Effects of homocysteine on acetylcholine- and adenosine-induced vasodilatation of pancreatic vascular bed in rats

ADMA and hyperhomocysteinemia

Hyperhomocysteinemia is a risk factor for cardiovascular disease and stroke. Like many other cardiovascular risk factors, hyperhomocysteinemia produces endothelial dysfunction due to impaired bioavailability of endothelium-derived nitric oxide (NO). The molecular mechanisms responsible for decreased NO bioavailabil ity in hyperhomocysteinemia are incompletely understood, but emerging evidence suggests that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may be a key mediator. Homocysteine is produced during the synthesis of ADMA and can alter ADMA metabolism by inhibiting dimethylarginine dimethy laminohydrolase (DDAH). Several animal and clinical studies have demonstrated a strong association between plasma total homocysteine, plasma ADMA, and endothelial dysfunction. These observations suggest a model in which elevation of ADMA may be a unifying mechanism for endothelial dysfunction during hyper homocysteinemia. The recent development of transgenic mice with altered ADMA metabolism should provide further mechanistic insights into the role of ADMA in hyperhomocysteinemia.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

Chronic pancreatitis is associated with hyperhomocysteinemia and derangements in transsulfuration and transmethylation pathways

OBJECTIVES

Homocysteine has been implicated in vascular dysfunction and thrombosis, as well as inflammatory conditions. This study was aimed to find out whether chronic pancreatitis (CP) is associated with hyperhomocysteinemia and derangements of transmethylation and transsulfuration pathways.

METHODS

We estimated homocysteine and its metabolites in 45 alcoholic CP patients, 45 tropical CP patients, and 48 healthy controls.

RESULTS

Significant increases in plasma total homocysteine and decreases in red blood cell folate, reduced glutathione, plasma methionine, cysteine, and urinary inorganic sulfate/creatinine ratio were observed in both alcoholic and tropical CP patients in comparison with healthy controls. Red blood cell glutathione and plasma cysteine levels were significantly lower in alcoholic than in tropical CP patients. However, plasma vitamin B12 levels were comparable between CP patients and controls. No significant differences in these parameters were observed between diabetic patients and nondiabetic patients. Multivariate regression analysis showed a significant negative correlation between homocysteine and folate (r = -0.415, P = 0.001) and a positive correlation between glutathione and cysteine levels (r = 0.37, P = 0.003).

CONCLUSIONS

Chronic pancreatitis is associated with hyperhomocysteinemia and derangements in transmethylation and transsulfuration pathways. Low folate levels observed in these patients seem to have a key role in this derangement.

Homocysteine and glutathione peroxidase-1

Mildly elevated homocysteine levels (Hcy) increase the risk for atherothrombotic vascular disease in the coronary, cerebrovascular, and peripheral arterial circulations. The molecular mechanisms responsible for decreased bioavailability of endothelium-derived nitric oxide (NO) by Hcy involve an increase of vascular oxidant stress and inhibition of important antioxidant capacity. Glutathione peroxidase-1 (GPx-1), a selenocysteine-containing antioxidant enzyme, may be a key target of Hcy's deleterious actions, and several experimental and clinical studies have demonstrated a complex relationship between plasma total homocysteine (tHcy), GPx-1, and endothelial dysfunction. Hcy may promote endothelial dysfunction, in part by decreasing GPx-1 expression; however, there is evidence to suggest that overexpression of GPx-1 can compensate for these effects. This review summarizes the current knowledge of the metabolism of Hcy, the effects of hyperhomocysteinemia observed in in vitro and in vivo models that lead to endothelial dysfunction and the possible mechanisms for these actions, and the role of GPx-1 in the pathogenesis of Hcy-induced cardiovascular disease (CVD).

Hyperhomocysteinemia-mediated DNA hypomethylation and its potential epigenetic role in rats

Hyperhomocysteinemia (HHcy), which is an independent risk factor for atherosclerosis, might cause dysregulation of gene expression, but the characteristics and key links involved in its pathogenic mechanisms are still poorly understood. The objective of the present study was to investigate the effect of HHcy on DNA methylation and the underlying mechanism of homocysteine (Hcy)-induced DNA methylation. HHcy was induced in Sprague-Dawley rats after 4 weeks of a low, medium or high methionine diet. The levels of total homocysteine, S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) were detected by high-performance liquid chromatography. The expression levels of genes and proteins of S-adenosylhomocysteine hydrolase, DNA methyltransferase and methyl-CpG-binding domain 2 were detected by real-time reverse transcription-polymerase chain reaction and Western blot analysis. A high-throughput quantitative methylation assay using fluorescence-based real-time polymerase chain reaction was employed to determine the levels of DNA methylation. The results indicated that HHcy induced the elevation of AdoHcy concentration, the decline of AdoMet concentration, the ratios of AdoMet/AdoHcy and the RNA and protein expression of S-adenosylhomocysteine hydrolase and methyl-CpG-binding domain 2, as well as an increase of DNA methyltransferase activity. With different methylation-dependent restriction endonucleases, the aberrant demethylation was found to prefer CCGG sequences to CpG islands. Increasing levels of HHcy significantly increased genome hypomethylation in B1 repetitive elements. The impacts of different levels of HHcy showed that the varied detrimental effects of HHcy could be attributed to different concentrations through different mechanisms. In mild and moderate HHcy, the Hcy might primarily influence the epigenetic regulation of gene expression through the interference of transferring methyl-group metabolism. However, at high Hcy concentrations, the impacts might be more injurious through oxidative stress, apoptosis and inflammation.

ADMA and hyperhomocysteinemia

Hyperhomocysteinemia is a risk factor for cardiovascular disease and stroke. Like many other cardiovascular risk factors, hyperhomocysteinemia produces endothelial dysfunction due to impaired bioavailability of endothelium-derived nitric oxide (NO). The molecular mechanisms responsible for decreased NO bioavailability in hyperhomocysteinemia are incompletely understood, but emerging evidence suggests that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may be a key mediator. Homocysteine is produced during the synthesis of ADMA and can alter ADMA metabolism by inhibiting dimethylarginine dimethylaminohydrolase (DDAH). Several animal and clinical studies have demonstrated a strong association between plasma total homocysteine, plasma ADMA, and endothelial dysfunction. These observations suggest a model in which elevation of ADMA may be a unifying mechanism for endothelial dysfunction during hyperhomocysteinemia. The recent development of transgenic mice with altered ADMA metabolism should provide further mechanistic insights into the role of ADMA in hyperhomocysteinemia.

ACUTE DILATORY AND NEGATIVE INOTROPIC EFFECTS OF HOMOCYSTEINE ARE INHIBITED BY AN ADENOSINE BLOCKER

1. Previous studies have shown that homocysteine elicits acute negative inotropic and coronary vasodilatory effects in rat hearts. In addition, this earlier work suggested that the inotropic action is mediated via an endothelium-derived agent that is neither nitric oxide (NO) nor a cyclooxygenase product, while the coronary actions were found to be antagonized by the NOS inhibitor l-NNA. Current experiments, which utilized coronary-perfused rat hearts, were designed to determine if muscarinic or adenosine receptors are involved in these acute actions of homocysteine. 2. Left ventricular developed pressure was used as a measure of systolic function in electrically paced, Langendorff-perfused heart with coronary pressure being used to monitor coronary vascular tone. Acute effects of homocysteine (10-300 micromol/L) were examined in the presence and absence of 1 yen 10(-6) mol atropine or 7 yen 10(-5) mol 8-(p-sulfophenyl) theophylline (SPT), a non-selective adenosine receptor antagonist. 3. Atropine had no effect on either the inotropic or vascular actions of homocysteine. In contrast, SPT partially antagonized both actions of the amino acid with the antagonism of the vasodilation being much greater than its inhibition of the negative inotropic effect. Experiments with adenosine demonstrated that the selected dose of SPT elicited marked rightward shifts in the dose-response curves for both the inotropic and vascular actions. 4. Current results suggest that adenosine plays a role in both the negative inotropic and vasodilatory actions of homocysteine. However, the relatively minor antagonistic action of SPT on the inotropic effect of homocysteine suggests that additional endothelium-derived mediators underlie its effects on contractility.

Homocysteine metabolism, hyperhomocysteinaemia and vascular disease: an overview

Hyperhomocysteinaemia has been regarded as a new modifiable risk factor for atherosclerosis and vascular disease. Homocysteine is a branch-point intermediate of methionine metabolism, which can be further metabolised via two alternative pathways: degraded irreversibly through the transsulphuration pathway or remethylated to methionine by the remethylation pathway. Both pathways are B-vitamin-dependent. Plasma homocysteine concentrations are determined by nongenetic and genetic factors. The metabolism of homocysteine, the role of B vitamins and the contribution of nongenetic and genetic determinants of homocysteine concentrations are reviewed. The mechanisms whereby homocysteine causes endothelial damage and vascular disease are not fully understood. Recently, a link has been postulated between homocysteine, or its intermediates, and an alterated DNA methylation pattern. The involvement of epigenetic mechanisms in the context of homocysteine and atherosclerosis, due to inhibition of transmethylation reactions, is briefly overviewed.

Elevated plasma homocysteine level in slow coronary flow

BACKGROUND

Slow flow velocity of dye in coronary arteries is not an infrequent finding during routine coronary angiography and its precise mechanism is unknown. In this study, we measured the plasma homocysteine level in patients with slow coronary flow (SCF) in comparison with subjects having normal coronary flow (NCF).

METHOD

The study consisted 39 patients (mean age, 47+/-8 years) with angiographically diagnosed SCF. SCF was defined according to TIMI frame count (TFC) method. Thirty subjects (mean age 46+/-8 years) with NCF served as control group. Plasma homocysteine levels were measured after 12 h fasting period in each subject.

RESULTS

Baseline demographic properties were similar in both groups. In patients with SCF, TFC was significantly higher than those with NCF. Similarly, in patients with SCF plasma homocysteine level was significantly higher than that of control group (14.1+/-2.2 vs. 5.5+/-1.3 micromol/l, respectively p < 0.001).

CONCLUSION

Elevated plasma homocysteine level supports the hypothesis that endothelial function is impaired in slow coronary flow.

Endothelial dysfunction: cardiovascular risk factors, therapy, and outcome

Endothelial dysfunction is a well established response to cardiovascular risk factors and precedes the development of atherosclerosis. Endothelial dysfunction is involved in lesion formation by the promotion of both the early and late mechanisms of atherosclerosis including up-regulation of adhesion molecules, increased chemokine secretion and leukocyte adherence, increased cell permeability, enhanced low-density lipoprotein oxidation, platelet activation, cytokine elaboration, and vascular smooth muscle cell proliferation and migration. Endothelial dysfunction is a term that covers diminished production/availability of nitric oxide and/or an imbalance in the relative contribution of endothelium-derived relaxing and contracting factors. Also, when cardiovascular risk factors are treated the endothelial dysfunction is reversed and it is an independent predictor of cardiac events. We review the literature concerning endothelial dysfunction in regard to its pathogenesis, treatment, and outcome.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Moderately elevated plasma homocysteine impairs functional endothelial recovery following denudation of mouse carotid arteries

Increased total plasma homocysteine is an independent risk factor for cardiovascular disease. This study was designed to determine whether it can impair endothelial function, by examining the recovery of acetylcholine-evoked relaxation following mechanical denudation of the endothelium in the arteries of cystathionine beta-synthase knockout (CbetaS(+/-)) mice. Heterozygous CbetaS(+/-) mice had total plasma homocysteine concentrations significantly higher (8.9 +/- 1.1 micromol/L, n = 12) than strain-matched wild-types (4.6 +/- 0.4 micromol/L, n = 5; P =.003). Left common carotid arteries were denuded of endothelium using a 250-microm polytetrafluoroethylene filament. After 10 days, when the endothelium had completely regrown, relaxation to acetylcholine was measured in precontracted segments of artery. Uninjured right carotid arteries from the same animals served as internal controls. Relaxation to acetylcholine was significantly attenuated in the injured arteries of the CbetaS(+/-) mice, compared to wild-types (P =.017); furthermore, there was a significant negative correlation between sensitivity to acetylcholine and total plasma homocysteine concentration measured in the same animal (r = -0.69, P <.003). These data suggest that even modest homocysteinemia has a deleterious effect on the function of healed endothelium in mouse arteries. This may account for its adverse influence on chronic cardiovascular disease.

Endothelium-Derived Hyperpolarizing Factor–Mediated Renal Vasodilatory Response Is Impaired During Acute and Chronic Hyperhomocysteinemia

Background— Endothelial dysfunction is an early event in the development of vascular complications in hyperhomocysteinemia. Endothelial cells release a number of vasodilators, including NO and prostacyclin. Several lines of evidence have indicated the existence of a third vasodilator pathway, mediated by endothelium-derived hyperpolarizing factor (EDHF). EDHF is a major determinant of vascular tone in small resistance vessels. The influence of hyperhomocysteinemia on EDHF is unknown. The present in vivo study evaluates the integrity of the EDHF pathway in the renal microcirculation of rats with acute and chronic hyperhomocysteinemia.

Methods and Results— EDHF-mediated vasodilation was evaluated as the renal blood flow (RBF) response to intrarenal acetylcholine during systemic NO synthase and cyclooxygenase inhibition. Acute hyperhomocysteinemia induced by intravenous homocysteine did not affect EDHF-mediated vasodilation. In contrast, intravenous methionine with subsequent hyperhomocysteinemia impaired the EDHF-mediated RBF response. When the methionine infusion was preceded by adenosine periodate oxidized to prevent the cleavage of S -adenosylhomocysteine to homocysteine and adenosine, a similar impairment of EDHF was observed, but with normal homocysteine levels. Animals with chronic hyperhomocysteinemia induced by a high-methionine, low–B vitamin diet during 8 weeks had a severely depressed EDHF-mediated vasodilation compared with those on a standard diet. Endothelium-independent vasodilation to deta-NONOate and pinacidil was not affected in acute and chronic hyperhomocysteinemia, demonstrating intact vascular smooth muscle reactivity.

Conclusions— EDHF-dependent responses are impaired in the kidney of hyperhomocysteinemic rats. Because EDHF is a major regulator of vascular function in small vessels, these findings have important implications for the development of microangiopathy in hyperhomocysteinemia.

Estrogen blocks homocysteine-induced endothelial dysfunction in porcine coronary arteries1,2

BACKGROUND

The objective of this study was to examine the effect of estrogen combined with homocysteine on vasomotor function and endothelial integrity in intact porcine coronary arteries.

MATERIALS AND METHODS

Pig coronary artery rings were incubated with estrogen, homocysteine, or estrogen and homocysteine for 24 h. Myographic analysis was performed with thromboxane A2 analog U46619 for contraction and bradykinin or sodium nitroprusside for relaxation. Endothelial nitric oxide synthase (eNOS) levels were determined by immunohistochemistry. Levels of superoxide anion were assessed by lucigenin-enhanced chemiluminescence analysis.

RESULTS

Endothelium-dependent vasorelaxation (bradykinin) for the homocysteine alone group was 62% compared with control (P < 0.05), and endothelium-dependent vasorelaxation for the estrogen alone group was 85% compared with control (P > 0.05). Endothelium-dependent vasorelaxation for the estrogen-homocysteine combined group was 79% compared with 89% for control (P > 0.05). There were no differences in endothelium-independent vasorelaxation (sodium nitroprusside) or in smooth muscle contractility (U46619) between all three groups and control. In addition, the eNOS immunoreactivity was declined in the homocysteine group and had no major change in the estrogen or estrogen plus homocysteine-treated group as compared with controls. The superoxide free radical measurement showed a marked increase in the homocysteine group, no major change from controls in the estrogen group, and a much-lessened effect in the combination of estrogen and homocysteine.

CONCLUSIONS

These data demonstrate that combining estrogen with homocysteine significantly blocks the effect of homocysteine on impairing endothelium-dependent vasorelaxation as well as on decreasing eNOS expression and increasing oxidative stress in porcine coronary arteries. This study suggests that estrogen may play a role in preventing homocysteine-mediated endothelial dysfunction and may be of benefit in the hyperhomocysteinemic patient.

Hyperhomocysteinemia, endothelial dysfunction, and cardiovascular risk: the potential role of ADMA

Hyperhomocysteinemia is an emerging risk factor for cardiovascular disease and stroke. The mechanisms underlying the pathophysiology of hyperhomocysteinemia are not completely defined, but endothelial dysfunction resulting from impaired bioavailability of nitric oxide is a consistent finding in experimental models. One potential mechanism for decreased nitric oxide bioavailability is inhibition of endothelial nitric oxide synthase by its endogenous inhibitor, asymmetric dimethylarginine (ADMA). Elevated plasma levels of ADMA have been found in association with hyperhomocysteinemia and endothelial dysfunction in both animals and humans. Additional studies are required to determine the mechanisms by which ADMA accumulates in hyperhomocysteinemia and to define the importance of ADMA in the endothelial dysfunction of hyperhomocysteinemia in vivo.

[Pharmacological study on the effects of the adenosine uptake inhibitor KF24345 on inflammatory diseases]

Adenosine protects against cellular damage and dysfunction under several adverse conditions, including inflammation. We examined the effects of KF24345, a novel adenosine uptake inhibitor, on inflammatory diseases to investigate whether the adenosine uptake inhibition is useful for the treatment of inflammation. KF24345 inhibited adenosine uptake into washed erythrocytes (in vitro) and sampled blood cells from mice after its oral administration (in vivo). KF24345 significantly suppressed lipopolysaccharide-induced tumor necrosis factor-alpha production and leukopenia in mice, and the effects of KF24345 were abolished by the treatment with a non-selective or an A(2A)-selective adenosine receptor antagonist. In the experimental glomerulonephritis induced in mice by anti-glomerular basement membrane antiserum, KF24345 significantly inhibited proteinuria and glomerular damage without exhibiting the side effects observed following the treatment with prednisolone and cyclophosphamide. In addition, KF24345 ameliorated the severity of experimental acute pancreatitis induced by cerulein or choline-deficient and ethionine-supplemented diet in mice, and it decreased mortality accompanying severe acute pancreatitis. The anti-pancreatitis effects of KF24345 were abolished by the treatment with a non-selective or an A(2A)-selective adenosine receptor antagonist. These results suggest that KF24345 and adenosine uptake inhibitors can be a new therapeutic approach for various inflammatory diseases, including glomerulonephritis and acute pancreatitis.

Adenosine uptake inhibition ameliorates cerulein-induced acute pancreatitis in mice

INTRODUCTION AND AIMS

Adenosine shows protective effects against cellular damage and dysfunction under several adverse conditions such as inflammation and ischemia. In the current study, we examined the effects of 3-[1-(6,7-diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1,3 )-quinazolinedione hydrochloride (KF24345), an adenosine uptake inhibitor, on cerulein-induced acute pancreatitis in mice to investigate whether inhibition of adenosine uptake could ameliorate the severity of acute pancreatitis.

METHODOLOGY

Acute pancreatitis was induced in mice with six intraperitoneal injections of cerulein (50 microg/kg each) at hourly intervals.

RESULTS

The cerulein injection increased activities of serum amylase and lipase and caused pathologic changes such as interstitial edema, polymorphonuclear cell infiltration, and acinar cell necrosis in the pancreas. KF24345 (10 mg/kg p.o.) ameliorated all these changes observed in mice with acute pancreatitis, and the suppressing effect of KF24345 on the elevation in serum amylase activity was abolished by the treatment with 8-(p-sulfophenyl)theophylline, an adenosine receptor antagonist. In addition, 2-(aminocarbonyl)- -(4-amino-2,6-dichlorophenyl)-4-[5,5-bis-(4-fluorophenyl)pentyl]-1-piperazineacetamide (R75231) and dipyridamole, other adenosine uptake inhibitors, also decreased the elevated serum amylase activity.

CONCLUSIONS

These are the first demonstrations that the adenosine uptake inhibitors ameliorate cerulein-induced acute pancreatitis in mice, and these data suggest that adenosine uptake inhibition could ameliorate the severity of acute pancreatitis in vivo.

KF24345, an adenosine uptake inhibitor, ameliorates the severity and mortality of lethal acute pancreatitis via endogenous adenosine in mice

Adenosine protects against cellular damage and dysfunction under several adverse conditions including inflammation and ischemia. In this study, we examined the effects of 3-[1-(6,7-diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345), an adenosine uptake inhibitor, on experimental acute pancreatitis induced by choline-deficient and ethionine-supplemented diet in mice. KF24345, administered with the diet onset and every 24 h thereafter, prevented hyperamylasemia, acinar cell injury and serum tumor necrosis factor-alpha elevation and ultimately decreased mortality. Therapeutic treatment with KF24345, which started 32 h after the diet onset, also decreased mortality. The beneficial effect of KF24345 on mortality was abolished by the pretreatment with 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385), a selective adenosine A(2A) receptor antagonist. An intravenous injection of KF24345 at 48 h after the diet onset increased plasma adenosine concentrations in mice with acute pancreatitis. These results suggest that KF24345 shows anti-pancreatitis effects via endogenous adenosine and adenosine A(2A) receptors. The adenosine uptake inhibition could be a new therapeutic approach for acute pancreatitis.

NO and the vasculature: where does it come from and what does it do?

Nitric oxide (NO) is involved in a large number of cellular processes and dysfunctions in NO production have been implicated in many different disease states. In the vasculature NO is released by endothelial cells where it modulates the underlying smooth muscle to regulate vascular tone. Due to the unique chemistry of NO, such as its reactive and free radical nature, it can interact with many different cellular constituents such as thiols and transition metal ions, which determine its cellular actions. In this review we also discuss many of the useful pharmacological tools that have been developed and used extensively to establish the involvement of NO in endothelium-derived relaxations. In addition, the recent literature identifying a potential source of NO in endothelial cells, which is not directly derived from endothelial nitric oxide synthase is examined. Finally, the photorelaxation phenomena, which mediates the release of NO from a vascular smooth muscle NO store, is discussed.

Effects of homocysteine on intracellular nitric oxide and superoxide levels in the renal arterial endothelium

The present study was designed to test the hypothesis that homocysteine (Hcys) reduces intracellular nitric oxide (NO) concentrations ([NO](i)) and stimulates superoxide (O.) production in the renal arterial endothelium, thereby resulting in endothelial dysfunction. With the use of fluorescence microscopic imaging analysis, a calcium ionophore, A-23187 (2 microM), and bradykinin (2 microM) were found to increase endothelial [NO](i) in freshly dissected lumen-opened small renal arteries loaded with 4,5-diaminofluorescein diacetate (DAF-2DA; 10 microM). Preincubation of the arteries with L-Hcys (20-40 microM) significantly attenuated the increase in endothelial [NO](i). However, L-Hcys had no effect on NO synthase activity in the renal arteries, as measured by the conversion rate of [(3)H]arginine to [(3)H]citrulline, but it concentration dependently decreased DAF-2DA-sensitive fluorescence induced by PAPA-NONOate in the solution, suggesting that L-Hcys reduces endothelial [NO](i) by its scavenging action. Because other thiol compounds such as L-cysteine and glutathione were also found to reduce [NO](i), it seems that decreased NO is not the only mechanism resulting in endothelial dysfunction or arteriosclerosis in hyperhomocysteinemia (hHcys). By analysis of intracellular O. levels using dihydroethidium trapping, we found that only L-Hcys among the thiol compounds studied markedly increased O. levels in the renal endothelium. These results indicate that L-Hcys inhibits the agonist-induced NO increase but stimulates O. production within endothelial cells. These effects of L-Hcys on [NO](i) and [O.] may contribute to endothelial injury associated with hHcys.

Vascular responses in male and female hypertensive rats with hyperhomocysteinemia

We studied vascular responses in spontaneously hypertensive rats (SHR) of both genders after methionine (Met) loading to test whether or not there were gender differences. SHRs were divided into 5 groups: male control (MSHR), female control (FSHR), methionine-loaded (+Met) males (MSHR[+Met]) and females (FSHR[+Met]), and male SHR with both 17beta-estradiol (E2) and Met administration (MSHR[+E2+Met]). Treated groups received Met (1g/kg body weight per day) in water for 6 weeks. Systolic blood pressure (SBP) was monitored weekly. Aortic contractile (phenylephrine-induced) and relaxant (acetylcholine-induced as endothelium-dependent relaxation, or EDR) responses as well as endothelial suppression (with nitric oxide synthase inhibitor) were evaluated at the end of experiments. Serum homocysteine (Hcy) level was also determined. Met overloading caused a nearly 3-fold increase in serum Hcy in each gender (moderate hyperhomocysteinemia, or HHcy). As age increased, SBP increased in all groups; FSHR(+Met) had the least elevation and significantly less increase of SBP than FSHR at the end of 6 weeks. There was also a significant increase of EDR in FSHR(+Met) compared with both FSHR and MSHR(+Met). FSHR(+Met) had the highest level of endothelium suppression. Furthermore, EDR in MSHR(+E2+Met) was significantly higher than that in MSHR(+Met). Direct Hcy feeding appeared to reduce the development of hypertension in female SHR in 3 weeks. Hence, SBP development was partially alleviated, whereas EDR and endothelium suppression were enhanced in female SHR with HHcy. E2 could mimic the gender-dependent effect of HHcy on EDR enhancement in MSHR; moreover, reduction of SBP development occurred in Hcy-fed FSHR.

Homocysteine decreases endothelium-dependent vasorelaxation in porcine arteries

BACKGROUND

Although hyperhomocysteinemia has long been recognized as an independent risk factor for vascular disease, the mechanisms of the pathogenesis of homocysteine are largely unknown. The objective of this study was to examine the effect of homocysteine on vasomotor function and endothelial integrity in intact porcine arteries.

METHODS

Pig coronary artery rings were incubated with homocysteine (10, 50, or 100 microM) for 24 h. Myograph analysis was performed with thromboxane A2 analogue U46619 for contraction and bradykinin or sodium nitroprusside for relaxation. Pig carotid arteries were perfusion-cultured in control and 50 and 100 microM homocysteine treatment groups. The diameter change was analyzed in response to norepinephrine and acetylcholine, respectively. Endothelial morphology and nitric oxide synthase (eNOS) levels were determined by histology analysis.

RESULTS

Endothelial-dependent vasorelaxation (bradykinin) was significantly reduced by 52, 87, and 97% in the pig coronary artery rings treated with 10, 50, and 100 microM homocysteine, respectively, compared to controls (P < 0.05). There were no differences in endothelium-independent vasorelaxation (sodium nitroprusside) or in smooth muscle contractility (U46619) between control and homocysteine-treated groups (P > 0.05). Acetylcholine-induced vasorelaxation was also significantly reduced by 44 and 98% in the pig carotid arteries treated with 50 and 100 microM homocysteine, respectively, compared to controls (P < 0.05). Variable degrees of endothelial cell injury, such as morphology change and detachment, were observed, and eNOS immunoreactivity was markedly reduced in both pig coronary and carotid arteries that were treated with high doses of homocysteine.

CONCLUSION

These data demonstrated that homocysteine significantly decreased endothelium-dependent vasorelaxation and eNOS immunoreactivity as well as induced marked endothelial injury in both porcine coronary and carotid arteries. This study suggests that homocysteine-mediated endothelial dysfunction and injury may play important roles in vascular lesion formation in the hyperhomocysteinemic patient.

Endothelial dysfunction and elevation of S-adenosylhomocysteine in cystathionine beta-synthase-deficient mice

Hyperhomocysteinemia is associated with increased risk for cardiovascular events, but it is not certain whether it is a mediator of vascular dysfunction or a marker for another risk factor. Homocysteine levels are regulated by folate bioavailability and also by the methyl donor S-adenosylmethionine (SAM) and its metabolite S-adenosylhomocysteine (SAH). We tested the hypotheses that endothelial dysfunction occurs in hyperhomocysteinemic mice in the absence of folate deficiency and that levels of SAM and SAH are altered in mice with dysfunction. Heterozygous cystathionine beta-synthase-deficient (CBS(+/-)) and wild-type (CBS(+/+)) mice were fed a folate-replete, methionine-enriched diet. Plasma levels of total homocysteine were elevated in CBS(+/-) mice compared with CBS(+/+) mice after 7 weeks (27.1+/-5.2 versus 8.8+/-1.1 micromol/L; P<0.001) and 15 weeks (23.9+/-3.0 versus 13.0+/-2.3 micromol/L; P<0.01). After 15 weeks, but not 7 weeks, relaxation of aortic rings to acetylcholine was selectively impaired by 35% (P<0.05) and thrombomodulin anticoagulant activity was decreased by 20% (P<0.05) in CBS(+/-) mice. Plasma levels of folate did not differ between groups. Levels of SAH were elevated approximately 2-fold in liver and brain of CBS(+/-) mice, and correlations were observed between plasma total homocysteine and SAH in liver (r=0.54; P<0.001) and brain (r=0.67; P<0.001). These results indicate that endothelial dysfunction occurs in hyperhomocysteinemic mice even in the absence of folate deficiency. Endothelial dysfunction in CBS(+/-) mice was associated with increased tissue levels of SAH, which suggests that altered SAM-dependent methylation may contribute to vascular dysfunction in hyperhomocysteinemia.

Endothelial dysfunction in a murine model of mild hyperhomocyst(e)inemia

Homocysteine is a risk factor for the development of atherosclerosis and its thrombotic complications. We have employed an animal model to explore the hypothesis that an increase in reactive oxygen species and a subsequent loss of nitric oxide bioactivity contribute to endothelial dysfunction in mild hyperhomocysteinemia. We examined endothelial function and in vivo oxidant burden in mice heterozygous for a deletion in the cystathionine beta-synthase (CBS) gene, by studying isolated, precontracted aortic rings and mesenteric arterioles in situ. CBS(-/+) mice demonstrated impaired acetylcholine-induced aortic relaxation and a paradoxical vasoconstriction of mesenteric microvessels in response to superfusion of methacholine and bradykinin. Cyclic GMP accumulation following acetylcholine treatment was also impaired in isolated aortic segments from CBS(-/+) mice, but aortic relaxation and mesenteric arteriolar dilation in response to sodium nitroprusside were similar to wild-type. Plasma levels of 8-epi-PGF(2alpha) (8-IP) were somewhat increased in CBS(-/+) mice, but liver levels of 8-IP and phospholipid hydroperoxides, another marker of oxidative stress, were normal. Aortic tissue from CBS(-/+) mice also demonstrated greater superoxide production and greater immunostaining for 3-nitrotyrosine, particularly on the endothelial surface. Importantly, endothelial dysfunction appears early in CBS(-/+) mice in the absence of structural arterial abnormalities. Hence, mild hyperhomocysteinemia due to reduced CBS expression impairs endothelium-dependent vasodilation, likely due to impaired nitric oxide bioactivity, and increased oxidative stress apparently contributes to inactivating nitric oxide in chronic, mild hyperhomocysteinemia.

Differential actions of L-cysteine on responses to nitric oxide, nitroxyl anions and EDRF in the rat aorta

1. The effects of L-cysteine were tested in rat aortic rings on responses to nitric oxide free radical (NO(*)), nitroxyl (NO(-)) derived from Angeli's salt and endothelium-derived relaxing factor (EDRF) activated by acetylcholine, ATP and the calcium ionophore A23187. Concentrations of 300 microM or less of L-cysteine had no effect on responses. 2. Relaxations produced by exogenous NO(*) (0.25 - 2.5 microM) were markedly prolonged and relaxations produced by sodium nitroprusside (0.001 - 0.3 microM) were enhanced by 1 and 3 mM L-cysteine. The enhancements by L-cysteine of responses to NO(*) and sodium nitroprusside may be attributed to the formation of S-nitrosocysteine. 3. Relaxations mediated by the nitroxyl anion (0.3 microM) donated from Angeli's salt were more prolonged than those produced by NO(*), and nitroxyl-induced relaxations were reduced by L-cysteine (1 and 3 mM). 4. EDRF-mediated relaxations produced by acetylcholine (0.01 - 10 microM), ATP (3 - 100 microM) and the calcium ionophore A23187 (0.1 microM) were significantly reduced by 3 mM L-cysteine. 5. The similarity between the inhibitory effects of L-cystei on responses to EDRF and on those to nitroxyl suggests that a component of the response to EDRF may be mediated by nitroxyl anion.

Homocysteine provokes leukocyte-endothelium interaction by downregulation of nitric oxide

Recent evidence indicates that chronic hyperhomocysteinemia, which is found in from 9 to 15% of the general population, is an independent risk factor for the development of atherosclerosis. We sought to elucidate the mechanism by which exposure of the vascular wall to high levels of homocysteine initiates this inflammatory reaction. We examined the acute effect of homocysteine on endothelial dysfunction in isolated rat arteries and on microcirculatory leukocyte-endothelium interaction in vivo. Intravital microscopy of rat mesenteric venules was performed by superfusing the mesentery with increasing concentrations of homocysteine (1-5 mmol/l). There was a significant concentration- and time-dependent increase in leukocyte rolling, adherence, and extravasation compared with control rats superfused with Krebs-Henseleit solution (p < 0.01). Moreover, immunohistochemical staining demonstrated significantly increased P-selectin and intercellular adhesion molecule-1 (ICAM-1) expression on intestinal venules after homocysteine superfusion. In contrast, mesenteric superfusion with the nitric oxide donor 4-hydroxymethyl-furazan-3-carboxylic acid oxide (CAS1609, 1 micromol/l) significantly attenuated homocysteine-induced leukocyte rolling, adherence, and transmigration to control levels (p < 0.01). CAS1609 also attenuated both P-selectin and ICAM-1 expression on mesenteric venules and decreased CD18 expression on isolated leukocytes. Superior mesenteric arteries incubated with 5 mmol/l homocysteine developed significant (p < 0.01) endothelial dysfunction (i.e., impaired relaxation to endothelium-dependent dilators). Acute hyperhomocysteinemia induces endothelial dysfunction, characterized by a loss of endothelium-derived nitric oxide, leading to an inflammatory state. This state results in increased leukocyte rolling, adherence, and transmigration by upregulation of cell adhesion molecules. Our data suggest that hyperhomocysteinemia inhibits the important homeostatic role of nitric oxide in preventing endothelial dysfunction.

Investigation of the inhibitory effects of homocysteine and copper on nitric oxide-mediated relaxation of rat isolated aorta

1. Elevated plasma levels of homocysteine (HC) and copper have both been associated with the development of inflammatory vascular diseases, such as atherosclerosis. In this study, the effects of a combination of HC and copper on nitric oxide (NO)-mediated relaxation of isolated rat aortic rings were investigated. 2. Exposure to HC (10-100 microM; 30 min) had no effect on relaxation to acetylcholine (ACh; 0.01-10 microM, n=4). Pre-incubation of aortic rings with a higher concentration of HC for an extended period (1 mM; 180 min) significantly inhibited endothelium-dependent relaxation (n=4), but this inhibition was prevented by the presence of the copper chelator bathocuprione (10 microM, 180 min, n=6). 3. Exposure to HC (100 microM) and copper (10-100 microM; 30 min) caused a copper concentration-dependent inhibition of endothelium-dependent relaxation (n=4). This inhibitory effect was reduced in the presence of either superoxide dismutase (SOD; 100 u ml(-1); n=4) or catalase (100 u ml(-1); n=4), and further reduced by the presence of both enzymes (n=5). 4. HC and copper (100 microM; 30 min) significantly inhibited endothelium-independent relaxation to glyceryl trinitrate (0.01-10 microM; n=8). In contrast, HC (1 mM), alone or in combination with copper (100 microM), did not inhibit relaxation to the endothelium-independent relaxant sodium nitroprusside (0.01-10 microM; n=4). 5. These data indicate that the presence of copper greatly enhances the inhibitory actions of HC on NO-mediated relaxation of isolated aortic rings. The reduction of inhibition by catalase and SOD indicates a possible role for copper-catalyzed generation of superoxide and hydrogen peroxide leading to an increased inactivation or decreased production of endothelium-derived NO.