Cross talk of shear-induced production of prostacyclin and nitric oxide in endothelial cells

Intermittent compression induces transitory hypoxic stimuli, upstream vasodilation and enhanced perfusion of skin capillaries, independent of age and diabetes

The benefit of enhanced shear stress to the vascular endothelium has been well-documented in conduit arteries but is less understood in skin microcirculation. The aim of this study was to provide physiological evidence of the vascular changes in skin microcirculation induced by intermittent pneumatic compression (IPC) of 1 s cuff inflation (130 mmHg) every 20 s to the palm of the hand for 30 min. The oxygenation and hemodynamics of dorsal mid-phalangeal finger skin microcirculation were assessed by laser Doppler fluximetry and reflectance spectroscopy before, during, and after IPC in 15 young (18-39 years old) and 39 older (40-80 years old) controls and 32 older subjects with type 2 diabetes mellitus. Each individual cuff inflation induced: 1) brief surge in flux immediately after cuff deflation followed by 2) transitory reduction in blood oxygen for ∼4 s, and 3) a second increase in perfusion and oxygenation of the microcirculation peaking ∼11 s after cuff deflation in all subject groups. With no significant change in blood volume observed by reflectance spectroscopy, despite the increased shear stress at the observed site, this second peak in flux and blood oxygen suggests a delayed vasoactive response upstream inducing increased arterial influx in the microcirculation that was higher in older controls and subjects with diabetes compared to young controls (P < 0.001, P < 0.001, respectively) and achieving maximum capillary recruitment in all subject groups. Transitory hypoxic stimuli with conducted vasodilation may be a mechanism through which IPC enhances capillary perfusion in skin microcirculation independent of age and type 2 diabetes mellitus.NEW & NOTEWORTHY This study demonstrates that hand intermittent pneumatic compression evokes transitory hypoxic stimuli in distal finger skin microcirculation inducing vasodilation of arterial inflow vessels, enhanced perfusion, and maximum capillary recruitment in young and older subjects and older subjects with type 2 diabetes mellitus. Enhanced shear stress in the microcirculation did not appear to induce local skin vasodilation.

Dependency of flow-mediated vasodilatation from basal nitric oxide activity

BACKGROUND

Flow-mediated vasodilatation (FMD) has become one of the most widely assessed parameters to analyse endothelial and vascular function in cardiovascular medicine. The degree of contribution of nitric oxide (NO) to FMD is inconclusive and varies widely depending on the device used. In this study, we used a semi-automatic ultrasound device to analyse to what extent basal NO activity contributes to FMD of the brachial artery.

METHODS

FMD was assessed with the UNEX EF device in a cross-over single blinded randomized study at baseline and then during infusion of either a NO-synthase-inhibitor (NG-monomethyl-L-arginine (L-NMMA)) or saline. The analysis was repeated after 1 week with the alternative infusion of L-NMMA or saline. All measurements were analysed both automatically and by a technician manually.

RESULTS

In total, 25 healthy men subjects completed the study. Diastolic blood pressure and heart rate significantly changed during infusion of L-NMMA. Infusion of L-NMMA reduced FMD significantly (-37%, p = 0.002). Saline solution had no effect on FMD (+14%, p = 0.392). Change in FMD was significantly different between the groups (ΔFMD_L-NMMA vs. ΔFMD_saline , p = 0.032). There was a statistically significant correlation between automatically analysed results and those obtained by an experienced technician (FMD_saline : r = 0.822, p < 0.001; FMD_L-NMMA : r = 0.645, p = 0.007).

CONCLUSION

The influence of NO on FMD is approximately 40% if assessed using the UNEX EF. Prior to use FMD as a marker of endothelial dysfunction, we should explore different methods including various duration of forearm ischaemia to increase NO dependency of FMD.

Reduction in MicroRNA-4488 Expression Induces NFκB Translocation in Venous Endothelial Cells Under Arterial Flow

PURPOSE

Little is known about the molecular interactions among inflammatory responses that damage venous endothelial cells (vECs) during venous-to-arterial flow transition in vein graft diseases. Because arterial flow triggers excessive autophagy and inflammation in vECs, this study aimed to investigate the mediator of inflammation and methods to prevent vEC damage.

METHODS

Arterial laminar shear stress (ALSS; 12 dynes/cm²) was applied to vECs via in vitro and ex vivo perfusion systems. Inflammation in vECs was measured using inflammatory protein markers, NFκB translocation, cyclooxygenase-2 (COX-2) and COX-2 and NFκB promoter assays. The involvement of microRNA-4488 (miR-4488) was measured and confirmed by altering the specific miR using a miR-4488 mimic or inhibitor. The potential anti-inflammatory drugs and/or nitric oxide (NO) donor L-arginine (L-Arg) to prevent damage to vECs under ALSS was investigated.

RESULTS

ALSS triggered reactive oxygen species production, excessive autophagy, COX-2 protein expression, and NFκB translocation during vEC inflammation. Reduction in miR-4488 expression was detected in inflamed vECs treated with LPS, lipopolysaccharide (LPS) TNFα, and ALSS. Transfection of miR-4488 mimic (50 nM) prior to ALSS application inhibited the accumulation of inflammatory proteins as well as the translocation of NFκB. Combined treatment of vECs with COX-2-specific inhibitor (SC-236) and L-Arg alleviated the ALSS-induced inflammatory responses. Protective effects of the combined treatment on vECs against ALSS-induced damage were abolished by the application of miR-4488 inhibitor.

CONCLUSION

We showed that ALSS triggered the COX-2/NFκB pathway to induce vEC inflammation with a reduction in miR-4488. Combination of SC-236 and L-Arg prevented ALSS-induced vEC damage, thus, shows high potential for preventing vein graft diseases.

Epithelial Na+ channel differentially contributes to shear stress-mediated vascular responsiveness in carotid and mesenteric arteries from mice

A potential "new player" in arteries for mediating shear stress responses is the epithelial Na⁺ channel (ENaC). The contribution of ENaC as shear sensor in intact arteries, and particularly different types of arteries (conduit and resistance), is unknown. We investigated the role of ENaC in both conduit (carotid) and resistance (third-order mesenteric) arteries isolated from C57Bl/6J mice. Vessel characteristics were determined at baseline (60 mmHg, no flow) and in response to increased intraluminal pressure and shear stress using a pressure myograph. These protocols were performed in the absence and presence of the ENaC inhibitor amiloride (10 µM) and after inhibition of endothelial nitric oxide synthase (eNOS) by N^ω-nitro-l-arginine methyl ester (l-NAME; 100 µM). Under no-flow conditions, amiloride increased internal and external diameters of carotid (13 ± 2%, P < 0.05) but not mesenteric (0.5 ± 0.9%, P > 0.05) arteries. In response to increased intraluminal pressure, amiloride had no effect on the internal diameter of either type of artery. However, amiloride affected the stress-strain curves of mesenteric arteries. With increased shear stress, ENaC-dependent effects were observed in both arteries. In carotid arteries, amiloride augmented flow-mediated dilation (9.2 ± 5.3%) compared with control (no amiloride, 6.2 ± 3.3%, P < 0.05). In mesenteric arteries, amiloride induced a flow-mediated constriction (-11.5 ± 6.6%) compared with control (-2.2 ± 4.5%, P < 0.05). l-NAME mimicked the effect of ENaC inhibition and prevented further amiloride effects in both types of arteries. These observations indicate that ENaC contributes to shear sensing in conduit and resistance arteries. ENaC-mediated effects were associated with NO production but may involve different (artery-dependent) downstream signaling pathways. NEW & NOTEWORTHY The epithelial Na⁺ channel (ENaC) contributes to shear sensing in conduit and resistance arteries. In conduit arteries ENaC has a role as a vasoconstrictor, whereas in resistance arteries ENaC contributes to vasodilation. Interaction of ENaC with endothelial nitric oxide synthase/nitric oxide signaling to mediate the effects is supported; however, cross talk with other shear stress-dependent signaling pathways cannot be excluded.

Relations among Adiposity and Insulin Resistance with Flow-Mediated Dilation, Carotid Intima-Media Thickness, and Arterial Stiffness in Children

OBJECTIVE

To determine the associations of adiposity and insulin resistance with measures of vascular structure and function in children.

STUDY DESIGN

A cross-sectional study included 252 children (age 15.1 ± 2.4 years; body mass index percentile 68.2 ± 26.5%; Tanner 2-5). Measurements of body fat percentage were obtained with dual-energy X-ray absorptiometry and visceral adipose tissue (VAT) with computed tomography. Insulin resistance was measured with hyperinsulinemic euglycemic clamp. Vascular measurements for endothelial function (brachial artery flow-mediated dilation [FMD]), vascular structure (carotid intima-media thickness [cIMT]), vascular stiffness (carotid incremental elastic modulus), and pulse wave velocity were analyzed by tertiles of adiposity and insulin resistance. Additional analyses with ANCOVA and linear regression were adjusted for Tanner, sex, race, and family relationship; FMD was also adjusted for baseline artery diameter.

RESULTS

FMD was positively associated with high adiposity (body mass index, body fat percentage, and VAT) (P < .01 all). Insulin resistance was not associated with FMD. cIMT was significantly, positively related to obesity, VAT, and insulin resistance (P < .05 all). No differences in carotid incremental elastic modulus and pulse wave velocity were observed in relation to adiposity or insulin resistance.

CONCLUSIONS

The findings suggest that adiposity is associated with higher FMD, and insulin resistance and VAT are associated with higher cIMT in children. Further research is needed to clarify the progression of these relations.

Preserved Microvascular Endothelial Function in Young, Obese Adults with Functional Loss of Nitric Oxide Signaling

Data indicate endothelium-dependent dilation (EDD) may be preserved in the skeletal muscle microcirculation of young, obese adults. Preserved EDD might be mediated by compensatory mechanisms, impeding insight into preclinical vascular dysfunction. We aimed to determine the functional roles of nitric oxide synthase (NOS) and cyclooxygenase (COX) toward EDD in younger obese adults. We first hypothesized EDD would be preserved in young, obese adults. Further, we hypothesized a reduced contribution of NOS in young, obese adults would be replaced by increased COX signaling. Microvascular EDD was assessed with Doppler ultrasound and brachial artery infusion of acetylcholine (ACh) in younger (27 ± 1 year) obese (n = 29) and lean (n = 46) humans. Individual and combined contributions of NOS and COX were examined with intra-arterial infusions of l-NMMA and ketorolac, respectively. Vasodilation was quantified as an increase in forearm vascular conductance (ΔFVC). Arterial endothelial cell biopsies were analyzed for protein expression of endothelial nitric oxide synthase (eNOS). ΔFVC to ACh was similar between groups. After l-NMMA, ΔFVC to ACh was greater in obese adults (p < 0.05). There were no group differences in ΔFVC to ACh with ketorolac. With combined NOS-COX inhibition, ΔFVC was greater in obese adults at the intermediate dose of ACh. Surprisingly, arterial endothelial cell eNOS and phosphorylated eNOS were similar between groups. Younger obese adults exhibit preserved EDD and eNOS expression despite functional dissociation of NOS-mediated vasodilation and similar COX signaling. Compensatory NOS- and COX-independent vasodilatory mechanisms conceal reduced NOS contributions in otherwise healthy obese adults early in life, which may contribute to vascular dysfunction.

Peripheral vascular reactivity and serum BDNF responses to aerobic training are impaired by the BDNF Val66Met polymorphism

Besides neuronal plasticity, the neurotrophin brain-derived neurotrophic factor (BDNF) is also important in vascular function. The BDNF has been associated with angiogenesis through its specific receptor tropomyosin-related kinase B (TrkB). Additionally, Val66Met polymorphism decreases activity-induced BDNF. Since BDNF and TrkB are expressed in vascular endothelial cells and aerobic exercise training can increase serum BDNF, this study aimed to test the hypotheses: 1) Serum BDNF levels modulate peripheral blood flow; 2) The Val66Met BDNF polymorphism impairs exercise training-induced vasodilation. We genotyped 304 healthy male volunteers (Val66Val, n = 221; Val66Met, n = 83) who underwent intense aerobic exercise training on a running track three times/wk for 4 mo. We evaluated pre- and post-exercise training serum BDNF and proBDNF concentration, heart rate (HR), mean blood pressure (MBP), forearm blood flow (FBF), and forearm vascular resistance (FVR). In the pre-exercise training, BDNF, proBDNF, BDNF/proBDNF ratio, FBF, and FVR were similar between genotypes. After exercise training, functional capacity (V̇o2 peak) increased and HR decreased similarly in both groups. Val66Val, but not Val66Met, increased BDNF (interaction, P = 0.04) and BDNF/proBDNF ratio (interaction, P < 0.001). Interestingly, FBF (interaction, P = 0.04) and the FVR (interaction, P = 0.01) responses during handgrip exercise (HG) improved in Val66Val compared with Val66Met, even with similar responses of HR and MBP. There were association between BDNF/proBDNF ratio and FBF (r = 0.64, P < 0.001) and FVR (r = -0.58, P < 0.001) during HG exercise. These results show that peripheral vascular reactivity and serum BDNF responses to exercise training are impaired by the BDNF Val66Met polymorphism and such responsiveness is associated with serum BDNF concentrations in healthy subjects.

Effects of exercise training on stress-induced vascular reactivity alterations: role of nitric oxide and prostanoids

BACKGROUND

Physical exercise may modify biologic stress responses.

OBJECTIVE

To investigate the impact of exercise training on vascular alterations induced by acute stress, focusing on nitric oxide and cyclooxygenase pathways.

METHOD

Wistar rats were separated into: sedentary, trained (60-min swimming, 5 days/week during 8 weeks, carrying a 5% body-weight load), stressed (2 h-immobilization), and trained/stressed. Response curves for noradrenaline, in the absence and presence of L-NAME or indomethacin, were obtained in intact and denuded aortas (n = 7-10).

RESULTS

None of the procedures altered the denuded aorta reactivity. Intact aortas from stressed, trained, and trained/stressed rats showed similar reduction in noradrenaline maximal responses (sedentary 3.54 ± 0.15, stressed 2.80 ± 0.10*, trained 2.82 ± 0.11*, trained/stressed 2.97 ± 0.21*, *P < 0.05 relate to sedentary). Endothelium removal and L-NAME abolished this hyporeactivity in all experimental groups, except in trained/stressed rats that showed a partial aorta reactivity recovery in L-NAME presence (L-NAME: sedentary 5.23 ± 0,26#, stressed 5.55 ± 0.38#, trained 5.28 ± 0.30#, trained/stressed 4.42 ± 0.41, #P < 0.05 related to trained/stressed). Indomethacin determined a decrease in sensitivity (EC50) in intact aortas of trained rats without abolishing the aortal hyporeactivity in trained, stressed, and trained/stressed rats.

CONCLUSIONS

Exercise-induced vascular adaptive response involved an increase in endothelial vasodilator prostaglandins and nitric oxide. Stress-induced vascular adaptive response involved an increase in endothelial nitric oxide. Beside the involvement of the endothelial nitric oxide pathway, the vascular response of trained/stressed rats involved an additional mechanism yet to be elucidated. These findings advance on the understanding of the vascular processes after exercise and stress alone and in combination.

Skeletal muscle vasodilation during systemic hypoxia in humans

In humans, the net effect of acute systemic hypoxia in quiescent skeletal muscle is vasodilation despite significant reflex increases in muscle sympathetic vasoconstrictor nerve activity. This vasodilation increases tissue perfusion and oxygen delivery to maintain tissue oxygen consumption. Although several mechanisms may be involved, we recently tested the roles of two endothelial-derived substances during conditions of sympathoadrenal blockade to isolate local vascular control mechanisms: nitric oxide (NO) and prostaglandins (PGs). Our findings indicate that 1) NO normally plays a role in regulating vascular tone during hypoxia independent of the PG pathway; 2) PGs do not normally contribute to vascular tone during hypoxia, however, they do affect vascular tone when NO is inhibited; 3) NO and PGs are not independently obligatory to observe hypoxic vasodilation when assessed as a response from rest to steady-state hypoxia; and 4) combined NO and PG inhibition abolishes hypoxic vasodilation in human skeletal muscle. When the stimulus is exacerbated via combined submaximal rhythmic exercise and systemic hypoxia to cause further red blood cell (RBC) deoxygenation, skeletal muscle blood flow is augmented compared with normoxic exercise via local dilator mechanisms to maintain oxygen delivery to active tissue. Data obtained in a follow-up study indicate that combined NO and PG inhibition during hypoxic exercise blunts augmented vasodilation and hyperemia compared with control (normoxic) conditions by ∼50%; however, in contrast to hypoxia alone, the response is not abolished, suggesting that other local substances are involved. Factors associated with greater RBC deoxygenation such as ATP release, or nitrite reduction to NO, or both likely play a role in regulating this response.

Contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during KIR channel and Na(+) /K(+) -ATPase inhibition

KEY POINTS

During exercise there is a balance between vasoactive factors that facilitate increases in blood flow and oxygen delivery to the active tissue and the sympathetic nervous system, which acts to limit muscle blood flow for the purpose of blood pressure regulation. Functional sympatholysis describes the ability of contracting skeletal muscle to blunt the stimulus for vasoconstriction, yet the underlying signalling of this response in humans is not well understood. We tested the hypothesis that activation of inwardly rectifying potassium channels and the sodium-potassium ATPase pump, two potential vasodilator pathways within blood vessels, contributes to the ability to blunt α1 -adrenergic vasoconstriction. Our results show preserved blunting of α1 -adrenergic vasconstriction despite blockade of these vasoactive factors. Understanding this complex phenomenon is important as it is impaired in a variety of clinical populations.

ABSTRACT

Sympathetic vasoconstriction in contracting skeletal muscle is blunted relative to that which occurs in resting tissue; however, the mechanisms underlying this 'functional sympatholysis' remain unclear in humans. We tested the hypothesis that α1 -adrenergic vasoconstriction is augmented during exercise following inhibition of inwardly rectifying potassium (KIR ) channels and Na(+) /K(+) -ATPase (BaCl2  + ouabain). In young healthy humans, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) at rest, during steady-state stimulus conditions (pre-phenylephrine), and after 2 min of phenylephrine (PE; an α1 -adrenoceptor agonist) infusion via brachial artery catheter in response to two different stimuli: moderate (15% maximal voluntary contraction) rhythmic handgrip exercise or adenosine infusion. In Protocol 1 (n = 11 subjects) a total of six trials were performed in three conditions: control (saline), combined enzymatic inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis (l-NMMA + ketorolac) and combined inhibition of NO, PGs, KIR channels and Na(+) /K(+) -ATPase (l-NMMA + ketorolac + BaCl2  + ouabain). In Protocol 2 (n = 6) a total of four trials were performed in two conditions: control (saline), and combined KIR channel and Na(+) /K(+) -ATPase inhibition. All trials occurred after local β-adrenoceptor blockade (propranolol). PE-mediated vasoconstriction was calculated (%ΔFVC) in each condition. Contrary to our hypothesis, despite attenuated exercise hyperaemia of ∼30%, inhibition of KIR channels and Na(+) /K(+) -ATPase, combined with inhibition of NO and PGs (Protocol 1) or alone (Protocol 2) did not enhance α1 -mediated vasoconstriction during exercise (Protocol 1: -27 ± 3%; P = 0.2 vs. control, P = 0.4 vs. l-NMMA + ketorolac; Protocol 2: -21 ± 7%; P = 0.9 vs. control). Thus, contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during combined KIR channel and Na(+) /K(+) -ATPase inhibition.

Peripheral conduit and resistance artery function are improved following a single, 1-h bout of peristaltic pulse external pneumatic compression

INTRODUCTION

External pneumatic compression (EPC) is being employed for a widening range of clinical and non-clinical populations. However, EPC devices vary markedly in treatment pressures, duty cycles and application sites, and the acute effects of whole limb, lower pressure EPC on peripheral vascular function have not been determined.

PURPOSE

The purpose of this study was to determine the acute effects of a single bout of peristaltic pulse EPC on peripheral conduit and resistance artery function.

METHODS

Twenty (n = 20; males = 12 and females = 8) young and apparently healthy subjects (aged 26.1 ± 8.2 years) participated in this study. A sequential EPC device with five inflation zones arranged linearly and inflating distal to proximal along the lower limbs was employed with target inflation pressures of 70 mmHg for 1 h. Flow-mediated dilation (FMD) of the brachial and popliteal arteries was evaluated with ultrasound before and after EPC. Venous occlusion plethysmography was employed to evaluate limb blood flow at rest and during reactive hyperemia (RH) in the forearm (FBF) and calf (CBF) before and after EPC.

RESULTS

Peak RH CBF was increased by 9 % after EPC (P < 0.05), whereas peak RH FBF (-10 %) did not change significantly (P > 0.25). Normalized popliteal artery FMD post-EPC (2.24 ± 1.41) was significantly higher than pre-EPC (1.36 ± 0.67, P = 0.015) and post-sham (1.58 ± 0.86, P = 0.032) values. Similarly, normalized brachial artery FMD post-EPC (1.47 ± 0.32) was significantly higher than pre-EPC (1.11 ± 0.41, P = 0.004) and post-sham (0.99 ± 0.27, P = 0.026) values.

CONCLUSION

Acutely, whole limb, lower pressure EPC improves conduit artery endothelial function systemically, but only improves RH blood flow locally (i.e., compressed limbs).

Capillary tone: cyclooxygenase, shear stress, luminal glycocalyx, and hydraulic conductivity (Lp)

Control of capillary hydraulic conductivity (L_p) is the physiological mechanism that underpins systemic hydration. Capillaries form the largest surface of endothelial cells in any species with a cardiovascular system and all capillaries are exposed to the flow-induced force, shear stress (τ). Vasoactive molecules such as prostacyclin (cyclooxygenase product, COX) are released from endothelial cells in response to τ. Little is known about how COX activity impacts capillary L_p. The purpose here was to assess L_p in situ following an acute Δτ stimulus and during COX1/COX2 inhibition. Mesenteric true capillaries (TC) of Rana pipiens (pithed) were cannulated for L_p assessment using the modified Landis technique. Rana were randomized into Control and Test groups. Two capillaries per animal were used (perfusate, 10 mg·mL⁻¹ BSA/frog Ringer's; superfusate, frog Ringer's or indomethacin (10⁻⁵ mol·L⁻¹) mixed in frog Ringer's solution). Three distinct responses of L_p to indomethacin (TC2) were demonstrated (TC1 and TC2 medians: Test Subgroup 1, 3.0 vs. 1.8; Test Subgroup 2, 18.2 vs. 2.2; Test Subgroup 3, 4.2 vs. 10.2 × 10⁻⁷ cm·sec⁻¹·cm H₂O⁻¹). Multiple regression analysis revealed a relationship between capillary L_p and systemic red blood cell concentration or hematocrit, plasma protein concentration, and Δτ (Test Subgroup 1, R² = 0.59, P < 0.0001; Test Subgroup 2, R² = 0.96, P = 0.002), but only during COX inhibition. Maintaining red blood cell and plasma protein levels within a normal range may control barrier function in a healthy state. Recovering barrier function may be an unrecognized benefit of transfusions during blood loss or edema formation.

Effects of age and sex on cerebrovascular function in the rat middle cerebral artery

BACKGROUND

Although the mechanisms underlying the beneficial effects of estrogen on cerebrovascular function are well known, the age-dependent deleterious effects of estrogen are largely unstudied. It was hypothesized that age and sex interact in modulating cerebrovascular reactivity to vasopressin (VP) by altering the role of prostanoids in vascular function.

METHODS

Female (F) Sprague-Dawley rats approximating key stages of "hormonal aging" in humans were studied: premenopausal (mature multigravid, MA, cyclic, 5-6 months) and postmenopausal (reproductively senescent, RS, acyclic, 10-12 months). Age-matched male (M) rats were also studied. Reactivity to VP (10(-12)-10(-7) M) was measured in pressurized middle cerebral artery segments in the absence or presence of selective inhibitors of COX-1 (SC560, SC, 1 μM) or COX-2 (NS398, NS, 10 μM). VP-stimulated release of PGI2 and TXA2 were measured using radioimmunoassay of 6-keto-PGF1α and TXB2 (stable metabolites, pg/mg dry wt/45 min).

RESULTS

In M, there were no changes in VP-induced vasoconstriction with age. Further, there were no significant differences in basal or in low- or high-VP-stimulated PGI2 or TXA2 production in younger or older M. In contrast, there were marked differences in cerebrovascular reactivity and prostanoid release with advancing age in F. Older RS F exhibited reduced maximal constrictor responses to VP, which can be attributed to enhanced COX-1 derived dilator prostanoids. VP-induced vasoconstriction in younger MA F utilized both COX-1 and COX-2 derived constrictor prostanoids. Further, VP-stimulated PGI2 and TXA2 production was enhanced by endogenous estrogen and decreased with advancing age in F, but not in M rats.

CONCLUSIONS

This is the first study to examine the effects of age and sex on the mechanisms underlying cerebrovascular reactivity to VP. Interestingly, VP-mediated constriction was reduced by age in F, but was unchanged in M rats. Additionally, it was observed that selective blockade of COX-1 or COX-2 produced age-dependent changes in cerebrovascular reactivity to VP and that VP-stimulated PGI2 and TXA2 production were enhanced by endogenous estrogen in younger F. A better understanding of the mechanisms by which estrogen exerts its effects may lead to new age- and sex-specific therapeutic agents for the prevention and/or treatment of cerebrovascular diseases.

The production of vasoconstriction-induced residual NO modulates perfusion pressure in rat mesenteric vascular bed

In the presence of nitric oxide synthase (NOS) inhibitors, the contribution of residual NO to endothelium-dependent relaxation induced by chemical agonists acetylcholine and bradykinin has been documented in resistance vessels. However, the contribution of residual NO to the vasodilatation in response to pressure and fluid shear stress is not well understood. In this study, to demonstrate the activity of residual NO, we applied a NO scavenger, hydroxocobalamin (HCX), on the phenylephrine-induced increase in perfusion pressure in the presence of NOS inhibitors, Nω-nitro-L-arginine (L-NA) or Nω-nitro-L-arginine methyl ester (L-NAME) in the rat perfused mesenteric bed. The perfusion pressure was increased by phenylephrine (1-2 µM), an α1-adrenoceptor agonist. This increase was augmented by the addition of L-NA or L-NAME. In the presence of any NOS inhibitors, the application of hydroxocobalamin (100 µM) further increased the perfusion pressure. The removal of endothelium by saponin (50 mg/L) and the use of a non-selective protein kinase inhibitor, staurosporine (5 nM), and a tyrosine kinase inhibitor, erbstatin A (30 µM), but not a calmodulin inhibitor, calmidazolium (0.5 µM), inhibited the additional pressor responses induced by L-NA or L-NAME and a combination of either of them with hydroxocobalamine. These findings show that there could be a NOS inhibitor-resistant residual NO production in response to pressure in the rat mesenteric vascular bed. This residual NO production may be associated with the activation of tyrosine kinase and protein kinases, but not calmodulin. Finally, this pressure-induced residual NO exerts a modulatory role against vasoconstriction induced by phenylephrine.

Is Flow-Mediated Dilation Nitric Oxide Mediated?

Flow-mediated dilation (FMD) is a noninvasive index of endothelial function and vascular health in humans. Studies examining the role of nitric oxide (NO) are not conclusive. In this article, we quantified the contribution of NO in FMD of conduit arteries and explored the effect of the protocol (ie, distal cuff, ≈5-minute ischemia) and method of analysis (ie, automated and continuous edge detection) on the NO dependency of this test. A systematic review and 3-stage meta-analysis of published crossover studies that measured FMD under local infusion of saline or the NO synthase blocker N G monomethyl-L-arginine (L-NMMA) was undertaken. Twenty studies met the inclusion criteria for stage 1 (374 individual comparisons). The meta-analyzed outcome was the difference in FMD between infusion of saline (ie, FMD saline ) and NO synthase blocker (ie, FMD L-NMMA ). Overall, FMD saline was 8.2% (95% confidence interval [CI], 6.8%–9.6%) compared with FMD L-NMMA of 3.7% (95% CI, 3.1%–4.3%; P <0.001). Stage 2 analysis focused on studies that used the most commonly adopted approach in healthy volunteers (ie, distal cuff placement, ≈5-minute occlusion), which similarly revealed a significant NO contribution to FMD (FMD saline , 6.5% [95% CI, 5.7%–7.3%]; FMD L-NMMA , 0.9% [95% CI, 0.5%–1.3%]; P <0.001). Stage 3 meta-analyzed the studies that adopted the commonly adopted approach and automated, continuous method of analysis, which also revealed a significant contribution of NO to the FMD (FMD saline , 6.9% [95% CI, 6.0%–7.8%]; FMD L-NMMA , 2.4% [95% CI, 1.1%–3.7%]; P <0.001). This comprehensive analysis demonstrates that FMD of conduit arteries in humans is, at least in part, mediated by NO.

Prostaglandins induce vasodilatation of the microvasculature during muscle contraction and induce vasodilatation independent of adenosine

Blood flow data from contracting muscle in humans indicates that adenosine (ADO) stimulates the production of nitric oxide (NO) and vasodilating prostaglandins (PG) to produce arteriolar vasodilatation in a redundant fashion such that when one is inhibited the other can compensate. We sought to determine whether these redundant mechanisms are employed at the microvascular level. First, we determined whether PGs were involved in active hyperaemia at the microvascular level. We stimulated four to five skeletal muscle fibres in the anaesthetized hamster cremaster preparation in situ and measured the change in diameter of 2A arterioles (maximum diameter 40 μm, third arteriolar level up from the capillaries) at a site of overlap with the stimulated muscle fibres before and after 2 min of contraction [stimulus frequencies: 4, 20 and 60 Hz at 15 contractions per minute (CPM) or contraction frequencies of 6, 15 or 60 CPM at 20 Hz; 250 ms train duration]. Muscle fibres were stimulated in the absence and presence of the phospholipase A2 inhibitor quinacrine. Further, we applied a range of concentrations of ADO (10(-7)-10(-5) M) extraluminally, (to mimic muscle contraction) in the absence and presence of L-NAME (NO synthase inhibitor), indomethacin (INDO, cyclooxygenase inhibitor) and L-NAME + INDO and observed the response of 2A arterioles. We repeated the latter experiment on a different level of the cremaster microvasculature (1A arterioles) and on the microvasculature of a different skeletal muscle (gluteus maximus, 2A arterioles). We observed that quinacrine inhibited vasodilatation during muscle contraction at intermediate and high contraction frequencies (15 and 60 CPM). L-NAME, INDO and L-NAME + INDO were not effective at inhibiting vasodilatation induced by any concentration of ADO tested in 2A and 1A arterioles in the cremaster muscle or 2A arterioles in the gluteus maximus muscle. Our data show that PGs are involved in the vasodilatation of the microvasculature in response to muscle contraction but did not obtain evidence that extraluminal ADO causes vasodilatation through NO or PG or both. Thus, we propose that PG-induced microvascular vasodilation during exercise is independent of ADO.

Metabolic actions of angiotensin II and insulin: a microvascular endothelial balancing act

Metabolic actions of insulin to promote glucose disposal are augmented by nitric oxide (NO)-dependent increases in microvascular blood flow to skeletal muscle. The balance between NO-dependent vasodilator actions and endothelin-1-dependent vasoconstrictor actions of insulin is regulated by phosphatidylinositol 3-kinase-dependent (PI3K)--and mitogen-activated protein kinase (MAPK)-dependent signaling in vascular endothelium, respectively. Angiotensin II acting on AT₂ receptor increases capillary blood flow to increase insulin-mediated glucose disposal. In contrast, AT₁ receptor activation leads to reduced NO bioavailability, impaired insulin signaling, vasoconstriction, and insulin resistance. Insulin-resistant states are characterized by dysregulated local renin-angiotensin-aldosterone system (RAAS). Under insulin-resistant conditions, pathway-specific impairment in PI3K-dependent signaling may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Similarly, excess AT₁ receptor activity in the microvasculature may selectively impair vasodilation while simultaneously potentiating the vasoconstrictor actions of insulin. Therapeutic interventions that target pathway-selective impairment in insulin signaling and the imbalance in AT₁ and AT₂ receptor signaling in microvascular endothelium may simultaneously ameliorate endothelial dysfunction and insulin resistance. In the present review, we discuss molecular mechanisms in the endothelium underlying microvascular and metabolic actions of insulin and Angiotensin II, the mechanistic basis for microvascular endothelial dysfunction and insulin resistance in RAAS dysregulated clinical states, and the rationale for therapeutic strategies that restore the balance in vasodilator and constrictor actions of insulin and Angiotensin II in the microvasculature.

Exercise training enhances multiple mechanisms of relaxation in coronary arteries from ischemic hearts

Exercise training of coronary artery disease patients is of considerable interest, since it has been shown to improve vascular function and, thereby, enhance blood flow into compromised myocardial regions. However, the mechanisms underlying exercise-induced improvements in vascular function have not been fully elucidated. We tested the hypothesis that exercise training increases the contribution of multiple mediators to endothelium-dependent relaxation of coronary arteries in the underlying setting of chronic coronary artery occlusion. To induce gradual occlusion, an ameroid constrictor was placed around the proximal left circumflex coronary artery in Yucatan miniature swine. At 8 wk postoperatively, pigs were randomly assigned to sedentary or exercise (treadmill, 5 days/wk) regimens for 14 wk. Exercise training significantly enhanced the contribution of nitric oxide, prostanoids, and large-conductance Ca(2+)-dependent K(+) (BKCa) channels to endothelium-dependent, bradykinin-mediated relaxation in nonoccluded and collateral-dependent arteries. Combined nitric oxide synthase, prostanoid, and BKCa channel inhibition ablated the enhanced relaxation associated with exercise training. Exercise training significantly increased nitric oxide levels in response to bradykinin in endothelial cells isolated from nonoccluded and collateral-dependent arteries. Bradykinin treatment significantly increased PGI2 levels in all artery treatment groups and tended to be further enhanced after nitric oxide synthase inhibition in exercise-trained pigs. No differences were found in whole cell BKCa channel currents, BKCa channel protein levels, or arterial cyclic nucleotide levels. Although redundant, upregulation of parallel vasodilator pathways appears to contribute to enhanced endothelium-dependent relaxation, potentially providing a more refined control of blood flow after exercise training.

Methodological considerations and practical recommendations for the application of peripheral arterial tonometry in children and adolescents

Endothelial dysfunction is recognized as the primum movens in the development of atherosclerosis. Its crucial role in both cardiovascular morbidity and mortality has been confirmed. In the past, research was hampered by the invasive character of endothelial function assessment. The development of non-invasive and feasible techniques to measure endothelial function has facilitated and promoted research in various adult and paediatric subpopulations. To avoid user dependence of flow-mediated dilation (FMD), which evaluates nitric oxide dependent vasodilation in large vessels, a semi-automated, method to assess peripheral microvascular function, called peripheral arterial tonometry (Endo-PAT(®)), was recently introduced. The number of studies using this technique in children and adolescents is rapidly increasing, yet there is no consensus with regard to either measuring protocol or data analysis of peripheral arterial tonometry in children and adolescents. Most paediatric studies simply applied measuring and analysing methodology established in adults, a simplification that may not be appropriate. This paper provides a detailed description of endothelial function assessment using the Endo-PAT for researchers and clinicians. We discuss clinical and methodological considerations and point out the differences between children, adolescents and adults. Finally, the main aim of this paper is to provide recommendations for a standardised application of Endo-PAT in children and adolescents, as well as for population-specific data analysis methodology.

Mechanisms of rapid vasodilation after a brief contraction in human skeletal muscle

A monophasic increase in skeletal muscle blood flow is observed after a brief single forearm contraction in humans, yet the underlying vascular signaling pathways remain largely undetermined. Evidence from experimental animals indicates an obligatory role of vasodilation via K⁺-mediated smooth muscle hyperpolarization, and human data suggest little to no independent role for nitric oxide (NO) or vasodilating prostaglandins (PGs). We tested the hypothesis that K⁺-mediated vascular hyperpolarization underlies the rapid vasodilation in humans and that combined inhibition of NO and PGs would have a minimal effect on this response. We measured forearm blood flow (Doppler ultrasound) and calculated vascular conductance 10 s before and for 30 s after a single 1-s dynamic forearm contraction at 10%, 20%, and 40% maximum voluntary contraction in 16 young adults. To inhibit K⁺-mediated vasodilation, BaCl₂ and ouabain were infused intra-arterially to inhibit inwardly rectifying K⁺ channels and Na⁺-K⁺-ATPase, respectively. Combined enzymatic inhibition of NO and PG synthesis occurred via NG-monomethyl-L-arginine (L-NMMA; NO synthase) and ketorolac (cyclooxygenase), respectively. In protocol 1 (n = 8), BaCl₂ + ouabain reduced peak vasodilation (range: 30-45%, P < 0.05) and total postcontraction vasodilation (area under the curve, ~55-75% from control) at all intensities. Contrary to our hypothesis, L-NMMA + ketorolac had a further impact (peak: ~60% and area under the curve: ~80% from control). In protocol 2 (n = 8), the order of inhibitors was reversed, and the findings were remarkably similar. We conclude that K⁺-mediated hyperpolarization and NO and PGs, in combination, significantly contribute to contraction-induced rapid vasodilation and that inhibition of these signaling pathways nearly abolishes this phenomenon in humans.

Assessment of endothelial dysfunction in childhood obesity and clinical use

The association of obesity with noncommunicable diseases, such as cardiovascular complications and diabetes, is considered a major threat to the management of health care worldwide. Epidemiological findings show that childhood obesity is rapidly rising in Western society, as well as in developing countries. This pandemic is not without consequences and can affect the risk of future cardiovascular disease in these children. Childhood obesity is associated with endothelial dysfunction, the first yet still reversible step towards atherosclerosis. Advanced research techniques have added further insight on how childhood obesity and associated comorbidities lead to endothelial dysfunction. Techniques used to measure endothelial function were further brought to perfection, and novel biomarkers, including endothelial progenitor cells, were discovered. The aim of this paper is to provide a critical overview on both in vivo as well as in vitro markers for endothelial integrity. Additionally, an in-depth description of the mechanisms that disrupt the delicate balance between endothelial damage and repair will be given. Finally, the effects of lifestyle interventions and pharmacotherapy on endothelial dysfunction will be reviewed.

Dysfunction of circulating endothelial progenitor cells in type 1 diabetic rats with diabetic retinopathy

PURPOSE

To investigate the role of endothelial progenitor cell (EPC) in the pathogenesis of diabetic retinopathy (DR) in streptozotocin (STZ)-induced type 1 diabetes mellitus (T1DM) rats.

METHODS

A total of 160 male Wistar rats (12 weeks old; 250-350 g) were randomly assigned into four groups (n = 40 for each), including control (group 1, no treatment), T1DM1 (group 2, 1 month after 50 mg/kg of STZ, single i.p.), T1DM3 (group 3, 3 months after 50 mg/kg of STZ, single i.p.), T1DM6 (group 4, 6 months after 50 mg/kg of STZ, single i.p.). Enumeration of circulating EPC from peripheral blood was measured by flow cytometry. EPC from bone marrow of rats was cultured in vitro to evaluate its function of proliferation, adhesion, and migration activities. Plasma levels of vascular endothelial growth factor (VEGF) and nitric oxide (NO) were measured by enzyme-linked immunosorbent assay (ELISA). Retinal sections were imaged by light microscopy and a transmission electron microscope (TEM).

RESULTS

The numbers of circulating EPC were significantly decreased in diabetic groups compared with the control group. Impaired proliferation, adhesion, and migratory activities of cultured EPC were observed in diabetic groups. There were significantly higher levels of plasma VEGF but lower levels of plasma NO in diabetic groups than those in non-diabetic controls. The significantly reduced thickness and obvious disorganized retinal cell layers were seen in T1DM DR rats. In the diabetic groups, we also found that T1DM rats developed telangiectatic vessels, vacuolar degeneration of ganglion cells, and thickened capillary basement membrane with capillary lumen stenosis in the retina. Significantly raised EPC numbers during DR formation and progression were also found.

CONCLUSIONS

The reduced numbers and impaired function of circulating EPC may contribute to the pathogenesis of DR in T1DM rats.

C-peptide-stimulated nitric oxide production in a cultured pulmonary artery endothelium is erythrocyte mediated and requires Zn(2+)

BACKGROUND

C-peptide has been shown to stimulate the production of nitric oxide (NO) in aortic endothelial cells via activation of endothelial nitric oxide synthase (eNOS) through an increased calcium influx. Here, results obtained using cultured bovine pulmonary artery endothelial cells (bPAECs) suggest that C-peptide does not induce eNOS activation directly in cultured pulmonary artery endothelium. However, C-peptide has been shown to stimulate the release of ATP from erythrocytes, a well-documented stimulus of eNOS activity in the pulmonary endothelium. Therefore, studies were performed to examine if C-peptide can indirectly stimulate NO production in a cultured pulmonary endothelium that is erythrocyte mediated.

METHODS

NO production and free intracellular calcium changes were monitored in immobilized bPAECs using specific intracellular fluorescent probes after stimulation with adenosine triphosphate (ATP), calcium ionophore A23187, or C-peptide. A microfluidic device enabled immobilized bPAECs to interact with flowing erythrocytes in the presence and absence of C-peptide to determine the role of the erythrocyte in C-peptide-stimulated NO production in cultured bPAECs.

RESULTS

ATP and the calcium ionophore stimulate significant increases in both intracellular NO production and influx of free calcium in cultured bPAECs. In contrast, C-peptide, ranging from physiological to above physiological concentrations, was unable to stimulate NO production or calcium influx in the bPAECs. However, when erythrocytes were pre-incubated with a mixture containing physiological concentrations of C-peptide with Zn(2+) and haemodynamically pumped beneath bPAECs cultured on a microfluidic device, an 88.6 ± 7.5% increase in endothelial NO production was observed.

CONCLUSIONS

C-peptide does not affect NO production in bPAECs directly but can impact NO production through an erythrocyte-mediated mechanism. Furthermore, in the absence of Zn(2+), C-peptide does not stimulate this NO production directly or indirectly. These results suggest that C-peptide, in the presence of Zn(2+), may be a determinant in purinergic receptor signalling via its ability to stimulate the release of ATP from erythrocytes.

Post-transcriptional regulation of placenta growth factor mRNA by hydrogen peroxide

In tissues containing pre-existing collateral vessels, occlusion of an upstream supply artery results in diversion of blood flow through these vessels, protecting the distal tissue from ischemia. The sudden rise in blood flow through collateral vessels exerts shear stress upon the vessel wall, thereby providing the initial stimulus for arteriogenesis. Arteriogenesis, the structural expansion of collateral circulation, involves smooth muscle cell (SMC) proliferation which leads to increased vessel diameter and wall thickness. Since shear is sensed at the level of endothelial cells (EC), communication from EC to the underlying SMC must occur as part of this process. We previously reported that endothelial cells (EC) exposed to shear stress release hydrogen peroxide (H(2)O(2)), and that H(2)O(2) can signal vascular SMC to increase gene and protein expression of placenta growth factor (PLGF), a known mediator of arteriogenesis. The purpose of the current study was to further elucidate the mechanism whereby PLGF is regulated by H(2)O(2). We found that a single, physiological dose of H(2)O(2) increases PLGF mRNA half-life, but has no effect on PLGF promoter activity, in human coronary artery SMC (CASMC). We further demonstrated that the H(2)O(2)-induced increase in PLGF mRNA levels partially relies on p38 MAPK, JNK and ERK1/2 pathways. Finally, we showed that chronic exposure to pathological levels of H(2)O(2) further increases PLGF mRNA levels, but does not result in a corresponding increase in PLGF secreted protein. These data suggest that PLGF regulation has an important translational component. To our knowledge, this is the first study to characterize post-transcriptional regulation of PLGF mRNA by H(2)O(2) in vascular SMC. These findings provide new insights into the regulation of this important growth factor and increase our understanding of PLGF-driven arteriogenesis.

Regulation of blood flow distribution in skeletal muscle: role of erythrocyte-released ATP

The maintenance of adequate tissue O(2) levels in skeletal muscle is vital for normal physiology and requires a well regulated and appropriately distributed convective O(2) supply. Inherent in this fundamental physiological process is the requirement for a mechanism which both senses tissue O(2) need and locally adjusts flow to appropriately meet that need. Over the past several years we and others have suggested that, in skeletal muscle, O(2) carrying erythrocytes participate in the regulation of total blood flow and its distribution by releasing ATP. Importantly, the release of this vasoactive molecule must be both rapid and well controlled if it is to serve an important physiological role. Here we provide insights into three distinct regulated signalling pathways within the erythrocyte that are activated by exposure to reduced O(2) tension or in response to binding of agonists to the prostacyclin or β-adrenergic receptors. Although much has been learned about the role of the erythrocyte in perfusion of skeletal muscle, much remains to be understood. However, what is clear is that the long established passive carrier of O(2) also contributes to the regulation of the distribution of microvascular perfusion in skeletal muscle by virtue of its capacity to release ATP.

Combined inhibition of nitric oxide and vasodilating prostaglandins abolishes forearm vasodilatation to systemic hypoxia in healthy humans

We tested the hypothesis that nitric oxide (NO) and vasodilating prostaglandins (PGs) contribute independently to hypoxic vasodilatation, and that combined inhibition would reveal a synergistic role for these two pathways in the regulation of peripheral vascular tone. In 20 healthy adults, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) responses to steady-state (SS) isocapnic hypoxia (O₂ saturation ~85%). All trials were performed during local α- and β-adrenoceptor blockade (via a brachial artery catheter) to eliminate sympathoadrenal influences on vascular tone and thus isolate local vasodilatory mechanisms. The individual and combined effects of NO synthase (NOS) and cyclooxygenase (COX) inhibition were determined by quantifying the vasodilatation from rest to SS hypoxia, as well as by quantifying how each inhibitor reduced vascular tone during hypoxia. Three hypoxia trials were performed in each subject. In group 1 (n = 10), trial 1, 5 min of SS hypoxia increased FVC from baseline (21 ± 3%; P < 0.05). Infusion of N(G)-nitro-L-arginine methyl ester (L-NAME) for 5 min to inhibit NOS during continuous SS hypoxia reduced FVC by -33 ± 3% (P < 0.05). In Trial 2 with continuous NOS inhibition, the increase in FVC from baseline to SS hypoxia was similar to control conditions (20 ± 3%), and infusion of ketorolac for 5 min to inhibit COX during continuous SS hypoxia reduced FVC by -15 ± 3% (P < 0.05). In Trial 3 with combined NOS and COX inhibition, the increase in FVC from baseline to SS hypoxia was abolished (~3%; NS vs. zero). In group 2 (n = 10), the order of NOS and COX inhibition was reversed. In trial 1, five minutes of SS hypoxia increased FVC from baseline (by 24 ± 5%; P < 0.05), and infusion of ketorolac during SS hypoxia had minimal impact on FVC (-4 ± 3%; NS). In Trial 2 with continuous COX inhibition, the increase in FVC from baseline to SS hypoxia was similar to control conditions (27 ± 4%), and infusion of L-NAME during continuous SS hypoxia reduced FVC by -36 ± 7% (P < 0.05). In Trial 3 with combined NOS and COX inhibition, the increase in FVC from baseline to SS hypoxia was abolished (~3%; NS vs. zero). Our collective findings indicate that (1) neither NO nor PGs are obligatory to observe the normal local vasodilatory response from rest to SS hypoxia; (2) NO regulates vascular tone during hypoxia independent of the COX pathway, whereas PGs only regulate vascular tone during hypoxia when NOS is inhibited; and (3) combined inhibition of NO and PGs abolishes local hypoxic vasodilatation (from rest to SS hypoxia) in the forearm circulation of healthy humans during systemic hypoxia.

Placenta growth factor expression is regulated by hydrogen peroxide in vascular smooth muscle cells

When supply arteries become occluded, blood is diverted through preexisting collateral vessels. Shear stress arising from this increase in blood flow provides the initial physiological stimulus for expansion of the collateral circulation, a process termed arteriogenesis. Endothelial cells (EC) respond to increased shear stress by releasing a variety of mediators that can act on underlying smooth muscle cells (SMC). Placenta growth factor (PLGF) is known to mediate certain aspects of arteriogenesis, such as recruitment of monocytes to the vessel wall. Therefore, we tested whether SMC PLGF expression is influenced by mediators released by EC. We used A10 SMC cultured with medium that had been conditioned by EOMA EC for 4 days as a model. We found that EC-conditioned medium is able to upregulate PLGF gene expression in A10 SMC. Further experiments identified hydrogen peroxide (H(2)O(2)) as a key mediator of this response. We confirmed the physiological relevance of this mechanism in primary human coronary artery SMCs by demonstrating that exogenous H(2)O(2) specifically upregulates PLGF gene and protein expression. We also demonstrated that the physiological stimulus of shear stress raises endogenous H(2)O(2) levels in media into the range found to increase PLGF expression. In this study, we demonstrate that EC-released H(2)O(2) acts as a positive regulator of PLGF gene and protein expression in vascular SMC. To our knowledge, this is the first study to describe H(2)O(2) as a regulator of PLGF expression and therefore an upstream mediator of PLGF-driven arteriogenesis.

The role of pulsatile flow in controlling microvascular retinal endothelial and pericyte cell apoptosis and proliferation

AIMS

Aberrant retinal blood flow is a hallmark of various retinopathies and may be a causative factor in the pathology associated with these conditions. We examined the effects of pulsatile flow on bovine retinal endothelial cell (BREC) and bovine retinal pericyte (BRP) apoptosis and proliferation.

METHODS AND RESULTS

Co-cultured BRECs and BRPs were exposed to low (0.3 mL/min) or high (25 mL/min) pulsatile flow for 72 h using a perfused transcapillary culture system. Pulsatile flow increased BREC nitric oxide synthase (eNOS) and cyclooxygenase-2 (COX-2) expression and activity concomitant with a significant decrease in pre-pro-endothelin-1 (ET-1) mRNA and peptide. BREC apoptosis was significantly attenuated following exposure to high flow. The inhibition of NOS, COX, and ET receptors significantly reduced the pro-survival effects of flow on BREC. In contrast, BRP apoptosis was significantly enhanced following exposure to high flow. The inhibition of COX and ET receptors significantly attenuated the high flow-induced increase in BRP apoptosis when compared with untreated controls. Treatment of static BREC with NO donor (S-nitroso-N-acetylpenicillamine, SNAP), ET-1, or iloprost inhibited serum deprivation-induced apoptosis, whereas treatment of BRP with ET-1 and iloprost, but not SNAP, was ineffective. High pulsatile flow decreased BRP proliferation, in the absence of any changes in BREC proliferation.

CONCLUSION

Increased pulsatile flow promotes BREC survival and enhances BRP apoptosis through the activation of endothelial-derived vasoactive substances. Altered pulsatile flow does not alter BREC proliferation in co-culture with BRP, whereas BRP proliferation was significantly decreased at high flow rates. These interactions have important implications for vessel growth and regression during retinal vascular pathogenesis.

Physical evidence for substrate binding in preventing cyclooxygenase inactivation under nitrative stress

Prostaglandin biosynthesis is catalyzed by two spatially and functionally distinct active sites in cyclooxygenase (COX) enzymes. Despite the crucial role of COXs in biology, molecular details regarding the function and regulation of these enzymes are incompletely defined. Reactive nitrogen species, formed during oxidative stress, produce modifications that alter COX functionalities and prostaglandin biosynthesis. We previously established that COX-1 undergoes selective nitration on Tyr385 via a mechanism that requires the presence of bound heme cofactor. As this is a critical residue for COX-1 catalysis, nitration at this site results in enzyme inactivation. We now show that occupancy of the COX-1 active site with substrate protects against Tyr385 nitration and redirects nitration to alternative Tyr residues on COX-1, preserving catalytic activity. This study reveals a novel role for the substrate in protecting COX-1 from inactivation by nitration in pathophysiological settings.

Exogenous irritant-induced airway hyperreactivity and inhibition of soluble guanylyl cyclase

The majority of nitric oxide (NO) effects in the respiratory system are caused by stimulation of soluble guanylyl cyclase (sGC) with subsequent increase of cyclic guanosine monophosphate (cGMP) production. The importance of this mechanism of NO action in airway hyperreactivity (AHR) pathogenesis is unknown. Therefore, the aim of our experiment was to examine the changes of airway reactivity enhanced by toluene vapor exposure in the presence or inhibition of sGC activity in guinea pigs. Animals were treated with a nonspecific sGC inhibitor, methylene blue, in a dose of 50 or 100 mg/kg body weight, administered by intraperitoneal injection 30 min before or after exposure to toluene vapors. The toluene exposure lasted 2 hr in each of 3 consecutive days under in vivo conditions. Thereafter, the tracheal and lung tissue smooth muscle response to cumulative doses of mediators (histamine or acetylcholine) was recorded under in vitro conditions. The exposure to toluene vapors significantly increased the airway reactivity to both mediators in comparison with the healthy animal group. The administration of methylene blue decreased the amplitude of airway smooth muscle contraction in toluene-induced hyperreactivity. The decreases were dependent on the inhibitor doses, on a regimen of administration (before or after toluene inhalation), the level of the respiratory system (trachea, lung), and the bronchoconstrictor mediators. Our results suggest that the interaction between NO and sGC may be important for airway reactivity changes, but other mechanisms of NO action are important in AHR pathogenesis, too.

The effect of nitric oxide synthase and cyclooxygenase inhibition on cutaneous microvascular reactivity

The role of nitric oxide (NO)- and prostacyclin (PGI(2))-independent mechanism, potentially attributable to endothelium-derived hyperpolarizing factor (EDHF), has not been extensively studied in human skin microcirculation. The aim of our study was to elucidate the contribution of the NO- and PGI(2)-independent mechanism to microvascular reactivity of cutaneous microcirculation. Skin perfusion was measured on the volar aspect of the forearm in 12 healthy male subjects (mean age 25.0 +/- 1.5), using laser Doppler (LD) fluxmetry. Combined endothelial nitric oxide synthase (eNOS) and cyclooxygenase (COX) inhibition was achieved by an intradermal injection (10 microl) of the eNOS inhibitor, L(omega)-monomethyl L-arginine (L-NMMA, 10 mM) and the COX inhibitor, diclofenac (10 mM); saline was injected as a control. LD flux was assessed at rest and after an iontophoretical application of acetylcholine (ACh, 1%), an endothelial agonist and sodium nitroprusside (SNP, 1%), an endothelium-independent agonist, respectively. Combined eNOS and COX inhibition had no effect on the baseline LDF (12.5 +/- 2.3 PU (perfusion units) in control vs. 10.9 +/- 1.8 PU in the treated site). On the other hand, the ACh-stimulated increase in LDF was significantly attenuated after eNOS and COX inhibition (390.5 +/- 43.5%), compared to the control (643.7 +/- 80.3% increase, t-test, P < 0.05). Nevertheless, at least 60% of ACh-mediated vasodilatation was preserved after combined eNOS and COX inhibition. eNOS and COX inhibition had no impact on the SNP-stimulated increase in LDF (768.8 +/- 70.5% in control vs. 733.5 +/- 54.6% in the treated site). These findings indicate that NO- and PGI(2)-independent mechanism plays an important role in the regulation of blood flow in the human skin microcirculation.

Human cutaneous reactive hyperaemia: role of BKCa channels and sensory nerves

Reactive hyperaemia is the increase in blood flow following arterial occlusion. The exact mechanisms mediating this response in skin are not fully understood. The purpose of this study was to investigate the individual and combined contributions of (1) sensory nerves and large-conductance calcium activated potassium (BKCa) channels, and (2) nitric oxide (NO) and prostanoids to cutaneous reactive hyperaemia. Laser-Doppler flowmetry was used to measure skin blood flow in a total of 18 subjects. Peak blood flow (BF) was defined as the highest blood flow value after release of the pressure cuff. Total hyperaemic response was calculated by taking the area under the curve (AUC) of the hyperaemic response minus baseline. Infusates were perfused through forearm skin using microdialysis in four sites. In the sensory nerve/BKCa protocol: (1) EMLA cream (EMLA, applied topically to skin surface), (2) tetraethylammonium (TEA), (3) EMLA + TEA (Combo), and (4) Ringer solution (Control). In the prostanoid/NO protocol: (1) ketorolac (Keto), (2) NG-nitro-l-arginine methyl ester (L-NAME), (3) Keto + l-NAME (Combo), and (4) Ringer solution (Control). CVC was calculated as flux/mean arterial pressure and normalized to maximal flow. Hyperaemic responses in Control (1389 +/- 794%CVC max s) were significantly greater compared to TEA, EMLA and Combo sites (TEA, 630 +/- 512, P = 0.003; EMLA, 421 +/- 216, P < 0.001; Combo, 201 +/- 200, P < 0.001%CVC max s). Furthermore, AUC in Combo (Keto + l-NAME) site was significantly greater than Control (4109 +/- 2777 versus 1295 +/- 368%CVC max s). These data suggest (1) sensory nerves and BKCa channels play major roles in the EDHF component of reactive hyperaemia and appear to work partly independent of each other, and (2) the COX pathway does not appear to have a vasodilatory role in cutaneous reactive hyperaemia.

Increased spontaneous tone in renal arteries of spontaneously hypertensive rats

The spontaneous tone of vascular smooth muscle is augmented in hypertension. The present study examined the role of nitric oxide (NO), cyclooxygenase (COX), thromboxane A2/prostanoid (TP) and PGE2/prostanoid (EP-1) receptors, reactive oxygen species, and large-conductance Ca2+-activated K+ (BKCa) channels in the regulation of spontaneous tone in renal arteries of young and mature Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Rings of arteries, with and without endothelium, were suspended in a myograph for isometric force recording. Spontaneous tone (increase above initial tension) was observed only in arteries of mature SHR and was greater in arteries without endothelium. Nω-nitro-l-arginine methyl ester (l-NAME, an inhibitor of NO synthases) induced larger contractions in arteries of SHR than WKY. Indomethacin (a COX inhibitor), SC-19220 (an EP-1 receptor antagonist), and terutroban (a TP receptor antagonist) reduced the l-NAME-evoked contractions. Tiron (a superoxide anion scavenger), catalase (an enzyme that degrades H2O2), and deferoxamine (a hydroxyl radical scavenger) augmented the l-NAME-induced contractions in arteries of mature SHR. Charybdotoxin (a BKCa channel blocker) caused contractions in arteries of mature SHR without endothelium and in arteries with endothelium incubated with l-NAME. A decreased protein level of endothelial NO synthase, an increased release of prostacyclin, and an increased expression of EP-1 receptors were observed in arteries of mature SHR. The present study suggests that spontaneous tone is precipitated by age and hypertension. The reduced production of NO, leading to decreased activation of BKCa channels, may leave the actions of endogenous vasoconstrictors unopposed. COX products that activate EP-1 and TP receptors are involved in the development of spontaneous tone.

Effects of the nitric oxide synthase inhibitor L-NMMA on cerebrovascular and cardiovascular responses to hypoxia and hypercapnia in humans

Cerebral blood flow is highly sensitive to alterations in the partial pressures of O(2) and CO(2) (P(O(2)) and P(CO(2)), respectively) in the arterial blood. In humans, the extent to which nitric oxide (NO) is involved in this regulation is unclear. We hypothesized that the NO synthase (NOS) inhibitor N(G)-monomethyl-l-arginine (l-NMMA), attenuates the sensitivity of middle cerebral artery blood velocity (V(p)) to isocapnic hypoxia (end-tidal P(O(2)) = 50 Torr) and euoxic hypercapnia (end-tidal P(CO(2)) = +9 Torr above resting values) in 10 volunteers (age, 28.7 +/- 1.3 years; height, 179.2 +/- 2.4 cm; weight, 78.0 +/- 3.7 kg; mean +/- s.e.m.). The techniques of transcranial Doppler ultrasound and dynamic end-tidal forcing were used to measure(V(p)), and control end-tidal P(O(2)) and end-tidal P(CO(2)), respectively. At baseline (isocapnic euoxia), following intravenous administration of l-NMMA, mean arterial blood pressure (MAP) increased (76.3 +/- 7.3 to 86.2 +/- 9.4 mmHg) and heart rate (HR) decreased (59.5 +/- 9.0 to 55.2 +/- 9.5 beats min(-1)) but (V(p)) was unchanged. Hypoxia-induced increases in MAP, HR and were similar with and without l-NMMA (5.0 +/- 0.7 versus 7.1 +/- 1.0 mmHg, 11.5 +/- 1.4 versus 12.4 +/- 1.5 beats min(-1), 6.5 +/- 0.8 versus 6.6 +/- 0.8 cm s(-1) for DeltaMAP, DeltaHR and Delta , respectively). Hypercapnia-induced increases in MAP, HR and (V(p)) were similar with and without l-NMMA (7.4 +/- 3.1 versus 8.1 +/- 2.2 mmHg, 10.4 +/- 4.6 versus 10.0 +/- 4.2 beats min(-1), 16.5 +/- 1.5 versus 17.6 +/- 1.5 cm s(-1) for DeltaMAP, DeltaHR and Delta(V(p)) , respectively) but the sensitivity of the(V(p)) response at the removal of hypercapnia was attenuated with l-NMMA. In young healthy humans, pharmacological blockade of nitric oxide synthesis does not affect the increases in cerebral blood flow with hypoxia and hypercapnia, suggesting that nitric oxide is not required for the cerbrovascular responses to hypoxia and hypercapnia.

Cardiovascular actions of insulin

Insulin has important vascular actions to stimulate production of nitric oxide from endothelium. This leads to capillary recruitment, vasodilation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin target tissues (e.g., skeletal muscle). Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways regulating endothelial production of nitric oxide share striking parallels with metabolic insulin-signaling pathways. Distinct MAPK-dependent insulin-signaling pathways (largely unrelated to metabolic actions of insulin) regulate secretion of the vasoconstrictor endothelin-1 from endothelium. These and other cardiovascular actions of insulin contribute to coupling metabolic and hemodynamic homeostasis under healthy conditions. Cardiovascular diseases are the leading cause of morbidity and mortality in insulin-resistant individuals. Insulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin. This cardinal feature of diabetes, obesity, and dyslipidemia is also a prominent component of hypertension, coronary heart disease, and atherosclerosis that are all characterized by endothelial dysfunction. Conversely, endothelial dysfunction is often present in metabolic diseases. Insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling that in vascular endothelium contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction. The clinical relevance of this coupling is highlighted by the findings that specific therapeutic interventions targeting insulin resistance often also ameliorate endothelial dysfunction (and vice versa). In this review, we discuss molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases.

Alterations in endothelial control of the pulmonary circulation in exercising swine with secondary pulmonary hypertension after myocardial infarction

Secondary pulmonary hypertension after myocardial infarction (MI) has been associated with endothelial dysfunction and activation of the endothelin (ET) system. Here, we investigated whether an increased ET-mediated pulmonary vasoconstrictor influence contributes to pulmonary hypertension after MI, and whether this increased ET vasoconstriction is caused by impaired nitric oxide (NO) and prostanoid production. For this purpose, chronically instrumented swine with and without MI ran on a treadmill at 0-4 km h(-1). Mixed ET(A)/ET(B) receptor blockade (tezosentan) was performed in the absence and presence of single or combined inhibition of endothelial NO synthase (eNOS, with N(omega)-nitro-l-arginine) and cyclo-oxygenase (COX, with indometacin). In normal swine, mixed ET(A)/ET(B) blockade decreased pulmonary vascular resistance, but only during exercise. In MI swine, an increased ET-mediated vasoconstrictor influence was observed in the pulmonary circulation both at rest and during exercise. Inhibition of COX resulted in pulmonary vasoconstriction at rest in MI, but not in normal swine; this vasoconstriction in MI swine was normalized by ET(A)/ET(B) receptor blockade. Inhibition of eNOS enhanced the vasodilator response to ET(A)/ET(B) blockade, indicating that NO blunts the pulmonary vasoconstrictor influence of ET. However, this vasodilator response was enhanced to a similar degree in MI and normal swine. In summary, swine with a recent MI are characterized by an exaggerated pulmonary vasoconstrictor influence of ET. This increased ET-mediated pulmonary vasoconstrictor influence is not caused by a loss of NO bioavailability, and is blunted by an increased prostanoid-mediated vasodilatation. In conclusion, an increased ET-mediated vasoconstriction, which does not appear to be the result of loss of endothelial vasodilators, contributes to pulmonary hypertension after MI.

Nitric oxide deficiency promotes vascular side effects of cyclooxygenase inhibitors

The cardiovascular safety of COX-2 selective and nonselective nonsteroidal anti-inflammatory drugs (NSAIDs) has recently been called into question. The factors that predispose to adverse events by NSAIDs are unknown. Because patients with arthritis have decreased nitric oxide (NO) bioavailability, the in vivo effects of NSAIDs on murine vascular tone and platelet activity in the presence or absence of NO were examined. Here, we show that acute hypertensive and prothrombotic activities of the COX-2-selective inhibitor celecoxib are revealed only after in vivo inhibition of NO generation. The nonselective NSAID indomethacin was hypertensive but antithrombotic when NO was absent. In vitro myography of aortic rings confirmed that vasoconstriction required inhibition of both NOS and COX-2 and was abolished by supplementation with exogenous NO. These data indicate that NO suppresses vascular side effects of NSAIDs, suggesting that risk will be greatest in patients with impaired vascular function associated with decreased NO bioavailability.

Effects of nitric oxide synthase inhibition with or without cyclooxygenase-2 inhibition on resting haemodynamics and responses to exendin-4

BACKGROUND AND PURPOSE

Interactions between the NO system and the cyclooxygenase systems may be important in cardiovascular regulation. Here we measured the effects of acute cyclooxygenase-2 inhibition (with parecoxib), alone and in combination with NOS inhibition (with NG-nitro-L-arginine methyl ester (L-NAME)), on resting cardiovascular variables and on responses to the glucagon-like peptide 1 agonist, exendin-4, which causes regionally-selective vasoconstriction and vasodilatation.

EXPERIMENTAL APPROACH

Rats were instrumented with flow probes and intravascular catheters to measure regional haemodynamics in the conscious, freely moving state. L-NAME was administered as a primed infusion 180 min after administration of parecoxib or vehicle, and exendin-4 was given 60 min after the onset of L-NAME infusion.

KEY RESULTS

Parecoxib had no effect on resting cardiovascular variables or on responses to L-NAME. Exendin-4 caused a pressor response accompanied by tachycardia, mesenteric vasoconstriction and hindquarters vasodilatation. Parecoxib did not affect haemodynamic responses to exendin-4, but L-NAME inhibited its hindquarters vasodilator and tachycardic effects. When combined, L-NAME and parecoxib almost abolished the hindquarters vasodilatation while enhancing the pressor response.

CONCLUSIONS AND IMPLICATIONS

Cyclooxygenase-2-derived products do not affect basal haemodynamic status in conscious normotensive rats, or influence the NO system acutely. The inhibitory effects of L-NAME on the hindquarters vasodilator and tachycardic effects of exendin-4 are consistent with a previous study that showed those events to be beta-adrenoceptor mediated. The additional effect of parecoxib on responses to exendin-4 in the presence of L-NAME, is consistent with other evidence for enhanced involvement of vasodilator prostanoids when NO production is reduced.

Signal transduction pathways involved in non-genomic action of estrone on vascular tissue

Previously we demonstrated that estrone non-genomically regulates rat aortic NOS and COX activity and that this effect depends on ovarian activity. The purpose of the present study was to characterize this effect and investigate the participation of phospholipase C and phophatidylinositol-3-kinase system in the intracellular transduction pathway involved in the response. Using aortic strips isolated from female fertile rats we showed that estrone stimulate nitric oxide synthase and cyclooxygenase in a short time interval (5-20 min), and that NO production was dependent in part on PGI2 production since 1 microM indomethacin significantly reduced this free radical production. Injection of 17-beta-estradiol to ovariectomized rats restored tissue capacity to rapidly increase NO production in response to "in vitro" treatment with 1 nM estrone. We also demonstrated that in aortic strips isolated from intact animals estrone elicited a rapid phospholipase C activation, inducing a biphasic increase in diacylglycerol generation (peaking at 45 s and 5 min). The presence of protein kinase C inhibitor chelerythrine did not prevent the increase of NO released in response to hormone treatment. We proved that PI3K-Akt system does not mediate NOS and COX activation. However, PLC activation was dependent on PI3K since presence of LY 294002 in the incubation medium abolished estrone-induced DAG increment. We concluded that, estrone rapid action on vascular tissue involves a cross talk between NOS and COX system, and the activation of PLC/DAG/PKC transduction pathways.

Blood viscosity maintains microvascular conditions during normovolemic anemia independent of blood oxygen-carrying capacity

Responses to exchange transfusion with red blood cells (RBCs) containing methemoglobin (MetRBC) were studied in an acute isovolemic hemodiluted hamster window chamber model to determine whether oxygen content participates in the regulation of systemic and microvascular conditions during extreme hemodilution. Two isovolemic hemodilution steps were performed with 6% dextran 70 kDa (Dex70) until systemic hematocrit (Hct) was reduced to 18% ( Level 2). A third-step hemodilution reduced the functional Hct to 75% of baseline by using either a plasma expander (Dex70) or blood adjusted to 18% Hct with all MetRBCs. In vivo functional capillary density (FCD), microvascular perfusion, and oxygen distribution in microvascular networks were measured by noninvasive methods. Methylene blue was administered intravenously to reduce methemoglobin (rRBC), which increased oxygen content with no change in Hct or viscosity from MetRBC. Final blood viscosities after the entire protocol were 2.1 cP for Dex70 and 2.8 cP for MetRBC (baseline, 4.2 cP). MetRBC had a greater mean arterial pressure (MAP) than did Dex70. FCD was substantially higher for MetRBC [82 (SD 6) of baseline] versus Dex70 [38 (SD 10) of baseline], and reduction of methemoglobin to oxyhemoglobin did not change FCD [84% (SD 5) of baseline]. Po2levels measured with palladium-meso-tetra(4-carboxyphenyl)porphyrin phosphorescence were significantly changed for Dex70 and MetRBC compared with Level 2 (Hct 18%). Reduction of methemoglobin to oxyhemoglobin partially restored Po2to Level 2. Wall shear rate and wall shear stress decreased in arterioles and venules for Dex70 and did not change for MetRBC or rRBC. Increased MAP and shear stress-mediated factors could be the possible mechanisms that improved perfusion flow and FCD after exchange for MetRBC. Thus the fall in systemic and microvascular conditions during extreme hemodilution with low-viscosity plasma expanders seems to be, in part, from the decrease in blood viscosity independent of the reduction in oxygen content.

NO-mediated regulation of NAD(P)H oxidase by laminar shear stress in human endothelial cells

The flowing blood generates shear stress at the endothelial cell surface. In endothelial cells, NAD(P)H oxidase complexes have been identified as major sources of superoxide anion (.O(2)(-)) formation. In this study, we analysed the effect of laminar shear stress on .O(2)(-) formation by cytochrome c reduction assay and on NAD(P)H oxidase subunit expression by standard calibrated competitive reverse transcription-polymerase chain reaction and Western blot in human endothelial cells. Primary cultures of human umbilical vein endothelial cells were exposed to laminar shear stress in a cone-and-plate viscometer for up to 24 h. Short-term application of shear stress transiently induced .O(2)(-) formation. This was inhibited by NAD(P)H oxidase inhibitor gp91ds-tat, but NAD(P)H oxidase subunit expression was unchanged. Long-term arterial laminar shear stress (30 dyne cm(-2), 24 h) down-regulated .O(2)(-) formation, and mRNA and protein expression of NAD(P)H oxidase subunits Nox2/gp91(phox) and p47(phox). In parallel, endothelial NO formation and eNOS, but not Cu/Zn SOD, protein expression was increased. Down-regulation of .O(2)(-) formation, gp91(phox) and p47(phox) expression by long-term laminar shear stress was blocked by l-NAME. NO donor DETA-NO down-regulates .O(2)(-) formation, gp91(phox) and p47(phox) expression in static cultures. In conclusion, our data suggest a transient activation of .O(2)(-) formation by short-term shear stress, followed by a down-regulation of endothelial NAD(P)H oxidase in response to long-term laminar shear stress. NO-mediated down-regulation by shear stress preferentially affects the gp91(phox)/p47(phox)-containing NAD(P)H oxidase complex. This mechanism might contribute to the regulation of endothelial NO/.O(2)(-) balance and the vasoprotective potential of physiological levels of laminar shear stress.

NO and prostanoids blunt endothelin-mediated coronary vasoconstrictor influence in exercising swine

Withdrawal of the endothelin (ET)-mediated vasoconstrictor influence contributes to metabolic coronary vasodilation during exercise. Because production of nitric oxide (NO) and prostanoids increases with increasing shear stress and because NO and prostanoids are able to modify the release of ET, we hypothesized that the withdrawal of ET-mediated coronary vasoconstriction during exercise is mediated through NO and/or prostanoids. To test this hypothesis, 19 chronically instrumented swine were studied at rest and while running on a treadmill up to 85-90% of maximal heart rate. Blockade of ET(A)/ET(B) receptors with tezosentan resulted in an increase in coronary venous O(2) levels (i.e., in coronary vasodilation) at rest, which waned at increasing levels of exercise intensity. Inhibition of either NO synthase [N(omega)-nitro-l-arginine (l-NNA)] or cyclooxygenase (indomethacin) did not affect the response to tezosentan under resting conditions but unmasked a vasodilator response to tezosentan during exercise. The vasodilator response to tezosentan during exercise increased progressively after combined administration of l-NNA and indomethacin. These findings suggest that NO and prostanoids act synergistically to inhibit the vasoconstrictor influence of ET on the coronary circulation during exercise, thereby facilitating the exercise-induced vasodilation of coronary resistance vessels.

Adventures in vascular biology: a tale of two mediators

I would like to thank the Royal Society for inviting me to deliver the Croonian Lecture. In so doing, the Society is adding my name to a list of very distinguished scientists who, since 1738, have preceded me in this task. This is, indeed, a great honour. For most of my research career my main interest has been the understanding of the normal functioning of the blood vessel wall and the way this is affected in pathology. During this time, our knowledge of these subjects has grown to such an extent that many people now believe that the conquering of vascular disease is a real possibility in the foreseeable future. My lecture concerns the discovery of two substances, prostacyclin and nitric oxide. I would like to describe the moments of insight and some of the critical experiments that contributed significantly to the uncovering of their roles in vascular biology. The process was often adventurous, hence the title of this lecture. It is the excitement of the adventure that I would like to convey in the text that follows.

Inducible nitric oxide synthase mediates prostaglandin h2 synthase nitration and suppresses eicosanoid production

Nitric oxide (NO) modulates the biological levels of arachidonate-derived cell signaling molecules by either enhancing or suppressing the activity of prostaglandin H(2) isoforms (PGHS-1 and PGHS-2). Whether NO activates or suppresses PGHS activity is determined by alternative protein modifications mediated by NO and NO-derived species. Here, we show that inducible NO synthase (iNOS) and PGHS-1 co-localize in atherosclerotic lesions of ApoE(-/-) mouse aortae. Immunoblotting and immunohistochemistry revealed Tyr nitration in PGHS-1 in aortic lesions but markedly less in adjacent nonlesion tissue. PGHS-2 was also found in lesions, but 3-nitrotyrosine incorporation was not detected. 3-Nitrotyrosine formation in proteins is considered a hallmark reaction of peroxynitrite, which can form via NO-superoxide reactions in an inflammatory setting. That iNOS-derived NO is essential for 3-nitrotyrosine modification of PGHS-1 was confirmed by the absence of 3-nitrotyrosine in lesions from ApoE(-/-)iNOS(-/-) mice. Mass spectrometric studies specifically identified the active site residue Tyr385 as a 3-nitrotyrosine modification site in purified PGHS-1 exposed to peroxynitrite. PGHS-mediated eicosanoid (PGE(2)) synthesis was more than fivefold accelerated in cultured iNOS(-/-) versus iNOS-expressing mouse aortic smooth muscle cells, suggesting that iNOS-derived NO markedly suppresses PGHS activity in vascular cells. These results further suggest a regulatory role of iNOS in eicosanoid biosynthesis in human atherosclerotic lesions.

A differential role for nitric oxide in two forms of physiological angiogenesis in mouse

NO plays a role in a variety of in vitro models of angiogenesis, although confounding effects of NO on non-endothelial tissues make its role during in vivo angiogenesis unclear. We therefore examined the effects of NO on two physiological models of angiogenesis in mouse skeletal muscle: (1) administration of prazosin (50 mg l-1) thereby increasing blood flow; and (2) muscle overload from surgical ablation of a functional synergist. These models induce angiogenesis via longitudinal splitting and capillary sprouting, respectively. Administration of NG-nitro-L-arginine (L-NNA) abolished the increase in capillary to fibre ratio (C:F) in response to prazosin administration, along with the increases in luminal filopodia and large endothelial vacuoles. L-NNA prevented luminal filopodia and vacuolisation in response to extirpation, but had no effect on abluminal sprouting, and little effect on C:F. Comparison of mice lacking endothelial (eNOS-/-) and neuronal NO synthase (nNOS-/-) showed that longitudinal splitting is eNOS-dependent, and Western blotting demonstrated an increase in eNOS but not inducible NOS (iNOS) expression. These data show that there are two pathways of physiological angiogenesis in skeletal muscle characterised by longitudinal splitting and capillary sprouting, respectively. NO generated by eNOS plays an essential role in splitting but not in sprouting angiogenesis, which has important implications for angiogenic therapies that target NO.

The influence of gender on parasympathetic vasodilatation in the submandibular gland of the rat

Parasympathetic vasodilatation in the rat submandibular gland is mediated by nitric oxide-dependent and -independent mechanisms (prostacyclin and endothelium-derived hyperpolarizing factor (EDHF)). The purpose of this study was to determine the influence of gender on the relative contributions of each pathway to nerve-stimulated vasodilatation. Absolute increases in perfusion (laser Doppler flowmetry) were similar in male and female rats (in arbitrary perfusion units: 6159+/-4530 and 5601+/-3877 at 2 Hz; 15645+/-6830 and 14848+/-6118 at 5 Hz; and 22418+/-7660 and 18878+/-5864 at 10 Hz). However, expressed as a percentage increase above resting values, stimulated perfusion was higher in males than in females (P<0.05). In males both Nomega-nitro-L-arginine methyl ester (L-NAME) and indomethacin partly blocked parasympathetic vasodilatation at all frequencies tested (P<0.05). In female rats significant reductions in nerve-stimulated perfusion were observed only at 2 and 5 Hz, but the effects of L-NAME were greater than in males (-64 compared with -45% at 2 Hz and -45 compared with -33% at 5 Hz, P<0.05). Indomethacin by itself had no apparent effect in females. The combined effects of L-NAME and indomethacin were dependent on the order of administration and on gender. Following L-NAME, indomethacin had no further effect in males or females. L-NAME reduced indomethacin-resistant vasodilatation in males and females, but the added effect of indomethacin was more pronounced in males. Finally, atropine-resistant vasodilatation was partly blocked by L-NAME, and the remaining vasodilatation was abolished by spantide I (substance P receptor antagonist). We conclude that NO, products of cyclo-oxygenase activity and EDHF all play a role in parasympathetic vasodilatation, but that NO and EDHF are the major endothelium-derived vasodilators in the rat submandibular gland. In addition, when other pathways are blocked EDHF makes a greater contribution in females. Lastly, both vasoactive intestinal peptide and substance P contribute to the atropine-resistant vasodilatation.

Vascular endothelium and blood flow

Major advances have been made over the last decade towards the elucidation of the molecular mechanisms involved in the endothelium-dependent regulation of vascular tone and blood flow. While the primary endothelium-derived vasodilator autacoid is nitric oxide, it is clear that epoxyeicosatrienoic acids and other endothelium-derived hyperpolarising factors, as well as endothelin-1 and reactive oxygen species, play a significant role in the regulation of vascular tone and gene expression. This review is intended as an overview of the signalling mechanisms that link haemodynamic stimuli (such as shear stress and cyclic stretch) and endothelial cell perturbation to the activation of enzymes generating vasoactive autacoids.

The role of prostaglandins in triggering the liver regeneration cascade

Following injury or surgical resection, the liver has the remarkable ability to regenerate. Despite over 100 years of research, the trigger of the liver regeneration cascade has only recently been identified. Shear stress-induced nitric oxide (NO), released secondary to a hemodynamic event following partial hepatectomy (PHX), has been implicated as the trigger of the liver regeneration cascade. However, it is also known that prostaglandins (PGs) are released following PHX, and in response to shear stress. Therefore, it is hypothesized that PGs, released secondary to an increase in the blood flow-to-liver mass ratio following PHX, trigger the liver regeneration cascade, and that NO and PGs interact during the triggering event. An index of initiation of the liver regeneration cascade, c-fos mRNA expression 15 min after PHX, has been employed. As expected, c-fos mRNA expression increased 15 min after PHX and this increase was inhibited by the NO synthase antagonist, l-NAME. This inhibition was reversed by the NO donors, SIN-1 and SNAP, and by the PGs, PGE2 and PGI2. Also, the increase in c-fos mRNA expression was inhibited by indomethacin, a cyclooxygenase antagonist. This inhibition was also reversed by the NO donors, SIN-1 and SNAP, and by the PGs, PGE2 and PGI2. These results suggest that there is interaction between NO and PGs in triggering the liver regeneration cascade, and that in a situation where either NO or COX is inhibited, provision of excess exogenous NO or PGs can reverse the inhibition. This suggests that exogenous NO and/or PGs may play a role in potentiation of the liver regeneration cascade.

Mechanisms of acetylcholine-mediated vasodilatation in young and aged human skin

Thermoregulatory cutaneous vasodilatation (VD) is attenuated in aged skin. While acetylcholine (ACh) plays a role in thermally mediated VD, the precise mechanisms through which ACh-mediated VD acts and whether those downstream mechanisms change with ageing are unclear. We tested the hypotheses that both nitric oxide (NO)- and prostanoid-mediated pathways contribute to exogenous ACh-mediated VD, and that both are attenuated with advanced age. Twelve young (Y: 23 +/- 1 years) and 10 older (O: 69 +/- 1 years) subjects underwent infusions of 137.5 mum ACh at four intradermal microdialysis sites: control (C, Ringer solution), NO synthase inhibited (NOS-I, 10 mm l-NAME), cyclooxygenase inhibited (COX-I, 10 mm ketorolac) and NOS-I + COX-I. Red blood cell flux was monitored using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC) was calculated (laser-Doppler flux/mean arterial pressure) and normalized to maximal CVC (%CVC(max)) (28 mm sodium nitroprusside + local heating to 43 degrees C). Baseline %CVC(max) was increased in the O at COX-I sites (COX-I 16 +/- 1, NOS-I + COX-I 16 +/- 2 versus C 10 +/- 1%CVC(max); P < 0.001) but not in the young, suggesting an age-related shift toward COX vasoconstrictors contributing to basal cutaneous vasomotor tone. There was no difference in peak %CVC(max) during ACh infusion between age groups, and the response was unchanged by NOS-I (O: NOS-I 35 +/- 5 versus C 38 +/- 5%CVC(max); P = 0.84) (Y: NOS-I 41 +/- 4 versus C 39 +/- 4%CVC(max); P = 0.67). COX-I and NOS-I + COX-I attenuated the peak CVC response to ACh in both groups (COX-I O: 29 +/- 3, Y: 22 +/- 2%CVC(max) versus C; P < 0.001 both groups; NOS-I + COX-I O: 32 +/- 3 versus Y: 29 +/- 2%CVC(max); versus C; P < 0.001 both groups). ACh mediates cutaneous VD through prostanoid and non-NO-, non-prostanoid-dependent pathways. Further, older subjects have a diminished prostanoid contribution to ACh-mediated VD.