Contribution of adenosine to compensatory dilation in hypoperfused contracting human muscles is independent of nitric oxide

We previously demonstrated that nitric oxide (NO) contributes to compensatory vasodilation in the contracting human forearm subjected to acute hypoperfusion. We examined the potential role of an adenosine-NO interaction to this response in 17 male subjects (25 ± 2 yr). In separate protocols subjects performed rhythmic forearm exercise (20% of maximum) while hypoperfusion was evoked by balloon inflation in the brachial artery above the elbow. Each trial included exercise before inflation, exercise with inflation, and exercise after deflation (3 min each). Forearm blood flow (FBF; ultrasound) and local [brachial artery catheter pressure (BAP)] and systemic [mean arterial pressure (MAP); Finometer] arterial pressure were measured. In protocol 1 (n = 10), exercise was repeated during nitric oxide synthase inhibition [N(G)-monomethyl-L-arginine (L-NMMA)] alone and during L-NMMA-aminophylline (adenosine receptor blockade) administration. In protocol 2, exercise was repeated during aminophylline alone and during aminophylline-L-NMMA. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from blood flow (ml/min) and BAP (mmHg). Percent recovery in FVC during inflation was calculated as (steady-state inflation + exercise value - nadir)/[steady-state exercise (control) value - nadir]. In protocol 1, percent recovery in FVC was 108 ± 8% during the control (no drug) trial. Percent recovery in FVC was attenuated with inhibition of NO formation alone (78 ± 9%; P < 0.01 vs. control) and was attenuated further with combined inhibition of NO and adenosine (58 ± 9%; P < 0.01 vs. L-NMMA). In protocol 2, percent recovery was reduced with adenosine receptor blockade (74 ± 11% vs. 113 ± 6%, P < 0.01) compared with control drug trials. Percent recovery in FVC was attenuated further with combined inhibition of adenosine and NO (48 ± 11%; P < 0.05 vs. aminophylline). Our data indicate that adenosine contributes to compensatory vasodilation in an NO-independent manner during exercise with acute hypoperfusion.     

Sex differences in salt sensitivity to nitric oxide dependent vasodilation in healthy young adults

Dietary sodium and blood pressure regulation differs between normotensive men and women, an effect which may involve endothelial production of nitric oxide (NO). Therefore, we tested the hypothesis that differences in the NO component of endothelium-dependent vasodilation between low and high dietary sodium intake depend on sex. For 5 days prior to study, healthy adults consumed a controlled low-sodium diet (10 mmol/day, n = 30, mean age ± SE: 30 ± 1 yr, 16 men) or high-sodium diet (400 mmol/day, n = 36, age 23 ± 1 yr, 13 men). Forearm blood flow (FBF, plethysmography) responses to brachial artery administration of acetylcholine (ACh, 4 μg·100 ml tissue(-1)·min(-1)) were measured before and after endothelial NO synthase inhibition with N(G)-monomethyl-l-arginine (l-NMMA, 50 mg bolus + 1 mg/min infusion). The NO component of endothelium-dependent dilation was calculated as the response to ACh before and after l-NMMA accounting for changes in baseline FBF: [(FBF ACh - FBF baseline) - (FBF ACh(L-NMMA) - FBF baseline(L-NMMA))]. This value was 5.7 ± 1.3 and 2.5 ± 0.8 ml·100 ml forearm tissue(-1)·min(-1) for the low- and high-sodium diets, respectively (main effect of sodium, P = 0.019). The sodium effect was larger for the men, with values of 7.9 ± 2.0 and 2.2 ± 1.4 for men vs. 3.1 ± 1.3 and 2.7 ± 1.0 ml·100 ml forearm tissue(-1)·min(-1) for the women (P = 0.034, sex-by-sodium interaction). We conclude that the NO component of endothelium-dependent vasodilation is altered by dietary sodium intake based on sex, suggesting that endothelial NO production is sensitive to dietary sodium in healthy young men but not women.     

Role of nitric oxide in the airway response to exercise in healthy and asthmatic subjects.

A role of nitric oxide (NO) has been suggested in the airway response to exercise. However, it is unclear whether NO may act as a protective or a stimulatory factor. Therefore, we examined the role of NO in the airway response to exercise by using N-monomethyl-L-arginine (L-NMMA, an NO synthase inhibitor), L-arginine (the NO synthase substrate), or placebo as pretreatment to exercise challenge in 12 healthy nonsmoking, nonatopic subjects and 12 nonsmoking, atopic asthmatic patients in a double-blind, crossover study. Fifteen minutes after inhalation of L-NMMA (10 mg), L-arginine (375 mg), or placebo, standardized bicycle ergometry was performed for 6 min using dry air, while ventilation was kept constant. The forced expiratory volume in 1-s response was expressed as area under the time-response curve (AUC) over 30 min. In healthy subjects, there was no significant change in AUC between L-NMMA and placebo treatment [28.6 +/- 17.0 and 1.3 +/- 20.4 (SE) for placebo and L-NMMA, respectively, P = 0.2]. In the asthmatic group, L-NMMA and L-arginine induced significant changes in exhaled NO (P < 0.01) but had no significant effect on AUC compared with placebo (geometric mean +/- SE: -204.3 +/- 1.5, -186.9 +/- 1.4, and -318.1 +/- 1.2%. h for placebo, L-NMMA, and L-arginine, respectively, P > 0.2). However, there was a borderline significant difference in AUC between L-NMMA and L-arginine treatment (P = 0.052). We conclude that modulation of NO synthesis has no effect on the airway response to exercise in healthy subjects but that NO synthesis inhibition slightly attenuates exercise-induced bronchoconstriction compared with NO synthase substrate supplementation in asthma. These data suggest that the net effect of endogenous NO is not inhibitory during exercise-induced bronchoconstriction in asthma.     

The impact of baseline artery diameter on flow-mediated vasodilation: a comparison of brachial and radial artery responses to matched levels of shear stress

An inverse relationship between baseline artery diameter (BAD) and flow-mediated vasodilation (FMD) has been identified using reactive hyperemia (RH) to create a shear stress (SS) stimulus in human conduit arteries. However, RH creates a SS stimulus that is inversely related to BAD. The purpose of this study was to compare FMD in response to matched levels of SS in two differently sized upper limb arteries [brachial (BA) and radial (RA) artery]. With the use of exercise, three distinct, shear rate (SR) stimuli were created (SR = blood velocity/vessel diameter; estimate of SS) in the RA and BA. Artery diameter and mean blood velocity were assessed with echo and Doppler ultrasound in 15 healthy male subjects (19-25 yr). Data are means ± SE. Subjects performed 6 min of adductor pollicis and handgrip exercise to increase SR in the RA and BA, respectively. Exercise intensity was modulated to achieve uniformity in SR between arteries. The three distinct SR levels were as follows: steady-state exercise 39.8 ± 0.6, 57.3 ± 0.7, and 72.4 ± 1.2 s(-1) (P < 0.001). %FMD and AbsFMD (mm) at the end of exercise were greater in the RA vs. the BA at each shear level [at the highest level: RA = 15.7 ± 1.5%, BA = 5.4 ± 0.8% (P < 0.001)]. The mean slope of the within-subject SR-%FMD regression line was greater in the RA (RA = 0.33 ± 0.04, BA = 0.13 ± 0.02, P < 0.001), and a strong within-subjects relationship between %FMD and SR was observed in both arteries (RA: r(2) = 0.92 ± 0.02; BA: r(2) = 0.90 ± 0.03). Within the RA, there was a significant relationship between baseline diameter and %FMD; however, this relationship was not present in the BA (RA: r(2) = 0.76, P < 0.001; BA: r(2) = 0.03, P = 0.541). These findings suggest that the response to SS is not uniform across differently sized vessels, which is in agreement with previous studies.     

Nitric oxide-mediated vasodilation becomes independent of beta-adrenergic receptor activation with increased intensity of hypoxic exercise

Hypoxic vasodilation in skeletal muscle at rest is known to include β-adrenergic receptor-stimulated nitric oxide (NO) release. We previously reported that the augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes β-adrenergic mechanisms. However, it is unclear whether a β-adrenergic receptor-stimulated NO component exists during hypoxic exercise. We hypothesized that NO-mediated vasodilation becomes independent of β-adrenergic receptor activation with increased exercise intensity during hypoxic exercise. Ten subjects (7 men, 3 women; 23 ± 1 yr) breathed hypoxic gas to titrate arterial O(2) saturation to 80% while remaining normocapnic. Subjects performed two consecutive bouts of incremental rhythmic forearm exercise (10% and 20% of maximum) with local administration (via a brachial artery catheter) of propranolol (β-adrenergic receptor inhibition) alone and with the combination of propranolol and nitric oxide synthase inhibition [N(G)-monomethyl-l-arginine (l-NMMA)] under normoxic and hypoxic conditions. Forearm blood flow (FBF, ml/min; Doppler ultrasound) and blood pressure [mean arterial pressure (MAP), mmHg; brachial artery catheter] were assessed, and forearm vascular conductance (FVC, ml·min(-1)·100 mmHg(-1)) was calculated (FBF/MAP). During propranolol alone, the rise in FVC (Δ from normoxic baseline) due to hypoxic exercise was 217 ± 29 and 415 ± 41 ml·min(-1)·100 mmHg(-1) (10% and 20% of maximum, respectively). Combined propranolol-l-NMMA infusion during hypoxic exercise attenuated ΔFVC at 20% (352 ± 44 ml·min(-1)·100 mmHg(-1); P < 0.001) but not at 10% (202 ± 28 ml·min(-1)·100 mmHg(-1); P = 0.08) of maximum compared with propranolol alone. These data, when integrated with earlier findings, demonstrate that NO contributes to the compensatory vasodilation during mild and moderate hypoxic exercise; a β-adrenergic receptor-stimulated NO component exists during low-intensity hypoxic exercise. However, the source of the NO becomes less dependent on β-adrenergic mechanisms as exercise intensity increases.     

Beta-adrenergic receptor blockade impairs NO-dependent dilation of large coronary arteries during exercise

Shear stress-dependent nitric oxide (NO) formation prevents immoderate vascular constriction. We examined whether shear stress-dependent NO formation limits exercise-induced coronary artery constriction after beta-adrenergic receptor blockade in dogs. Control exercise led to increases (P < 0.01) in coronary blood flow (CBF) by 38 +/- 5 ml/min from 41 +/- 5 ml/min and in the external diameter of epicardial coronary arteries (CD) by 0.24 +/- 0.03 mm from 3.33 +/- 0.20 mm. CD and shear stress were linearly related. After propranolol, CD fell (P < 0.01) during exercise (0.08 +/- 0.03 from 3.23 +/- 0.19 mm), and the slope of the relationship between CD and shear stress was reduced (P < 0.01). This slope was not further altered by the additional blockade of NO formation. In propranolol-treated resting dogs, flow-dependent effects of intracoronary adenosine to mimic exercise-induced increases in shear stress (after propranolol) led to increases (P < 0.01) in CD (0.09 +/- 0.02 from 3.68 +/- 0.27 mm). Thus both shear stress-dependent NO formation and beta-adrenergic receptor activation are required to cause CD dilation during exercise. Suppression of beta-adrenergic receptor activation leads to impaired shear stress-dependent NO formation and allows alpha-adrenergic constriction to become dominant.     

Effects of nitric oxide synthase inhibitor on decrease in peripheral arterial stiffness with acute low-intensity aerobic exercise

We previously reported that even low-intensity, short-duration acute aerobic exercise decreases arterial stiffness. We aimed to test the hypothesis that the exercise-induced decrease in arterial stiffness is caused by the increased production of NO in vascular endothelium with exercise. Nine healthy men (age: approximately 22-28 yr) performed a 5-min single-leg cycling exercise (30 W) in the supine position under an intravenous infusion of NG-monomethyl-L-arginine (L-NMMA; 3 mg/kg during the initial 5 min and subsequent continuous infusion of 50 mug.kg(-1).min(-1) in saline) or vehicle (saline) in random order on separate days. The pulse wave velocity (PWV) from the femoral to posterior tibial artery was measured on both legs before and after the infusion at rest and 2 min after exercise. Under the control condition, exercised leg PWV significantly decreased after exercise (P <0.05), whereas nonexercised leg PWV did not show a significant change throughout the experiment. Under L-NMMA administration, exercised leg PWV was increased significantly by the infusion (P <0.05) but decreased significantly after the exercise (P <0.05). Nonexercised leg PWV increased with L-NMMA administration and maintained a significantly higher level during the administration compared with baseline (before the infusion, all P <0.05). The NO synthase blockade x time interaction on exercised leg PWV was not significant (P=0.706). These results suggest that increased production of NO is not a major factor in the decrease of regional arterial stiffness with low-intensity, short-duration aerobic exercise.     

Systemic alpha-adrenergic and nitric oxide inhibition on basal limb blood flow: effects of endurance training in middle-aged and older adults

Endurance training improves endothelium-dependent vasodilation, yet it does not increase basal blood flow in the legs. We determined the effects of a 3-mo aerobic exercise intervention on basal leg blood flow and alpha-adrenergic vasoconstriction and nitric oxide (NO) release in seven apparently healthy middle-aged and older adults (60 +/- 3 yr). Basal femoral artery blood flow (via Doppler ultrasound) (pretraining: 354 +/- 29; posttraining: 335 +/- 34 ml/min) and vascular conductance did not change significantly with the exercise training. Before the exercise intervention, femoral artery blood flow increased 32 +/- 16% with systemic alpha-adrenergic blockade (with phentolamine) (P < 0.05), and the addition of nitric oxide synthase (NOS) inhibition using N(G)-monomethyl-L-arginine (L-NMMA) did not affect femoral artery blood flow. After training was completed, femoral artery blood flow increased 47 +/- 7% with alpha-adrenergic blockade (P < 0.01) and then decreased 18 +/- 7% with the subsequent administration of L-NMMA (P < 0.05). Leg vascular conductance showed a greater alpha-adrenergic blockade-induced vasodilation (+1.7 +/- 0.5 to +3.0 +/- 0.5 units, P < 0.05) as well as NOS inhibition-induced vasoconstriction (-0.8 +/- 0.4 to -2.7 +/- 0.7 units, P < 0.05) after the exercise intervention. Resting plasma norepinephrine concentration significantly increased after the training. These results suggest that regular aerobic exercise training enhances NO bioavailability in middle-aged and older adults and that basal limb blood flow does not change with exercise training because of the contrasting influences of sympathetic nervous system activity and endothelium-derived vasodilation on the vasculature.     

Mechanisms of ATP-mediated vasodilation in humans: modest role for nitric oxide and vasodilating prostaglandins

ATP is an endothelium-dependent vasodilator, and findings regarding the underlying signaling mechanisms are equivocal. We sought to determine the independent and interactive roles of nitric oxide (NO) and vasodilating prostaglandins (PGs) in ATP-mediated vasodilation in young, healthy humans and determine whether any potential role was dependent on ATP dose or the timing of inhibition. In protocol 1 (n = 18), a dose-response curve to intrabrachial infusion of ATP was performed before and after both single and combined inhibition of NO synthase [N(G)-monomethyl-L-arginine (L-NMMA)] and cyclooxygenase (ketorolac). Forearm blood flow (FBF) was measured via venous occlusion plethysmography and forearm vascular conductance (FVC) was calculated. In this protocol, neither individual nor combined NO/PG inhibition had any effect on the vasodilatory response (P = 0.22-0.99). In protocol 2 (n = 16), we determined whether any possible contribution of both NO and PGs to ATP vasodilation was greater at low vs. high doses of ATP and whether inhibition during steady-state infusion of the respective dose of ATP impacted the dilation. FBF in this protocol was measured via Doppler ultrasound. In protocol 2, infusion of low (n = 8)- and high-dose (n = 8) ATP for 5 min evoked a significant increase in FVC above baseline (low = 198 ± 24%; high = 706 ± 79%). Infusion of L-NMMA and ketorolac together reduced steady-state FVC during both low- and high-dose ATP (P < 0.05), and in a subsequent trial with continuous NO/PG blockade, the vasodilator response from baseline to 5 min of steady-state infusion was similarly reduced for both low (ΔFVC = -31 ± 11%)- and high-dose ATP (ΔFVC -25 ± 11%; P = 0.70 low vs. high dose). Collectively, our findings indicate a potential modest role for NO and PGs in the vasodilatory response to exogenous ATP in the human forearm that does not appear to be dose or timing dependent; however, this is dependent on the method for assessing forearm vascular responses. Importantly, the majority of ATP-mediated vasodilation is independent of these putative endothelium-dependent pathways in humans.     

Evidence of preserved endothelial function and vascular plasticity with age

We sought to identify the relationship between shear stimuli and flow-mediated vasodilation and to determine whether small muscle mass exercise training could provoke limb-specific improvements in endothelial function in older subjects. In five young (22 +/- 1 yr old) and six old (71 +/- 2 yr old) subjects, ultrasound Doppler measurements were taken in the arm (brachial artery) and leg (deep and superficial femoral arteries) after suprasystolic cuff occlusion with and without ischemic exercise to evaluate flow-mediated dilation (FMD) in both limbs. Older subjects were reevaluated after 6 wk of single-leg knee extensor exercise training. Before the training, a significant FMD was observed in the arm of young (3 +/- 1%) but not old (1 +/- 1%) subjects, whereas a significant leg FMD was observed in both groups (5 +/- 1% old vs. 3 +/- 1% young). However, arm vasodilation was similar between young and old when normalized for shear rate, and cuff occlusion with superimposed handgrip exercise provoked additional shear, which proportionately improved the FMD response in both groups. Exercise training significantly improved arm FMD (5 +/- 1%), whereas leg FMD was unchanged. However, ischemic handgrip exercise did not provoke additional arm vasodilation after training, which may indicate an age-related limit to shear-induced vasodilation. Together, these data demonstrate that vascular reactivity is dependent on limb and degree of shear stimuli, challenging the convention of diminished endothelial function typically associated with age. Likewise, exercise training improved arm vasodilation, indicating some preservation of vascular plasticity with age.     

Male-female differences in the relative contribution of endothelial vasodilators released by rat tail artery

Several different vasodilator substances can be released by vascular endothelium in response to mechanical stimuli and vasoactive agents. The purpose of this study was to determine whether there is a male-female difference in the relative contributions of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent vasodilation. Perfusion pressure was measured in isolated tail arteries from male and female rats. Vasodilators released by mechanical shear stress were assessed by constricting the artery with methoxamine; acetylcholine was applied to induce receptor-mediated vasodilation. We used an inhibitor of NO synthase, N(G)-monomethyl-L-arginine acetate (L-NMMA), and elevated levels of K(+) (27 mM) to reveal the relative contributions of NO and EDHF, respectively. Indomethacin was present in all experiments to block prostanoid production. The results indicate that NO was the primary vasodilator released by male tail arteries in response to both mechanical stress and acetylcholine (the L-NMMA-sensitive component of the combined L-NMMA/K(+) effect was 83 +/- 8% and 101 +/- 4%, respectively). However female tail arteries appeared to utilize both NO and EDHF for vascular relaxation (e.g., L-NMMA sensitivity: 58 +/- 9%; K+-sensitivity: 42 +/- 9% in mechanical stress experiments). These findings suggest endothelial regulation differs between males and females.     

Interaction between prostanoids and nitric oxide in regulation of systemic, pulmonary, and coronary vascular tone in exercising swine

Prostacyclin and nitric oxide (NO) are produced by the endothelium in response to physical forces such as shear stress. Consequently, both NO and prostacyclin may increase during exercise and contribute to metabolic vasodilation. Conversely, NO has been hypothesized to inhibit prostacyclin production. We therefore investigated the effect of cyclooxygenase (COX) inhibition on exercise-induced vasodilation of the porcine systemic, pulmonary, and coronary beds before and after inhibition of NO production. Swine were studied at rest and during treadmill exercise at 1-5 km/h, before and after COX inhibition with indomethacin (10 mg/kg iv), and in the absence and presence of NO synthase inhibition with N(omega)-nitro-l-arginine (l-NNA; 20 mg/kg iv). COX inhibition produced systemic vasoconstriction at rest, which waned during exercise. The systemic vasoconstriction by COX inhibition was enhanced after l-NNA, particularly at rest. In the coronary circulation, COX inhibition also resulted in vasoconstriction at rest and during exercise. However, vasoconstriction was not modified by pretreatment with l-NNA. In contrast, COX inhibition had no effect on the pulmonary circulation, either at rest or during exercise. Moreover, a prostanoid influence in the pulmonary circulation could not be detected after l-NNA. In conclusion, endogenous prostanoids contribute importantly to systemic and coronary tone in awake swine at rest but are not mandatory for exercise-induced vasodilation in these beds. Endogenous prostanoids are not mandatory for the regulation of pulmonary resistance vessel tone. Finally, NO blunts the contribution of prostanoids to vascular tone regulation in the systemic but not in the coronary and pulmonary beds.     

Impact of repeated increases in shear stress via reactive hyperemia and handgrip exercise: no evidence of systematic changes in brachial artery FMD

Reactive hyperemia (RH) creates an uncontrolled, transient increase in brachial artery (BA) shear stress (SS) for flow-mediated dilation (FMD) assessment. In contrast, handgrip exercise (HGEX) can create similar, sustained SS increases over repeated trials. The purpose of this study was to examine the impact of repeated SS elevation via RH or HGEX and the relationship between RH and HGEX %FMD. BA diameter and blood velocity were assessed with echo and Doppler ultrasound in 20 healthy subjects. Visit A consisted of four 6-min HGEX trials (HGEX trials 1-4) at the intensity required to achieve a shear rate (SR = mean blood velocity/BA diameter; an estimate of SS) of 65 s(-1). Visit B consisted of four RH trials (RH trials 1-4). The RH SR area under the curve (AUC) was higher in trial 1 versus trial 3 and trial 4 (P = 0.019 and 0.047). The HGEX mean SR was similar across trials (mean SR = 66.1 ± 5.8 s(-1), P = 0.152). There were no differences in %FMD across trials or tests (RH trial 1: 6.9 ± 3.5%, trial 2: 6.9 ± 2.3%, trial 3: 7.1 ± 3.5%, and trial 4: 7.0 ± 2.8%; HGEX trial 1: 7.3 ± 3.6%, trial 2: 7.0 ± 3.6%, trial 3: 6.5 ± 3.5%, and trial 4: 6.8 ± 2.9%, P = 0.913). No relationship between subject's RH %FMD and HGEX %FMD was detected (r(2) = 0.12, P = 0.137). However, with response normalization, a relationship emerged (RH %FMD/SR AUC vs. HGEX %FMD/mean SR, r(2) = 0.44, P = 0.002). In conclusion, with repeat trials, there were no systematic changes in RH or HGEX %FMD. The relationship between normalized RH and HGEX %FMD suggests that endothelial responses to different SS profiles provide related information regarding endothelial function.     

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.     

Brachial artery flow-mediated dilation during handgrip exercise: evidence for endothelial transduction of the mean shear stimulus

Exercise elevates shear stress in the supplying conduit artery. Although this is the most relevant physiological stimulus for flow-mediated dilation (FMD), the fluctuating pattern of shear that occurs may influence the shear stress-FMD stimulus response relationship. This study tested the hypothesis that the brachial artery FMD response to a step increase in shear is influenced by the fluctuating characteristics of the stimulus, as evoked by forearm exercise. In 16 healthy subjects, we examined FMD responses to step increases in shear rate in three conditions: stable shear upstream of heat-induced forearm vasodilation (FHStable); fluctuating shear upstream of heat-induced forearm vasodilation and rhythmic forearm cuff inflation/deflation (FHFluctuating); and fluctuating shear upstream of exercise-induced forearm vasodilation (FEStep Increase). The mean increase in shear rate (+/-SD) was the same in all trials (FHFluctuating): 51.69 +/- 15.70 s(-1); FHStable: 52.16 +/- 14.10 s(-1); FEStep Increase: 50.14 +/- 13.03 s(-1) P = 0.131). However, the FHFluctuating and FEStep Increase trials resulted in a fluctuating shear stress stimulus with rhythmic high and low shear periods that were 96.18 +/- 24.54 and 11.80 +/- 7.30 s(-1), respectively. The initial phase of FMD (phase I) was followed by a second, delayed-onset FMD and was not different between conditions (phase I: FHFluctuating: 5.63 +/- 2.15%; FHStable: 5.33 +/- 1.85%; FEStep Increase: 5.30 +/- 2.03%; end-trial: FHFluctuating: 7.76 +/- 3.40%; FHStable: 7.00 +/- 3.03%; FEStep Increase: 6.68 +/- 3.04%; P = 0.196). Phase I speed also did not differ (P = 0.685). In conclusion, the endothelium transduced the mean shear when exposed to shear fluctuations created by a typical handgrip protocol. Muscle activation did not alter the FMD response. Forearm exercise may provide a viable technique to investigate brachial artery FMD in humans.     

Nitric oxide exerts feedback inhibition on EDHF-induced coronary arteriolar dilation in vivo

We tested the hypothesis that nitric oxide (NO) inhibits endothelium-derived hyperpolarizing factor (EDHF)-induced vasodilation via a negative feedback pathway in the coronary microcirculation. Coronary microvascular diameters were measured using stroboscopic fluorescence microangiography. Bradykinin (BK)-induced dilation was mediated by EDHF, when NO and prostaglandin syntheses were inhibited, or by NO when EDHF and prostaglandin syntheses were blocked. Specifically, BK (20, 50, and 100 ng. kg(-1). min(-1) ic) caused dose-dependent vasodilation similarly before and after administration of N(G)-monomethyl-L-arginine (L-NMMA) (3 micromol/min ic for 10 min) and indomethacin (Indo, 10 mg/kg iv). The residual dilation to BK with L-NMMA and Indo was completely abolished by suffusion of miconazole or an isosmotic buffer containing high KCl (60 mM), suggesting that this arteriolar vasodilation is mediated by the cytochrome P-450 derivative EDHF. BK-induced dilation was reduced by 39% after inhibition of EDHF and prostaglandin synthesis, and dilation was further inhibited by combined blockade with L-NMMA to a 74% reduction in the response. This suggests an involvement for NO in the vasodilation. After dilation to BK was assessed with L-NMMA and Indo, sodium nitroprusside (SNP, 1-3 microgram. kg(-1). min(-1) ic), an exogenous NO donor, was administered in a dose to increase the diameter to the original control value. Dilation to BK was virtually abolished when administered concomitantly with SNP during L-NMMA and Indo (P < 0.01 vs. before SNP), suggesting that NO inhibits EDHF-induced dilation. SNP did not affect adenosine- or papaverine-induced arteriolar dilation in the presence of L-NMMA and Indo, demonstrating that the effect of SNP was not nonspecific. In conclusion, our data are the first in vivo evidence to suggest that NO inhibits the production and/or action of EDHF in the coronary microcirculation.     

Nitric oxide regulates retinal vascular tone in humans

The purpose of the present study was to investigate the contribution of basal nitric oxide (NO) on retinal vascular tone in humans. In addition, we set out to elucidate the role of NO in flicker-induced retinal vasodilation in humans. Twelve healthy young subjects were studied in a three-way crossover design. Subjects received an intravenous infusion of either placebo or NG-monomethyl-L-arginine (L-NMMA; 3 or 6 mg/kg over 5 min), an inhibitor of NO synthase. Thereafter, diffuse luminance flicker was consecutively performed for 16, 32, and 64 s at a frequency of 8 Hz. The effect of L-NMMA on retinal arterial and venous diameter was assessed under resting conditions and during the hyperemic flicker response. Retinal vessel diameter was measured with a Zeiss retinal vessel analyzer. L-NMMA significantly reduced arterial diameter (3 mg/kg: -2%; 6 mg/kg: -4%, P < 0.001) and venous diameter (3 mg/kg: -5%; 6 mg/kg: -8%, P < 0.001). After placebo infusion, flicker induced a significant increase in retinal vessel diameter (P < 0.001). At a flicker duration of 64 s, arterial diameter increased by 4% and venous diameter increased by 3%. L-NMMA did not abolish these hyperemic responses but blunted venous vasodilation (P = 0.017) and arterial vasodilation (P = 0.02) in response to flicker stimulation. Our data indicate that NO contributes to basal retinal vascular tone in humans. In addition, NO appears to play a role in flicker-induced vasodilation of the human retinal vasculature.     

Prostaglandins do not contribute to the nitric oxide-mediated compensatory vasodilation in hypoperfused exercising muscle

We tested the hypothesis that 1) prostaglandins (PGs) contribute to compensatory vasodilation in contracting human forearm subjected to acute hypoperfusion, and 2) the combined inhibition of PGs and nitric oxide would attenuate the compensatory vasodilation more than PG inhibition alone. In separate protocols, subjects performed forearm exercise (20% of maximum) during hypoperfusion evoked by intra-arterial balloon inflation. Each trial included baseline, exercise before inflation, exercise with inflation, and exercise after deflation. Forearm blood flow (FBF; ultrasound) and local (brachial artery) and systemic arterial pressure [mean arterial pressure (MAP); Finometer] were measured. In protocol 1 (n = 8), exercise was repeated during cyclooxygenase (COX) inhibition (Ketorolac) alone and during Ketorolac-NOS inhibition [N(G)-monomethyl-l-arginine (l-NMMA)]. In protocol 2 (n = 8), exercise was repeated during l-NMMA alone and during l-NMMA-Ketorolac. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from FBF (ml/min) and local MAP (mmHg). The percent recovery in FVC during inflation was calculated as (steady-state inflation + exercise value - nadir)/[steady-state exercise (control) value - nadir] × 100. In protocol 1, COX inhibition alone did not reduce the %FVC recovery compared with the control (no drug) trial (92 ± 11 vs. 100 ± 10%, P = 0.83). However, combined COX-nitric oxide synthase (NOS) inhibition caused a substantial reduction in %FVC recovery (54 ± 8%, P < 0.05 vs. Ketorolac alone). In protocol 2, the percent recovery in FVC was attenuated with NOS inhibition alone (69 ± 9 vs. 107 ± 10%, P < 0.01) but not attenuated further during combined NOS-COX inhibition (62 ± 10%, P = 0.74 vs. l-NMMA alone). Our data indicate that PGs are not obligatory to the compensatory dilation observed during forearm exercise with hypoperfusion.     

Nitric oxide production is maintained in exercising swine with chronic left ventricular dysfunction

Left ventricular (LV) dysfunction caused by myocardial infarction (MI) is accompanied by endothelial dysfunction, most notably a loss of nitric oxide (NO) availability. We tested the hypothesis that endothelial dysfunction contributes to impaired tissue perfusion during increased metabolic demands as produced by exercise, and we determined the contribution of NO to regulation of regional systemic, pulmonary, and coronary vasomotor tone in exercising swine with LV dysfunction produced by a 2- to 3-wk-old MI. LV dysfunction resulted in blunted systemic and coronary vasodilator responses to ATP, whereas the responses to nitroprusside were maintained. Exercise resulted in blunted systemic and pulmonary vasodilator responses in MI that resembled the vasodilator responses in normal (N) swine following blockade of NO synthase with N(omega)-nitro-L-arginine (L-NNA, 20 mg/kg iv). However, L-NNA resulted in similar decreases in systemic (43 +/- 3% in N swine and 49 +/- 4% in MI swine), pulmonary (45 +/- 5% in N swine and 49 +/- 4% in MI swine), and coronary (28 +/- 4% in N and 35 +/- 3% in MI) vascular conductances in N and MI swine under resting conditions; similar effects were observed during treadmill exercise. Selective inhibition of inducible NO synthase with aminoguanidine (20 mg/kg iv) had no effect on vascular tone in MI. These findings indicate that while agonist-induced vasodilation is already blunted early after myocardial infarction, the contribution of endothelial NO synthase-derived NO to regulation of vascular tone under basal conditions and during exercise is maintained.     

Rapid non-genomic effects of aldosterone on the renal vasculature

There is increasing evidence for the importance of rapid non-genomic effects of aldosterone on the human vasculature including renal vessels. Arima and colleagues by examining isolated perfused afferent and efferent arterioles from rabbit kidneys found a vasoconstriction in both. In another study the same group showed that endothelium-derived nitric oxide (NO) modulates the vasoconstrictor response to aldosterone in rabbit preglomerular afferent arterioles. Disrupting the endothelium as well as blockade of endothelial NO synthase (eNOS) augmented aldosterone-induced vasoconstriction in this study. Uhrenholt et al. found no effect of aldosterone alone to afferent arterioles but a suppression of depolarisation-induced vasoconstriction. After the blockade of eNOS the aldosterone effect was completely suppressed. In a clinical study in healthy male volunteers injection of aldosterone had no statistically significant effects. Co-infusion of the eNOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) changed the effect of aldosterone on renal hemodynamics. Aldosterone in co-infusion with L-NMMA decreased renal plasma flow (RPF) much stronger than L-NMMA alone. Infusion of L-NMMA alone increased GFR whereas aldosterone/L-NMMA lowered GFR slightly. Aldosterone co-infused with L-NMMA strongly increased renal vascular resistance (RVR). The increase was on top of the smaller increase that was induced by L-NMMA infusion. These data indicate that aldosterone acts via rapid non-genomic effects in vivo in humans at the renal vasculature. Antagonizing the endothelial nitric oxide synthase unmasks these effects. Therefore, rapid non-genomic aldosterone effects increase renal vascular resistance and thereby may mediate arterial hypertension if endothelial dysfunction is present.