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

Abstract

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 increased NO bioavailability via 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 180s 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: 13.7 ± 0.5, BR: 19 ± 1.6 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 trueV.O2 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.

Dietary nitrate supplementation enhances short but not longer duration running time-trial performance

“…Interestingly, recent murine model studies have identified effects of NO 3 − which appear to be selective to type II muscle (for review, see Jones et al 2016), and may be of mechanistic relevance in the present study given the likely high recruitment of type II fibers during the 1500 m TT (Essén 1978;Sale 1987). NO 3 − supplementation was reported to augment blood flow and thus O 2 delivery to type II muscle in exercising rats (Ferguson et al 2013(Ferguson et al , 2015. It was suggested that these local blood flow changes might be beneficial by increasing the microvascular partial pressure of O 2 (PO 2 ), and concomitantly decreasing muscle metabolic perturbations (e.g.…”

“…Recent data from murine models suggest that NO 3 − supplementation elevates blood flow and oxygenation (Ferguson et al 2013(Ferguson et al , 2015, and increases calcium (Ca 2+ ) handling protein expression and force production (Hernandez et al 2012) in type II muscle only (for review, see Jones et al 2016). Therefore, the beneficial effects of NO 3 − supplementation might be restricted to, or most pronounced during, exercise where type II fibers are predominantly recruited (Breese et al 2013;Bailey et al 2015;Coggan et al 2015), such as during short duration exercise where a higher relative work rate can be adopted (Essén 1978;Sale 1987).…”

Dietary nitrate supplementation enhances high-intensity running performance in moderate normobaric hypoxia, independent of aerobic fitness

“…Nitrate supplementation had no effect on severe-intensity cycling TTE in normoxia, but improved TTE by ~ 8.6 % in hypoxia relative to placebo. Further, there is evidence from murine model investigations that nitrate supplementation may enhance tissue blood flow [41,42] and muscle contractile function [9] preferentially in type II muscle fibres. It is therefore possible that nitrate supplementation is more effective during shorter more high-intensity exercise, during which these muscle fibres are more heavily recruited [9,[43][44][45]64], or else in individuals with greater distribution of type II muscle fibres [43].…”

“…Evidence from murine models suggests that the effects of dietary nitrate supplementation may be fibre type specific, increasing muscle perfusion and oxygenation [41,42], and enhancing calcium handling and force production [9] in type II muscle fibres only, as may be recruited during high-intensity exercise (for review, see Jones et al [43]). Moreover, nitrate supplementation has been demonstrated to elicit beneficial effects when the contribution of type II muscle towards force generation is high, including at fast but not slow muscle contraction frequencies [44], and during the transition from moderate to severe but not unloaded to moderate intensity exercise [45].…”

Inorganic nitrate supplementation improves muscle oxygenation, O2uptake kinetics, and exercise tolerance at high but not low pedal rates

“…There is also evidence to suggest that pulmonary V O 2 kinetics is slower (11, 18) and the V O 2 slow component is increased (11, 52) when cycling at very high compared with very low pedal cadences. Dietary NO 3 Ϫ supplementation has been shown to increase muscle blood flow (24), lower the O 2 (3,4,42,43,58,61) and ATP (3) requirements of muscle contraction, and improve the matching between muscle O 2 supply and muscle O 2 utilization (4,25,26). Therefore, this potential for enhanced contractile efficiency and perfusion distribution with NO 3 Ϫ supplementation might improve muscle oxygenation, V O 2 kinetics, and exercise toler-…”