA-treated rats but not of control animals. LPS also increased plasma levels of nitrate plus nitrite and aortic levels of cGMP. Dexamethasone inhibited the de novo synthesis of NO synthase in the aorta and other tissues and reduced the depressor effect of LPS. These and other findings suggest that induction of nitric oxide synthase in resistant arteries might underlie the pathogenesis of LPS-induced hypotension in PA-sensitized animals and the mechanism responsible for the susceptibility to endotoxin.

Failure to prevent selective CA1 neuronal death and reduce cortical infarction following cerebral ischemia with inhibition of nitric oxide synthase

We investigated the putative role of nitric oxide in the expression of neuronal injury following both transient severe forebrain ischemia (CA1 neuronal injury) and transient or permanent middle cerebral artery occlusion (neocortical pannecrosis). Using the four-vessel occlusion model and increasing doses of N-omega-nitro-L-arginine, 2-40 mg/kg, we were unable to demonstrate any reduction in the percentage of CA1 cells injured following 10 min of transient severe forebrain ischemia followed by seven days of reperfusion. Higher doses proved toxic insofar as they increased the mortality following the ischemic insult. Saline-treated animals (n = 8) had 77 +/- 10% CA1 injury while those treated with 2 mg/kg of nitro-arginine i.v. had 80 +/- 7% (n = 7), and those with 10 mg/kg i.v. had 78 +/- 11% (n = 8). Two of five rats given 20 mg/kg i.v., three of eight given 40 mg/kg i.v., and two of six given 10 mg/kg i.v. followed by 3 x 10 mg/kg i.p., died. Of those treated with high-dose nitro-arginine and which survived ischemia and seven days' reperfusion, no significant reduction in CA1 injury was detected. Wistar rats and spontaneously hypertensive rats treated with either saline or nitro-arginine i.v. were exposed to 2 h of transient middle cerebral artery occlusion followed by 22 h of reperfusion. There were seven animals in each group. Wistars treated with saline had 198 +/- 67 mm3 (mean +/- S.D.) of neocortical infarction, and those treated with 10 m/kg of nitro-arginine i.v. had 199 +/- 93 mm3. Spontaneously hypertensive rats, transiently ischemic, treated with saline had 164 +/- 25 mm3 of infarct volume, while those treated with 2 mg/kg i.v. had 151 +/- 53 mm3, and those treated with 10 mg/kg i.v. had 145 +/- 29 mm3. Animals treated with 40 mg/kg i.v. had a nonsignificantly larger mean infarct volume (191 +/- 81 mm3). High dose nitro-arginine caused an increase in hypertension in the spontaneously hypertensive rats and increased the severity of focal ischemia as measured by intra-ischemic regional cerebral blood flows. A final group of seven spontaneously hypertensive rats underwent permanent middle cerebral artery occlusion and repeated dosing with N-omega-nitro-L-arginine i.p. In these animals an infarct volume of 234 +/- 60 mm3 was observed, which was again not statistically different from saline-treated controls (208 +/- 43 mm3, n = 7).(ABSTRACT TRUNCATED AT 400 WORDS)

[Study on roles of L-arginine to nitric oxide pathway for the cardiovascular control: assessment with a new model of hypertension produced by the chronic administration of nitric oxide synthase inhibitor]

This study was aimed to make a new stable model of chronic hypertension by administration of a nitric oxide synthase inhibitor. L-Nw-nitroarginine methylester (NAME), and using this model, to investigate the roles of nitric oxide for the cardiovascular regulation. Male Wistar rats were implanted of osmotic pumps filled with saline (control group: n = 8), 0.2M NAME (low NAME group: n = 13) or 1M NAME (high NAME group: n = 12) intraperitoneally. After 4-week observation of blood pressure (BP) and heart rate (HR), blood concentrations of cathecholamine, active renin, L-arginine and L-Nw-nitroarginine were measured and histological changes in aorta and heart were examined. Age-matched SHRSP (n = 9) served as positive controls. BP elevated in the low NAME and high NAME group (113.9 +/- 3.1 to 144.0 +/- 4.4 mmHg, and 114.1 +/- 7.3 to 181.4 +/- 9.0 mmHg, respectively; mean +/- S.E.), while BP remained constant in the control group and SHRSP group (116.8 +/- 5.5 to 120.6 +/- 1.9 mmHg, 200.3 +/- 5.1 to 213.8 +/- 7.4 mmHg, respectively). HR in the low NAME and high NAME group rapidly decreased (not equal to 410 to not equal to 340 bpm) and then slowly returned to the control level. HR in the control group and SHRSP group remained constant (not equal to 420 and not equal to 450 bpm, respectively). Noradrenaline increased significantly in the high NAME group (0.21 +/- 0.03 ng/ml), and there were no significant changes in the control, low NAME and SHR group (0.13 +/- 0.02, 0.14 +/- 0.02, 0.15 +/- 0.03 ng/ml, respectively). Adrenaline, dopamine and active renin concentrations did not differ among 4 groups. Aortic wall/lumen area ratio in the high NAME group was similar to that in SHRSP group, in spite of its lower BP and shorter duration of hypertension compared with SHRSP. Left ventricular wall of the high NAME group was significantly thicker than that of SHRSP group. These findings suggest that, in addition to endothelium-derived NO, NO produced in the brain and peripheral neurons may function to regulate cardiovascular system by inhibiting noradrenaline release from sympathetic nerves or by inhibiting cardiac and vascular cell proliferation.

Nitric oxide synthase in the enteric nervous system of the guinea-pig: a quantitative description

The distribution and abundance of nitric oxide synthase (NOS)-containing neurons and their terminals in the gastrointestinal tract of the guinea-pig were examined in detail using NADPH diaphorase histochemistry and NOS immunohistochemistry. NOS-containing cell bodies were found in the myenteric plexus throughout the gastrointestinal tract and in the submucous plexus of the stomach, colon and rectum. NOS-containing neurons comprised between 12% (in the duodenum) and 54% (in the esophagus) of total myenteric neurons. In the ileum, NOS neurons represented 19% of total myenteric neurons. Most of the NOS neurons throughout the gastrointestinal tract possessed lamellar dendrites and a single axon. NOS-containing terminals were abundant in the circular muscle, including that of the sphincters, but were rare in the longitudinal muscle, except for the taeniae of the caecum. The muscularis mucosae of the esophagus, stomach, colon and rectum received a medium to dense innervation by NOS terminals. Within myenteric ganglia, NOS-containing terminals were extremely sparse in the esophagus, stomach and duodenum, common in the ileum and distal colon and extremely dense in the proximal colon and rectum. The submucous plexus in the ileum and large intestine contained a sparse plexus of NOS-containing terminals. NOS terminals were not observed in the mucosa of any region. We conclude that throughout the gastrointestinal tract of the guinea-pig, NOS neurons are inhibitory motor neurons to the circular muscle; in the ileum and large intestine, NOS neurons may also function as interneurons.

Age and development-related changes in osteopontin and nitric oxide synthase mRNA levels in human kidney proximal tubule epithelial cells: contrasting responses to hypoxia and reoxygenation

Osteopontin (OPN) encodes a secreted glycosylated phosphoprotein containing a GRGDS motif that can mediate cell attachment through the alpha v beta 3 integrin, and has recently been shown to down-regulate nitric oxide synthase (NOS) expression. We report here that primary cultures of renal proximal tubule epithelial (PTE) cells prepared from human kidneys of different developmental stages and ages show a positive correlation between developmental age and the expression, at the mRNA level, of both OPN and constitutive NOS. However, OPN and NOS responded in different manners, as assessed by mRNA measurements, to hypoxia-reoxygenation injury. The OPN mRNA level, assessed by Northern blotting, increased slightly during 60 min of hypoxia and more substantially during subsequent reoxygenation of primary PTE cells derived from the kidneys of young but not of aged donors. The abundance of NOS mRNA, measured using a cDNA probe to the constitutive form of the enzyme, was enhanced during hypoxia in kidneys derived from humans of all ages, and then decreased during reoxygenation--possibly as the result of increased OPN expression. PTE cells from aged kidneys are more susceptible to cell death under hypoxic conditions that PTE cells from young kidneys. An investigation of the effect of an oxidant on OPN and NOS mRNA levels revealed that within 30 min of exposure to H2O2, NOS mRNA levels decreased simultaneously with an increase in OPN mRNA levels. Nitric oxide (NO), the product of NOS, is at low levels an important signal transduction molecule participating in the regulation of vascular tone and renal reabsorption; at high levels it is cytotoxic. We suggest that the diminished ability of cells from old kidneys to down-regulate NO production and to increase OPN expression after hypoxia-reoxygenation may contribute to their increased susceptibility to oxidant injury.

NADPH-diaphorase histochemistry and nitric oxide synthase activity in deutocerebrum of the crayfish, Pacifastacus leniusculus (Crustacea, Decapoda)

The activity of an nitric oxide synthase in the deutocerebrum of the crayfish Pacifastacus leniusculus was investigated with histochemical and biochemical methods. By using the NADPH-diaphorase histochemical reaction, known as a selective marker for NO synthase in mammals, it was possible to localize specific neuronal elements in the crayfish. Pronounced diaphorase-staining was observed in peripheral olfactory sensory cells and in the neuropil of the olfactory lobes. Less intense diaphorase-staining also occurred in other deutocerebral neuropils, such as the accessory lobes, the lateral antennular neuropil and in the deutocerebral commissure neuropil. The biochemical assay revealed a calcium/calmodulin-dependent formation of citrulline from L-arginine in brain homogenate. It was also possible to show that the selective NO synthase inhibitor L-NOARG decreased the formation of citrulline. These data indicate a role for NO as an intercellular messenger in the crayfish.

Three distinct subpopulations of GABAergic neurons in rat frontal agranular cortex

GABAergic interneurons in the rat frontal cortex were subdivided on the basis of immunoreactivity for calcium binding proteins, neuropeptides and nitric oxide synthase, using double immunofluorescence and mirror image immunohistochemical methods. The results indicate that in this region of the neocortex there are at least three distinct subpopulations of local circuit neurons. The first subgroup consists of parvalbumin-immunoreactive cells. Those do not contain neuropeptide, calretinin or nitric oxide synthase immunoreactivity. A substantial number of parvalbumin-immunoreactive cells in layer II/III were also immunoreactive for calbindin D28k. The second subgroup consists of cells immunoreactive for calretinin. Most were usually immunoreactive for vasoactive intestinal polypeptide as well, but a few cells in layer II/III were immunoreactive for one or the other only. Calretinin-immunoreactive cells do not colocalize parvalbumin, somatostatin or nitric oxide synthase, and only a few colocalize calbindin D28k. The third subgroup consists of cells most of which contain somatostatin, and is entirely separate from the parvalbumin- and calretinin-immunoreactive populations. There was substantial colocalization of somatostatin and calbindin D28k and of somatostatin and neuropeptide Y. Some somatostatin-immunoreactive cells showed nitric oxide synthase immunoreactivity. All of the populations of immunoreactive cells examined in the present study also showed GABA immunoreactivity. About 10% of calbindin D28k-immunoreactive cells and all of those strongly stained for calbindin D28k in layer II/III showed GABA immunoreactivity. Most calbindin D28k-positive cells in deep layers also showed GABA immunoreactivity. These results support that almost all calbindin D28k-immunoreactive non-pyramidal cells are probably GABAergic.

Nitric oxide is required for tactile learning in Octopus vulgaris

Nitric oxide, produced by nitric oxide synthase in brain tissue, is essential for several different kinds of learning in vertebrates. We present the first evidence that it is also essential for learning in an invertebrate. Intramuscular injections of an inhibitor of the enzyme completely block touch learning in Octopus vulgaris. Eight control animals learned a touch paradigm, but none of eight synthase-inhibited ones learned it.

Importance of nitric oxide for local increases of blood flow in rat cerebellar cortex during electrical stimulation

The endothelium-derived relaxing factor, probably nitric oxide (NO), is a potent vasodilator that regulates the vascular tone in several vascular beds, including the brain. We explored the possibility that NO might be of importance for the increase of cerebral blood flow (CBF) associated with activity of the well-defined neuronal circuits of the rat cerebellar cortex. Laser-Doppler flowmetry was used to measure increases of cerebellar blood flow evoked by trains of electrical stimulations of the dorsal surface. The evoked increases of CBF were frequency-dependent, being larger on than off the parallel fiber tracts, suggesting that conduction along parallel fibers and synaptic activation of target cells were important for the increase of CBF. This was verified experimentally since the evoked CBF increases were abolished by tetrodotoxin and reduced by 10 mM Mg2+ and selective antagonists for non-N-methyl-D-aspartate receptors. The cerebellar cortex contains high levels of NO synthase. This raised the possibility that NO was involved in the increase of CBF associated with neuronal activation. NO synthase inhibition by topical application of NG-nitro-L-arginine attenuated the evoked CBF increase by about 50%. This effect was partially reversed by pretreatment with L-arginine, the natural substrate for the enzyme, while NG-nitro-D-arginine, the inactive enantiomer, had no effect on the evoked CBF increases. Simultaneous blockade of non-N-methyl-D-aspartate receptors and NO synthase had no further suppressing effect on the blood flow increase than either substance alone, suggesting that the NO-dependent flow rise was dependent on postsynaptic mechanisms. These findings are consistent with the idea that local synthesis of NO is involved in the transduction mechanism between neuronal activity and increased CBF.

[Clinical significance of nitric oxide in hypertension]

Vascular endothelial cells produce various biologically active factors regulating blood pressure, coagulation, and possibly cell growth of the vascular wall. Of the factors, nitric oxide (NO) has been the object of attention because of its quite simple molecular structure and variety of biological functions. In the present review, we focused on the physiologic and pathologic aspects of NO in hypertension. In experimental animals, both acute and chronic inhibition of NO synthase (NOS) with arginine derivatives produce a significant rise in blood pressure, indicating that tonic production of NO regulates basal vascular tonus. The chronic hypertension caused by NOS inhibitor is associated with cardiac hypertrophy and renal insufficiency. Sodium retention, though transient, and the plasma and tissue renin/angiotensin system in addition to the reduced production of NO have been implicated in the development of hypertension. Hypertension and the associated target organ failure can be reversed by co-administration of L-arginine or blockades of the renin/angiotensin system. Studies in which L-arginine as the substrate of NO or NOS inhibitor was administered demonstrated an important role of NO in the regulation of tonic vascular tonus also in normal subjects. In hypertensive subjects, however, endothelium-dependent vasorelaxation and production of NO are impaired, possibly due to a deficiency of L-arginine and/or a disorder of its utilization. Recent advances in the methods of detecting NO enabled us to demonstrate its diminished production from endothelial cells of hypertensive rats in vitro, although no definite biochemical evidence has been obtained in hypertensive subjects. The endothelial dysfunction, however, is not a primary cause of hypertension but a secondary result since it is commonly observed in various types of hypertension and can be reversed by correcting the blood pressure. Other common diseases including atherosclerosis and diabetes mellitus are also associated with similar abnormalities of the endothelium. NO has anti-atherogenic actions: inhibition of platelet functions and proliferation of vascular smooth muscle cells. Therefore, potentiation of endogenous NO and/or supplement of exogenous NO donors could be novel therapeutic approaches for the treatment of hypertension and atherosclerosis, while potential adverse effects of NO including cytotoxicity, immunosuppressibility, and hypotensive shock should be taken into account.

Dorsal root ganglion neurons of embryonic chicks contain nitric oxide synthase and respond to nitric oxide

We investigated the function of nitric oxide (NO) in dorsal root ganglion (DRG) neurons from 10 day embryonic chicks and adult birds. NADPH-diaphorase activity, a histochemical marker for nitric oxide synthase (NOS) in paraformaldehyde-fixed neurons, and NOS-like immunoreactivity were localized in all neurons in thoracic and lumbar ganglia from embryos. However, only a subset of neurons from adults contained NOS-like immunoreactivity and NADPH-diaphorase activity. Thus, embryonic chick DRG neurons have the potential to synthesize NO in response to elevated cytoplasmic Ca2+. We also investigated the ability of dissociated embryonic chick DRG neurons to respond to NO by examining the effects of NO donors and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) on Ca2+ current (ICa) using the amphotericin-permeabilized patch-clamp technique: sodium nitroprusside (5 microM) reduced ICa to 0.68 +/- 0.06 (mean +/- S.D., n = 5) of control, S-nitroso-N-acetylpenicillamine (1 microM) reduced ICa to 0.44 +/- 0.06 (n = 4) of control, while 8-Br-cGMP (1 mM) reduced ICa to 0.58 +/- 0.22 (n = 5) of control. ICa was reduced in every neuron tested and this effect was partially reversed after approximately 10 min of washing. Thus, ICa of embryonic chick DRG neurons is inhibited by NO, possibly by a cGMP-dependent mechanism. These results indicate that all DRG neurons in embryonic chicks contain NOS-like immunoreactivity and respond to NO. Further, the percentage of NADPH-diaphorase positive neurons is reduced during development.

Presence of constitutive type nitric oxide synthase in cultured astrocytes isolated from rat cerebra

To define whether astrocytes express a constitutive nitric oxide synthase (NOS), nitric oxide (NO) producing activity in astrocytes derived fetal rat cerebra was examined. We found that the addition of an NOS inhibitor to cultures caused decrease in the basal cGMP levels in unstimulated astrocytes and the decrease was dose-dependent. Further, the expression of cNOS activity in unstimulated astrocytes was confirmed by histochemical staining for NADPH diaphorase and by measuring conversion of L-arginine to L-citrulline. We conclude that cultured astrocytes express constitutive NOS, in addition to inducible one.

VIP enhances and nitric oxide synthase inhibitor reduces survival of rat lungs perfused ex vivo

VIP delayed the onset of edematous lung injury in isolated perfused rat lungs by 64%, and was more effective than PGI2 in prolonging lung survival ex vivo. While PGI2 increased survival by 37 min versus Krebs/BSA only, VIP increased it by 2 h and 17 min. On the other hand, inhibition of NO. synthase, which protected the lung against oxidant injury caused by paraquat or X/XO,3 actually hastened the onset of injury caused by prolonged perfusion ex vivo, suggesting opposite roles for NO. in different forms of oxidant injury.

Astrocyte-derived lipoxygenase product evokes endothelium-dependent relaxation of the basilar artery

The goal of this study was to examine the possible production of vasoactive factors by astrocytes. We consistently observe that rat astroglial cells in suspension produce marked relaxation when added to precontracted rings of intact (but not endothelium-denuded) rabbit basilar artery. The ultimate mediator of this relaxation was endothelium-derived nitric oxide whose synthesis is activated by an as yet unidentified factor(s) produced tonically by astrocytes. The factor is relatively stable, and is not arachidonate, or a product of cyclooxygenase or P450 metabolism. Based upon studies with selective inhibitors, the factor appears to result from 12- or 15-lipoxygenase metabolism, the products of which are known to be vasoactive. In a separate series of experiments, astrocyte-conditioned medium stimulated the production of citrulline from L-arginine by nitric oxide synthase in bovine aortic endothelial cells. The possible significance for central nervous system (CNS) pathophysiology of an astrocyte-derived vasodilator is discussed.

Modification of metabolism of transplantable and spontaneous murine tumors by the nitric oxide synthase inhibitor, nitro-L-arginine

PURPOSE

To determine the effects of the nitric oxide synthase inhibitor, nitro-L-arginine on energy metabolism in transplantable and spontaneous murine tumors.

METHODS AND MATERIALS

The responses of the transplantable murine tumor SCCVII/Ha and a range of spontaneously arising murine mammary adenocarcinomas to 10 mg/kg IV nitro-L-arginine were examined using in vivo 31P magnetic resonance spectroscopy (MRS). The influence of Hypnorm/Hypnovel anesthesia on the response to nitro-L-arginine was also determined in the SCCVII/Ha tumors. Data were expressed as changes in the inorganic phosphate peak area relative to the sum of all peak areas from the 31P MR spectrum, or Pi/total.

RESULTS

Nitro-L-arginine at 10 mg/kg IV increased Pi/total 2-3-fold in the SCCVII/Ha tumors for at least 2 h after administration, in both anesthetized and nonanesthetized mice, consistent with increased tumor hypoxia. Similar increases in Pi/total were observed after 10 mg/kg IV nitro-L-arginine in 13 spontaneous murine tumors from three different mouse strains, where anesthetic was used.

CONCLUSION

The results indicate that tumor metabolism may be modified by an inhibitor of nitric oxide synthesis, that this modification occurs in both transplantable and spontaneous murine tumors and is not affected by anesthetic.

Nitric oxide limits transcriptional induction of nitric oxide synthase in CNS glial cells

Nitric oxide (NO) can modulate directly and indirectly the activity of a variety of enzymes, including nitric oxide synthase (NOS). In addition, it appears that the NO formed by astroglial cells in which transcriptional induction of NOS has been evoked by cytokines affects NOS mRNA level. We have observed that induction in the presence of NOS inhibitors or NO-trapping agents amplifies NOS mRNA expression and that this effect is reversed by an NO donor. Rather than promoting mRNA instability, NO appears to inhibit the transcriptional induction of NOS. This effect of NO on induction of the NOS gene provides a mechanism by which the temporal and perhaps spatial production in the nervous system of NO, a reactive and potentially toxic mediator, can be finely regulated.

Activation of myosin light chain kinase and nitric oxide synthase activities by calmodulin fragments

We have investigated the abilities of calmodulin (CaM) tryptic fragments 1-75 (TRCI) or 78-148 (TRCII) to activate gizzard smooth muscle myosin light chain kinase (gMLCK), rabbit skeletal muscle myosin light chain kinase (skMLCK), and neural nitric oxide synthase (nNOS) activities. Our results indicate for all three enzymes that binding of CaM follows an ordered mechanism wherein the C-terminal lobe, represented by TRCII, binds specifically to a site we designated as A, followed by binding of the N-terminal lobe, represented by TRCI, to a site designated as B. With TRCII and TRCI bound to their respective sites, skMLCK and gMLCK activities are both activated to about 80% of their maximum levels. Occupancy of both sites in the MLCK enzymes by TRCI results in only low levels of enzyme activation; occupancy of both sites by TRCII also results in low levels of gMLCK activity, but activates skMLCK activity to 65% of the maximum level. With TRCI bound at site B and either TRCII or TRCI bound at site A, nNOS activity is 50% of the maximum level. Apparent dissociation constants for TRCII binding to site A and TRCI binding to site B are, respectively; 0.3 and 3 microM (skMLCK); 1.2 and 0.8 microM (gMLCK); 10 nM and 150 microM (nNOS). Our results demonstrate that the CaM lobes can make distinct contributions to binding and/or activation of different CaM-dependent enzymes and that the tethering function of the central helix can be mimicked by sufficiently high concentrations of the CaM fragments. We have modeled tethering as if it stabilizes the CaM-enzyme complex by creating a high effective concentration of the N-terminal lobe. Calculated values for this concentration term indicate essentially identical contributions by the central helix to the observed nanomolar dissociation constants of the three CaM-enzyme complexes examined.

Vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide-dependent activation of membrane-bound NO synthase in smooth muscle mediated by pertussis toxin-sensitive Gi1-2

Plasma membranes isolated from dispersed gastric muscle cells exhibited calmodulin-dependent NOS activity that was stimulated by Ca2+ in the range 0.1-1 mM (maximum 10 microM). Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) (in the presence of GTP), and GTP gamma S (guanosine 5'-O-(gamma-thio)triphosphate) stimulated NOS activity in a concentration-dependent fashion above that maximally stimulated by Ca2+. The increase in NOS activity induced by VIP, PACAP, and GTP gamma S was abolished by GDP beta S (guanosine 5'-O-(beta-thio)diphosphate), which had no effect on NOS activity stimulated by Ca2+. The NOS inhibitor NG-nitro-L-arginine and the calmodulin antagonist calmidazolium abolished NOS activity stimulated by all agents including Ca2+. NOS activity stimulated by GTP gamma S, VIP, and PACAP was inhibited by Gi alpha 1-2 antibody but not by Gq alpha, Gs alpha, and Gi alpha 3 antibodies. NOS activity stimulated by VIP and PACAP was inhibited by 80-83% in membranes derived from pertussis toxin-treated cells. We conclude that a Ca2+/calmodulin-dependent NOS present in plasma membranes of gastric muscle cells is activated by two homologous peptide transmitters, VIP and PACAP, via a common receptor coupled to pertussis toxin (PTx)-sensitive Gi1-2. The study provides the first evidence of receptor-mediated G protein activation of NOS in smooth muscle cells.

Two distinct signaling pathways trigger the expression of inducible nitric oxide synthase in rat renal mesangial cells

The expression of nitric oxide synthase (NOS; EC 1.14.13.39) is induced in rat glomerular mesangial cells by exposure to the inflammatory cytokine interleukin 1 beta (IL-1 beta) or cAMP-elevating agents. Stimulation with IL-1 beta alone leads to an approximately 40-fold increase in NOS activity and nitrite synthesis, whereas the elevation of cAMP with forskolin, cholera toxin, salbutamol, or dibutyryl-cAMP for 24 h resulted in a 2- to 12-fold increase in NOS activity. Moreover, the combinations of IL-1 beta with each of the cAMP-elevating agents greatly enhanced NOS activity in a synergistic fashion. Northern-blot analysis demonstrated a single band of approximately 4.5 kb for the NOS mRNA in rat mesangial cells. IL-1 beta increased NOS mRNA levels in a dose- and time-dependent fashion with a peak of NOS mRNA at 24 h. Dibutyryl-cAMP also increased NOS mRNA levels in mesangial cells in a dose- and time-dependent manner. Furthermore, combination of IL-1 beta and forskolin revealed a strong synergy with maximal mRNA levels 12 h after stimulation. Nuclear run-on transcription experiments suggest that IL-1 beta and cAMP synergistically interact to increase NOS gene expression at the transcriptional level. Furthermore, message stability studies established that NOS mRNA induced by cAMP has a longer half-life than the IL-1 beta-induced message. Moreover, cAMP exposure markedly prolonged the half-life of NOS mRNA from 1 h to 3 h. These data suggest that the level of NOS mRNA is controlled by at least two different signaling pathways, one involving cAMP and the other being triggered by cytokines such as IL-1 beta. The two pathways act synergistically and thus potently up-regulate the expression of inducible NOS in rat mesangial cells.