Responses of carotid artery in mice deficient in expression of the gene for endothelial NO synthase

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

Structure of cerebral arterioles in mice deficient in expression of the gene for endothelial nitric oxide synthase

We examined effects of pharmacological inhibition of nitric oxide synthase (NOS) and genetic deficiency of the endothelial isoform of NOS (eNOS) on structure and mechanics of cerebral arterioles. We measured pressure, diameter, and cross-sectional area (CSA) of the vessel wall (histologically) in maximally dilated cerebral arterioles in mice that were untreated or treated for 3 months with the NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg per day in drinking water). Treatment with L-NAME increased systemic arterial mean pressure (SAP; 143+/-4 versus 121+/-4 mm Hg, P<0.05) and CSA (437+/-27 versus 310+/-34 microm2, P<0.05). These findings suggest that hypertension induced in mice by NOS inhibition is accompanied by hypertrophy of cerebral arterioles. To determine the role of the eNOS isoform in regulation of cerebral vascular growth, we examined mice with targeted disruption of one (heterozygous) or both (homozygous) genes encoding eNOS. Wild-type littermates served as controls. SAP and CSA were significantly increased in homozygous (SAP, 141+/-5 versus 122+/-3 mm Hg in wild-type mice, P<0.05; CSA, 410+/-18 versus 306+/-15 microm2 in wild-type mice, P<0.05), but not in heterozygous (SAP, 135+/-4 mm Hg; CSA, 316+/-32 microm2) eNOS-deficient mice. Carotid ligation normalized cerebral arteriolar pulse pressure did not prevent increases in CSA in homozygous eNOS-deficient mice. Thus, cerebral arterioles undergo hypertrophy in homozygous eNOS-deficient mice, even in the absence of increases in arteriolar pulse pressure. These findings suggest that eNOS plays a major role in regulation of cerebral vascular growth.

Overexpression of CuZn-SOD Prevents Lipopolysaccharide-Induced Endothelial Dysfunction

Background and Purpose— Inflammation is thought to be a major contributor to carotid artery disease. Lipopolysaccharide (LPS) activates inflammatory mechanisms thought to contribute to endothelial dysfunction by mechanisms that are not well defined. The goal of this study was to determine whether overexpression of CuZn-SOD protects against LPS-induced increases in superoxide and endothelial dysfunction.

Methods— Carotid arteries from CuZn-SOD transgenic (SOD-Tg) and nontransgenic (non-Tg) littermates were examined in vitro. Superoxide levels were measured using lucigenin-enhanced chemiluminescence.

Results— In non-Tg mice, LPS (0.5 μg/mL for 22 hours) produced marked impairment of vasorelaxation in response to the endothelium-dependent dilator acetylcholine (ACh). For example, 100 μmol/L ACh relaxed carotid arteries from non-Tg mice by 86±6% and 38±8% after treatment with vehicle and LPS, respectively. In contrast, LPS did not significantly impair responses of carotid artery to ACh in SOD-Tg mice, and LPS had no effect on relaxation responses to the endothelium-independent dilator nitroprusside in carotid artery from non-Tg or SOD-Tg mice. LPS-induced increases in superoxide, as measured using lucigenin-enhanced chemiluminescence, were higher in vessels from non-Tg mice than from SOD-Tg mice.

Conclusions— These results indicate that LPS increases superoxide and impairs endothelium-dependent relaxation. Overexpression of the CuZn isoform of SOD effectively prevents LPS-induced oxidative stress and endothelial dysfunction in the carotid artery.

Evidence for a role of protein kinase C-alpha in urine concentration

In mouse kidney, the conventional protein kinase C (PKC) isoenzyme alpha is expressed in glomeruli, the cortical collecting duct (intercalated cells only), and medullary collecting duct. To get insights on its function, PKC-alpha knockout (-/-) and wild-type (+/+) mice were studied. When provided free access to water, PKC-alpha -/- mice showed approximately 50% greater urine flow rate and lower urinary osmolality in 24-h metabolic cage experiments despite a greater urinary vasopressin-to-creatinine ratio vs. PKC-alpha +/+ mice. Renal albumin excretion was not different. Clearance experiments under inactin/ketamine anesthesia revealed a modestly reduced glomerular filtration rate and showed a reduced absolute and fractional renal fluid reabsorption in PKC-alpha -/- mice. The sodium-restricting response to a low-sodium diet was unaffected in PKC-alpha -/- mice. Urinary osmolality was reduced to similar hypotonic levels in PKC-alpha -/- and +/+ mice during acute oral water loading or application of the vasopressin V(2)-receptor antagonist SR-121463. In comparison, the lower urinary osmolality observed in PKC-alpha -/- mice vs. wild-type mice under basal conditions persisted during water restriction for 36 h. In conclusion, PKC-alpha appears not to play a major role in renal sodium reabsorption but, consistent with its expression in the medullary collecting duct, contributes to urinary concentration in mice. Considering that PKC-beta I and -beta II are coexpressed with PKC-alpha in mouse medullary collecting duct, the present results indicate that conventional PKC isoenzymes cannot fully compensate for each other.

Gene-targeted mice reveal a critical role for inducible nitric oxide synthase in vascular dysfunction during diabetes

BACKGROUND AND PURPOSE

Inducible nitric oxide synthase (iNOS) is a mediator of vascular dysfunction during inflammation. The purpose of this study was to test the hypothesis that vascular dysfunction during diabetes is dependent on expression of iNOS.

METHODS

Diabetes was produced in mice with streptozotocin (150 mg/kg IP). After 4 to 6 months of diabetes, vasomotor function was examined in vitro in carotid arteries from mice with targeted disruption of the gene for iNOS (iNOS-deficient mice) and from normal, wild-type (WT) mice.

RESULTS

Contractile responses of carotid arteries to U46619, a thromboxane A2 analogue, were not altered by diabetes in WT mice. Responses to U46619 were increased in arteries from diabetic iNOS-deficient mice compared with diabetic WT and nondiabetic mice (iNOS-deficient and WT mice). These results indicate that expression of iNOS inhibits an increased vasoconstrictor response during diabetes. Arteries from nondiabetic WT mice relaxed 83+/-2% (mean+/-SE) in response to acetylcholine (1 micromol/L) compared with 58+/-6% in arteries from diabetic WT mice (P<0.05 versus nondiabetic mice). In contrast, relaxation of carotid arteries to acetylcholine was similar (81+/-4% versus 76+/-6%; P>0.05) in iNOS-deficient mice under nondiabetic and diabetic conditions, respectively. Thus, diabetes produced impairment of endothelium-dependent relaxation in arteries from WT but not iNOS-deficient mice. Endothelium-independent relaxation in response to nitroprusside was similar in arteries from all mice.

CONCLUSIONS

These results provide the first direct evidence that impairment of endothelium-dependent relaxation during diabetes is dependent on expression of iNOS.

The relationship between insulin resistance and polymorphisms of the endothelial nitric oxide synthase gene in patients with coronary artery disease

Nitric oxide (NO) regulates endothelial function and is believed to prevent atherogenesis. In endothelial cells, endothelial nitric oxide synthase (eNOS) is expressed constitutively, and regulates NO synthesis. A mutation of the eNOS gene has been associated with the development of coronary artery disease (CAD). The development of CAD is also influenced by insulin resistance, and recent studies suggest that NO might affect cellular insulin activity. We investigated the association between eNOS polymorphisms and insulin resistance in patients with CAD. We screened 45 patients with a history of myocardial infarction (MI), angina pectoris (AP), or coronary spasm. Genotypes were determined by polymerase chain reaction-restriction fragment-length polymorphism analysis. We examined two polymorphisms of the eNOS gene (The T(-786)-->C variant and the missense Glu298Asp variant). Insulin resistance was measured by determining the plasma immunoreactive insulin concentration at the 120 min time point (IRI 120) of a 75 g oral glucose tolerance test. The IRI 120 of the T(-786)-->C variant group was higher than that for the control group (p<0.05). This finding demonstrates that the T(-786)-->C mutation in the eNOS gene decreases insulin sensitivity.

Enhanced vasoconstrictor responses in eNOS deficient mice

Previous studies suggest that vasoconstriction is modulated by nitric oxide (NO). Contractions to ET-1 and/or thromboxane may be enhanced during chronic deficiency in expression or activity of NO synthase (NOS). Multiple isoforms of NOS are expressed within the vessel wall and purely pharmacological approaches cannot define the role of each. We tested the hypothesis that vasoconstriction to endothelin-1 (ET-1) and/or the thromboxane mimetic, U46619, is enhanced under conditions of chronic, selective deficiency in endothelial NOS (eNOS-/-) by examining responses in aorta from eNOS-/- mice compared to wild type (eNOS+/+). ET-1 produced dose-dependent contraction of aorta from eNOS+/+ mice that was increased twofold following acute inhibition of all NOS isoforms with N(G)-nitro-L-arginine (L-NNA). In eNOS-/- mice, contractions to ET-1 were increased twofold compared to eNOS+/+. L-NNA had no effect. Although contraction of the aorta to thromboxane mimetic U46619 was increased at lower concentrations, maximal contractions to U46619 were not increased following acute inhibition of NOS or in eNOS-/- mice. These studies provide direct evidence that vasoconstriction to ET-1 and thromboxane is augmented in the face of eNOS deficiency, demonstrating that eNOS normally inhibits vascular contractile responses.

Deficiency of glutathione peroxidase-1 sensitizes hyperhomocysteinemic mice to endothelial dysfunction

OBJECTIVE

We tested the hypothesis that deficiency of cellular glutathione peroxidase (GPx-1) enhances susceptibility to endothelial dysfunction in mice with moderate hyperhomocysteinemia.

METHODS AND RESULTS

Mice that were wild type (Gpx1+/+), heterozygous (Gpx1+/-), or homozygous (Gpx1-/-) for the mutated Gpx1 allele were fed a control diet or a high-methionine diet for 17 weeks. Plasma total homocysteine was elevated in mice on the high-methionine diet compared with mice on the control diet (23+/-3 versus 6+/-0.3 micromol/L, respectively; P<0.001) and was not influenced by Gpx1 genotype. In mice fed the control diet, maximal relaxation of the aorta in response to the endothelium-dependent dilator acetylcholine (10(-5) mol/L) was similar in Gpx1+/+, Gpx1+/-, and Gpx1-/- mice, but relaxation to lower concentrations of acetylcholine was selectively impaired in Gpx1-/- mice (P<0.05 versus Gpx1+/+ mice). In mice fed the high-methionine diet, relaxation to low and high concentrations of acetylcholine was impaired in Gpx1-/- mice (maximal relaxation 73+/-6% in Gpx1-/- mice versus 90+/-2% in Gpx1+/+ mice, P<0.05). No differences in vasorelaxation to nitroprusside or papaverine were observed between Gpx1+/+ and Gpx1-/- mice fed either diet. Dihydroethidium fluorescence, a marker of superoxide, was elevated in Gpx1-/- mice fed the high-methionine diet (P<0.05 versus Gpx1+/+ mice fed the control diet).

CONCLUSIONS

These findings demonstrate that deficiency of GPx-1 exacerbates endothelial dysfunction in hyperhomocysteinemic mice and provide support for the hypothesis that hyperhomocysteinemia contributes to endothelial dysfunction through a peroxide-dependent oxidative mechanism.

Increased superoxide and vascular dysfunction in CuZnSOD-deficient mice

Increased superoxide is thought to play a major role in vascular dysfunction in a variety of disease states. Superoxide dismutase (SOD) limits increases in superoxide; however, the functional significance of selected isoforms of SOD within the vessel wall are unknown. We tested the hypothesis that selective loss of CuZnSOD results in increased superoxide and altered vascular responsiveness in CuZnSOD-deficient (CuZnSOD(-/-)) mice compared with wild-type (CuZnSOD(+/+)) littermates. Total SOD activity was reduced (P<0.05) by approximately 60% and CuZnSOD protein was absent in aorta from CuZnSOD(-/-) as compared with wild-type mice. Vascular superoxide levels, measured using lucigenin (5 micro mol/L)-enhanced chemiluminescence and hydroethidine (2 micro mol/L)-based confocal microscopy, were increased (approximately 2-fold; P<0.05) in CuZnSOD(-/-) mice as compared with wild-type mice. Relaxation of the carotid artery in response to acetylcholine and authentic nitric oxide was impaired (P<0.05) in CuZnSOD(-/-) mice. For example, maximal relaxation to acetylcholine (100 micro mol/L) was 50+/-6% and 69+/-5% in CuZnSOD(-/-) and wild-type mice, respectively. Contractile responses of the carotid artery were enhanced (P<0.05) in CuZnSOD(-/-) mice in response to phenylephrine and serotonin, but not to potassium chloride or U46619. In vivo, dilatation of cerebral arterioles (baseline diameter=31+/-1 micro m) to acetylcholine was reduced by approximately 50% in CuZnSOD(-/-) mice as compared with wild-type mice (P<0.05). These findings provide the first direct insight into the functional importance of CuZnSOD in blood vessels and indicate that this specific isoform of SOD limits increases in superoxide under basal conditions. CuZnSOD-deficiency results in altered responsiveness in both large arteries and microvessels.

Inhibition of renin-angiotensin system ameliorates endothelial dysfunction associated with aging in rats

OBJECTIVE

Endothelial vasodilator functions are progressively impaired with aging, which may account in part for the increased incidence of cardiovascular events in elderly people. We examined what treatment could ameliorate the endothelial dysfunction associated with aging in rats.

METHODS AND RESULTS

Aged (12-month-old) Wistar-Kyoto rats were treated with vehicle, temocapril, CS-866 (an angiotensin II type 1 receptor antagonist), cerivastatin, or hydralazine for 2 weeks. Endothelium-dependent relaxations (EDRs) of aortas from aged rats were markedly impaired compared with EDRs of aortas from young (3-month-old) rats. Indomethacin, NS-398 (a cyclooxygenase [COX]-2 inhibitor), and SQ-29548 (a thromboxane A2/prostaglandin H2 receptor antagonist) acutely restored EDRs in aged rats, suggesting an involvement of COX-2-derived vasoconstricting eicosanoids. Tiron, a superoxide scavenger, also partially improved EDRs, suggesting an involvement of superoxide. EDRs were significantly ameliorated in aged rats after long-term treatment with temocapril or CS-866 but not after treatment with cerivastatin or hydralazine. Indomethacin induced no further improvement of EDRs after treatment with temocapril or CS-866. COX-2 protein expression and superoxide production were increased in the aortas of aged rats and were also attenuated by treatment with temocapril or CS-866.

CONCLUSIONS

These results demonstrate that long-term inhibition of the renin-angiotensin system ameliorates endothelial dysfunction associated with aging through the inhibition of the synthesis of COX-2-derived vasoconstricting factors and superoxide anions.

Interleukin-10 protects nitric oxide-dependent relaxation during diabetes: role of superoxide

Interleukin (IL)-10, an anti-inflammatory cytokine, preserves endothelial function during acute inflammation. We tested the hypotheses that IL-10 plays a protective role in blood vessels during diabetes by suppressing impairment of endothelium-dependent relaxation and that protection by IL-10 is mediated by effects on superoxide (O(2-)). Streptozotocin (150 mg/kg i.p.) or citrate buffer was injected into IL-10-deficient (IL-10(-/-)) mice and wild-type controls (IL-10(+/+)). In IL-10(+/+) and IL-10(-/-) mice, blood glucose levels were approximately 120 mg/dl after citrate administration and approximately 400 mg/dl after streptozotocin administration. Vasorelaxation was examined in arteries in vitro 12-16 weeks later. Maximum relaxation to acetylcholine (30 micromol/l) was 88 +/- 3% (means +/- SE) in nondiabetic mice and 84 +/- 3% in diabetic IL-10(+ /+) mice (P > 0.05). Thus, at this time point, diabetes did not impair endothelium-dependent relaxation in vessels in wild-type mice. In contrast, maximum relaxation in vessels from diabetic IL-10(-/-) mice was significantly decreased (74 +/- 5%) compared with nondiabetic IL-10(-/-) mice (93 +/- 2%, P < 0.05). Superoxide dismutase with polyethylene glycol (PEG-SOD) restored impaired responses to acetylcholine to levels seen in controls. Responses to acetylcholine also were improved by allopurinol (an inhibitor of xanthine oxidase) in vessels from diabetic IL-10(- /-) mice. Thus, diabetes produces greater impairment of relaxation to acetylcholine in IL-10(-/-) mice than in IL-10(+/ +) mice. These findings provide direct evidence that IL-10 impedes mechanisms of endothelial dysfunction during diabetes. Restoration of vasorelaxation with PEG-SOD or allopurinol suggests that the mechanism(s) by which IL-10 preserves endothelium-dependent vasorelaxation involves O(2-), perhaps by reducing production of O(2-) by xanthine oxidase.

Interactions between endothelial nitric oxide synthase and sex hormones in vascular protection in mice

The vasculoprotective effects of sex hormones, particularly estrogens, have been attributed to their ability to increase the bioavailability of nitric oxide through activation of endothelial nitric oxide synthase (eNOS). To dissect the relative contribution in vivo of eNOS, sex hormones, and their interaction in two complex vascular phenotypes, hypertension and atherosclerosis, we used mice doubly deficient in eNOS and apoE (nnee) or lacking only apoE (NNee). Females and males were gonadectomized at 1 month of age and implanted either with control pellets or pellets releasing 17beta-estradiol (E2). Hormonally intact nnee mice have elevated blood pressure (BP) and increased atherosclerosis compared with NNee mice, but on removal of gonads, BP and atherosclerosis decreased significantly in nnee mice but not in NNee mice. Three months of treatment with exogenous E2 dramatically reduced atherosclerosis and significantly lowered BP in both NNee and nnee mice compared with animals treated with control pellets. Thus exogenous E2 has strong BP-lowering and atheroprotective effects in apoE-deficient mice, but eNOS is not essential for either effect. Endogenous sex hormones, on the other hand, cause significant damage to the vasculature in the absence of eNOS, but these effects are overridden by interactions between eNOS and sex hormones.

Endothelial dysfunction and blood pressure variability in selected inbred mouse strains

The genetic regulation of blood pressure (BP) and endothelial function is likely to be polygenic. Because there is considerable variability in basal BP among inbred mouse strains, the purpose of this study was to determine whether a similar variability in vascular function exists among 7 "normotensive" strains. We tested the hypothesis that compared with mice with higher BPs, mice with lower BPs would have greater aortic endothelial responses to acetylcholine (ACh). Mean BP ranged from 117 to 145 mm Hg among the 7 strains. The responses of aortic rings to ACh, sodium nitroprusside, and papaverine were assessed after submaximal precontraction with prostaglandin F(2alpha). The aortas from all strains relaxed in a concentration-dependent manner to sodium nitroprusside and papaverine, but responses to ACh were markedly impaired in the aortas, but not carotid arteries, from 129P3/J and 129X1/SvJ mice. Aortas from the other strains relaxed normally to ACh. Furthermore, the endothelium-dependent dilators ADP and A23187 caused similar relaxation in 129P3/J, 129X1/SvJ, and C57BL/6J mice. Although the data do not support the initial hypothesis, the impaired aortic response to ACh in the 129 strains is a novel finding and illustrates the potential impact that genetic background can have on vascular responsiveness.

Cholinergic dilation of cerebral blood vessels is abolished in M(5) muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.

Neuronal nitric oxide synthase (NOS) regulates leukocyte-endothelial cell interactions in endothelial NOS deficient mice

1. The present study was designed to examine the possible role of neuronal nitric oxide synthase (nNOS) in regulation of leukocyte - endothelial cell interactions in the absence of endothelial nitric oxide synthase (eNOS), using intravital microscopy of the cremasteric microcirculation of eNOS(-/-) mice. 2. Baseline leukocyte rolling and adhesion revealed no differences between wild-type and eNOS(-/-) mice in either the cremasteric or intestinal microcirculations. 3. Superfusion with L-NAME (100 microM) caused a progressive and significant increase in leukocyte adhesion in both wild-type and eNOS(-/-) mice, without detecting differences between the two strains of mice. 4. Superfusion with 7-nitroindazole (100 microM), a selective inhibitor of nNOS, had no effect on leukocyte adhesion in wild-type animals. However, it increased leukocyte adhesion significantly in eNOS(-/-) mice, which was reversed by systemic L-arginine pre-administration. 5. Stimulation of the microvasculature with H(2)O(2) (100 microM) induced a transient elevation in leukocyte rolling in wild-type mice. Conversely, the effect persisted during the entire 60 min of experimental protocol in eNOS(-/-) mice either with or without 7-nitroindazole. 6. Semi-quantitative analysis by RT - PCR of the mRNA for nNOS levels in eNOS(-/-) and wild-type animals, showed increased expression of nNOS in both brain and skeletal muscle of eNOS(-/-) mice. 7. In conclusion, we have demonstrated that leukocyte-endothelial cell interactions are predominantly modulated by eNOS isoform in postcapillary venules of normal mice, whereas nNOS appears to assume the same role in eNOS(-/-) mice. Interestingly, unlike eNOS there was insufficient NO produced by nNOS to overcome leukocyte recruitment elicited by oxidative stress, suggesting that nNOS cannot completely compensate for eNOS.

NO-dependent vasorelaxation is impaired after gene transfer of inducible NO-synthase

Proinflammatory stimuli produce expression of inducible NO-synthase (iNOS) within blood vessels and are associated with impaired endothelium-dependent relaxation. Gene transfer of iNOS was used to test the hypothesis that expression of iNOS in blood vessels produces impairment of NO-dependent relaxation as well as contraction. An adenoviral vector containing cDNA for murine iNOS, AdCMViNOS, and a control virus, AdCMVBglII, were used for gene transfer to rabbit carotid arteries in vitro and in vivo. After gene transfer of iNOS in vitro, contractile responses to KCl, phenylephrine, and U46619 were impaired. Relaxation in response to acetylcholine, ADP, A23187, and nitroprusside was also impaired. For example, maximum relaxation of vessels to acetylcholine (10 micromol/L) was 78+/-4% (mean+/-SE) after AdBglII (10(10.5) plaque-forming units) and 34+/-5% after AdiNOS (10(10.5) plaque-forming units, P<0.05). NO-independent relaxation in response to 8-bromo-cGMP and papaverine was not impaired after AdiNOS. Contraction and relaxation were improved in carotid arteries expressing iNOS by aminoguanidine and L-N-iminoethyl lysine, inhibitors of iNOS. After intraluminal gene transfer of iNOS in vivo, contraction of vessels in vitro was normal, but responses to acetylcholine were impaired. In summary, the major finding is that NO-dependent relaxation is impaired in arteries after gene transfer of iNOS in vitro and in vivo. Thus, expression of iNOS per se impairs NO-dependent relaxation.

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.

Role of sex differences and effects of endothelial NO synthase deficiency in responses of carotid arteries to serotonin

We examined the hypothesis that contraction of the carotid arteries to serotonin is normally inhibited by endothelial NO synthase (eNOS) and is enhanced in mice lacking the gene for eNOS. Because the influence of eNOS may vary with the sex of the mouse, we also tested whether responses to serotonin were dependent on sex. We studied carotid arteries in vitro from littermate control (eNOS(+/+)) mice, heterozygous (eNOS(+/-)) mice, and homozygous eNOS-deficient (eNOS(-/-)) mice (male and female). Contraction to serotonin was greater in male eNOS(+/+) mice than in female eNOS(+/+) mice. In male mice, contraction to serotonin increased by approximately 40% and 2.5-fold in male eNOS(+/-) and eNOS(-/-) mice, respectively. Contraction to serotonin was more than doubled in female eNOS(+/-) mice and increased >5-fold in arteries from eNOS(-/-) mice. In contrast, maximum vasoconstriction to U46619 was similar in male and female eNOS(+/+), eNOS(+/-), and eNOS(-/-) mice. Relaxation to acetylcholine was not different in male and female eNOS(+/+) or eNOS(+/-) mice but was absent in eNOS(-/-) mice. These findings suggest that the contraction of carotid arteries to serotonin is influenced by the sex of the animal. eNOS deficiency in gene-targeted mice is associated with enhanced contraction to serotonin, particularly in female mice, providing direct evidence that eNOS is a major determinant of vascular effects of serotonin. The results with eNOS(+/-) mice suggest a "gene-dosing" effect for vascular responses to serotonin.

Mechanisms that produce nitric oxide-mediated relaxation of cerebral arteries during atherosclerosis

BACKGROUND AND PURPOSE

The first goal of the present study was to examine the hypothesis that relaxation of cerebral arteries to nitric oxide in primates is dependent on activation of soluble guanylate cyclase (sGC). The second goal was to determine whether the role of sGC in mediating responses to nitric oxide is altered in atherosclerosis.

METHODS

Basilar arteries from normal and atherosclerotic monkeys were studied in vitro. After precontraction with prostaglandin F(2alpha) (0.1 to 1 micromol/L), concentration-response curves to authentic nitric oxide (1 nmol/L to 1 micromol/L), sodium nitroprusside (10 nmol/L to 10 micromol/L; a nitric oxide donor), and papaverine (10 nmol/L to 10 micromol/L; a non-nitric oxide, non-sGC-dependent stimulus) were generated in the presence and absence of 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 and 10 micromol/L; an inhibitor of sGC). The effect of ODQ on basal tone of basilar arteries from normal and atherosclerotic monkeys was also examined.

RESULTS

Nitric oxide, sodium nitroprusside, and papaverine produced relaxation that was similar (P:>0.05) in normal and atherosclerotic monkeys. ODQ produced marked inhibition (P:<0.05) of vasorelaxation in response to nitric oxide and nitroprusside but not papaverine. For example, relaxation of the basilar artery in response to nitric oxide (0.1 micromol/L) was inhibited by approximately 85% and 73% by ODQ (1 micromol/L) in normal and atherosclerotic monkeys, respectively. ODQ produced contraction of the basilar arteries, and the increase in tension to ODQ was greater in normal (2.7+/-0.3 g; mean+/-SE) than in atherosclerotic monkeys (1.4+/-0.4 g; P:<0.05). In contrast, contraction to prostaglandin F(2alpha) was similar in the basilar artery from normal and atherosclerotic monkeys.

CONCLUSIONS

These findings suggest that (1) relaxation of cerebral arteries in primates in response to nitric oxide is normally dependent, in large part, on activation of sGC and (2) the influence of sGC (via reduced production and/or activity of basal nitric oxide) on cerebral vascular tone is reduced in atherosclerosis.

Reciprocal regulation of cGMP-mediated vasorelaxation by soluble and particulate guanylate cyclases

Nitric oxide (NO) and atrial natriuretic peptides (ANP) activate soluble (sGC) and particulate guanylate cyclase (pGC), respectively, and play important roles in the maintenance of cardiovascular homeostasis. However, little is known about potential interactions between these two cGMP-generating pathways. Here we demonstrate that sGC and pGC cooperatively regulate cGMP-mediated relaxation in human and murine vascular tissue. In human vessels, the potency of spermine-NONOate (SPER-NO) and ANP was increased after inhibition of endogenous NO synthesis and decreased by prior exposure to glyceryl trinitrate (GTN). Aortas from endothelial NO synthase (eNOS) knockout (KO) mice were more sensitive to ANP than tissues from wild-type (WT) animals. However, in aortas from WT mice, the potency of ANP was increased after pretreatment with NOS or sGC inhibitor. Vessels from eNOS KO animals were less sensitive to ANP after GTN pretreatment, an effect that was reversed in the presence of an sGC inhibitor. cGMP production in response to SPER-NO and ANP was significantly greater in vessels from eNOS KO animals compared with WT animals. This cooperative interaction between NO and ANP may have important implications for human pathophysiologies involving deficiency in either mediator and the clinical use of nitrovasodilators.

Potassium channel function in vascular disease

Potassium ion (K(+)) channel activity is a major regulator of vascular muscle cell membrane potential (E(m)) and is therefore an important determinant of vascular tone. There is growing evidence that the function of several types of vascular K(+) channels is altered during major cardiovascular diseases, such as chronic hypertension, diabetes, and atherosclerosis. Vasoconstriction and the compromised ability of an artery to dilate are likely consequences of defective K(+) channel function in blood vessels during these disease states. In some instances, increased K(+) channel function may help to compensate for increased vascular tone. Endothelial cell dysfunction is commonly associated with cardiovascular disease, and altered activity of nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor could also contribute to changes in resting K(+) channel activity, E(m), and K(+) channel-mediated vasodilatation. Our current knowledge of the effects of disease on vascular K(+) channel function almost exclusively relies on interpretation of data obtained by using pharmacological modulators of K(+) channels. As further progress is made in the development of more selective drugs and through molecular approaches such as gene targeting technology in mice, specific K(+) channel abnormalities and their causes in particular diseases should be more readily identified, providing novel directions for vascular therapy.

Reduction of obesity, as induced by leptin, reverses endothelial dysfunction in obese (Lep(ob)) mice

Obesity is a major health care problem and is associated with significant cardiovascular morbidity. Leptin, a neuroendocrine hormone released by adipose tissue, is important in modulating obesity by signaling satiety and increasing metabolism. Moreover, leptin receptors are expressed on vascular endothelial cells (ECs) and mediate angiogenesis. We hypothesized that leptin may also play an important role in vasoregulation. We investigated vasoregulatory mechanisms in the leptin-deficient obese (ob/ob) mouse model and determined the influence of leptin replacement on endothelial-dependent vasorelaxant responses. The direct effect of leptin on EC nitric oxide (NO) production was also tested by using 4, 5-diaminofluorescein-2 diacetate staining and measurement of nitrate and nitrite concentrations. Vasoconstrictor responses to phenylephrine, norepinephrine, and U-46619 were markedly enhanced in aortic rings from ob/ob mice and were modulated by NO synthase inhibition. Vasorelaxant responses to ACh were markedly attenuated in mesenteric microvessels from ob/ob mice. Leptin replacement resulted in significant weight loss and reversal of the impaired endothelial-dependent vasorelaxant responses observed in ob/ob mice. Preincubation of ECs with leptin enhanced the release of NO production. Thus leptin-deficient ob/ob mice demonstrate marked abnormalities in vasoregulation, including impaired endothelial-dependent vasodilation, which is reversed by leptin replacement. These findings may be partially explained by the direct effect of leptin on endothelial NO production. These vascular abnormalities are similar to those observed in obese, diabetic, leptin-resistant humans. The ob/ob mouse may, therefore, be an excellent new model for the study of the cardiovascular effects of obesity.

Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in mice

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several endothelium-derived relaxing factors, such as prostacyclin, nitric oxide (NO), and the previously unidentified endothelium-derived hyperpolarizing factor (EDHF). In this study, we examined our hypothesis that hydrogen peroxide (H(2)O(2)) derived from endothelial NO synthase (eNOS) is an EDHF. EDHF-mediated relaxation and hyperpolarization in response to acetylcholine (ACh) were markedly attenuated in small mesenteric arteries from eNOS knockout (eNOS-KO) mice. In the eNOS-KO mice, vasodilating and hyperpolarizing responses of vascular smooth muscle per se were fairly well preserved, as was the increase in intracellular calcium in endothelial cells in response to ACh. Antihypertensive treatment with hydralazine failed to improve the EDHF-mediated relaxation. Catalase, which dismutates H(2)O(2) to form water and oxygen, inhibited EDHF-mediated relaxation and hyperpolarization, but it did not affect endothelium-independent relaxation following treatment with the K(+) channel opener levcromakalim. Exogenous H(2)O(2) elicited similar relaxation and hyperpolarization in endothelium-stripped arteries. Finally, laser confocal microscopic examination with peroxide-sensitive fluorescence dye demonstrated that the endothelium produced H(2)O(2) upon stimulation by ACh and that the H(2)O(2) production was markedly reduced in eNOS-KO mice. These results indicate that H(2)O(2) is an EDHF in mouse small mesenteric arteries and that eNOS is a major source of the reactive oxygen species.

Tumor necrosis factor-alpha impairs contraction but not relaxation in carotid arteries from iNOS-deficient mice

We used mice deficient in expression of inducible NO synthase (iNOS -/-) to directly examine the role of iNOS in impaired vasoconstrictor responses following tumor necrosis factor-alpha (TNF-alpha). In iNOS +/+ mice, contraction of carotid arteries in response to prostaglandin F(2alpha) (PGF(2alpha)) was impaired following TNF-alpha (100 microg/kg ip)(n = 10, P < 0.01). In contrast to responses in wild-type mice, contraction to low concentrations of PGF(2alpha) were normal, but maximum contraction to PGF(2alpha) was impaired in arteries from iNOS -/- mice treated with TNF-alpha [0.35 +/-.0.02 g (n = 8) following vehicle and 0.25 +/- 0.02 g (n = 7) following TNF-alpha (P < 0.05)]. Aminoguanidine, a relatively selective inhibitor of iNOS, partially restored contraction to PGF(2alpha) in vessels from iNOS +/+ mice but had no effect in iNOS -/- mice injected with TNF-alpha, suggesting that a mechanism(s) other than iNOS contributes to impaired responses. In contrast to contractile responses, relaxation of the carotid artery in response to acetylcholine and nitroprusside was not altered following TNF-alpha in iNOS +/+ or iNOS -/-mice. Responses of carotid arteries from iNOS -/- mice and effects of aminoguanidine suggest that both iNOS-dependent and iNOS-independent mechanisms contribute to impaired contractile responses following TNF-alpha.

Vasodilator mechanisms in the coronary circulation of endothelial nitric oxide synthase-deficient mice

Previous studies have demonstrated that responses to endothelium-dependent vasodilators are absent in the aortas from mice deficient in expression of endothelial nitric oxide synthase (eNOS -/- mice), whereas responses in the cerebral microcirculation are preserved. We tested the hypothesis that in the absence of eNOS, other vasodilator pathways compensate to preserve endothelium-dependent relaxation in the coronary circulation. Diameters of isolated, pressurized coronary arteries from eNOS -/-, eNOS heterozygous (+/-), and wild-type mice (eNOS +/+ and C57BL/6J) were measured by video microscopy. ACh (an endothelium-dependent agonist) produced vasodilation in wild-type mice. This response was normal in eNOS +/- mice and was largely preserved in eNOS -/- mice. Responses to nitroprusside were also similar in arteries from eNOS +/+, eNOS +/-, and eNOS -/- mice. Dilation to ACh was inhibited by N(G)-nitro-L-arginine, an inhibitor of NOS in control and eNOS -/- mice. In contrast, trifluoromethylphenylimidazole, an inhibitor of neuronal NOS (nNOS), decreased ACh-induced dilation in arteries from eNOS-deficient mice but had no effect on responses in wild-type mice. Indomethacin, an inhibitor of cyclooxygenase, decreased vasodilation to ACh in eNOS-deficient, but not wild-type, mice. Thus, in the absence of eNOS, dilation of coronary arteries to ACh is preserved by other vasodilator mechanisms.

IL-10 deficiency increases superoxide and endothelial dysfunction during inflammation

Little is known about the role of interleukin-10 (IL-10), an anti-inflammatory cytokine, in blood vessels. We used IL-10-deficient mice (IL-10 -/-) to examine the hypothesis that IL-10 protects endothelial function after lipopolysaccharide (LPS) treatment. The responses of carotid arteries were studied in vitro 6 h after injection of a relatively low dose of LPS (10 microgram ip). In IL-10 -/- mice, the maximum relaxation to ACh (3 microM) was 56 +/- 6% (means +/- SE) after LPS injection and 84 +/- 4% after vehicle injection (P < 0.05). Thus endothelium-dependent relaxation was impaired in carotid arteries from IL-10 -/- mice after LPS injection. In contrast, this dose of LPS did not alter relaxation to ACh in vessels from wild-type (IL-10 +/+) mice. Relaxation to nitroprusside and papaverine was similar in arteries from both IL-10 -/- and IL-10 +/+ mice after vehicle or LPS injection. Because inflammation is associated with increased levels of reactive oxygen species, we also tested the hypothesis that superoxide contributes to the impairment of endothelial function by LPS in the absence of IL-10. Results using confocal microscopy and hydroethidine indicated that levels of superoxide are elevated in carotid arteries from IL-10 -/- mice compared with IL-10 +/+ mice after LPS injection. The impaired relaxation of arteries from IL-10 -/- mice after LPS injection was restored to normal by polyethylene glycol-suspended superoxide dismutase (50 U/ml) or allopurinol (1 mM), an inhibitor of xanthine oxidase. These data provide direct evidence that IL-10 protects endothelial function after an acute inflammatory stimulus by limiting local increases in superoxide. The source of superoxide in this model may be xanthine oxidase.

Folate dependence of hyperhomocysteinemia and vascular dysfunction in cystathionine beta-synthase-deficient mice

Hyperhomocysteinemia is a risk factor for stroke, myocardial infarction, and venous thrombosis. Moderate hyperhomocysteinemia is associated with impaired endothelial function, but the mechanisms responsible for endothelial dysfunction in hyperhomocysteinemia are poorly understood. We have used genetic and dietary approaches to produce hyperhomocysteinemia in mice. Heterozygous cystathionine beta-synthase-deficient mice (CBS +/-), which have a selective defect in homocysteine transsulfuration, and wild-type (CBS +/+) littermates were fed either a control diet or a diet that is relatively deficient in folic acid for 6 wk. Plasma total homocysteine was 5.3 +/- 0.7 microM in CBS +/+ mice and 6.4 +/- 0.6 microM in CBS +/- mice (P = 0.3) given the control diet. Plasma total homocysteine was 11.6 +/- 4.5 microM in CBS +/+ mice and 25.1 +/- 3.2 microM in CBS +/- mice (P = 0.004) given a low-folate diet. In mice fed the control diet, relaxation of aortic rings in response to the endothelium-dependent vasodilator acetylcholine did not differ significantly between CBS +/+ mice and CBS +/- mice. In contrast, in mice fed a low-folate diet, maximal relaxation to acetylcholine was markedly impaired in CBS +/- mice (58 +/- 9%) compared with CBS +/+ mice (84 +/- 4%) (P = 0.01). No differences in relaxation to the endothelium-independent vasodilator sodium nitroprusside were observed among the four groups of mice. These data indicate that CBS-deficient mice are predisposed to hyperhomocysteinemia during dietary folate deficiency, and moderate hyperhomocysteinemia is associated with marked impairment of endothelial function in mice.

Mechanisms of reduced nitric oxide/cGMP-mediated vasorelaxation in transgenic mice overexpressing endothelial nitric oxide synthase

NO, constitutively produced by endothelial NO synthase (eNOS), plays a key regulatory role in vascular wall homeostasis. We generated transgenic (Tg) mice overexpressing eNOS in the endothelium and reported the presence of reduced NO-elicited relaxation. The purpose of this study was to clarify mechanisms of the reduced response to NO-mediated vasodilators in eNOS-Tg mice. Thoracic aortas of Tg and control mice were surgically isolated for vasomotor studies. Relaxations to acetylcholine and sodium nitroprusside were significantly reduced in Tg vessels compared with control vessels. Relaxations to atrial natriuretic peptide and 8-bromo-cGMP were also significantly reduced in Tg vessels. Reduced relaxations to these agents were restored by chronic N(G)-nitro-L-arginine methyl ester treatment. Basal cGMP levels of aortas were higher in Tg mice than in control mice, whereas soluble guanylate cyclase (sGC) activity in Tg vessels was approximately 50% of the activity in control vessels. Moreover, cGMP-dependent protein kinase (PKG) protein levels and PKG enzyme activity were decreased in Tg vessels. These observations indicate that chronic overexpression of eNOS in the endothelium resulted in resistance to the NO/cGMP-mediated vasodilators and that at least 2 distinct mechanisms might be involved: one is reduced sGC activity, and the other is a decrease in PKG protein levels. We reported for the first time that increased NO release from the endothelium reduces sGC and PKG activity in mice. These data may provide a new insight into the mechanisms of nitrate tolerance and cross tolerance to nitrovasodilators.

Temporal events underlying arterial remodeling after chronic flow reduction in mice: correlation of structural changes with a deficit in basal nitric oxide synthesis

To define the cellular events of vascular remodeling in mice, we measured blood flow and analyzed the morphology of remodeled vessels at defined points after a flow-reducing remodeling stimulus for 3, 7, 14, and 35 days. Acute ligation of the left external carotid artery reduced blood flow in the left common carotid artery (LC) compared with sham and contralateral right common carotid arteries (RCs). In morphometric analyses, the decrease in diameter in LCs was reversible by vasodilator perfusion 3 days after ligation, whereas ligation for 7 days or greater resulted in a permanent diameter reduction. Coincident with structural remodeling at day 7 was an increase in cell death in remodeled LCs. Functionally, rings from remodeled LCs contracted to prostaglandin F(2alpha) and relaxed to acetylcholine in a manner identical to that of control arteries. However, remodeled LCs were hypersensitive to the nitrovasodilator sodium nitroprusside (at day 7) and exhibited a marked reduction in basal NO synthesis at 7 and 14 days after ligation. The impairment of endothelial NO synthase function was likely due to post-translational mechanisms, given that endothelial NO synthase mRNA and protein levels did not change in remodeled LCs. These data define the ontogeny of flow-triggered luminal remodeling in adult mice and suggest that endothelial dysfunction occurs during reorganization of the vessel wall.

In eNOS knockout mice skeletal muscle arteriolar dilation to acetylcholine is mediated by EDHF

The mechanisms that account for acetylcholine (ACh)-induced responses of skeletal muscle arterioles of mice lacking endothelial nitric oxide (NO) synthase (eNOS-KO) were investigated. Isolated, cannulated, and pressurized arterioles of gracilis muscle from male eNOS-KO (74.1 +/- 2.3 microm) and wild-type (WT, 87.2 +/- 2.1 microm) mice developed spontaneous tone accounting for 63 and 61% of their passive diameter (116.8 +/- 3.4 vs. 143.2 +/- 2.8 microm, respectively) and dilated dose-dependently to ACh (10(-9)-10(-7) M). These dilations were significantly smaller in vessels of eNOS-KO compared with WT mice (29.2 +/- 2.0 microm vs. 46.3 +/- 2.1 microm, at maximum concentration) but responses to the NO donor, sodium nitrite (NaNO(2), 10(-6)-3 x 10(-5) M), were comparable in the vessels of the two strains. N(G)-nitro-L-arginine (L-NNA, 10(-4) M), an inhibitor of eNOS, inhibited ACh-induced dilations by 60-90% in arterioles of WT mice but did not affect responses in those of eNOS-KO mice. In arterioles of eNOS-KO mice, dilations to ACh were not affected by indomethacin but were essentially abolished by inhibitors of cytochrome P-450, clotrimazole (CTZ, 2 x 10(-6) M) or miconazole (MCZ, 2 x 10(-6) M), as well as by either high K(+) (40 mM) or iberiotoxin [10(-7) M, a blocker of Ca(2+)-dependent K(+) channels (K(Ca) channels)]. On the other hand, in WT arterioles CTZ or MCZ inhibited ACh-induced dilations only by approximately 10% and only in the presence of L-NNA. These results indicate that in arterioles of eNOS-KO mice, endothelium-derived hyperpolarizing factor (EDHF), synthesized via cytochrome P-450, accounts entirely for the mediation of ACh-induced dilation via an increase in K(Ca)-channel activity. In contrast, in arterioles of WT mice, endothelium-derived NO predominantly mediates ACh-induced dilation in which participation of EDHF becomes apparent only after inhibition of NO synthesis.

nNOS and eNOS modulate cGMP formation and vascular response in contracting fast-twitch skeletal muscle

Nitric oxide (NO) from Ca(2+)-dependent neuronal nitric oxide synthase (nNOS) in skeletal muscle fibers may modulate vascular tone by a cGMP-dependent pathway similar to NO derived from NOS in endothelial cells (eNOS). In isolated fast-twitch extensor digitorum longus (EDL) muscles from control mice, cGMP formation increased approximately 166% with electrical stimulation (30 Hz, 15 s). cGMP levels were not altered in slow-twitch soleus muscles. The NOS inhibitor N(omega)-nitro-l-arginine abolished the contraction-induced increase in cGMP content in EDL muscles, and the NO donor sodium nitroprusside (SNP) increased cGMP content approximately 167% in noncontracting EDL muscles. SNP treatment but not electrical stimulation increased cGMP formation in muscles from nNOS(-/-) mice. cGMP formation in control and stimulated EDL muscles from eNOS(-/-) mice was less than that obtained with similarly treated muscles from control mice. Arteriolar relaxation in contracting fast-twitch mouse cremaster muscle was attenuated in muscles from mice lacking either nNOS or eNOS. These findings suggest that increases in cGMP and NO-dependent vascular relaxation in contracting fast-twitch skeletal muscle may require both nNOS and eNOS.

Increased nitrovasodilator sensitivity in endothelial nitric oxide synthase knockout mice: role of soluble guanylyl cyclase

Endogenously produced nitric oxide (NO) modulates nitrovasodilator-induced relaxation. We investigated the underlying mechanism in wild-type (WT) mice and endothelial NO synthase knockout (eNOS(-/-)) mice to determine whether a chronic lack of endothelial NO alters the soluble guanylyl cyclase (sGC) pathway. In aortic segments from eNOS(-/-) mice, the vasodilator sensitivity to sodium nitroprusside (SNP) was significantly greater than that in WT mice. There was no difference in sensitivity to the G-kinase I activator 8-para-chlorophenylthio-cGMP or to cromakalim. N(omega)-Nitro-L-arginine had no effect on the SNP-induced relaxation in eNOS(-/-) but increased the sensitivity in WT mice so it was no longer different than that of eNOS(-/-). Basal cGMP levels in aortic rings were significantly lower in eNOS(-/-) mice than in WT mice. SNP (300 nmol/L) induced a significantly greater cGMP accumulation in eNOS(-/-) mice than in WT mice. The maximal SNP-induced (10 micromol/L) increase in cGMP was similar in both strains. SNP-stimulated sGC activity was significantly greater in eNOS(-/-) mice than in WT mice. Incubation of aortic segments from WT mice with N(omega)-nitro-L-arginine increased sGC activity, an effect prevented by coincubation with SNP (10 micromol/L). The aortic expressions of the sGC alpha1 and beta1 subunits in WT and eNOS(-/-) mice were identical as determined with Western blot analysis. These data suggest that chronic exposure to endothelium-derived NO, as well as acute exposure to nitrovasodilator-derived NO, desensitizes sGC to activation by NO but does not alter sGC expression. Both the acute cessation of endothelial NO formation in WT mice and the chronic deficiency of NO in eNOS(-/-) mice restore the NO sensitivity of sGC and enhance vascular smooth muscle relaxation in response to nitrovasodilator agents.

Vascular effects of lipopolysaccharide are enhanced in interleukin-10-deficient mice

BACKGROUND AND PURPOSE

The role in blood vessels of interleukin-10 (IL-10), a potent anti-inflammatory cytokine, is not known. Using mice with targeted deletion of the gene for IL-10 (IL-10(-/-)), we examined the hypothesis that IL-10 is a major modulator of the vascular effects of lipopolysaccharide (LPS). Methods-We examined in vitro responses of carotid arteries obtained from wild-type (129/SvEv or C57BL/6; IL-10(+/+)) and IL-10-deficient mice 6 hours after injection of a relatively low dose of LPS (10 microgram).

RESULTS

Contraction of the carotid artery in response to U46619 was impaired in IL-10-deficient mice treated with LPS compared with LPS-treated controls. After LPS, U46619 (0.03 and 0.1 microgram/mL) contracted the carotid artery by 0.11+/-0.02 (mean+/-SEM) and 0.38+/-0.03 g in wild-type (n=10) and 0.03+/-0.01 and 0.19+/-0.03 g in IL-10-deficient (n=8) mice (P<0.05 versus control). Aminoguanidine, an inhibitor of inducible nitric oxide synthase (iNOS), had no significant effect on contraction of the carotid artery from LPS-treated control mice but restored contraction of the carotid artery in response to U46619 in IL-10-deficient mice to levels seen in wild-type mice. Similar findings were obtained when phenylephrine was used as a vasoconstricting agent. These findings indicate that LPS produces much greater impairment of contractile responses of the carotid artery in IL-10-deficient mice than in control mice. Impaired contractile function was eliminated by aminoguanidine, suggesting that expression of iNOS is enhanced in arteries from IL-10-deficient mice. In carotid arteries from animals injected with LPS, reverse transcription-polymerase chain reaction (RT-PCR) products for iNOS were found more frequently in IL-10-deficient mice than in wild-type mice. RT-PCR products for iNOS were not present in arteries from vehicle-treated animals (IL-10-deficient or wild-type mice).

CONCLUSIONS

This is the first evidence that endogenous IL-10 is a major determinant of the effects of LPS on vascular tone. The results suggest that impaired constrictor responses of the carotid artery after LPS in IL-10-deficient mice are mediated by enhanced expression of iNOS.

Acetylcholine-induced relaxation of peripheral arteries isolated from mice lacking endothelial nitric oxide synthase

1. Acetycholine-mediated relaxations in phenylephrine-contracted aortas, femoral and mesenteric resistance arteries were studied in vessels from endothelial nitric oxide synthase knock-out (eNOS -/-) and the corresponding wild-type strain (eNOS +/+) C57BL6/SV19 mice. 2. Aortas from eNOS (+/+) mice relaxed to acetylcholine in an endothelium-dependent NG-nitro-L-arginine (L-NOARG) sensitive manner. Aortas from eNOS (-/-) mice did not relax to acetylcholine but demonstrated enhanced sensitivity to both authentic NO and sodium nitroprusside. 3. Relaxation to acetylcholine in femoral arteries was partially inhibited by L-NOARG in vessels from eNOS (+/+) mice, but relaxation in eNOS (-/-) mice was insensitive to a combination of L-NOARG and indomethacin and the guanylyl cyclase inhibitor 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The L-NOARG/ODQ/indomethacin-insensitive relaxation to acetylcholine in femoral arteries was inhibited in the presence of elevated (30 mM) extracellular KCl. 4. In mesenteric resistance vessels from eNOS (+/+) mice, the acetylcholine-mediated relaxation response was completely inhibited by a combination of indomethacin and L-NOARG or by 30 mM KCl alone. In contrast, in mesenteric arteries from eNOS (-/-) mice, the acetylcholine-relaxation response was insensitive to a combination of L-NOARG and indomethacin, but was inhibited in the presence of 30 mM KCl. 5. These data indicate arteries from eNOS (-/-) mice demonstrate a supersensitivity to exogenous NO, and that acetylcholine-induced vasorelaxation of femoral and mesenteric vessels from eNOS (-/-) mice is mediated by an endothelium-derived factor that has properties of an EDHF but is neither NO nor prostacyclin. Furthermore, in mesenteric vessels, there is an upregulation of the role of EDHF in the absence of NO.

Enhanced release of prostaglandins contributes to flow-induced arteriolar dilation in eNOS knockout mice

Nitric oxide and prostaglandins were shown to contribute to the endothelial mediation of flow-induced dilation of skeletal muscle arterioles of rats. Thus, we hypothesized that flow-induced dilation and its mediation are altered in gracilis muscle arterioles of mice deficient in the gene for endothelial nitric oxide synthase (eNOS-KO) compared with control wild-type (WT) mice. Gracilis muscle arterioles ( approximately 80 micrometer) of male mice were isolated, then cannulated and pressurized in a vessel chamber. The increases in diameter elicited by increases in perfusate flow from 0 to 10 microq/min were similar in arterioles from eNOS-KO (n=28) and WT (n=22) mice ( approximately 20 micrometer at 10 microL/min flow). Removal of the endothelium eliminated flow-induced dilations in vessels of both strains of mice. N(omega)-nitro-L-arginine (L-NNA, 10(-4) mol/L) significantly inhibited flow-induced dilation in arterioles of WT mice by approximately 51% but had no effect on responses of arterioles from eNOS-KO mice. Indomethacin (INDO, 10(-5) mol/L) inhibited flow-induced dilation of WT mice by approximately 49%, whereas it completely abolished this response in arterioles of eNOS-KO mice. Simultaneous administration of INDO and L-NNA eliminated flow-induced responses in arterioles of WT mice. Dilations to carbaprostacyclin were similar at concentrations of 10(-8) and 3x10(-8) mol/L but decreased significantly at 10(-7) mol/L in arterioles of eNOS-KO compared with those of WT mice. These findings demonstrate that, despite the lack of nitric oxide mediation, flow-induced dilation is close to normal in arterioles of eNOS-KO mice because of an enhanced release of endothelial dilator prostaglandins and suggest that this vascular adaptation may contribute to the regulation of peripheral resistance in eNOS-KO mice.

Endogenous endothelial nitric oxide synthase-derived nitric oxide is a physiological regulator of myocardial oxygen consumption

Our objective was to determine the precise role of endothelial nitric oxide synthase (eNOS) as a modulator of cardiac O2 consumption and to further examine the role of nitric oxide (NO) in the control of mitochondrial respiration. Left ventricle O2 consumption in mice with defects in the expression of eNOS [eNOS (-/-)] and inducible NOS [iNOS (-/-)] was measured with a Clark-type O2 electrode. The rate of decreases in O2 concentration was expressed as a percentage of the baseline. Baseline O2 consumption was not significantly different between groups of mice. Bradykinin (10(-4) mol/L) induced significant decreases in O2 consumption in tissues taken from iNOS (-/-) (-28+/-4%), wild-type eNOS (+/+) (-22+/-4%), and heterozygous eNOS(+/-) (-22+/-5%) but not homozygous eNOS (-/-) (-3+/-4%) mice. Responses to bradykinin in iNOS (-/-) and both wild-type and heterozygous eNOS mice were attenuated after NOS blockade with N-nitro-L-arginine methyl ester (L-NAME) (-2+/-5%, -3+/-2%, and -6+/-5%, respectively, P<0.05). In contrast, S-nitroso-N-acetyl-penicillamine (SNAP, 10(-4) mol/L), which releases NO spontaneously, induced decreases in myocardial O2 consumption in all groups of mice, and such responses were not affected by L-NAME. In addition, pretreatment with bacterial endotoxin elicited a reduction in basal O2 consumption in tissues taken from normal but not iNOS (-/-)-deficient mice. Our results indicate that the pivotal role of eNOS in the control of myocardial O2 consumption and modulation of mitochondrial respiration by NO may have an important role in pathological conditions such as endotoxemia in which the production of NO is altered.

Inhibitory effect of 4-aminopyridine on responses of the basilar artery to nitric oxide

1. Voltage-dependent K+ channels are present in cerebral arteries and may modulate vascular tone. We used 200 microM 4-aminopyridine (4-AP), thought to be a relatively selective inhibitor of voltage-dependent K+ channels at this concentration, to test whether activation of these channels may influence baseline diameter of the basilar artery and dilator responses to nitric oxide (NO) and cyclic GMP in vivo. 2. Using a cranial window in anaesthetized rats, topical application of 4-AP to the basilar artery (baseline diameter = 240+/-5 microm, mean +/- s.e.mean) produced 10+/-1% constriction. Sodium nitroprusside (a NO donor), acetylcholine (which stimulates endothelial release of NO), 8-bromo cyclic GMP (a cyclic GMP analogue), cromakalim (an activator of ATP-sensitive K+ channels) and papaverine (a non-NO, non-K+ channel-related vasodilator) produced concentration-dependent vasodilator responses that were reproducible. 3. Responses to 10 and 100 nM nitroprusside were inhibited by 4-AP (20+/-4 vs 8+/-2% and 51+/-5 vs 33+/-5%, respectively, n=10; P<0.05). Responses to acetylcholine and 8-bromo cyclic GMP were also partially inhibited by 4-AP. In contrast, 4-AP had no effect on vasodilator responses to cromakalim or papaverine. These findings suggest that NO/cyclic GMP-induced dilator responses of the basilar artery are selectively inhibited by 4-aminopyridine. 4. Responses to nitroprusside were also markedly inhibited by 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (an inhibitor of soluble guanylate cyclase; 16+/-4 vs 1+/-1% and 44+/-7 vs 7+/-1%; n=10; P<0.05). 5. Thus, dilator responses of the rat basilar artery to NO appear to be mediated by activation of soluble guanylate cyclase and partially by activation of a 4-aminopyridine-sensitive mechanism. The most likely mechanism would appear to be activation of voltage-dependent K+ channels by NO/cyclic GMP.

The pulmonary circulation of homozygous or heterozygous eNOS-null mice is hyperresponsive to mild hypoxia

Acute hypoxic vasoconstriction and development of hypoxic pulmonary hypertension (PHTN) are unique properties of the pulmonary circulation. The pulmonary endothelium produces vasoactive factors, including nitric oxide (NO), that modify these phenomena. We tested the hypothesis that NO produced by endothelial nitric oxide synthase (eNOS) modulates pulmonary vascular responses to hypoxia using mice with targeted disruption of the eNOS gene (eNOS-/-). Marked PHTN was found in eNOS-/- mice raised in mild hypoxia when compared with either controls or eNOS-/- mice raised in conditions simulating sea level. We found an approximate twofold increase in partially and fully muscularized distal pulmonary arteries in eNOS-/- mice compared with controls. Consistent with vasoconstriction being the primary mechanism of PHTN, however, acute inhalation of 25 ppm NO resulted in normalization of RV pressure in eNOS-/- mice. In addition to studies of eNOS-/- mice, the dose-effect of eNOS was tested using heterozygous eNOS+/- mice. Although the lungs of eNOS+/- mice had 50% of normal eNOS protein, the response to hypoxia was indistinguishable from that of eNOS-/- mice. We conclude that eNOS-derived NO is an important modulator of the pulmonary vascular response to chronic hypoxia and that more than 50% of eNOS expression is required to maintain normal pulmonary vascular tone.

Acetylcholine-induced relaxation in blood vessels from endothelial nitric oxide synthase knockout mice

1. Isometric tension was recorded in isolated rings of aorta, carotid, coronary and mesenteric arteries taken from endothelial nitric oxide synthase knockout mice (eNOS(-/-) mice) and the corresponding wild-type strain (eNOS(+/+) mice). The membrane potential of smooth muscle cells was measured in coronary arteries with intracellular microelectrodes. 2. In the isolated aorta, carotid and coronary arteries from the eNOS(+/+) mice, acetylcholine induced an endothelium-dependent relaxation which was inhibited by N(omega)-L-nitro-arginine. In contrast, in the mesenteric arteries, the inhibition of the cholinergic relaxation required the combination of N(omega)-L-nitro-arginine and indomethacin. 3. The isolated aorta, carotid and coronary arteries from the eNOS(-/-) mice did not relax in response to acetylcholine. However, acetylcholine produced an indomethacin-sensitive relaxation in the mesenteric artery from eNOS(-/-) mice. 4. The resting membrane potential of smooth muscle cells from isolated coronary arteries was significantly less negative in the eNOS(-/-) mice (-64.8 +/- 1.8 mV, n = 20 and -58.4 +/- 1.9 mV, n = 17, for eNOS(+/+) and eNOS(-/-) mice, respectively). In both strains, acetylcholine, bradykinin and substance P did not induce endothelium-dependent hyperpolarizations whereas cromakalim consistently produced hyperpolarizations (- 7.9 +/- 1.1 mV, n = 8 and -13.8 +/- 2.6 mV, n = 4, for eNOS(+/+) and eNOS(-/-) mice, respectively). 5. These findings demonstrate that in the blood vessels studied: (1) in the eNOS(+/+) mice, the endothelium-dependent relaxations to acetylcholine involve either NO or the combination of NO plus a product of cyclo-oxygenase but not EDHF; (2) in the eNOS(-/-) mice, NO-dependent responses and EDHF-like responses were not observed. In the mesenteric arteries acetylcholine releases a cyclo-oxygenase derivative.

Hypotension and reduced nitric oxide-elicited vasorelaxation in transgenic mice overexpressing endothelial nitric oxide synthase

Nitric oxide (NO), constitutively produced by endothelial nitric oxide synthase (eNOS), plays a major role in the regulation of blood pressure and vascular tone. We generated transgenic mice overexpressing bovine eNOS in the vascular wall using murine preproendothelin-1 promoter. In transgenic lineages with three to eight transgene copies, bovine eNOS-specific mRNA, protein expression in the particulate fractions, and calcium-dependent NOS activity were confirmed by RNase protection assay, immunoblotting, and L-arginine/citrulline conversion. Immunohistochemical studies revealed that eNOS protein was predominantly localized in the endothelial cells of aorta, heart, and lung. Blood pressure was significantly lower in eNOS-overexpressing mice than in control littermates. In the transgenic aorta, basal NO release (estimated by Nomega-nitro-L-arginine-induced facilitation of the contraction by prostaglandin F2alpha) and basal cGMP levels (measured by enzyme immunoassay) were significantly increased. In contrast, relaxations of transgenic aorta in response to acetylcholine and sodium nitroprusside were significantly attenuated, and the reduced vascular reactivity was associated with reduced response of cGMP elevation to these agents as compared with control aortas. Thus, our novel mouse model of chronic eNOS overexpression demonstrates that, in addition to the essential role of eNOS in blood pressure regulation, tonic NO release by eNOS in the endothelium induces the reduced vascular reactivity to NO-mediated vasodilators, providing several insights into the pathogenesis of nitrate tolerance.