Role of endothelium-derived relaxing factor and prostaglandins in responses of coronary arteries to thromboxane in vivo

Activation of Nrf2 in Mice Causes Early Microvascular Cyclooxygenase-Dependent Oxidative Stress and Enhanced Contractility

Nuclear factor erythroid factor E2-related factor 2 (Nrf2) transcribes antioxidant genes that reduce the blood pressure (BP), yet its activation with tert-butylhydroquinone (tBHQ) in mice infused with angiotensin II (Ang II) increased mean arterial pressure (MAP) over the first 4 days of the infusion. Since tBHQ enhanced cyclooxygenase (COX) 2 expression in vascular smooth muscle cells (VSMCs), we tested the hypothesis that tBHQ administration during an ongoing Ang II infusion causes an early increase in microvascular COX-dependent reactive oxygen species (ROS) and contractility. Mesenteric microarteriolar contractility was assessed on a myograph, and ROS by RatioMaster™. Three days of oral tBHQ administration during the infusion of Ang II increased the mesenteric microarteriolar mRNA for p47^phox, the endothelin type A receptor and thromboxane A₂ synthase, and increased the excretion of 8-isoprostane F_2α and the microarteriolar ROS and contractions to a thromboxane A₂ (TxA₂) agonist (U-46,619) and endothelin 1 (ET1). These were all prevented in Nrf2 knockout mice. Moreover, the increases in ROS and contractility were prevented in COX1 knockout mice with blockade of COX2 and by blockade of thromboxane prostanoid receptors (TPRs). In conclusion, the activation of Nrf2 over 3 days of Ang II infusion enhances microarteriolar ROS and contractility, which are dependent on COX1, COX2 and TPRs. Therefore, the blockade of these pathways may diminish the early adverse cardiovascular disease events that have been recorded during the initiation of Nrf2 therapy.

TP receptor-mediated vasoconstriction in microperfused afferent arterioles: roles of O(2)(-) and NO

Thromboxane A(2) (TxA(2)) preferentially constricts the renal afferent arteriole. Nitric oxide (NO) modulates vasoconstriction and is rapidly degraded by superoxide radical (O(2)(-)). We investigated the roles of NO and O(2)(-) in rabbit isolated, perfused renal afferent arteriole responses to the TxA(2)/prostaglandin H(2) (TP) receptor agonist U-46,619. U-46,619 (10(-10)-10(-6) M) dose-dependently reduced afferent arteriolar luminal diameter (ED(50) = 7.5 +/- 5.0 nM), which was blocked by the TP receptor antagonist ifetroban (10(-6) M). Tempol (10(-3) M) pretreatment, which prevented paraquat-induced vasoconstriction in afferent arterioles, blocked the vasoconstrictor responses to U-46,619. To test whether U-46,619 stimulates NO and whether tempol prevents U-46, 619-induced vasoconstriction by enhancing the biological activity of NO, we examined the luminal diameter response to U-46,619 in arterioles pretreated with N(w)-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or L-NAME + tempol. During L-NAME, the sensitivity and maximal responses of the afferent arteriole to U-46, 619 were significantly (P < 0.05) enhanced. Moreover, L-NAME restored a vasoconstrictor response to U-46,619 in vessels pretreated with tempol. In conclusion, in isolated perfused renal afferent arterioles TP receptor activation stimulates NO production, which buffers the vasoconstriction, and stimulates O(2)(-) production, which mediates the vasoconstriction, in part, through interaction with NO.

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

Regional involvement of an endothelium-derived contractile factor in the vasoactive actions of neuropeptide Y in bovine isolated retinal arteries

1. In vitro experiments in a microvascular myograph were designed in order to investigate the effects of human neuropeptide Y (NPY), its receptor subtype and the mechanisms underlying NPY actions in bovine isolated retinal proximal (PRA) and distal (DRA) arteries. 2. A single concentration of NPY (10 nM) induced a prompt and reproducible contraction which reached a plateau within 1-4 min, after which the response returned to baseline over the next 2-10 min. Cumulative addition of NPY induced concentration-dependent contractions of bovine retinal arteries, with an EC50[M] of 1.7 nM and a maximal response equal to 54 +/- 8% of Emax (absolute maximal contractile levels of vessels) and not different from that obtained by a single addition of the peptide. There were no significant differences in either sensitivity or maximal response to NPY between PRA and DRA. 3. Porcine NPY and the selective Y1-receptor agonist, [Pro34]NPY, also induced concentration-dependent contractions of the retinal arteries with a potency and maximal response not significantly different from those of human NPY; in contrast, the selective Y2-receptor agonist, NPY(13-36), caused only a 5% contraction at the highest concentration used. 4. Removal of extracellular Ca2+ or pretreatment with the 1,4-dihydropyridine Ca(2+)-channel blocker, nifedipine (1 microM), reduced the contractile response of 10 nM NPY to 18.4 +/- 3.3% (n = 6) and 18.6 +/- 3.9% (n = 6); respectively, of the controls. 5. Mechanical removal of the endothelium depressed the maximal contraction elicited by NPY in PRA but did not affect either sensitivity or maximal response to the peptide in DRA. In endothelium-intact arteries, blockade of the cyclo-oxygenase pathway with 3 microM indomethacin increased resting tension in both PRA and DRA and significantly inhibited sensitivity and maximal contraction to NPY of PRA and DRA, respectively. The thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist, SQ30741, reduced both sensitivity and maximal contraction to NPY in PRA but not in DRA. 6. In endothelium-denuded PRA, indomethacin but not SQ30741 significantly reduced NPY maximal response and induced a marked increase in resting tension suggesting a basal release of a vasodilator prostanoid from smooth muscle cells. 7. Superoxide dismutase (SOD) (150 u ml-1) reduced the maximal contraction to NPY in PRA. Inhibition of the nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NOARG) (30 microM), enhanced sensitivity and maximal contraction to NPY in both PRA and DRA. In the presence of L-NOARG, SOD did not further inhibit NPY responses in PRA. 8. NPY (10 nM) induced a 2.9 fold leftwards shift of the noradrenaline concentration-response curves in PRA and increased maximal response by 50 +/- 16%. Neither 1 nor 10 nM NPY affected noradrenaline responses in DRA. [Pro34]NPY (10 nM), but not NPY(13-36), mimicked the potentiating effect of NPY on noradrenaline responses in PRA. 9. TXA2 analogue, U46619, at 10 nM elicited 3.6 fold leftwards shift of the noradrenaline concentration-responses curves in PRA and increased the maximal contraction by 32 +/- 3%, whereas in the presence of 1 microM SQ30741, 10 nM NPY did not potentiate noradrenaline responses. 10. The present results indicate that NPY may play a role in the regulation of retinal blood flow through both a direct contractile action, independent of the vessel size and a potentiation of the responses induced by noradrenaline in the proximal part of the retinal circulation, both effects being mediated by Y1 receptors. NPY promotes Ca2+ influx through voltage-dependent Ca2+ channels and stimulates the synthesis of contractile prostanoids in PRA and DRA, although only in PRA does the peptide trigger the release of an endothelium-derived contractile factor which facilitates the contraction and also seems to account for the potentiating effect of NPY.

Coronary artery spasm. Multiple causes and multiple roles in heart disease

Myocardial infarction and sudden cardiac death may be initiated by a sudden intense localized contraction of coronary artery smooth muscle. When this event occurs around a vulnerable eccentric lipid-filled plaque, rupture and extrusion of plaque contents and exposure of collagen occur. This may sometimes be a silent and self-limiting event; other times it leads to thrombus formation. A second wave of spasm due to accumulated platelet and inflammatory mediators may compound the contractile consequences of the initiating event. Spasm involves intrinsic smooth muscle cell electrical mechanisms, hyper-responsive cells, and multiple agonists that synergize their actions, and the involvement of each mechanism varies at different times in the sequence of vascular occlusion. Study of spasm requires vascular systems that adequately model coronary artery responses of the ageing human heart. As previously emphasized, tissues obtained postmortem, and when possible from recipients during heart transplants, must be integral to theory building, alongside animal models, despite the experimental limitations such tissues impose. A multidisciplinary approach, at all levels of vascular physiology and pharmacology, will be necessary to understand coronary motor activity and human heart disease.

Regional vascular responses to thromboxane A2 analogue and their blockade with vapiprost, a selective thromboxane receptor blocking drug, in anesthetized dogs

Regional vascular responses to the thromboxane A2 analogue U46619 and effects of the selective thromboxane receptor blocking drug vapiprost on these responses were examined in anesthetized dogs. Hemodynamic responses to U46619 (0.5 micrograms/kg into the left atrium), norepinephrine (NE, 0.3 microgram/kg, i.v.) and angiotensin II (AII, 30 or 60 ng/kg, i.v.) were periodically tested before and after administration of vapiprost (10, 30 or 100 micrograms/kg, i.v.) or its vehicle. In the absence of vapiprost, U46619 increased total peripheral (TPR), vertebral (VR), coronary (CR) and renal (RR) vascular resistance by 60.1 +/- 4.7%, 33.6 +/- 4.9%, 15.3 +/- 1.3% and 120.8 +/- 17.4%, respectively, indicating that vasoconstrictor responses to U46619 were most prominent in the renal vascular bed as compared to those in the vertebral or coronary vasculatures. Vapiprost as well as the vehicle did not affect the base-line hemodynamics. However, vapiprost apparently inhibited the U46619-induced vasoconstriction in all measured vascular beds in a dose-related manner without attenuating vasoconstrictor responses to NE compared to the inhibitions of VR and CR. These results demonstrate that there was a regional difference both in the vasoconstrictor responses to U46619 and in the blocking effects of vapiprost, and indicate that vapiprost is a potent and selective antagonist for thromboxane receptors in vivo.

Progression of coronary endothelial dysfunction in man and its potential clinical significance

Endothelial injury represents an important factor in the initiation of atherosclerosis and is associated with abnormal vasomotor responses (= dysfunctional endothelium) to a variety of stimuli. Therefore, the evaluation of endothelial function may provide a means to detect early vascular alteration preceding overt atherosclerotic lesions. To test this clinically important hypothesis, we studied the coronary vasomotor responses to three different endothelium-dependent stimuli in patients with different early stages of coronary artery disease: first, increasing doses of intracoronary infusion of acetylcholine (ACH) (10(-8), 10(-7), 10(-6) M); second, transient increases in coronary flow causing flow-dependent dilation by injection of papaverine into the midportion of the left descending artery (exposing the proximal segment of this vessel to increased flow, but not to papaverine); and third, sympathetic stimulation by cold pressor test. Coronary diameters were assessed by repeated coronary angiography and quantitative angiography, blood flow velocity (and subsequent calculation of blood flow) was obtained by intracoronary Doppler. In normal individuals, all three stimuli elicited epicardial artery dilation. In patients with smooth coronary arteries, but hypercholesterolemia, a substantial vasoconstriction was observed in response to ACH, whereas the response to cold pressor test and increases in flow was normal, that is, vasodilation occurred. In patients with angiographically visible atherosclerosis, acetylcholine and cold pressor test exerted coronary vasoconstriction. Moreover, flow-dependent, endothelium-mediated dilation was attenuated in those patients demonstrating visible luminal irregularities in the vessel under study. In patients with hypercholesterolemia, substantial endothelial dysfunction (as assessed by attenuated blood-flow increase in response to acetylcholine) was demonstrated in the coronary microcirculation. Thus, progression of endothelial dysfunction occurs during the early course of development of coronary atherosclerosis. Utilizing quantitative coronary angiography and the intracoronary Doppler technique, these early functional alterations can be identified safely at a stage when atherosclerotic lesions are not detectable by angiography. This may be useful in designing early effective interventions which restore endothelial function and prevent the occurrence of overt atherosclerosis.