Diffusion of nitric oxide and scavenging by blood in the vasculature

It has been deduced (Lancaster, Proc. Natl. Acad. Sci. USA 91 (1994) 8137–8141), from a consideration of Fick’s law of diffusion, that the very effective scavenging of nitric oxide (NO) by haemoglobin in red blood cells prevents any NO from endothelial cells migrating outwards into vascular smooth muscle. This conclusion has led some authors to suggest that endothelium-derived relaxing factor (EDRF) is not free NO. We have reconsidered the application of Fick’s law to the migration of NO in the vasculature, making allowance for the reaction of NO with guanylate cyclase and for the layer of red blood-free plasma next to the endothelium. The source of NO is taken as an infinite cylinder. Calculations for vessels of various diameters indicate that a substantial amount of NO migrates outwards in spite of very effective scavenging by haemoglobin and that the relative amount of NO migrating outwards depends upon the radius of the vessel. The view that locally produced NO is not responsible for vascular dilation has not been sustained.     

Midostaurin upregulates eNOS gene expression and preserves eNOS function in the microcirculation of the mouse

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) is a powerful vasodilator and possesses vasoprotective effects. Therefore, augmentation of eNOS expression and -activity by pharmacological means could provide protection against cardiovascular disease. However, this concept has been questioned recently, because in several disease models, eNOS upregulation was associated with a dysfunctional enzyme (referred to as eNOS uncoupling). In contrast, the present study demonstrates that an eNOS gene expression-enhancing compound with additional protein kinase C (PKC) inhibitory properties can upregulate eNOS while preserving its enzymatic function. Apolipoprotein E-knockout mice were treated for 7 days with midostaurin (4'-N-benzoyl staurosporine, compound CGP 41251, 50-125 mg/kg/day), a PKC inhibitor previously shown to increase eNOS expression and NO production in cultured human endothelial cells. Midostaurin treatment enhanced eNOS mRNA expression (RNase protection assay) in mouse aorta, kidney, and heart in a dose-dependent fashion. In the dorsal skinfold microcirculation, midostaurin produced an arteriolar vasorelaxation (intravital microscopy), which could be prevented by the NOS inhibitor L-NAME, indicating that the upregulated eNOS remained functional. In organ chamber experiments, the aorta from midostaurin-treated mice showed an enhanced NO-mediated relaxation in response to acetylcholine. Accordingly, serum levels of nitrite/nitrate (NO-Analyzer) were increased, and the production of reactive oxygen species in the aorta (L-012 chemiluminescence) was reduced by midostaurin. Thus, in mice in vivo, midostaurin treatment results in enhanced expression of eNOS with preserved enzyme function and enhanced production of bioactive NO. Given the beneficial effects of endothelial-derived NO, vasoprotective and anti-atherosclerotic effects are likely to ensue.     

Endothelium-Derived Relaxing Factor: Discovery, Early Studies, and Identification as Nitric Oxide

The discovery, early studies and identification of endothelium-derived relaxing factor (EDRF) as nitric oxide, are described.     

The Reaction of no With Superoxide

The rate constant for the reaction of NO with ·O₂^? was determined to be (6.7 ± 0.9) × 10⁹ 1 mol^?1 s^?1, considerably higher than previously reported. Rate measurements were made from pH 5.6 to 12.5 both by monitoring the loss of ·O₂^? and the formation of the product ^?OONO. The decay rate of ^?OONO, in the presence of 0.1 moll^?1 formate, ranges from 1.2s^?1 at pH 5 to about 0.2s^?1 in strong base, the latter value probably reflecting catalysis by formate.     

Cyclic GMP Regulates Nicotine-Induced Secretion from Cultured Bovine Adrenal Chromaffin Cells: Effects of 8–Bromo–Cyclic GMP, Atrial Natriuretic Peptide, and Nitroprusside (Nitric Oxide)

Methacholine, atrial natriuretic peptide (ANP), nitroprusside (nitric oxide), angiotensin II, and bradykinin raised cyclic GMP (cGMP) levels in cultured bovine adrenal chromaffin cells. The role of cGMP in secretion from chromaffin cells was examined using 8-bromo-cGMP. This analogue had no effect on basal secretion or secretion due to angiotensin II, bradykinin, or a high K+ level but potentiated secretion due to low doses of nicotine. At supramaximal doses of nicotine, 8-bromo-cGMP inhibited secretion. These effects of 8-bromo-cGMP were not due to changes in the nicotine-induced rise in cytosolic calcium concentration. A potentiation of secretion due to low doses of nicotine was also found following simultaneous addition of ANP or nitroprusside, a result suggesting that ANP and nitric oxide (endothelium-derived relaxing factor) could be important regulators of secretion from adrenal chromaffin cells.     

Role of nitric oxide and endothelium-derived hyperpolarizing factor (EDHF) in the regulation of blood pressure by leptin in lean and obese rats

We investigated the role of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in hemodynamic action of leptin. The effect of leptin (1 mg/kg i.p.) on systolic blood pressure (SBP) was examined in lean rats and in rats made obese by feeding highly palatable diet for either 1 or 3 months. Separate groups received NO synthase inhibitor, L-NAME, or EDHF inhibitors, the mixture of apamin+charybdotoxin or sulfaphenazole, before leptin administration. Leptin increased NO production, as evidenced by increase in plasma and urinary NO metabolites and cyclic GMP. This effect was impaired in both obese groups. In lean rats either leptin or EDHF inhibitors had no effect on blood pressure. L-NAME increased blood pressure in lean animals and this effect was prevented by leptin. However, when leptin was administered to animals pretreated with both L-NAME and EDHF inhibitors, blood pressure increased even more than after L-NAME alone. In the 1-month obese group leptin had no effect on SBP, however, pressor effect of leptin was observed in animals pretreated with EDHF inhibitors. In the 3-month obese group leptin alone increased SBP, and EDHF inhibitors did not augment its pressor effect. The results suggest that leptin may stimulate EDHF when NO becomes deficient, e.g. after NOS blockade or in short-term obesity. Although the effect of leptin on NO production is impaired in the 1-month obese group, BP does not increase, probably because EDHF compensates for NO deficiency. In contrast, leptin increases BP in 3-month obesity because its effect on EDHF is also attenuated.     

EDRF(NO)-mediated modulation of collagen-induced platelet accumulation in rat pulmonary microcirculation

Endothelium-derived relaxing factor (EDRF) or nitric oxide (NO) biosynthesis fromL-arginine occurs in the endothelium and platelets and may modulate plate-let function and contribute to thromboresistance in the vessel wall. A rat model was used to evaluate selective accumulation of¹¹¹In-labeled platelets in the pulmonary microcirculation following the administration of collagen, adenosine 5-diphosphate (ADP) or thrombin. Platelet aggregation was monitored continuously over the thorax using a microcomputer-based system. Sodium nitroprusside, a stimulator of soluble guanylate cyclase and zaprinast, a phosphodiesterase V inhibitor, both known to cause accumulation of cyclic guanosine monophosphate, exhibited moderate inhibitory activity, which was shared byL-arginine. N^G-monomethyl-L-arginine (L-NMMA; 1 mg/kg/min), an inhibitor of EDRF(NO), potentiated the aggregatory response to collagen at an intravenous dose of 100 µg/kg but not at one of 30 µg/kg. D-NMMA had no such effect. The augmenting effect of L-NMMA was abolished byL-arginine. N^G-nitro-L-arginine methyl ester (L-NAME; 0.1 mg/kg/min) also markedly augmented the collagen-induced platelet response, and, at higher doses, all treated animals died upon collagen challenge. Both L-NMMA and L-NAME did not affect the responses to ADP and thrombin. The results suggest that in the intact vascular system, basal release of EDRF(NO) is not critically involved in modulation of platelet function but becomes a significant factor when platelets are exposed to great amounts of collagen fibrils.The results were presented in part at the annual meeting of the Society of Chinese Bioscientists of America, Tri-State Chapter, New York, N.Y., on May 16, 1992.     

Production of the endocannabinoids anandamide and 2-arachidonoylglycerol by endothelial progenitor cells

Recent studies have highlighted the importance of paracrine growth factors as mediators of pro-angiogenic effects by endothelial progenitor cells (EPCs), but little is known about the release of lipid-based factors like endocannabinoids by EPCs. In the current study, the release of the endocannabinoids anandamide and 2-arachidonoylglycerol by distinct human EPC sub-types was measured using HPLC/tandem mass-spectrometry. Anandamide release was highest by adult blood colony-forming EPCs at baseline and they also demonstrated increased 2-arachidonoylglycerol release with TNF-alpha stimulation. Treatment of mature endothelial cells with endocannabinoids significantly reduced the induction of the pro-inflammatory adhesion molecule CD106 (VCAM-1) by TNF-alpha.     

Chronic nitric oxide exposure alters the balance between endothelium-derived relaxing factors released from rat renal arteries: prevention by treatment with NOX-100, a NO scavenger

We investigated whether nitric oxide (NO) exposure alters the balance between NO and endothelium-derived hyperpolarizing factor (EDHF) released from rat renal arteries. To produce states of acutely or chronically excessive NO, lipopolysaccharide (LPS) was administered intraperitoneally to rats in a single dose of 4 mg/kg (LPS-single group) or in stepwise doses of 0.5, 1.0 and 2.0 mg/kg every other day (LPS-repeated group). On the day after LPS treatment, the protein levels of inducible NO synthase (iNOS) and endothelial NOS (eNOS) were measured, and the relaxation responses were determined in the renal arteries. The protein levels of iNOS markedly increased in both LPS-treated groups, while those of eNOS significantly increased in the LPS-repeated group compared with those in the respective control groups. In both LPS-treated groups, the relaxations in response to acetylcholine (ACh) and sodium nitroprusside remained unchanged. The ACh-induced relaxations in the presence of N(G)-nitro-L-arginine methyl ester, a NOS inhibitor, or by 1H-[1, 2, 4-] oxadiazole [4, 3-a] quinoxalin-1-one, a soluble guanylyl cyclase inhibitor, i.e. EDHF-mediated relaxations were significantly impaired in the LPS-repeated group but not in the LPS-single group, indicating increase in NO-mediated relaxation in the LPS-repeated group. These changes in the protein levels and EDHF-mediated relaxations induced by ACh observed in the LPS-repeated group were restored by treatment with NOX-100, a NO scavenger. These results suggest that persistent but not acute excessive NO exposure in rats impairs EDHF-mediated relaxation in renal arteries, leading to a compensatory upregulation of the eNOS/NO pathway.     

Nitric oxide and prostacyclin-dependent pathways involvement on in vitro induced hypothermia

Nitric oxide and prostacyclin are endogenous endothelium-derived vasodilators, but little information is available on their release during hypothermia. This study was carried out to test the hypothesis that endothelium may modulate vascular reactivity to decreased temperature changes. Segments of contracted (prostaglandin F(2alpha), 2x10(-6)M) canine coronary, femoral, and renal arteries, with and without endothelium, were in vitro ("organ chambers") exposed to progressive hypothermia (from 37 to 10 degrees C) in graded steps. The study is limited to physiological measurements of vascular tone, in the presence or absence of PGI(2) and/or NOS inhibitors, which show correlation with the relaxation. Hypothermia induced vasodilatation of vessels with intact endothelium, which became endothelium-independent below 20 degrees C. This vasodilatation began at 35 degrees C and, in the presence of indomethacin (2x10(-6)M), at 30 degrees C. Endothelium-dependent vasodilatation to hypothermia was blocked by L-NMMA or L-NOARG (10(-5)M), two competitive inhibitors of nitric oxide synthase (n=5 each, P<0.05). Oxyhemoglobin (2x10(-6)M) also inhibited vasodilatation induced by hypothermia (n=6, P<0.05). Pretreatment with either atropine or pirenzepine (10(-6)M) inhibited hypothermia-mediated vasodilatation (n=5 each, P<0.05). The present in vitro study concluded that the endothelium is sensitive to temperature variations and indicated that PGI(2) and NO-dependent pathways may be involved endothelium-dependent relaxation to hypothermia. The endothelium-dependent vasodilatation to hypothermia, in systemic and coronary arteries, is mediated by the M1 muscarinic receptor.     

The role of arterial smooth muscle in vasorelaxation

The concept of endothelium-derived relaxing factor (EDRF) implies that nitric oxide (NO) produced by NO synthase (NOS) in the endothelium in response to vasorelaxants such as acetylcholine (ACh) acts on the underlying vascular smooth muscle cells (VSMC) inducing vascular relaxation. The EDRF concept was derived from experiments on denuded blood vessel strips and, in frames of this concept, VSMC were regarded as passive recipients of NO from endothelial cells. However, it was later found that VSMC express NOS by themselves, but the principal question remained unanswered, is the NO generation by VSMC physiologically relevant? We hypothesized that the destruction of the vascular wall anatomical integrity by rubbing off the endothelial layer might increase vascular superoxides that, in turn, reduced the NO bioactivity as a relaxing factor. To test our hypothesis, we examined ACh-induced vasorelaxation under protection against oxidative stress and found that superoxide scavengers restored vasodilatory responses to ACh in endothelium-deprived blood vessels. These findings imply that VSMC can release NO in amounts sufficient to account for the vasorelaxatory response and challenge the concept of the obligatory role of endothelial cells in the relaxation of arterial smooth muscle.     

Ghrelin improves endothelial dysfunction through growth hormone-independent mechanisms in rats

Ghrelin is a novel growth hormone (GH)-releasing peptide which was isolated from the stomach. We have reported that ghrelin causes vasorelaxation in rats through GH-independent mechanisms. We investigated whether ghrelin improves endothelial dysfunction. Ghrelin was subcutaneously administered to GH-deficient rats for three weeks. After isolation of the thoracic aorta, aortic ring tension was measured to evaluate vasorelaxation. Acetylcholine-induced vasorelaxation was impaired in GH-deficient rats given placebo compared to that in normal rats given placebo. GH-deficient rats treated with ghrelin, however, showed a significant increase in the maximal relaxation as compared with those given placebo. This improvement by ghrelin was inhibited by N(G)-nitro-L-arginine methyl ester, a nonselective nitric oxide synthase (NOS) inhibitor. Western blot analysis demonstrated that treatment with ghrelin increased endothelial NOS (eNOS) expression in the aorta of GH-deficient rats. These results suggest that administration of ghrelin improves endothelial dysfunction and increases eNOS expression in rats through GH-independent mechanisms.     

AMP-activated protein kinase mediates VEGF-stimulated endothelial NO production

Vascular endothelial growth factor (VEGF) is an important regulator of endothelial cell function. VEGF stimulates NO production, proposed to be a result of phosphorylation and activation of endothelial NO synthase (eNOS) at Ser1177. Phosphorylation of eNOS at this site also occurs after activation of AMP-activated protein kinase (AMPK) in cultured endothelial cells. We therefore determined whether AMPK mediates VEGF-stimulated NO synthesis in endothelial cells. VEGF caused a rapid, dose-dependent stimulation of AMPK activity, with a concomitant increase in phosphorylation of eNOS at Ser1177. Infection of endothelial cells with an adenovirus expressing a dominant negative mutant AMPK partially inhibited both VEGF-stimulated eNOS Ser1177 phosphorylation and NO production. VEGF-stimulated AMPK activity was completely inhibited by the Ca(2+)/calmodulin-dependent protein kinase kinase inhibitor, STO-609. Stimulation of AMPK via Ca(2+)/calmodulin-dependent protein kinase kinase represents a novel signalling mechanism utilised by VEGF in endothelial cells that contributes to eNOS phosphorylation and NO production.     

The interaction between nitrogen oxides and hemoglobin and endothelium-derived relaxing factor

Among nitrogen oxides, NO and NO₂ are free radicals and show a variety of biological effects. NO₂ is a strongly oxidizing toxicant, although NO, not oxidizing as NO₂, is toxic in that it interacts with hemoglobin to form nitrosyl-and methemoglobin. Nitrosylhemoglobin shows a characteristic electron spin resonance (ESR) signal due to an odd electron localized on the nitrogen atom of NO and reacts with oxygen to yield nitrate and methemoglobin, which is rapidly reduced by methemoglobin reductase in red cells. NO was found to inhibit the reductase activity. Part of NO inhaled in the body is oxidized by oxygen to NO₂, which easily dissolves in water and converts to nitrite. The nitrite oxyhemoglobin autocatalytically after a lag. The mechanism of the oxidation, particularly the involvement of superoxide, was controversial. The stoichiometry of the reaction has now been established using nitrate ion electrode and a methemoglobin free radical was detected by ESR during the oxidation. Complete inhibition of the autocatalysis by aniline or aminopyrine suggests that the radical catalyzes conversion of nitrite to NO₂, which oxidizes oxyhemoglobin. Recently NO was shown to be one of endothelium- derived relaxing factors and the relaxation induced by the factor was inhibited by hemoglobin and potentiated by superoxide dismutase.     

Cultured Astrocytes Release a Factor that Decreases Endothelin-1 Secretion by Brain Microvessel Endothelial Cells

Abstract: Endothelin-1 (ET-1), originally characterized as a potent vasoconstrictor peptide secreted by vascular endothelial cells, has now been described to possess a wide range of biological activities within the cardiovascular system and in other organs. Brain microvessel endothelial cells, which, together with perivascular astrocytes, constitute the blood-brain barrier, have been shown to secrete ET-1, whereas specific ET-1 receptors are expressed on astrocytes. It is reported here that conditioned medium from primary cultures of mouse embryo astrocytes could significantly, and reversibly, attenuate the accumulation of both ET-1 and its precursor big ET-1 in the supernatant of rat brain microvessel endothelial cells by up to 59 and 76%, respectively, as assessed by immunometric assay. This inhibitor of ET-1 production was purified by gel-exclusion and ion-exchange chromatography as a 280-Da iron-containing molecule, able to release nitrites upon degradation. These results suggest that astrocytes, via release of an iron-nitrogen oxide complex, may be involved in a regulatory loop of ET-1 production at the level of the blood-brain barrier.     

Plasma nitrite response and arterial reactivity differentiate vascular health and performance

NO is crucial for endothelial function and vascular health. Plasma nitrite (NO₂⁻) is the main oxidation product of NO and has been shown to reflect changes in eNOS activity. We hypothesized that plasma NO₂⁻ response to physical exercise stress along with physiological endothelial function would be reduced with increasing severity of vascular disease. Subject groups were: (a) risk factors but no vascular disease (RF); (b) Type 2 diabetes with no vascular disease (DM); (c) diagnosed peripheral arterial disease (PAD); and (d) DM + PAD. Venous blood was drawn at rest and 10 min following maximal exercise. Plasma samples were analyzed by reductive chemiluminescence. Brachial diameters were imaged prior to, during and following 5 min of forearm occlusion (BAFMD). There were no differences in resting plasma NO₂⁻ or BA diameters between groups. The PAD groups had lower age adjusted BAFMD responses (p  0.05). Within group analysis revealed an increase in NO₂⁻ in the RF group (+39.3%), no change in the DM (−15.51%), and a decrease in the PAD (−44.20%) and PAD + DM (−39.95%). This was maintained after adjusting for age and VO_2peak (p  0.05). ΔNO₂⁻ and BAFMD were the strongest independent predictors of VO_2peak in multivariate linear regression. These findings suggest ΔNO₂⁻ discriminates severity of cardiovascular disease risk, is related to endothelial function and predicts exercise capacity.     

Pregnancy-associated increase in rat systemic arteries endothelial nitric oxide production diminishes vasoconstrictor but does not enhance vasodilator responses

Late pregnancy in rats is characterized by a decrease in arterial pressure and in isolated arterial vessels response to vasoconstrictors. In uterine arteries the pregnancy-associated attenuation of the response to vasoconstrictors has been attributed to an increase in basal and agonist-induced endothelial NO production. However, the role of NO in pregnancy-associated changes of systemic arteries reactivity to vasoactive agents remains to be fully elucidated. We examined whether pregnancy influences the reactivity of systemic arteries to vasodilator or vasoconstrictor agents through NO-dependent mechanisms. Thoracic aortic rings and mesenteric arterial bed of late pregnant rats showed refractoriness to phenylephrine-induced vasoconstriction that was abolished by NO synthase inhibition. The potency of L-NNA to enhance tension of aortic rings preconstricted with phenylephrine (10–20% of their maximal response) was significantly lower in preparations from pregnant animals. In phenylephrine-contracted aortas and mesenteric bed, the effects of the endothelium-dependent vasodilators acetylcholine, A23187 and bradykinin, were not influenced by pregnancy. Similarly, pregnancy did not affect the vasodilator responses of adenosine, isoproterenol, capsaicin, nitroprusside, forskolin, and Hoe234 in the mesenteric bed. NO synthase activity measured by determining the conversion of L−[³H]-arginine to L−[³H]-citrulline in aorta and mesenteric arteries homogenates was not altered by pregnancy. These findings show that endothelial-dependent and -independent vasodilators action as well as NO synthase activity in systemic arteries is uninfluenced by pregnancy, whereas pregnancy-associated hyporeactivity of systemic arteries to vasoconstrictors is related to an enhanced endothelial NO production either spontaneous or elicited directly or indirectly by vasoconstrictor agents. This interpretation implies that the enhanced NO production observed in systemic arteries during late pregnancy involves cellular pathways other than the ones involved in the response to endothelium-dependent vasodilators such as acetylcholine.     

Comparisons of the release of vasodilator substances from left and right cardiac chambers of the isolated perfused rabbit heart: Implications for intraventricular thrombus formation

Prostacyclin (PgI₂) and endothelium-derived nitric oxide (EDNO) are produced by the arterial and venous endothelium. In addition to their vasodilator action on vascular smooth muscle, both act together to inhibit platelet aggregation and promote platelet disaggregation. EDNO also inhibits platelet adhesion to the endothelium. EDNO and PgI₂ have been shown to be released from the cultured endocardial cells. In this study, we examined the release of vasoactive substances from the intact endocardium by using isolated rabbit hearts perfused with physiological salt solution (95% O₂/5% CO₂, T = 37 °C). The right and left cardiac chambers were perfused through separate constant-flow perfusion loops (physiological salt solution, 8 ml min⁻¹). Effluent from left and right cardiac, separately, was bioassayed on canine coronary artery smooth muscle, which had been contracted with prostaglandin F_{2α_}(2 × 10⁻⁶ M) and no change in tension was exhibit. However, addition of calcium ionophore A23187 (10⁻⁶ M) to the cardiac chambers’ perfusion line induced vasodilation of the bioassay coronary ring, 61.4 ± 7.4% versus 70.49 ± 6.1% of initial prostaglandin F_2α contraction for the left and right cardiac chambers perfusate, respectively (mean ± SEM, n = 10, p > 0.05). Production of vasodilator was blocked totally in the left heart but, only partially blocked in the right heart by adding indomethacin (10⁻⁵ M) to the perfusate, respectively, 95.2 ± 2.2% versus 41.5 ± 4.8% (mean ± SEM, n = 10, p < 0.05). 6-Keto prostaglandin F_1α, measured in the endocardial superfusion effluent was also higher for the left cardiac chambers than for the right at the time of stimulation with the A23187, respectively, 25385.88 ± 5495 pg/ml (n = 8) versus 13,132.45 ± 1839.82 pg/ml (n = 8), (p < 0.05). These results showed that cyclooxygenase pathway plays major role in generating vasoactive substances for the left cardiac chamber endocardium; while it is not the main pathway for the right ventricular endocardium at which EDNO and PgI₂ could act together and potentiate their antithrombogenic activities in isolated perfused rabbit heart. This may be an explanation for the intraventricular thrombus mostly seen in left ventricle rather than in right ventricle as a complication of myocardial infarction.