Hydroxyl radical-dependent inactivation of guanylate cyclase in cerebral arterioles by methylene blue and by LY83583

Roles for NAD(P)H oxidases and reactive oxygen species in vascular oxygen sensing mechanisms

Observations that physiological levels of O2 control the rates of production of reactive O2 species by systems including NAD(P)H oxidases and that certain of these species have signalling mechanisms that regulate vascular tone has resulted in consideration of these systems in processes that mediate the sensing of changes in P(O2). Evidence exists for the participation of hydrogen peroxide-dependent regulation of prostaglandin production and soluble guanylate cyclase activity, resulting from the metabolism of peroxide by cyclooxygenase and catalase, respectively, in P(O2)-elicited signalling mechanisms that regulate vascular force generation. A microsomal NADH oxidase whose activity is controlled by the redox status of cytosolic NAD(H) appears to function as a P(O2) sensor in bovine pulmonary and coronary arteries where changes in O2 levels control the production of superoxide anion-derived hydrogen peroxide and a cGMP-mediated relaxation response. Interactions with nitric oxide and superoxide anion, and the activity of glutathione peroxidase appear to influence the function of these O2 sensing systems, and some of these interactions, along with the activation of other oxidases, may contribute to alterations in P(O2) sensing mechanisms under pathophysiological conditions that affect vascular function.

Antioxidant therapy against cerebral vasospasm following aneurysmal subarachnoid hemorrhage

1. Approximately one-third of the morbidity and mortality due to aneurysmal subarachnoid hemorrhage (SAH) is caused by delayed ischemic neurological deficit (DIND) due to cerebral vasospasm. 2. Compared to prolonged arterial constriction in other parts of the body, cerebral vasospasm is characterized by its long duration and refractoriness to vasodilators such as calcium antagonists. 3. Whereas oxyhemoglobin (oxyHb) liberated into the CSF from the subarachnoid clot has been deemed the causative agent of vasoconstriction, the biochemical mechanisms whereby oxyHb elicits prolonged constriction of the cerebral arteries has remained elusive. Here, we suggest that oxyHb triggers the generation of reactive oxygen intermediates (ROI) within the CSF. 4. Multiple lines of evidence indicate that the occurrence of vasospasm, namely, prolonged smooth muscle contraction, is due to the following intracellular events. 5. First, hydroxyl radicals (OH*), the most reactive species of ROI, are generated within the cerebral arterial wall via the Fenton and Haber-Weiss reactions catalyzed by oxyHb. Second, subsequent peroxidative membrane damage in the arterial smooth muscle cell enhances the metabolism of phosphatidylcholine and phosphatidylethanolamine, leading to a rise in the intracellular level of diacylglycerol, an endogenous activator of protein kinase C. 6. The prolonged arterial contraction that occurs during vasospasm is attributable primarily to the activation of protein kinase C, not to the Ca2+/calmodulin system. In this article, literature relevant to the above thesis is reviewed, and the rationale for the antioxidant therapy against cerebral vasospasm is discussed.

Effects of the superoxide dismutase-mimic compound TEMPOL on oxidant stress-mediated endothelial dysfunction

The aim of this study was to investigate the effects of oxidant stress on endothelium-dependent and endothelium-independent arterial relaxation. For this, oxidant stress was generated by preincubation of rat aortic rings (RARs) in either 25 mM glucose (mimicking hyperglycemic stress) or 0.5 mM pyrogallol (a superoxide generator) and the effects of the superoxide dismutase (SOD)-mimetic compound 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy free radical (TEMPOL) on the vasorelaxant and cGMP-producing effects of acetylcholine (ACh) and glyceryl trinitrate (GTN) in control RARs and RARs exposed to oxidant stress were examined. Pyrogallol, and to a lesser extent high glucose concentration, enhanced the contractile response of RARs to phenylephrine and markedly inhibited the vasorelaxant response to ACh. Although they existed, the inhibitory effects of high glucose and pyrogallol on the vasorelaxant response to GTN were less profound, especially with pyrogallol. Moreover, both pyrogallol and high glucose concentration inhibited the basal and the ACh-induced vascular cyclic guanosine monophosphate (cGMP) production. Treatment with TEMPOL (1-5 mM) slightly increased the ACh and GTN-induced cGMP levels in control RARs but had a significant effect in high glucose and pyrogallol-pretreated RARs. Additionally, concomitant treatment of RARs with TEMPOL (5 mM) abolished the difference in the relaxation response between control RARs and RARs exposed to either pyrogallol or high glucose concentration. These results further support the theory that reactive oxygen species (ROS), especially superoxide, play a key role in mediation of endothelial dysfunction accompanying diabetes, probably through their effects on the ability of the endothelium to synthesize, release or respond to endogenous nitric oxide (NO) or NO donated by nitrovasodilators.

Studies of the role of endothelium-dependent nitric oxide release in the sustained vasodilator effects of corticotrophin releasing factor and sauvagine

1. The mechanisms of the sustained vasodilator actions of corticotrophin-releasing factor (CRF) and sauvagine (SVG) were studied using rings of endothelium de-nuded rat thoracic aorta (RTA) and the isolated perfused rat superior mesenteric arterial vasculature (SMA). 2. SVG was approximately 50 fold more potent than CRF on RTA (EC40: 0.9 +/- 0.2 and 44 +/- 9 nM respectively, P < 0.05), and approximately 10 fold more active in the perfused SMA (ED40: 0.05 +/- 0.02 and 0.6 +/- 0.1 nmol respectively, P < 0.05). Single bolus injections of CRF (100 pmol) or SVG (15 pmol) in the perfused SMA caused reductions in perfusion pressure of 23 +/- 1 and 24 +/- 2% that lasted more than 20 min. 3. Removal of the endothelium in the perfused SMA with deoxycholic acid attenuated the vasodilatation and revealed two phases to the response; a short lasting direct action, and a sustained phase which was fully inhibited. 4. Inhibition of nitric oxide synthase with L-NAME (100 microM) L-NMMA (100 microM) or 2-ethyl-2-thiopseudourea (ETPU, 100 microM) had similar effects on the vasodilator responses to CRF as removal of the endothelium, suggesting a pivotal role for nitric oxide. However the selective guanylate cyclase inhibitor 1H-[l,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ, 10 microM) did not affect the response to CRF. 5. High potassium (60 mM) completely inhibited the vasodilator response to CRF in the perfused SMA, indicating a role for K channels in this response. 6. Compared to other vasodilator agents acting via the release of NO, the actions of CRF and SVG are strikingly long-lasting, suggesting a novel mechanism of prolonged activation of nitric oxide synthase.

Inhibition of nitric oxide formation and superoxide generation during reduction of LY83583 by neuronal nitric oxide synthase

6-Anilino-5,8-quinolinedione (LY83583) has been widely used as an agent to reduce levels of nitric oxide (NO)-dependent cGMP in tissues. We report here that suppression of NO formation and production of superoxide during enzymatic reduction of LY83583 by neuronal NO synthase appeared to be potentially involved in the pharmacological action caused by LY83583. LY83583 suppressed neuronal NO synthase activity of 20,000 x g rat cerebellar supernatant preparation in a concentration-dependent manner (IC50 value = 12.9 microM). A kinetic study revealed that LY83583 is a competitive inhibitor with respect to NADPH, with a Ki value of 2.57 microM. With purified neuronal NO synthase it was found that LY83583 was a potent inhibitor of NO formation by the enzyme and served as efficient substrate for reduction with a specific activity of 173 nmol of NADPH oxidized per mg of protein per minute. The reductase activity was stimulated about 19.8-fold by addition of CaCl2/calmodulin, indicating that the presence of CaCl2/calmodulin is essential to express maximal activity of LY83583 reduction. Although LY83583 was a good substrate for both NADPH-cytochrome P450 reductase (P450 reductase) and DT-diaphorase, these flavin enzymes-catalyzed reductions of LY83583 were less than the neuronal NO synthase-mediated reduction in the presence of CaCl2/calmodulin. Enzymatic generation of superoxide during reduction of LY83583 by neuronal NO synthase, P450 reductase or DT-diaphorase was confirmed by electron spin resonance (ESR) experiments. Thus the present results indicate that a benzoquinone derivative LY83583 appears to interact with the P450 reductase domain on neuronal NO synthase, resulting in inhibition of NO formation and superoxide generation, which is involved in suppression of intracellular cGMP content.

Comparison of two soluble guanylyl cyclase inhibitors, methylene blue and ODQ, on sodium nitroprusside-induced relaxation in guinea-pig trachea

To clarify further the role of cyclic GMP in mediating the relaxant response in guinea-pig trachea induced by sodium nitroprusside (SNP), the effects of soluble guanylyl cyclase inhibitors, methylene blue and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) on SNP-induced muscle relaxation and cyclic GMP accumulation were determined. SNP (0.3-100 microM) evoked a concentration-dependent relaxation of guinea-pig isolated tracheas precontracted with 0.3 microM carbachol. Preincubation of the preparations with methylene blue (10, 30 and 100 microM) resulted in a slight but concentration-dependent prevention of the relaxant response to SNP. In contrast, the relaxation to SNP was extensively prevented by 3 microM ODQ and almost abolished by 10 microM ODQ. SNP (30 microM) induced a significant elevation of cyclic GMP accumulation (from 1.34+/-0.14 to 5.39+/-0.28 pmol mg(-1) protein, n= 5; P<0.001), which was partially attenuated by 100 microM methylene blue (3.11+/-0.51 pmol mg(-1) protein, n=5; P<0.05), whereas completely abolished by 10 microM ODQ (1.31+/-0.28 pmol mg(-1) protein, n = 5; P<0.001). Methylene blue, but not ODQ and Nomega-nitro-L-arginine methyl ester (L-NAME), caused a concentration-dependent contraction in the tracheal preparation. The tension produced by 100 microM methylene blue was 41.8+/-4.3% (0.3 microM carbachol as 100%; n = 12). Moreover, the non-selective muscarinic receptor antagonist atropine and the M3-selective antagonist 4-diphenylacetoxy-N-methylpiperidine methiodine greatly inhibited the contractile effect evoked by methylene blue (100 microM). In conclusion, this study provides substantial evidence that SNP-induced muscle relaxation in guinea-pig trachea is completely via a cyclic GMP-dependent mechanism. Furthermore, ODQ, but not methylene blue, will likely become an important tool in differentiating between cyclic GMP-dependent and -independent effects of nitric oxide.

Responses of human basilar and other isolated arteries to novel nitric oxide donors

The adducts of nitric oxide (NO), diethylamine/NO (DEA/NO) and diethylenetriamine/NO (DETA/NO), are new NO donors that spontaneously release NO in aqueous solutions. These donors may have therapeutic advantages over sodium nitroprusside (SNP), which depends on metabolism to yield NO. This study was performed to define and compare the pharmacodynamic properties of these NO donors on isolated rings of human, canine, and porcine basilar arteries and further to compare canine and porcine common carotid arteries precontracted with KCl. The median effective concentration (EC50) and the basic effect of 100 microM were determined for each NO donor. On basilar arteries, DEA/NO was the most potent but the maximal dilatation produced by 100 microM did not persist for 60 min, whereas that of DETA/NO and SNP did. DETA/NO was more potent than SNP on all three species of basilar arteries but was the least potent on peripheral (carotid) arteries. Methylene blue in equimolar concentrations significantly inhibited the vasorelaxant effects of DEA/NO and DETA/NO, suggesting a common mechanism of action. Of the NO donors studied, the pharmacodynamic properties of DETA/NO seemed most relevant clinically as a cerebrovascular dilator in being more potent than SNP while producing sustained vasorelaxation.

Nitric oxide-independent activation of soluble guanylyl cyclase contributes to endotoxin shock in rats

We investigated whether a complete inhibition of nitric oxide (NO) formation caused by bacterial endotoxin (lipopolysaccharide, LPS) in vivo prevents the hypotension and restores the vascular hyporeactivity to normal in vivo and ex vivo. The combination of dexamethasone (Dex; 3 mg/kg at 30 min before LPS) plus aminoguanidine (AG; 15 mg/kg at 2 h after LPS) inhibited the overproduction of nitrate (an indicator of NO) in the plasma and aortic smooth muscle and also prevented the development of the delayed hypotension in rats treated with LPS for 6 h. However, the vascular hyporeactivity to norepinephrine (NE) was only partially improved either in vivo or ex vivo in endotoxemic rats treated with Dex plus AG. Pretreatment of aortic rings with Nomega-nitro-L-arginine methyl ester (L-NAME) or 1H-[1,2, 4]oxidazolo[4,3-a]quinoxalin-1-one (ODQ) enhanced the contraction to NE in rings obtained from LPS-treated rats, but not in those from Dex plus AG-treated endotoxemic rats. Methylene blue, an inhibitor of soluble guanylyl cyclase (GC), completely restored contractions to NE and aortic cGMP levels to normal either in LPS-treated rats or in Dex plus AG-treated endotoxemic rats, whereas the cGMP level was partially inhibited by ODQ in LPS-treated rats only. These results suggest that non-NO mediator(s) also activates soluble GC during endotoxemia. Interestingly, we found that in the presence of tetraethylammonium (an inhibitor of K+ channels) plus L-NAME or charybdotoxin [a specific inhibitor of large-conductance Ca2+-activated K+ (KCa) channels] plus ODQ, the vascular hyporeactivity to NE in the LPS-treated group was also completely restored to normal. In addition, in the presence of L-NAME or ODQ, the vascular hyporeactivity to high K+ was abolished in rings from the LPS-treated group. These results suggest that LPS causes the production of other mediator(s), in addition to NO, which also stimulates soluble GC (i.e., increases the formation of cGMP) and then activates the large-conductance KCa channels in the vascular smooth muscle causing vascular hyporeactivity.

Effect of reduced cerebral perfusion pressure on cerebral blood flow following inhibition of nitric oxide synthesis

OBJECT

The authors tested the hypothesis in a porcine model that inhibition of nitric oxide synthesis during reduced cerebral perfusion pressure (CPP) affected the relative cerebral blood flow (CBF) and the cerebrovascular resistance.

METHODS

The CPP was reduced by inducing high cerebrospinal fluid pressure and hemorrhagic hypotension. With continuous blood and intracranial pressure monitoring, relative CPP was estimated using the laser Doppler flowmetry technique in nine pigs that received 40 mg/kg nitro-L-arginine methyl ester (L-NAME) and in nine control animals. The L-NAME caused a decrease in relative CBF (p < 0.01) and increases in cerebrovascular resistance (p < 0.01), blood pressure (p < 0.05), and CPP (p < 0.001). During high intracranial pressure there were no significant differences between the treated animals and the controls. After hemorrhage, there was no significant difference between the groups initially, but 30 minutes later the cerebrovascular resistance was decreased in the control group and increased in the L-NAME group relative to baseline (p < 0.05). Combined hemorrhage and high intracranial pressure increased the difference between the two groups with regard to cerebrovascular resistance (p < 0.05).

CONCLUSIONS

These results suggest that nitric oxide synthesis inhibition affects the autoregulatory response of the cerebral circulation after cardiovascular compensation has taken place. Nitric oxide synthesis inhibition enhanced the undesirable effects of high intracranial pressure during hypovolemia.

Reversal of cerebral vasospasm using an intrathecally administered nitric oxide donor

OBJECT

Intrathecal bolus administration of (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)aminio]diazen++ +-1-ium-1,2-diolate (DETA/NO), a long half-life diazeniumdiolate-class nitric oxide (NO) donor, was evaluated for safety and efficacy in the treatment of delayed cerebral vasospasm in a canine model of subarachnoid hemorrhage (SAH).

METHODS

The baseline basilar artery (BA) diameter of 25 dogs was measured with the aid of angiography on Day 0. Vasospasm was then induced by intracisternal injection of autologous arterial blood on Days 0 and 2. Repeated arteriography on Day 7 revealed an average BA diameter of 58% of baseline. Each dog was then randomized to one of four groups: a pathology control group (SAH only, four animals); a treatment control group (SAH plus 2 micromol of the inactive drug carrier DETA, eight animals); a low-dose treatment group (SAH plus 0.2 micromol DETA/NO, six animals); or a high-dose treatment group (SAH plus 2 micromol DETA/NO, six animals). The drugs were administered in a 2-ml intrathecal bolus via the cisterna magna. Arterial caliber was monitored by angiography over the subsequent 4 hours. A 2-micromol dose of the drug was then given and serial arteriography continued for an additional hour to screen for tachyphylaxis. Intracranial pressure and respiratory and hemodynamic parameters were continuously monitored. Histopathological analyses of the animals' brains were performed after the dogs were killed on Day 8. The drug DETA/NO produced reversal of vasospasm in a dose-dependent fashion that roughly followed a double exponential time course. Doses of 2 micromol DETA/NO resulted in restoration of the angiographically monitored BA diameter to the prevasospasm size at 1.5 hours posttreatment, and this was sustained at 88% of baseline at 4 hours (p < 0.01, independent samples t-test). By contrast, the treatment control group remained on average at 54% of baseline diameter. The low-dose treatment group achieved only partial and more transitory relaxation. Histopathological analyses showed findings consistent with chronic SAH but did not demonstrate any toxicity associated with the NO donor. No adverse physiological changes were seen.

CONCLUSIONS

This study indicates that long-acting NO donors are potentially useful as agents to restore circulation in patients suffering from cerebral vasospasm.

The nitric oxide/cyclic GMP pathway: a potential major regulator of cochlear physiology

The nitric oxide (NO)/cyclic guanosine monophosphate (GMP) pathway is now recognized as a major regulatory system in cell physiology and tissue homeostasis. This pathway may control processes as diverse as muscle relaxation, gut peristalsis, neurotransmission and hormonal secretion. It is also involved in the development and function of sensory systems such as vision and olfaction. This review will detail the NO/cyclic GMP pathway, evaluate studies in the auditory system and discuss its potential participation in cochlear blood flow, supporting cell physiology and excitotoxicity.

Mild hypothermia can attenuate nitroglycerin-induced vasodilation of pial arterioles in the cat

The purpose of the present study was to investigate the effect of mild hypothermia on nitroglycerin-induced vasodilation of cerebral vessels. The cranial window technique, combined with microscopic video recording, was used in an experiment involving 26 cats anesthetized with isoflurane. Animals were randomly assigned to either a normothermic or a mildly hypothermic group (33 degrees C). We administered three different concentrations of nitroglycerin (10[-6], 10[-5], 10[-4] M) under the window and measured the diameter of small (< 100 microm) and large (100-200 microm) pial arterioles. In the normothermic group (n = 13), nitroglycerin produced a significant dilation of both small and large arterioles in a dose-dependent manner. In the hypothermic group (n = 13), a significant dilation of arterioles was observed only after topical application of nitroglycerin at a concentration of 10(-4) M. The percent increase in diameter of small and large arterioles was less in the hypothermic group than the normothermic group. Our in vivo study demonstrates that topically applied nitroglycerin produces a dose-dependent dilation of pial arterioles in normothermic cats anesthetized with isoflurane, but the reduction of temperature to 33 degrees C significantly attenuates nitroglycerin-induced vasodilation of pial arterioles.

IMPLICATIONS

Although nitroglycerin may be used in hypothermic patients, the effect of mild hypothermia on nitroglycerin-induced vasodilation of cerebral vessels is unknown. In this study, we investigated the effects of nitroglycerin on pial arteriolar diameter in normothermic and hyperthermic cats. Hypothermia was found to attenuate nitroglycerin-induced vasodilation.

Roles of calcium-activated and voltage-gated delayed rectifier potassium channels in endothelium-dependent vasorelaxation of the rabbit middle cerebral artery

1. The cellular mechanism(s) of action of endothelium-derived vasodilator substances in the rabbit middle cerebral artery (RMCA) were investigated. Specifically, the subtypes of potassium channels involved in the effects of endothelium-derived relaxing factors (EDRFs) in acetylcholine (ACh)-induced endothelium-dependent vasorelaxation in this vessel were systematically compared. 2. In the endothelium-intact RMCA precontracted with histamine (3 microM), ACh induced a concentration-dependent vasorelaxation, which was sensitive to indomethacin (10 microM) or N(G)-nitro-L-arginine (L-NOARG; 100 microM); pD2 values 8.36 vs 7.40 and 6.38, P < 0.01 for both, n = 6 and abolished by a combination of both agents. ACh caused relaxation in the presence of high K+ PSS (40 mM KCl), which was not affected by indomethacin, but abolished by L-NOARG and a combination of indomethacin and L-NOARG. 3. In the presence of indomethacin, relaxation to ACh in the endothelium-intact RMCA precontracted with histamine was unaffected by either glibenclamide (10 microM), an ATP-sensitive K+ channel (K[ATP]) blocker, 4-aminopyridine (4-AP, 1 mM) or dendrotoxin (DTX, 0.1 microM), delayed rectifier K channel (Kv) blockers. However, relaxation responses to ACh were significantly inhibited by either LY83583 (10 microM) and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 microM), guanylyl cyclase inhibitors, or charybdotoxin (CTX; 0.1 microM), iberiotoxin (ITX, 0.1 microM) and apamin (APA, 0.1 microM), large conductance Ca2+-activated K+ channels (BK[Ca]) blocker and small conductance Ca2+-activated K+ channel (SK[Ca]) blocker, respectively. 4. In the presence of L-NOARG, relaxation to ACh was unaffected by glibenclamide or the cytochrome P450 mono-oxygenase inhibitor, clotrimazole (1 microM), but was significantly inhibited by either 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 10 microM) and 2',3'-dideoxyadenosine (2',3'-DDA, 30 microM), adenylyl cyclase inhibitors, or 4-AP, DTX, CTX, ITX and APA. 5. In the endothelium-denuded RMCA precontracted with histamine, authentic NO-induced relaxation was unaffected by glibenclamide, 4-AP and DTX, but significantly reduced by ODQ, ITX and APA. Authentic prostaglandin I2 (PGI2)-induced relaxation was unaffected by glibenclamide, but significantly reduced by 2',3'-DDA, 4-AP, DTX, ITX and APA. Forskolin-induced relaxation was significantly inhibited by high K+, CTX and 4-AP. 6. These results indicate that: (1) in the RMCA the EDRFs released by ACh are NO and a prostanoid (presumably PGI2), and there is no evidence for the release of a non-NO/PGI2 endothelium-derived hyperpolarizing factor (EDHF), (2) K(Ca) channels are involved in NO-mediated relaxation of the RMCA but both K(Ca) and Kv channels are involved in PGI2-mediated relaxation.

Guanylyl cyclase inhibition reduces contractility and decreases cGMP and cAMP in isolated rat hearts

OBJECTIVE

Several studies have shown that increasing cGMP in the heart reduces contractility, however, decreasing basal cGMP levels have also been shown in some studies to reduce contractility. This study was designed to evaluate the hypothesis that decreasing basal levels of cGMP decreases ventricular contractility, and that this depressed function is associated with a decrease in cAMP.

METHODS

Using paced, constant flow, buffer-perfused rat hearts, we determined the effects of intracoronary infusions of the guanylyl cyclase inhibitor, LY83583 (10(-5) M), on ventricular function, oxygen consumption, and ventricular content of cGMP and cAMP. These experiments were conducted in the absence and presence of isoproterenol (ISO) to increase baseline left ventricular developed pressure (LVDP) and cAMP.

RESULTS

LY83583, infused for 25 min, decreased LVDP by 44 +/- 3 (SE), 77 +/- 20 and 120 +/- 38 mmHg, in control, 10(-9) M, and 10(-8) M ISO-stimulated hearts, respectively. Regardless of the level of ISO stimulation, LY83583 reduced LVDP to the same sub-basal level. Oxygen consumption also decreased, but proportionately less than LVDP. ISO increased cAMP without changing cGMP. LY83583 decreased cGMP by about 25% at all levels of ISO, and decreased cAMP by 22% in the 10(-8) M ISO-stimulated group.

CONCLUSION

Guanylyl cyclase inhibition by LY83583 decreased cGMP, cAMP and ventricular contractility. However, LY83583 depression of contractility was not always associated with a reduction in cAMP, suggesting that LY83583 can depress contractility by both cAMP-dependent and independent mechanisms.

Activation of soluble guanylyl cyclase by YC-1 in aortic smooth muscle but not in ventricular myocardium from rat

1. The effects of YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole), an activator of soluble guanylyl cyclase, on tension, levels of cyclic GMP and cyclic AMP, and cardiac L-type Ca2+-current (I[Ca(L)]) were investigated in aortic smooth muscle and ventricular heart muscle from rat. 2. YC-1 (0.1-30 microM) induced a concentration-dependent relaxation in aortic rings precontracted with phenylephrine (3 microM). The relaxant effects of YC-1 were reversed by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (30 microM; ODQ), potentiated by zaprinast (10 microM) and antagonized by Rp-8-Br-cGMPS (100 microM). 3. In ventricular heart muscle strips, YC-1 (30 microM) exhibited no effects on force of contraction (Fc) in the absence or presence of either zaprinast (10 microM) or 3-isobutyl-1-methylxanthine (30 microM). Fc was slightly increased by YC-1 (30 microM) in the presence of isoprenaline (100 nM), but this effect was not influenced by ODQ (30 microM). 4. Cardiac I[Ca(L)] was not significantly affected by YC-1 (30 microM), either in the absence or presence of isoprenaline (30 nM). 5. In aortic rings, cyclic GMP levels were increased almost 3 fold by YC-1 (30 microM); this effect was abolished by ODQ (30 microM). In isolated ventricular cardiomyocytes, cyclic GMP levels were not affected by YC-1 (30 microM) but almost doubled by activation of particular guanylyl cyclase with atriopeptin II (100 nM). 6. YC-1 (30 microM) did not increase cyclic AMP levels either in aortic rings or in ventricular cardiomyocytes. In contrast, isoprenaline (3 microM) increased cyclic AMP levels about two fold in both tissues. In cardiomyocytes, the effect of isoprenaline (3 microM) was slightly enhanced by YC-1 (30 microM). 7. It is concluded that relaxation of smooth muscle preparations by YC-1 is mediated mainly by activation of soluble guanylyl cyclase and subsequent increase in cyclic GMP levels. The failure of YC-1 to affect cardiac Fc, levels of cyclic GMP, and I[Ca(L)] suggests that soluble guanylyl cyclase is not influenced by YC-1 in rat heart muscle or only barely present in this tissue.

Denitrification of the genotoxicant 2-nitropropane: relationship to its mechanism of toxicity

1. The stabilities of the industrial chemical and constituent of cigarette smoke 2-nitropropane (2-NP) and its aci tautomer propane 2-nitronate (P2N) towards hepatic enzymes and proteins such as serum proteins and oxyhaemoglobin were investigated in vitro in biological (hepatocytes and subcellular liver fractions) and model systems (serum proteins, oxyhaemoglobin, methylene blue). 2. Denitrification of 2-NP, P2N and 2-deutero 2-nitropropane (2H-NP) occurred in murine hepatocytes significantly faster than in rat cells. For 2-NP the rates were 1271 +/- 167 versus 820 +/- 125 pmol nitrite x min-1 x 10(6) cells-1. 3. A similar observation was made in microsomes, where 2-NP denitrification was 1460 +/- 110 (mouse) versus 480 +/- 80 pmol nitrite x min-1 x mg protein-1 (rat). 4. The major NO2(-)-forming activity was found to be localized in the microsomal fraction. 5. Conversion of 2-NP into P2N, either chemically or enzymatically, was a prerequisite for rapid denitrification. 6. Serum proteins and oxyhaemoglobin proved to be capable of denitrifying P2N (198 +/- 24 pmol nitrite x min-1 x mg protein-1 and 7.1 +/- 1.0 nmol nitrite x min-1 x nmol HbO2(-1) respectively), but were much less active towards 2-NP (24 +/- 2 pmol nitrite x min-1 x mg protein-1 and none respectively). 7. Methylene blue decomposed 2-NP and P2N at rates of 11 +/- 3 and 192 +/- 4 pmol nitrite x min-1 x nmol, methylene blue-1 respectively. The dye also enhanced NO2- formation from P2N and 2-NP in the presence of hepatocytes or serum proteins, with a concomitant enhancement of both 2-NP and P2N toxicity. 8. The results presented report species differences in the denitrification rate of 2-NP and highlight the crucial nitro-aci tautomerism of 2-NP as a pivotal determinant of 2-NP toxicity.

Differential effects of isoliquiritigenin and YC-1 in rat aortic smooth muscle

We investigated the effects of isoliquiritigenin and YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole) on tension in endothelial-free rat aortic rings precontracted with phenylephrine (3 microM). Both compounds induced a concentration-dependent relaxation (EC50 of YC-1 1.9 microM and of isoliquiritigenin 9.4 microM). The effects developed faster with YC-1 than with isoliquiritigenin, and the effects of YC-1 were potentiated by isoliquiritigenin (10 microM). 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (30 microM) inhibited the effect of YC-1, but not of isoliquiritigenin. These results suggest that the effects of YC-1 are due to stimulation of soluble guanylyl cyclase activity, whereas the effects of isoliquiritigenin are rather related to inhibition of phosphodiesterase activity.

Dual cell cycle-specific mechanisms mediate the antimitogenic effects of nitric oxide in vascular smooth muscle cells

OBJECTIVE

To determine the cell cycle specificity and intracellular mechanisms involved in inhibition by nitric oxide (NO) of vascular smooth muscle cell mitogenesis.

METHODS

Cultured rat aortic smooth muscle cells were synchronized by serum withdrawal, treated with the NO donor S-nitroso-N-acetylpenicillamine and the cyclic GMP analog 8-Br-cGMP at various times during cell cycle progression, and DNA synthesis measured during the S phase. Two additional NO donors, 5-nitroso-glutathione and diethylamine NONOate, were used to confirm the inhibition of DNA synthesis by S-nitroso-N-acetylpenicillamine, and the ability of two antagonists of free NO to reverse the effects of NO donors was also evaluated. Bypass of ribonucleotide reductase by use of exogenous deoxynucleosides was attempted to determine whether inhibition of this S-phase enzyme was the mechanism by which NO inhibited DNA synthesis during the S phase.

RESULTS

Vascular smooth muscle cell mitogenesis was inhibited by cyclic GMP (cGMP) up to late G1 phase of the cell cycle, which corresponded to the point of greatest sensitivity to exogenous NO. In contrast to cGMP, three different NO donors inhibited DNA synthesis when added to cells synchronized in S phase, beyond the restriction point of cell cycle control in late G1 phase. This S-phase inhibition was reversible by removal of the NO donor or addition of two NO antagonists and was not observed with non-NO analogs of the donors. Inhibition by NO donors in S phase was neither reversed by the guanylate cyclase inhibitor methylene blue nor mimicked by exogenous cGMP. The S-phase inhibition by all three NO donors was reversed partially by bypass of ribonucleotide reductase, establishing this enzyme as an S-phase target of NO.

CONCLUSIONS

These findings demonstrate that NO inhibits smooth muscle mitogenesis by cGMP-dependent and -independent mechanisms acting at distinct points in the cell cycle. NO is the first endogenous substance to have been shown to inhibit mitogenesis beyond the restriction point in late G1 phase, suggesting that it plays a role in regulation of cells that have lost normal mechanisms of G1 growth control, such as the hyperproliferative smooth muscle cells noted in hypertension and restenosis.

NO/PGI2-independent vasorelaxation and the cytochrome P450 pathway in rabbit carotid artery

1. The nature and cellular mechanisms that are responsible for endothelium-dependent relaxations resistant to indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) were investigated in phenylephrine (PE) precontracted isolated carotid arteries from the rabbit. 2. In the presence of the cyclo-oxygenase inhibitor, indomethacin (10 microM), acetylcholine (ACh) induced a concentration- and endothelium-dependent relaxation of PE-induced tone which was more potent than the calcium ionophore A23187 with pD2 values of 7.03 +/- 0.12 (n = 8) and 6.37 +/- 0.12 (n = 6), respectively. The ACh-induced response was abolished by removal of the endothelium, but was not altered when indomethacin was omitted (pD2 value 7.00 +/- 0.10 and maximal relaxation 99 +/- 3%, n = 6). Bradykinin and histamine (0.01-100 microM) had no effect either upon resting or PE-induced tone (n = 5). 3. In the presence of indomethacin plus the NO synthase inhibitor, L-NAME (30 microM), the response to A23187 was abolished. However, the response to ACh was not abolished, although it was significantly inhibited with the pD2 value and the maximal relaxation decreasing to 6.48 +/- 0.10 and 67 +/- 3%, respectively (for both P < 0.01, n = 8). The L-NAME/indomethacin insensitive vasorelaxation to ACh was completely abolished by preconstriction of the tissues with potassium chloride (40 mM, n = 8). 4. The Ca(2+)-activated K+ (KCa) channel blockers, tetrabutylammonium (TBA, 1 mM, n = 5) and charybdotoxin (CTX, 0.1 microM, n = 5), completely inhibited the nitric oxide (NO) and prostacyclin (PGI2)-independent relaxation response to ACh. However, iberiotoxin (ITX, 0.1 microM, n = 8) or apamin (1-3 microM, n = 6) only partially inhibited the relaxation. 5. Inhibitors of the cytochrome P450 mono-oxygenase, SKF-525A (1-10 microM, n = 6), clotrimazole (1 microM, n = 5) and 17-octadecynoic acid (17-ODYA, 3 microM, n = 7) also reduced the NO/PGI2-independent relaxation response to ACh. 6. In endothelium-denuded rings of rabbit carotid arteries, the relaxation response to exogenous NO was not altered by either KCa channel blockade with apamin (1 microM, n = 5) or CTX (0.1 microM, n = 5), or by the cytochrome P450 mono-oxygenase blockers SKF-525A (10 microM, n = 4) and clotrimazole (10 microM, n = 5). However, the NO-induced response was shifted to the right by LY83583 (10 microM, n = 4), a guanylyl cyclase inhibitor, with the pD2 value decreasing from 6.95 +/- 0.14 to 6.04 +/- 0.09 (P < 0.01). 7. ACh (0.01-100 microM) induced a concentration-dependent relaxation of PE-induced tone in endothelium-denuded arterial segments sandwiched with endothelium-intact donor segments. This relaxation to ACh was largely unaffected by indomathacin (10 microM) plus L-NAME (30 microM), but abolished by the combination of indomethacin, L-NAME and TBA (1 mM, n = 5). 8. These data suggest that in the rabbit carotid artery: (a) ACh can induce the release of both NO and EDHF, whereas A23187 only evokes the release of NO from the endothelium, (b) the diffusible EDHF released by ACh may be a cytochrome P450-derived arachidonic acid metabolite, and (c) EDHF-induced relaxation involves the opening of at least two types of KCa channels, whereas NO mediates vasorelaxation via a guanosine 3': 5'-cyclic monophosphate (cyclic GMP)-mediated pathway, in which a cytochrome P450 pathway and KCa channels do not seem to be involved.

A cyclic GMP-dependent housekeeping Cl- channel in rabbit gastric parietal cells activated by a vasodilator ecabapide

1. The membrane potential of rabbit gastric parietal cells is dominated by a Cl- channel with a subpicosiemens single channel conductance in the basolateral membrane. The effects of 3-[[[2-(3,4-dimethoxyphenyl)ethyl]carbamoyl]amino-N-methylbenzamide++ + (DQ-2511: ecabapide), a vasodilator, on the opening of this Cl-1 channel, the cyclic GMP content and the intracellular free Ca2+ concentration ([Ca2+]i) of parietal cells were investigated by whole-cell patch-clamp technique, enzyme immunoassay and Fura 2-fluorescence measurement. 2. Ecabapide stimulated the opening of the Cl-1 channel as determined by the reversal potential. This stimulation was concentration-dependent, and its EC50 value was 0.2 microM. Both the basal and ecabapide-induced openings of the channel were inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 500 microM), a Cl- channel blocker. Another Cl- channel blocker, niflumic acid (500 microM) was much less effective. 3. The power spectra of the currents before and after the addition of ecabapide (10 microM) were analysed. Both spectra contained only one Lorentzian (1/f2) component. 4. 6-Anilino-5,8-quinolinedione (LY83583; 5 microM) which prevents activation of soluble guanylate cyclase, significantly inhibited both the basal and ecabapide (10 microM)-induced openings of the Cl- channel. 5. Ecabapide (0.01-100 microM) concentration-dependently elevated the cyclic GMP content in the parietal cell-rich suspension. The EC50 value was 0.2 microM. 6. In single Fura 2-loaded parietal cells, ecabapide (10-100 microM) did not increase [Ca2+]i. 7. These results indicate that ecabapide stimulates an intracellular production of cyclic GMP in the parietal cell without increasing [Ca2+]i, and leads to an activation of the housekeeping Cl- channel.

Actions of nitric oxide and expression of the mRNA encoding nitric oxide synthase in rat vagal afferent neurons

The present study has investigated whether nitric oxide (NO) is involved in neurotransmission of rat vagal afferent neurons. The diethylamine-NO complex (diethylamine-NO, 10-100 microM) and S-nitroso-N-acetylpenicillamine (3-100 microM) both elicited a concentration-dependent depolarisation of the isolated rat nodose ganglion preparation. Pre-treatment with 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 300 nM), 6-(phenylamino)-5,8-quinolinedione (LY83,583, 30 microM) and Methylene blue (100 microM) all caused a significant shift to the right in the concentration-response curve to diethylamine-NO. Incubation of rat nodose ganglion sections with a 35S-labeled antisense oligonucleotide to neuronal NO synthase resulted in visualisation of the mRNA encoding NO synthase over vagal afferent perikarya. The anatomical findings, therefore, suggest that a number of rat vagal afferent perikarya possess the ability to produce the enzyme required for the biosynthesis of NO. Collectively, these data suggest that NO may be functionally important as a neuromodulator of rat vagal afferent neurons.

Endothelium-dependent and NO-mediated desensitization to vasopressin in rat aorta

1. The present study was performed to characterize the tachyphylaxis of rat aortae to vasopressin. Isometric tension generated by rat thoracic aorta sliced in 4 mm rings, was recorded. 2. Tension generated by intact rings increased with cumulative additions of vasopressin up to 10 nM (1.51 +/- 0.15 g). After this concentration, most rings lost their tension and relaxed to 1.09 +/- 0.17 g (P < 0.001) despite further addition of vasopressin. This tachyphylaxis was not observed in endothelium-denuded rings (from 2.87 +/- 0.12 g to 2.68 +/- 0.17 g). 3. Repeated administrations of supramaximal concentration (100 nM) of vasopressin confirmed an enhanced desensitization in intact rings, compared to endothelium-denuded rings. No desensitization to phenylephrine was observed in intact or in endothelium-denuded rings. 4. Dose-response curves to a V1 receptor agonist, [Phe2, Ile3, Orn8]-vasopressin, and to a V2 receptor agonist, [deamino-Cys1,D-Arg8]-vasopressin, were performed in intact preparations. An increase in tension, followed by a desensitization was observed with the V1 receptor agonist. In contrast, the V2 receptor agonist did not induce any response. 5. Pretreatment of intact aortic rings with the cyclo-oxygenase inhibitor, diclofenac (1 microM), did not prevent the desensitization to vasopressin. In contrast, NO synthase inhibition with NG-nitro-L-arginine (30 microM) resulted in an attenuated desensitization to vasopressin in intact rings (from 2.46 +/- 0.17 to 2.25 +/- 0.22 g, NS). 6. To confirm the involvement of NO, endothelium-denuded rings were pretreated with sodium nitroprusside (SNP). At a concentration of 10 nM, SNP induced a desensitization to vasopressin comparable with that observed in intact rings. 7. Pretreatment of endothelium-denuded rings with 8-bromo-cyclic GMP (100 microM) reduced maximum contraction to vasopressin without producing any desensitization. In contrast, guanylate cyclase inhibition with either LY 83,583 (10 microM) or methylene blue (10 microM) blocked completely the desensitization of intact rings to vasopressin. 8. The results suggest that the endothelium-dependent tachyphylaxis to vasopressin is due to rapid desensitization and is mediated by NO. However, it is unclear whether this effect of NO involves cyclic GMP.

In vivo microdialysis study of a specific inhibitor of soluble guanylyl cyclase on the glutamate receptor/nitric oxide/cyclic GMP pathway

1. Nitric oxide (NO) is known to stimulate soluble guanylyl cyclase, thereby eliciting an elevation of guanosine 3':5'-cyclic monophosphate (cyclic GMP) in target cells. Recently, a selective inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), has been identified and characterized in vitro. We have investigated the in vivo effects of ODQ on the glutamate receptor/NO/ cyclic GMP pathway by monitoring extracellular cyclic GMP during microdialysis of the cerebellum or the hippocampus of freely-moving adult rats. 2. Intracerebellar administration of ODQ (1-100 microM) via the microdialysis probe inhibited, in a concentration-dependent manner, the basal extracellular level of cyclic GMP. The maximal inhibition, measured after a 20 min perfusion with 100 microM ODQ, amounted to 80% and persisted unchanged as long as ODQ was perfused. When ODQ was removed from the perfusion stream after 20 min, the levels of cyclic GMP started to recover, suggesting reversibility of guanylyl cyclase inhibition by ODQ. 3. The cyclic GMP response evoked in the cerebellum by NMDA (200 microM) or by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA; 100 microM) was largely attenuated by 100 microM ODQ. The pattern of the inhibition curves suggests competition for guanylyl cyclase between ODQ and the NO generated by NMDA or AMPA receptor activation. 4. ODQ (100 microM) prevented the elevation of extracellular cyclic GMP levels provoked by intracerebellar infusion of the NO generator S-nitroso-N-acetylpenicillamine (SNAP; 1 mM). The inhibition of the SNAP effect was rapidly relieved when ODQ was removed from the perfusion fluid. However, ODQ (100 microM) was unable to affect the cyclic GMP response elicited by 5 mM SNAP, in keeping with the proposed idea that ODQ binds to the "NO receptor' in a reversible and competitive manner. 5. Infusion of ODQ (10, 100 or 300 microM) into the hippocampus of freely-moving rats diminished the basal extracellular level of cyclic GMP. The maximal inhibition amounted to 50% and was produced by 100 microM ODQ. 6. The cyclic GMP response observed when 1 mM SNAP was perfused in the hippocampus, similar in percentage terms to that seen in cerebellum, was dramatically reduced during co-infusion of 100 microM ODQ. 7. ODQ appears to act in vivo as a selective, reversible and possibly competitive inhibitor of the soluble guanylyl cyclase targeted by NO. This enzyme may generate most (about 80%) of the cyclic GMP found under basal conditions in the extracellular space of the cerebellum. In the hippocampus, about 50% of the basal cyclic GMP does not seem to originate from the ODQ-sensitive soluble guanylyl cyclase.

Effect of sodium nitroprusside on compound action potential thresholds in the gerbil cochlea

The presence of active nitric oxide synthase (NOS) in the spiral ganglion cells of the cochlea suggests that the neuromodulator nitric oxide (NO) may play a role in hearing. This study investigated the effects of sodium nitroprusside (SNP), an NO donor, upon cochlear function mediated through its activation of guanylate cyclase. In gerbils, cochlear compound action potential (CAP) thresholds were recorded after cochlear perfusions of control and test solutions in four experimental groups. Perfusions were performed using the following: artificial perilymph solution (APS); the NO donor SNP; the guanylate cyclase inhibitor methylene blue (MB); and sodium dodecyl sulfate (SDS), which facilitates MB entrance into cells. SNP caused significant elevations of CAP thresholds from baseline (25 dB SPL +/- 1.54 dB to 64.3 dB SPL +/- 2.54 dB). SNP with MB also resulted in significant CAP threshold elevations (29.4 dB SPL +/- 4.27 dB to 38.1 dB SPL +/- 4.0 dB); however, these elevations were significantly lower than those seen in SNP perfusions without MB. Drilling perfusion holes and perfusion of APS, APS/SDS, and MB/SDS/APS solutions did not significantly affect CAP thresholds. These results suggest that the NO donor nitroprusside does affect cochlear neuromodulation and effects this mediation in part through NO activation of guanylate cyclase.

Localization of nitric oxide synthase I and III in the cochlea

Nitric oxide synthase (NOS) isoforms I and III were localized in the guinea pig cochlea by indirect immunohistochemistry using frozen sections and paraffin sections. NOS I staining was observed in the cytoplasm of outer hair cells, in nerve cell somata and fibers of the spiral ganglion, and in axonal profiles of the spiral lamina next to the base of inner hair cells. In addition, lining cells of the inner sulcus and limbus, and cells of the spiral ligament stained for NOS I but vascular walls remained unstained. NOS III reactivity was seen in the cytoplasm of outer and inner hair cell, in lining cells of the limbus, and on the endolymphatic surface of marginal cells. Staining for NOS III of spiral ganglion perikarya showed varying intensity. Endothelial cells of cochlear glomeruli reacted for NOS III. NOS III in vascular endothelial cells implies regulatory effects of nitric oxide (NO) on vascular wall tonus and cochlear blood supply. NOS I in cochlear neurons indicates these cells as possible sources for NO during neuronal activity. Activated neurons may provide NO that adjusts cochlear perfusion to neuronal activity. Finally, NO that is liberated from hair cells or afferent synaptic terminals may act as an inhibitor on N-methyl-D-aspartate (NMDA) receptors (negative feed-back inhibition).

Induction of guinea pig airway hyperresponsiveness by inactivation of guanylate cyclase

To examine the role of cyclic 3', 5'-guanosine monophosphate (cGMP) in airway responsiveness the effects of substances known to interfere with nitric oxide (NO) or cGMP were investigated on guinea pig airways. Using a perfused organ bath system, it was possible to apply the chemicals from either the serosal or the mucosal side independently. In addition, levels of intracellular cGMP were determined in tissues after various treatments. Sodium nitroprusside (a donor of NO), zaprinast (a specific inhibitor of cGMP phosphodiesterase) and 8-bromo-cGMP (8-Br-cGMP) caused a concentration-dependent relaxation of guinea pig trachea. These results indicate that cGMP is an important second messenger mediating tracheal relaxations. The above mentioned drugs caused a more profound relaxation when applied to the serosal side compared to the mucosal side, suggesting a barrier function of the epithelial layer. Incubation on the mucosal side of the tissues with 100 microM pyrogallol (a generator of superoxide that may inactivate NO) increased the contractile response to histamine at concentrations 0.3-3.2 microM (P < 0.05). Treatment of the preparations with 1 mM cystamine (an inactivator of guanylate cyclase) caused a 5-fold increase in the sensitivity to histamine (P < 0.05), indicating the involvement of the NO/cGMP pathway in the development of airway hyperresponsiveness. Incubation of the tissues with 100 microM histamine elevated the intracellular cGMP levels 10-fold; this effect was completely prevented by incubation of the tissues with methylene blue (a potent inactivator of guanylate cyclase). Mucosal incubation of the tracheal tubes with 10 microM methylene blue induced an 8-fold increase in sensitivity to histamine (P < 0.01) and the Emax was slightly increased. 25 min after instillation of 0.4 mumol methylene blue into the airways of anaesthetized guinea pigs, the lung resistance in response to histamine was elevated up to 395 +/- 82% (P < 0.001). The present study revealed that inactivation of NO or guanylate cyclase enhances the histamine-induced contractions of guinea pig tracheas. Therefore, it is suggested that the NO/cGMP pathway may be implicated in the pathogenesis of airway hyperresponsiveness and that drugs which enhance cGMP levels in airway smooth muscle may be of significance in the treatment of airway obstruction and enhanced reactivity.

Methylene blue induces cytotoxicity in human brain tumor cells

Methylene blue (MB), a known inhibitor of guanylyl cyclase, induced cytotoxicity in SK-N-MC human neuroblastoma and U-373 MG human astrocytoma cells in a dose-dependent manner. MB did not significantly alter cellular levels of cGMP in both cells. 8-Br cGMP, a membrane-permeable analogue of cGMP, did not decrease MB-induced cytotoxicity, indicating that cGMP may not be a major target of the cytotoxic action of MB. However, hydroxyl radical scavengers or intracellular Ca2+ modulators effectively blocked the MB-induced cytotoxicity. These results suggest that hydroxyl radical and intracellular Ca2+ may have an important involvement in the cytotoxic action of MB. These results further suggest that the treatment with MB may be useful for the therapeutic applications of human brain tumors.

Mechanism of potentiation of LY83583-induced growth inhibition by sodium nitroprusside in human brain tumor cells

The effect of 6-anilino-5,8-quinolinedione (LY83583), an inhibitor of guanylyl cyclase (GC), on the growth of human brain tumor cells (U-373 MG astrocytoma and SK-N-MC neuroblastoma) was evaluated. LY83583 inhibited the growth of these cells in a dose-dependent manner. This growth inhibition was found to be the result of decreased cell viability as assessed by the trypan blue exclusion method. The LY83583-induced decrease in cell viability was not altered by dibutyryl cyclic GMP, but significantly was reversed by superoxide dismutase and catalase, indicating that these effects of LY83583 may not be due to the inhibition of GC, but due to the formation of superoxide anion. The LY83583-induced decrease in cell viability was potentiated by cotreatment with sodium nitroprusside (SNP), a nitric oxide (NO) donor. This SNP-induced potentiation was significantly blocked by various scavengers for hydroxyl radicals or by intracellular Ca2+ release blockers. These results suggest that the potentiation effects of SNP may be mediated through the generation of hydroxyl radicals which can be formed by the interaction of superoxide anion (from LY83583) and NO (from SNP), and that intracellular Ca2+ release from internal stores may play an important role in the cytotoxic mechanism of hydroxyl radicals.

Nitric oxide donor SIN-1 inhibits mammalian cardiac calcium current through cGMP-dependent protein kinase

The effect of the nitric oxide (NO) donor SIN-1 (3-morpholino-sydnonimine) on the calcium current (ICa) was examined in guinea pig ventricular myocytes. SIN-1 had little effect on basal ICa. After moderate stimulation of ICa with 10 nM isoproterenol (ISO), 10 microM SIN-1 caused either stimulation or inhibition of ICa; 100 microM SIN-1 consistently caused inhibition. SIN-1 (1-100 microM) inhibited ICa equally following considerable enhancement of ICa by either 1 microM ISO or 100 microM 3-isobutyl-1-methylxanthine, a nonspecific phosphodiesterase (PDE) inhibitor. SIN-1 (100 microM) also inhibited ICa equally following enhancement by either 10 microM pipette adenosine 3',5'-cyclic monophosphate (cAMP) or hydrolysis-resistant 8-bromo-cAMP. Thus the inhibitory effect of SIN-1 appears independent of PDEs. Addition of LY-83583 (a blocker of guanylate cyclase) to the pipette or superfusion with KT-5823 [a blocker of the guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase] suppressed the inhibitory effect of SIN-1. We conclude that NO is an important modulator of beta-adrenergic effects on ICa and that the mechanism of NO inhibition of ICa in mammalian cardiac cells involves the cGMP-dependent protein kinase.

Cerebral vasospasm and free radicals

The literature implicating free radical reactions in the genesis of cerebral vasospasm following aneurysmal subarachnoid hemorrhage is reviewed. While this condition has features of a prototypical free radical-mediated disease and a plausible theory can be outlined, data to support the theory are limited. An association of lipid peroxidation with vasospasm has been observed, but more sophisticated techniques for detection of free radicals and for detection of free radical damage to arterial wall proteins and nucleic acids have not been used. There are conflicting reports about efficacy of various antioxidant treatments for vasospasm. In these studies, concomitant experiments have usually not confirmed that the treatments have decreased free radicals or lipid peroxides in cerebrospinal fluid. Because smooth muscle contraction is involved in vasospasm, it would be interesting to investigate the actions of free radicals on smooth muscle cells using, for example, isometric tension recordings and patch clamp techniques. Studies of cardiac myocytes indicate that free radicals alter conductances through potassium and calcium channels and through the sodium-calcium exchanger and may result in elevations in intracellular calcium. Few studies have been performed on cerebral smooth muscle cells. In one study, exposure of cerebrovascular smooth muscle cells to free radicals resulted in increased outward currents, decreased membrane resistance, cell contraction, appearance of membrane blebs, and cell death. In summary, more investigations using better experimental techniques are required before free radicals and reactions induced by them can be said with certainty to be the primary cause of vasospasm.

Nitric oxide has no chronotropic effect in right atria isolated from rat heart

This study was designed to determine if nitric oxide (NO) has direct effects on heart rate or if it is involved in the chronotropic actions of adrenergic or cholinergic stimulation. Right atria were isolated from hearts of adult male rats, bathed in Krebs-Henseleit buffer (37 degrees C), and used to monitor spontaneous rate. For comparison, ring segments of thoracic aorta were also suspended in the Krebs-Henseleit solution and used to examine vascular actions of various agents. The dose-dependent chronotropic effects of acetylcholine (10(-7)-10(-3) M) and norepinephrine (10(-8)-3 x 10(-4) M) in right atria were not affected by pretreatment with 10(-4) M N-nitro-L-arginine or 10(-3) M N-nitro-L-arginine-methyl ester, inhibitors of L-arginine-derived NO production. SIN-1 (3-morpholino-sydnonimine), an agent which releases NO in aqueous solution, elicited a dose-dependent (0.3-100 microM) vasorelaxation in aortic preparations constricted with 60 mM KCl; the ED50 value for this effect was increased by pretreatment with methylene blue (10 microM) and LY-83,583 (6-(phenylamino)-5,8- quinolinedione; 1 and 3 microM), compounds which inhibit NO-induced stimulation of guanylate cyclase. SIN-1 produced a negative chronotropic effect in right atria; however, this action was not observed at concentrations less than 300 microM and was not antagonized by methylene blue or LY-83,583. 8-Bromo cyclic GMP produced a dose-dependent (10-3000 microM) decrease in KCl-induced tension in aortic rings. In right atria, 8-bromo cyclic GMP elicited a positive chronotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

The hemispheric cerebrovascular response to hemodilution is attenuated by a focal cryogenic brain injury

Experimental brain injury attenuates the normal increase in cerebral blood flow (CBF) that accompanies a fall in PaO2, and this may contribute to the well-known detrimental effects of hypoxia following closed head injury. Anemia is also known to adversely affect posttraumatic survival, and it is reasonable to hypothesize that this too may be related to an altered cerebrovascular response. Therefore, to examine this possibility, pentobarbital-anesthetized rabbits were subjected to a left posterior parietal cryogenic cortical injury, followed 90 min later by isovolemic hemodilution with hetastarch. Unlesioned control animals underwent an identical degree of hemodilution. CBF was measured using radioactive microspheres. In control animals, hemodilution was accompanied by a marked increase in CBF in all brain regions. For example, in the left posterior cortex, CBF increased from 30 +/- 14 mL/100 g/min (baseline Hct = 42 +/- 2%, mean +/- SD) to 100 +/- 20 mL/100 g/min at Hct = 12 +/- 1%. By contrast, there was a markedly attenuated response throughout the left (ipsilateral) hemisphere of injured animals, even in cortical regions distant from the injury. For example, in the left posterior cortex, CBF changed from a baseline of 32 +/- 21 mL/100 g/min (baseline) to 40 +/- 14 mL/100 g/min at the lowest Hct. CBF responses to hemodilution were unaltered in the contralateral hemisphere and in the hindbrain. These data indicate that a localized brain lesion can produce widespread ipsilateral alterations in the CBF response to hemodilution, with resultant compromise in cerebral O2 delivery. These data support the argument that the CBF increase produced by hemodilution is an active vasodilatory process rather than a passive response to changing blood viscosity.