Combined NO and PG inhibition augments alpha-adrenergic vasoconstriction in contracting human skeletal muscle

The effect of self-identified arm dominance on exercising forearm hemodynamics and skeletal muscle desaturation

The human forearm model is commonly employed in physiological investigations exploring local vascular function and oxygen delivery; however, the effect of arm dominance on exercising forearm hemodynamics and skeletal muscle oxygen saturation (SmO2) in untrained individuals is poorly understood. Therefore, the purpose of this study was to explore the effect of self-identified arm dominance on forearm hemodynamics and SmO2 in untrained individuals during submaximal, non-ischemic forearm exercise. Twenty healthy individuals (23±4 years, 50% female; 80% right-handed) completed three-minute bouts of supine rhythmic (1 second contraction: 2 second relaxation duty cycle) forearm handgrip exercise at both absolute (10kg; 98N) and relative (30% of maximal voluntary contraction) intensities in each forearm. Beat-by-beat measures of forearm blood flow (FBF; ml/min), mean arterial blood pressure (MAP; mmHg) and flexor digitorum superficialis SmO2 (%) were obtained throughout and averaged during the final 30 seconds of rest, exercise, and recovery while forearm vascular conductance was calculated (FVC; ml/min/100mmHg). Data are Δ from rest (mean±SD). Absolute force production did not differ between non-dominant and dominant arms (97±11 vs. 98±13 N, p = 0.606) whereas relative force production in females did (69±24 vs. 82±25 N, p = 0.001). At both exercise intensities, FBFRELAX, FVCRELAX, MAPRELAX, and the time constant tau for FBF and SmO2 were unaffected by arm dominance (all p>0.05). While arm dominance did not influence SmO2 during absolute intensity exercise (p = 0.506), the non-dominant arm in females experienced an attenuated reduction in SmO2 during relative intensity exercise (-14±10 vs. -19±8%, p = 0.026)-though exercise intensity was also reduced (p = 0.001). The present investigation has demonstrated that arm dominance in untrained individuals does not impact forearm hemodynamics or SmO2 during handgrip exercise.

Exercise pressor responses are exaggerated relative to force production during, but not following, thirty-minutes of rhythmic handgrip exercise

PURPOSE

This study tested the hypothesis that blood pressure responses would increase relative to force production in response to prolonged bouts of muscular work.

METHODS

Fifteen individuals performed two minutes of static handgrip (SHG; 35% MVC), followed by three minutes of post-exercise-cuff-occlusion (PECO), before and after thirty minutes of rest (control), or rhythmic handgrip exercise (RHG) of the contralateral and ipsilateral forearms. Beat-by-beat recordings of mean arterial pressure (MAP), heart rate (HR), and handgrip force (kg) were averaged across one-minute periods at baseline, and minutes 5, 10, 15, 20, 25, and 30 of RHG. MAP was also normalized to handgrip force, providing a relative measure of exercise pressor responses (mmHg/kg). Hemodynamic responses to SHG and PECO were also compared before and after contralateral RHG, ipsilateral RHG, and control, respectively. Similar to the RHG trial, areas under the curve were calculated for MAP (blood pressure index; BPI) and normalized to the time tension index (BPInorm).

RESULTS

HR and MAP significantly increased during RHG (15.3 ± 1.4% and 20.4 ± 3.2%, respectively, both p < 0.01), while force output decreased by up to 36.6 ± 8.0% (p < 0.01). This resulted in a 51.6 ± 9.4% increase in BPInorm during 30 min of RHG (p < 0.01). In contrast, blood pressure responses to SHG and PECO were unchanged following RHG (all p ≥ 0.07), and only the mean HR (4.2 ± 1.5%, p = 0.01) and ΔHR (67.2 ± 18.1%, p < 0.01) response to SHG were exaggerated following ipsilateral RHG.

CONCLUSIONS

The magnitude of exercise pressor responses relative to force production progressively increases during, but not following, prolonged bouts of muscular work.

Therapeutic Options for Migraines in the Microsurgical Patient: A Scoping Review

BACKGROUND

There exists an increasing array of treatments proposed to prevent, alleviate, and abort symptoms of a migraine; however, for patients who undergo reconstructive microsurgery, caution must be taken to preserve vascular integrity. This study is the first-to-date scoping review of vascular and bleeding risk of current migraine therapies, with the purpose of identifying potential therapeutic agents for postoperative migraine management appropriate for microsurgical patients.

METHODS

Currently available migraine therapeutics were compiled from the UpToDate software system and the American Academy of Family Physicians. A PubMed literature review was performed for each therapeutic's effect on bleeding or vascular involvement. Data were compiled into tables of abortive, symptom-controlling and prophylactic, and nonpharmacologic treatments. Expert microsurgeons reviewed the data to provide recommendations for optimized patient care.

RESULTS

Triptans and other ergot derivatives demonstrated strong evidence of vasoconstriction and were greatly advised against for immediate postmicrosurgical use. Novel pharmaceutical therapies such as lasmiditan and calcitonin gene-related peptide antagonists have no literature indicating potential for vasoconstriction or hematoma and remain an investigational option for abortive medical treatment. For symptom control, acetaminophen appears the safest option, with clinical judgment and further research needed for use of nonsteroidal antiinflammatory drugs. Alternative treatment techniques may include migraine prophylaxis with botulinum toxin injection or nutraceutical treatment by means of magnesium supplementation or coenzyme Q10 administration, minimizing the need for additional medication in the postoperative setting.

CONCLUSIONS

Patients undergoing reconstructive microsurgery have a unique medical profile limiting the therapeutic options available to treat migraines. This review provides preliminary evidence to be considered as a guide for prescribing therapeutics for migraine in the postoperative setting.

A single high-fat Western meal modulates vascular responsiveness to sympathetic activation at rest and during exercise in humans: a randomized controlled trial

A single high-fat Western meal transiently reduces endothelium-dependent vasodilation at rest, but the interaction with sympathetic vasoconstrictor activity during exercise remains unknown. Herein, we tested the hypothesis that a single high-fat Western meal would impair the ability of contracting skeletal muscle to offset vascular responsiveness to sympathetic activation during exercise, termed functional sympatholysis. In 18 (10 females/8 males) healthy young adults, forearm blood flow (Doppler ultrasound) and beat-to-beat arterial pressure (photoplethysmography) were measured during lower-body negative pressure (LBNP; -20 mmHg) applied at rest and simultaneously during low (15% maximum contraction) and moderate (30% maximum contraction)-intensity rhythmic handgrip exercise. The magnitude of sympatholysis was calculated as the difference of LBNP-induced changes in forearm vascular conductance (FVC) between handgrip and rest. Experiments were performed preprandial and 1 h, 2 h, and 3 h after a high- or low-fat meal. In the preprandial state, LBNP decreased resting FVC (Δ-54 ± 10%), and these responses were attenuated during low (Δ-17 ± 7%)- and moderate (Δ-8 ± 6%)-intensity handgrip exercise. Following a high-fat meal, LBNP induced attenuated decreases in resting FVC (3 h postprandial, Δ-47 ± 10%, P = 0.002 vs. preprandial) and blunted attenuation of FVC during low (3 h postprandial, Δ-23 ± 8%, P = 0.001 vs. preprandial)- and moderate (3 h postprandial, Δ-16 ± 6%, P < 0.001 vs. preprandial)-intensity handgrip exercise. The high-fat meal attenuated the magnitude of sympatholysis during low (preprandial, 38 ± 7 vs. 3 h postprandial, 23 ± 8%, P < 0.001)- and moderate (preprandial, 46 ± 11 vs. 3 h postprandial, 31 ± 10%, P < 0.001)-intensity handgrip exercise. The low-fat meal had no impact on these responses. In conclusion, a single high-fat Western meal modulates sympathetic vasoconstriction at rest and during low- and moderate-intensity handgrip exercise in young healthy adults.NEW & NOTEWORTHY We observed that a single high-fat Western meal, but not an isocaloric low-fat meal, attenuated the sympathetic vasoconstriction at rest and the ability of the active skeletal muscle to counteract the vascular responsiveness to sympathetic activation (i.e., functional sympatholysis) during low- and moderate-intensity rhythmic handgrip exercise in healthy young adults. Our findings highlight the potential deleterious vascular effect associated with the consumption of a Western diet.

Rho-kinase inhibition improves haemodynamic responses and circulating ATP during hypoxia and moderate intensity handgrip exercise in healthy older adults

Abstract

Skeletal muscle haemodynamics and circulating adenosine triphosphate (ATP) responses during hypoxia and exercise are blunted in older (OA) vs. young (YA) adults, which may be associated with impaired red blood cell (RBC) ATP release. Rho‐kinase inhibition improves deoxygenation‐induced ATP release from OA isolated RBCs. We tested the hypothesis that Rho‐kinase inhibition (via fasudil) in vivo would improve local haemodynamic and ATP responses during hypoxia and exercise in OA. Healthy YA (25 ± 3 years; n = 12) and OA (65 ± 5 years; n = 13) participated in a randomized, double‐blind, placebo‐controlled, crossover study on two days (≥5 days between visits). A forearm deep venous catheter was used to administer saline/fasudil and sample venous plasma ATP ([ATP]_V). Forearm vascular conductance (FVC) and [ATP]_V were measured at rest, during isocapnic hypoxia (80% SpO2), and during graded rhythmic handgrip exercise that was similar between groups (5, 15 and 25% maximum voluntary contraction (MVC)). Isolated RBC ATP release was measured during normoxia/hypoxia. With saline, ΔFVC was lower (P < 0.05) in OA vs. YA during hypoxia (∼60%) and during 15 and 25% MVC (∼25–30%), and these impairments were abolished with fasudil. Similarly, [ATP]_V and ATP effluent responses from normoxia to hypoxia and rest to 25% MVC were lower in OA vs. YA and improved with fasudil (P < 0.05). Isolated RBC ATP release during hypoxia was impaired in OA vs. YA (∼75%; P < 0.05), which tended to improve with fasudil in OA (P = 0.082). These data suggest Rho‐kinase inhibition improves haemodynamic responses to hypoxia and moderate intensity exercise in OA, which may be due in part to improved circulating ATP.

Key points

Skeletal muscle blood flow responses to hypoxia and exercise are impaired with age. Blunted increases in circulating ATP, a vasodilator, in older adults may contribute to age‐related impairments in haemodynamics.

Red blood cells (RBCs) are a primary source of circulating ATP, and treating isolated RBCs with a Rho‐kinase inhibitor improves age‐related impairments in deoxygenation‐induced RBC ATP release.

In this study, treating healthy older adults systemically with the Rho‐kinase inhibitor fasudil improved blood flow and circulating ATP responses during hypoxia and moderate intensity handgrip exercise compared to young adults, and also tended to improve isolated RBC ATP release.

Improved blood flow regulation with fasudil was also associated with increased skeletal muscle oxygen delivery during hypoxia and exercise in older adults.

This is the first study to demonstrate that Rho‐kinase inhibition can significantly improve age‐related impairments in haemodynamic and circulating ATP responses to physiological stimuli, which may have therapeutic implications.

Neurovascular Dysregulation During Exercise in Type 2 Diabetes

Emerging evidence suggests that type 2 diabetes (T2D) may impair the ability to properly adjust the circulation during exercise with augmented blood pressure (BP) and an attenuated contracting skeletal muscle blood flow (BF) response being reported. This review provides a brief overview of the current understanding of these altered exercise responses in T2D and the potential underlying mechanisms, with an emphasis on the sympathetic nervous system and its regulation during exercise. The research presented support augmented sympathetic activation, heightened BP, reduced skeletal muscle BF, and impairment in the ability to attenuate sympathetically mediated vasoconstriction (i.e., functional sympatholysis) as potential drivers of neurovascular dysregulation during exercise in T2D. Furthermore, emerging evidence supporting a contribution of the exercise pressor reflex and central command is discussed along with proposed future directions for studies in this important area of research.

Greater α1-adrenergic-mediated vasoconstriction in contracting skeletal muscle of patients with type 2 diabetes

Findings presented in this article are the first to show patients with type 2 diabetes mellitus have blunted hyperemic and vasodilatory responses to dynamic handgrip exercise. Moreover, we illustrate greater α1-adrenergic-mediated vasoconstriction may contribute to our initial observations. Collectively, these data suggest patients with type 2 diabetes may have impaired functional sympatholysis, which can contribute to their reduced exercise capacity.

Effect of acute dietary nitrate supplementation on sympathetic vasoconstriction at rest and during exercise

Dietary nitrate ( NO3- ) supplementation has been shown to reduce resting blood pressure. However, the mechanism responsible for the reduction in blood pressure has not been identified. Dietary NO3- supplementation may increase nitric oxide (NO) bioavailability, and NO has been shown to inhibit sympathetic vasoconstriction in resting and contracting skeletal muscle. Therefore, the purpose of this study was to investigate the hypothesis that acute dietary NO3- supplementation would attenuate sympathetic vasoconstrictor responsiveness at rest and during exercise. In a double-blind randomized crossover design, 12 men (23 ± 5 yr) performed a cold-pressor test (CPT) at rest and during moderate- and heavy-intensity alternate-leg knee-extension exercise after consumption of NO3- rich beetroot juice (~12.9 mmol NO3- ) or a NO3- -depleted placebo (~0.13 mmol NO3- ). Venous blood was sampled before and 2.5 h after the consumption of beetroot juice for the measurement of total plasma nitrite/ NO3- [NO_x]. Beat-by-beat blood pressure was measured by Finometer. Leg blood flow was measured at the femoral artery via Doppler ultrasound, and leg vascular conductance (LVC) was calculated. Sympathetic vasoconstrictor responsiveness was calculated as the percentage decrease in LVC in response to the CPT. Total plasma [NO_x] was greater (P < 0.001) in the NO3- (285 ± 120 µM) compared with the placebo (65 ± 30 µM) condition. However, mean arterial blood pressure and plasma catecholamines were not different (P > 0.05) between NO3- and placebo conditions at rest or during moderate- and heavy-intensity exercise. Sympathetic vasoconstrictor responsiveness (Δ% LVC) was not different (P > 0.05) between NO3- and placebo conditions at rest ( NO3- : -33 ± 10%; placebo: -35 ± 11%) or during moderate ( NO3- : -18 ± 8%; placebo: -20 ± 10%)- and heavy ( NO3- : -12 ± 8%; placebo: -11 ± 9%)-intensity exercise. These data demonstrate that acute dietary NO3- supplementation does not alter sympathetic vasoconstrictor responsiveness at rest or during exercise in young healthy males. NEW & NOTEWORTHY Dietary nitrate may increase nitric oxide bioavailability, and nitric oxide has been shown to attenuate sympathetic vasoconstriction in resting and contracting skeletal muscle and enhance functional sympatholysis. However, the effect of dietary nitrate on sympathetic vasoconstrictor responsiveness is unknown. Acute dietary nitrate supplementation did not alter blood pressure or sympathetic vasoconstrictor responsiveness at rest or during exercise in young healthy males.

Functional sympatholysis is impaired in end-stage renal disease

Patients with end-stage renal disease (ESRD) have decreased exercise capacity and exercise intolerance that contribute to cardiovascular risk. One potential mechanism underlying exercise intolerance in ESRD is impaired ability to oppose sympathetically mediated vasoconstriction within exercising skeletal muscle (i.e., functional sympatholysis, FS). We hypothesized that ESRD patients have impaired FS compared with healthy (CON) and hypertensive (HTN) controls and that impaired FS is related to circulating levels of the uremic toxin asymmetric dimethyl arginine (ADMA), an endogenous nitric oxide synthase inhibitor. Near-infrared spectroscopy-derived oxygen tissue saturation index (TSI) of the forearm muscle was measured continuously in 33 participants (9 CON, 14 HTN, 10 ESRD) at rest and during low-dose (-20 mmHg) lower body negative pressure (LBNP), moderate rhythmic handgrip exercise, and LBNP with concomitant handgrip exercise (LBNP+handgrip). Resting muscle TSI was lower in ESRD than in CON and HTN groups (CON = 67.8 ± 1.9%, HTN = 67.2 ± 1.1%, ESRD = 62.7 ± 1.5%, P = 0.03). Whereas CON and HTN groups had an attenuation in sympathetically mediated reduction in TSI during LBNP + handgrip compared with LBNP alone (P ≤ 0.05), this response was not present in ESRD (P = 0.71), suggesting impaired FS. There was no difference in plasma [ADMA] between groups (CON = 0.47 ± 0.05 µmol/l, HTN = 0.42 ± 0.06 µmol/l, ESRD = 0.63 ± 0.14 µmol/l, P = 0.106) and no correlation between plasma [ADMA] and resting muscle TSI (P = 0.84) or FS (P = 0.75). Collectively, these findings suggest that ESRD patients have lower muscle perfusion at rest and impaired FS but that these derangements are not related to circulating [ADMA].

Acute ingestion of dietary nitrate increases muscle blood flow via local vasodilation during handgrip exercise in young adults

Dietary nitrate (NO3−) is converted to nitrite (NO2−) and can be further reduced to the vasodilator nitric oxide (NO) amid a low O₂ environment. Accordingly, dietary NO3− increases hind limb blood flow in rats during treadmill exercise; however, the evidence of such an effect in humans is unclear. We tested the hypothesis that acute dietary NO3− (via beetroot [BR] juice) increases forearm blood flow (FBF) via local vasodilation during handgrip exercise in young adults (n = 11; 25 ± 2 years). FBF (Doppler ultrasound) and blood pressure (Finapres) were measured at rest and during graded handgrip exercise at 5%, 15%, and 25% maximal voluntary contraction (MVC) lasting 4 min each. At the highest workload (25% MVC), systemic hypoxia (80% SaO₂) was induced and exercise continued for three additional minutes. Subjects ingested concentrated BR (12.6 mmol nitrate (n = 5) or 16.8 mmol nitrate (n = 6) and repeated the exercise bout either 2 (12.6 mmol) or 3 h (16.8 mmol) postconsumption. Compared to control, BR significantly increased FBF at 15% MVC (184 ± 15 vs. 164 ± 15 mL/min), 25% MVC (323 ± 27 vs. 286 ± 28 mL/min), and 25% + hypoxia (373 ± 39 vs. 343 ± 32 mL/min) and this was due to increases in vascular conductance (i.e., vasodilation). The effect of BR on hemodynamics was not different between the two doses of BR ingested. Forearm VO₂ was also elevated during exercise at 15% and 25% MVC. We conclude that acute increases in circulating NO3− and NO2− via BR increases muscle blood flow during moderate‐ to high‐intensity handgrip exercise via local vasodilation. These findings may have important implications for aging and diseased populations that demonstrate impaired muscle perfusion and exercise intolerance.

Nitric oxide-dependent attenuation of noradrenaline-induced vasoconstriction is impaired in the canine model of Duchenne muscular dystrophy

KEY POINTS

We developed a novel method to study sympatholysis in dogs. We showed abolishment of sarcolemmal nNOS, and reduction of total nNOS and total eNOS in the canine Duchenne muscular dystrophy (DMD) model. We showed sympatholysis in dogs involving both nNOS-derived NO-dependent and NO-independent mechanisms. We showed that the loss of sarcolemmal nNOS compromised sympatholysis in the canine DMD model. We showed that NO-independent sympatholysis was not affected in the canine DMD model.

ABSTRACT

The absence of dystrophin in Duchenne muscular dystrophy (DMD) leads to the delocalization of neuronal nitric oxide synthase (nNOS) from the sarcolemma. Sarcolemmal nNOS plays an important role in sympatholysis, a process of attenuating reflex sympathetic vasoconstriction during exercise to ensure blood perfusion in working muscle. Delocalization of nNOS compromises sympatholysis resulting in functional ischaemia and muscle damage in DMD patients and mouse models. Little is known about the contribution of membrane-associated nNOS to blood flow regulation in dystrophin-deficient DMD dogs. We tested the hypothesis that the loss of sarcolemmal nNOS abolishes protective sympatholysis in contracting muscle of affected dogs. Haemodynamic responses to noradrenaline in the brachial artery were evaluated at rest and during contraction in the absence and presence of NOS inhibitors. We found sympatholysis was significantly compromised in DMD dogs, as well as in normal dogs treated with a selective nNOS inhibitor, suggesting that the absence of sarcolemmal nNOS underlies defective sympatholysis in the canine DMD model. Surprisingly, inhibition of all NOS isoforms did not completely abolish sympatholysis in normal dogs, suggesting sympatholysis in canine muscle also involves NO-independent mechanism(s). Our study established a foundation for using the dog model to test therapies aimed at restoring nNOS homeostasis in DMD.

Inhibition of Na+ /K+ -ATPase and KIR channels abolishes hypoxic hyperaemia in resting but not contracting skeletal muscle of humans

KEY POINTS

Increasing blood flow (hyperaemia) to exercising muscle helps match oxygen delivery and metabolic demand. During exercise in hypoxia, there is a compensatory increase in muscle hyperaemia that maintains oxygen delivery and tissue oxygen consumption. Nitric oxide (NO) and prostaglandins (PGs) contribute to around half of the augmented hyperaemia during hypoxic exercise, although the contributors to the remaining response are unknown. In the present study, inhibiting NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels did not blunt augmented hyperaemia during hypoxic exercise beyond previous observations with NO/PG block alone. Furthermore, although inhibition of only Na⁺ /K⁺ -ATPase and K_IR channels abolished hyperaemia during hypoxia at rest, it had no effect on augmented hyperaemia during hypoxic exercise. This is the first study in humans to demonstrate that Na⁺ /K⁺ -ATPase and K_IR channel activation is required for augmented muscle hyperaemia during hypoxia at rest but not during hypoxic exercise, thus providing new insight into vascular control.

ABSTRACT

Exercise hyperaemia in hypoxia is augmented relative to the same exercise intensity in normoxia. During moderate-intensity handgrip exercise, endothelium-derived nitric oxide (NO) and vasodilating prostaglandins (PGs) contribute to ∼50% of the augmented forearm blood flow (FBF) response to hypoxic exercise (HypEx), although the mechanism(s) underlying the remaining response are unclear. We hypothesized that combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels would abolish the augmented hyperaemic response in HypEx. In healthy young adults, FBF responses were measured (Doppler ultrasound) and forearm vascular conductance was calculated during 5 min of rhythmic handgrip exercise at 20% maximum voluntary contraction under regional sympathoadrenal inhibition in normoxia and isocapnic HypEx (O₂ saturation ∼80%). Compared to control, combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and K_IR channels (l-NMMA + ketorolac + ouabain + BaCl_2; Protocol 1; n = 10) blunted the compensatory increase in FBF during HypEx by ∼50% (29 ± 6 mL min^-1 vs. 62 ± 8 mL min^-1 , respectively, P < 0.05). By contrast, ouabain + BaCl₂ alone (Protocol 2; n = 10) did not affect this augmented hyperaemic response (50 ± 11 mL min^-1 vs. 60 ± 13 mL min^-1 , respectively, P > 0.05). However, the blocked condition in both protocols abolished the hyperaemic response to hypoxia at rest (P < 0.05). We conclude that activation of Na⁺ /K⁺ -ATPase and K_IR channels is involved in the hyperaemic response to hypoxia at rest, although it does not contribute to the augmented exercise hyperaemia during hypoxia in humans.

Exercise intolerance in Type 2 diabetes: is there a cardiovascular contribution?

Physical activity is critically important for Type 2 diabetes management, yet adherence levels are poor. This might be partly due to disproportionate exercise intolerance. Submaximal exercise tolerance is highly sensitive to muscle oxygenation; impairments in exercising muscle oxygen delivery may contribute to exercise intolerance in Type 2 diabetes since there is considerable evidence for the existence of both cardiac and peripheral vascular dysfunction. While uncompromised cardiac output during submaximal exercise is consistently observed in Type 2 diabetes, it remains to be determined whether an elevated cardiac sympathetic afferent reflex could sympathetically restrain exercising muscle blood flow. Furthermore, while deficits in endothelial function are common in Type 2 diabetes and are often cited as impairing exercising muscle oxygen delivery, no direct evidence in exercise exists, and there are several other vasoregulatory mechanisms whose dysfunction could contribute. Finally, while there are findings of impaired oxygen delivery, conflicting evidence also exists. A definitive conclusion that Type 2 diabetes compromises exercising muscle oxygen delivery remains premature. We review these potentially dysfunctional mechanisms in terms of how they could impair oxygen delivery in exercise, evaluate the current literature on whether an oxygen delivery deficit is actually manifest, and correspondingly identify key directions for future research.

Phosphodiesterase-5 inhibition preserves exercise-onset vasodilator kinetics when NOS activity is reduced

Nitric oxide (NO)-mediated vasodilation contributes to the rapid rise in muscle blood flow at exercise onset. This occurs via increased cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase-5 (PDE-5). Whether PDE-5 limits exercise vasodilation onset kinetics is unknown. We hypothesized the time course of exercise vasodilation would be 1) accelerated during PDE-5 inhibition (sildenafil citrate, SDF) and 2) decelerated during NO synthase inhibition ( N^G-monomethyl-l-arginine, l-NMMA), and 3) the effect of SDF on vasodilation onset kinetics would be attenuated with concurrent l-NMMA. Data from 29 healthy adults were analyzed. Individuals completed 5 min of moderate-intensity forearm exercise under control conditions and during 1) oral SDF ( n = 8), 2) intra-arterial l-NMMA ( n = 15), or 3) combined SDF + l-NMMA ( n = 6). Forearm blood flow (FBF; Doppler ultrasound of the brachial artery) and mean brachial artery blood pressure (MAP) were measured continuously. Forearm vascular conductance (FVC, FBF ÷ MAP) was curve-fit with a monoexponential model, and vasodilation onset kinetics were assessed by mean response time (MRT, time to achieve 63% of steady state). SDF had no effect on MRT ( P = 0.90). NOS inhibition increased MRT ( P = 0.01). MRT during SDF+l-NMMA was not different from control exercise ( P = 0.76). PDE-5 inhibition alone has no effect on rapid-onset vasodilation. Whereas NOS inhibition decelerates vasodilator kinetics, when combined with SDF, vasodilator kinetics do not differ from control. These data suggest NO-independent activation of cGMP occurs at exercise onset; thus PDE-5 inhibition may improve vasodilation in pathologies where NO bioavailability is impaired. NEW & NOTEWORTHY We show that when NO bioavailability is reduced, PDE-5 inhibition can restore vasodilation onset kinetics of exercise-mediated vasodilation via NO-independent cGMP pathways. These data suggest PDE-5 inhibition may improve exercise vasodilation onset kinetics in pathologies where NO bioavailability is impaired.

Sympatholytic effect of intravascular ATP is independent of nitric oxide, prostaglandins, Na+ /K+ -ATPase and KIR channels in humans

KEY POINTS

Intravascular ATP attenuates sympathetic vasoconstriction (sympatholysis) similar to what is observed in contracting skeletal muscle of humans, and may be an important contributor to exercise hyperaemia. Similar to exercise, ATP-mediated vasodilatation occurs via activation of inwardly rectifying potassium channels (KIR ), and synthesis of nitric oxide (NO) and prostaglandins (PG). However, recent evidence suggests that these dilatatory pathways are not obligatory for sympatholysis during exercise; therefore, we tested the hypothesis that the ability of ATP to blunt α1 -adrenergic vasoconstriction in resting skeletal muscle would be independent of KIR , NO, PGs and Na+ /K+ -ATPase activity. Blockade of KIR channels alone or in combination with NO, PGs and Na+ /K+ -ATPase significantly reduced the vasodilatatory response to ATP, although intravascular ATP maintained the ability to attenuate α1 -adrenergic vasoconstriction. This study highlights similarities in the vascular response to ATP and exercise, and further supports a potential role of intravascular ATP in blood flow regulation during exercise in humans.

ABSTRACT

Exercise and intravascular ATP elicit vasodilatation that is dependent on activation of inwardly rectifying potassium (KIR ) channels, with a modest reliance on nitric oxide (NO) and prostaglandin (PG) synthesis. Both exercise and intravascular ATP attenuate sympathetic α-adrenergic vasoconstriction (sympatholysis). However, KIR channels, NO, PGs and Na+ /K+ -ATPase activity are not obligatory to observe sympatholysis during exercise. To further determine similarities between exercise and intravascular ATP, we tested the hypothesis that inhibition of KIR channels, NO and PG synthesis, and Na+ /K+ -ATPase would not alter the ability of ATP to blunt α1 -adrenergic vasoconstriction. In healthy subjects, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to intra-arterial infusion of phenylephrine (PE; α1 -agonist) during ATP or control vasodilatator infusion, before and after KIR channel inhibition alone (barium chloride; n = 7; Protocol 1); NO (l-NMMA) and PG (ketorolac) inhibition alone, or combined NO, PGs, Na+ /K+ -ATPase (ouabain) and KIR channel inhibition (n = 6; Protocol 2). ATP attenuated PE-mediated vasoconstriction relative to adenosine (ADO) and sodium nitroprusside (SNP) (PE-mediated ΔFVC: ATP: -16 ± 2; ADO: -38 ± 6; SNP: -59 ± 6%; P < 0.05 vs. ADO and SNP). Blockade of KIR channels alone or combined with NO, PGs and Na+ /K+ -ATPase, attenuated ATP-mediated vasodilatation (∼35 and ∼60% respectively; P < 0.05 vs. control). However, ATP maintained the ability to blunt PE-mediated vasoconstriction (PE-mediated ΔFVC: KIR blockade alone: -6 ± 5%; combined blockade:-4 ± 14%; P > 0.05 vs. control). These findings demonstrate that intravascular ATP modulates α1 -adrenergic vasoconstriction via pathways independent of KIR channels, NO, PGs and Na+ /K+ -ATPase in humans, consistent with a role for endothelium-derived hyperpolarization in functional sympatholysis.

Postsynaptic α1-Adrenergic Vasoconstriction Is Impaired in Young Patients With Vasovagal Syncope and Is Corrected by Nitric Oxide Synthase Inhibition

BACKGROUND

Syncope is a sudden transient loss of consciousness and postural tone with spontaneous recovery; the most common form is vasovagal syncope (VVS). During VVS, gravitational pooling excessively reduces central blood volume and cardiac output. In VVS, as in hemorrhage, impaired adrenergic vasoconstriction and venoconstriction result in hypotension. We hypothesized that impaired adrenergic responsiveness because of excess nitric oxide can be reversed by reducing nitric oxide.

METHODS AND RESULTS

We recorded cardiopulmonary dynamics in supine syncope patients and healthy volunteers (aged 15-27 years) challenged with a dose-response using the α1-agonist phenylephrine (PE), with and without the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine, monoacetate salt (L-NMMA). Systolic and diastolic pressures among control and VVS were the same, although they increased after L-NMMA and saline+PE (volume and pressor control for L-NMMA). Heart rate was significantly reduced by L-NMMA (P<0.05) for control and VVS compared with baseline, but there was no significant difference in heart rate between L-NMMA and saline+PE. Cardiac output and splanchnic blood flow were reduced by L-NMMA for control and VVS (P<0.05) compared with baseline, while total peripheral resistance increased (P<0.05). PE dose-response for splanchnic flow and resistance were blunted for VVS compared with control after saline+PE, but enhanced after L-NMMA (P<0.001). Postsynaptic α1-adrenergic vasoconstrictive impairment was greatest in the splanchnic vasculature, and splanchnic blood flow was unaffected by PE. Forearm and calf α1-adrenergic vasoconstriction were unimpaired in VVS and unaffected by L-NMMA.

CONCLUSIONS

Impaired postsynaptic α1-adrenergic vasoconstriction in young adults with VVS can be corrected by nitric oxide synthase inhibition, demonstrated with our use of L-NMMA.

Endothelial dysfunction correlates with exaggerated exercise pressor response during whole body maximal exercise in chronic kidney disease

Chronic kidney disease (CKD) patients have exercise intolerance associated with increased cardiovascular mortality. Previous studies demonstrate that blood pressure (BP) and sympathetic nerve responses to handgrip exercise are exaggerated in CKD. These patients also have decreased nitric oxide (NO) bioavailability and endothelial dysfunction, which could potentially lead to an impaired ability to vasodilate during exercise. We hypothesized that CKD patients have exaggerated BP responses during maximal whole body exercise and that endothelial dysfunction correlates with greater exercise pressor responses in these patients. Brachial artery flow-mediated dilation (FMD) was assessed before maximal treadmill exercise in 56 participants: 38 CKD (56.7 ± 1.2 yr old, 38 men) and 21 controls (52.8 ± 1.8 yr old, 20 men). During maximal treadmill exercise, the slope-of-rise in systolic BP (+10.32 vs. +7.75 mmHg/stage, P < 0.001), mean arterial pressure (+3.50 vs. +2.63 mmHg/stage, P = 0.004), and heart rate (+11.87 vs. +10.69 beats·min^-1·stage^-1, P = 0.031) was significantly greater in CKD compared with controls. Baseline FMD was significantly lower in CKD (2.76 ± 0.42% vs. 5.84 ± 0.97%, P = 0.008). Lower FMD values were significantly associated with a higher slope-of-rise in systolic BP (+11.05 vs. 8.71 mmHg/stage, P = 0.003) during exercise in CKD, as well as poorer exercise capacity measured as peak oxygen uptake (V̇o_2peak; 19.47 ± 1.47 vs. 24.57 ± 1.51 ml·min^-1·kg^-1, P < 0.001). These findings demonstrate that low FMD in CKD correlates with augmented BP responses during exercise and lower V̇o_2peak, suggesting that endothelial dysfunction may contribute to exaggerated exercise pressor responses and poor exercise capacity in CKD patients.

Potentiation of the NO-cGMP pathway and blood flow responses during dynamic exercise in healthy humans

PURPOSE

Previous work has shown nitric oxide (NO) contributes to ~15% of the hyperemic response to dynamic exercise in healthy humans. This NO-mediated vasodilation occurs, in part, via increases in intracellular cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase. We sought to examine the effect of phosphodiesterase-5 (PDE-5) inhibition on forearm blood flow (FBF) responses to dynamic handgrip exercise in healthy humans and the role of NO. We hypothesized exercise hyperemia would be augmented by sildenafil citrate (SDF, PDE-5 inhibitor). We further hypothesized any effect of SDF on exercise hyperemia would be abolished with intra-arterial infusion of the NO synthase (NOS) inhibitor L-N^G-monomethyl arginine (L-NMMA).

METHODS

FBF (Doppler ultrasound) was assessed at rest and during 5 min of dynamic forearm handgrip exercise at 15% of maximal voluntary contraction under control (saline) conditions and during 3 experimental protocols: (1) oral SDF (n = 10), (2) intra-arterial L-NMMA (n = 20), (3) SDF and L-NMMA (n = 10). FBF responses to intra-arterial sodium nitroprusside (NTP, NO donor) were also assessed.

RESULTS

FBF increased with exercise (p < 0.01). Intra-arterial infusion of L-NMMA resulted in a reduction in exercise hyperemia (17 ± 1 to 15 ± 1 mL/dL/min, p < 0.01). Although the hyperemic response to NTP was augmented by SDF (area under the curve: 41 ± 7 vs 61 ± 11 AU, p < 0.01), there was no effect of SDF on exercise hyperemia (p = 0.33).

CONCLUSIONS

Despite improving NTP-mediated vasodilation, oral SDF failed to augment exercise hyperemia in young, healthy adults. These observations reflect a minor contribution of NO and the cGMP pathway during exercise hyperemia in healthy young humans.

Endothelium-dependent vasodilatory signalling modulates α1 -adrenergic vasoconstriction in contracting skeletal muscle of humans

KEY POINTS

'Functional sympatholysis' describes the ability of contracting skeletal muscle to attenuate sympathetic vasoconstriction, and is critical to ensure proper blood flow and oxygen delivery to metabolically active skeletal muscle. The signalling mechanism responsible for sympatholysis in healthy humans is unknown. Evidence from animal models has identified endothelium-derived hyperpolarization (EDH) as a potential mechanism capable of attenuating sympathetic vasoconstriction. In this study, increasing endothelium-dependent signalling during exercise significantly enhanced the ability of contracting skeletal muscle to attenuate sympathetic vasoconstriction in humans. This is the first study in humans to identify endothelium-dependent regulation of sympathetic vasoconstriction in contracting skeletal muscle, and specifically supports a role for EDH-like vasodilatory signalling. Impaired functional sympatholysis is a common feature of cardiovascular ageing, hypertension and heart failure, and thus identifying fundamental mechanisms responsible for sympatholysis is clinically relevant.

ABSTRACT

Stimulation of α-adrenoceptors elicits vasoconstriction in resting skeletal muscle that is blunted during exercise in an intensity-dependent manner. In humans, the underlying mechanisms remain unclear. We tested the hypothesis that stimulating endothelium-dependent vasodilatory signalling will enhance the ability of contracting skeletal muscle to blunt α₁ -adrenergic vasoconstriction. Changes in forearm vascular conductance (FVC; Doppler ultrasound, brachial intra-arterial pressure via catheter) to local intra-arterial infusion of phenylephrine (PE; α₁ -adrenoceptor agonist) were calculated during (1) infusion of the endothelium-dependent vasodilators acetylcholine (ACh) and adenosine triphosphate (ATP), the endothelium-independent vasodilator (sodium nitroprusside, SNP), or potassium chloride (KCl) at rest; (2) mild or moderate intensity handgrip exercise; and (3) combined mild exercise + ACh, ATP, SNP, or KCl infusions in healthy adults. Robust vasoconstriction to PE was observed during vasodilator infusion alone and mild exercise, and this was blunted during moderate intensity exercise (ΔFVC: -34 ± 4 and -34 ± 3 vs. -13 ± 2%, respectively, P < 0.05). Infusion of ACh or ATP during mild exercise significantly attenuated PE vasoconstriction similar to levels observed during moderate exercise (ACh: -3 ± 4; ATP: -18 ± 4%). In contrast, infusion of SNP or KCl during mild exercise did not attenuate PE-mediated vasoconstriction (-32 ± 5 and -46 ± 3%). To further study the role of endothelium-dependent hyperpolarization (EDH), ACh trials were repeated with combined nitric oxide synthase and cyclooxygenase inhibition. Here, PE-mediated vasoconstriction was blunted at rest (blockade: -20 ± 5 vs.

CONTROL

-31 ± 3% vs.; P < 0.05) and remained blunted during exercise (blockade: -15 ± 5 vs.

CONTROL

-14 ± 5%). We conclude that stimulation of EDH-like vasodilatation can blunt α₁ -adrenergic vasoconstriction in contracting skeletal muscle of humans.

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.

Regulation of skeletal muscle blood flow during exercise in ageing humans

The regulation of skeletal muscle blood flow and oxygen delivery to contracting skeletal muscle is complex and involves the mechanical effects of muscle contraction; local metabolic, red blood cell and endothelium-derived substances; and the sympathetic nervous system (SNS). With advancing age in humans, skeletal muscle blood flow is typically reduced during dynamic exercise and this is due to a lower vascular conductance, which could ultimately contribute to age-associated reductions in aerobic exercise capacity, a primary predictor of mortality in both healthy and diseased ageing populations. Recent findings have highlighted the contribution of endothelium-derived substances to blood flow control in contracting muscle of older adults. With advancing age, impaired nitric oxide availability due to scavenging by reactive oxygen species, in conjunction with elevated vasoconstrictor signalling via endothelin-1, reduces the local vasodilatory response to muscle contraction. Additionally, ageing impairs the ability of contracting skeletal muscle to blunt sympathetic vasoconstriction (i.e. 'functional sympatholysis'), which is critical for the proper regulation of tissue blood flow distribution and oxygen delivery, and could further reduce skeletal muscle perfusion during high intensity and/or large muscle mass exercise in older adults. We propose that initiation of endothelium-dependent hyperpolarization is the underlying signalling event necessary to properly modulate sympathetic vasoconstriction in contracting muscle, and that age-associated impairments in red blood cell adenosine triphosphate release and stimulation of endothelium-dependent vasodilatation may explain impairments in both local vasodilatation and functional sympatholysis with advancing age in humans.

Norepinephrine responses in rat renal and femoral veins are reinforced by vasoconstrictor prostanoids

Norepinephrine (NE) responses are larger in renal and femoral veins compared to phenylephrine (PE). These differences may be due to the subtypes of adrenoceptor involved in these responses or to the involvement of local modulatory mechanisms. Therefore, the present study investigated in organ bath the adrenoceptor subtypes involved in the NE and PE responses in both renal and femoral veins as well as the influence of local mechanisms related to NO and to prostanoids upon these responses. The obtained data showed that the NE responses in these veins were not significantly modified by the selective inhibition of β1 or β2-adrenoceptors as well as AT1 or AT2 receptors. However, yohimbine reduced the NE Rmax in renal veins and, in parallel, right shifted the NE concentration-response curves in femoral veins. In both veins, prazosin reduced the NE Rmax and the clonidine induced a measurable contraction. The endothelium removal attenuated the NE responses in femoral veins, thereby abolishing the differences of NE and PE responses. Furthermore, the NE responses in renal and femoral veins were attenuated by indomethacin, which suppressed the statistical difference in relation to the PE response. In conclusion, a synergism between α1- and α2-adrenoceptors is essential to assure full NE contractile responses in both renal and femoral veins. Thus, by acting simultaneously in these adrenoceptors, NE induces more pronounced contractile responses, in comparison to PE, not only in renal but also in femoral veins. Moreover, this pronounced NE response in both renal and femoral veins appears to involve endothelium-derived vasoconstrictor prostanoids.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

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.

Acute ascorbic acid ingestion increases skeletal muscle blood flow and oxygen consumption via local vasodilation during graded handgrip exercise in older adults

Human aging is associated with reduced skeletal muscle perfusion during exercise, which may be a result of impaired endothelium-dependent dilation and/or attenuated ability to blunt sympathetically mediated vasoconstriction. Intra-arterial infusion of ascorbic acid (AA) increases nitric oxide-mediated vasodilation and forearm blood flow (FBF) during handgrip exercise in older adults, yet it remains unknown whether an acute oral dose can similarly improve FBF or enhance the ability to blunt sympathetic vasoconstriction during exercise. We hypothesized that 1) acute oral AA would improve FBF (Doppler ultrasound) and oxygen consumption (V̇o2) via local vasodilation during graded rhythmic handgrip exercise in older adults (protocol 1), and 2) AA ingestion would not enhance sympatholysis in older adults during handgrip exercise (protocol 2). In protocol 1 (n = 8; 65 ± 3 yr), AA did not influence FBF or V̇o2 during rest or 5% maximal voluntary contraction (MVC) exercise, but increased FBF (199 ± 13 vs. 248 ± 16 ml/min and 343 ± 24 vs. 403 ± 33 ml/min; P < 0.05) and V̇o2 (26 ± 2 vs. 34 ± 3 ml/min and 43 ± 4 vs. 50 ± 5 ml/min; P < 0.05) at both 15 and 25% MVC, respectively. The increased FBF was due to elevations in forearm vascular conductance (FVC). In protocol 2 (n = 10; 63 ± 2 yr), following AA, FBF was similarly elevated during 15% MVC (∼ 20%); however, vasoconstriction to reflex increases in sympathetic activity during -40 mmHg lower-body negative pressure at rest (ΔFVC: -16 ± 3 vs. -16 ± 2%) or during 15% MVC (ΔFVC: -12 ± 2 vs. -11 ± 4%) was unchanged. Our collective results indicate that acute oral ingestion of AA improves muscle blood flow and V̇o2 during exercise in older adults via local vasodilation.

Exercise vasodilation is greater in women: contributions of nitric oxide synthase and cyclooxygenase

PURPOSE

We hypothesized exercise vasodilation would be greater in women due to nitric oxide synthase (NOS) and cyclooxygenase (COX) signaling.

METHODS

45 healthy adults (23 women, W, 22 men, M, 26 ± 1 years) completed two 10-min trials of dynamic forearm exercise at 15 % intensity. Forearm blood flow (FBF; Doppler ultrasound), arterial pressure (brachial catheter), and forearm lean mass were measured to calculate relative forearm vascular conductance (FVCrel) = FBF 100 mmHg(-1) 100 g(-1) lean mass. Local intra-arterial infusion of L-NMMA or ketorolac acutely inhibited NOS and COX, respectively. In Trial 1, the first 5 min served as control exercise (CON), followed by 5 min of L-NMMA or ketorolac over the last 5 min of exercise. In Trial 2, the remaining drug was infused during 5-10 min, to achieve combined NOS-COX inhibition (double blockade, DB).

RESULTS

Are mean ± SE. Women exhibited 29 % greater vasodilation in CON (ΔFVCrel, 19 ± 1 vs. 15 ± 1, p = 0.01). L-NMMA reduced ΔFVCrel (p < 0.001) (W: Δ -2.3 ± 1.3 vs. M: Δ -3.7 ± 0.8, p = 0.25); whereas, ketorolac modestly increased ΔFVCrel (p = 0.04) similarly between sexes (W: Δ 1.6 ± 1.1 vs. M: Δ 2.0 ± 1.6, p = 0.78). DB was also found to be similar between the sexes (p = 0.85).

CONCLUSION

These data clearly indicate women produce a greater exercise vasodilator response. Furthermore, contrary to experiments in animal models, these data are the first to demonstrate vascular control by NOS and COX is similar between sexes.

Acute inhibition of ATP-sensitive K+ channels impairs skeletal muscle vascular control in rats during treadmill exercise

The ATP-sensitive K(+) (KATP) channel is part of a class of inward rectifier K(+) channels that can link local O2 availability to vasomotor tone across exercise-induced metabolic transients. The present investigation tested the hypothesis that if KATP channels are crucial to exercise hyperemia, then inhibition via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance during treadmill exercise. In 27 adult male Sprague-Dawley rats, mean arterial pressure, blood lactate concentration, and hindlimb muscle BF (radiolabeled microspheres) were determined at rest (n = 6) and during exercise (n = 6-8, 20, 40, and 60 m/min, 5% incline, i.e., ~60-100% maximal O2 uptake) under control and GLI conditions (5 mg/kg intra-arterial). At rest and during exercise, mean arterial pressure was higher (rest: 17 ± 3%, 20 m/min: 5 ± 1%, 40 m/min: 5 ± 2%, and 60 m/min: 5 ± 1%, P < 0.05) with GLI. Hindlimb muscle BF (20 m/min: 16 ± 7%, 40 m/min: 30 ± 9%, and 60 m/min: 20 ± 8%) and vascular conductance (20 m/min: 20 ± 7%, 40 m/min: 33 ± 8%, and 60 m/min: 24 ± 8%) were lower with GLI during exercise at 20, 40, and 60 m/min, respectively (P < 0.05 for all) but not at rest. Within locomotory muscles, there was a greater fractional reduction present in muscles comprised predominantly of type I and type IIa fibers at all exercise speeds (P < 0.05). Additionally, blood lactate concentration was 106 ± 29% and 44 ± 15% higher during exercise with GLI at 20 and 40 m/min, respectively (P < 0.05). That KATP channel inhibition reduces hindlimb muscle BF during exercise in rats supports the obligatory contribution of KATP channels in large muscle mass exercise-induced hyperemia.

Interactive effect of acute sympathetic activation and exercise intensity on the dynamic response characteristics of vascular conductance in the human calf muscle

PURPOSE

The effect of acute activation of the sympathetic nervous system on the dynamic response of muscle hyperaemia during exercise at different intensities is not clear.

METHODS

To explore this, six men performed 16, 5-min bouts of intermittent calf contractions at two intensities (25 and 50 % MVC) and two levels of sympathetic activation (CPT cold pressor test, CON control). Mean arterial pressure (MAP) and leg vascular conductance (LVC leg blood flow/MAP) were measured during rest and contractions (3 s intervals), and dynamic response characteristics of LVC were estimated using curve-fitting and empirical modeling.

RESULTS

MAP was ~20 % greater (P ≤ 0.05) during CPT than CON before and during initial contractions at both intensities. At 25 % MVC, CPT reduced the exercise-induced change in LVC (0.109 vs 0.125 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), an effect attributed to the reduction in the amplitude of the fast growth phase (0.091 vs 0.128 1 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05). At 50 % MVC, CPT also blunted the fast growth phase (0.147 vs 0.189 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), but the total change in LVC during exercise was unaffected because of a significant reduction in the amplitude of the rapid decay phase and tendency (P = 0.1) for a lower amplitude of the slow decay phase.

CONCLUSION

Increased sympathetic constraint of vasodilation persists during initial contractions but is overcome at the high intensity by a mechanism apparently related to hyperaemic decay.

Acute dietary nitrate supplementation enhances compensatory vasodilation during hypoxic exercise in older adults

We have previously demonstrated that aging reduces the compensatory vasodilator response during hypoxic exercise due to blunted nitric oxide (NO) signaling. Recent evidence suggests that NO bioavailability can be augmented by dietary nitrate through the nitrate-nitrite pathway. Thus we tested the hypothesis that acute dietary nitrate supplementation increases the compensatory vasodilator response to hypoxic exercise, particularly in older adults. Thirteen young (25 ± 1 yr) and 12 older (64 ± 2 yr) adults performed rhythmic forearm exercise at 20% of maximum voluntary contraction during normoxia and hypoxia (∼80% O2 saturation); both before (control) and 3 h after beetroot juice (BR) consumption. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from forearm blood flow (ml/min) and blood pressure (mmHg). Compensatory vasodilation was defined as the relative increase in FVC due to hypoxic exercise (i.e., % increase compared with respective normoxic exercise trial). Plasma nitrite was determined from venous blood samples obtained before the control trials and each of the exercise trials (normoxia and hypoxia) after BR. Consumption of BR increased plasma nitrite in both young and older adults (P < 0.001). During the control condition, the compensatory vasodilator response to hypoxic exercise was attenuated in older compared with young adults (3.8 ± 1.7% vs. 14.2 ± 1.2%, P < 0.001). Following BR consumption, compensatory vasodilation did not change in young (13.7 ± 3.3%, P = 0.81) adults but was substantially augmented in older adults (11.4 ± 2.1%, P < 0.01). Our data suggest that acute dietary nitrate supplementation increases the compensatory vasodilator response to hypoxic exercise in older but not young adults.

Abnormal neurocirculatory control during exercise in humans with chronic renal failure

Abnormal neurocirculatory control during exercise is one important mechanism leading to exercise intolerance in patients with both end-stage renal disease (ESRD) and earlier stages of chronic kidney disease (CKD). This review will provide an overview of mechanisms underlying abnormal neurocirculatory and hemodynamic responses to exercise in patients with kidney disease. Recent studies have shown that ESRD and CKD patients have an exaggerated increase in blood pressure (BP) during both isometric and rhythmic exercise. Subsequent studies examining the role of the exercise pressor reflex in the augmented pressor response revealed that muscle sympathetic nerve activity (MSNA) was not augmented during exercise in these patients, and metaboreflex-mediated increases in MSNA were blunted, while mechanoreflex-mediated increases were preserved under basal conditions. However, normalizing the augmented BP response during exercise via infusion of nitroprusside (NTP), and thereby equalizing baroreflex-mediated suppression of MSNA, an important modulator of the final hemodynamic response to exercise, revealed that CKD patients had an exaggerated increase in MSNA during isometric and rhythmic exercise. In addition, mechanoreflex-mediated control was augmented, and metaboreceptor blunting was no longer apparent in CKD patients with baroreflex normalization. Factors leading to mechanoreceptor sensitization, and other mechanisms underlying the exaggerated exercise pressor response, such as impaired functional sympatholysis, should be investigated in future studies.

Exercise training augments neuronal nitric oxide synthase-mediated inhibition of sympathetic vasoconstriction in contracting skeletal muscle of rats

We tested the hypothesis that exercise training would increase neuronal nitric oxide synthase (nNOS)-mediated inhibition of sympathetic vasoconstriction in resting and contracting skeletal muscle. Sprague-Dawley rats (n = 18) were randomized to sedentary or exercise-trained (40 m min(-1), 5° grade; 5 days week(-1) for 4 weeks) groups. Following completion of sedentary behaviour or exercise training, rats were anaesthetized and instrumented with a brachial artery catheter, femoral artery flow probe and stimulating electrodes on the lumbar sympathetic chain. The percentage change of femoral vascular conductance (%FVC) in response to sympathetic chain stimulations delivered at 2 and 5 Hz was determined at rest and during triceps surae muscle contraction before (control) and after selective nNOS blockade with S-methyl-l-thiocitrulline (SMTC, 0.6 mg kg(-1), i.v.) and subsequent non-selective NOS blockade with l-NAME (5 mg kg(-1), i.v.; SMTC + l-NAME). At rest, sympathetic vasoconstrictor responsiveness was greater (P < 0.05) in exercise-trained compared to sedentary rats in control, SMTC and SMTC + l-NAME conditions. During contraction, the constrictor response was not different (P > 0.05) between exercise trained (2 Hz: -11 ± 4%FVC; 5 Hz: -21 ± 5%FVC) and sedentary rats (2 Hz: -7 ± 6%FVC; 5 Hz: -18 ± 10%FVC) in control conditions. SMTC augmented (P < 0.05) sympathetic vasoconstriction in sedentary and exercise-trained rats; however, sympathetic vasoconstrictor responsiveness was greater (P < 0.05) in exercise-trained (2 Hz: -27 ± 5%FVC; 5 Hz: -39 ± 5%FVC) compared to sedentary (2 Hz: -17 ± 6%FVC; 5 Hz: -27 ± 8%FVC) rats during selective nNOS inhibition. SMTC + l-NAME further augmented (P < 0.05) sympathetic vasoconstrictor responsiveness by a similar magnitude (P > 0.05) in exercise-trained and sedentary rats. These data demonstrate that exercise training augmented nNOS-mediated inhibition of sympathetic vasoconstriction in contracting muscle.

Role of α-adrenergic vasoconstriction in regulating skeletal muscle blood flow and vascular conductance during forearm exercise in ageing humans

In healthy humans, ageing is typically associated with reduced skeletal muscle blood flow and vascular conductance during exercise. Further, there is a marked increase in resting sympathetic nervous system (SNS) activity with age, yet whether augmented SNS-mediated α-adrenergic vasoconstriction contributes to the age-associated impairment in exercising muscle blood flow and vascular tone in humans is unknown. We tested the hypothesis that SNS-mediated vasoconstriction is greater in older than young adults and limits muscle (forearm) blood flow (FBF) during graded handgrip exercise (5, 15, 25% maximal voluntary contraction (MVC)). FBF was measured (Doppler ultrasound) and forearm vascular conductance (FVC) was calculated in 11 young (21 ± 1 years) and 12 older (62 ± 2 years) adults in control conditions and during combined local α- and β-adrenoreceptor blockade via intra-arterial infusions of phentolamine and propranolol, respectively. Under control conditions, older adults exhibited significantly lower FBF and FVC at 15% MVC exercise (22.6 ± 1.3 vs. 29 ± 3.3 ml min(-1) 100 g forearm fat-free mass (FFM)(-1) and 21.7 ± 1.2 vs. 33.6 ± 4.0 ml min(-1) 100 g FFM(-1) 100 mmHg(-1); P < 0.05) and 25% MVC exercise (37.4 ± 1.4 vs. 46.0 ± 4.9 ml min(-1) 100 g FFM(-1) and 33.7 ± 1.4 vs. 49.0 ± 5.7 ml min(-1) 100 g FFM(-1) 100 mmHg(-1); P < 0.05), whereas there was no age group difference at 5% MVC exercise. Local adrenoreceptor blockade increased FBF and FVC at rest and during exercise in both groups, although the increase in FBF and FVC from rest to steady-state exercise was similar in young and older adults across exercise intensities, and thus the age-associated impairment in FBF and FVC persisted. Our data indicate that during graded intensity handgrip exercise, the reduced FVC and subsequently lower skeletal muscle blood flow in older healthy adults is not due to augmented sympathetic vasoconstriction, but rather due to impairments in local signalling or structural limitations in the peripheral vasculature with advancing age.

KIR channel activation contributes to onset and steady-state exercise hyperemia in humans

We tested the hypothesis that activation of inwardly rectifying potassium (KIR) channels and Na(+)-K(+)-ATPase, two pathways that lead to hyperpolarization of vascular cells, contributes to both the onset and steady-state hyperemic response to exercise. We also determined whether after inhibiting these pathways nitric oxide (NO) and prostaglandins (PGs) are involved in the hyperemic response. Forearm blood flow (FBF; Doppler ultrasound) was determined during rhythmic handgrip exercise at 10% maximal voluntary contraction for 5 min in the following conditions: control [saline; trial 1 (T1)]; with combined inhibition of KIR channels and Na(+)-K(+)-ATPase alone [via barium chloride (BaCl2) and ouabain, respectively; trial 2 (T2)]; and with additional combined nitric oxide synthase (N(G)-monomethyl-l-arginine) and cyclooxygenase inhibition [ketorolac; trial 3 (T3)]. In T2, the total hyperemic responses were attenuated ~50% from control (P < 0.05) at exercise onset, and there was minimal further effect in T3 (protocol 1; n = 11). In protocol 2 (n = 8), steady-state FBF was significantly reduced during T2 vs. T1 (133 ± 15 vs. 167 ± 17 ml/min; Δ from control: -20 ± 3%; P < 0.05) and further reduced during T3 (120 ± 15 ml/min; -29 ± 3%; P < 0.05 vs. T2). In protocol 3 (n = 8), BaCl2 alone reduced FBF during onset (~50%) and steady-state exercise (~30%) as observed in protocols 1 and 2, respectively, and addition of ouabain had no further impact. Our data implicate activation of KIR channels as a novel contributing pathway to exercise hyperemia in humans.

Redundant vasodilator pathways underlying radial artery flow-mediated dilation are preserved in healthy aging

Background. Blocking nitric oxide (NO) and vasodilator prostanoids (PN) does not consistently reduce flow-mediated dilation (FMD) in young adults. The impact of aging on the contribution of NO and PG to FMD is unknown. Methods. FMD was measured in older adults (n = 10, 65 ± 3 y) after arterial infusion of saline, N(G)-monomethyl-L-arginine (L-NMMA), and ketorolac + L-NMMA. Data were compared to published data in young adults. Results. L-NMMA reduced FMD in older adults (8.9 ± 3.6 to 5.9 ± 3.7%) although this was not statistically significant (P = 0.08) and did not differ (P = 0.74) from the reduction observed in young adults (10.0 ± 3.8 to 7.6 ± 4.7%; P = 0.03). Blocking PN did not affect FMD in young or older adults. In older adults, L-NMMA reduced (n = 6; range = 36-123% decrease), augmented (n = 3; 10-122% increase), or did not change FMD (n = 1; 0.4% increase). After PN blockade, FMD responses were reduced (n = 2), augmented (n = 6), or unaffected (n = 1). Conclusions. NO or PN blockade did not consistently reduce FMD in healthy older adults, suggesting the existence of redundant vasodilator phenotypes as observed previously in young adults.

Influence of regular exercise training on post-exercise hemodynamic regulation to orthostatic challenge

To prevent orthostatic hypotension, arterial blood pressure (BP) is neurally and hormonally regulated via increases in heart rate (HR) and peripheral vascular tone. After dynamic exercise, however, the latter arm is blunted because of the increased vasodilators in exercised muscles. Orthostatic tachycardia is likely a more important compensatory mechanism for post-exercise orthostatic intolerance in individuals who have higher leg vasodilator capacity, such as endurance-trained athletes. To test the hypothesis that regular endurance training was associated with the greater augmentation of tachycardia response to post-exercise orthostasis, we compared hemodynamic responses to 5-min 60° head-up tilt (HUT) before and after 60 min of cycling at 70% of HR reserve in the endurance-trained (n = 8) and sedentary men (n = 9). Calf peak vascular conductance was 62% greater in the endurance-trained than the sedentary (P < 0.001). After the exercise, the HUT-induced reduction of SV was significantly augmented in the endurance-trained (from −27.7 ± 6.9 to −33.7 ± 7.7 ml, P = 0.03) but not in their sedentary peers. Nevertheless, MAP was well maintained during post-exercise HUT even in the endurance-trained (from 81 ± 10 to 80 ± 8 mmHg). Tachycardia responses during sustained orthostasis were significantly increased in the sedentary (1.3-fold vs. pre-exercise) and more in the endurance-trained (2.0-fold). The augmented response of HUT-induced tachycardia was greater in the endurance-trained than the sedentary (P = 0.04). Additionally, cardiovagal baroreflex sensitivity (BRS), evaluated by the HR response to the hypotensive perturbation, was improved after the exercise in the endurance-trained (from −0.56 ± 0.32 to −1.03 ± 0.26 bpm/mmHg, P = 0.007) but not in the sedentary. These results suggest that in the endurance-trained men the increased orthostatic tachycardia and augmented cardiovagal BRS may favorably mitigate accumulated risks for orthostatic intolerance in the early phase of post-exercise.

Exercise training improves functional sympatholysis in spontaneously hypertensive rats through a nitric oxide-dependent mechanism

Functional sympatholysis is impaired in hypertensive animals and patients. Exercise training (ET) improves functional sympatholysis through a nitric oxide (NO)-dependent mechanism in normotensive rats. However, whether ET has similar physiological benefits in hypertension remains to be elucidated. Thus we tested the hypothesis that the impairment in functional sympatholysis in hypertension is reversed by ET through a NO-dependent mechanism. In untrained normotensive Wistar-Kyoto rats (WKYUT; n = 13), untrained spontaneously hypertensive rats (SHRUT; n = 13), and exercise-trained SHR (SHRET; n = 6), changes in femoral vascular conductance (FVC) were examined during lumbar sympathetic nerve stimulation (1, 2.5, and 5 Hz) at rest and during muscle contraction. The magnitude of functional sympatholysis (Δ%FVC = Δ%FVC muscle contraction - Δ%FVC rest) in SHRUT was significantly lower than WKYUT (1 Hz: -2 ± 4 vs. 13 ± 3%; 2.5 Hz: 9 ± 3 vs. 21 ± 3%; and 5 Hz: 12 ± 3 vs. 26 ± 3%, respectively; P < 0.05). Three months of voluntary wheel running significantly increased maximal oxygen uptake in SHRET compared with nontrained SHRUT (78 ± 6 vs. 62 ± 4 ml·kg(-1)·min(-1), respectively; P < 0.05) and restored the magnitude of functional sympatholysis in SHRET (1 Hz: 9 ± 2%; 2.5 Hz: 20 ± 4%; and 5 Hz: 34 ± 5%). Blockade of NO synthase (NOS) by N(G)-nitro-l-arginine methyl ester attenuated functional sympatholysis in WKYUT but not SHRUT. Furthermore, NOS inhibition significantly diminished the improvements in functional sympatholysis in SHRET. These data demonstrate that impairments in functional sympatholysis are normalized via a NO mechanism by voluntary wheel running in hypertensive rats.

Role of neuronal nitric oxide in the inhibition of sympathetic vasoconstriction in resting and contracting skeletal muscle of healthy rats

Isoform-specific nitric oxide (NO) synthase (NOS) contributions to NO-mediated inhibition of sympathetic vasoconstriction in resting and contracting skeletal muscle are incompletely understood. The purpose of the present study was to investigate the role of neuronal NOS (nNOS) in the inhibition of sympathetic vasoconstriction in resting and contracting skeletal muscle of healthy rats. We hypothesized that acute pharmacological inhibition of nNOS would augment sympathetic vasoconstriction in resting and contracting skeletal muscle, demonstrating that nNOS is primarily responsible for NO-mediated inhibition of sympathetic vasoconstriction. Sprague-Dawley rats ( n = 13) were anesthetized and instrumented with an indwelling brachial artery catheter, femoral artery flow probe, and lumbar sympathetic chain stimulating electrodes. Triceps surae muscles were stimulated to contract rhythmically at 60% of maximal contractile force. In series 1 ( n = 9), the percent change in femoral vascular conductance (%FVC) in response to sympathetic stimulations delivered at 2 and 5 Hz was determined at rest and during muscle contraction before and after selective nNOS blockade with S-methyl-l-thiocitrulline (SMTC, 0.6 mg/kg iv) and subsequent nonselective NOS blockade with Nω-nitro-l-arginine methyl ester (l-NAME, 5 mg/kg iv). In series 2 ( n = 4), l-NAME was injected first, and then SMTC was injected to determine if the effect of l-NAME on constrictor responses was influenced by selective nNOS inhibition. Sympathetic stimulation decreased FVC at rest (−25 ± 7 and −44 ± 8%FVC at 2 and 5 Hz, respectively) and during contraction (−7 ± 3 and −19 ± 5%FVC at 2 and 5 Hz, respectively). The decrease in FVC in response to sympathetic stimulation was greater in the presence of SMTC at rest (−32 ± 6 and −49 ± 8%FVC at 2 and 5 Hz, respectively) and during contraction (−21 ± 4 and −28 ± 4%FVC at 2 and 5 Hz, respectively). l-NAME further increased ( P < 0.05) the sympathetic vasoconstrictor response at rest (−47 ± 4 and −60 ± 6%FVC at 2 and 5 Hz, respectively) and during muscle contraction (−33 ± 3 and −40 ± 6%FVC at 2 and 5 Hz, respectively). The effect of l-NAME was not altered by the order of nNOS inhibition. These data demonstrate that NO derived from nNOS and endothelial NOS contribute to the inhibition of sympathetic vasoconstriction in resting and contracting skeletal muscle.

Persistence of functional sympatholysis post-exercise in human skeletal muscle

Blunting of sympathetic vasoconstriction in exercising muscle is well-established. Whether it persists during the early post-exercise period is unknown. This study tested the hypothesis that it persists in human skeletal muscle during the first 10 min of recovery from exercise. Eight healthy young males (21.4 ± 0.8 yrs, SE) performed 7 min of forearm rhythmic isometric handgrip exercise at 15% below forearm critical force (fCF). In separate trials, a cold pressor test (CPT) of 2 min duration was used to evoke forearm sympathetic vasoconstriction in each of Rest (R), Steady State Exercise (Ex), 2-4 min Post-Exercise (PEearly), and 8-10 min Post-Exercise (PElate). A 7 min control exercise trial with no CPT was also performed. Exercising forearm brachial artery blood flow, arterial blood pressure, cardiac output (CO), heart rate (HR), forearm deep venous catecholamine concentration, and arterialized venous catecholamine concentration were obtained immediately prior to and following the CPT in each trial. CPT resulted in a significant increase in forearm venous plasma norepinephrine concentration in all trials (P = 0.007), but no change in arterialized plasma norepinephrine (P = 0.32). CPT did not change forearm venous plasma epinephrine (P = 0.596) or arterialized plasma epinephrine concentration (P = 0.15). As assessed by the %reduction in forearm vascular conductance (FVC) the CPT evoked a robust vasoconstriction at rest that was severely blunted in exercise (R = -39.9 ± 4.6% vs. Ex = 5.5 ± 7.4%, P < 0.001). This blunting of vasoconstriction persisted at PEearly (-12.3 ± 10.1%, P = 0.02) and PElate (-18.1 ± 8.2%, P = 0.03) post-exercise. In conclusion, functional sympatholysis remains evident in human skeletal muscle as much as 10 min after the end of a bout of forearm exercise. Persistence of functional sympatholysis may have important implications for blood pressure regulation in the face of a challenge to blood pressure following exercise.

Update on the theory and management of orthostatic intolerance and related syndromes in adolescents and children

Orthostasis means standing upright. One speaks of orthostatic intolerance (OI) when signs, such as hypotension, and symptoms, such as lightheadedness, occur when upright and are relieved by recumbence. The experience of transient mild OI is part of daily life. 'Initial orthostatic hypotension' on rapid standing is a normal form of OI. However, other people experience OI that seriously interferes with quality of life. These include episodic acute OI, in the form of postural vasovagal syncope, and chronic OI, in the form of postural tachycardia syndrome. Less common is neurogenic orthostatic hypotension, which is an aspect of autonomic failure. Normal orthostatic physiology and potential mechanisms for OI are discussed, including forms of sympathetic hypofunction, forms of sympathetic hyperfunction and OI that results from regional blood volume redistribution. General and specific treatment options are proposed.

Short-term exercise training enhances functional sympatholysis through a nitric oxide-dependent mechanism

We tested the hypothesis that short-term mild- (M) and heavy-intensity (H) exercise training would enhance sympatholysis through a nitric oxide (NO)-dependent mechanism. Sprague-Dawley rats (n = 36) were randomly assigned to sedentary (S) or to M (20 m min(-1) 5% gradient) or H exercise training groups (40 m min(-1) 5% gradient). Rats assigned to M and H groups trained on 5 days week(-1) for 4 weeks, with the volume of training being matched between groups. Rats were anaesthetized and instrumented for stimulation of the lumbar sympathetic chain and the measurement of arterial blood pressure and femoral artery blood flow. The triceps surae muscle group was stimulated to contract rhythmically at 30 and 60% of maximal contractile force (MCF). The percentage change of femoral vascular conductance (%FVC) in response to sympathetic stimulation delivered at 2 and 5 Hz was determined at rest and during contraction at 30 and 60% MCF. The vascular response to sympathetic stimulation was reduced as a function of MCF in all rats (P < 0.05). At 30% MCF, the magnitude of sympatholysis (%FVC rest - contraction; %FVC) was greater in H compared with M and S groups (%FVC at 2 Hz, S, 9 ± 5; M, 11 ± 8; and H, 18 ± 7; and %FVC at 5 Hz, S, 6 ± 6; M, 12 ± 9; and H, 18 ± 7; P < 0.05) and was greater in H and M compared with S at 60% MCF (%FVC at 2 Hz, S, 15 ± 5; M, 25 ± 3; and H, 36 ± 6; and %FVC at 5 Hz, S, 22 ± 6; M, 33 ± 9; and H, 39 ± 9; P < 0.05). Blockade of NO synthase did not alter the magnitude of sympatholysis in S during contraction at 30 or 60% MCF. In contrast, NO synthase inhibition diminished sympatholysis in H at 30% MCF and in M and H at 60% MCF (P < 0.05). The present findings indicate that short-term exercise training augments sympatholysis in a training-intensity-dependent manner and through an NO-dependent mechanism.

Roles of nitric oxide and prostaglandins in the hyperemic response to a maximal metabolic stimulus: redundancy prevails

Vasodilatory mechanisms controlling post-exercise or post-ischemic hyperemia are thought to be under redundant control and remain incompletely understood. A maximal metabolic stimulus evoked by ischemic exercise (IE) might limit redundancy by full activation of multiple pathways. We tested whether nitric oxide (NO) and/or prostaglandins contribute to the hyperemic response to IE. 17 subjects were randomized into two groups and performed three trials of IE during control (saline), N (G)-monomethyl-L-arginine (L-NMMA; NOS inhibition) (protocol 1) or ketorolac (cyclooxygenase inhibition) infusion (protocol 2), and combined L-NMMA/ketorolac infusion via a brachial arterial catheter. Forearm blood flow (FBF) was measured with venous occlusion plethysmography following IE trials consisting of 5 min of ischemia and simultaneous rhythmic handgrip exercise (final 2 min). Peak and total (area under the curve) FBF and blood pressure (MAP) were measured for 3 min after each trial. Forearm vascular conductance (FVC) was calculated as FBF/MAP. Change (Δ) in peak FBF and FVC from baseline differed only between peak FBF for the saline and L-NMMA + ketorolac trials in protocol 1. Peak ΔFBF was 26.8 ± 2.5, 30.0 ± 2.8, and 33.9 ± 3.6 ml 100 ml(-1) min(-1) for saline, L-NMMA, and L-NMMA + ketorolac trials (P = 0.04). For protocol 1 (n = 8), total ΔFVC was 59.6 ± 4.3, 57.8 ± 6.0, and 59.9 ± 5.6 ml 100 ml(-1) 100 mmHg(-1) for saline, L-NMMA, and L-NMMA + ketorolac trials, (P = 0.82). For protocol 2 (n = 9), total ΔFVC was 54.2 ± 5.0, 56.9 ± 4.5, and 56.5 ± 5.3 ml 100 ml(-1) 100 mmHg(-1) for saline, ketorolac, and ketorolac + L-NMMA trials, (P = 0.69). These results suggest that NO and PGs are not obligatory for the hyperemic response to IE, and other vasodilator mechanisms predominate.

Inefficient functional sympatholysis is an overlooked cause of malperfusion in contracting skeletal muscle

Contracting skeletal muscle can overcome sympathetic vasoconstrictor activity (functional sympatholysis), which allows for a blood supply that matches the metabolic demand. This ability is thought to be mediated by locally released substances that modulate the effect of noradrenaline (NA) on the α-receptor. Tyramine induces local NA release and can be used in humans to investigate the underlying mechanisms and physiological importance of functional sympatholysis in the muscles of healthy and diseased individuals as well as the impact of the active muscles' training status. In sedentary elderly men, functional sympatholysis and muscle blood flow are impaired compared to young men, but regular physical activity can prevent these age related impairments. In young subjects, two weeks of leg immobilization causes a reduced ability for functional sympatholysis, whereas the trained leg maintained this function. Patients with essential hypertension have impaired functional sympatholysis in the forearm, and reduced exercise hyperaemia in the leg, but this can be normalized by aerobic exercise training. The effect of physical activity on the local mechanisms that modulate sympathetic vasoconstriction is clear, but it remains uncertain which locally released substance(s) block the effect of NA and how this is accomplished. NO and ATP have been proposed as important inhibitors of NA mediated vasoconstriction and presently an inhibitory effect of ATP on NA signalling via P2 receptors appears most likely.

Heat and α1-adrenergic responsiveness in human skeletal muscle feed arteries: the role of nitric oxide

Increased local temperature exerts a sympatholytic effect on human skeletal muscle feed arteries. We hypothesized that this attenuated α(1)-adrenergic receptor responsiveness may be due to a temperature-induced increase in nitric oxide (NO) bioavailability, thereby reducing the impact of the α(1)-adrenergic receptor agonist phenylephrine (PE). Thirteen human skeletal muscle feed arteries were harvested, and wire myography was used to generate PE concentration-response curves at 37 °C and 39 °C, with and without the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA). A subset of arteries (n = 4) were exposed to 37 °C or 39 °C, and the protein content of endothelial NOS (eNOS) and α(1)-adrenergic receptors was determined by Western blot analysis. Additionally, cultured bovine endothelial cells were exposed to static or shear stress conditions at 37 °C and 39 °C and assayed for eNOS activation (phosphorylation at Ser(1177)), eNOS expression, and NO metabolites [nitrate + nitrite (NOx)]. Maximal PE-induced vasocontraction (PE(max)) was lower at 39 °C than at 37 °C [39 ± 10 vs. 84 ± 30% maximal response to 100 mM KCl (KCl(max))]. NO blockade restored vasocontraction at 39 °C to that achieved at 37 °C (80 ± 26% KCl(max)). Western blot analysis of the feed arteries revealed that heating increased eNOS protein, but not α(1)-adrenergic receptors. Heating of bovine endothelial cells resulted in greater shear stress-induced eNOS activation and NOx production. Together, these data reveal for the first time that, in human skeletal muscle feed arteries, NO blockade can restore the heat-attenuated α(1)-adrenergic receptor-mediated vasocontraction and implicate endothelium-derived NO bioavailability as a major contributor to heat-induced sympatholysis. Consequently, these findings highlight the important role of vasodilators in modulating the vascular response to vasoconstrictors.

Vasoconstrictor responsiveness during hyperbaric hyperoxia in contracting human muscle

Large increases in systemic oxygen content cause substantial reductions in exercising forearm blood flow (FBF) due to increased vascular resistance. We hypothesized that 1) functional sympatholysis (blunting of sympathetic α-adrenergic vasoconstriction) would be attenuated during hyperoxic exercise and 2) α-adrenergic blockade would limit vasoconstriction during hyperoxia and increase FBF to levels observed under normoxic conditions. Nine male subjects (age 28 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Studies were performed in a hyperbaric chamber at 1 atmosphere absolute (ATA; sea level) while breathing 21% O(2) and at 2.82 ATA while breathing 100% O(2) (estimated change in arterial O(2) content ∼6 ml O(2)/100 ml). FBF (ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated from FBF and blood pressure (arterial catheter). Vasoconstrictor responsiveness was determined using intra-arterial tyramine. FBF and FVC were substantially lower during hyperoxic exercise than normoxic exercise (∼20-25%; P < 0.01). At rest, vasoconstriction to tyramine (% decrease from pretyramine values) did not differ between normoxia and hyperoxia (P > 0.05). During exercise, vasoconstrictor responsiveness was slightly greater during hyperoxia than normoxia (-22 ± 3 vs. -17 ± 2%; P < 0.05). However, during α-adrenergic blockade, hyperoxic exercise FBF and FVC remained lower than during normoxia (P < 0.01). Therefore, our data suggest that although the vasoconstrictor responsiveness during hyperoxic exercise was slightly greater, it likely does not explain the majority of the large reductions in FBF and FVC (∼20-25%) during hyperbaric hyperoxic exercise.

Impact of dietary nitrate supplementation via beetroot juice on exercising muscle vascular control in rats

Dietary nitrate (NO(3)(-)) supplementation, via its reduction to nitrite (NO(2)(-)) and subsequent conversion to nitric oxide (NO) and other reactive nitrogen intermediates, reduces blood pressure and the O(2) cost of submaximal exercise in humans. Despite these observations, the effects of dietary NO(3)(-) supplementation on skeletal muscle vascular control during locomotory exercise remain unknown. We tested the hypotheses that dietary NO(3)(-) supplementation via beetroot juice (BR) would reduce mean arterial pressure (MAP) and increase hindlimb muscle blood flow in the exercising rat. Male Sprague-Dawley rats (3-6 months) were administered either NO(3)(-) (via beetroot juice; 1 mmol kg(-1) day(-1), BR n = 8) or untreated (control, n = 11) tap water for 5 days. MAP and hindlimb skeletal muscle blood flow and vascular conductance (radiolabelled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). BR resulted in significantly lower exercising MAP (control: 137 ± 3, BR: 127 ± 4 mmHg, P < 0.05) and blood [lactate] (control: 2.6 ± 0.3, BR: 1.9 ± 0.2 mm, P < 0.05) compared to control. Total exercising hindlimb skeletal muscle blood flow (control: 108 ± 8, BR: 150 ± 11 ml min(-1) (100 g)(-1), P < 0.05) and vascular conductance (control: 0.78 ± 0.05, BR: 1.16 ± 0.10 ml min(-1) (100 g)(-1) mmHg(-1), P < 0.05) were greater in rats that received BR compared to control. The relative differences in blood flow and vascular conductance for the 28 individual hindlimb muscles and muscle parts correlated positively with their percentage type IIb + d/x muscle fibres (blood flow: r = 0.74, vascular conductance: r = 0.71, P < 0.01 for both). These data support the hypothesis that NO(3)(-) supplementation improves vascular control and elevates skeletal muscle O(2) delivery during exercise predominantly in fast-twitch type II muscles, and provide a potential mechanism by which NO(3)(-) supplementation improves metabolic control.

Mechanisms of sympathetic regulation in orthostatic intolerance

Sympathetic circulatory control is key to the rapid cardiovascular adjustments that occur within seconds of standing upright (orthostasis) and which are required for bipedal stance. Indeed, patients with ineffective sympathetic adrenergic vasoconstriction rapidly develop orthostatic hypotension, prohibiting effective upright activities. One speaks of orthostatic intolerance (OI) when signs, such as hypotension, and symptoms, such as lightheadedness, occur when upright and are relieved by recumbence. The experience of transient mild OI is part of daily life. However, many people experience episodic acute OI as postural faint or chronic OI in the form of orthostatic tachycardia and orthostatic hypotension that significantly reduce the quality of life. Potential mechanisms for OI are discussed including forms of sympathetic hypofunction, forms of sympathetic hyperfunction, and OI that results from regional blood volume redistribution attributable to regional adrenergic hypofunction.

A comparison of the changes in cardiac output and systemic vascular resistance during exercise following high-fat meals containing DHA or EPA

Long-chain n-3 PUFA can lower blood pressure (BP) but their acute effects on cardiac output, BP and systemic vascular resistance (SVR) in response to dynamic exercise are uncertain. We compared the effects of high-fat meals rich in EPA (20 : 5n-3), DHA (22 : 6n-3) or oleic acid (control) on cardiac output, BP and SVR in response to exercise stress testing. High-fat meals (50 g fat) containing high-oleic sunflower oil enriched with 4·7 g of either EPA or DHA v. control (high-oleic sunflower oil only) were fed to twenty-two healthy males using a randomised cross-over design. Resting measurements of cardiac output, heart rate and BP were made before and hourly over 5 h following the meal. A standardised 12 min exercise test was then conducted with further measurements made during and post-exercise. Blood samples were collected at fasting, 5 h postprandially and immediately post-exercise for the analysis of lipid, glucose and 8-isoprostane-F2α (8-iso-PGF2α). Plasma concentrations of EPA and DHA increased by 0·22 mmol/l 5 h following the EPA and DHA meals, respectively, compared with the control (P < 0·001). Resting cardiac output and 8-iso-PGF2α increased similarly following all meals and there were no significant differences in cardiac output during exercise between the meals. SVR was lower at 5 h and during exercise following the DHA but not EPA meal, compared with the control meal, by 4·9 % (95 % CI 1·3, 8·4; P < 0·01). Meals containing DHA appear to differ from EPA with regard to their effects on cardiovascular haemodynamics during exercise.

Muscle contraction duration and fibre recruitment influence blood flow and oxygen consumption independent of contractile work during steady-state exercise in humans

We tested the hypothesis that, among conditions of matched contractile work, shorter contraction durations and greater muscle fibre recruitment result in augmented skeletal muscle blood flow and oxygen consumption ( ) during steady-state exercise in humans. To do so, we measured forearm blood flow (FBF; Doppler ultrasound) during 4 min of rhythmic hand-grip exercise in 24 healthy young adults and calculated forearm oxygen consumption ( ) via blood samples obtained from a catheter placed in retrograde fashion into a deep vein draining the forearm muscle. In protocol 1 (n = 11), subjects performed rhythmic isometric hand-grip exercise at mild and moderate intensities during conditions in which time-tension index (isometric analogue of work) was held constant but contraction duration was manipulated. In this protocol, shorter contraction durations led to greater FBF (184 ± 25 versus 164 ± 25 ml min(-1)) and (23 ± 3 versus 17 ± 2 ml min(-1); both P < 0.05) among mild workloads, whereas this was not the case for moderate-intensity exercise. In protocol 2 (n = 13), subjects performed rhythmic dynamic hand-grip exercise at mild and moderate intensities in conditions of matched total work, but muscle fibre recruitment was manipulated. In this protocol, greater muscle fibre recruitment led to significantly greater FBF (152 ± 15 versus 127 ± 13 ml min(-1)) and (20 ± 2 versus 17 ± 2 ml min(-1); both P < 0.05) at mild workloads, and there was a trend for similar responses at the moderate intensity but this was not statistically significant. In both protocols, the ratio of the change in FBF to change in was similar across all exercise intensities and manipulations, and the strongest correlation among all variables was between and blood flow. Our collective data indicate that, among matched workloads, shorter contraction duration and greater muscle fibre recruitment augment FBF and during mild-intensity forearm exercise, and that muscle blood flow is more closely related to metabolic cost ( ) rather than contractile work per se during steady-state exercise in humans.