The L-arginine-nitric oxide pathway

Regulation of prostaglandin production by nitric oxide; an in vivo analysis

1. Endotoxin E. Coli lipopolysaccharide (LPS)-treatment in conscious, restrained rats increased plasma and urinary prostaglandin (PG) and nitric oxide (NO) production. Inducible cyclo-oxygenase (COX-2) and nitric oxide synthase (iNOS) expression accounted for the LPS-induced PG and NO release since the glucocorticoid, dexamethasone inhibited both effects. Thus, LPS (4 mg kg-1) increased the plasma levels of nitrite/nitrate from 14 +/- 1 to 84 +/- 7 microM within 3 h and this rise was inhibited to 35 +/- 1 microM by dexamethasone. Levels of 6-keto PGF1 alpha in the plasma were below the detection limit of the assay (< 0.2 ng ml-1). However, 3 h after the injection of LPS these levels rose to 2.6 +/- 0.2 ng ml-1 and to 0.7 +/- 0.01 ng ml-1 after LPS in rats that received dexamethasone. 2. The induced enzymes were inhibited in vivo with selective COX and NOS inhibitors. Furthermore, NOS inhibitors, that did not affect COX activity in vitro markedly suppressed PG production in the LPS-treated animals. For instance, the LPS-induced increased in plasma nitrite/nitrate and 6-keto PGF1 alpha at 3 h was decreased to 18 +/- 2 microM and 0.5 +/- 0.02 ng ml-1, 23 +/- 1 microM and 0.7 +/- 0.01 ng ml-1, 29 +/- 2 microM and 1 +/- 0.01 ng ml-1 in rats treated with LPS in the presence of the NOS inhibitors NG-monomethyl-L-arginine, NG-nitro arginine methyl ester and aminoguanidine, respectively. 3. The intravenous infusion of the NO donors sodium nitroprusside (SNP) or glyceryl trinitrate (GTN)increased prostaglandin production in normal animals (for instance urinary PGE2 excretion was increased from 96 +/- 10 to 576 +/- 12 pg min-1 and 400 +/- 24 pg min-1 in the presence of GTN or SNP respectively).4. Proteinuria was measured in order to evaluate the roles of NO and PG in renal damage associated with the in vivo injection of LPS. Interestingly, dexamethasone and the NOS inhibitors attenuated proteinuria in the LPS-treated rats. The COX inhibitors had no effect. It therefore appears that NO and not PG contributes to the LPS-induced renal damage; these findings support the potential use of NOS inhibitors in the treatment of renal inflammation.5. This study demonstrates the regulatory contribution of NO on the in vivo production of prostanoids and suggests that in inflammatory diseases that are driven by both NO and the prostaglandins, NOS inhibitors may act to reduce inflammation by the dual inhibition of cytotoxic NO and pro-inflammatory PG.

Na(+)-Ca2+ exchange in intact endothelium of rabbit cardiac valve

A new method of measuring cytoplasmic free Ca2+ ([Ca2+]i) of individual intact cardiovascular endothelial cells by using imaging fluorescence microscopy was designed. Application of agonist to the aortic or pulmonary valve of the rabbit triggered an increase in [Ca2+]i, which depended on the existence of endothelium on the surface of the valve. Under resting conditions, sudden reversal of the Na+ gradient by substituting external Na+ with N-methyl D-glucamine (NMDG) resulted in a [Ca2+]i spike, which then returned toward the resting level. Increasing intracellular Na+ concentration ([Na+]i) by application of ouabain or monensin induced a sustained [Ca2+]i increase. Na+ substitution by NMDG during the agonist- or monensin-induced [Ca2+]i increase gave rise to a further [Ca2+]i spike, which subsequently declined to a level higher than that before removal of external Na+. A selective inhibitor of Na(+)-Ca2+ exchange, 3',4'-dichlorobenzamyl (DCB), abolished the transient [Ca2+]i increase induced by Na+ substitution, and Mg2+, an inorganic inhibitor of Na(+)-Ca2+ exchanger, markedly reduced this transient [Ca2+]i increase. On the other hand, the selective Na(+)-H+ exchanger blocker 5-(N,N-hexamethylene)amiloride (HMA) did not abolish the transient [Ca2+]i increase caused by Na+ substitution. In summary, decreasing the Na+ gradient of the endothelial cells through either receptor stimulation (agonist), Na(+)-K+ pump inhibition (ouabain), pretreatment with Na+ ionophore (monensin), or reversing the Na+ gradient through Na+ substitution (NMDG) all increased [Ca2+]i. This raised [Ca2+]i was antagonized by agents such as DCB or Mg2+, which are thought to inhibit Na(+)-Ca2+ exchange, but not by HMA, an inhibitor of Na(+)-H+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

The value and limitations of L-arginine infusion on glomerular and tubular function in the ischemic/reperfused kidney

PURPOSE

The nitric oxide precursor, L-arginine, has been shown to have a salutary effect on ischemia and reperfusion injury in skeletal muscle, skin, and intestines. Because L-arginine also increases renal blood flow, glomerular filtration, and urine flow in experimental animals with normal renal function, we postulated that L-arginine may also improve renal function after renal ischemic injury.

METHODS

Eighteen adult New Zealand white rabbits weighing 3 to 3.5 kg were subjected to bilateral normothermic renal ischemia by clamping both renal pedicles for 1 hour followed by 2 hours of reperfusion. The animals were randomized into three groups: group I (control, n = 6) received no additional treatment; group II (pretreatment, n = 6) received systemic intravenous L-arginine at 150 mg/kg over 20 minutes before induction of ischemia; group III (posttreatment, n = 6) received systemic intravenous L-arginine at 150 mg/kg over 20 minutes from the onset of reperfusion. Urine flow, creatinine clearance (CCR), fractional excretion of sodium (FENa), and renal failure index (RFI) were calculated before ischemia and 2 hours after reperfusion, by use of standard formulas. The changes of the various renal parameters were compared among the three groups.

RESULTS

Bilateral normothermic renal ischemia for 1 hour produced a significant deterioration of glomerular filtration as evidenced by a CCR decrease from 11.1 +/- 1.8 to 2.49 +/- 0.9 ml/min (p < 0.01), FENa increase from 2.9% +/- 1.0% to 20.8% +/- 1.5% (p < 0.01) and RFI increase from 4.0 +/- 1.3 to 28.8 +/- 2.6 (p < 0.01). Pretreatment with L-arginine (group II) minimized the deleterious effects caused by ischemia on glomerular filtration (CCR of 2.49 +/- 0.9 ml/min in group I vs 4.95 +/- 2.5 ml/min in group II, p < 0.05) and tubular function (FENa of 20.8% +/- 1.5% in group I vs 13.0% +/- 5.6% in group II and RFI of 28.8 +/- 2.6 in group I vs 18.6 +/- 8.0 in group II, p < 0.05). Infusion of L-arginine at the onset of reperfusion (group III) produced a significant diuretic effect (urine flow from 32.6 +/- 13.4 ml/hr in group I to 63.3 +/- 18.8 ml/hr in group III, p < 0.05) and also minimized glomerular damage (CCR from 2.49 +/- 0.9 ml/min in group I to 4.80 +/- 1.2 ml/min in group III, p < 0.05); however, no beneficial effect was observed on tubular function.

CONCLUSION

Induction of nitric oxide production by systemic L-arginine infusion can best preserve glomerular and tubular function in the ischemic/reperfused kidney when given before the ischemic insult.

Nitric oxide and asthmatic inflammation

Asthmatic patients show an increased expression of inducible nitric oxide synthase (iNOS) in airway epithelial cells and an increased level of nitric oxide (NO) in exhaled air. The NO derived from airway epithelial cells may be a mechanism for amplifying and perpetuating asthmatic inflammation, through inhibition of T helper 1 (Th1) cells and their production of interferon gamma (IFN-gamma). This would result in an increase in the number of Th2 cells and the cytokines interleukin 4 (IL-4) (which is important for IgE expression) and IL-5 (which plays a critical role in the recruitment of eosinophils into the airways). Although this mechanism may be part of our nonspecific airway defence against parasite invasion, it appears to have been activated inappropriately in asthma. Here, Peter Barnes and Eddy Liew argue that the development of specific iNOS inhibitors may represent a novel therapeutic approach for asthma and other allergic diseases.

Inhibition of nitric oxide synthesis reduces infarct size by an adenosine-dependent mechanism

BACKGROUND

Nitric oxide (NO) is both a potent endogenous vasodilator with potential to attenuate ischemia-reperfusion injury and a mediator of tissue injury. The aim of the present study was to investigate the mechanism by which prior inhibition of NO synthesis can lessen ischemia-reperfusion injury in the isolated rabbit heart.

METHODS AND RESULTS

We examined the effects of inhibition of NO synthesis on infarct size using a model of coronary artery ligation in isolated rabbit hearts perfused at a constant flow rate of 35 mL/min. Infarct size averaged 65% of the zone at risk after 45 minutes of ischemia and 180 minutes of reperfusion. The addition of 30 mumol/L NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthesis, to the perfusate reduced the infarct-to-risk (I/R) ratio to an average of 41% (P < .05 versus control). This effect was abolished by pretreatment with 75.5 mumol/L 8-p-sulfophenyl theophylline (SPT), an adenosine receptor antagonist (I/R ratio, 63%). Ischemic preconditioning (5 minutes of ischemia and 10 minutes of reperfusion) before 45 minutes of ischemia and 3 hours of reperfusion reduced the I/R ratio to an average of 21%, and this was not augmented by pretreatment with L-NAME (I/R ratio, 20%). However, all protection due to preconditioning and L-NAME was lost in hearts pretreated with SPT (I/R ratio, 59%). In a separate set of experiments, adenosine concentration in the coronary perfusate and myocardial lactate concentrations were measured. Treatment with L-NAME increased the average adenosine concentration in the perfusate from 5.7 mumol/L per 100 g of heart (control) to a peak of 24.0 mumol/L per 100 g of heart; however, there was no effect on average myocardial lactate concentration (control, 4.6 mumol/g dry wt; L-NAME, 5.5 mumol/g dry wt). In contrast, after 5 minutes of global ischemia, the average adenosine concentration peaked at 139.0 mumol/L per 100 g of heart, and the average myocardial lactate concentration increased to 27.1 mumol/g dry wt.

CONCLUSIONS

Infarct size limitation after inhibition of NO synthesis shares a common mechanism with that of ischemic preconditioning and is dependent on the release of adenosine. However, in this model, adenosine release after inhibition of NO synthesis is not secondary to myocardial ischemia. The protection of the heart against ischemic injury by adenosine appears to be concentration dependent.

Inducible nitric oxide synthase in rat hepatic lipocytes and the effect of nitric oxide on lipocyte contractility

In liver injury, perisinusoidal cells known as lipocytes (Ito cells) undergo "activation," acquiring smooth muscle-like features and a contractile phenotype. To assess whether contraction of these cells is regulated by nitric oxide (NO), we examined the production of NO by lipocytes and the effect of NO on lipocyte contractility. Cultured lipocytes were exposed to cytokines and/or LPS. Single agents had little or no effect on the level of inducible NO synthase (iNOS) mRNA. However, interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha), or LPS in combination with interferon-gamma (IFN-gamma) stimulated iNOS mRNA, which was present within 4 h after exposure. iNOS mRNA levels were paralleled by changes in nitrite (a metabolic product of NO). Intraperitoneal administration of IFN-gamma, TNF-alpha, and LPS led to rapid induction of iNOS mRNA in lipocytes, confirming in vivo the culture findings. Ligation of the common hepatic bile duct, which induces periportal-based liver injury, stimulated iNOS mRNA in lipocytes. Transforming growth factor-beta 1 decreased IFN-gamma/TNF-alpha--stimulated iNOS mRNA and nitrite. Finally, the effect of NO on lipocyte contractility was examined. In cells incubated with IFN-gamma and TNF-alpha, the contractile response to either serum or endothelin-1 was blocked. Contraction was restored entirely by an inhibitor of NO synthase, NG-monomethylarginine. Furthermore, 8-bromoguanosine 3':5'-cyclic monophosphate and sodium nitroprusside inhibited lipocyte contractility, consistent with the effect of NO induced by cytokines. We conclude that NO is a potent modulator of lipocyte contractility and may regulate this function by autocrine (or intracrine) mechanisms. Moreover, NO may play an important role in liver injury, countering the effect of contractile agonists on lipocytes.

Glibenclamide-induced inhibition of the expression of inducible nitric oxide synthase in cultured macrophages and in the anaesthetized rat

1. We have investigated whether glibenclamide, an inhibitor of ATP-sensitive potassium channels, influences the induction of the calcium-independent isoform of nitric oxide synthase (iNOS) in cultured J774.2 macrophages activated by bacterial endotoxin (E.coli lipopolysaccharide; LPS), as well as in the lung and aorta of rats with endotoxic shock. 2. Pretreatment of J774.2 macrophages with glibenclamide (10(-7) to 10(-5) M for 30 min) dose-dependently inhibited the accumulation of nitrite caused by LPS (1 microgram ml-1). In contrast, pretreatment of macrophages with tetraethylammonium (10(-4) to 10(-2) M for 30 min), a non-selective inhibitor of potassium channels, did not affect the rise in nitrite caused by LPS. At the highest concentration (10(-5) M) used, cromakalim, an opener of ATP-sensitive potassium channels, caused a small, but significant inhibition of nitrite formation in macrophages activated with LPS, while lower concentrations (10(-7) to 3 x 10(-6) M) were without effect. 3. The inhibition by glibenclamide (3 microM) of the increase in nitrite induced by LPS in J774.2 macrophages was weaker when glibenclamide was given several hours after LPS, indicating that glibenclamide inhibits the induction, but not the activity, of iNOS. In contrast, the degree of inhibition of nitrite formation caused by the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) was similar when this agent was given up to 10 h after LPS. 4.In anaesthetized rats, LPS caused a fall in mean arterial blood pressure (MAP) from 120 +/-(time 0)to 98 +/- mmHg at 180 min (P<0.05, n = 6). Treatment of LPS-rats with glibenclamide (1 mg kg-1, i.v.at 60 min after LPS) caused a rapid and sustained rise in MAP (e.g. MAP at 180 min after LPS:122 +/-4 mmHg; n =6, P <0.05 when compared to LPS-rats). The maximum of the rise in MAP produced by glibenclamide (1 mg kg-1 , i.v.) was similar when the drug was given either at 60 or 180 min after LPS. However, the duration of the pressor response was significantly longer when glibenclamide was given at 60 min, rather than at 180 min after LPS.5. LPS-treatment caused a significant reduction of the pressor responses elicited by noradrenaline (NA,1 microg kg-1, i.v.) from 35 +/- 2 to 19 +/- 1 mmHg at 60 min and 20 +/- 2 mmHg at 180 min (P<0.05).Treatment of LPS-rats with glibenclamide (1 mg kg-1, i.v. at 60 min) caused a significant restoration of the pressor responses elicited by NA from 19 +/- 1 mmHg at 60 min (prior to glibenclamide injection) to 29 +/- 3 mmHg at 180 min (P<0.05).6. Endotoxaemia for 180 min resulted in a significant increase in a calcium-independent NOS activity(which was taken to represent iNOS activity) in the lung from 0.17 +/- 0.1 (control, n =4) to 6.21 +/- 0.48 pmol mg-1 min-1 (n =6, P<0.05). Injection of glibenclamide (1 mg kg-1, i.v.) at 60 min after LPS attenuated the increase in iNOS activity caused by endotoxaemia in the lung by 43 +/- 7%(n = 6, P <0.05). In contrast, injection of glibenclamide at 180 min after LPS did not result in a significant inhibition of iNOS activity (n = 6, P <0.05. 7. Thoracic aortae obtained from rats at 180 min after LPS showed a significant reduction in the contractions elicited by noradrenaline (NA, 10-9 to 10-6 M). Treatment of LPS-rats with glibenclamide(1 mg kg-1, i.v. at 60 min after LPS) significantly alleviated this LPS-induced hyporeactivity to NA ex vivo. In contrast, when aortic rings from LPS-rats were incubated in vitro with glibenclamide (10 microM for 20 min), glibenclamide did not reverse the vascular hyporeactivity to NA. However, L-NAME (300 microM for 20 min) significantly enhanced the contractile response to NA in aortic rings obtained from LPS-rats(P<0.05, n=6).8. No significant amounts of tumour necrosis factor-alpha (TNF alpha) were detectable in the plasma before the injection of LPS. Endotoxaemia for 90 min resulted in a significant rise in plasma TNFalpha levels(0.05 +/- 0.05 ng ml-1 at time 0, 3.78 +/- 0.24 ng ml-1 at 90 min, n = 6, P < 0.05). Treatment of LPS-rats with glibenclamide (1 mg kg-1, i.v. at 15 min prior to LPS, n = 5) did not significantly reduce the rise in plasma TNF alpha levels caused by endotoxin.9. Thus, glibenclamide inhibits the induction, but not the activity, of iNOS in vitro and in vivo. This inhibition of iNOS induction may contribute to the beneficial haemodynamic effects of glibenclamide in endotoxic shock.

Pulmonary catabolism of interferon-gamma evaluated by lung perfusion of both normal and smoke-exposed rats

The role of the lungs in the catabolism of rat recombinant interferon-gamma, either in normal rats or in rats subjected to an acute cigarette smoking episode, was evaluated using an isolated and perfused lung preparation. After administration of interferon-gamma into the lung perfusion medium, there was no clearance of the cytokine in either control or smoke-exposed rat lungs, and only 0.1 +/- 0.2% of the total dose was recovered in the bronchoalveolar lavage fluid. When the same amount of interferon-gamma was instilled into the bronchial alveolar tree, concentrations of the cytokine in the perfusate increased progressively so that after 3 h up to 71.2 +/- 4.3 and 62 +/- 5.7% of the administered dose, as measured by ELISA test, had been transferred from the bronchial lumen to the perfusion medium of either control or smoke-exposed rat lungs, respectively, the latter values being significantly lower (p < or = 0.05) than those obtained in control lungs. Moreover, total recoveries of interferon-gamma evaluated in smoke-exposed rat lungs (78.4 +/- 8.6%) were also significantly lower than those observed in control rat lungs (91.4 +/- 11.8%). Biologic activity evaluations on the same samples gave values significantly lower than those obtained using ELISA, indicating a partial loss of biologic activity during transalveolar transit. In conclusion, it appears that the transfer of interferon-gamma is almost exclusively unidirectional from the alveolar space to the plasmatic pool, with partial degradation during transalveolar passage.

The emerging multifaceted roles of nitric oxide

Nitric oxide (NO) is a highly reactive free radical with a multitude of organ specific regulatory functions. Since 1985, NO has been the subject of numerous research efforts and as a result, has been found to play a major role in the cardiovascular, pulmonary, gastrointestinal, immune, and central nervous systems. In addition, deranged NO synthesis is the basis for a number of pathophysiologic states, such as atherosclerosis, pulmonary hypertension, pyloric stenosis, and the hypertension associated with renal failure. Traditional NO donors such as sodium nitroprusside and new pharmacologic NO adducts such as S-nitrosothiols may serve as exogenous sources of NO for the treatment of NO-deficient pathologic states. This review is an attempt to acquaint the surgical community with the fundamentals of NO biochemistry and physiology. Increased knowledge of its functions in normal homeostasis and pathologic states will enable physicians to better understand these disease processes and utilize new pharmacologic therapies.

Bone cell function, regulation, and communication: a role for nitric oxide

A large array of factors serve as vital communication links between cells and the characterization, regulation, and mechanisms of action of such factors are topics of intense research efforts. Most intercellular messenger molecules which have been described over the years are represented by proteins, small peptides, amino acids or their derivatives, ions, lipid metabolites, or steroids. However, a small uncharged free radical, nitric oxide, has recently garnered much attention as a potent multifunctional signal molecule with widespread actions within and between diverse tissues. Biochemical, molecular, and regulatory studies of the family of enzymes responsible for nitric oxide synthesis, nitric oxide synthases, have established that there are at least three distinct isoforms of this enzyme which are differentially expressed and regulated in various cells or tissues. Modulation of these isoenzyme levels or activities by diverse signals is mediated via transcriptional, translational, and/or post-translational mechanisms, and consequently, alterations in such control may influence normal or pathological processes. Nitric oxide appears to exert pronounced effects on skeletal physiology and its production by various bone cells, elicited target cell responses, modulation by other signalling molecules (e.g., cytokines, hormones, fatty acid derivatives), and chemical interactions with other free radicals (e.g., superoxide anions, hydroxyl radicals) may form one important facet of the many complicated communication pathways controlling bone cell physiology and remodeling. Further cell and molecular studies are needed to address the precise roles that nitric oxide plays in bone development and in the formation and degradation of bone during ordinary bone metabolism. In addition, alterations in the regulation and action of the bone nitric oxide system as a function of certain bone disorders may be manifested by perturbations in bone integrity or mineral homeostasis. In this article, we review the current evidence implicating nitric oxide as an important messenger molecule in bone intercellular communication, speculate on potential roles for this radical in bone biology, and discuss possible future directions for advanced research into the function of nitric oxide in skeletal physiology.

The effect of cholesterol-lowering and antioxidant therapy on endothelium-dependent coronary vasomotion

BACKGROUND

Patients with coronary artery disease and abnormalities of serum lipids often have endothelial vasodilator dysfunction, which may contribute to ischemic cardiac events. Whether cholesterol-lowering or antioxidant therapy can restore endothelium-dependent coronary vasodilation is unknown.

METHODS

We randomly assigned 49 patients (mean serum cholesterol level, 209 +/- 33 mg per deciliter [5.40 +/- 0.85 mmol per liter]) to receive one of three treatments: an American Heart Association Step 1 diet (the diet group, 11 patients); lovastatin and cholestyramine (the low-density lipoprotein [LDL]-lowering group, 21 patients); or lovastatin and probucol (the LDL-lowering-antioxidant group, 17 patients). Endothelium-dependent coronary-artery vasomotion in response to an intracoronary infusion of acetylcholine (10(-8) to 10(-6) M) was assessed at base line and after one year of therapy. Vasoconstrictor responses to these doses of acetylcholine are considered to be abnormal.

RESULTS

Treatment resulted in significant reductions in LDL cholesterol levels of 41 +/- 22 percent in the LDL-lowering-antioxidant group and 38 +/- 20 percent in the LDL-lowering group (P < 0.001 vs. the diet group). The maximal changes in coronary-artery diameter with acetylcholine at base line and at follow-up were -19 and -2 percent, respectively, in the LDL-lowering-antioxidant group, -15 and -6 percent in the LDL-lowering group, and -14 and -19 percent in the diet group (P < 0.01 for the LDL-lowering-antioxidant group vs. the diet group; P = 0.08 for the LDL-lowering group vs. the diet group). (The negative numbers indicate vasoconstriction). Thus, the greatest improvement in the vasoconstrictor response was seen in the LDL-lowering-antioxidant group.

CONCLUSIONS

The improvement in endothelium-dependent vasomotion with cholesterol-lowering and antioxidant therapy may have important implications for the activity of myocardial ischemia and may explain in part the reduced incidence of adverse coronary events that is known to result from cholesterol-lowering therapy.

Systemic nature of endothelial dysfunction in atherosclerosis

Vascular endothelium plays a pivotal role in the maintenance of vasodilation and the inhibition of platelet aggregation and smooth muscle cell proliferation through the release of nitric oxide and other factors. Extensive data have demonstrated abnormalities in coronary endothelial function in the epicardial coronary arteries in patients with atherosclerosis or risk factors for atherosclerosis. This dysfunction leads to abnormal vasoconstriction and platelet aggregation, which likely play a role in producing ischemia in patients with coronary artery disease. Invasive techniques have been available to assess endothelium-dependent vasodilator responses in the coronary arteries. However, until recently little has been known about endothelial responses in the peripheral vasculature, as methods to assess this have not been readily available. The hypothesis that endothelial dysfunction is a systemic process is explored, and new noninvasive methods of assessing endothelial function are discussed.

Dilatation of cerebral arterioles in response to lipopolysaccharide in vivo

BACKGROUND AND PURPOSE

Bacterial lipopolysaccharide can increase nitric oxide (NO) production by expression of an inducible form of NO synthase. Bacterial infections of the central nervous system dilate cerebral vessels and increase blood flow. We hypothesized that topical application of bacterial lipopolysaccharide would increase production of NO, causing dilatation of cerebral arterioles.

METHODS

Cranial windows were implanted in anesthetized rabbits. Windows were flushed with artificial cerebrospinal fluid, artificial cerebrospinal fluid with lipopolysaccharide, or artificial cerebrospinal fluid with lipopolysaccharide and NG-monomethyl-L- arginine (an inhibitor of NO synthase) for 4 hours. Other rabbits received either dexamethasone or indomethacin intravenously 1 hour before lipopolysaccharide treatment of cranial windows.

RESULTS

Application of lipopolysaccharide in cranial windows produced marked, progressive vasodilation, with diameter increased by 58 +/- 7% (mean +/- SEM) after 4 hours. The cerebral vasodilator response was inhibited by NG-monomethyl-L-arginine, dexamethasone, or indomethacin. Excess L-arginine reversed the inhibitory effect of NG-monomethyl-L-arginine.

CONCLUSIONS

Inhibition of lipopolysaccharide-induced dilatation of cerebral arterioles by NG-monomethyl-L-arginine and dexamethasone suggests that a portion of the vasodilation was mediated by inducible NO synthase. Indomethacin also inhibited lipopolysaccharide-induced vasodilatation. These findings suggest an important role for both nitric oxide and cyclooxygenase products in lipopolysaccharide-induced cerebral arteriolar dilatation in vivo.

Cytokines regulate endotoxin stimulation of endothelial cell arginine transport

BACKGROUND

Endotoxin (lipopolysaccharide) stimulates transmembrane L-arginine transport in pulmonary artery endothelial cells (PAECs). The proinflammatory cytokines tumor necrosis factor (TNF) and interleukin-1 (IL-1) mediate many of the pathophysiologic effects of endotoxemia and sepsis. Endothelial cells secrete TNF and IL-1 in response to endotoxin. We hypothesize that lipopolysaccharide stimulation of plasma membrane L-arginine transport is mediated via an autocrine cytokine loop involving TNF and IL-1.

METHODS

Confluent porcine PAECs were incubated with various concentrations of lipopolysaccharide, TNF, or IL-1, and arginine uptake was determined by assaying the uptake of 3H-L-arginine in the presence or absence of Na+ at different time points. PAECs were then incubated with lipopolysaccharide or saline solution after pretreatment with either anti-TNF antibody or IL-1-receptor antagonist, and transport was measured 12 hours later.

RESULTS

Lipopolysaccharide, IL-1, and TNF all increased both Na+-dependent and Na+-independent carrier-mediated L-arginine transport in a fashion that was both time and dose dependent. Maximal increases in stimulated arginine uptake occurred 8 hours after exposure to the cytokines and 12 hours after exposure to lipopolysaccharide. Pretreatment of endothelial cells with anti-TNF antibody blocked lipopolysaccharide stimulation of both Na+-independent and Na+-dependent transport by 100% and 90%, respectively. In addition, IL-1-receptor antagonist inhibited lipopolysaccharide stimulation of both Na+-independent and Na+-dependent transport by 65% and 85%, respectively.

CONCLUSIONS

The marked increase in carrier-mediated L-arginine transport activity produced by lipopolysaccharide, IL-1, and TNF may represent an adaptive response by the pulmonary endothelium to support arginine-dependent biosynthetic pathways during sepsis. Furthermore, lipopolysaccharide stimulation of arginine transport is mediated in part through an autocrine mechanism involving IL-1 and TNF.

Plasma arginine, citrulline, and ornithine kinetics in adults, with observations on nitric oxide synthesis

The plasma fluxes of ornithine (Orn), arginine (Arg), and citrulline (Cit) and rate of conversion of labeled ornithine-to-citrulline (QOrn-->Cit) were estimated in six healthy adult men receiving an arginine-rich or arginine-free L-amino acid-based diet, each for 6 days. On day 7 an 8-h (3-h fast, 5-h fed) primed continuous intravenous infusion of L-[guanido-15N,15N] arginine, L-[ureido-13C]citrulline, L-[5,5,2H2]ornithine, and L-[5,5,5-2H3]leucine was conducted. Mean citrulline fluxes (mumol.kg-1.h-1) were 10.4-13.6 for the various conditions and/or diets and remained unchanged (P > 0.05). Arginine flux was lowered (P < 0.01) by 38% for fed state during arginine-free period. Ornithine fluxes for arginine-rich were (P < 0.01) reduced with the arginine-free diet. Rates of QOrn-->Cit declined by 30% (P < .05) during the fed arginine-free period. Short-term restriction in the dietary supply of arginine did not alter the rate of whole body nitric oxide synthesis. One subject showed a very high output of nitrate on arginine-free diet (6 times average for remaining subjects).

Inhibition of nitric oxide synthase--potential for a novel class of therapeutic agent?

The free-radical gas nitric oxide (NO) plays an important role in a wide and diverse range of physiological processes. As progress is made in understanding the biological function of NO, there is growing interest in the possibility that inhibitors of NO synthase (NOS) may be of clinical use in the therapy of certain disease states. The search for novel and clinically relevant inhibitors of this enzyme represents a truly multidisciplinary approach to drug screening and will no doubt benefit from the application of recombinant DNA (rDNA) technology.

Bioenergetic and oxidative stress in neurodegenerative diseases

Aging is a major risk factor for several common neurodegenerative diseases, including Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and Huntington's disease (HD). Recent studies have implicated mitochondrial dysfunction and oxidative stress in the aging process and also in the pathogenesis of neurodegenerative diseases. In brain and other tissues, aging is associated with progressive impairment of mitochondrial function and increased oxidative damage. In PD, several studies have demonstrated decreased complex I activity, increased oxidative damage, and altered activities of antioxidant defense systems. Some cases of familial ALS are associated with mutations in the gene for Cu, Zn superoxide dismutase (Cu, Zn SOD) and decreased Cu, Zn SOD activity, while in sporadic ALS oxidative damage may be increased. Defects in energy metabolism and increased cortical lactate levels have been detected in HD patients. Studies of AD patients have identified decreased complex IV activity, and some patients with AD and PD have mitochondrial DNA mutations. The age-related onset and progressive course of these neurodegenerative diseases may be due to a cycling process between impaired energy metabolism and oxidative stress.

Increased production of nitric oxide in patients infected with the European variant of hantavirus

Serum nitrate levels, a measure of nitric oxide (NO) production in vivo, were very high (95 +/- 14 microM) in 13 patients infected with Puumala virus, the European variant of Hantavirus (HTV), as compared to those in healthy subjects (33 +/- 3 microM). Serum nitrate levels showed a high and significant correlation with scores on the Acute Physiological And Chronic Health Evaluation (APACHE II) scale and with serum creatinine, and an inverse correlation with platelet counts. Serial serum measurements of nitrate in 2 severe cases showed very high levels at the onset of arterial hypotension and acute renal failure. We conclude that excessive amounts of NO are produced in patients infected with Puumala virus and that this reactive nitrogen intermediate could play an important role in the pathogenesis of the disease.

Endotoxin inhibition of distension-stimulated gastric acid secretion in rat: mediation by NO in the central nervous system

1. The involvement of nitric oxide in the acute inhibitory effects of low doses of endotoxin, following intracerebroventricular (i.c.v.) or intravenous (i.v.) administration, on gastric acid secretion stimulated by distension or i.v. infusion of pentagastrin has been investigated in the continuously perfused stomach of the anaesthetized rat. 2. The i.c.v. administration of E. coli endotoxin (800 ng kg-1) abolished the acid secretory response induced by gastric distension (20 cm water intragastric pressure) within 30 min of administration. 3. By contrast, submaximal rates of acid secretion induced by i.v. infusion of pentagastrin (8 micrograms kg-1 h-1) were not inhibited by i.c.v. administration of endotoxin (800 ng kg-1). 4. Prior i.c.v. administration of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 800 micrograms kg-1) restored the acid secretory responses to distension in rats treated with endotoxin (i.c.v.). 5. Likewise, i.v. administration of endotoxin (5 micrograms kg-1) abolished the acid secretory response induced by gastric distension within 30 min of administration. Prior i.c.v. injection of L-NAME (800 micrograms kg-1) or its i.v. administration (10 mg kg-1) restored acid secretory responses in rats receiving i.v. endotoxin. 6. The reversal by L-NAME (i.v.) of the acid inhibitory effects of endotoxin (i.v.) was prevented by L-arginine (12 mg kg-1, i.c.v. or 100 mg kg-1, i.v.), but not by its enantiomer D-arginine. 7. The present results imply the existence of an acute response to endotoxin involving NO synthesis in the brain. NO may act as a neuromodulator or neurotransmitter in a nervous reflex leading to the inhibition of acid secretion stimulated by gastric distension.

Alterations in chemically induced tissue injury related to all-trans-retinol pretreatment in rodents

Retinol (vitamin A) is an essential nutrient which has many physiological effects throughout the body. Our studies have demonstrated that retinol modulation of immune response, through alteration of macrophage and neutrophil function, can have dramatic effects on the toxicity of some compounds. Based on these studies, our current hypothesis for retinol potentiation of chemical-induced liver injury is that retinol administered to rats prior to the hepatotoxicant (CCl4 and AA in rats; and AA, APAP, and GalN in mice) primes the Kupffer cells to a more active state. This may occur in part as a result of increases in chemical mediators such as TNF from these Kupffer cells. Following hepatocyte damage by a toxicant, Kupffer cells are activated to release reactive oxygen species, immune mediators, and chemotactic factors which all serve to enhance the inflammatory response. This increased inflammatory response then results in increased injury to the already toxicant-damaged hepatocytes. In addition, retinol modulation of toxicant activation and detoxification may also make important contributions to the potentiation of some toxicants such as AA. Retinol protection of CCl4 hepatotoxicity in mice is more difficult to explain at this time but is possibly related to alterations in CCl4 metabolism in this species. Differences in response between pulmonary and liver macrophages (Kupffer cells) may explain the retinol protection from 1-NN pulmonary toxicity. Retinol may decrease the inflammatory response through downregulation of pulmonary macrophage function, thus resulting in decreased pulmonary injury. Finally, since retinol protection of cadmium toxicity in the liver and testis requires 7 days of retinol pretreatment, we suspect that retinol is inducing protective protein(s) in these organs. Aside from its normal biological role in rhe body, clinical medicine has found new uses for retinol in the treatment and prevention of some cancers, and in the treatment of certain dermatologic conditions. Since these patients are frequently administered or exposed to other potentially toxic compounds, it is obviously prudent and necessary to continue research into the effects of retinol on immune modulation and interaction with other compounds. More importantly, these studies demonstrate the modulation of immune function is one mechanism by which one chemical can influence the toxicity of another.

Liver cell necrosis: cellular mechanisms and clinical implications

Based on our current understanding, we have developed a provisional model for hepatocyte necrosis that may be applicable to cell necrosis in general (Figure 6). Damage to mitochondria appears to be a key early event in the progression to necrosis. At least two pathways may be involved. In the first, inhibition of oxidative phosphorylation in the absence of the MMPT leads to ATP depletion, ion dysregulation, and enhanced degradative hydrolase activity. If oxygen is present, toxic oxygen species may be generated and lipid peroxidation can occur. Subsequent cytoskeleton and plasma membrane damage result in plasma membrane bleb formation. These steps are reversible if the insult to the cell is removed. However, if injury continues, bleb rupture and cell lysis occur. In the second pathway, mitochondrial damage results in an MMPT. This step is irreversible and leads to cell death by as yet uncertain mechanisms. It is important to note that MMPT may occur secondary to changes in the first pathway (e.g. oxidative stress, increased Cai2+, and ATP depletion) and that all the "downstream events" occurring in the first pathway may result from MMPT (e.g., ATP depletion, ion dysregulation, or hydrolase activation). Proof of this model's applicability to cell necrosis in general awaits further validation. In this review, we have attempted to highlight the advances in our understanding of the cellular mechanisms of necrotic injury. Recent advances in this understanding have allowed scientists and clinicians a better comprehension of liver pathophysiology. This knowledge has provided new avenues of therapy and played a key role in the practice of hepatology as evidenced by advances in organ preservation. Understanding the early reversible events leading to cellular and subcellular damage will be key to prevention and treatment of liver disease. Hopefully, disease and injury specific preventive or pharmacological strategies can be developed based on this expanding data base.

A review of the evidence supporting melatonin's role as an antioxidant

This survey summarizes the findings, accumulated within the last 2 years, concerning melatonin's role in defending against toxic free radicals. Free radicals are chemical constituents that have an unpaired electron in their outer orbital and, because of this feature, are highly reactive. Inspired oxygen, which sustains life, also is harmful because up to 5% of the oxygen (O2) taken in is converted to oxygen-free radicals. The addition of a single electron to O2 produces the superoxide anion radical (O2-.); O2-. is catalytic-reduced by superoxide dismutase, to hydrogen peroxide (H2O2). Although H2O2 is not itself a free radical, it can be toxic at high concentrations and, more importantly, it can be reduced to the hydroxyl radical (.OH). The .OH is the most toxic of the oxygen-based radicals and it wreaks havoc within cells, particularly with macromolecules. In recent in vitro studies, melatonin was shown to be a very efficient neutralizer of the .OH; indeed, in the system used to test its free radical scavenging ability it was found to be significantly more effective than the well known antioxidant, glutathione (GSH), in doing so. Likewise, melatonin has been shown to stimulate glutathione peroxidase (GSH-Px) activity in neural tissue; GSH-PX metabolizes reduced glutathione to its oxidized form and in doing so it converts H2O2 to H2O, thereby reducing generation of the .OH by eliminating its precursor. More recent studies have shown that melatonin is also a more efficient scavenger of the peroxyl radical than is vitamin E. The peroxyl radical is generated during lipid peroxidation and propagates the chain reaction that leads to massive lipid destruction in cell membranes. In vivo studies have demonstrated that melatonin is remarkably potent in protecting against free radical damage induced by a variety of means. Thus, DNA damage resulting from either the exposure of animals to the chemical carcinogen safrole or to ionizing radiation is markedly reduced when melatonin is co-administered. Likewise, the induction of cataracts, generally accepted as being a consequence of free radical attack on lenticular macromolecules, in newborn rats injected with a GSH-depleting drug are prevented when the animals are given daily melatonin injections. Also, paraquat-induced lipid peroxidation in the lungs of rats is overcome when they also receive melatonin during the exposure period. Paraquat is a highly toxic herbicide that inflicts at least part of its damage by generating free radicals.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological and toxicological actions of nitric oxide in the central nervous system

NO has clearly revolutionized our thinking about aspects of neurotransmission and neuronal signaling. It has also radically altered our thoughts about how synaptic transmission takes place. NO is emerging as an important regulator of a variety of physiological processes; however, under certain conditions of excessive formation, NO is emerging as an important mediator of pathological nervous tissue damage. Understanding the role of NO in these processes will hopefully lead to the development of selective therapeutic agents and to a better understanding of basic processes underlying normal and pathological neuronal functions.

The pharmacology and toxicology of three new biologic agents used in pulmonary medicine

Biological agents have played an important role in the evolution of modern medical therapeutics. Recent advances in biologicals have in part been stimulated by the biotechnology revolution seen over the last several years. Toxicologists need to be aware of the proposed mechanisms and approved and experimental uses of these new biologic agents. Further, controversies about their use, efficacy, cost issues and potential toxicities should be known. Often these drugs are designed for small patient populations thus limiting the availability of human toxicological data bases. This paper reviews the pharmacology and toxicology of three new biologics (recombinant human DNase I, alpha 1-protease inhibitor, and nitric oxide). These agents appear to have important roles in treating specific diseases or disease states seen in pulmonary medicine.

Sulfamethoxazole-trimethoprim double-blind, placebo-controlled, crossover trial in Machado-Joseph disease: sulfamethoxazole-trimethoprim increases cerebrospinal fluid level of biopterin

We performed a double-blind, placebo-controlled, crossover trial of sulfamethoxazole-trimethoprim (S-T) in 8 patients with Machado-Joseph disease (MJD), and measured the blood and cerebrospinal fluid levels of biopterins, biogenic amines or metabolites, and folate. The clinical results were as follows; mild improvements of hyperreflexia of knee jerks and of rigospasticity of the legs during S-T treatment period. In addition, S-T significantly reduced the times of 8 motor activities on the timed tests. The biochemical results showed that basal levels of all biopterins and homovanillic acid in the cerebrospinal fluid (CSF) were reduced to less than half the levels of those of controls with other neurological diseases. After S-T treatment, total and oxidized form of biopterins in the CSF increased significantly. Therefore, S-T may be effective to neurologic deficits through its mechanism of increasing the level of brain biopterins.

Platelet-vessel wall interactions, focal adhesions, and the mechanism of action of endothelial factors

Endothelial cells produce a variety of vasoactive substances including prostacyclin (PGI2) and endothelium-derived relaxing factor (EDRF/NO) which are potent inhibitors of platelet adhesion/aggregation and vascular smooth muscle cell contraction/proliferation. PGI2 and EDRF elevate cAMP or cGMP, respectively, in vascular cells and other targets. The intracellular effects of cAMP and cGMP in vascular smooth muscle cells and platelets are primarily mediated by the family of cAMP- and cGMP-dependent protein kinases and their substrates. Important effector systems include enzymes, channels and regulatory proteins responsible for the regulation of intracellular Ca++. Other evidence suggests that VASP, a focal adhesion protein phosphorylated in platelets and smooth muscle cells in response to PGI2 and EDRF, is important for the regulation of integrins and cell-matrix interactions.

Significance of nitric oxide in the stimulation of intestinal fluid absorption in the rat jejunum in vivo

1. The effects of inhibiting nitric oxide (NO)-synthase on fluid transport, mucosal cyclic GMP and cyclic AMP levels and intraluminal prostaglandin E2 (PGE2)-release were studied in a model of ligated jejunal loops of anaesthetized rats in vivo. Experiments were performed under basal conditions as well as under conditions, when net fluid secretion was induced by Escherichia coli heat stable enterotoxin a (E. coli STa) or PGE2. 2. Intravenous infusion of the NO-synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 0.25-50 mg kg-1, 45 min) dose-dependently reversed net fluid absorption to net secretion, whereas infusion of D-NAME, the inactive enantiomer of L-NAME, in corresponding doses did not influence net fluid transport. N omega-nitro-L-arginine (L-NOARG, 25 mg kg-1), another NO-synthase inhibitor, also elicited net secretion of fluid. 3. L-NAME (25 mg kg-1)-induced net fluid secretion was reversed to net absorption by infusion of L-arginine (400 mg kg-1) or sodium nitroprusside (1 mg kg-1) and s.c. administration of indomethacin (10 mg kg-1). Hexamethonium (1 mg kg-1, s.c.), a ganglionic blocker and granisetron (100 micrograms kg-1, s.c.), a 5-HT3-receptor antagonist, did not influence L-NAME-induced net secretion. 4. Net fluid secretion induced by intraluminal instillation of E. coli STa (10 units ml-1) was enhanced by infusion of L-NAME (25 mg kg-1) and was inhibited by infusion of L-arginine (400 mg kg-1) and sodium nitroprusside (1 mg kg-1). D-Arginine (400 mg kg-1) did not influence E. coli STa-induced fluid secretion. Likewise, net fluid secretion induced by i.a. infusion of PGE2 (79 ng ml-1, 30 min) was enhanced by infusion of L-NAME and was inhibited by L-arginine and sodium nitroprusside. D-Arginine(400 mg kg-1) did not influence PGE2-induced fluid secretion.5. PGE2 levels in intraluminal fluid were not elevated after infusion of L-NAME (25mgkg-1) compared to controls.6. Mucosal cyclic GMP and cyclic AMP levels after L-NAME-treatment were not different from control values.7. These results indicate that nitric oxide plays an important role in the regulation of intestinal fluid transport. The data suggest a nitric oxide-dependent proabsorptive tone in the intestine, which possibly involves the enteric nervous system and suppression of prostaglandin formation. This proabsorptive tone also may downregulate fluid secretion induced by E. coli STa or PGE2.

Microvascular responses to inhibition of nitric oxide production. Role of active oxidants

The objective of this study was to assess the potential contribution of hydrogen peroxide (H2O2) to the leukocyte-endothelial cell adhesion and increased microvascular permeability observed in rat mesenteric venules after inhibition of nitric oxide synthesis with NG-nitro-L-arginine methyl ester (L-NAME). Leukocyte adherence and emigration and leakage of fluorescein isothiocyanate-labeled albumin were monitored in postcapillary venules before and after exposure of the tissue to L-NAME. H2O2 production in mesenteric tissue was monitored by using dihydrorhodamine 123 (DHR), the H2O2-sensitive fluorochrome. L-NAME elicited a rapid increase in both the rate of albumin extravasation and oxidation of DHR, which was followed by an increased adherence and emigration of leukocytes in postcapillary venules. Treatment with either catalase or dimethylthiourea attenuated the L-NAME-induced oxidative stress, albumin leakage, and leukocyte-endothelial cell adhesion. Oxidation of DHR was enhanced in animals treated with either 3-amino-1,2,4-triazole (ATZ), an inhibitor of endogenous catalase, or a combination of ATZ and maleic acid diethyl ester, which depletes intracellular glutathione. Animals receiving a CD11/CD18-specific antibody to prevent leukocyte adhesion/emigration exhibited a reduced oxidation of DHR in response to L-NAME. These findings indicate that most of the H2O2 (and secondarily derived oxidants) generated in mesenteric tissue exposed to an inhibitor of nitric oxide production is due to accumulation of activated leukocytes.

Detection of endothelial cell-derived nitric oxide: current trends and future directions

The vascular endothelium is a significant site of NO release that inhibits cellular adhesion and maintains a non-thrombogenic surface. Use of newly described technology suggests for the first time that the maximal release of NO induced by cNOS and iNOS activation may be quite similar, implying that it is the duration of NO release and not the concentration of NO produced from stimulated endothelial cells that accounts for the different biological activities of the enzymes. The respective roles of cNOS and iNOS must be carefully evaluated since both enzymes may have potent biological effects at local sites of production.

Effect of palytoxin on endothelium-dependent and -independent relaxation in rat aortic rings

Endothelium-intact rat aortic rings were incubated with palytoxin (PTX, 10(-11)-10(-9)M, 10 min) in oxygenated (O2 95%, CO2 5%) baths. Phenylephrine (PE)-contracted vascular rings demonstrated decreasing relaxation to acetylcholine (ACh), depending upon PTX incubation in a dose-dependent manner; however, sodium nitroprusside (NaNP) persisted in returning the ring to its pre-PE tension. After incubation with PTX, relaxation to the receptor-independent, endothelium-dependent relaxant A23187 was also attenuated. Thus, endothelium-dependent mechanism(s) normally responsive to both ACh and A23187, stimulators of nitric oxide (NO) release, were disrupted. Following incubation with PTX, endothelium-independent relaxation to NaNP remained intact but relaxation to atriopeptin II (APII) decreased. Electron microscopy demonstrated microvesiculation of endothelial cell cytoplasm and an irregular luminal surface following incubation with PTX. The intact response to NaNP, despite the loss of relaxation to ACh, indicated that soluble guanylate cyclase was not affected by PTX. However, loss of relaxation to AP-II, involving particulate guanylate cyclase of vascular smooth muscle (VSM), was inhibited by PTX pre-incubation. Determination of the site(s) of action of PTX requires further study.

Peroxynitrite induces both vasodilatation and impaired vascular relaxation in the isolated perfused rat heart

The effects of the oxidant species peroxynitrite (ONOO-) on coronary perfusion pressure and vasodilatation elicited by acetylcholine, isoproterenol, and S-nitroso-N-acetyl-DL-penicillamine were investigated in the isolated perfused rat heart. ONOO- (0.3-1000 microM) caused a concentration-dependent vasodilatation of the coronary vasculature. This dilator response was inhibited by oxyhemoglobin, indicating that it was due to the generation of nitric oxide. Tachyphylaxis to ONOO- developed rapidly, so that the response disappeared after three or four applications of this compound. ONOO- not only induced tachyphylaxis but also inhibited the vasodilatation induced by the three vasodilators studied. This latter effect of ONOO- was critically dependent on its concentration, since it occurred at 3 microM, which was subthreshold as a dilator, and at 1000 microM, which was supramaximal, but not at 30 and 100 microM. These latter concentrations inhibited the responses to vasodilators only in the presence of oxyhemoglobin. Thus, a wide range of concentrations of ONOO- induce a vascular dysfunction, as evidenced by the tachyphylaxis to its own vasodilator actions and the long-lasting impairment of the responses to other vasodilators. However, at the same time ONOO- generates nitric oxide, which at certain concentrations of ONOO- is sufficient to counteract its deleterious action. Coinfusion of S-nitroso-N-acetyl-DL-penicillamine or prostacyclin at low concentrations that did not produce vasodilatation also protected against ONOO(-)-induced vascular dysfunction: these compounds may be protective through a common mechanism, as yet undefined.

Nitric oxide-synthesizing neurons in the hamster suprachiasmatic nucleus: a combined NOS- and NADPH- staining and retinohypothalamic tract tracing study

Neuronal nitric oxide (NO), thought to be a neuroactive substance of high potency, is produced by the enzyme nitric oxide synthase (NOS) which has been demonstrated to additionally exhibit a so-called NADPH-diaphorase (NADPH-d) activity. Since physiological results pointed to the involvement of NO in circadian regulation, and morphological descriptions are not available, we sought to study the distribution of NO-producing cells in the hypothalamic suprachiasmatic nucleus (SCN) in Djungarian hamsters (Phodopus sungorus) by means of histochemistry and immunohistochemistry (IHC). In the SCN, NADPH-d stained perikarya of varying intensity and number were found predominantly in the ventrolateral subdivision. Diaphorase staining combined with the IHC demonstration of NOS revealed a complete overlapping of both. The combination of NADPH-d staining with the demonstration of the retinohypothalamic tract using the anterograde neuronal transport of cholera toxin B (CTB) following intraocular injection showed CTB terminals accumulating at NADPH-d cell bodies mainly in the ventrolateral region of the SCN. These data provide morphological evidence for the involvement of nitric oxide in the mediation of photic stimulation of the circadian oscillator located in the SCN.

Evidence for reduction of bradykinin-induced bronchoconstriction in guinea-pigs by release of nitric oxide

1. In this study the influence of nitric oxide (NO) on the bronchoconstriction induced by bradykinin in anaesthetized and artifically ventilated guinea-pigs pretreated with atropine was investigated. 2. Aerosol administration of bradykinin (0.1-1 mM, 40 breaths) caused a dose-dependent increase in lung resistance (RL): maximum increase in RL was 2.5 fold the baseline value. Pretreatment with aerosolized NG-nitro-L-arginine methyl ester (L-NAME) or NG-monomethyl-L-arginine (L-NMMA) (1 mM, 10 breaths every 5 min for 30 min), NO synthase inhibitors, markedly increased the bronchoconstrictor response to bradykinin. L-Arginine, but not D-arginine, (3 mM, 10 breaths every 5 min for 30 min) reversed the hyperresponsiveness to aerosolized bradykinin caused by L-NAME and L-NMMA. 3. L-NAME (1 mM, 10 breaths every 5 min for 30 min) increased the bronchoconstriction induced by intravenous bradykinin (1-10 nmol kg-1). L-Arginine, but not D-arginine, (10 breaths every 5 min for 30 min) reversed the hyperresponsiveness to intravenous bradykinin caused by L-NAME. 4. The increase in RL induced by capsaicin, either aerosol (10 microM, 10 breaths) or i.v. (20 nmol kg-1) was not affected by L-NAME (1 mM, 10 breaths every 5 min for 30 min). Acute resection of the vagi did not affect the bronchoconstriction evoked by bradykinin in guinea-pigs, either in the absence or presence of L-NAME (1 mM, 10 breaths every 5 min for 30 min). 4. These results suggest that, irrespective of the route of administration, bradykinin releases NO or a related molecule which exerts a bronchodilator action that opposes the bronchoconstrictor mechanisms activated by bradykinin itself.

Inhibition of sympathetic vasoconstriction is a major principle of vasodilation by nitric oxide in vivo

The objective of this study was to determine whether vasodilator effects of nitric oxide (NO) can be explained by the inhibition of vasoconstriction caused by peripheral sympathetic nerve activity (SNA) in vivo. For this purpose, we studied the effects of systemic inhibition of NO synthesis during experimental variation of SNA in anesthetized cats. Intravenous infusion of NG-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) in baroreceptor-intact animals (n = 6) caused increases in mean arterial blood pressure (MAP) from 105.8 +/- 3.4 to 192.0 +/- 4.3 mm Hg that were associated with slight decreases in preganglionic SNA recorded from the white ramus of the third thoracic segment. Higher SNA appeared in completely baroreceptor-denervated cats (n = 10) than in the intact cats, but no changes in nerve activity occurred after the subsequent administration of L-NAME. In contrast, MAP increased from 123.3 +/- 4.0 to 245.8 +/- 5.1 mm Hg. In baroreceptor-denervated cats, reversible suppression of peripheral SNA produced by cooling of the ventral surface of the rostral ventrolateral medulla oblongata (RVLM) caused significant hypotension (61.1 +/- 2.6 mm Hg) and almost completely reversed the hypertension caused by L-NAME (76.0 +/- 3.7 mm Hg). Intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin after L-NAME reduced MAP to a similar extent. In contrast, hypertension induced by angiotensin II could not be reversed by RVLM cooling. The pressor effects of intravenously administered noradrenaline during RVLM cooling were markedly potentiated by L-NAME and attenuated by the NO-donor compound S-nitroso-N-acetylpenicillamine (SNAP).(ABSTRACT TRUNCATED AT 250 WORDS)

Pteridine biosynthesis and nitric oxide synthase in Physarum polycephalum

Physarum polycephalum, an acellular slime mould, serves as a model system to study cell-cycle-dependent events since nuclear division is naturally synchronous. This organism was shown to release isoxanthopterin which is structurally related to tetrahydrobiopterin, a cofactor of aromatic amino acid hydroxylases and of nitric oxide synthases (NOSs) (EC 1.14.13.39). Here, we studied Physarum pteridine biosynthesis in more detail and found that high amounts of tetrahydrobiopterin are produced and NOS activity is expressed. Physarum pteridine biosynthesis is peculiar in as much as 7,8-dihydroneopterin aldolase (EC 4.1.2.25), an enzyme of folic acid biosynthesis usually not found in organisms producing tetrahydrobiopterin, is detected in parallel. NOS purified from Physarum depends on NADPH, tetrahydrobiopterin and flavins. Enzyme activity is independent of exogenous Ca2+ and is inhibited by arginine analogues. The purified enzyme (with a molecular mass of 130 kDa) contains tightly bound tetrahydrobiopterin and flavins. During the synchronous cell cycle of Physarum, pteridine biosynthesis increases during S-phase whereas NOS activity peaks during mitosis, drops at telophase and peaks again during early S-phase. Our results characterize Physarum pteridine biosynthesis and NOS and suggest a possible link between NOS activity and mitosis.

Oxidized lipoprotein(a) inhibits endothelium-dependent dilation: prevention by high density lipoprotein

We assessed the effects of human native and oxidized lipoprotein(a) (150 min, 30 and 100 micrograms/ml) on endothelium-dependent vasodilation of isolated rabbit renal arteries. Vasodilation was not attenuated after incubation of arteries with native lipoprotein(a). However, when the arteries were exposed to oxidized lipoprotein(a), acetylcholine-induced vasodilation was dose dependently significantly impaired. Concomitant incubation of segments with high density lipoprotein (HDL, 0.5 mg/ml) prevented the attenuation of dilations induced by oxidized lipoprotein(a). Thus, we report for the first time that oxidized lipoprotein(a) impairs endothelium-dependent vasodilation, and that HDL prevents its inhibitory effect.

Dual role of nitric oxide in focal cerebral ischemia

The importance of nitric oxide (NO) in the pathophysiology of cerebral ischemia was examined following middle cerebral artery occlusion in rats. A significant increase in infarct size developed following inhibition of NO synthase (NOS) activity by L-arginine analogues whereas intravenous L-arginine dose-dependently decreased infarct volume in the same models. Protection after L-arginine administration was associated with enhanced blood flow within the perinfarct zone as demonstrated by simultaneous recording of rCBF and electrocorticogram activity within subjacent brain. Selective NOS inhibition by 7-nitroindazole (7-NI) significantly reduced infarct volume at doses of 25 and 50 mg kg and in amounts that did not decrease the response of pial vessels to topical acetylcholine. Together these data suggest that enhanced NO production within the cerebrovasculature protects brain tissue during focal ischemia via hemodynamic mechanisms whereas neuronal overproduction may facilitate or mediate neurotoxicity. Recent data using transgenic animals lacking NOS activity support the latter conclusion.

Effects of the cytokine synthesis inhibitor CGP 47969A on nitric oxide production by lipopolysaccharide-stimulated J774A.1 macrophages

CGP 47969A is a novel inhibitor of the biosynthesis of interleukin-1 and other cytokines, being developed as an anti-arthritic. The effect of the compound on lipopolysaccharide (LPS; 1 microgram/ml) stimulated nitric oxide (NO) production by the mouse macrophage cell line, J774A.1, was examined in the present study. CGP 47969A inhibited NO production in a concentration-dependent fashion (0.1-10 microM; IC50 = 2 microM) in a 24 h assay. Dexamethasone (Dex), which inhibits cytokine and inducible nitric oxide synthase (iNOS) gene transcription, and N-methyl arginine (NMA), a substrate analogue inhibitor of NOS activity, also inhibited NO production in this assay system with IC50 values of approximately 5 nM and 100 microM, respectively. When iNOS expression was induced by LPS for 24 h, CGP 47969A and Dex did not inhibit NO production, whereas NMA retained activity (IC50 = 40 microM). In time course experiments, CGP 47969A (10 microM) or Dex (1 microM) were added to J774A.1 cultures at t = 0, 1, 3 or 6 h after LPS. Dex inhibited NO production by 86%, 57%, 35% and 15% at these time points, while CGP 47969A inhibited by 90%, 91%, 89% and 76%. Taken together, the results indicate that CGP 47969A inhibits NO production by an effect similar to the inhibitory effect on cytokine production rather than by inhibition of iNOS enzyme activity per se or iNOS gene expression. The ability of CGP 47969A to inhibit cytokine and NO production may explain its efficacy in animal models of arthritis.