Effects of neuronal nitric oxide synthase inhibition on resting and exercising hindlimb muscle blood flow in the rat

Does sympathetic vasoconstriction contribute to metabolism: Perfusion matching in exercising skeletal muscle?

The process of matching skeletal muscle blood flow to metabolism is complex and multi-factorial. In response to exercise, increases in cardiac output, perfusion pressure and local vasodilation facilitate an intensity-dependent increase in muscle blood flow. Concomitantly, sympathetic nerve activity directed to both exercising and non-active muscles increases as a function of exercise intensity. Several studies have reported the presence of tonic sympathetic vasoconstriction in the vasculature of exercising muscle at the onset of exercise that persists through prolonged exercise bouts, though it is blunted in an exercise-intensity dependent manner (functional sympatholysis). The collective evidence has resulted in the current dogma that vasoactive molecules released from skeletal muscle, the vascular endothelium, and possibly red blood cells produce local vasodilation, while sympathetic vasoconstriction restrains vasodilation to direct blood flow to the most metabolically active muscles/fibers. Vascular smooth muscle is assumed to integrate a host of vasoactive signals resulting in a precise matching of muscle blood flow to metabolism. Unfortunately, a critical review of the available literature reveals that published studies have largely focused on bulk blood flow and existing experimental approaches with limited ability to reveal the matching of perfusion with metabolism, particularly between and within muscles. This paper will review our current understanding of the regulation of sympathetic vasoconstriction in contracting skeletal muscle and highlight areas where further investigation is necessary.

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.

Nitric oxide synthase inhibition with N(G)-monomethyl-l-arginine: Determining the window of effect in the human vasculature

Nitric oxide synthase (NOS) inhibition with N(G)-monomethyl-l-arginine (L-NMMA) is often used to assess the role of NO in human cardiovascular function. However, the window of effect for L-NMMA on human vascular function is unknown, which is critical for designing and interpreting human-based studies. This study utilized the passive leg movement (PLM) assessment of vascular function, which is predominantly NO-mediated, in 7 young male subjects under control conditions, immediately following intra-arterial L-NMMA infusion (0.24 mg⋅dl^-1⋅min^-1), and at 45-60 and 90-105 min post L-NMMA infusion. The leg blood flow (LBF) and leg vascular conductance (LVC) responses to PLM, measured with Doppler ultrasound and expressed as the change from baseline to peak (ΔLBF_peak and ΔLVC_peak) and area under the curve (LBF_AUC and LVC_ACU), were assessed. PLM-induced robust control ΔLBF_peak (1135 ± 324 ml⋅min^-1) and ΔLVC_peak (10.7 ± 3.6 ml⋅min^-1⋅mmHg^-1) responses that were significantly attenuated (704 ± 196 ml⋅min^-1 and 6.7 ± 2 ml⋅min^-1⋅mmHg^-1) immediately following L-NMMA infusion. Likewise, control condition PLM ΔLBF_AUC (455 ± 202 ml) and ΔLVC_AUC (4.0 ± 1.4 ml⋅mmHg^-1) were significantly attenuated (141 ± 130 ml and 1.3 ± 1.2 ml⋅mmHg^-1) immediately following L-NMMA infusion. However, by 45-60 min post L-NMMA infusion all PLM variables were not significantly different from control, and this was still the case at 90-105 min post L-NMMA infusion. These findings reveal that the potent reduction in NO bioavailability afforded by NOS inhibition with L-NMMA has a window of effect of less than 45-60 min in the human vasculature. These data are particularly important for the commonly employed approach of pharmacologically inhibiting NOS with L-NMMA in the human vasculature.

Systemic NOS inhibition reduces contracting muscle oxygenation more in intact female than male rats

Females respond to baroreceptor stimulation with enhanced modulation of heart rate (HR) to regulate blood pressure and also express greater reliance on nitric oxide (NO) for vascular control compared to males. Sex differences in muscle oxygenation consequent to central hemodynamic challenge induced by systemic NO synthase (NOS) inhibition are unknown. We tested the hypotheses that systemic NOS inhibition would induce lower contracting skeletal muscle oxygenation in females compared to males. The spinotrapezius of Sprague-Dawley rats (females (♀) = 9, males (♂) = 9) was surgically exposed and contracted by electrical stimulation (180s, 1 Hz, ~6 V) under pentobarbital sodium anesthesia. Oxyphor G4 was injected into the muscle and phosphorescence quenching was used to measure the interstitial PO₂ (PO₂is, determined by O₂ delivery-to-utilization matching) under control (Krebs-Henseleit solution) and after intra-arterial infusion of nitro-l-arginine methyl ester (l-NAME; NOS blockade; 10 mg kg^-1). At rest, females showed a greater PO₂is increase (ΔPO₂is/ΔMAP) and HR (ΔHR/ΔMAP) reduction than males in response to the elevated MAP induced by systemic NOS inhibition (both p < 0.05). Following l-NAME, during the contracting steady-state, females exhibited lower PO₂is than males (♂: 17.1 ± 1.4 vs ♀: 10.8 ± 1.4 mmHg, p < 0.05). The rate pressure product was lower in females than males (♂: 482 ± 14 vs ♀: 392 ± 29, p < 0.05) and correlated with the steady-state PO₂is (r = 0.66, p < 0.05). These results support that females express greater reductions in HR than males in response to l-NAME-induced elevation of MAP via the baroreceptor reflex and provide new insights on how central hemodynamics affect skeletal muscle oxygenation in a sex-specific manner.

Limb blood flow and muscle oxygenation responses during handgrip exercise above vs. below critical force

This study compared the brachial artery blood flow (Q̇_BA) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). Q̇_BA and microvascular oxygen delivery are important determinants of oxygen utilization and metabolite accumulation during exercise, both of which increase progressively during exercise above CF. However the Q̇_BA and microvascular oxygen delivery responses above vs. below CF remain unknown. We hypothesized that Q̇_BA, deoxygenated-heme (deoxy-[heme]; an estimate of microvascular fractional oxygen extraction), and total-heme concentrations (total-[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort test (MET) to determine CF and 2) two constant target-force tests above (severe-intensity; S1 and S2) and two constant target-force tests below (heavy-intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6%MVC). At end-exercise, Q̇_BA was greater for tests above CF (S1: 418 ± 147 mL/min; S2: 403 ± 137 mL/min) compared to tests below CF (H1: 287 ± 97 mL/min; H2: 340 ± 116 mL/min; all p < 0.05) but was not different between S1 and S2. Further, end-test Q̇_BA during both tests above CF was not different from Q̇_BA estimated at CF (392 ± 37 mL/min). At end-exercise, deoxy-[heme] was not different between tests above CF (S1: 150 ± 50 μM; S2: 155 ± 57 μM), but was greater during tests above CF compared to tests below CF (H1: 101 ± 24 μM; H2: 111 ± 21 μM; all p < 0.05). At end-exercise, total-[heme] was not different between tests above CF (S1: 404 ± 58 μM; S2: 397 ± 73 μM), but was greater during tests above CF compared to H1 (352 ± 58 μM; p < 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations exist and maximal levels of muscle microvascular oxygen delivery and extraction occur during exercise above, but not below, CF.

Rapid changes in the microvascular circulation of skeletal muscle impair insulin delivery during sepsis

Sepsis costs the healthcare system $23 billion annually and has a mortality rate between 10 and 40%. An early indication of sepsis is the onset of hyperglycemia, which is the result of sepsis-induced insulin resistance in skeletal muscle. Previous investigations have focused on events in the myocyte (e.g., insulin signaling and glucose transport and subsequent metabolism) as the causes for this insulin-resistant state. However, the delivery of insulin to the skeletal muscle is also an important determinant of insulin action. Skeletal muscle microvascular blood flow, which delivers the insulin to the muscle, is known to be decreased during sepsis. Here we test whether the reduced capillary blood flow to skeletal muscle belies the sepsis-induced insulin resistance by reducing insulin delivery to the myocyte. We hypothesize that decreased capillary flow and consequent decrease in insulin delivery is an early event that precedes gross cardiovascular alterations seen with sepsis. This hypothesis was examined in mice treated with either lipopolysaccharide (LPS) or polymicrobial sepsis followed by intravital microscopy of the skeletal muscle microcirculation. We calculated insulin delivery to the myocyte using two independent methods and found that LPS and sepsis rapidly reduce insulin delivery to the skeletal muscle by ~50%; this was driven by decreases in capillary flow velocity and the number of perfused capillaries. Furthermore, the changes in skeletal muscle microcirculation occur before changes in both cardiac output and arterial blood pressure. These data suggest that a rapid reduction in skeletal muscle insulin delivery contributes to the induction of insulin resistance during sepsis.

Acute Nitric Oxide Synthase Inhibition Accelerates Transendothelial Insulin Efflux In Vivo

Before insulin can stimulate glucose uptake in muscle, it must be delivered to skeletal muscle (SkM) through the microvasculature. Insulin delivery is determined by SkM perfusion and the rate of movement of insulin across the capillary endothelium. The endothelium therefore plays a central role in regulating insulin access to SkM. Nitric oxide (NO) is a key regulator of endothelial function and stimulates arterial vasodilation, which increases SkM perfusion and the capillary surface area available for insulin exchange. The effects of NO on transendothelial insulin efflux (TIE), however, are unknown. We hypothesized that acute reduction of endothelial NO would reduce TIE. However, intravital imaging of TIE in mice revealed that reduction of NO by l-N^G-nitro-l-arginine methyl ester (l-NAME) enhanced the rate of TIE by ∼30% and increased total extravascular insulin delivery. This accelerated TIE was associated with more rapid insulin-stimulated glucose lowering. Sodium nitroprusside, an NO donor, had no effect on TIE in mice. The effects of l-NAME on TIE were not due to changes in blood pressure alone, as a direct-acting vasoconstrictor (phenylephrine) did not affect TIE. These results demonstrate that acute NO synthase inhibition increases the permeability of capillaries to insulin, leading to an increase in delivery of insulin to SkM.

Role of neuronal nitric oxide synthase (nNOS) in Duchenne and Becker muscular dystrophies - Still a possible treatment modality?

Neuronal nitric oxide synthase (nNOS) is involved in nitric oxide (NO) production and suggested to play a crucial role in blood flow regulation of skeletal muscle. During activation of the muscle, NO helps attenuate the sympathetic vasoconstriction to accommodate increased metabolic demands, a phenomenon known as functional sympatholysis. In inherited myopathies such as the dystrophinopathies Duchenne and Becker muscle dystrophies (DMD and BMD), nNOS is lost from the sarcolemma. The loss of nNOS may cause functional ischemia contributing to skeletal and cardiac muscle cell injury. Effects of NO is augmented by inhibiting degradation of the second messenger cyclic guanosine monophosphate (cGMP) using sildenafil and tadalafil, both of which inhibit the enzyme phosphodiesterase 5 (PDE5). In animal models of DMD, PDE5-inhibitors prevent functional ischemia, reduce post-exercise skeletal muscle pathology and fatigue, show amelioration of cardiac muscle cell damage and increase cardiac performance. However, effect on clinical outcomes in DMD and BMD patients have been disappointing with minor effects on upper limb performance and none on ambulation. This review aims to summarize the current knowledge of nNOS function related to functional sympatholysis in skeletal muscle and studies on PDE5-inhibitor treatment in nNOS-deficient animal models and patients.

Impact of cell-free hemoglobin on contracting skeletal muscle microvascular oxygen pressure dynamics

Free hemoglobin (Hb) associated with hemolysis extravasates into vascular tissue and depletes nitric oxide (NO), which leads to impaired vascular function and could impair skeletal muscle metabolic control during exercise. We tested the hypothesis that: 1) free Hb would extravasate into skeletal muscle tissue, reducing the contracting skeletal muscle O₂ delivery/O₂ utilization ratio (microvascular PO₂, PO₂mv) to a similar extent as that observed following NO synthase (NOS) blockade, and 2) that the Hb scavenging protein haptoglobin (Hp) would prevent Hb extravasation and inhibit these skeletal muscle tissue effects. PO₂mv was measured in eight rats (phosphorescence quenching) at rest and during 180 s of electrically induced (1-Hz) twitch spinotrapezius muscle contractions (experiment 1). A second group of seven rats was also used to investigate the effects of Hb + Hp (experiment 2). For both experiments, measurements were made: 1) during control conditions, 2) following a bolus infusion of either Hb (50 mg/kg) or Hb + Hp (50 mg/kg), and 3) following local superfusion of NG-nitro-l-arginine methyl ester (L-NAME; 10 mg/kg). Additional experiments were completed to visualize Hb extravasation into the muscular tissue using Click chemistry techniques. There were no significant differences in the PO₂mv observed at rest for any condition in either experiment (p > 0.05 for all). In experiment 1, both Hb and L-NAME reduced the PO₂mv significantly during the steady-state of muscle contractions when compared to control conditions with no differences between Hb and L-NAME (control: 24 ± 1, Hb: 21 ± 1, L-NAME: 20 ± 1 mmHg, p < 0.05). In experiment 2, only L-NAME resulted in a significantly lower PO₂mv during the steady-state of muscle contractions (control: 25 ± 1, Hb + Hp: 22 ± 2, L-NAME: 18 ± 1 mmHg, p < 0.05). Free Hb lowered the blood-myocyte O₂ driving force to a level not significantly different from L-NAME. However, infusing Hb bound to Hp resulted in no significant differences in steady-state PO₂mv during muscle contractions when compared to control. Surprisingly, we did not observe Hb accumulation in skeletal muscle tissue. Taken together these data suggests that free Hb impairs O₂ delivery/utilization via a NO scavenging effect. Furthermore, the unchanged PO₂mv steady-state observed following Hb + Hp further indicates that vascular compartmentalization of Hb by the scavenger protein haptoglobin may improve skeletal muscle metabolic control and potentially exercise tolerance in those afflicted with hemolytic diseases.

Neuronal nitric oxide synthase regulation of skeletal muscle functional hyperemia: exercise training and moderate compensated heart failure

Nitric oxide (NO) modulates oxygen delivery-utilization matching in resting and contracting skeletal muscle. Recent reports indicate that neuronal NO synthase (nNOS)-mediated vasoregulation during contractions is enhanced with exercise training and impaired with chronic heart failure (HF). Consequently, we tested the hypothesis that selective nNOS inhibition (S-methyl-l-thiocitrulline; SMTC, 2.1 μmol/kg) would produce attenuated reductions in muscle blood flow during moderate/heavy submaximal exercise in sedentary HF rats compared to their healthy counterparts. In addition, SMTC was expected to evoke greater reductions in exercising muscle blood flow in trained compared to sedentary healthy and HF rats. Blood flow during submaximal treadmill running (20 min/m, 5% grade) was determined via radiolabeled microspheres pre- and post-SMTC administration in healthy sedentary (Healthy + Sed, n = 8), healthy exercise trained (Healthy + ExT, n = 8), HF sedentary (HF + Sed, left ventricular end-diastolic pressure (LVEDP) = 12 ± 1 mmHg, n = 8), and HF exercise trained (HF + ExT, LVEDP = 16 ± 2 mmHg, n = 7) rats. nNOS contribution to exercising total hindlimb blood flow (ml/min/100 g) was not increased by training in either healthy or HF groups (Healthy + Sed: 105 ± 11 vs. 108 ± 16; Healthy + ExT: 96 ± 9 vs. 91 ± 7; HF + Sed: 124 ± 6 vs. 110 ± 12; HF + ExT: 107 ± 13 vs. 101 ± 8; control vs. SMTC, respectively; p > .05 for all). Similarly, SMTC did not reduce exercising blood flow in the majority of individual hindlimb muscles in any group (p > .05 for all, except for the semitendinosus and adductor longus in HF + Sed and the adductor longus in HF + ExT; p < .05). Contrary to our hypothesis, we find no support for either upregulation of nNOS function contributing to exercise hyperemia after training or its dysregulation with chronic HF.

The effect of nitric oxide synthase inhibition with and without inhibition of prostaglandins on blood flow in different human skeletal muscles

PURPOSE

Animal studies suggest that the inhibition of nitric oxide synthase (NOS) affects blood flow differently in different skeletal muscles according to their muscle fibre type composition (oxidative vs glycolytic). Quadriceps femoris (QF) muscle consists of four different muscle parts: vastus intermedius (VI), rectus femoris (RF), vastus medialis (VM), and vastus lateralis (VL) of which VI is located deep within the muscle group and is generally regarded to consist mostly of oxidative muscle fibres.

METHODS

We studied the effect of NOS inhibition on blood flow in these four different muscles by positron emission tomography in eight young healthy men at rest and during one-leg dynamic exercise, with and without combined blockade with prostaglandins.

RESULTS

At rest blood flow in the VI (2.6 ± 1.1 ml/100 g/min) was significantly higher than in VL (1.9 ± 0.6 ml/100 g/min, p = 0.015) and RF (1.7 ± 0.6 ml/100 g/min, p = 0.0015), but comparable to VM (2.4 ± 1.1 ml/100 g/min). NOS inhibition alone or with prostaglandins reduced blood flow by almost 50% (p < 0.001), but decrements were similar in all four muscles (drug × muscle interaction, p = 0.43). During exercise blood flow was also the highest in VI (45.4 ± 5.5 ml/100 g/min) and higher compared to VL (35.0 ± 5.5 ml/100 g/min), RF (38.4 ± 7.4 ml/100 g/min), and VM (36.2 ± 6.8 ml/100 g/min). NOS inhibition alone did not reduce exercise hyperemia (p = 0.51), but combined NOS and prostaglandin inhibition reduced blood flow during exercise (p = 0.002), similarly in all muscles (drug × muscle interaction, p = 0.99).

CONCLUSION

NOS inhibition, with or without prostaglandins inhibition, affects blood flow similarly in different human QF muscles both at rest and during low-to-moderate intensity exercise.

Effect of sodium nitrite on local control of contracting skeletal muscle microvascular oxygen pressure in healthy rats

Exercise intolerance characteristic of diseases such as chronic heart failure (CHF) and diabetes is associated with reduced nitric oxide (NO) bioavailability from nitric oxide synthase (NOS), resulting in an impaired microvascular O₂ driving pressure (Po₂mv; O₂ delivery/O₂ utilization) and metabolic control. Infusions of the potent NO donor sodium nitroprusside augment NO bioavailability yet decrease mean arterial pressure (MAP) thereby reducing its potential efficacy for patient populations. To eliminate or reduce hypotensive sequelae, [Formula: see text] was superfused onto the spinotrapezius muscle. It was hypothesized that local [Formula: see text] administration would elevate resting Po₂mv and slow Po₂mv kinetics [increased time constant (τ) and mean response time (MRT)] following the onset of muscle contractions without decreasing MAP. In 12 anesthetized male Sprague-Dawley rats, Po₂mv of the circulation-intact spinotrapezius muscle was measured by phosphorescence quenching during 180 s of electrically induced twitch contractions (1 Hz) before and after superfusion of sodium nitrite (NaNO₂ 30 mM). [Formula: see text] superfusion elevated resting Po₂mv (control: 28.4 ± 1.1 vs. [Formula: see text]: 31.6 ± 1.2 mmHg; P ≤ 0.05), τ (control: 12.3 ± 1.2 vs. [Formula: see text]: 19.7 ± 2.2 s; P ≤ 0.05), and MRT (control: 19.3 ± 1.9 vs. [Formula: see text]: 25.6 ± 3.3 s; P ≤ 0.05). Importantly, these effects occurred in the absence of any reduction in MAP (103 ± 4 vs. 105 ± 4 mmHg, pre- and postsuperfusion respectively; P > 0.05). These results indicate that [Formula: see text] supplementation delivered to the muscle directly through [Formula: see text] superfusion enhances the blood-myocyte oxygen driving pressure without compromising MAP at rest and following the onset of muscle contraction. This strategy has substantial clinical utility for a range of ischemic conditions.

NEW & NOTEWORTHY

Ischemic conditions as diverse as chronic heart failure (CHF) and frostbite inflict tissue damage via inadequate O₂ delivery. Herein we demonstrate that direct application of sodium nitrite enhances the O₂ supply-O₂ demand relationship, raising microvascular O₂ pressure in healthy skeletal muscle. This therapeutic action of nitrite-derived nitric oxide occurred without inducing systemic hypotension and has the potential to relieve focal ischemia and preserve tissue vitality by enhancing O₂ delivery.

Heterogeneity of Muscle Blood Flow and Metabolism: Influence of Exercise, Aging, and Disease States

The systematic increase in V˙O2 uptake and O2 extraction with increasing work rates conceals a substantial heterogeneity of O2 delivery (Q˙O2)-to- V˙O2 matching across and within muscles and other organs. We hypothesize that whether increased/decreased Q˙O2/V˙O2 heterogeneity can be judged as "good" or "bad," for example, after exercise training or in aged individuals or with disease (heart failure, diabetes) depends on the resultant effects on O2 transport and contractile performance.

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.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

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 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.

Involvement of N-methyl-D-aspartate glutamate receptor and nitric oxide in cardiovascular responses to dynamic exercise in rats

Dynamic exercise evokes sustained cardiovascular responses, which are characterized by arterial pressure and heart rate increases. Although it is well accepted that there is central nervous system mediation of cardiovascular adjustments during exercise, information on the role of neural pathways and signaling mechanisms is limited. It has been reported that glutamate, by acting on NMDA receptors, evokes the release of nitric oxide through activation of neuronal nitric oxide synthase (nNOS) in the brain. In the present study, we tested the hypothesis that NMDA receptors and nNOS are involved in cardiovascular responses evoked by an acute bout of exercise on a rodent treadmill. Moreover, we investigated possible central sites mediating control of responses to exercise through the NMDA receptor-nitric oxide pathway. Intraperitoneal administration of the selective NMDA glutamate receptor antagonist dizocilpine maleate (MK-801) reduced both the arterial pressure and heart rate increase evoked by dynamic exercise. Intraperitoneal treatment with the preferential nNOS inhibitor 7-nitroindazole reduced exercise-evoked tachycardiac response without affecting the pressor response. Moreover, treadmill running increased NO formation in the medial prefrontal cortex (MPFC), bed nucleus of the stria teminalis (BNST) and periaqueductal gray (PAG), and this effect was inhibited by systemic pretreatment with MK-801. Our findings demonstrate that NMDA receptors and nNOS mediate the tachycardiac response to dynamic exercise, possibly through an NMDA receptor-NO signaling mechanism. However, NMDA receptors, but not nNOS, mediate the exercise-evoked pressor response. The present results also provide evidence that MPFC, BNST and PAG may modulate physiological adjustments during dynamic exercise through NMDA receptor-NO signaling.

Muscle fibre-type dependence of neuronal nitric oxide synthase-mediated vascular control in the rat during high speed treadmill running

We have recently shown that nitric oxide (NO) derived from neuronal NO synthase (nNOS) does not contribute to the hyperaemic response within rat hindlimb skeletal muscle during low-speed treadmill running. This may be attributed to low exercise intensities recruiting primarily oxidative muscle and that vascular effects of nNOS-derived NO are manifest principally within glycolytic muscle. We tested the hypothesis that selective nNOS inhibition via S-methyl-l-thiocitrulline (SMTC) would reduce rat hindlimb skeletal muscle blood flow and vascular conductance (VC) during high-speed treadmill running above critical speed (asymptote of the hyperbolic speed versus time-to-exhaustion relationship for high-speed running and an important glycolytic fast-twitch fibre recruitment boundary in the rat) principally within glycolytic fast-twitch muscle. Six rats performed three high-speed treadmill runs to exhaustion to determine critical speed. Subsequently, hindlimb skeletal muscle blood flow (radiolabelled microspheres) and VC (blood flow/mean arterial pressure) were determined during supra-critical speed treadmill running (critical speed + 15%, 52.5 ± 1.3 m min(-1)) before (control) and after selective nNOS inhibition with 0.56 mg kg(-1) SMTC. SMTC reduced total hindlimb skeletal muscle blood flow (control: 241 ± 23, SMTC: 204 ± 13 ml min(-1) (100 g)(-1), P < 0.05) and VC (control: 1.88 ± 0.20, SMTC: 1.48 ± 0.13 ml min(-1) (100 g)(-1) mmHg(-1), P < 0.05) during high-speed running. The relative reductions in blood flow and VC were greater in the highly glycolytic muscles and muscle parts consisting of 100% type IIb+d/x fibres compared to the highly oxidative muscles and muscle parts consisting of 35% type IIb+d/x muscle fibres (P < 0.05). These results extend our understanding of vascular control during exercise by identifying fibre-type-selective peripheral vascular effects of nNOS-derived NO during high-speed treadmill running.

Neuronal nitric oxide synthase inhibition and regional sympathetic nerve discharge: implications for peripheral vascular control

Neuronal nitric oxide (NO) synthase (nNOS) inhibition with systemically administered S-methyl-l-thiocitrulline (SMTC) elevates mean arterial pressure (MAP) and reduces rat hindlimb skeletal muscle and renal blood flow. We tested the hypothesis that those SMTC-induced cardiovascular effects resulted, in part, from increased sympathetic nerve discharge (SND). MAP, HR, and lumbar and renal SND (direct nerve recordings) were measured in 9 baroreceptor (sino-aortic)-denervated rats for 20min each following both saline and SMTC (0.56mg/kg i.v.). SMTC increased MAP (peak ΔMAP: 50±8mmHg, p<0.01) compared to saline. Lumbar and renal SND were not different between saline and SMTC conditions at any time (p>0.05). The ΔSND between saline and SMTC conditions for the lumbar and renal nerves were not different from zero (peak ΔSND, lumbar: 2.0±6.8%; renal: 9.7±9.0%, p>0.05 versus zero for both). These data support that SMTC-induced reductions in skeletal muscle and renal blood flow reported previously reflect peripheral nNOS-derived NO vascular control as opposed to increased sympathetic vasoconstriction.

Sympathetic activation increases NO release from eNOS but neither eNOS nor nNOS play an essential role in exercise hyperemia in the human forearm

Nitric oxide (NO) release from endothelial NO synthase (eNOS) and/or neuronal NO synthase (nNOS) could be modulated by sympathetic nerve activity and contribute to increased blood flow after exercise. We examined the effects of brachial-arterial infusion of the nNOS selective inhibitor S-methyl-l-thiocitrulline (SMTC) and the nonselective NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) on forearm arm blood flow at rest, during sympathetic activation by lower body negative pressure, and during lower body negative pressure immediately after handgrip exercise. Reduction in forearm blood flow by lower body negative pressure during infusion of SMTC was not significantly different from that during vehicle (-28.5 ± 4.02 vs. -34.1 ± 2.96%, respectively; P = 0.32; n = 8). However, l-NMMA augmented the reduction in forearm blood flow by lower body negative pressure (-44.2 ± 3.53 vs. -23.4 ± 5.71%; n = 8; P < 0.01). When lower body negative pressure was continued after handgrip exercise, there was no significant effect of either l-NMMA or SMTC on forearm blood flow immediately after low-intensity exercise (P = 0.91 and P = 0.44 for l-NMMA vs. saline and SMTC vs. saline, respectively; each n = 10) or high-intensity exercise (P = 0.46 and P = 0.68 for l-NMMA vs. saline and SMTC vs. saline, respectively; each n = 10). These results suggest that sympathetic activation increases NO release from eNOS, attenuating vasoconstriction. Dysfunction of eNOS could augment vasoconstrictor and blood pressure responses to sympathetic activation. However, neither eNOS nor nNOS plays an essential role in postexercise hyperaemia, even in the presence of increased sympathetic activation.

Neuronal nitric oxide synthase is indispensable for the cardiac adaptive effects of exercise

Exercise results in beneficial adaptations of the heart that can be directly observed at the ventricular myocyte level. However, the molecular mechanism(s) responsible for these adaptations are not well understood. Interestingly, signaling via neuronal nitric oxide synthase (NOS1) within myocytes results in similar effects as exercise. Thus, the objective was to define the role NOS1 plays in the exercise-induced beneficial contractile effects in myocytes. After an 8-week aerobic interval training program, exercise-trained (Ex) mice had higher VO(2max) and cardiac hypertrophy compared to sedentary (Sed) mice. Ventricular myocytes from Ex mice had increased NOS1 expression and nitric oxide production compared to myocytes from Sed mice. Remarkably, acute NOS1 inhibition normalized the enhanced contraction (shortening and Ca(2+) transients) in Ex myocytes to Sed levels. The NOS1 effect on contraction was mediated via greater Ca(2+) cycling that resulted from increased phospholamban phosphorylation. Intriguingly, a similar aerobic interval training program on NOS1 knockout mice failed to produce any beneficial cardiac adaptations (VO(2max), hypertrophy, and contraction). These data demonstrate that the beneficial cardiac adaptations observed after exercise training were mediated via enhanced NOS1 signaling. Therefore, it is likely that beneficial effects of exercise may be mimicked by the interventions that increase NOS1 signaling. This pathway may provide a potential novel therapeutic target in cardiac patients who are unable or unwilling to 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.

Effects of neuronal nitric oxide synthase inhibition on microvascular and contractile function in skeletal muscle of aged rats

Advanced age is associated with derangements in skeletal muscle microvascular function during the transition from rest to contractions. We tested the hypothesis that, contrary to what was reported previously in young rats, selective neuronal nitric oxide (NO) synthase (nNOS) inhibition would result in attenuated or absent alterations in skeletal muscle microvascular oxygenation (Po(2)(mv)), which reflects the matching between muscle O(2) delivery and utilization, following the onset of contractions in old rats. Spinotrapezius muscle blood flow (radiolabeled microspheres), Po(2)(mv) (phosphorescence quenching), O(2) utilization (Vo(2); Fick calculation), and submaximal force production were measured at rest and following the onset of contractions in anesthetized old male Fischer 344 × Brown Norway rats (27 to 28 mo) pre- and postselective nNOS inhibition (2.1 μmol/kg S-methyl-l-thiocitrulline; SMTC). At rest, SMTC had no effects on muscle blood flow (P > 0.05) but reduced Vo(2) by ∼23% (P < 0.05), which elevated basal Po(2)(mv) by ∼18% (P < 0.05). During contractions, steady-state muscle blood flow, Vo(2), Po(2)(mv), and force production were not altered after SMTC (P > 0.05 for all). The overall Po(2)(mv) dynamics following onset of contractions was also unaffected by SMTC (mean response time: pre, 19.7 ± 1.5; and post, 20.0 ± 2.0 s; P > 0.05). These results indicate that the locus of nNOS-derived NO control in skeletal muscle depends on age and metabolic rate (i.e., rest vs. contractions). Alterations in nNOS-mediated regulation of contracting skeletal muscle microvascular function with aging may contribute to poor exercise capacity in this population.

Effects of chronic heart failure on neuronal nitric oxide synthase-mediated control of microvascular O2 pressure in contracting rat skeletal muscle

Chronic heart failure (CHF) impairs nitric oxide (NO)-mediated regulation of the skeletal muscle microvascular O(2) delivery/V(O(2)) ratio (which sets the microvascular O(2) pressure, PO(2)mv). Given the pervasiveness of endothelial dysfunction in CHF, this NO-mediated dysregulation is attributed generally to eNOS. It is unknown whether nNOS-mediated PO(2)mv regulation is altered in CHF. We tested the hypothesis that CHF impairs nNOS-mediated PO(2)mv control. In healthy and CHF (left ventricular end diastolic pressure (LVEDP): 6 ± 1 versus 14 ± 1 mmHg, respectively, P < 0.05) rats spinotrapezius muscle blood flow (radiolabelled microspheres), PO(2)mv (phosphorescence quenching), and V(O(2)) (Fick calculation) were measured before and after 0.56 mg kg(-1)i.a. of the selective nNOS inhibitor S-methyl-l-thiocitrulline (SMTC). In healthy rats, SMTC increased baseline PO(2)mv (

CONTROL

29.7 ± 1.4, SMTC: 34.4 ± 1.9 mmHg, P < 0.05) by reducing V(O(2)) (↓20%) without any effect on blood flow and speeded the mean response time (MRT, time to reach 63% of the overall kinetics response,

CONTROL

24.2 ± 2.0, SMTC: 18.5 ± 1.3 s, P < 0.05). In CHF rats, SMTC did not alter baseline PO(2)mv (

CONTROL

25.7 ± 1.6, SMTC: 28.6 ± 2.1 mmHg, P > 0.05), V(O(2)) at rest, or the MRT (CONTROL: 22.8 ± 2.6, SMTC: 21.3 ± 3.0 s, P > 0.05). During the contracting steady-state, SMTC reduced blood flow (↓15%) and V(O(2)) (↓15%) in healthy rats such that PO(2)mv was unaltered (

CONTROL

19.8 ± 1.7, SMTC: 20.7 ± 1.8 mmHg, P > 0.05). In marked contrast, in CHF rats SMTC did not change contracting steady-state blood flow, V(O(2)), or PO(2)mv (

CONTROL

17.0 ± 1.4, SMTC: 17.7 ± 1.8 mmHg, P > 0.05). nNOS-mediated control of skeletal muscle microvascular function is compromised in CHF versus healthy rats. Treatments designed to ameliorate microvascular dysfunction in CHF may benefit by targeting improvements in nNOS function.

Exercise-induced angiogenesis correlates with the up-regulated expression of neuronal nitric oxide synthase (nNOS) in human skeletal muscle

The contribution of neuronal nitric oxide synthase (nNOS) to angiogenesis in human skeletal muscle after endurance exercise is controversially discussed. We therefore ascertained whether the expression of nNOS is associated with the capillary density in biopsies of the vastus lateralis (VL) muscle that had been derived from 10 sedentary male subjects before and after moderate training (four 30-min weekly jogging sessions for 6 months, with a heart-rate corresponding to 75% VO(2)max). In these biopsies, nNOS was predominantly expressed as alpha-isoform with exon-mu and to a lesser extent without exon-mu, as determined by RT-PCR. The mRNA levels of nNOS were quantified by real-time PCR and related to the capillary-to-fibre ratio and the numerical density of capillaries specified by light microscopy. If the VL biopsies of all subjects were co-analysed, mRNA levels of nNOS were non-significantly elevated after training (+34%; P > 0.05). However, only five of the ten subjects exhibited significant (P ≤ 0.05) elevations in the capillary-to-fibre ratio (+25%) and the numerical density of capillaries (+21%) and were thus undergoing angiogenesis. If the VL biopsies of these five subjects alone were evaluated, the mRNA levels of nNOS were significantly up-regulated (+128%; P ≤ 0.05) and correlated positively (r = 0.8; P ≤ 0.01) to angiogenesis. Accordingly, nNOS protein expression in VL biopsies quantified by immunoblotting was significantly increased (+82%; P ≤ 0.05) only in those subjects that underwent angiogenesis. In conclusion, the expression of nNOS at mRNA and protein levels was statistically linked to capillarity after exercise suggesting that nNOS is involved in the angiogenic response to training in human skeletal muscle.

Critical speed in the rat: implications for hindlimb muscle blood flow distribution and fibre recruitment

Critical speed (CS) constitutes an important metabolic and performance demarcator. However, active skeletal muscle blood flow distribution specifically surrounding CS remains unknown. We tested the hypotheses that CS could be accurately determined in the running rat and that measurement of hindlimb inter- and intramuscular blood flow below and above CS would support that the greatest muscle fibre recruitment above, relative to below, CS occurs in the predominantly glycolytic muscles. Seven male Sprague-Dawley rats performed five constant-speed tests to exhaustion at speeds between 95 and 115% of the speed that elicited to determine CS. Subsequent constant-speed tests were performed at speeds incrementally surrounding CS to determine time to exhaustion, V(O2), and hindlimb muscle blood flow distribution. Speed and time to exhaustion conformed to a hyperbolic relationship (r(2) = 0.92 ± 0.03) which corresponded to a linear 1/time function (r(2) = 0.93 ± 0.02) with a CS of 48.6 ± 1.0 m min(-1). Time to exhaustion below CS was ∼ 5× greater (P < 0.01) than that above. Below CS V(O2) stabilized at a submaximal value (58.5 ± 2.5 ml kg(-1) min(-1)) whereas above CS (81.7 ± 2.5 ml kg(-1) min(-1)) increased to (84.0 ± 1.8 ml kg(-1) min(-1), P > 0.05 vs. above CS). The 11 individual muscles or muscle parts that evidenced the greatest blood flow increases above, relative to below, CS were composed of ≥ 69% Type IIb/d/x muscle fibres. Moreover, there was a significant correlation (P < 0.05, r = 0.42) between the increased blood flow above expressed relative to below CS and the percentage Type IIb/d/x fibres found in the individual muscles or muscle parts. These data validate the powerful CS construct in the rat and identify that running above CS, relative to below CS, incurs disproportionate blood flow increases (indicative of recruitment) in predominantly highly glycolytic muscle fibres.

Erythrocyte-dependent regulation of human skeletal muscle blood flow: role of varied oxyhemoglobin and exercise on nitrite, S-nitrosohemoglobin, and ATP

The erythrocyte is proposed to play a key role in the control of local tissue perfusion via three O(2)-dependent signaling mechanisms: 1) reduction of circulating nitrite to vasoactive NO, 2) S-nitrosohemoglobin (SNO-Hb)-dependent vasodilatation, and 3) release of the vasodilator and sympatholytic ATP; however, their relative roles in vivo remain unclear. Here we evaluated each mechanism to gain insight into their roles in the regulation of human skeletal muscle blood flow during hypoxia and hyperoxia at rest and during exercise. Arterial and femoral venous hemoglobin O(2) saturation (O(2)Hb), plasma and erythrocyte NO and ATP metabolites, and leg and systemic hemodynamics were measured in 10 healthy males exposed to graded hypoxia, normoxia, and graded hyperoxia both at rest and during submaximal one-legged knee-extensor exercise. At rest, leg blood flow and NO and ATP metabolites in plasma and erythrocytes remained unchanged despite large alterations in O(2)Hb. During exercise, however, leg and systemic perfusion and vascular conductance increased in direct proportion to decreases in arterial and venous O(2)Hb (r(2) = 0.86-0.98; P = 0.01), decreases in venous plasma nitrite (r(2) = 0.93; P < 0.01), increases in venous erythrocyte nitroso species (r(2) = 0.74; P < 0.05), and to a lesser extent increases in erythrocyte SNO (r(2) = 0.59; P = 0.07). No relationship was observed with plasma ATP (r(2) = 0.01; P = 0.99) or its degradation compounds. These in vivo data indicate that, during low-intensity exercise and hypoxic stress, but not hypoxic stress alone, plasma nitrite consumption and formation of erythrocyte nitroso species are associated with limb vasodilatation and increased blood flow in the human skeletal muscle vasculature.