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

• Dietary nitrate (NO3−) increases skeletal muscle function during exercise.
• Improvements were seen in muscles comprised predominantly of fast twitch fibers.
• Improved vascular function would be expected to raise microvascular PO2 (PO2mv).
• The impacts of NO3− on the PO2mv profile of fast versus slow twitch muscles are unknown.
• NO3− supplementation preferentially elevates PO2mv in fast twitch muscle.

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 V˙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.