Increased expression of iNOS is associated with endothelial dysfunction and impaired pressor responsiveness in streptozotocin-induced diabetes

Role of inducible nitric oxide synthase in induction of RhoA expression in hearts from diabetic rats

AIMS

Recent studies from our laboratory demonstrated that increased expression of the small GTP-binding protein RhoA and activation of the RhoA/rho kinase (ROCK) pathway play an important role in the contractile dysfunction associated with diabetic cardiomyopathy in hearts from streptozotocin (STZ)-induced diabetic rats. Nitric oxide (NO) has been reported to be a positive regulator of RhoA expression in vascular smooth muscle, and we have previously found that the expression of inducible NO synthase (iNOS) is increased in hearts from STZ-diabetic rats. Therefore, in this study, we investigated the hypothesis that induction of iNOS positively regulates RhoA expression in diabetic rat hearts.

METHODS AND RESULTS

To determine whether NO and iNOS could increase RhoA expression in the heart, cardiomyocytes from non-diabetic rats were cultured in the presence of the NO donor sodium nitroprusside (SNP) or lipopolysaccharide (LPS) in the absence and presence of the selective iNOS inhibitor, N(6)-(1-iminoethyl)-l-lysine dihydrochloride (L-NIL). In a second study, 1 week after induction of diabetes with STZ, rats were treated with L-NIL (3 mg/kg/day) for 8 more weeks to determine the effect of iNOS inhibition in vivo on RhoA expression and cardiac contractile function. Expression of iNOS was elevated in cardiomyocytes isolated from diabetic rat hearts. Both SNP and LPS increased RhoA expression in non-diabetic cardiomyocytes. The LPS-induced elevation in RhoA expression was accompanied by an increase in iNOS expression and prevented by L-NIL. Treatment of diabetic rats with L-NIL led to a significant improvement in left ventricular developed pressure and rates of contraction and relaxation concomitant with normalization of total cardiac nitrite levels, RhoA expression, and phosphorylation of the ROCK targets LIM (Lin-11, Isl-1, Mec-3) kinase and ezrin/radixin/moesin.

CONCLUSION

These data suggest that iNOS is involved in the increased expression of RhoA in diabetic hearts and that one of the mechanisms by which iNOS inhibition improves cardiac function is by preventing the upregulation of RhoA and its availability for activation.

Effects of alpha-lipoic acid on endothelial function in aged diabetic and high-fat fed rats

BACKGROUND AND PURPOSE

This study was conducted to investigate the effects of alpha-lipoic acid (alpha-LA) on endothelial function in diabetic and high-fat fed animal models and elucidate the potential mechanism underlying the benefits of alpha-LA.

EXPERIMENTAL APPROACH

Plasma metabolites reflecting glucose and lipid metabolism, endothelial function, urinary albumin excretion (UAE), plasma and aortic malondialdehyde (MDA) and urinary 8-hydroxydeoxyguanosine (8-OHdG) were assessed in non-diabetic controls (Wistar rats), untreated Goto-Kakizaki (GK) diabetic and high-fat fed GK rats (fed with atherogenic diet only, treated with alpha-LA and treated with vehicle, for 3 months). Vascular eNOS, nitrotyrosine, carbonyl groups and superoxide anion were also assessed in the different groups.

KEY RESULTS

alpha-LA and soybean oil significantly reduced both total and non-HDL serum cholesterol and triglycerides induced by atherogenic diet. MDA, carbonyl groups, vascular superoxide and 8-OHdG levels were higher in GK and high-fat fed GK groups and fully reversed with alpha-LA treatment. High-fat fed GK diabetic rats showed significantly reduced endothelial function and increased UAE, effects ameliorated with alpha-LA. This endothelial dysfunction was associated with decreased NO production, decreased expression of eNOS and increased vascular superoxide production and nitrotyrosine expression.

CONCLUSIONS AND IMPLICATIONS

alpha-LA restores endothelial function and significantly improves systemic and local oxidative stress in high-fat fed GK diabetic rats. Improved endothelial function due to alpha-LA was at least partially attributed to recoupling of eNOS and increased NO bioavailability and represents a pharmacological approach to prevent major complications associated with type 2 diabetes.

Cardiac Oxidative Stress Is Elevated at the Onset of Dilated Cardiomyopathy in Streptozotocin-Diabetic Rats

The association between nitric oxide synthase (eNOS and iNOS) status, oxidative stress, and cardiac function was evaluated in streptozotocin (STZ)-diabetic rats to understand the etiology of diabetic cardiomyopathy. Cardiac function was determined by echocardiography. eNOS and iNOS status and superoxide production were assessed by immunohistochemistry and chemiluminescence, respectively. In STZ-diabetic rats, stroke volume, cardiac output, and left ventricular ejection fraction were significantly lower than in controls (CT, P < .05), whereas left ventricular end-systolic volume was higher. Cardiac NOS activity increased from 161 ± 18 cpm/mg tissue in CT rats to 286 ± 20 cpm/mg tissue ( P < .001) in STZ-diabetic rats. Furthermore, superoxide production and cardiac eNOS and iNOS levels were higher in STZ-diabetic rats than in CT rats ( P < .05). An increased activation of cardiac eNOS and iNOS is observed concomitantly with decreased cardiac function. Thus, increased oxidative stress in the heart may be implicated in the development of dilated cardiomyopathy in STZ-diabetic rats.

Temporal changes in cardiac force- and flow-generation capacity, loading conditions, and mechanical efficiency in streptozotocin-induced diabetic rats

Diabetes mellitus may result in impaired cardiac contractility, but the underlying mechanisms remain unclear. We aimed to investigate the temporal alterations in cardiac force- and flow-generation capacity and loading conditions as well as mechanical efficiency in the evolution of systolic dysfunction in streptozotocin (STZ)-induced diabetic rats. Adult male Wistar rats were randomized into control and STZ-induced diabetic groups. Invasive hemodynamic studies were done at 8, 16, and 22 wk post-STZ injection. Maximal systolic elastance (Emax) and maximum theoretical flow (Qmax) were assessed by curve-fitting techniques, and ventriculoarterial coupling and mechanical efficiency were assessed by a single-beat estimation technique. In contrast to early occurring and persistently depressed Emax, Qmax progressively increased with time but was decreased at 22 wk post-STZ injection, which temporally correlated with the changes in cardiac output. The favorable loading conditions enhanced stroke volume and Qmax, whereas ventriculoarterial uncoupling attenuated the cardiac mechanical efficiency in diabetic animals. The changes in Emax and Qmax are discordant during the progression of contractile dysfunction in the diabetic heart. In conclusion, our study showed that depressed Qmax and cardiac mechanical efficiency, occurring preceding overt systolic heart failure, are two major determinants of deteriorating cardiac performance in diabetic rats.

Selective iNOS inhibition reduces renal damage induced by cisplatin

Cisplatin is a chemotherapeutic agent used in the treatment of several cancer tumors; however, nephrotoxicity has restricted its use. Reactive oxygen species and peroxynitrite, which is formed by the reaction between superoxide anion and nitric oxide (NO*), are implicated in cisplatin-induced nephrotoxicity. In contrast, both toxic and beneficial effects of NO* have been suggested in cisplatin-induced nephrotoxicity. Therefore, nowadays the role of NO* in this experimental model remains controversial. The aim of the present work was to elucidate the role of NO* in cisplatin-induced renal damage using N-[3-(aminomethyl)benzyl]acetamidine (1400W), a selective and irreversible inhibitor of iNOS. The mRNA levels of iNOS were increased in cisplatin-treated rats. The administration of 1400W reduced the cisplatin induced histological damage, renal dysfunction (increase in proteinuria and kidney injury molecule expression and decrease in creatinine clearance), tubulointerstitial infiltration, oxidative stress (increase in renal malondialdehyde and inmmunostaining for 4-hydroxy-2-nonenal) and nitrosative stress (immunostaining for 3-nitrotyrosine). In addition, the administration of 1400W was unable to modify systolic blood pressure in control rats. Our data demonstrate that selective iNOS inhibition reduces the cisplatin-induced nephrotoxicity and nitrosative stress which strongly suggest that in this experimental model (1) the NO* production is toxic and (2) iNOS is the main source of NO*.

Chronic hyperglicemia and nitric oxide bioavailability play a pivotal role in pro-atherogenic vascular modifications

Diabetes is associated with accelerated atherosclerosis and macrovascular complications are a major cause of morbidity and mortality in this disease. Although our understanding of vascular pathology has lately greatly improved, the mechanism(s) underlying enhanced atherosclerosis in diabetes remain unclear. Endothelial cell dysfunction is emerging as a key component in the pathophysiology of cardiovascular abnormalities associated with diabetes. Although it has been established that endothelium plays a critical role in overall homeostasis of the vessels, vascular smooth muscle cells (vSMC) in the arterial intima have a relevant part in the development of atherosclerosis in diabetes. However, high glucose induced alterations in vSMC behaviour are not fully characterized. Several studies have reported that impaired nitric oxide (NO) synthesis and/or actions are often present in diabetes and endothelial dysfunction. Furthermore, although endothelial cells are by far the main site of vascular NO synthesis, vSMC do express nitric oxyde synthases (NOSs) and NO synthesis in vSMC might be important in vessel's function. Although it is known that vSMC contribute to vascular pathology in diabetes by their change from a quiescent state to an activated proliferative and migratory phenotype (termed phenotypic modulation), whether this altered phenotypic modulation might also involve alterations in the nitrergic systems is still controversial. Our recent data indicate that, in vivo, chronic hyperglycemia might induce an increased number of vSMC proliferative clones which persist in culture and are associated with increased eNOS expression and activity. However, upregulation of eNOS and increased NO synthesis occur in the presence of a marked concomitant increase of O(2-) production. Since NO bioavailabilty might not be increased in high glucose stimulated vSMC, it is tempting to hypothesize that the proliferative phenotype observed in cells from diabetic rats is associated with a redox imbalance responsible quenching and/or trapping of NO, with the consequent loss of its biological activity. This might provide new insight on the mechanisms responsible for accelerated atherosclerosis in diabetes.

Impaired vascular function in normoglycemic mice prone to autoimmune diabetes: Role of nitric oxide

Type 1 diabetes is an immuno-inflammatory condition which increases the risk of cardiovascular disease, particularly in young adults. This study investigated whether vascular function is altered in mice prone to autoimmune diabetes and whether the nitric oxide (NO)-cyclic GMP axis is involved. Aortic rings suspended in organ chambers and precontracted with phenylephrine were exposed to cumulative concentrations of acetylcholine. To investigate the role of NO, some experiments were performed in the presence of either 1400W (N-(3-aminomethyl)benzyl-acetamidine hydrochloride), a selective inhibitor of the iNOS-isoform, L-NAME (N(G)-nitro-L-arginine methyl ester hydrochloride), an inhibitor of all three NOS-isoforms, or ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), a selective inhibitor of guanylate cyclase. Moreover, contractility to phenylephrine, big endothelin-1, and endothelin-1 was assessed and histological analysis and iNOS immunohistochemistry were performed. Endothelium-dependent relaxation was reduced in prediabetic NOD mice (78+/-4 vs. 88+/-2%, respectively, P<0.05 vs. control) despite normal plasma glucose levels (n.s. vs. control). Preincubation with 1400W further attenuated responses in prediabetic (P<0.05 vs. untreated) but not in diabetic or in control mice. In contrast, basal NO bioactivity remained unaffected until the onset of diabetes in NOD mice. Contractile responses to big endothelin-1 and endothelin-1 were reduced in prediabetic animals (P<0.05 vs. control), whereas in diabetic mice only responses to big endothelin-1 were decreased (P<0.05 vs. control). These data demonstrate that endothelium-dependent and -independent vascular function in NOD mice is abnormal already in prediabetes in the absence of structural injury. Early proinflammatory activation due to iNOS in diabetes-prone NOD mice appears to be one of the mechanisms contributing to impaired vasoreactivity.

Nitric oxide production is paradoxically decreased after weight reduction surgery in morbid obesity patients

Obesity is associated with vascular endothelial cell dysfunction (ECD). Studies on nitric oxide (NO) production of vascular system in these subjects may help delineate the pathogenesis of obesity-associated ECD. In this study, we recruited 69 severely obese patients who were treated with gastric partition surgery for weight reduction and 69 matched healthy controls for comparison. The following parameters were obtained in the healthy control subjects and in the obese subjects both before and after gastric partition surgery: body mass index, blood pressure, serum lipids, high sensitivity C-reactive protein (hs-CRP), adiponectin, total nitrite and nitrate (NO(x)), and 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha), and insulin resistance index (as measured by homeostasis model assessment (HOMA-IR). At baseline, serum lipids, glucose, insulin, hs-CRP and 8-iso-PGF2alpha and HOMA-IR were all higher while adiponectin lower in the obese group than in the control group. The serum NO(x) levels were not different between the two groups. In the obese subjects, the adiponectin levels were significantly elevated but NO(x) markedly decreased after surgery. All other measurements, except for systolic blood pressure, were decreased after surgery. For healthy controls, the serum NO(x) levels were negatively associated with HOMA-IR and positively associated with serum adiponectin levels as analyzed by multiple linear regression analysis. In obese patients, the baseline serum NO(x) was positively associated with the serum TG levels. The changes of serum NO(x) levels after weight reduction surgery were positively associated with the changes of body mass index and serum TG levels. These observations suggested that, in the extremely obese patients, there might be excessive production and/or inactivation of NO and, after weight reduction surgery, the NO production was down-regulated. In conclusion, in the severely obese patients, the apparently normal NO production might be due to over-expression of iNOS. After gastric partition surgery, the NO production was significantly decreased which might be reflecting the usual status of NO production in obese subjects. The positive correlation between NO(x) and serum TG level might suggest that the metabolism of TG plays a role in the regulation of NO production.

Inhibition of nitric oxide synthase enhances contractile response of ventricular myocytes from streptozotocin-diabetic rats

The contractile hyporesponsiveness of the streptozotocin diabetic rat heart in vitro to beta-adrenergic agonists is eliminated when the heart is perfused with N(G)-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of nitric oxide synthase (NOS). The following study evaluated the hypothesis that an increased production of NO/cGMP within the diabetic myocyte inhibits the beta-adrenergic-stimulated increase in calcium current and contractile response. Male Sprague-Dawley rats were given an intravenous injection of streptozotocin (60 mg/kg). After 8 weeks, L-type calcium currents were recorded in ventricular myocytes using the whole cell voltage-clamp method. Shortening of isolated myocytes was determined using a video edge detection system. cAMP and cGMP were measured using radioimmunoassay. Nitric oxide production was determined using the Griess assay kit. Basal cGMP levels and nitric oxide production were elevated in diabetic myocytes. Shortening of the diabetic myocytes in response to isoproterenol (1 microM) was markedly diminished. However, there was no detectable difference in the isoproterenol-stimulated L-type calcium current or cAMP levels between control and diabetic myocytes. Acute superfusion of the diabetic myocyte with L-NAME (1 mM) decreased basal cGMP and markedly enhanced the shortening response to isoproterenol but did not alter isoproterenol-stimulated calcium current. These data suggest that increased production of NO/cGMP within the diabetic myocyte suppressed beta-adrenergic stimulated shortening of the myocyte. However, NO/cGMP apparently does not suppress shortening of the myocyte by inhibition of the beta-stimulated calcium current.

Endothelin A receptor blockade reduces diabetic renal injury via an anti-inflammatory mechanism

Endothelin (ET) receptor blockade delays the progression of diabetic nephropathy; however, the mechanism of this protection is unknown. Therefore, the aim of this study was to test the hypothesis that ET(A) receptor blockade attenuates superoxide production and inflammation in the kidney of diabetic rats. Diabetes was induced by streptozotocin (diabetic rats with partial insulin replacement to maintain modest hyperglycemia [HG]), and sham rats received vehicle treatments. Some rats also received the ETA antagonist ABT-627 (sham+ABT and HG+ABT; 5 mg/kg per d; n = 8 to 10/group). During the 10-wk study, urinary microalbumin was increased in HG rats, and this effect was prevented by ET(A) receptor blockade. Indices of oxidative stress, urinary excretion of thiobarbituric acid reactive substances, 8-hydroxy--deoxyguanosine, and H2O2 and plasma thiobarbituric acid reactive substances were significantly greater in HG rats than in sham rats. These effects were not prevented by ABT-627. In addition, renal cortical expression of 8-hydroxy--deoxyguanosine and NADPH oxidase subunits was not different between HG and HG+ABT rats. ETA receptor blockade attenuated increases in macrophage infiltration and urinary excretion of TGF-beta and prostaglandin E2 metabolites in HG rats. Although ABT-627 did not alleviate oxidative stress in HG rats, inflammation and production of inflammatory mediators were reduced in association with prevention of microalbuminuria. These observations indicate that ETA receptor activation mediates renal inflammation and TGF-beta production in diabetes and are consistent with the postulate that ETA blockade slows progression of diabetic nephropathy via an anti-inflammatory mechanism.

N-acetylcysteine attenuates PKCbeta2 overexpression and myocardial hypertrophy in streptozotocin-induced diabetic rats

OBJECTIVE

Oxidative stress-mediated activation of protein kinase C (PKC) beta(2) in the myocardium has been implicated in the development of cardiomyopathy. Overexpression of PKCbeta(2) is associated with increased expression of connective tissue growth factor (CTGF) in myocardium, resulting in myocardial hypertrophy. We hypothesized that chronic treatment with the antioxidant N-acetylcysteine (NAC) would normalize oxidative stress-mediated overexpression of myocardial PKCbeta(2) and CTGF and attenuate the development of myocardial hypertrophy.

METHODS

Control and streptozotocin-induced diabetic rats were treated with NAC in drinking water for 8 weeks. At termination rats were surgically prepared for hemodynamic measurement, subsequent to which their hearts were removed to evaluate cardiac performance and histological and biochemical changes. Further, the role of PKCbeta(2) in hyperglycemia-induced cardiomyocyte hypertrophy was tested in cultured neonatal cardiomyocytes.

RESULTS

Myocardial hypertrophy, characterized by an increased ratio of ventricle weight to body weight and cardiomyocyte cross-sectional area was found to be higher in untreated diabetic rats. Further, in myocardium, increased levels of 15-F(2t)-isoprostane were accompanied by an increased expression of membrane-bound PKCbeta(2) and CTGF. N-acetylcysteine treatment not only attenuated these changes but also prevented hyperglycemia-induced hypertrophy in cultured neonatal rat cardiomyocytes.

CONCLUSIONS

The results suggest that PKCbeta(2) overexpression represents a mechanism causing hyperglycemia-mediated myocardial hypertrophy, which can be prevented by the antioxidant N-acetylcysteine.

N-acetylcysteine prevents nitrosative stress-associated depression of blood pressure and heart rate in streptozotocin diabetic rats

Previous studies have indicated that cardiovascular abnormalities such as depressed blood pressure and heart rate occur in streptozotocin (STZ) diabetic rats. Chronic diabetes, which is associated with increased expression of inducible nitric oxide synthase (iNOS) and oxidative stress, may produce peroxynitrite/nitrotyrosine and cause nitrosative stress. We hypothesized that nitrosative stress causes cardiovascular depression in STZ diabetic rats and therefore can be corrected by reducing its formation. Control and STZ diabetic rats were treated orally for 9 weeks with N-acetylcysteine (NAC), an antioxidant and inhibitor of iNOS. At termination, the mean arterial blood pressure (MABP) and heart rate (HR) were measured in conscious rats. Nitrotyrosine and endothelial nitric oxide synthase (eNOS) and iNOS expression were assessed in the heart and mesenteric arteries by immunohistochemistry and Western blot experiments. Untreated diabetic rats showed depressed MABP and HR that was prevented by treatment with NAC. In untreated diabetic rats, levels of 15-F(2t)-isoprostane, an indicator of lipid peroxidation increased, whereas plasma nitric oxide and antioxidant concentrations decreased. Furthermore, decreased eNOS and increased iNOS expression were associated with elevated nitrosative stress in blood vessel and heart tissue of untreated diabetic rats. N-acetylcysteine treatment of diabetic rats not only restored the antioxidant capacity but also reduced the expression of iNOS and nitrotyrosine and normalized the expression of eNOS to that of control rats in heart and superior mesenteric arteries. The results suggest that nitrosative stress depress MABP and HR following diabetes. Further studies are required to elucidate the mechanisms involved in nitrosative stress mediated depression of blood pressure and heart rate.

Impairment of dilator responses of cerebral arterioles during diabetes mellitus: role of inducible NO synthase

BACKGROUND AND PURPOSE

During diabetes, expression of inducible nitric oxide synthase (iNOS) plays an important role in the development of endothelial dysfunction in extracranial blood vessels. Progression of vascular dysfunction after the onset of diabetes differs among vascular beds. In this study, the effects of hyperglycemia/diabetes on vasomotor function were examined in cerebral arterioles at 2 different times in control and iNOS-deficient mice and compared with the effects on carotid arteries.

METHODS

Streptozotocin (150 mg/kg IP) was given to induce diabetes. The diameter of cerebral arterioles was measured through a cranial window in diabetic and nondiabetic mice in vivo. Vasomotor function of the carotid artery was examined in vitro.

RESULTS

In diabetic mice, responses of the cerebral arterioles to acetylcholine (1 mumol/L) were normal after 3 weeks of diabetes but were significantly impaired after 5 to 6 weeks of diabetes (4+/-1% [mean+/-SEM] increase in diameter) compared with control mice (14+/-1; P=0.0002). Responses to sodium nitroprusside were similar in diabetic and nondiabetic mice at both time points. In contrast, the vasomotor function of the carotid artery was not affected after 5 to 6 weeks of diabetes. In diabetic iNOS-deficient mice, cerebral arteriolar vasomotor function was not impaired, even after 4 months of diabetes.

CONCLUSIONS

During diabetes, endothelial dysfunction of cerebral arterioles requires expression of iNOS and develops earlier than in carotid arteries.

Antioxidant N-acetylcysteine restores myocardial Mn-SOD activity and attenuates myocardial dysfunction in diabetic rats

Manganese-containing superoxide dismutase (Mn-SOD) plays a critical role in guarding against mitochondrial oxidative stress. Abnormal myocardial mitochondrial metabolism of reactive oxygen species plays an important role in the pathogenesis of diabetic cardiac dysfunction. We hypothesised that chronic treatment with N-acetylcysteine, an antioxidant and glutathione (GSH) precursor, would normalize hyperglycemia induced inactivation of Mn-SOD and attenuate myocardial dysfunction. Control and streptozotozin-induced diabetic rats were treated or untreated with N-acetylcysteine in drinking water for 8 weeks, initiated 1 week after streptozotozin injection. Myocardial performance was determined using the isolated perfused working heart preparation. Myocardial Mn-SOD activity, but not Mn-SOD protein expression, in diabetic rats was significantly reduced while levels of oxidative stress as determined by myocardial free 15-F2t-isoprostane were increased in diabetic rats and were normalized by N-acetylcysteine treatment. However, compensatory increases in myocardial Cu/Zn-SOD and GSH content were seen in diabetic rats accompanied by an increase in tissue antioxidant capacity as compared to control. N-acetylcysteine abolished the compensatory increase in myocardial Cu/Zn-SOD. The left ventricular developed pressure and rates of left ventricular pressure development and relaxation were decreased in diabetic rats as compared to control. These effects were attenuated, but not prevented by N-acetylcysteine treatment. N-acetylcysteine attenuation of diabetic myocardial dysfunction could be attributed to the restoration of myocardial Mn-SOD activity.

Antioxidant N-acetylcysteine restores systemic nitric oxide availability and corrects depressions in arterial blood pressure and heart rate in diabetic rats

Increased oxidative stress and reduced nitric oxide (NO) bioactivity are key features of diabetes mellitus that eventually result in cardiovascular abnormalities. We assessed whether N-acetylcysteine (NAC), an antioxidant and glutathione precursor, could prevent the hyperglycaemia induced increase in oxidative stress, restore NO availability and prevent depression of arterial blood pressure and heart rate in vivo in experimental diabetes. Control (C) and streptozotocin-induced diabetic (D) rats were treated or not treated with NAC in drinking water for 8 weeks, initiated 1 week after induction of diabetes. At termination, plasma levels of free 15-F2t-isoprostane, a specific marker of oxygen free radical induced lipid peroxidation, was increased while the plasma total antioxidant concentration was decreased in untreated diabetic rats as compared to control rats (P<0.05). This was accompanied by a significant reduction of plasma levels of nitrate and nitrite, stable metabolites of NO, (P<0.05, D vs. C) and a reduced endothelial NO synthase protein expression in the heart and in aortic and mesenteric artery tissues. Systolic, diastolic and mean arterial blood pressures (SBP, DBP and MAP) and heart rate (HR) were reduced in diabetic rats (P<0.05 vs. C) and NAC normalised the changes that occurred in the diabetic rats. The protective effects may be attributable to restoration of NO bioavailability in the circulation.

Nitric oxide and the vascular endothelium

The vascular endothelium synthesises the vasodilator and anti-aggregatory mediator nitric oxide (NO) from L-arginine. This action is catalysed by the action of NO synthases, of which two forms are present in the endothelium. Endothelial (e)NOS is highly regulated, constitutively active and generates NO in response to shear stress and other physiological stimuli. Inducible (i)NOS is expressed in response to immunological stimuli, is transcriptionally regulated and, once activated, generates large amounts of NO that contribute to pathological conditions. The physiological actions of NO include the regulation of vascular tone and blood pressure, prevention of platelet aggregation and inhibition of vascular smooth muscle proliferation. Many of these actions are a result of the activation by NO of the soluble guanylate cyclase and consequent generation of cyclic guanosine monophosphate (cGMP). An additional target of NO is the cytochrome c oxidase, the terminal enzyme in the electron transport chain, which is inhibited by NO in a manner that is reversible and competitive with oxygen. The consequent reduction of cytochrome c oxidase leads to the release of superoxide anion. This may be an NO-regulated cell signalling system which, under certain circumstances, may lead to the formation of the powerful oxidant species, peroxynitrite, that is associated with a variety of vascular diseases.