Scott K. Ferguson

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

•  Inorganic nitrate (NO3−) supplementation with beetroot juice (BR) in humans lowers blood pressure and the O2 cost of exercise and may improve exercise tolerance following its reduction to nitrite (NO2−) and nitric oxide (NO). •  The effect of inorganic NO3− supplementation with BR on skeletal muscle blood flow (BF) and vascular conductance (VC) within and among locomotory muscles during exercise is unknown. •  Inorganic NO3− supplementation with BR in rats resulted in lower exercising mean arterial pressure, lower blood [lactate], and higher total skeletal muscle hindlimb BF and VC during submaximal treadmill running. •  The greater BF and VC was found in muscles and muscle parts containing primarily type IIb + d/x muscle fibres. •  These data demonstrate that inorganic NO3− supplementation improves vascular control and elevates skeletal muscle O2 delivery during exercise predominantly in fast‐twitch type II muscles, and provide a potential mechanism by which NO3− supplementation improves metabolic control.

Skeletal muscle capillary function: contemporary observations and novel hypotheses

•  What is the topic of this review? This review presents the paradigm shift in our understanding of capillary structure and function that has occurred since 1920 (August Kroghs Nobel Prize‐winning work). •  What advances does it highlight? The compelling weight of evidence supports the concept that most capillaries support red blood cell (RBC) flux in resting muscle. Increased blood–myocyte flux during contractions thus occurs via elevated RBC flux, velocity and haematocrit in already flowing capillaries, with capillary surface area being recruited along the length of already flowing capillaries. Heart failure, diabetes and sepsis impair blood–myocyte O2 (and glucose) flux by increasing the proportion of non‐RBC/plasma flowing capillaries and compromising the matching of O2 delivery to O2 requirements.

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.

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.

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.

Exercise training and muscle microvascular oxygenation: functional role of nitric oxide

Exercise training induces multiple adaptations within skeletal muscle that may improve local O(2) delivery-utilization matching (i.e., Po(2)mv). We tested the hypothesis that increased nitric oxide (NO) function is intrinsic to improved muscle Po(2)mv kinetics from rest to contractions after exercise training. Healthy young Sprague-Dawley rats were assigned to sedentary (n = 18) or progressive treadmill exercise training (n = 10; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min, -14% grade) groups. Po(2)mv 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 rats had greater peak oxygen uptake (Vo(2 peak)) than their sedentary counterparts (81 ± 1 vs. 72 ± 2 ml · kg(-1) · min(-1), respectively; P < 0.05). Exercise-trained rats had significantly slower Po(2)mv fall throughout contractions (τ(1); time constant for the first component) during control (sedentary: 8.1 ± 0.6; trained: 15.2 ± 2.8 s). Compared with control, SNP slowed τ(1) to a greater extent in sedentary rats (sedentary: 38.7 ± 5.6; trained: 26.8 ± 4.1 s; P > 0.05) whereas l-NAME abolished the differences in τ(1) between sedentary and trained rats (sedentary: 12.0 ± 1.7; trained: 11.2 ± 1.4 s; P < 0.05). Our results indicate that endurance exercise training leads to greater muscle microvascular oxygenation across the metabolic transient following the onset of contractions (i.e., slower Po(2)mv kinetics) partly via increased NO-mediated function, which likely constitutes an important mechanism for training-induced metabolic adaptations.

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

•  Neuronal nitric oxide (NO) synthase (nNOS) inhibition does not impact skeletal muscle blood flow or vascular conductance (VC) during low‐speed (20 m min−1) treadmill running. •  This may be due to the fact that low exercise intensities recruit primarily oxidative muscle and that nNOS‐derived NO contributes to vascular control primarily within glycolytic muscle. •  Rats ran in the severe‐intensity domain at 15% above critical speed (an important glycolytic fast‐twitch fibre recruitment boundary in the rat) before and after selective nNOS inhibition with S‐methyl‐l‐thiocitrulline (SMTC). •  SMTC reduced blood flow and VC during supra‐critical speed treadmill running (52.5 ± 1.3 m min−1) with the greatest proportional reductions observed in glycolytic fast‐twitch compared to oxidative slow‐ and fast‐twitch muscle. There were no effects of SMTC on muscle blood flow or VC during low‐speed running (20 m min−1). •  The present data reveal important fibre‐type‐ and exercise intensity‐dependent peripheral vascular effects of nNOS‐derived NO during whole‐body exercise.

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.

Role of neuronal nitric oxide synthase in modulating microvascular and contractile function in rat skeletal muscle.

Please cite this paper as: Copp, Hirai, Ferguson, Musch and Poole (2011). Role of Neuronal Nitric Oxide Synthase in Modulating Microvascular and Contractile Function in Rat Skeletal Muscle. Microcirculation 18(6), 501–511.

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 NG-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.