Effects of nitric oxide synthase inhibition on vascular conductance during high speed treadmill exercise in rats

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.

An Overview of NO Signaling Pathways in Aging

Nitric Oxide (NO) is a potent signaling molecule involved in the regulation of various cellular mechanisms and pathways under normal and pathological conditions. NO production, its effects, and its efficacy, are extremely sensitive to aging-related changes in the cells. Herein, we review the mechanisms of NO signaling in the cardiovascular system, central nervous system (CNS), reproduction system, as well as its effects on skin, kidneys, thyroid, muscles, and on the immune system during aging. The aging-related decline in NO levels and bioavailability is also discussed in this review. The decreased NO production by endothelial nitric oxide synthase (eNOS) was revealed in the aged cardiovascular system. In the CNS, the decline of the neuronal (n)NOS production of NO was related to the impairment of memory, sleep, and cognition. NO played an important role in the aging of oocytes and aged-induced erectile dysfunction. Aging downregulated NO signaling pathways in endothelial cells resulting in skin, kidney, thyroid, and muscle disorders. Putative therapeutic agents (natural/synthetic) affecting NO signaling mechanisms in the aging process are discussed in the present study. In summary, all of the studies reviewed demonstrate that NO plays a crucial role in the cellular aging processes.

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.

Vascular function is related to blood flow during high-intensity, but not low-intensity, knee extension exercise

While vascular function, assessed as the ability of the vasculature to dilate in response to a stimulus, is related to cardiovascular health, its relationship to exercise hyperemia is unclear. This study sought to determine if blood flow during submaximal and maximal exercise is related to vascular function. Nineteen healthy adults completed multiple assessments of vascular function specific to the leg, including passive leg movement (PLM), rapid onset vasodilation (ROV), reactive hyperemia (RH), and flow-mediated dilation (FMD). On a separate day, exercise blood flow (Doppler ultrasound) was assessed in the same leg during various intensities of single-leg, knee-extension (KE) exercise. Vascular function, determined by PLM, ROV, and RH, was related to exercise blood flow at high intensities, including maximum work rate (WRmax) ( r = 0.58–0.77, P < 0.001), but not low intensities, like ~21% WRmax ( r = 0.12–0.34, P = 0.12–0.62). Relationships between multiple indices of vascular function and peak exercise blood flow persisted when controlling for quadriceps mass and exercise work rate ( P < 0.05), indicating vascular function is independently related to the blood flow response to intense exercise. When divided into two groups based upon the magnitude of the PLM response, subjects with a lower PLM response exhibited lower exercise flow at several absolute work rates, as well as lower peak flow ( P < 0.05). In conclusion, leg flow during dynamic exercise is independently correlated with multiple different indices of microvascular function. Thus microvascular function appears to modulate the hyperemic response to high-intensity, but not low-intensity, exercise.

NEW & NOTEWORTHY While substantial evidence indicates that individuals with lower vascular function are at greater risk for cardiovascular disease, with many redundant vasodilator pathways present during exercise, it has been unclear if low vascular function actually impacts blood flow during exercise. This study provides evidence that vascular function, assessed by multiple noninvasive methods, is related to the blood flow response to high-intensity leg exercise in healthy young adults. Importantly, healthy young adults with lower levels of vascular function, particularly microvascular function, exhibit lower blood flow during high-intensity, and maximal knee extension exercise. Thus it appears that in addition to increasing one’s risk of cardiovascular disease, lower vascular function is also related to a blunted blood flow response during high-intensity exercise.

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.

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.

Effects of aging and exercise training on the dynamics of vasoconstriction in skeletal muscle resistance vessels

It is unknown whether aging or exercise training affect the dynamics of arteriolar vasoconstriction.

PURPOSE

We hypothesized that old age will slow, and exercise training will speed, the dynamics of skeletal muscle arteriolar vasoconstriction in resistance vessels of aged rats.

METHOD

Young (6 month old) and aged (24 month old) male Fischer-344 rats were assigned to sedentary (Sed: n = 6/age group) or exercise-trained (ET: n = 5 aged and 6 young; via treadmill running for 10-12 weeks) groups. After completion of training, arterioles from the red portion of the gastrocnemius muscle were removed, cannulated, and exposed to 10^-4 M norepinephrine (NE) or 20 mM caffeine. Changes in luminal diameter were recorded for analysis of constrictor dynamics.

RESULT

Old age blunted all kinetic parameters (i.e., time delay, time constant) resulting in vasoconstriction taking ~3 times as long to reach a steady state (SS) versus younger counterparts for NE (aged-sed: 15.6 ± 6.0 versus young-sed: 4.6 ± 0.5 s; P < 0.05) with a similar time course to caffeine. Exercise training resulted in a similar time to SS between age groups for NE (aged-ET: 6.8 ± 1.6 versus young-ET: 7.0 ± 0.6 s) and caffeine (aged-ET: 7.8 ± 0.6 versus young-ET: 8.6 ± 1.0 s).

CONCLUSION

The results of this study demonstrate that aging blunts the rate of vasoconstriction in skeletal muscle resistance vessels to the sympathetic neurotransmitter NE due, in part, to an attenuated rate of contraction from intracellular calcium release. Further, exercise training speeds the dynamics of constriction to both NE and caffeine with old age.

Skeletal Muscle Vascular Control During Exercise: Impact of Nitrite Infusion During Nitric Oxide Synthase Inhibition in Healthy Rats

The nitric oxide synthase (NOS)-independent pathway of nitric oxide (NO) production in which nitrite (NO2 (-)) is reduced to NO may have therapeutic applications for those with cardiovascular diseases in which the NOS pathway is downregulated. We tested the hypothesis that NO2 (-) infusion would reduce mean arterial pressure (MAP) and increase skeletal muscle blood flow (BF) and vascular conductance (VC) during exercise in the face of NOS blockade via L-NAME. Following infusion of L-NAME (10 mg kg(-1), L-NAME), male Sprague-Dawley rats (3-6 months, n = 8) exercised without N(G)-nitro-L arginine methyl ester (L-NAME) and after infusion of sodium NO2 (-) (7 mg kg(-1); L-NAME + NO2 (-)). MAP and hindlimb skeletal muscle BF (radiolabeled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). Across group comparisons were made with a published control data set (n = 11). Relative to L-NAME, NO2 (-) infusion significantly reduced MAP (P < 0.03). The lower MAP in L-NAME+NO2 (-) was not different from healthy control animals (control: 137 ± 3 L-NAME: 157 ± 7, L-NAME + NO2 (-): 136 ± 5 mm Hg). Also, NO2 (-) infusion significantly increased VC when compared to L-NAME (P < 0.03), ultimately negating any significant differences from control animals (control: 0.78 ± 0.05, L-NAME: 0.57 ± 0.03, L-NAME + NO2 (-); 0.69 ± 0.04 mL min(-1) 100 g(-1) mm Hg(-1)) with no apparent fiber-type preferential effect. Overall, hindlimb BF was decreased significantly by L-NAME; however, in L-NAME + NO2 (-), BF improved to a level not significantly different from healthy controls (control: 108 ± 8, L-NAME: 88 ± 3, L-NAME + NO2 (-): 94 ± 6 mL min(-1) 100 g(-1), P = 0.38 L-NAME vs L-NAME + NO2 (-)). Individuals with diseases that impair NOS activity, and thus vascular function, may benefit from a NO2 (-)-based therapy in which NO bioavailability is elevated in an NOS-independent manner.

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.

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.

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.

Role of nitric oxide and adenosine in the onset of vasodilation during dynamic forearm exercise

We tested the hypothesis that nitric oxide (NO) and adenosine contribute to the onset of vasodilation during dynamic forearm exercise. Twenty-two subjects performed rhythmic forearm exercise (20 % of maximum) during control and NO synthase (NOS) inhibition (N (G)-monomethyl-L-arginine; L-NMMA) trials. A subset of subjects performed a third trial of forearm exercise during combined inhibition of NOS and adenosine (aminophylline; n = 9). Additionally, a separate group of subjects (n = 7) performed rhythmic forearm exercise during control, inhibition of adenosine alone and combined inhibition of adenosine and NOS. Forearm vascular conductance (FVC; ml min(-1) · 100 mmHg(-1)) was calculated from blood flow and mean arterial pressure (mmHg). The onset of vasodilation was assessed by calculating the slope of the FVC response for every duty cycle between baseline and steady state, and the number of duty cycles (1-s contraction/2-s relaxation) to reach steady state. NOS inhibition blunted vasodilation at the onset of exercise (11.1 ± 0.8 vs. 8.5 ± 0.6 FVC units/duty cycle; P < 0.001 vs. control) and increased the time to reach steady state (25 ± 1 vs. 32 ± 1 duty cycles; P < 0.001 vs. control). Vasodilation was blunted further with combined inhibition of NOS and adenosine (7.5 ± 0.6 vs. 6.2 ± 0.8 FVC units/duty cycle; P < 0.05 vs. L-NMMA alone), but not with aminophylline alone (16.0 ± 2.2 vs. 14.7 ± 2.0 FVC units/duty cycle; P = 0.67 vs. control). Our data indicate that NO and adenosine (in the absence of NO) contribute to the onset of vasodilation during dynamic forearm exercise.

Aging alters the contribution of nitric oxide to regional muscle hemodynamic control at rest and during exercise in rats

Advanced age is associated with altered skeletal muscle hemodynamic control during the transition from rest to exercise. This study investigated the effects of aging on the functional role of nitric oxide (NO) in regulating total, inter-, and intramuscular hindlimb hemodynamic control at rest and during submaximal whole body exercise. We tested the hypothesis that NO synthase inhibition (N(G)-nitro-l-arginine methyl ester, l-NAME; 10 mg/kg) would result in attenuated reductions in vascular conductance (VC) primarily in oxidative muscles in old compared with young rats. Total and regional hindlimb muscle VCs were determined via radiolabeled microspheres at rest and during treadmill running (20 m/min, 5% grade) in nine young (6-8 mo) and seven old (27-29 mo) male Fisher 344 × Brown Norway rats. At rest, l-NAME increased mean arterial pressure (MAP) significantly by ∼17% and 21% in young and old rats, respectively. During exercise, l-NAME increased MAP significantly by ∼13% and 19% in young and old rats, respectively. Compared with young rats, l-NAME administration in old rats evoked attenuated reductions in 1) total hindlimb VC during exercise (i.e., down by ∼23% in old vs. 43% in young rats; P < 0.05), and 2) VC in predominantly oxidative muscles both at rest and during exercise (P < 0.05). Our results indicate that the dependency of highly oxidative muscles on NO-mediated vasodilation is markedly diminished, and therefore mechanisms other than NO-mediated vasodilation control the bulk of the increase in skeletal muscle VC during the transition from rest to exercise in old rats. Reduced NO contribution to vasomotor control with advanced age is associated with blood flow redistribution from highly oxidative to glycolytic muscles during exercise.

Adrenergic control of vascular resistance varies in muscles composed of different fiber types: influence of the vascular endothelium

The influence of the sympathetic nervous system (SNS) upon vascular resistance is more profound in muscles comprised predominately of low-oxidative type IIB vs. high-oxidative type I fiber types. However, within muscles containing high-oxidative type IIA and IIX fibers, the role of the SNS on vasomotor tone is not well established. The purpose of this study was to examine the influence of sympathetic neural vasoconstrictor tone in muscles composed of different fiber types. In adult male rats, blood flow to the red and white portions of the gastrocnemius (Gast(Red) and Gast(White), respectively) and the soleus muscle was measured pre- and postdenervation. Resistance arterioles from these muscles were removed, and dose responses to α₁-phenylephrine or α₂-clonidine adrenoreceptor agonists were determined with and without the vascular endothelium. Denervation resulted in a 2.7-fold increase in blood flow to the soleus and Gast(Red) and an 8.7-fold increase in flow to the Gast(White). In isolated arterioles, α₂-mediated vasoconstriction was greatest in Gast(White) (∼50%) and less in Gast(Red) (∼31%) and soleus (∼17%); differences among arterioles were abolished with the removal of the endothelium. There was greater sensitivity to α(1)-mediated vasoconstriction in the Gast(White) and Gast(Red) vs. the soleus, which was independent of whether the endothelium was present. These data indicate that 1) control of vascular resistance by the SNS in high-oxidative, fast-twitch muscle is intermediate to that of low-oxidative, fast-twitch and high-oxidative, slow-twitch muscles; and 2) the ability of the SNS to control blood flow to low-oxidative type IIB muscle appears to be mediated through postsynaptic α₁- and α₂-adrenoreceptors on the vascular smooth muscle.

Nitric oxide bioavailability modulates the dynamics of microvascular oxygen exchange during recovery from contractions

AIM

lowered microvascular PO(2) (PO(2) mv) during the exercise off-transient likely impairs muscle metabolic recovery and limits the capacity to perform repetitive tasks. The current investigation explored the impact of altered nitric oxide (NO) bioavailability on PO(2) mv during recovery from contractions in healthy skeletal muscle. We hypothesized that increased NO bioavailability (sodium nitroprusside: SNP) would enhance PO(2) mv and speed its recovery kinetics while decreased NO bioavailability (l-nitro arginine methyl ester: l-NAME) would reduce PO(2) mv and slow its recovery kinetics.

METHODS

  PO(2) mv was measured by phosphorescence quenching during transitions (rest-1 Hz twitch-contractions for 3 min-recovery) in the spinotrapezius muscle of Sprague-Dawley rats under SNP (300 microm), Krebs-Henseleit (CONTROL) and l-NAME (1.5 mm) superfusion conditions.

RESULTS

relative to recovery in CONTROL, SNP resulted in greater overall microvascular oxygenation as assessed by the area under the PO(2) mv curve (PO(2 AREA) ;

CONTROL

3471 ± 292 mmHg s; SNP: 4307 ± 282 mmHg s; P < 0.05) and faster off-kinetics as evidenced by the mean response time (MRToff;

CONTROL

60.2 ± 6.9 s; SNP: 34.8 ± 5.7 s; P < 0.05), whereas l-NAME produced lower PO(2 AREA) (2339 ± 444 mmHg s; P < 0.05) and slower MRToff (86.6 ± 14.5s; P < 0.05).

CONCLUSION

no bioavailability plays a key role in determining the matching of O(2) delivery-to-O(2) uptake and thus the upstream O(2) pressure driving capillary-myocyte O(2) flux (i.e. PO(2) mv) following cessation of contractions in healthy skeletal muscle. Additionally, these data support a mechanistic link between reduced NO bioavailability and prolonged muscle metabolic recovery commonly observed in ageing and diseased populations.

Crucial importance of the endothelial K+ channel SK3 and connexin40 in arteriolar dilations during skeletal muscle contraction

Skeletal muscle activity requires substantial increases in blood flow, and the underlying vasodilation involves endothelial activity, but the contribution of the endothelium-dependent hyperpolarizing factor (EDHF) is only poorly defined. In EDHF signaling, endothelial hyperpolarization mediated by the Ca(2+)-activated K(+) channels SK3 and IK1 is a key step and also initiates gap junction-dependent conducted dilations. We assessed the role of SK3, IK1, and connexin40 (Cx40) in muscular contraction-induced dilations in the microcirculation in vivo. Hitherto, arterioles were observed in the electrically stimulated cremaster skeletal muscle of anesthetized mice lacking SK3, IK1, or Cx40 using intravital microscopy. Genetic deficiency of SK3, but not of IK1, strongly attenuated dilations to muscular contraction. Similarly, pharmacologic blockade of SK3 by the specific blocker UCL1684 impaired such dilations in wild-type and IK1-deficient mice. In contrast, IK1 was required for acetylcholine-induced dilations. Genetic deficiency of Cx40 also attenuated dilations induced by muscular contraction but not by acetylcholine. These data support the concept that endothelial hyperpolarization through activation of SK3 contributes to exercise hyperemia and the hyperpolarization ascends the vascular tree through gap junctions formed by Cx40 to orchestrate dilation. The differential impact of SK3- and IK1-deficiency on dilations to distinct stimuli suggests stimulus-dependent activation of these endothelial channels.

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

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) is an integral mediator of vascular control during muscle contractions. However, it is not known whether neuronal NOS (nNOS)-derived NO regulates tissue hyperaemia in healthy subjects, particularly during exercise. We tested the hypothesis that selective nNOS inhibition would reduce blood flow and vascular conductance (VC) in rat hindlimb locomotor muscle(s), kidneys and splanchnic organs at rest and during dynamic treadmill exercise (20 m min(-1), 10% grade). Nineteen male Sprague-Dawley rats (555 +/- 23 g) were assigned to either rest (n = 9) or exercise (n = 10) groups. Blood flow and VC were determined via radiolabelled microspheres before and after the intra-arterial administration of the selective nNOS inhibitor S-methyl-L-thiocitrulline (SMTC, 2.1 +/- 0.1 micromol kg(-1)). Total hindlimb muscle blood flow (control: 20 +/- 2 ml min(-1) 100g(-1), SMTC: 12 +/- 2 ml min(-1) 100g(-1), P < 0.05) and VC (control: 0.16 +/- 0.02 ml min(-1) 100 g(-1) mmHg(1), SMTC: 0.09 +/- 0.01 ml min(-1) 100 g(-1) mmHg(-1), P < 0.05) were reduced substantially at rest. Moreover, the magnitude of the absolute reduction in blood flow and VC correlated (P < 0.05) with the proportion of oxidative muscle fibres found in the individual muscles or muscle parts of the hindlimb. During exercise, total hindlimb blood flow (control: 108 +/- 7 ml min(-1) 100 g(-1), SMTC: 105 +/- 8 ml min(-1) 100 g(-1)) and VC (control: 0.77 +/- 0.06 ml min(-1) 100g(-1) mmHg(-1); SMTC: 0.70 +/- 0.05 ml min(-1) 100g(-1) mmHg(-1)) were not different (P > 0.05) between control and SMTC conditions. SMTC reduced (P < 0.05) blood flow and VC at rest and during exercise in the kidneys, adrenals and liver. These results enhance our understanding of the role of NO-mediated circulatory control by demonstrating that nNOS does not appear to subserve an obligatory role in the exercising muscle hyperaemic response in the rat.

Decreased arteriolar tetrahydrobiopterin is linked to superoxide generation from nitric oxide synthase in mice fed high salt

OBJECTIVE

Impaired endothelium-dependent arteriolar dilation in mice fed high salt (HS) is due to local oxidation of nitric oxide (NO) by superoxide anion (O(2) (-)). We explored the possibility that "uncoupled" endothelial nitric oxide synthase (eNOS) is the source of this O(2) (-).

METHODS

Levels of L-arginine (L-Arg), tetrahydrobiopterin (BH(4)), and O(2) (-) (hydroethidine oxidation) were measured in spinotrapezius muscle arterioles of mice fed normal salt (0.45%, NS) or (4%, HS) diets for 4 weeks, with or without dietary L-Arg supplementation. The contribution of NO to endothelium-dependent dilation was determined from the effect of N(omega)-nitro-L-arginine methyl ester (L-NAME) on responses to acetylcholine (ACh).

RESULTS

Arterioles in HS mice had lower [BH(4)] and higher O(2) (-) levels than those in NS mice. ACh further increased arteriolar O(2) (-) in HS mice only. L-Arg supplementation prevented the reduction in [BH(4)] in arterioles of HS mice, and O(2) (-) was not elevated in these vessels. Compared to NS mice, arteriolar ACh responses were diminished and insensitive to L-NAME in HS mice, but not in HS mice supplemented with L-Arg.

CONCLUSIONS

These findings suggest that eNOS uncoupling due to low [BH(4)] is responsible for O(2) (-) generation and reduced NO-dependent dilation in arterioles of mice fed a HS diet.

Nitric oxide synthase inhibition during treadmill exercise reveals fiber-type specific vascular control in the rat hindlimb

The control of vascular tone during exercise is highly complex and integrated. Specifically, in regards to the contribution of nitric oxide (NO), the observed magnitude and muscle fiber-type dependency of the NO contribution to exercise hyperemia may differ depending on the timing of NO synthase (NOS) inhibition with respect to the exercise bout (i.e., administration prior to vs. during exercise). We tested the hypothesis that, in the presence of prior cyclooxygenase inhibition (indomethacin, 5 mg/kg(-1)), NOS inhibition (N(G)-nitro-L-arginine methyl ester, L-NAME; 10 mg/kg) administered during submaximal treadmill exercise would blunt blood flow and vascular conductance (VC) in the hindlimb muscle(s) of the rat with the greatest reductions in blood flow and VC occurring in the predominantly oxidative muscles. Adult female Wistar rats (n = 10, age: 3-4 mo) ran on a motor-driven treadmill (20 m/min, 10% grade). Total and regional hindlimb muscle blood flow and VC were determined via radiolabeled microspheres before (control) and after L-NAME administration during exercise. L-NAME reduced (P < 0.05) total hindlimb muscle blood flow (control: 123 + or - 10, L-NAME: 103 + or - 7 ml x min(-1) x 100g(-1)) and VC (control: 0.95 + or - 0.09, L-NAME: 0.63 + or - 0.05 ml x min(-1) x 100g(-1) x mmHg(-1)). There was a significant correlation (r = 0.51, P < 0.05) between the absolute reductions in VC after L-NAME and the percent sum of type I and IIa fibers in the individual muscles and muscle parts; however, there was no correlation (P = 0.62) when expressed as blood flow. Surprisingly, the highly oxidative muscles demonstrated a marked ability to maintain oxygen delivery, which differs substantially from previous reports of L-NAME infusion prior to exercise in these muscles. The demonstration that NO is an important regulator of blood flow and VC in the rat hindlimb during treadmill exercise, but that the fiber-type dependency of NO is altered markedly when NOS inhibition is performed during, vs. prior to, exercise, lends important insights into the integrated nature of vascular control during exercise.

Endothelial nitric oxide synthase phosphorylation in treadmill-running mice: role of vascular signalling kinases

The intracellular signalling kinases Akt/protein kinase B (Akt), protein kinase A (PKA) and adenosine monophosphate-activated protein kinase (AMPK) are phosphorylated in response to increased mechanical force or perfusion rate in cultured endothelial cells or isolated blood vessels. All three kinases phosphorylate endothelial nitric oxide synthase (eNOS) on serine (S) 1177, while Akt and PKA additionally phosphorylate eNOS on S617 and S635 respectively. Although these kinases might contribute to subsequent activation of eNOS during dynamic exercise, the specific mediators of exercise-induced eNOS phosphorylation and activation in vivo are unknown. We determined the impact of 50 min of treadmill running on the phosphorylation of Akt, AMPK, cyclic adenosine monophosphate response element binding protein (CREB - a target of PKA) and eNOS (S 1177, 635 and 617 and threonine (T) 495) in the presence or absence of pharmacological inhibition of PI3 kinase (PI3K) and Akt signalling using wortmannin. Compared to arteries from sedentary mice, eNOS enzyme activity was greater in vessels from treadmill-running animals and was associated with increased phosphorylation of Akt (S473), CREB (S133), AMPK (T172), and eNOS at S1177 and S617 but not at S635 or T495. These data suggest that Akt signalling is a major mediator of eNOS activation. To confirm this, treadmill-running was performed in the presence of vehicle (DMSO) or PI3K inhibition. Compared to results from sedentary mice, vascular Akt phosphorylation and eNOS phosphorylation at S617 during treadmill-running were prevented by wortmannin but not vehicle treatment, whereas exercise-related increases in AMPK and CREB phosphorylation were similar between groups. Arterial eNOS phosphorylation at S1177 increased during exercise after wortmannin treatment relative to values obtained from sedentary animals, but the elevation was blunted by approximately 50% compared to results from vehicle-treated mice. These findings indicate that Akt and AMPK contribute importantly to vascular eNOS S1177 phosphorylation during treadmill-running, and that AMPK is sufficient to activate p-eNOS S1177 in the presence of PI3K inhibition.

Current concepts of oxygen transport during exercise

This brief review examines the athletic potential of mammals in general and the horse in particular as it relates to oxygen (O2) transport and utilization. The horse has been bred selectively for over six millennia based upon its ability to run fast. Whereas this has optimized cardiovascular and muscle function and the capacity to deliver and utilize O2, it has resulted in lung failure during intense exercise. Horses in their athletic prime are considered and attention is focused on their maximal capacities as related to O2transport, irrespective of ageper se. Following a few comments on the history of O2, this review moves from established principles of O2transport at the integrative organ level to the microcirculation and the processes and principles that govern O2offloading, where much remains to be discovered. Four principal questions are addressed: (1) as an athlete, what are the most outstanding physiological characteristics of the horse? (2) what anatomical and physiological capacities facilitate this superlative performance and such prodigious O2fluxes (i.e. maximal VO2)? (3) do cardiovascular dynamics or intramuscular energetic processes limit VO2kinetics (i.e. the speed at which VO2increases at the onset of exercise)? VO2kinetics determine the size of the O2deficit and as such represent an important determinant of muscle metabolism and fatigue; and (4) what determines the efficacy of muscle microcirculatory O2exchange?

Changes in capillary shear stress in skeletal muscles exposed to long-term activity: role of nitric oxide

OBJECTIVE

The purpose of this study was to establish whether suppression of angiogenesis by nitric oxide synthase (NOS) inhibition in skeletal muscles exposed to long-term activity can be explained by changes in capillary shear stress linked to the lack of nitric oxide production.

METHODS

Capillary shear stress was calculated from diameters (d) and red blood cell velocities (V(rbc)) measured at rest and after acute contractions in epi-illuminated extensor digitorum longus muscles of control rats and those in which ankle flexors had been stimulated via implanted electrodes (10 Hz, 8 h x day(-1)) for 2 or 7 days without and with inhibition of nitric oxide synthase activity by N(omega)-nitro-L-arginine (L-NNA, 3-4 mg x day(-1) in drinking water).

RESULTS

Neither chronic electrical stimulation nor L-NNA treatment altered capillary diameters. Capillary V(rbc) and shear stress (SS) were doubled in muscles after 2 days stimulation (298 +/- 22 microm x s(-1) and 11.4 +/- 1.0 dyne x cm(-2), respectively, p < .005) compared to controls (148 +/- 18 microm x s(-1) and 5.6 +/- 0.8 dyne x cm(-2)) but normalized after 7 days (153 +/- 27 microm x s(-1) and 6.2 +/- 1.0 dyne x cm(-2)), when the capillary bed is known to be enlarged. L-NNA, which increased blood pressure in all treated animals, abolished the increase in capillary SS after 2 days stimulation and decreased SS after 7 days.

CONCLUSIONS

These data support a role for NO in the early elevation of capillary shear stress that initiates angiogenesis in stimulated muscles, likely via modulation of upstream vascular resistance, and could explain the lack of capillary growth in stimulated muscles when nitric oxide generation is suppressed.

Ibuprofen Inhibits Skeletal Muscle Hypertrophy in Rats

PURPOSE

We sought to determine whether cyclooxygenase (COX) activity is necessary for overload-induced growth of adult rat skeletal muscle, and whether nitric oxide synthase (NOS) activity is involved in upregulation of COX messenger RNA (mRNA) expression in skeletal muscle.

METHODS

Unilateral surgical removal of the gastrocnemius and soleus was performed on the right hindlimb of 16 female Sprague-Dawley rats (approximately 230 g) to induce chronic overload (OL) of the plantaris for 14 d, with sham surgeries performed on the contralateral leg as a normally loaded (NL) control. Half of the rats were treated with the nonspecific COX inhibitor, ibuprofen (0.2 mg.mL(-1) in drinking water; approximately 20 mg.kg(-1).d(-1)). In a second experiment, the plantaris was unilaterally overloaded for 5 or 14 d in male rats (approximately 350 g; N = 16 rats per time point) and half of the animals were treated with the NOS inhibitor, L-NAME (0.75 mg.mL(-1) in drinking water; approximately 90 mg.kg(-1).d(-1)).

RESULTS

Ibuprofen treatment inhibited plantaris hypertrophy by approximately 50% (P < 0.05) following 14 d of OL, as did L-NAME treatment (P < 0.05). COX-1 and COX-2 mRNA did not differ between any groups at 5 d. At 14 d, however, L-NAME caused a 30-fold increase in plantaris COX-1 mRNA expression independent of loading condition. Additionally, OL induced a 20-fold increase in COX-2 mRNA expression compared with NL (P < 0.05) at 14 d, without affecting COX-1 mRNA level. L-NAME treatment significantly inhibited OL-induced expression of COX-2 mRNA.

CONCLUSION

COX activity is important for in vivo muscle hypertrophy, and plantaris overload is associated with NOS activity-dependent COX-2 expression.

Nitric oxide, VEGF, and VEGFR-2: interactions in activity-induced angiogenesis in rat skeletal muscle

Vascular endothelial growth factor (VEGF) is considered to be important in promotion of capillary growth in skeletal muscles exposed to increased activity. We studied its interactions with nitric oxide (NO) by examining the expression of endothelial NO synthase (NOS), VEGF, and VEGF receptor-2 (VEGFR-2) proteins in relation to capillary growth in rat extensor digitorum longus muscles electrically stimulated for 2, 4, or 7 days with and without NOS inhibition by N(omega)-nitro-L-arginine (L-NNA, 3 mg/day). Stimulation increased all proteins from 2 days onward, concomitantly with capillary proliferation (labeling for proliferating cell nuclear antigen). Capillary-to-fiber ratio was elevated by 25% after 7 days. Concurrent oral administration of L-NNA did not affect the increase in endothelial NOS but depressed its activity, as shown by increased blood pressure and decreased arteriolar diameters in 2-day-stimulated muscles. NOS inhibition eliminated the increased expression of VEGFR-2 and VEGF proteins in muscles stimulated for 2 and 4 days but not for 7 days. However, it depressed capillary proliferation and the increase in C/F at all time points. We conclude that, in stimulated muscles, NO, generated by activation of neuronal NOS by muscle activity or endothelial NOS by increased blood flow and capillary shear stress, may increase capillary proliferation in the early stages of stimulation through upregulation of VEGFR-2 and VEGF. With longer stimulation, capillary growth appears to require NO, and high levels of VEGF and VEGFR-2 may be contributing to maintenance of the increased capillary bed.

Endothelial modulation and tolerance development in the vasorelaxant responses to nitrate of rabbit aorta

We investigated the endothelial modulations in nitrate tolerance in isolated rabbit aorta. Nitrate tolerance was induced by a 72-h treatment with transdermal nitroglycerin (NTG, 0.4 mg/h) in conscious rabbits, which was verified by a 20-fold increase in the EC50 values [NTG tolerance (6.1 +/- 0.8) x 10(-7) M vs control (3.0 +/- 0.6) x 10(-8) M]. The relaxations to NTG in tolerant and nontolerant aortic strips were enhanced when their endothelia were denuded [E(-)]. In the presence of endothelium [E(+)], NTG-tolerant vessels were not tolerant to acetylcholine (ACh), which can release endothelial nitric oxide (NO), exogenous NO or 8-bromo (Br)-cGMP. In NTG-tolerant and nontolerant vessels with endothelium, concentration-response curves for NO were the same as those in endothelium-absent tolerant vessels. In both NTG-tolerant and nontolerant vessels, treatment with superoxide dismutase (SOD, 20 units/ml), an O2-. scavenger, unaffected the responses to NTG reduced in the presence of endothelium, but treatment with NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), an NO synthase (NOS) inhibitor, reversed these reductions. Thus, our data did not indicate that an increased endothelial superoxide O2-. production contributes to nitrate tolerance. Our study suggested that (i) an impaired biotransformation process from NTG to NO is responsible for the occurrence of nitrate tolerance and (ii) vascular response to NTG enhanced by endothelial removal is related to blocked endothelial NO release.

Involvement of nitric oxide synthase in skeletal muscle adaptation to chronic overload

The purpose of this study was to determine the necessity of nitric oxide (NO) for hypertrophy and fiber-type transition in overloaded (OL) skeletal muscle. Endogenous NO production was blocked by administering N(G)-nitro-L-arginine methyl ester (L-NAME; 0.75 mg/ml; approximately 100 mg x kg-1 x day-1) in drinking water. Thirty-eight female Sprague-Dawley rats (approximately 250 g) were randomly divided into four groups: control-nonoverloaded (Non-OL), control-OL, L-NAME-Non-OL, and L-NAME-OL. Chronic overload of the plantaris was induced bilaterally by surgical removal of the gastrocnemius and soleus. Rats in the Non-OL groups received sham surgeries. L-NAME treatment began 24 h before surgery and continued until the rats were killed 14 days postsurgery. Although OL induced hypertrophy in both control (+76%) and L-NAME (+39%) conditions (P < 0.05), mean plantaris-to-body mass ratio in the L-NAME-OL group was significantly lower (P < 0.05) than that in the control-OL group. Microphotometric analysis of histochemically determined fiber types revealed increases in cross-sectional area (P < 0.05) for all fiber types (types I, IIA, and IIB/X) in the OL plantaris from control rats, whereas L-NAME-OL rats exhibited increases only in type I and IIB/X fibers. SDS-PAGE analysis of myosin heavy chain (MHC) composition in the plantaris indicated a significant (P < 0.05) OL effect in the control rats. Specifically, the mean proportion of type I MHC increased 6% (P < 0.05), whereas the proportion of type IIb MHC decreased approximately 9% (P < 0.05). No significant OL effects on MHC profile were observed in the L-NAME rats. These data support a role of NO in overload-induced skeletal muscle hypertrophy and fiber-type transition.