The Role of Nitric Oxide in Digoxin-Induced Arrhythmias in Guinea-Pigs*

Abstract: We have investigated the effects of nitric oxide synthase inhibitor (L-NAME), nitric oxide precursor (L-arginine) and nitric oxide donor (sodium nitroprusside) on digoxin-induced arrhythmias both in guinea-pig isolated hearts and in anaesthetised animals. Sodium nitroprusside (0.1 μmol kg^?1 min.^?1 for 70 min.) caused a marked inhibition in mortality and arrhythmia score but L-NAME (10 mg kg^?1) and L-arginine (30 mg kg^?1 intravenous bolus followed by 10 mg kg^?1 min.^?1 for 60 min.) treatments were ineffective in anaesthetised guinea-pigs. None of the drugs markedly affected the time of onset of first arrhythmias or ventricular fibrillation incidence. In isolated heart experiments, nitric oxide generated by either L-arginine (1 mM) or sodium nitroprusside (1 mM) significantly reduced the arrhythmia score whereas L-NAME (1 mM) had no effect. Ventricular fibrillation incidence was totally abolished by sodium nitroprusside and none of the hearts treated with L-arginine had an irreversible ventricular fibrillation. L-NAME decreased ventricular tachycardia duration but increased ventricular fibrillation duration. There were no marked changes in the time of onset of first arrhythmias with these drugs in in vitro experiments. These results suggest that nitric oxide may play a modulatory role in the digoxin-induced arrhythmias in guinea-pigs.     

The effect of aspirin, indomethacin and sodium meclofenamate on coronary artery ligation arrhythmias in anaesthetized rats

The intravenous administration of either aspirin, (in doses of 1 and 3 mg/kg) sodium meclofenamate (1 and 3 mg/kg) or indomethacin (1 and 3 mg/kg) markedly reduced ventricular arrhythmias (VT and VF) induced by coronary artery ligation in anaesthetised rats. Their effect were very pronounced on ventricular ectopic activity occurring between the sixteenth and thirtieth min. These effects suggest the desirability of combining the non steroidal anti-inflammatory drugs with beta-adrenergic blockers in the treatment of ventricular arrhythmias following acute myocardial infarction.     

Resistance of cholestatic rats against epinephrine-induced arrhythmia: the role of nitric oxide and endogenous opioids

Short-term ligation of bile duct has been used as a model to study acute cholestasis and is associated with various cardiovascular abnormalities. We examined the role of nitric oxide (NO) and endogenous opioids on epinephrine-induced arrhythmia in 7-day bile duct-ligated (BDL) rats. Six groups of rats, each of which was subdivided into two subgroups (sham-operated and BDL), were examined. First group of animals were chronically treated with normal saline. In the second and third groups, single intraperitoneal administration of N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) or naltrexone (20 mg/kg) was performed 30 min before evaluation of epinephrine-induced arrhythmia. Two groups received chronic administration of low dose (3 mg/kg/day) or high dose (10 mg/kg/day) L-NAME; and the last group was treated chronically with naltrexone (20 mg/kg/day). Chronic drug administration was performed subcutaneously for 6 consecutive days following BDL or sham operation. After induction of arrhythmia by intravenous injection of 10 microg/kg epinephrine, mean arterial pressure and electrocardiogram were recorded for 1 min. Heart rate and mean arterial pressure were significantly lower in BDL rats (P<0.01). Chronic injection of naltrexone increased heart rate and mean arterial pressure in BDL (P<0.05). Chronic low dose L-NAME administration had no effect on baseline hemodynamic parameters. High dose L-NAME injection corrected hypotension in BDL rats, but not bradycardia (P<0.05). Epinephrine induced less arrhythmia in BDL rats (P<0.05). Acute and chronic injection of naltrexone had no effect on the resistance of BDL rats against epinephrine-induced arrhythmia. Although acute L-NAME administration enhanced arrhythmias in sham-operated rats (P<0.001), it had no effect on BDL animals. Chronic injection of low dose or high dose L-NAME, without having any effect on sham-operated animals, increased arrhythmias in BDL rats (P<0.01). This study showed that BDL animals are resistant against epinephrine-induced arrhythmia and this resistance depends on long-term NO overproduction.     

MAGNOLOL REDUCES INFARCT SIZE AND SUPPRESSES VENTRICULAR ARRHYTHMIA IN RATS SUBJECTED TO CORONARY LIGATION

1. Magnolol is an active component of Magnolia officinalis. It is 1000-times more potent than α-tocopherol in inhibiting lipid peroxidation in rat heart mitochondira. In the present study, the in vivo antiarrhythmic and anti-ischaemic effects of magnolol in coronary ligated rats were investigated. 2. Male Sprague-Dawley rats were anaesthetized with urethane. Magnolol, at dosages of 10^?7, 10^?8 and 10^?9 g/kg, was adminstered intravenously 15 min before ligation of the coronary artery. 3. The incidence and duration of ventricular tachycardia and ventricular fibrillation during 30 min coronary ligation were significantly reduced by magnolol. Ventricular arrhythmias during 10 min reperfusion after the relief of coronary ligation were also reduced. 4. In rats subjected to 4h coronary ligation, 10^?7 and 10^?8 g/kg magnolol significantly reduced infarct size. 5. We conclude that magnolol may protect the myocardium against ischaemic injury and suppress ventricular arrhythmia during ischaemia and reperfusion.     

Nitric oxide and sensory afferent neurones modulate the protective effects of low-dose endotoxin on rat gastric mucosal damage

Pretreatment (1 h) with low doses (5–40 μg/kg i.p.) of Escherichia coli endotoxin dose dependently reduced the gastric mucosal damage induced by a 10 min challenge with 1 ml ethanol (50% and 100%) in conscious rats. Treatment with the nitric oxide synthesis inhibitor, oxide synthesis inhibitor, N^G-nitro- -arginine methyl ester (L-NAME, 5 and 10 mg/kg i.p.), significantly inhibited the protective effects of endotoxin (40 μg/kg i.p.). The actions of L-NAME were reversed by the prior administration of -arginine (100 mg/kg i.p.). The protective effects of endotoxin were not influenced by pretreatment with dexamethasone (5 mg/kg s.c. twice) or indomethacin (5 mg/kg s.c.). However, ablation of sensory afferent neurones by capsaicin pretreatment (20, 30 and 50 mg/kg s.c.) abolished the mucosa protective effects of endotoxin (40 μg/kg). These findings suggest that the protection elicited by low doses of endotoxin against ethanol-induced mucosal damage involves synthesis of nitric oxide and activation of sensory neurones.     

Electrophysiological evidence for a role of nitric oxide in prolonged chemical nociception in the rat.

The role of nitric oxide in the periphery and the spinal cord, during acute electrically-evoked and prolonged chemically-evoked nociceptive stimulation, was investigated in rats anaesthetised with halothane. The responses of single dorsal horn neurones to electrically-evoked A beta fibre and C fibre inputs were reduced by topical application (directly onto the spinal cord) of both the nitric oxide inhibitor, nitro-L-arginine methyl ester (L-NAME; 500-1500 micrograms) and the precursor of nitric oxide, L-arginine (4500 micrograms). Administration of L-NAME, either directly into the receptive field (500-1500 micrograms) or intravenously (10-100 mg/kg) had little or no effect on the acute electrically-evoked activity. Intravenous injection of L-NAME, administered 40 min prior to injection of formalin, significantly reduced the prolonged second peak of firing, with only a small effect on the short-duration first peak. Administration of L-NAME, directly into the site of injection of formalin, as a 10 min pretreatment, significantly reduced the second but not the first peak of the response. Topical application of L-NAME onto the spinal cord, as a 30 min pretreatment, significantly reduced both the first and second peaks of the response. This inhibition was not reversed by the coadministration of L-arginine, which was inhibitory by itself. Thus, nitric oxide may be involved, in a complex way, in nociceptive events both in the periphery and within the spinal cord.     

Magnolol reduces myocardial ischemia/reperfusion injury via neutrophil inhibition in rats.

The accumulation of oxygen-free radicals and activation of neutrophils are strongly implicated as important pathophysiological mechanisms mediating myocardial ischemia/reperfusion injury. It has been proven that various antioxidants have cardioprotective effects. Magnolol, an active component extracted from the Chinese medicinal herb Magnolia officinalis, possesses potent antioxidant and free radical scavenging activities. In this study, the cardioprotective activity of magnolol was evaluated in an open-chest anesthetized rat model of myocardial ischemia/reperfusion injury. The results demonstrated that pretreatment with magnolol (0.2 and 0.5 microg/kg, i.v. bolus) at 10 min before 45 min of left coronary artery occlusion, significantly suppressed the incidence of ventricular fibrillation and mortality when compared with the control group. Magnolol (0.2 and 0.5 microg/kg) also significantly reduced the total duration of ventricular tachycardia and ventricular fibrillation. After 1 h of reperfusion, pretreatment with magnolol (0.2 and 0.5 microg/kg) caused a significant reduction in infarct size. In addition, magnolol (0.2 microg/kg) significantly reduced superoxide anion production and myeloperoxidase activity, an index of neutrophil infiltration in the ischemic myocardium. In addition, pretreatment with magnolol (0.2 and 0.5 microg/kg) suppressed ventricular arrhythmias elicited by reperfusion following 5 min of ischemia. In vitro studies of magnolol (5, 20 and 50 microM) significantly suppressed N-formylmethionyl-leucyl-phenylalanine (fMLP; 25 nM)-activated human neutrophil migration in a concentration-dependent manner. It is concluded that magnolol suppresses ischemia- and reperfusion-induced ventricular arrhythmias and reduces the size of the infarct resulting from ischemia/reperfusion injury. This pronounced cardioprotective activity of magnolol may be mediated by its antioxidant activity and by its capacity for neutrophil inhibition in myocardial ischemia/reperfusion.     

Nitric oxide modulation affects the tissue distribution and toxicity of bupivacaine

We have previously demonstrated that inhibition of nitric oxide synthase (NOS) alters the toxicity of local anesthetics including bupivacaine. Because significant changes in blood distribution are associated with the use of nonselective NOS inhibitors, the purpose of this study was to determine whether modification of bupivacaine toxicity by nonselective NOS inhibition is due to alteration in tissue disposition of bupivacaine. Rats were anesthetized with halothane and pretreated with either: 1) a nonselective NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 2 mg/kg/min, IV for 30 min); 2) a neuronal NOS inhibitor, 7-nitroindazole (7-NI, 30 mg/kg, IP); or 3) vehicle (control). Thirty minutes later, bupivacaine 2 mg/kg/min IV was infused until onset of seizures, arrhythmias, or asystole. L-NAME caused a rapid increase in plasma bupivacaine concentrations (3-4 times faster than in the other groups), which was associated with markedly lower bupivacaine doses (mg/kg) required to produce arrhythmias in L-NAME (4.2 +/- 0.5) vs. control (26 +/- 3, p < 0.01) and 7-NI groups (17 +/- 3, p < 0.01). Myocardial bupivacaine concentrations at arrhythmia onset were slightly lower in the L-NAME group. Bupivacaine seizure doses in 7-NI and L-NAME pretreated animals were similar to control but significantly different from each other. Brain bupivacaine concentrations at seizure onset were similar among the groups. There were no significant differences between 7-NI and control groups in any parameter observed. We conclude that enhanced cardiotoxicity of bupivacaine by nonselective NOS inhibition is primarily due to rapid increases in plasma and myocardial distribution of bupivacaine.     

Nitric oxide synthase inhibition blocks amphetamine-induced locomotor activity in mice.

Effects of N(G)-nitroarginine methyl ester (L-NAME), a nonspecific inhibitor of nitric oxide (NO) synthase, on amphetamine-induced locomotor activity were investigated in Swiss-Webster mice. Locomotor activity was measured for 30 min immediately following amphetamine (1, 2 and 4 mg/kg, i.p.) or saline treatments. L-NAME (15 and 30 mg/kg) and a combination of L-arginine (1000 mg/kg) and L-NAME (30 mg/kg) were injected 30 min before amphetamine (2 mg/kg) to other groups of the mice. L-Arginine was injected 30 min before L-NAME treatment when they were combined. L-NAME (30 mg/kg) and L-arginine (1000 mg/kg) were also tested for ability to depress or stimulate locomotor activity in the absence of amphetamine. Amphetamine caused a dose-dependent increase in locomotor activity of the mice. L-NAME blocked the amphetamine-induced locomotor stimulation dose dependently. L-Arginine pretreatment prevented the inhibitory effects of L-NAME on amphetamine-induced locomotor stimulation. L-NAME and L-arginine did not cause any significant change in locomotor activity in mice not treated with amphetamine. These results suggest that amphetamine-induced locomotor stimulation in mice is modulated by NO.     

Evidence for an involvement of nitric oxide and gamma aminobutyric acid in the anticonvulsant action of L-arginine on picrotoxin-induced convulsions in rats

Five, 30, and 60 min pretreatment of 1000 mg/kg and not 500 mg/kg of L-arginine inhibited convulsions induced by picrotoxin. The concentrations of nitric oxide (NO) and gamma aminobutyric acid (GABA) were increased in the brain 5, 30, and 60 min after administration of 1000 mg/kg and not 500 mg/kg of L-arginine. A much higher dose of L-arginine (2000 mg/kg), 30 min after administration, produced a lesser anticonvulsant and NO and GABA increasing actions as compared to that produced by 1000 mg/kg of L-arginine. The same dose of L-arginine, 60 min after administration, decreased the concentrations of both NO and GABA and increased the convulsion frequency of picrotoxin. An NO decreasing dose of nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (L-NAME) decreased brain GABA concentration and increased the convulsant action of picrotoxin. Further, L-NAME pretreatment prevented L-arginine (1000 mg/kg) from producing anticonvulsant and NO and GABA increasing effects. An interpretation of these results suggests that NO synthesized from systemically administered L-arginine inhibits convulsions by increasing the concentration of GABA in the brain. However, the effects of L-arginine are reversible, if it is administered at a higher dose (2000 mg/kg) 60 min prior to the test. It is concluded that L-arginine produces anticonvulsant or proconvulsant action depending upon the dose and time of its administration-related changes in the concentrations of NO and GABA in the brain.     

The effect of cyclosporin A on morphine tolerance and dependence: involvement of L-arginine/nitric oxide pathway

Cyclosporin A is known to decrease nitric oxide (NO) production in nervous tissues. The effects of systemic cyclosporine A on the induction and expression of morphine tolerance and dependence, acute morphine-induced antinociception, and the probable involvement of the L-arginine/nitric oxide pathway in these effects were assessed in mice. Cyclosporin A (20 mg/kg), N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) and a combination of the two at lower and per se non-effective doses (5 and 3 mg/kg, respectively) showed a similar pattern of action, inhibiting the induction of tolerance to morphine-induced antinociception and increasing the antinociception threshold in the expression phase of morphine tolerance. These agents also inhibited the expression of morphine dependence as assessed by naloxone-precipitated withdrawal signs, while having no effect on the induction of morphine dependence. L-Arginine, at a per se non-effective dose (60 mg/kg), inhibited the effects of Cyclosporin A. Moreover, acute administration of Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) enhanced the antinociception induced by acute administration of morphine (5 mg/kg), while chronic pretreatment with Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) for 2 days (twice daily) did not affect morphine-induced antinociception. The inducible nitric oxide synthase inhibitor, aminoguanidine (100 mg/kg), did not alter morphine antinociception, tolerance or dependence. In conclusion, decreasing NO production through constitutive nitric oxide synthase may be a mechanism through which cyclosporin A differentially modulates morphine tolerance, dependence and antinociception.     

Delayed cardioprotection by intestinal preconditioning is mediated by calcitonin gene-related peptide

Previous studies have shown that nitric oxide and calcitonin gene-related peptide (CGRP) are involved in mediation of the delayed cardioprotection of ischemic or pharmacological preconditioning, and nitric oxide can evoke the release of CGRP. In the present study, we examined the role of CGRP in nitric oxide-mediated delayed cardioprotection by brief intestinal ischemia in rats. The serum concentration of creatine kinase and infarct size were measured after 45-min coronary artery occlusion and 180-min reperfusion. Ischemic preconditioning was induced by six cycles of 4-min ischemia and 4-min reperfusion of the small intestine. Pretreatment with intestinal ischemic preconditioning for 24, 48, or 72 h significantly reduced infarct size and creatine kinase release, and the effects of ischemic preconditioning were completely abolished by L-nitroarginine methyl ester (L-NAME, 10 mg/kg, i.p.), an inhibitor of nitric oxide synthase, or by pretreatment with capsaicin (50 mg/kg, s.c.), which selectively depletes transmitters in capsaicin-sensitive sensory nerves. Intestinal preconditioning caused a significant increase in plasma concentrations of CGRP, and the effect was also abolished by L-NAME or capsaicin. These results suggest that the delayed cardioprotection afforded by intestinal ischemic preconditioning is mediated by endogenous CGRP via the nitric oxide pathway.     

Edaravone reduces myocardial infarct size and improves cardiac function and remodelling in rabbits

1. In the present study, we investigated the effect of 3-methyl-1-phenyl-2-pyrazolin-5-one (edaravone), a free radical scavenger, on myocardial infarct (MI) size and cardiac function in an in vivo model of MI in rabbits. We further investigated the contribution of hydroxyl radicals, superoxide and nitric oxide (NO) to its effects. 2. Anaesthetized open-chest Japanese white male rabbits were subjected to 30 min coronary occlusion and 48 h reperfusion. The control group (n = 10) was injected with saline 10 min before reperfusion. The edaravone group (n = 10) was injected with a bolus of 3 mg/kg edaravone 10 min before reperfusion. The edaravone + N(G)-nitro-L-arginine methyl ester (L-NAME) group (n = 5) was given 10 mg/kg, i.v., L-NAME 10 min before the administration of 3 mg/kg edaravone. The L-NAME group (n = 5) was given 10 mg/kg, i.v., L-NAME 20 min before reperfusion. Infarct size was measured using the triphenyl tetrazolium chloride method and is expressed as a percentage of area at risk. Cardiac function was assessed by echocardiography 14 days after infarction. 3. In another series of experiments, rabbits were subjected to 30 min coronary occlusion and 30 min reperfusion and myocardial interstitial 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA levels, indicators of hydroxyl radical, were measured using a microdialysis technique. 4. Infarct size in the edaravone group was significantly reduced compared with that in the control group (27.4 +/- 6.8 vs 43.4 +/- 6.8%, respectively; P < 0.05). The edaravone-induced reduction of infarct size was abolished by pretreatment with L-NAME. Myocardial interstitial levels of 2,3-DHBA and 2,5-DHBA increased 20 and 30 min after ischaemia and peaked at 10 min reperfusion in the control group. Edaravone significantly inhibited the increase in 2,3-DHBA and 2,5-DHBA levels seen during reperfusion. Dihydroethidium staining showing in situ detection of superoxide was less intense in ischaemic myocardium in the edaravone-treated group compared with the control group. Edaravone improved cardiac function and left ventricular remodelling 14 days after infarction. 5. In conclusion, edaravone significantly reduces MI size and improves cardiac function and LV remodelling by decreasing hydroxyl radicals and superoxide in the myocardium and increasing the production of NO during reperfusion in rabbits.     

Antiarrhythmic effects of MS-551, a new class III antiarrhythmic agent, on canine models of ventricular arrhythmia.

The antiarrhythmic effects of MS-551, which prolongs cardiac action potential duration without affecting the maximum upstroke velocity of the action potential, were assessed in three different canine ventricular arrhythmia models: 1) ventricular tachycardia (VT) induced by electrical stimuli 3-5 days after myocardial infarction, 2) spontaneous ventricular tachyarrhythmias 24-48 hr after two-stage coronary ligation and 3) ventricular tachyarrhythmias induced by digitalis. Intravenous administration of MS-551 (0.1-1 mg/kg) decreased the susceptibility in 10 dogs out of 13 to VT or ventricular fibrillation evoked by programmed electrical stimulation (PES) delivered to the ventricular septum 3-5 days after myocardial infarction. Oral administration of MS-551 (3 mg/kg) also decreased the susceptibility to VT evoked by PES in 7 out of 10 conscious postinfarction dogs. Concurrently, intravenous (0.1-1 mg/kg) or oral (3 mg/kg) administration of MS-551 produced increases in the ventricular effective refractory periods (ERP) by 7 +/- 1% - 17 +/- 3% or 13 +/- 2%, respectively. Similarly, d-sotalol (0.3-3 mg/kg, i.v. and 10 mg/kg, p.o.) decreased the susceptibility to VT with increased ERP. However, MS-551 (1 and 10 mg/kg, i.v.) failed to inhibit both canine two-stage coronary ligation arrhythmia and digitalis arrhythmia. These results suggest that MS-551 is a pure class III antiarrhythmic drug which may be effective in the treatment of life-threatening reentrant tachyarrhythmias, but not in automaticity arrhythmias.     

Interaction of cyclooxygenase-2 inhibitors and salicylate in gastric mucosal damage.

The interactions of sodium salicylate and the selective cyclooxygenase-2 inhibitors N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS-398) and 5.5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulfonyl)phenyl-2(5II)-furanone (DFU), dexamethasone and the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methylester (L-NAME) were examined in ischaemia-reperfusion damage and adaptive protection in the rat stomach. Ischaemia-reperfusion damage was substantially aggravated by pretreatment with NS-398 (4 mg/kg), DFU (2 mg/kg), dexamethasone (1 mg/kg) or L-NAME (3 and 10 mg/kg). Salicylate (0.01-0.05 mg/kg) reversed the aggravating effect of NS-398, DFU and dexamethasone, while the effect of L-NAME was counteracted by L-arginine (twice 400 mg/kg) but not salicylate (0.05 or 10 mg/kg). Instillation of 20% ethanol prevented mucosal damage induced by 70% ethanol. This adaptive gastroprotection was abolished by pretreatment with NS-398 (1 mg/kg), DFU (0.2 mg/kg) or L-NAME (10 mg/kg). Salicylate (0.01-0.05 mg/kg) reversed the inhibition of protection by NS-398 and DFU, while the effect of L-NAME (10 mg/kg) was antagonized by L-arginine (100 mg/kg) but not salicylate (0.05 mg/kg). The precise mechanism of the functional antagonism between extremely low doses of salicylate and selective cyclooxygenase-2 inhibitors remains to be investigated.     

Anti-inflammatory effect of propolis through inhibition of nitric oxide production on carrageenin-induced mouse paw edema

The anti-inflammatory effect of propolis was compared with that of diclofenac, a non-steroidal anti-inflammatory drug, and L-N(G)-nitro arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, using carrageenin-induced mouse paw edema. When administered 10 min prior to carrageenin injection, propolis (1 : 1000, 1 : 100, p.o.), diclofenac (12.5, 50 mg/kg, p.o.) and L-NAME (10, 100 mg/kg, s.c.) showed a significant anti-inflammatory effect. The anti-inflammatory effects of propolis and L-NAME were significantly inhibited by L-arginine, a precursor of nitric oxide, but not by D-arginine. In contrast, the anti-inflammatory effect produced by diclofenac was not inhibited by either D-arginine or L-arginine. These results indicate that the anti-inflammatory effect of propolis on mouse paw edema acts via the inhibition of nitric oxide production, similar to that of L-NAME but not diclofenac.     

Differential renal response to Nomega-nitro-L-arginine methyl ester and L-arginine in rats with hypertensive or diabetic nephropathy

The present experiments were designed to assess the renal functional response to alterations in nitric oxide formation in animals with different forms of nephropathy. To address this issue, the effects of Nomega-nitro-L-arginine methyl ester (L-NAME) or L-arginine were assessed in animal models exhibiting arterial hypertension due to chronic nitric oxide inhibition (L-NAME, 50 mg/l in drinking water for 12 weeks) or diabetes mellitus (streptozotocin, 60 mg/kg IP). Vehicle-treated, age-matched animals served as controls. Following 12 weeks of pretreatment, mean arterial pressure (MAP), renal hemodynamics, urinary albumin, and electrolyte excretion were determined in standard clearance experiments prior to and following infusion of L-NAME (50 microg/kg/min), l-arginine (5 mg/kg/min), or saline vehicle. In control animals, L-NAME resulted in an increase in MAP and renal vascular resistance and a decline in glomerular filtration rate and renal plasma flow, as expected. L-arginine had no effect on renal hemodynamics. In nitric oxide-depleted hypertensive animals, L-NAME had no additional effect on MAP or renal hemodynamics. Infusion of L-arginine reduced elevated MAP but did not reverse changes in renal hemodynamics. Diabetic rats demonstrated glomerular hyperfiltration and proteinuria. No significant changes in MAP or renal hemodynamics were observed following infusion of L-NAME or L-arginine, respectively. However, L-NAME increased urinary albumin excretion in the absence of hemodynamic changes. The effects of nitric oxide on vascular tone were shown to be dependent on the vascular bed and the underlying disease. Variations in local nitric oxide formation and susceptibility may account for the differential response of the systemic and renal vasculature and contribute to the degree of renal functional impairment observed in different systemic diseases.     

The role of nitric oxide in the reversal of hemorrhagic shock by oxotremorine.

In the present study, the effect of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME), on the antishock actions of oxotremorine was investigated in rats subjected to hemorrhagic shock under urethane anesthesia. L-citrulline production in the AV3V region, as an indicator of nitric oxide (NO) synthesis, was assayed by high-performance liquid chromatography (HPLC) with fluorescent detection throughout the experiment. The rats were pretreated with either intravenous (i.v.) physiological saline or L-NAME (2.5 mg/kg) before bleeding. L-NAME potentiated the reversal of hypotension by oxotremorine (25 microg/kg, i.v.). However, oxotremorine either alone or in combination with L-NAME did not produce any significant change in 60-min survival rate at this low dose. Analysis of microdialysis samples collected from the AV3V region showed that L-citrulline concentration increased during bleeding and that this increase was abolished by L-NAME pretreatment. These results may suggest that nitric oxide production contributes to hypotension in rats bled to shock since nitric oxide levels in the AV3V region increased in response to bleeding and nitric oxide synthase (NOS) inhibition abolished this increase and potentiated the oxotremorine-induced reversal of hypotension.     

Effects of antiaggregant and antiinflammatory doses of aspirin on coronary hemodynamics and myocardial reactive hyperemia in conscious dogs

Clinical studies have shown that low doses of aspirin (<300 mg/day) inhibit thromboxane A2 production and platelet aggregation but preserve prostacyclin synthesis. In contrast, high doses of aspirin (>1,000 mg/day) suppress the synthesis of both eicosanoids. Because the consequences of aspirin administration have never been investigated on coronary vasomotor tone in vivo, we investigated the effects of low and high doses of aspirin on systemic and coronary hemodynamics under basal conditions and after myocardial reactive hyperemia in conscious dogs. Dogs were instrumented with a Doppler flow probe and a hydraulic occluder. Coronary blood flow was measured in the conscious state at baseline and during myocardial reactive hyperemia after 10, 20, and 30 s of coronary occlusion. Thromboxane B2 serum concentrations, an index of platelet aggregation, decreased by >90% after long-term i.v. administration of aspirin, 100 mg/day for 7 days (low dose). Neither systemic and coronary hemodynamics nor reactive hyperemia were affected by the drug. After combined administration of this low dose of aspirin and of the nitric oxide synthase (NOS) inhibitor, N(omega)-nitro-L-arginine (L-NNA, 30 mg/kg/day/7 days), reactive hyperemia decreased to the same extent as when L-NNA was administered alone. After administration of a unique high-dose aspirin (1,000 mg, i.v.), myocardial reactive hyperemia was markedly reduced, and this effect was still observed after previous blockade of NOS and cyclooxygenase by L-NNA and diclofenac, respectively. Thus long-term treatment with a low antiaggregant dose of aspirin does not alter the ability of coronary vessels to dilate during myocardial reactive hyperemia in conscious dogs. In contrast, short-term administration of a high antiinflammatory dose of aspirin severely blunts myocardial reactive hyperemia through a mechanism that is independent of both cyclooxygenase and nitric oxide metabolic pathways.     

Inhibition of nitric oxide synthase enhances cocaine's developmental toxicity: vascular and CNS effects.

Ischemia and/or reperfusion injury from free radicals may cause cocaine's toxicity, including its effect upon neurobehavioral development. We previously used salicylate to measure hydroxyl free radicals in chick embryos exposed to cocaine. The combination was more toxic than cocaine alone. We postulated that salicylate enhanced the vasoconstriction and toxicity via inhibition of compensatory processes (eg by inhibition of the synthesis of vasodilatory prostanoids and/or nitric oxide). A nontoxic dose of N(G)-nitro-L-arginine methyl ester (L-NAME) was used to inhibit nitric oxide synthase to test this hypothesis. In one experiment, cocaine was injected every 1.5 h (total dose =67.5 mg/kg egg) on day 15 of development, 1 h after injection of L-NAME (200 mg/kg egg) to determine viability and hatchability, which are measures of toxicity. Another experiment measured diameters of blood vessels after L-NAME injection, followed by NaCl or cocaine infusion (0.23 mg/egg/min; total dose=67.5 mg/kg egg) at 15 and 5 min afterwards. Lastly, brains of embryos pretreated with L-NAME before cocaine injections were analyzed for nitric oxide synthase activity. Cocaine decreased viability and hatchability. L-NAME enhanced cocaine's effect upon both parameters. Blood vessel diameters were decreased by cocaine after 15 min of infusion. L-NAME+cocaine caused a decrease in vessel diameter as soon as 5 min into the infusion and was greater with time, compared with other groups. Enzyme activity in brains was decreased only in the L-NAME+cocaine group. Thus, inhibition of nitric oxide synthesis interferes with the embryos' capacity to mount a compensatory vasodilatory response.