Effects of acute phosphodiesterase type 5 inhibition on skeletal muscle interstitial PO2 during contractions and recovery

The oxygen partial pressure within the interstitial space (PO2is; mmHg) provides the driving force for oxygen diffusion into the myocyte thereby supporting oxidative phosphorylation. We tested the hypothesis that potentiation of the nitric oxide pathway with sildenafil (phosphodiesterase type 5 inhibitor) would enhance PO2is during muscle metabolic transitions, thereby slowing PO2is on- and accelerating PO2is off-kinetics. The rat spinotrapezius muscle (n = 17) was exposed for PO2is measurements via phosphorescence quenching under control (CON), low-dose sildenafil (1 mg/kg i.a., SIL1) and high-dose sildenafil (7 mg/kg i.a., SIL7). Data were collected at rest and during submaximal twitch contractions (1 Hz, 4-6 V, 3 min) and recovery (3 min). Mean arterial blood pressure (MAP; mmHg) was reduced with both SIL1 (pre:132 ± 5; post:99 ± 5) and SIL7 (pre:111 ± 6; post:99 ± 4) (p < 0.05). SIL7 elevated resting PO2is (18.4 ± 1.1) relative to both CON (15.7 ± 0.7) and SIL1 (15.2 ± 0.7) (p < 0.05). In addition, SIL7 increased end-recovery PO2is (17.7 ± 1.6) compared to CON (12.8 ± 0.9) and SIL1 (13.4 ± 0.8) (p < 0.05). The overall PO2is response during recovery (i.e., area under the PO2is curve) was greater in SIL7 (4107 ± 444) compared to CON (3493 ± 222) and SIL1 (3114 ± 205 mmHg s) (p < 0.05). Contrary to our hypothesis, there was no impact of acute SIL (1 or 7 mg/kg) on the speed of the PO2is response during contractions or recovery (p > 0.05). However, sildenafil lowered MAP and improved skeletal muscle interstitial oxygenation in healthy rats. Specifically, SIL7 enhanced PO2is at rest and during recovery from submaximal muscle contractions. Potentiation of the nitric oxide pathway with sildenafil enhances microvascular blood-myocyte O2 transport and is expected to improve repeated bouts of contractile activity.

HDAC6 modulates myofibril stiffness and diastolic function of the heart

Passive stiffness of the heart is determined largely by extracellular matrix and titin, which functions as a molecular spring within sarcomeres. Titin stiffening is associated with the development of diastolic dysfunction (DD), while augmented titin compliance appears to impair systolic performance in dilated cardiomyopathy. We found that myofibril stiffness was elevated in mice lacking histone deacetylase 6 (HDAC6). Cultured adult murine ventricular myocytes treated with a selective HDAC6 inhibitor also exhibited increased myofibril stiffness. Conversely, HDAC6 overexpression in cardiomyocytes led to decreased myofibril stiffness, as did ex vivo treatment of mouse, rat, and human myofibrils with recombinant HDAC6. Modulation of myofibril stiffness by HDAC6 was dependent on 282 amino acids encompassing a portion of the PEVK element of titin. HDAC6 colocalized with Z-disks, and proteomics analysis suggested that HDAC6 functions as a sarcomeric protein deacetylase. Finally, increased myofibril stiffness in HDAC6-deficient mice was associated with exacerbated DD in response to hypertension or aging. These findings define a role for a deacetylase in the control of myofibril function and myocardial passive stiffness, suggest that reversible acetylation alters titin compliance, and reveal the potential of targeting HDAC6 to manipulate the elastic properties of the heart to treat cardiac diseases.

Role of nitric oxide in convective and diffusive skeletal microvascular oxygen kinetics

Progress in understanding physiological mechanisms often consists of discrete discoveries made across different models and species. Accordingly, understanding the mechanistic bases for how altering nitric oxide (NO) bioavailability impacts exercise tolerance (or not) depends on integrating information from cellular energetics and contractile regulation through microvascular/vascular control of O₂ transport and pulmonary gas exchange. This review adopts state-of-the-art concepts including the intramyocyte power grid, the Wagner conflation of perfusive and diffusive O₂ conductances, and the Critical Power/Critical Speed model of exercise tolerance to address how altered NO bioavailability may, or may not, affect physical performance. This question is germane from the elite athlete to the recreational exerciser and particularly the burgeoning heart failure (and other clinical) populations for whom elevating O₂ transport and/or exercise capacity translates directly to improved life quality and reduced morbidity and mortality. The dearth of studies in females is also highlighted, and areas of uncertainty and questions for future research are identified.

Hemopexin dosing improves cardiopulmonary dysfunction in murine sickle cell disease

Hemopexin (Hpx) is a crucial defense protein against heme liberated from degraded hemoglobin during hemolysis. High heme stress creates an imbalance in Hpx bioavailability, favoring heme accumulation and downstream pathophysiological responses leading to cardiopulmonary disease progression in sickle cell disease (SCD) patients. Here, we evaluated a model of murine SCD, which was designed to accelerate red blood cell sickling, pulmonary hypertension, right ventricular dysfunction, and exercise intolerance by exposure of the mice to moderate hypobaric hypoxia. The sequence of pathophysiology in this model tracks with circulatory heme accumulation, lipid oxidation, extensive remodeling of the pulmonary vasculature, and fibrosis. We hypothesized that Hpx replacement for an extended period would improve exercise tolerance measured by critical speed as a clinically meaningful therapeutic endpoint. Further, we sought to define the effects of Hpx on upstream cardiopulmonary function, histopathology, and tissue oxidation. Our data shows that tri-weekly administrations of Hpx for three months dose-dependently reduced heme exposure and pulmonary hypertension while improving cardiac pressure-volume relationships and exercise tolerance. Furthermore, Hpx administration dose-dependently attenuated pulmonary fibrosis and oxidative modifications in the lung and myocardium of the right ventricle. Observations in our SCD murine model are consistent with pulmonary vascular and right ventricular pathology at autopsy in SCD patients having suffered from severe pulmonary hypertension, right ventricular dysfunction, and sudden cardiac death. This study provides a translational evaluation supported by a rigorous outcome analysis demonstrating therapeutic proof-of-concept for Hpx replacement in SCD.

Exercise training decreases intercostal and transversus abdominis muscle blood flows in heart failure rats during submaximal exercise

Diaphragm muscle blood flow (BF) and vascular conductance (VC) are elevated with chronic heart failure (HF) during exercise. Exercise training (ExT) elicits beneficial respiratory muscle and pulmonary system adaptations in HF. We hypothesized that diaphragm BF and VC would be lower in HF rats following ExT than their sedentary counterparts (Sed). Respiratory muscle BFs and mean arterial pressure were measured via radiolabeled microspheres and carotid artery catheter, respectively, during submaximal treadmill exercise (20 m/min, 5 % grade). During exercise, no differences were present between HF + ExT and HF + Sed in diaphragm BFs (201 ± 36 vs. 227 ± 44 mL/min/100 g) or VCs (both, p > 0.05). HF + ExT compared to HF + Sed had lower intercostal BF (27 ± 3 vs. 41 ± 5 mL/min/100 g) and VC (0.21 ± 0.02 vs. 0.31 ± 0.04 mL/min/mmHg/100 g) during exercise (both, p < 0.05). Further, HF + ExT compared to HF + Sed had lower transversus abdominis BF (20 ± 1 vs. 35 ± 6 mL/min/100 g) and VC (0.14 ± 0.02 vs. 0.27 ± 0.05 mL/min/mmHg/100 g) during exercise (both, p < 0.05). These data suggest that exercise training lowers the intercostal and transversus abdominis BF responses in HF rats during submaximal treadmill exercise.

Effects of inorganic nitrate supplementation on cardiovascular function and exercise tolerance in heart failure

Heart failure (HF) results in a myriad of central and peripheral abnormalities that impair the ability to sustain skeletal muscle contractions and, therefore, limit tolerance to exercise. Chief among these abnormalities is the lowered maximal oxygen uptake, which is brought about by reduced cardiac output and exacerbated by O2 delivery-utilization mismatch within the active skeletal muscle. Impaired nitric oxide (NO) bioavailability is considered to play a vital role in the vascular dysfunction of both reduced and preserved ejection fraction HF (HFrEF and HFpEF, respectively), leading to the pursuit of therapies aimed at restoring NO levels in these patient populations. Considering the complementary role of the nitrate-nitrite-NO pathway in the regulation of enzymatic NO signaling, this review explores the potential utility of inorganic nitrate interventions to increase NO bioavailability in the HFrEF and HFpEF patient population. Although many preclinical investigations have suggested that enhanced reduction of nitrite to NO in low Po2 and pH environments may make a nitrate-based therapy especially efficacious in patients with HF, inconsistent results have been found thus far in clinical settings. This brief review provides a summary of the effectiveness (or lack thereof) of inorganic nitrate interventions on exercise tolerance in patients with HFrEF and HFpEF. Focus is also given to practical considerations and current gaps in the literature to facilitate the development of effective nitrate-based interventions to improve exercise tolerance in patients with HF.

The effect of dietary nitrate supplementation on the speed-duration relationship in mice with sickle cell disease

Sickle cell disease (SCD) causes exercise intolerance likely due to impaired skeletal muscle function and low nitric oxide (NO) bioavailability. Dietary nitrate improves hemodynamic and metabolic control during exercise in humans and animals. The purpose of this investigation was to assess the impact of nitrate supplementation on exercise capacity as measured by the running speed to exercise duration relationship [critical speed (CS)]in mice with SCD. We tested the hypothesis that nitrate supplementation via beetroot juice (BR) would attenuate the exercise intolerance observed in mice with SCD. Ten wild-type (WT) and 18 Berkley sickle-cell mice (BERK) received water (WT: n = 10, BERK: n = 10) or nitrate-rich BR (BERK+BR: n = 8, nitrate dose 1 mmol/kg/day) for 5 days. Following the supplementation period, all mice performed 3-5 constant-speed treadmill tests that resulted in exhaustion within 1.5 to 20 min. Time to exhaustion vs. treadmill speed was fit to a hyperbolic model to determine CS. CS was significantly lower in BERK vs. WT and BERK+BR with no significant difference between WT and BERK+BR (WT: 36.6 ± 1.6, BERK: 23.8 ± 1.5, BERK+BR: 31.1 ± 2.1 m/min, P < 0.05). Exercise tolerance, measured via CS, was significantly lower in BERK mice relative to WT. However, BERK mice receiving 5 days of nitrate supplementation exhibited no difference in exercise tolerance when compared with WT. These results support the potential utility of a dietary nitrate intervention to improve functionality in SCD patients.NEW & NOTEWORTHY Sickle cell disease compromises muscle O₂ delivery resulting in exercise intolerance. Dietary nitrate supplementation increases skeletal muscle blood flow during exercise and may improve exercise capacity in a mouse model of sickle cell disease. We investigated the effects of dietary nitrate supplementation on exercise tolerance in a mouse model of sickle cell disease using the treadmill speed-duration relationship (critical speed). Mice with sickle cell disease provided with a dietary nitrate supplement had a critical speed not significantly different from healthy wild-type mice.

Transcapillary PO2 gradients in contracting muscles across the fibre type and oxidative continuum

KEY POINTS

Within skeletal muscle the greatest resistance to oxygen transport is thought to reside across the short distance at the red blood cell-myocyte interface. These structures generate a significant transmural oxygen pressure (PO₂ ) gradient in mixed fibre-type muscle. Increasing O₂ flux across the capillary wall during exercise depends on: (i) the transmural O₂ pressure gradient, which is maintained in mixed-fibre muscle, and/or (ii) elevating diffusing properties between microvascular and interstitial compartments resulting, in part, from microvascular haemodynamics and red blood cell distribution. We evaluated the PO₂ within the microvascular and interstitial spaces of muscles spanning the slow- to fast-twitch fibre and high- to low-oxidative capacity spectrums, at rest and during contractions, to assess the magnitude of transcapillary PO₂ gradients in rats. Our findings demonstrate that, across the metabolic rest-contraction transition, the transcapillary pressure gradient for O₂ flux is: (i) maintained in all muscle types, and (ii) the lowest in contracting highly oxidative fast-twitch muscle.

ABSTRACT

In mixed fibre-type skeletal muscle transcapillary PO₂ gradients (PO₂ mv-PO₂ is; microvascular and interstitial, respectively) drive O₂ flux across the blood-myocyte interface where the greatest resistance to that O₂ flux resides. We assessed a broad spectrum of fibre-type and oxidative-capacity rat muscles across the rest-to-contraction (1 Hz, 120 s) transient to test the novel hypotheses that: (i) slow-twitch PO₂ is would be greater than fast-twitch, (ii) muscles with greater oxidative capacity have greater PO₂ is than glycolytic counterparts, and (iii) whether PO₂ mv-PO₂ is at rest is maintained during contractions across all muscle types. PO₂ mv and PO₂ is were determined via phosphorescence quenching in soleus (SOL; 91% type I+IIa fibres and CSa: ∼21 μmol min^-1 g^-1 ), peroneal (PER; 33% and ∼20 μmol min^-1 g^-1 ), mixed (MG; 9% and ∼26 μmol min^-1 g^-1 ) and white gastrocnemius (WG; 0% and ∼8 μmol min^-1 g^-1 ) across the rest-contraction transient. PO₂ mv was higher than PO₂ is in each muscle (∼6-13 mmHg; P < 0.05). SOL PO₂ is_area was greater than in the fast-twitch muscles during contractions (P < 0.05). Oxidative muscles had greater PO₂ is_nadir (9.4 ± 0.8, 7.4 ± 0.9 and 6.4 ± 0.4; SOL, PER and MG, respectively) than WG (3.0 ± 0.3 mmHg, P < 0.05). The magnitude of PO₂ mv-PO₂ is at rest decreased during contractions in MG only (∼11 to 7 mmHg; time × (PO₂ mv-PO₂ is) interaction, P < 0.05). These data support the hypothesis that, since transcapillary PO₂ gradients during contractions are maintained in all muscle types, increased O₂ flux must occur via enhanced intracapillary diffusing conductance, which is most extreme in highly oxidative fast-twitch muscle.

An Hb-mediated circulating macrophage contributing to pulmonary vascular remodeling in sickle cell disease

Circulating macrophages recruited to the lung contribute to pulmonary vascular remodeling in various forms of pulmonary hypertension (PH). In this study we investigated a macrophage phenotype characterized by intracellular iron accumulation and expression of antioxidant (HO-1), vasoactive (ET-1), and proinflammatory (IL-6) mediators observed in the lung tissue of deceased sickle cell disease (SCD) patients with diagnosed PH. To this end, we evaluated an established rat model of group 5 PH that is simultaneously exposed to free hemoglobin (Hb) and hypobaric hypoxia (HX). Here, we tested the hypothesis that pulmonary vascular remodeling observed in human SCD with concomitant PH could be replicated and mechanistically driven in our rat model by a similar macrophage phenotype with iron accumulation and expression of a similar mixture of antioxidant (HO-1), vasoactive (ET-1), and inflammatory (IL-6) proteins. Our data suggest phenotypic similarities between pulmonary perivascular macrophages in our rat model and human SCD with PH, indicating a potentially novel maladaptive immune response to concomitant bouts of Hb and HX exposure. Moreover, by knocking out circulating macrophages with gadolinium trichloride (GdCl3), the response to combined Hb and hypobaric HX was significantly attenuated in rats, suggesting a critical role for macrophages in the exacerbation of SCD PH.

Pre-clinical assessment of a water-in-fluorocarbon emulsion for the treatment of pulmonary vascular diseases

Hypoxic pulmonary vasoconstriction (HPV) is a well-characterized vascular response to low oxygen pressures and is involved in life-threatening conditions such as high-altitude pulmonary edema (HAPE) and pulmonary arterial hypertension (PAH). While the efficacy of oral therapies can be affected by drug metabolism, or dose-limiting systemic toxicity, inhaled treatment via pressured metered dose inhalers (pMDI) may be an effective, nontoxic, practical alternative. We hypothesized that a stable water-in-perfluorooctyl bromide (PFOB) emulsion that provides solubility in common pMDI propellants, engineered for intrapulmonary delivery of pulmonary vasodilators, reverses HPV during acute hypoxia (HX). Male Sprague Dawley rats received two 10-min bouts of HX (13% O₂) with 20 min of room air and drug application between exposures. Treatment groups: intrapulmonary delivery (PUL) of (1) saline; (2) ambrisentan in saline (0.1 mg/kg); (3) empty emulsion; (4) emulsion encapsulating ambrisentan or sodium nitrite (NaNO₂) (0.1 and 0.5 mg/kg each); and intravenous (5) ambrisentan (0.1 mg/kg) or (6) NaNO₂ (0.5 mg/kg). Neither PUL of saline or empty emulsion, nor infusions of drugs prevented pulmonary artery pressure (PAP) elevation (32.6 ± 3.2, 31.5 ± 1.2, 29.3 ± 1.8, and 30.2 ± 2.5 mmHg, respectively). In contrast, PUL of aqueous ambrisentan and both drug emulsions reduced PAP by 20–30% during HX, compared to controls. IL6 expression in bronchoalveolar lavage fluid and whole lung 24 h post-PUL did not differ among cohorts. We demonstrate proof-of-concept for delivering pulmonary vasodilators via aerosolized water-in-PFOB emulsion. This concept opens a potentially feasible and effective route of treating pulmonary vascular pathologies via pMDI.

Effects of living at moderate altitude on pulmonary vascular function and exercise capacity in mice with sickle cell anaemia

KEY POINTS

Sickle cell disease (SCD) results in cardiopulmonary dysfunction, which may be exacerbated by prolonged exposure to environmental hypoxia. It is currently unknown whether exposure to mild and moderate altitude exacerbates SCD associated cardiopulmonary and systemic complications. Three months of exposure to mild (1609 m) and moderate (2438 m) altitude increased rates of haemolysis and right ventricular systolic pressures in mice with SCD compared to healthy wild-type cohorts and SCD mice at sea level. The haemodynamic changes in SCD mice that had lived at mild and moderate altitude were accompanied by changes in the balance between pulmonary vascular endothelial nitric oxide synthase and endothelin receptor expression and impaired exercise tolerance. These data demonstrate that chronic altitude exposure exacerbates the complications associated with SCD and provides pertinent information for the clinical counselling of SCD patients.

ABSTRACT

Exposure to high altitude worsens symptoms and crises in patients with sickle cell disease (SCD). However, it remains unclear whether prolonged exposure to low barometric pressures exacerbates SCD aetiologies or impairs quality of life. We tested the hypothesis that, relative to wild-type (WT) mice, Berkley sickle cell mice (BERK-SS) residing at sea level, mild (1609 m) and moderate (2438 m) altitude would have a higher rate of haemolysis, impaired cardiac function and reduced exercise tolerance, and that the level of altitude would worsen these decrements. Following 3 months of altitude exposure, right ventricular systolic pressure was measured (solid-state transducer). In addition, the adaptive balance between pulmonary vascular endothelial nitric oxide synthase and endothelin was assessed in lung tissue to determine differences in pulmonary vascular adaptation and the speed/duration relationship (critical speed) was used to evaluate treadmill exercise tolerance. At all altitudes, BERK-SS mice had a significantly lower percentage haemocrit and higher total bilirubin and free haemoglobin concentration (P < 0.05 for all). right ventricular systolic pressures in BERK-SS were higher than WT at moderate altitude and also compared to BERK-SS at sea level (P < 0.05, for both). Critical speed was significantly lower in BERK-SS at mild and moderate altitude (P < 0.05). BERK-SS demonstrated exacerbated SCD complications and reduced exercise capacity associated with an increase in altitude. These results suggest that exposure to mild and moderate altitude enhances the progression of SCD in BERK-SS mice compared to healthy WT cohorts and BERK-SS mice at sea level and provides crucial information for the clinical counselling of SCD patients.

Discrete physiological effects of beetroot juice and potassium nitrate supplementation following 4-wk sprint interval training

The physiological and exercise performance adaptations to sprint interval training (SIT) may be modified by dietary nitrate ([Formula: see text]) supplementation. However, it is possible that different types of [Formula: see text] supplementation evoke divergent physiological and performance adaptations to SIT. The purpose of this study was to compare the effects of 4-wk SIT with and without concurrent dietary [Formula: see text] supplementation administered as either [Formula: see text]-rich beetroot juice (BR) or potassium [Formula: see text] (KNO3). Thirty recreationally active subjects completed a battery of exercise tests before and after a 4-wk intervention in which they were allocated to one of three groups: 1) SIT undertaken without dietary [Formula: see text] supplementation (SIT); 2) SIT accompanied by concurrent BR supplementation (SIT + BR); or 3) SIT accompanied by concurrent KNO3 supplementation (SIT + KNO3). During severe-intensity exercise, V̇o2peak and time to task failure were improved to a greater extent with SIT + BR than SIT and SIT + KNO3 ( P < 0.05). There was also a greater reduction in the accumulation of muscle lactate at 3 min of severe-intensity exercise in SIT + BR compared with SIT + KNO3 ( P < 0.05). Plasma [Formula: see text] concentration fell to a greater extent during severe-intensity exercise in SIT + BR compared with SIT and SIT + KNO3 ( P < 0.05). There were no differences between groups in the reduction in the muscle phosphocreatine recovery time constant from pre- to postintervention ( P > 0.05). These findings indicate that 4-wk SIT with concurrent BR supplementation results in greater exercise capacity adaptations compared with SIT alone and SIT with concurrent KNO3 supplementation. This may be the result of greater NO-mediated signaling in SIT + BR compared with SIT + KNO3.

NEW & NOTEWORTHY We compared the influence of different forms of dietary nitrate supplementation on the physiological and performance adaptations to sprint interval training (SIT). Compared with SIT alone, supplementation with nitrate-rich beetroot juice, but not potassium [Formula: see text], enhanced some physiological adaptations to training.

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.

Dietary nitrate supplementation opposes the elevated diaphragm blood flow in chronic heart failure during submaximal exercise

Chronic heart failure (CHF) results in a greater cost of breathing and necessitates an elevated diaphragm blood flow (BF). Dietary nitrate (NO₃‾) supplementation lowers the cost of exercise. We hypothesized that dietary NO₃‾ supplementation would attenuate the CHF-induced greater cost of breathing and thus the heightened diaphragm BF during exercise. CHF rats received either 5days of NO₃‾-rich beetroot (BR) juice (CHF+BR, n=10) or a placebo (CHF, n=10). Respiratory muscle BFs (radiolabeled microspheres) were measured at rest and during submaximal exercise (20m/min, 5% grade). Infarcted left ventricular area and normalized lung weight were not significantly different between groups. During submaximal exercise, diaphragm BF was markedly lower for CHF+BR than CHF (CHF+BR: 195±28; CHF: 309±71mL/min/100g, p=0.04). The change in diaphragm BF from rest to exercise was less (p=0.047) for CHF+BR than CHF. These findings demonstrate that dietary NO₃‾ supplementation reduces the elevated diaphragm BF during exercise in CHF rats thus providing additional support for this therapeutic intervention in CHF.

Hemoglobin induced cell trauma indirectly influences endothelial TLR9 activity resulting in pulmonary vascular smooth muscle cell activation

It is now well established that both inherited and acquired forms of hemolytic disease can promote pulmonary vascular disease consequent of free hemoglobin (Hb) induced NO scavenging, elevations in reactive oxygen species and lipid peroxidation. It has recently been reported that oxidative stress can activate NFkB through a toll-like receptor 9 (TLR9) mediated pathway; further, TLR9 can be activated by either nuclear or mitochondrial DNA liberated by stress induced cellular trauma. We hypothesis that Hb induced lipid peroxidation and subsequent endothelial cell trauma is linked to TLR9 activation, resulting in IL-6 mediated pulmonary smooth muscle cell proliferation. We examined the effects of Hb on rat pulmonary artery endothelial and smooth muscle cells (rPAEC and rPASMC, respectively), and then utilized TLR9 and IL6 inhibitors, as well as the Hb and heme binding proteins (haptoglobin (Hp) and hemopexin (Hpx), respectively) to further elucidate the aforementioned mediators. Further, we explored the effects of Hb in vivo utilizing endothelial cell (EC) specific myeloid differentiation primary response gene-88 (MyD88) and TLR9 null mice. Our data show that oxidized Hb induces lipid peroxidation, cellular toxicity (5.5 ± 1.7 fold; p≤0.04), increased TLR9 activation (60%; p = 0.01), and up regulated IL6 expression (1.75±0.3 fold; p = 0.04) in rPAEC. Rat PASMC exhibited a more proliferative state (13 ± 1%; p = 0.01) when co-cultured with Hb activated rPAEC. These effects were attenuated with the sequestration of Hb or heme by Hp and Hpx as well as with TLR9 an IL-6 inhibition. Moreover, in both EC-MyD88 and TLR9 null mice Hb-infusion resulted in less lung IL-6 expression compared to WT cohorts. These results demonstrate that Hb-induced lipid peroxidation can initiate a modest TLR9 mediated inflammatory response, subsequently generating an activated SMC phenotype.

Fiber Type-Specific Effects of Dietary Nitrate

Dietary nitrate supplementation increases circulating nitrite concentration, and the subsequent reduction of nitrite to nitric oxide is promoted in hypoxic environments. Given that PO2 is lower in Type II compared with Type I muscle, this article examines the hypothesis that the ergogenicity of nitrate supplementation is linked to specific effects on vascular, metabolic, and contractile function in Type II muscle.

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.

Dietary nitrate supplementation: impact on skeletal muscle vascular control in exercising rats with chronic heart failure

Chronic heart failure (CHF) results in central and peripheral derangements that ultimately reduce skeletal muscle O2 delivery and impair exercise tolerance. Dietary nitrate (NO3 (-)) supplementation improves skeletal muscle vascular function and tolerance to exercise. We tested the hypothesis that NO3 (-) supplementation would elevate exercising skeletal muscle blood flow (BF) and vascular conductance (VC) in CHF rats. Myocardial infarction (MI) was induced (coronary artery ligation) in young adult male rats. After 21 days of recovery, rats randomly received 5 days of NO3 (-)-rich beetroot juice (CHF + BR, n = 10) or a placebo (CHF, n = 10). Mean arterial pressure (carotid artery catheter) and skeletal muscle BF (radiolabeled microspheres) were measured during treadmill exercise (20 m/min, 5% grade). CHF-induced dysfunction, as determined by myocardial infarction size (29 ± 3% and 33 ± 4% in CHF and CHF + BR, respectively) and left ventricular end-diastolic pressure (18 ± 2 and 18 ± 2 mmHg in CHF and CHF + BR, respectively), and exercising mean arterial pressure (131 ± 3 and 128 ± 4 mmHg in CHF and CHF + BR, respectively) were not different (P > 0.05) between groups. Total exercising hindlimb skeletal muscle BF (95 ± 5 and 116 ± 9 ml·min(-1)·100 g(-1) in CHF and CHF + BR, respectively) and VC (0.75 ± 0.05 and 0.90 ± 0.05 ml·min(-1)·100 g(-1)·mmHg(-1) in CHF and CHF + BR, respectively) were 22% and 20% greater in BR-supplemented rats, respectively (P < 0.05). During exercise, BF in 9 and VC in 10 hindlimb muscles and muscle portions were significantly greater in the CHF + BR group. These results provide strong evidence that dietary NO3 (-) supplementation improves skeletal muscle vascular function during exercise in rats with CHF and, thus, support the use of BR as a novel therapeutic modality for the treatment of CHF.

Effects of nitrite infusion on skeletal muscle vascular control during exercise in rats with chronic heart failure

Chronic heart failure (CHF) reduces nitric oxide (NO) bioavailability and impairs skeletal muscle vascular control during exercise. Reduction of NO2 (-) to NO may impact exercise-induced hyperemia, particularly in muscles with pathologically reduced O2 delivery. We tested the hypothesis that NO2 (-) infusion would increase exercising skeletal muscle blood flow (BF) and vascular conductance (VC) in CHF rats with a preferential effect in muscles composed primarily of type IIb + IId/x fibers. CHF (coronary artery ligation) was induced in adult male Sprague-Dawley rats. After a >21-day recovery, mean arterial pressure (MAP; carotid artery catheter) and skeletal muscle BF (radiolabeled microspheres) were measured during treadmill exercise (20 m/min, 5% incline) with and without NO2 (-) infusion. The myocardial infarct size (35 ± 3%) indicated moderate CHF. NO2 (-) infusion increased total hindlimb skeletal muscle VC (CHF: 0.85 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1) and CHF + NO2 (-): 0.93 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1), P < 0.05) without changing MAP (CHF: 123 ± 4 mmHg and CHF + NO2 (-): 120 ± 4 mmHg, P = 0.17). Total hindlimb skeletal muscle BF was not significantly different (CHF: 102 ± 7 and CHF + NO2 (-): 109 ± 7 ml·min(-1)·100 g(-1) ml·min(-1)·100 g(-1), P > 0.05). BF increased in 6 (∼21%) and VC in 8 (∼29%) of the 28 individual muscles and muscle parts. Muscles and muscle portions exhibiting greater BF and VC after NO2 (-) infusion comprised ≥63% type IIb + IId/x muscle fibers. These data demonstrate that NO2 (-) infusion can augment skeletal muscle vascular control during exercise in CHF rats. Given the targeted effects shown herein, a NO2 (-)-based therapy may provide an attractive "needs-based" approach for treatment of the vascular dysfunction in CHF.

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.

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

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

Microvascular oxygen pressures in muscles comprised of different fiber types: Impact of dietary nitrate supplementation

Nitrate (NO3(-)) supplementation via beetroot juice (BR) preferentially improves vascular conductance and O2 delivery to contracting skeletal muscles comprised predominantly of type IIb + d/x (i.e. highly glycolytic) fibers following its reduction to nitrite and nitric oxide (NO). To address the mechanistic basis for NO3(-) to improve metabolic control we tested the hypothesis that BR supplementation would elevate microvascular PO2 (PO2mv) in fast twitch but not slow twitch muscle. Twelve young adult male Sprague-Dawley rats were administered BR ([NO3(-)] 1 mmol/kg/day, n = 6) or water (control, n = 6) for 5 days. PO2mv (phosphorescence quenching) was measured at rest and during 180 s of electrically-induced 1-Hz twitch contractions (6-8 V) of the soleus (9% type IIb +d/x) and mixed portion of the gastrocnemius (MG, 91% type IIb + d/x) muscles. In the MG, but not the soleus, BR elevated contracting steady state PO2mv by ~43% (control: 14 ± 1, BR: 19 ± 2 mmHg (P < 0.05)). This higher PO2mv represents a greater blood-myocyte O2 driving force during muscle contractions thus providing a potential mechanism by which NO3(-) supplementation via BR improves metabolic control in fast twitch muscle. Recruitment of higher order type II muscle fibers is thought to play a role in the development of the VO2 slow component which is inextricably linked to the fatigue process. These data therefore provide a putative mechanism for the BR-induced improvements in high-intensity exercise performance seen in humans.

Dose dependent effects of nitrate supplementation on cardiovascular control and microvascular oxygenation dynamics in healthy rats

High dose nitrate (NO3(-)) supplementation via beetroot juice (BR, 1 mmol/kg/day) lowers mean arterial blood pressure (MAP) and improves skeletal muscle blood flow and O2 delivery/utilization matching thereby raising microvascular O2 pressure (PO2mv). We tested the hypothesis that a low dose of NO3(-) supplementation, consistent with a diet containing NO3(-) rich vegetables (BRLD, 0.3 mmol/kg/day), would be sufficient to cause these effects. Male Sprague-Dawley rats were administered a low dose of NO3(-) (0.3 mmol/kg/day; n=12), a high dose (1 mmol/kg/day; BRHD, n=6) or tap water (control, n=10) for 5 days. MAP, heart rate (HR), blood flow (radiolabeled microspheres) and vascular conductance (VC) were measured during submaximal treadmill exercise (20 m/min, 5% grade, equivalent to ~60% of maximal O2 uptake). Subsequently, PO2mv (phosphorescence quenching) was measured at rest and during 180 s of electrically-induced twitch contractions (1 Hz, ~6 V) of the surgically-exposed spinotrapezius muscle. BRLD and BRHD lowered resting (control: 139 ± 4, BRLD: 124 ± 5, BRHD: 128 ± 9 mmHg, P<0.05, BRLD vs. control) and exercising (control: 138 ± 3, BRLD: 126 ± 4, BRHD: 125 ± 5 mmHg, P<0.05) MAP to a similar extent. For BRLD this effect occurred in the absence of altered exercising hindlimb muscle(s) blood flow or spinotrapezius PO2mv (rest and across the transient response at the onset of contractions, all P>0.05), each of which increased significantly for the BRHD condition (all P<0.05). Whereas BRHD slowed the PO2mv kinetics significantly (i.e., >mean response time, MRT; control: 16.6 ± 2.1, BRHD: 23.3 ± 4.7s) following the onset of contractions compared to control, in the BRLD group this effect did not reach statistical significance (BRLD: 20.9 ± 1.9s, P=0.14). These data demonstrate that while low dose NO3(-) supplementation lowers MAP during exercise it does so in the absence of augmented muscle blood flow, VC and PO2mv; all of which are elevated at a higher dose. Thus, in healthy animals, a high dose of NO3(-) supplementation seems necessary to elicit significant changes in exercising skeletal muscle O2 delivery/utilization.

Skeletal muscle microvascular oxygenation dynamics in heart failure: exercise training and nitric oxide-mediated function

Chronic heart failure (CHF) impairs nitric oxide (NO)-mediated regulation of skeletal muscle O2 delivery-utilization matching such that microvascular oxygenation falls faster (i.e., speeds PO2mv kinetics) during increases in metabolic demand. Conversely, exercise training improves (slows) muscle PO2mv kinetics following contractions onset in healthy young individuals via NO-dependent mechanisms. We tested the hypothesis that exercise training would improve contracting muscle microvascular oxygenation in CHF rats partly via improved NO-mediated function. CHF rats (left ventricular end-diastolic pressure = 17 ± 2 mmHg) were assigned to sedentary (n = 11) or progressive treadmill exercise training (n = 11; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min; -14% grade downhill running) groups. PO2mv was measured via phosphorescence quenching in the spinotrapezius muscle at rest and during 1-Hz twitch contractions under control (Krebs-Henseleit solution), sodium nitroprusside (SNP; NO donor; 300 μM), and N(G)-nitro-l-arginine methyl ester (L-NAME, nonspecific NO synthase blockade; 1.5 mM) superfusion conditions. Exercise-trained CHF rats had greater peak oxygen uptake and spinotrapezius muscle citrate synthase activity than their sedentary counterparts (p < 0.05 for both). The overall speed of the PO2mv fall during contractions (mean response time; MRT) was slowed markedly in trained compared with sedentary CHF rats (sedentary: 20.8 ± 1.4, trained: 32.3 ± 3.0 s; p < 0.05), and the effect was not abolished by L-NAME (sedentary: 16.8 ± 1.5, trained: 31.0 ± 3.4 s; p > 0.05). Relative to control, SNP increased MRT in both groups such that trained CHF rats had slower kinetics (sedentary: 43.0 ± 6.8, trained: 55.5 ± 7.8 s; p < 0.05). Improved NO-mediated function is not obligatory for training-induced improvements in skeletal muscle microvascular oxygenation (slowed PO2mv kinetics) following contractions onset in rats with CHF.

Skeletal muscle capillary function: contemporary observations and novel hypotheses

The capillary bed constitutes a vast surface that facilitates exchange of O2, substrates and metabolites between blood and organs. In contracting skeletal muscle, capillary blood flow and O2 diffusing capacity, as well as O2 flux, may increase two orders of magnitude above resting values. Chronic diseases, such as heart failure and diabetes, and also sepsis impair these processes, leading to compromised energetic, metabolic and, ultimately, contractile function. Among researchers seeking to understand blood-myocyte exchange in health and the basis for dysfunction in disease, there is a fundamental disconnect between microcirculation specialists and many physiologists and physiologist clinicians. While the former observe capillaries and capillary function directly (muscle intravital microscopy), the latter generally use indirect methodologies (e.g. post-mortem tissue analysis, 1-methyl xanthine, contrast-enhanced ultrasound, permeability-surface area product) and interpret their findings based upon August Krogh's observations made nearly a century ago. 'Kroghian' theory holds that only a small fraction of capillaries support red blood cell (RBC) flux in resting muscle, leaving the vast majority to be 'recruited' (i.e. to initiate RBC flux) during contractions, which would constitute the basis for increasing surface area for capillary exchange and reducing capillary-mitochondrial diffusion distances. Experimental techniques each have their strengths and weaknesses, and often the correct or complete answer to a problem emerges from integration across multiple technologies. Today, Krogh's entrenched 'capillary recruitment' hypothesis is challenged by direct observations of capillaries in contracting muscle, which is something that he and his colleagues could not do. Moreover, in the peer-reviewed scientific literature, application of a range of contemporary physiological technologies, including intravital microscopy of contracting muscle, magnetic resonance, near-infrared spectroscopy and phosphorescence quenching, combined with elegant in situ and in vivo models, suggest that the role of the capillary bed, at least in contracting muscle, is subserved without the necessity for de novo capillary recruitment of previously non-flowing capillaries. When viewed within the context of the capillary recruitment hypothesis, this evidence casts serious doubt on the interpretation of those data that are based upon Kroghian theory and indirect methodologies. Thus, today a wealth of evidence calls for a radical revision of blood-muscle exchange theory to one in which most capillaries support RBC flux at rest and, during contractions, capillary surface area is 'recruited' along the length of previously flowing capillaries. This occurs, in part, by elevating capillary haematocrit and extending the length of the capillary available for blood-myocyte exchange (i.e. longitudinal recruitment). Our understanding of blood-myocyte O2 and substrate/metabolite exchange in health and the mechanistic basis for dysfunction in disease demands no less.

The effects of dietary fish oil on exercising skeletal muscle vascular and metabolic control in chronic heart failure rats

Impaired vasomotor control in chronic heart failure (CHF) is due partly to decrements in nitric oxide synthase (NOS) mediated vasodilation. Exercising muscle blood flow (BF) is augmented with polyunsaturated fatty acid (PUFA) supplementation via fish oil (FO) in healthy rats. We hypothesized that FO would augment exercising muscle BF in CHF rats via increased NO-bioavailability. Myocardial infarction (coronary artery ligation) induced CHF in Sprague-Dawley rats which were subsequently randomized to dietary FO (20% docosahexaenoic acid, 30% eicosapentaenoic acid, n = 15) or safflower oil (SO, 5%, n = 10) for 6-8 weeks. Mean arterial pressure (MAP), blood [lactate], and hindlimb muscles BF (radiolabeled microspheres) were determined at rest, during treadmill exercise (20 m·min(-1), 5% incline) and exercise + N(G)-nitro-l-arginine-methyl-ester (l-NAME) (a nonspecific NOS inhibitor). FO did not change left ventricular end-diastolic pressure (SO: 14 ± 2; FO: 11 ± 1 mm Hg, p > 0.05). During exercise, MAP (SO: 128 ± 3; FO: 132 ± 3 mm Hg) and blood [lactate] (SO: 3.8 ± 0.4; FO: 4.6 ± 0.5 mmol·L(-1)) were not different (p > 0.05). Exercising hindlimb muscle BF was lower in FO than SO (SO: 120 ± 11; FO: 93 ± 4 mL·min(-1)·100 g(-1), p < 0.05) but was not differentially affected by l-NAME. Specifically, 17 of 28 individual muscle BF's were lower (p < 0.05) in FO demonstrating that PUFA supplementation with FO in CHF rats does not augment muscle BF during exercise but may lower metabolic cost.

Effects of nitrate supplementation via beetroot juice on contracting rat skeletal muscle microvascular oxygen pressure dynamics

NO3(-) supplementation via beetroot juice (BR) augments exercising skeletal muscle blood flow subsequent to its reduction to NO2(-) then NO. We tested the hypothesis that enhanced vascular control following BR would elevate the skeletal muscle O2 delivery/O2 utilization ratio (microvascular PO2, PmvO2) and raise the PmvO2 during the rest-contractions transition. Rats were administered BR (~0.8 mmol/kg/day, n=10) or water (control, n=10) for 5 days. PmvO2 was measured during 180 s of electrically induced (1 Hz) twitch spinotrapezius muscle contractions. There were no changes in resting or contracting steady-state PmvO2. However, BR slowed the PmvO2 fall following contractions onset such that time to reach 63% of the initial PmvO2 fall increased (MRT1; control: 16.8±1.9, BR: 24.4±2.7 s, p<0.05) and there was a slower relative rate of PmvO2 fall (Δ1PmvO2/τ1; control: 1.9±0.3, BR: 1.2±0.2 mmHg/s, p<0.05). Despite no significant changes in contracting steady state PmvO2, BR supplementation elevated the O2 driving pressure during the crucial rest-contractions transients thereby providing a potential mechanism by which BR supplementation may improve metabolic control.

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.

The NO donor sodium nitroprusside: evaluation of skeletal muscle vascular and metabolic dysfunction

The nitric oxide (NO) donor sodium nitroprusside (SNP) may promote cyanide-induced toxicity and systemic and/or local responses approaching maximal vasodilation. The hypotheses were tested that SNP superfusion of the rat spinotrapezius muscle exerts 1) residual impairments in resting and contracting blood flow, oxygen utilization (VO(2)) and microvascular O(2) pressure (PO(2)mv); and 2) marked hypotension and elevation in resting PO(2)mv. Two superfusion protocols were performed: 1) Krebs-Henseleit (control 1), SNP (300 μM; a dose used commonly in superfusion studies) and Krebs-Henseleit (control 2), in this order; 2) 300 and 1200 μM SNP in random order. Spinotrapezius muscle blood flow (radiolabeled microspheres), VO(2) (Fick calculation) and PO(2)mv (phosphorescence quenching) were determined at rest and during electrically-induced (1 Hz) contractions. There were no differences in spinotrapezius blood flow, VO(2) or PO(2)mv at rest and during contractions pre- and post-SNP condition (control 1 and control 2; p>0.05 for all). With regard to dosing, SNP produced a graded elevation in resting PO(2)mv (p<0.05) with a reduction in mean arterial pressure only at the higher concentration (p<0.05). Contrary to our hypotheses, skeletal muscle superfusion with the NO donor SNP (300 μM) improved microvascular oxygenation during the transition from rest to contractions (PO(2)mv kinetics) without precipitating residual impairment of muscle hemodynamic or metabolic control or compromising systemic hemodynamics. These data suggest that SNP superfusion (300 μM) constitutes a valid and important tool for assessing the functional roles of NO in resting and contracting skeletal muscle function without incurring residual alterations consistent with cyanide accumulation and poisoning.

(-)-Epicatechin administration and exercising skeletal muscle vascular control and microvascular oxygenation in healthy rats

Consumption of the dietary flavanol (-)-epicatechin (EPI) is associated with enhanced endothelial function and augmented skeletal muscle capillarity and mitochondrial volume density. The potential for EPI to improve peripheral vascular function and muscle oxygenation during exercise is unknown. We tested the hypothesis that EPI administration in healthy rats would improve treadmill exercise performance secondary to elevated skeletal muscle blood flow and vascular conductance [VC, blood flow/mean arterial pressure (MAP)] and improved skeletal muscle microvascular oxygenation. Rats received water (control, n = 12) or 4 mg/kg EPI (n = 12) via oral gavage daily for 24 days. Exercise endurance capacity and peak O(2) uptake (Vo(2) peak) were measured via treadmill runs to exhaustion. MAP (arterial catheter) and blood flow (radiolabeled microspheres) were measured and VC was calculated during submaximal treadmill exercise (25 m/min, 5% grade). Spinotrapezius muscle microvascular O(2) pressure (Po(2mv)) was measured (phosphorescence quenching) during electrically induced twitch (1 Hz) contractions. In conscious rats, EPI administration resulted in lower (↓~5%) resting (P = 0.03) and exercising (P = 0.04) MAP. There were no differences in exercise endurance capacity, Vo(2) peak, total exercising hindlimb blood flow (control, 154 ± 13; and EPI, 159 ± 8 ml·min(-1)·100 g(-1), P = 0.68), or VC (control, 1.13 ± 0.10; and EPI, 1.24 ± 0.08 ml·min(-1)·100 g(-1)·mmHg(-1), P = 0.21) between groups. Following anesthesia, EPI resulted in lower MAP (↓~16%) but did not impact resting Po(2mv) or any kinetics parameters (P > 0.05 for all) during muscle contractions compared with control. EPI administration (4 mg·kg(-1)·day(-1)) improved modestly cardiovascular function (i.e., ↓MAP) with no impact on exercise performance, total exercising skeletal muscle blood flow and VC, or contracting muscle microvascular oxygenation in healthy rats.

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.