Different modulation by Ca2+-activated K+ channel blockers and herbimycin of acetylcholine- and flow-evoked vasodilatation in rat mesenteric small arteries

Age-dependent impact of CaV 3.2 T-type calcium channel deletion on myogenic tone and flow-mediated vasodilatation in small arteries

KEY POINTS

Blood pressure and flow exert mechanical forces on the walls of small arteries, which are detected by the endothelial and smooth muscle cells, and lead to regulation of the diameter (basal tone) of an artery. CaV 3.2 T-type calcium channels are expressed in the wall of small arteries, although their function remains poorly understood because of the low specificity of T-type blockers. We used mice deficient in CaV 3.2 channels to study their role in pressure- and flow-dependent tone regulation and the possible impact of ageing on this role. In young mice, CaV 3.2 channels oppose pressure-induced vasoconstriction and participate in endothelium-dependent, flow-mediated dilatation. These effects were not seen in mature adult mice. The results of the present study demonstrate an age-dependent impact of CaV 3.2 T-type calcium channel deletion in rodents and suggest that the loss of CaV 3.2 channel function leads to more constricted arteries, which is a risk factor for cardiovascular disease.

ABSTRACT

The myogenic response and flow-mediated vasodilatation are important regulators of local blood perfusion and total peripheral resistance, and are known to entail a calcium influx into vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), respectively. CaV 3.2 T-type calcium channels are expressed in both VSMCs and ECs of small arteries. The T-type channels are important drug targets but, as a result of the lack of specific antagonists, our understanding of the role of CaV 3.2 channels in vasomotor tone at various ages is scarce. We evaluated the myogenic response, flow-mediated vasodilatation, structural remodelling and mRNA + protein expression in small mesenteric arteries from CaV 3.2 knockout (CaV 3.2KO) vs. wild-type mice at a young vs. mature adult age. In young mice only, deletion of CaV 3.2 led to an enhanced myogenic response and a ∼50% reduction of flow-mediated vasodilatation. Ni2+ had both CaV 3.2-dependent and independent effects. No changes in mRNA expression of several important K+ and Ca2+ channel genes were induced by CaV 3.2KO However, the expression of the other T-type channel isoform (CaV 3.1) was reduced at the mRNA and protein level in mature adult compared to young wild-type arteries. The results of the present study demonstrate the important roles of the CaV 3.2 T-type calcium channels in myogenic tone and flow-mediated vasodilatation that disappear with ageing. Because increased arterial tone is a risk factor for cardiovascular disease, we conclude that CaV 3.2 channels, by modulating pressure- and flow-mediated vasomotor responses to prevent excess arterial tone, protect against cardiovascular disease.

Pressure Myography to Study the Function and Structure of Isolated Small Arteries

Small arteries play an important role in regulation of peripheral resistance and organ perfusion. Here we describe a series of methods allowing measurements in pressurized segments of small arteries from the systemic and coronary circulation of mice as well as other species. The pressure myography techniques described include measurements of wall structure, wall stress, strain, and myogenic tone. The pressurized perfused small arteries also allow evaluation of responses to increases in pressure, flow, and drugs, where the main readout is changes in vascular diameter.

Impaired NO-mediated vasodilatation in rat coronary arteries after asphyxial cardiac arrest

BACKGROUND

Cardiovascular dysfunction after cardiac arrest is a common finding. It is unknown whether altered endothelium-mediated vasoreactivity contributes to this dysfunction. We hypothesised that cardiac arrest and resuscitation results in impaired endothelial function. This was addressed by measurements of inflammatory and endothelial plasma markers and of endothelium-dependent vasodilatation in coronary and mesenteric arteries in rats after cardiac arrest and resuscitation.

METHODS

Male Sprague Dawley rats underwent either asphyxia-induced cardiac arrest for 5 min and subsequent resuscitation (n = 30) or a sham procedure (control animals, n = 39). Animals were euthanised after 30 min or 2 h. Blood was analysed for TNF-α, IL-1β, IL-6, IL-10, sE-selectin, sP-selectin, sVCAM-1, ICAM-1, VEGF-α and vWF. Arterial rings of the left anterior descending coronary artery and mesenteric resistance arteries were mounted in microvascular myographs, and concentration-response curves were constructed.

RESULTS

The plasma levels of the endothelial markers, sP-selectin and vWF increased following cardiac arrest at both 30 min and 2 h. Acetylcholine-induced endothelium-dependent and mainly nitric oxide (NO)-mediated vasodilatation was impaired in the coronary arteries at 30 min, but not 2 h after resuscitation. Endothelium-derived hyperpolarisation (EDH)-type vasodilatation induced by NS309 and vasodilatation induced by the NO donor sodium nitroprusside was unaltered. In parallel with systemic hypotension, mesenteric arteries exhibited a larger response to NS309 2 h after resuscitation.

CONCLUSION

The present results show marked endothelial alterations after cardiac arrest and resuscitation reflected by increased endothelial plasma markers, impaired NO-mediated coronary vasodilatation in the early post-resuscitation phase and enhanced EDH-type vasodilatation in mesenteric arteries later in the post-resuscitation phase.

Effects of acute transmural pressure elevation on endothelium-dependent vasodilation in isolated rat mesenteric veins

BACKGROUND/AIMS

The vascular regulatory function of the endothelium can be impaired by increases in transmural pressure (TMP). We tested the hypothesis that increasing TMP impairs the endothelial dilator function of rat mesenteric small veins (MSVs).

METHODS

In PGF2α-preconstricted MSVs, bradykinin (BK), sodium nitroprusside (SNP) and S-Nitroso-N-acetylpenicillamine (SNAP) concentration-response curves were generated at intermediate (6 mm Hg) and high (12 mm Hg) pressures. BK-induced vasodilation was examined in the absence and presence of nitric oxide synthase inhibitor [N(ω)-nitro-L-arginine (L-NNA), 100 µM], cyclooxygenase inhibitor (indomethacin, 1 µM), and large (BKCa, paxilline, 500 nM) and small (SKCa, apamin, 300 nM) conductance Ca(2+)-activated K(+) channel blockers.

RESULTS

BK, SNP and SNAP responses were not altered by TMP increases. BK-induced vasodilation was significantly reduced by L-NNA, indomethacin, apamin and paxilline at 6 mm Hg and L-NNA at 12 mm Hg, and was further reduced by coapplication of apamin and/or paxilline with L-NNA compared with responses obtained with either blocker. Endothelium removal completely abolished BK-induced vasodilation.

CONCLUSION

Venous endothelial dilator function is not affected by TMP elevation. BK-induced vasodilation is completely dependent on the presence of functional endothelial cells and mediated in part by nitric oxide, BKCa and SKCa channels, while the participation of prostacyclin may be important at intermediate pressures.

Ageing alters perivascular nerve function of mouse mesenteric arteries in vivo

Abstract  Mesenteric arteries (MAs) are studied widely in vitro but little is known of their reactivity in vivo. Transgenic animals have enabled Ca(2+) signalling to be studied in isolated MAs but the reactivity of these vessels in vivo is undefined. We tested the hypothesis that ageing alters MA reactivity to perivascular nerve stimulation (PNS) and adrenoreceptor (AR) activation during blood flow control. First- (1A), second- (2A) and third-order (3A) MAs of pentobarbital-anaesthetized Young (3-6 months) and Old (24-26 months) male and female Cx40(BAC)-GCaMP2 transgenic mice (C57BL/6 background; positive or negative for the GCaMP2 transgene) were studied with intravital microscopy. A segment of jejunum was exteriorized and an MA network was superfused with physiological salt solution (pH 7.4, 37°C). Resting tone was 10% in MAs of Young and Old mice; diameters were ∼5% (1A), 20% (2A) and 40% (3A) smaller (P 0.05) in Old mice. Throughout MA networks, vasoconstriction increased with PNS frequency (1-16 Hz) but was ∼20% less in Young vs. Old mice (P 0.05) and was inhibited by tetrodotoxin (1 μm). Capsaicin (10 μm; to inhibit sensory nerves) enhanced MA constriction to PNS (P 0.05) by ∼20% in Young but not Old mice. Phenylephrine (an α1AR agonist) potency was greater in Young mice (P 0.05) with similar efficacy (∼60% constriction) across ages and MA branches. Constrictions to UK14304 (an α2AR agonist) were less (∼20%; P 0.05) and were unaffected by ageing. Irrespective of sex or transgene expression, ageing consistently reduced the sensitivity of MAs to α1AR vasoconstriction while blunting the attenuation of sympathetic vasoconstriction by sensory nerves. These findings imply substantive alterations in splanchnic blood flow control with ageing.

Heterogeneous cytoplasmic calcium response in microvascular endothelial cells

We investigated changes in calcium concentration in response to the administration of ATP and the onset of shear stress with cultured rat adrenomedulary endothelial cells (RAMECs, microvascular). A substantial heterogeneity in time and space in the calcium response was observed. The onset of shear stress induced calcium waves that originated from one or several cells and propagated to neighboring cells The application of uniform exogenous ATP produced similar heterogeneous calcium transients. The size of the responding groups was dependent on ATP concentration. The propagation of calcium waves induced by either ATP or shear stress challenge was significantly suppressed by suramin, a non-specific purinergic receptor blocker. We investigated some of the mechanisms leading to the heterogeneity, and the results indicated that the main source of variation is the heterogeneous distribution of purinergic receptor. The application of ATP or shear stress stimulates cells to release ATP causing an increase of [Ca²⁺]<inf>i</inf>via purinergic receptor in the cells that have high sensitivity. Subsequently, additional ATP is released and the elevation of ATP concentration in the vicinity of the initially responding cells mediates the calcium propagation. These data suggest a mechanism by which ATP acts as an autocrine and paracrine mediator to integrate individual cell responses that result in coordination of vascular functions in situ.

Mesenteric arteries responsiveness to acute variations of wall shear stress is impaired in rats with liver cirrhosis

OBJECTIVE

In liver cirrhosis, excessive splanchnic vasodilation is due to abnormal synthesis of endogenous vasodilators and to decreased sensitivity to vasoconstrictors. The role of mechanical stimuli such as wall shear stress (WSS) on splanchnic circulation remains unclear. The aim of this study was to assess the vasodilation induced by wall shear stress (WSS) and acute changes in blood flow in the mesenteric arteries in an experimental model of liver cirrhosis.

MATERIALS AND METHODS

The effect of acute changes in intraluminal flow (0, 10, and 20 μl/min) and WSS on the diameter of the mesenteric arteries (diameters <500 μm) of control and cirrhotic rats was assessed, at baseline and after the inhibition of nitric oxide synthase, cyclooxygenase and hemeoxygenase. Concentration-response curves to phenylephrine were also obtained.

RESULTS

In controls, the increase in intraluminal flow led to a significant increase in arterial diameter (p < 0.05), while WSS remained stable; the effect was maintained in vessels pre-constricted with phenylephrine, blocked by the exposure to indomethacin and L-NAME and restored by the subsequent addition of chromium mesoporphyrin (p < 0.05). In cirrhotic arteries, arterial diameters did not change in response to acute increase in flow, neither at baseline nor after exposure to indomethacin and L-NAME, while WSS increased (p < 0.01). Responsiveness to flow was partially restored (p < 0.05) after exposure of the arteries to chromium mesoporphyrin in addition to indomethacin and L-NAME.

CONCLUSIONS

Arteries from cirrhotic rats showed an abolished responsiveness to acute variations in flow, which exposes the mesenteric endothelium to sudden variations in WSS.

Opening of small and intermediate calcium-activated potassium channels induces relaxation mainly mediated by nitric-oxide release in large arteries and endothelium-derived hyperpolarizing factor in small arteries from rat

This study was designed to investigate whether calcium-activated potassium channels of small (SK(Ca) or K(Ca)2) and intermediate (IK(Ca) or K(Ca)3.1) conductance activated by 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) are involved in both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in large and small rat mesenteric arteries. Segments of rat superior and small mesenteric arteries were mounted in myographs for functional studies. NO was recorded using NO microsensors. SK(Ca) and IK(Ca) channel currents and mRNA expression were investigated in human umbilical vein endothelial cells (HUVECs), and calcium concentrations were investigated in both HUVECs and mesenteric arterial endothelial cells. In both superior (∼1093 μm) and small mesenteric (∼300 μm) arteries, NS309 evoked endothelium- and concentration-dependent relaxations. In superior mesenteric arteries, NS309 relaxations and NO release were inhibited by both N(G),N(G)-asymmetric dimethyl-l-arginine (ADMA) (300 μM), an inhibitor of NO synthase, and apamin (0.5 μM) plus 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) (1 μM), blockers of SK(Ca) and IK(Ca) channels, respectively. In small mesenteric arteries, NS309 relaxations were reduced slightly by ADMA, whereas apamin plus an IK(Ca) channel blocker almost abolished relaxation. Iberiotoxin did not change NS309 relaxation. HUVECs expressed mRNA for SK(Ca) and IK(Ca) channels, and NS309 induced increases in calcium, outward current, and NO release that were blocked by apamin and TRAM-34 or charybdotoxin. These findings suggest that opening of SK(Ca) and IK(Ca) channels leads to endothelium-dependent relaxation that is mediated mainly by NO in large mesenteric arteries and by EDHF-type relaxation in small mesenteric arteries. NS309-induced calcium influx appears to contribute to the formation of NO.

Large and small artery endothelial function in patients with essential hypertension – Effect of ACE inhibition and beta‐blockade

Hypertension has been associated with changes in endothelial function in both large muscular arteries and small resistance arteries. We evaluated the relationship between blood flow velocity and dilatation of the brachial artery following transient forearm ischemia and acetylcholine-induced relaxation in subcutaneous small arteries and the influence of antihypertensive therapy on both in patients with essential hypertension. Thirty-one previously untreated hypertensive patients were randomized in a double-blind fashion to treatment with either the angiotensin-converting enzyme (ACE) inhibitor perindopril or the beta-blocker atenolol and compared with 17 healthy normotensive controls. Before and after 1 year of treatment, while still on active medication, flow-mediated dilatation (FMD) was measured in the brachial artery using ultrasound while relaxation to acetylcholine in small arteries was tested in vitro in a myograph. FMD correlated inversely to resting brachial artery diameter (r = -0.38, p<0.05). FMD corrected for resting diameter (FMD(corr)) was lower in patients (3.0+/-0.2%) compared with controls (4.2+/-0.3%, p<0.01). In both patients and controls, FMD(corr) was related to flow velocity in a non-linear way with FMD(corr) reaching a maximum despite increasing flow velocities, and in the patients, FMD(corr) was only reduced at high flow velocities. Furthermore, patients had a reduced acetylcholine-induced relaxation in small arteries (p = 0.04). Perindopril and atenolol reduced blood pressure to similar levels and both drugs improved FMD(corr) to a similar degree without any effects on relaxation to acetylcholine in small arteries. The present study demonstrates the role of correcting for differences in baseline diameter during measurements of FMD and a non-linear relationship between flow velocity and FMD in the brachial artery. Furthermore, the results suggest different effects of antihypertensive treatment on endothelial function in large and small arteries.

Novel approaches to improving endothelium-dependent nitric oxide-mediated vasodilatation

Endothelial dysfunction, which is defined by decreased endothelium-dependent vasodilatation, is associated with an increased number of cardiovascular events. Nitric oxide (NO) bioavailability is reduced by altered endothelial signal transduction or increased formation of radical oxygen species reacting with NO. Endothelial dysfunction is therapeutically reversible and physical exercise, calcium channel blockers, angiotensin converting enzyme inhibitors, and angiotensin receptor antagonists improve flow-evoked endothelium-dependent vasodilation in patients with hypertension and diabetes. We have investigated three different approaches, with the aim of correcting endothelial dysfunction in cardiovascular disease. Thus, (1) we evaluated the effect of a cell permeable superoxide dismutase mimetic, tempol, on endothelial dysfunction in small arteries exposed to high pressure, (2) investigated the endothelial signal transduction pathways involved in vasorelaxation and NO release induced by an olive oil component, oleanolic acid, and (3) investigated the role of calcium-activated K channels in the release of NO induced by receptor activation. Tempol increases endothelium-dependent vasodilatation in arteries from hypertensive animals most likely through the lowering of radical oxygen species, but other mechanisms also appear to contribute to the effect. While oleanolic acid leads to the release of NO by calcium-independent phosphorylation of endothelial NO synthase, endothelial calcium-activated K channels and an influx of calcium play an important role in G-protein coupled receptor-evoked release of NO. Thus, all three approaches increase bioavailability of NO in the vascular wall, but it remains to be addressed whether these actions have any direct benefit at a clinical level.

Oleanolic acid induces relaxation and calcium-independent release of endothelium-derived nitric oxide

BACKGROUND AND PURPOSE

The present study investigated the mechanisms by which oleanolic acid, a component of olive oil, increases release of nitric oxide (NO).

EXPERIMENTAL APPROACH

Measurements of isometric tension, NO concentration, or endothelial cell calcium were made in rat isolated mesenteric arteries. Immunoblotting for endothelial NOS (eNOS) and Akt kinase were performed in primary cultures of human umbilical vein endothelial cells (HUVECs).

KEY RESULTS

Oleanolic acid (3-30 microM) evoked endothelium-dependent relaxations in noradrenaline-contracted rat superior and small mesenteric arteries. In rat superior mesenteric arteries, oleanolic acid induced simultaneous increases in NO concentration and relaxation, and these responses were inhibited by an inhibitor of NOS, asymmetric dimethyl-L-arginine (300 microM) and by the NO scavenger, oxyhaemoglobin (10 microM). Oleanolic acid-evoked NO increases were not reduced in Ca(2+)-free solution and in the presence of an inhibitor of endoplasmic reticulum calcium-ATPase, thapsigargin (1 microM). Oleanolic acid evoked relaxation without changes in endothelial cell calcium, but decreased smooth muscle calcium in arterial segments. Oleanolic acid failed to increase calcium in HUVECs, but increased time-dependently phosphorylation of Akt kinase at Serine(473) (Akt-Ser(473)) and eNOS at Serine(1177) (eNOS-Ser(1177)), which was attenuated by inhibitors of phosphoinositide-3-kinase.

CONCLUSIONS AND IMPLICATIONS

This study provides direct evidence that a component of olive oil, oleanolic acid, activated endothelium-dependent release of NO and decreased smooth muscle cell calcium followed by relaxation. The oleanolic acid-evoked endothelium-derived NO release was independent of endothelial cell calcium and involved phosphoinositide-3-kinase-dependent phosphorylation of Akt-Ser(473) followed by phosphorylation of eNOS-Ser(1177).

Regulation of endothelial-dependent relaxation in human systemic arteries by SKCa and IKCa channels

Blockade of small-conductance Ca (2)(+)-activated K(+) channels (SK(Ca)) and intermediate conductance Ca(2)(+)-activated K(+) channels (IK(Ca)) can cause inhibition of endothelium-dependent hyperpolarizing factor (EDHF) in many vascular beds from animals, but there is a relative paucity of data in human vessels. Systemic arteries, isolated from women with healthy pregnancies, relax to the endothelial-dependent agonist bradykinin via a nonprostacyclin and non-nitric oxide pathway attributable to EDHF. Therefore, in this study, the authors investigated the effect of pharmacological blockade of SK(Ca) and IK(Ca) on EDHF-mediated relaxation of human omental and myometrial arteries preconstricted with either arginine vasopressin or U46619. Human arteries were isolated from omental and myometrial biopsies taken from healthy women undergoing planned cesarean section at term. Endothelial function was assessed using wire myography. In all vessels examined, nonspecific blockade of IK(Ca) with charybdotoxin attenuated EDHF-attributed relaxation. However, when Tram 34 was used to block IK(Ca), the attenuation of relaxation was evident only with U46619 preconstriction. In arteries from both vascular beds, and with either preconstrictor, a combination of either apamin and charybdotoxin or apamin plus Tram 34 almost ablated EDHF-attributable relaxation. These data support the notion that in human systemic arteries, activation of, primarily, SK(Ca) and IK(Ca)K(+) channel subtypes underlies EDHF-mediated relaxation. These results have important implications for future studies ascertaining the molecular mechanisms of hypertensive disorders (eg, preeclampsia, in which EDHF is thought to be aberrant).

Flow- and acetylcholine-induced dilatation in small arteries from rats with renovascular hypertension — effect of tempol treatment

We investigated whether renovascular hypertension alters vasodilatation mediated by nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) and the influence of the superoxide dismutase mimetic tempol on vasodilatation. One-kidney one-clip hypertensive Sprague-Dawley rats, treated with either vehicle or tempol (from weeks 5 to 10 after placement of the clip), and uninephrectomized control rats were investigated. In renal hypertensive rats systolic blood pressure increased to 171+/-6 mmHg (n=10), while in tempol-treated rats systolic blood pressure remained normal (139+/-7 mmHg, n=5). In isolated pressurized mesenteric small arteries NO-mediated dilatation was obtained by increasing flow rate and EDHF-mediated dilatation by acetylcholine. In arteries from hypertensive rats, flow-induced dilatation was blunted, as compared to normotensive and tempol-treated rats, while acetylcholine-induced dilatation remained normal. Measured by dihydroethidium staining there was an increased amount of superoxide in arteries from vehicle-treated rats, but not from tempol-treated rats. Expression by immunoblotting of endothelial NO synthase and the NAD(P)H oxidase subunit p47phox remained unaffected by high blood pressure and tempol treatment. Simultaneous measurements of NO-concentration and relaxation were performed in isolated coronary arteries from the same animals. As compared to vehicle-treated rats, both acetylcholine-induced relaxation and NO-concentration increased in arteries from tempol-treated animals, while only the relaxation was improved by the NO donor, S-nitroso-N-acetylpenicillamine (SNAP). In conclusion renovascular hypertension selectively inhibits flow-induced NO-mediated vasodilatation, while EDHF-type vasodilatation remains unaffected, suggesting that high blood pressure leads to increased generation of superoxide contributing to decreased NO bioavailability. Furthermore, the abnormal endothelium function can be corrected by tempol treatment, but this seems to involve mechanisms partly independent of NO.

Elevated pressure selectively blunts flow-evoked vasodilatation in rat mesenteric small arteries

BACKGROUND AND PURPOSE

The present study investigated mechanisms underlying impaired endothelium-dependent vasodilatation elicited by elevating the intraluminal pressure in rat mesenteric small arteries.

EXPERIMENTAL APPROACH

Arterial segments (internal diameter 316+/-2 microm, n=86) were mounted in a pressure myograph. The effect of elevating pressure from 50 to 120 mmHg for 1 h before resetting it to 50 mmHg was studied on endothelium-dependent vasodilatation.

KEY RESULTS

In arteries constricted with U46619 in the presence of indomethacin, shear stress generated by flow, evoked vasodilatation that was abolished by an inhibitor of nitric oxide (NO) synthase, asymmetric dimethylarginine (1 mM), whereas acetylcholine-induced vasodilatation was unchanged. After elevation of intraluminal pressure for 1 h and then resetting it to 50 mmHg, vasodilatation induced by shear stress and the NO donor, S-nitrosopenicillamine was inhibited, while vasodilatation induced by a guanylyl cyclase activator, BAY 412272, and acetylcholine was unaltered. Superoxide levels sensitive to polyethylene glycol superoxide dismutase were increased in segments exposed to elevated pressure. A superoxide scavenger, tempol (300 microM), a general endothelin receptor antagonist, SB 217242 and the selective ET(A) receptor antagonist, BQ 123 preserved shear stress-evoked vasodilatation.

CONCLUSIONS AND IMPLICATIONS

The present study shows that transient exposure to an elevated intraluminal pressure selectively inhibits flow-evoked NO-mediated vasodilatation, probably through activation of endothelin receptors and increased formation of superoxide. In contrast, elevation of pressure did not affect the acetylcholine-evoked endothelium-derived hyperpolarizing factor type vasodilatation in mesenteric small arteries.

Recent advances in understanding endothelial dysfunction in atherosclerosis

Over the last two decades, it has become evident that decreased bioavailability of endothelial nitric oxide (NO) produced from endothelial NO synthase (eNOS), referred to as endothelial dysfunction, plays a crucial role in the development and progression of atherosclerosis. Much progress has been made in understanding the mechanisms of decreased endothelial NO bioavailability at the levels of regulation of eNOS gene expression, eNOS enzymatic activity and NO inactivation. Initial studies suggest that increasing eNOS gene expression would improve endothelial NO release in the hope of inhibiting the progression of atherosclerosis. Recent experimental studies, however, do not always support this therapeutic concept and show some evidence that overexpression of eNOS in atherosclerosis may be even harmful for the disease progression.Thus, recent research to improve endothelial function in atherosclerosis has focused on regulation of eNOS enzymatic activity and prevention of NO inactivation by oxidative stress. Since the role of oxidative stress in endothelial NO bioavailability has been reviewed in a large number of comprehensive articles, this article focuses on the relevant regulatory mechanisms of eNOS enzymatic activity that are emerging to play a role in endothelial dysfunction in atherosclerosis.

Chronic Mild Hyperhomocysteinemia Induces Aortic Endothelial Dysfunction but Does Not Elevate Arterial Pressure in Rats

Mild hyperhomocysteinemia is prevalent in the general population and has been linked to endothelial dysfunction and high arterial pressure (AP) in clinical studies. The present study was designed to determine whether a rise in AP was induced by mild hyperhomocysteinemia and whether the potential rise in AP is secondary or prior to endothelial dysfunction. Experiments were performed in a rat model of mild hyperhomocysteinemia induced by oral administration of homocysteine for 1-4 months. Aortic endothelial dysfunction was observed 2 months after homocysteine treatment while endothelium-independent vasodilation was normal. In parallel, homocysteine treatment increased phenylephrine-induced contraction in aortas with endothelium, but did not modify the contraction in aortas without endothelium, suggesting a decrease of basal NO production. In conscious unrestrained rats, AP was not significantly different 1, 2, 3 and 4 months after homocysteine treatment. In correlation, endothelial function of a resistance vessel (mesenteric artery), mainly non-NO nonprostanoid factor mediated, was preserved, indicating that homocysteine treatment only affected the NO pathway. In conclusion, mild hyperhomocysteinemia alone is not sufficient to elevate arterial blood pressure, at least in the rat model. Aortic endothelial dysfunction produced by mild hyperhomocysteinemia is independent of hemodynamic factors.

The impact of subchronic lead poisoning on the vascular effect of nitric oxide in rats

Lead-induced arterial hypertension is suggested to have resulted mainly from a reduction in nitric oxide (NO) bioactivity in vessel walls. The aim of this study was to evaluate the impact of poisoning by lead in so-called hypertensive doses on the basal and stimulated released NO effect in the rat mesenteric bed. Male Buffalo rats were given lead in a dose of 50 or 100ppm in drinking water for three months. The isolated mesenteric bed preconstricted by norepinephrine (0.5μg/ml) was used to determine the changes in vascular resistance induced by N-ω-nitro-l-arginine injected in increasing doses from 1.0 to 200.0μg or by acetylcholine administered in doses from 0.05 × 10(-10) to 5.0 × 10(-10)mol. These changes were measured as an increase or decrease in perfusion pressure in the constant flow system. In comparison with controls rats given 50ppm of lead, an increase in maximal response to N-ω-nitro-l-arginine (P < 0.01) and acetylcholine (P < 0.05) and a shift to the left of the dose-response curve for acetylcholine were demonstrated. Vascular responses in rats, who were given 100ppm of lead, were similar to those observed in the control group. It is concluded that lead induces NO-mediated changes of vascular tone and vascular reactivity only in the small range of doses known as hypertensive.

Spreading vasodilatation in resistance arteries

Focal application of vasodilators such as acetylcholine (ACh), which evoke arterial hyperpolarization, cause coordinated dilatation along the length of an artery with minimal decay with distance from the site of application. This phenomenon is called spreading vasodilatation. In an artery wall, the endothelium is separated from the surrounding smooth muscle cell layers by an internal elastic lamina (IEL). Adjacent endothelial cells are strongly connected via gap junctions, which can allow direct communication between the cells, including the passage of small molecules and electrical current. Direct communication between an endothelial cell and a smooth muscle cell, through a hole in the IEL, has recently been observed in arteries. Spreading vasodilatation is associated with a spread of hyperpolarization which may be a key mechanism responsible for this spreading arterial vasodilatation. Endothelial cells appear to play an important role in such spread, even though the facilitating mechanisms underlying this spread are as yet unclear. These spreading responses are likely to have an important physiological role in the coordination of blood flow within a vascular network.

Spreading dilatation in rat mesenteric arteries associated with calcium-independent endothelial cell hyperpolarization

Both ACh and levcromakalim evoke smooth muscle cell hyperpolarization and associated relaxation in rat mesenteric resistance arteries. We investigated if they could evoke conducted vasodilatation along isolated arteries, whether this reflected spreading hyperpolarization and the possible mechanism involved. Focal micropipette application of either ACh, to stimulate endothelial cell muscarinic receptors, or levcromakalim, to activate smooth muscle K(ATP) channels, each evoked a local dilatation (88 +/- 14%, n= 6 and 92 +/- 6% reversal of phenylephrine-induced tone, n= 11, respectively) that rapidly spread upstream (at 1.5 mm 46 +/- 19%, n= 6 and 57 +/- 13%, n= 9) to dilate the entire isolated artery. The local dilatation to ACh was associated with a rise in endothelial cell [Ca(2+)](i) (F/F(t = 0)= 1.22 +/- 0.33, n= 14) which did not spread beyond 0.5 mm (F/F(t = 0)= 1.01 +/- 0.01, n= 14), while the local dilatation to levcromakalim was not associated with any change in endothelial cell [Ca(2+)](i). In contrast, ACh and levcromakalim both stimulated local (12.7 +/- 1.2 mV, n= 10 and 13.5 +/- 4.7 mV, n= 10) and spreading (at 2 mm: 3.0 +/- 1.1 mV, n= 5 and 4.1 +/- 0.7 mV, n= 5) smooth muscle hyperpolarization. The spread of hyperpolarization could be prevented by cutting the artery, so was not due to a diffusible agent. Both the spreading dilatation and hyperpolarization were endothelium dependent. The injection of propidium iodide into either endothelial or smooth muscle cells revealed extensive dye coupling between the endothelial cells, but limited coupling between the smooth muscle cells. Some evidence for heterocellular spread of dye was also evident. Together, these data show that vasodilatation can spread over significant distances in mesenteric resistance arteries, and suggest this reflects an effective coupling between the endothelial cells to facilitate [Ca(2+)](i)-independent spread of hyperpolarization.