Constriction to hypoxia-reoxygenation in isolated mouse coronary arteries: role of endothelium and superoxide

Folic acid improves acetylcholine-induced vasoconstriction of coronary vessels isolated from hyperhomocysteinemic mice: an implication to coronary vasospasm

Human atherosclerotic coronary vessels elicited vasoconstriction to acetylcholine (Ach) and revealed a phenomenon of vasospasm. Homocysteine (Hcy) levels are elevated in the atherosclerotic plaque tissue, suggesting its pathological role in endothelial damage in atherosclerotic diseases. Accordingly, we examined the role hyperhomocysteinemia in coronary endothelial dysfunction, vessel wall thickness, lumen narrowing, leading to acute/chronic coronary vasospasm. The therapeutic potential and mechanisms of folic acid (FA) using hyperhomocysteinemic cystathionine beta synthase heterozygote (CBS-/+) and wild type (CBS+/+) mice were addressed. The CBS-/+ and CBS+/+ mice were treated with or without a Hcy lowering agent FA in drinking water (0.03 g/L) for 4 weeks. The isolated mouse septum coronary artery was cannulated and pressurized at 60 mmHg. The wall thickness and lumen diameters were measured by Ion-Optic. The vessels were treated with Ach (10(-8) -10(-5)  M) and, for comparison, with non-endothelial vasodilator sodium nitroprusside (10(-5)  M). The endothelium-impaired arteries from CBC-/+ mice constricted in response to Ach and this vasoconstriction was mitigated with FA supplementation. The level of endothelial nitric oxide synthase (eNOS) was lower in coronary artery in CBS-/+ than of CBS+/+ mice. Treatment with FA increased the levels of Ach-induced NO generation in the coronary artery of CBS-/+ mice. The results suggest that Ach induced coronary vasoconstriction in CBS-/+ mice and this vasoconstriction was ameliorated by FA treatment. The mechanisms for the impairment of vascular function and therapeutic effects of FA may be related to the regulation of eNOS expression, NO availability and tissue homocysteine.

Rho-kinase contributes to hypoxia/reoxygenation-induced cerebral endothelial dysfunction

Hypoxia/reoxygenation (H/R) in vitro induced cerebral endothelial dysfunction is mediated by superoxide production. However, the intracellular pathways involved remain unclear. The present study was designed to investigate the involvement of Rho-kinase and its interaction with nitric oxide (NO) in cerebral endothelial dysfunction after H/R. Arterial diameter and intraluminal pressure were simultaneously measured in vitro on rat posterior cerebral arteries. Vascular NO production was determined by measuring stable NO metabolites nitrate/nitrite. H/R selectively inhibited cerebral vasodilation to the endothelium-dependent agonist acetylcholine (ACh, 0.01 to 10 micromol/L) and caused NO deficiency. H/R-impaired vasodilation to ACh was reversed by Y27632 (1 micromol/L), a specific inhibitor of Rho-kinase, but not by chelerythrine (1 micromol/L), a selective inhibitor of protein kinase C. Y27632 had no protective effect in the presence of N-nitro-L-arginine methyl ester (L-NAME; 100 micromol/L), a specific endothelial NO synthase inhibitor. L-NAME (100 micromol/L) alone failed to modulate H/R-impaired vasodilation, so did L-arginine (3 mmol/L), a substrate for NO synthase. However, a stable NO donor diethylenetetra amine-NONOate (5 micromol/L) normalized H/R-impaired dilation to ACh. In conclusion, H/R-induced endothelial dysfunction is associated with activation of Rho-kinase-dependent pathway and NO deficiency. Pretreatment with either Y27632 or the stable NO donor profoundly prevented H/R-mediated cerebral endothelial dysfunction.

Metabolic coronary flow regulation--current concepts

The concept of metabolic coronary flow control provides a rationale for the close relationship of coronary flow and myocardial metabolic rate of oxygen. The concept is based on the presence of an oxygen (metabolic) sensor coupled functionally to effector mechanisms, which control vascular tone. Four modes of metabolic control models have been proposed. 1) An oxygen sensor located in the wall of coronary vessels coupling to smooth muscle tension. Endothelial prostaglandin production may support this concept. 2) An oxygen sensing mechanism located in the myocardium and changing metabolism in response to changes of local pO(2). Adenosine is a metabolite produced at an accelerated rate when the supply-to-demand relationship for oxygen falls. 3) Sensing of oxygen turnover may be achieved by carbon dioxide production and, potentially, by mitochondrial production of reactive oxygen species. 4) The red blood cell might serve as an oxygen sensor in response to changes of haemoglobin oxygenation. A potential link to vessel relaxation may be red cell ATP release. A large body of experimental evidence supports the notion that K(ATP) channels play a significant role causing smooth muscle hyper-polarization. However, additional yet unknown effector mechanisms must exist, because block of K(ATP) channels does not lead to deterioration of coronary flow control under conditions of exercise. Thus, although several lines of evidence show that metabolic flow regulation is effective during hypoxic conditions,mechanisms mediating normoxic metabolic flow control still await further clarification.

Hypoxia imaging-directed radiation treatment planning

Increasing evidence supports the role of the tumor microenvironment in modulating cancer behavior. Tissue hypoxia, an important and common condition affecting the tumor microenvironment, is well established as a resistance factor in radiotherapy. Increasing evidence points to the ability of hypoxia to induce the expression of gene products, which confer aggressive tumor behavior and promote broad resistance to therapy. These factors suggest that determining the presence or absence of tumor hypoxia is important in planning cancer therapy. Recent advances in PET hypoxia imaging, conformal radiotherapy, and imaging-directed radiotherapy treatment planning now make it possible to perform hypoxia-directed radiotherapy. We review the biological aspects of tumor hypoxia and PET imaging approaches for measuring tumor hypoxia, along with methods for conformal radiotherapy and image-guided treatment, all of which provide the underpinnings for hypoxia-directed therapy. As a case example, we review emerging data on PET imaging of hypoxia to direct radiotherapy.

NF-κB Activation Plays a Role in Superoxide-Mediated Cerebral Endothelial Dysfunction After Hypoxia/Reoxygenation

Background and Purpose— Cerebral vascular injury occurs in response to hypoxia/reoxygenation (H/R). However, the cellular signaling pathways that regulate this event remain unclear. The present study was designed to determine whether reactive oxygen species (ROS) mediate endothelial dysfunction after H/R in cerebral resistance arteries and, if so, the relative contribution of ROS, NADPH oxidase, and a nuclear factor-κB (NF-κB) pathway.

Methods— Arterial diameter and intraluminal pressure were simultaneously measured on rat posterior cerebral arteries (PCA). Superoxide was measured by 5-μmol/L lucigenin-enhanced chemiluminescence.

Results— Hypoxia/reoxygenation selectively inhibited cerebral vasodilation to the endothelium-dependent agonist acetylcholine (Ach) (0.01 to 10 μmol/L) by ≈50%. Impaired vasodilation after H/R was reversed by 2,2,6,6-tetramethylpiperidine- N -oxyl (Tempo) (100 μmol/L), a cell-permeable superoxide dismutase mimetic, and partially by ebselen (10 μmol/L), a peroxynitrite scavenger. H/R-impaired vasodilation to Ach was also preserved by apocynin (1 mmol/L), a specific inhibitor for NADPH oxidase. Correspondingly, H/R significantly increased lucigenin-detectable superoxide, which was reduced by either Tempo or apocynin, but not by allopurinol (10 μmol/L), an inhibitor of xanthine oxidase. Finally, the NF-κB inhibitors helenalin (10 μmol/L) and MG-132 (1 μmol/L) independently antagonized H/R-impaired Ach-induced vasodilation without affecting dilator response to sodium nitroprusside, an endothelium-independent vasodilator.

Conclusions— These results indicate that superoxide mediates cerebral endothelial dysfunction after hypoxia/reoxygenation largely via activation of NADPH oxidase and possibly activation of NF-κB pathway.

Extracellular superoxide enhances 5-HT-induced murine pulmonary artery vasoconstriction

Accumulating evidence suggests that changes in both 5-hydroxytryptamine (5-HT) receptor activity and in the levels of reactive oxygen species (ROS) play an important role in regulating pulmonary artery (PA) vascular responsiveness, particularly in the setting of pulmonary hypertension. Therefore, we hypothesized that increased levels of superoxide enhance 5-HT-induced PA constriction. With the use of a small-vessel bioassay, 5-HT (0.01–10 μM) induced a concentration-dependent vasoconstriction in isolated wild-type murine intrapulmonary arteries (100–150 μm diameter) that was enhanced by both removal of the endothelium and by treatment with either NG-nitro-l-arginine methyl ester (30 μM) or xanthine (10 μM) + xanthine oxidase (0.005 U/ml). PA isolated from extracellular superoxide dismutase (EC-SOD) knockout mice also showed enhanced constriction. On the other hand, PA constriction to 5-HT was attenuated by either the addition of GR-127935 (0.1 μM, a selective inhibitor of 5-HT1B/1Dreceptor) or copper/zinc-containing superoxide dismutase (Cu/Zn SOD, 150 U/ml) and in PA isolated from transgenic mice overexpressing human EC-SOD. With the use of both oxidative fluorescent confocal microscopy and lucigenin-enhanced chemiluminescence, superoxide levels were increased significantly after 5-HT-induced PA vasoconstriction. This increase in superoxide levels could be blocked by the exogenous addition of Cu/Zn SOD (150 U/ml) or by apocynin (30 μM, an inhibitor of NADPH oxidase) but was not affected by gp91phoxknockout mice. Overall, our results are consistent with 5-HT increasing vascular smooth muscle superoxide production via an NADPH oxidase pathway that is independent of gp91phox, which leads to increases in extracellular superoxide levels, which in turn enhances 5-HT-induced murine pulmonary vasoconstriction.

Reoxygenation-induced constriction in murine coronary arteries: the role of endothelial NADPH oxidase (gp91phox) and intracellular superoxide

Previous work suggests that superoxide mediates hypoxia/reoxygenation (H/R)-induced constriction of isolated mouse coronary arteries (CA). To determine the source of superoxide overproduction during H/R we studied CA obtained from transgenic (Tg) mice overexpressing human CuZn-superoxide dismutase (SOD) and mice lacking gp91(phox) using an in vitro vascular ring bioassay. We found that under normoxic conditions CA isolated from wild type (wt) mice, CuZn-SOD Tg mice and gp91(phox) knock-out mice had similar contractile responses to U46619 and hypoxia and similar dilation responses to acetylcholine. In wt CA, 30 min of hypoxia (1% O(2)) followed by reoxygenation (16% O(2)) resulted in further coronary vasoconstriction (internal diameter from 105 +/- 11 to 84.5 +/- 17.9 microm), whereas this response was completely blocked in both CuZn-SOD Tg and gp91(phox) knock-out CA (104.3 +/- 10.5 to 120.7 +/- 14 microm and 143.3 +/- 15.3 to 172.7 +/- 12.5 microm, respectively, p < 0.01). Furthermore, we show that H/R enhances the generation of superoxide radicals in wt CA (25.8 +/- 0.7 relative light units per second (RLU/s)), whereas CuZn-SOD Tg CA (12.2 +/- 0.8 RLU/s, p < 0.01) and gp91(phox) CA (12.5 +/- 0.9 RLU/s, p < 0.01) show reduced levels. These results demonstrate that H/R-induced vasoconstriction is mediated by intracellular superoxide overproduction via endothelial NADPH oxidase gp91(phox). Therefore, increasing endogenous levels of CuZn-SOD in CA may provide a novel cardioprotective strategy for maintaining coronary perfusion under conditions of H/R.

Coronary vasospasm and the regulation of coronary blood flow

Under physiologic conditions, epicardial arteries contribute minimally to coronary vascular resistance. However, in the presence of endothelial dysfunction, stimuli that normally produce vasodilation may instead cause constriction. Examples include neural release of acetylcholine or norepinephrine, platelet activation and production of serotonin and thrombin, and release of local factors such as bradykinin. This shift from a primary endothelial-mediated vasodilator influence to one of endothelial dysfunction and unchecked vasoconstriction is precisely the milieu in which coronary vasospasm is observed. This condition, which typically occurs during periods of relatively sedentary activity, is associated with focal and transient obstruction of an epicardial arterial segment resulting in characteristic echocardiographic changes and symptoms of myocardial ischemia. This review highlights the current understanding of mechanisms regulating the coronary circulation during health and examines the pathophysiologic changes that occur with coronary spasm. Genetic and other predisposing conditions are addressed, as well as novel therapies based on recent mechanistic insights of the coronary contractile dysfunction associated with coronary spasm.

Involvement of nitric oxide and potassium channels in the bradykinin-induced vasodilatation in the rat kidney perfused ex situ

The role of nitric oxide (NO), K(+) channels, and arachidonic acid metabolism, via cytochrome P450 and cyclooxygenase pathways, in the renal vasodilatory effect of bradykinin was examined in the isolated rat kidney perfused ex situ with a blood-free solution. Bradykinin (BK, 0.25-1.0 microM) induced a dose-dependent reduction of 10-35% in the relative renal vascular resistance (rRVR) of isolated kidneys preconstricted with phenylephrine (PHE, 0.17-0.35 microM). The vasodilating effect of 0.5 microM bradykinin was significantly inhibited by the nitric oxide synthase inhibitors, N(G)-nitro-L-arginine (95% inhibition) and N(G)-nitro-L-arginine methyl ester (45-75% inhibition). Clotrimazole, an inhibitor of cytochrome P450 pathway but not indomethacin, a cyclooxygenase inhibitor, reduced the renal vasodilator response to bradykinin by 84%. The nonspecific K(+) channel inhibitor, tetraethylammonium ion (TEA) and the selective inhibitor of Ca(2+)-activated K(+) channels, charybdotoxin (ChTX) greatly attenuated the vasodilator response to bradykinin by approximately 84% and 79%, respectively. These two K(+) channel inhibitors showed similar effects on vasodilatation induced by S-nitroso-acetyl-D,L-penicillamine (1 microM), a nitric oxide donor. The results suggest that bradykinin releases nitric oxide which, by opening potassium channels specifically the Ca(+)-dependent type, mediates the renal vasodilator response to bradykinin in the isolated kidney perfused ex situ.

Endothelium-dependent vasoactive modulation in the ophthalmic circulation

The vascular endothelium is strategically located between the circulating blood and the vascular smooth muscle cells. Different agonists or stimuli transported by the circulating blood can trigger the endothelium to release potent relaxing (nitric oxide, prostacyclin, endothelium-derived hyperpolarizing factor) or contracting factors (endothelin, cycloxygenase products). These endothelium-derived vasoactive factors can modulate blood flow locally. Heterogeneity exists from one vascular bed to the other, or even between vessels, in the agonists able to stimulate the release of endothelium-derived vasoactive factors. In the ophthalmic circulation, nitric oxide and endothelin are strong vasoactive modulators. In many vascular diseases that are of importance in ophthalmology (hypercholesterolemia, arteriosclerosis, hypertension, diabetes, vasospastic syndrome, ischemia and reperfusion, etc) the function of the endothelium can be impaired. There exist different drugs that can modulate the vasoactive function of the vascular endothelium. In other words, it appears that the vascular endothelium plays an important role in both the physiology and pathophysiology of the regulation of blood flow. The modulation of this regulatory system by different drugs might open new therapeutical approaches to treat vascular disorders in ophthalmology.