The effects of nitrate and nitrite supplementation on mitochondrial respiration in permeabilized muscle fibres in young healthy adults

Nitric oxide (NO) is a direct regulator of mitochondrial respiration. Nitrate (NO 3 - ) and nitrite (NO 2 - ) are good sources of NO, but whether their effects on mitochondrial respiration differ between in vivo and in vitro administration remains unclear. In Study 1, 8 participants consumed NO 3 - rich beetroot juice (BR) (∼12.8 mmol NO 3 - ) and NO 3 - depleted placebo beetroot juice (PL) (∼0.08 mmol NO 3 - ) acutely and chronically for 2 weeks in a randomised, double-blind, crossover design. A substrate-uncoupler-inhibitor titration (SUIT) protocol was used to assess mitochondrial respiration using high-resolution respirometry (oxygen tension: ∼200-450 μM) in permeabilized muscle fibres. In Study 2, skeletal muscle samples were collected from 11 participants. In a randomised, crossover design, different doses (0, 1.5, and 3.0 μM) of sodium nitrite (NaNO 2 ) were administered to permeabilized muscle fibres. Mitochondrial respiration was measured using the same SUIT protocol under lower oxygen tension (∼50-200 μM). Although muscle NO 3 - concentration significantly increased after both acute and chronic BR supplementation, mitochondrial respiration and exercise performance did not differ between PL and BR in either condition. Similarly, absolute oxygen flux across different respiratory states were not different between different doses of NaNO 2 . However, the leak control ratio, reflecting the degree of uncoupling of mitochondrial respiration, was significantly higher with 3.0 μM NaNO 2 administration (0.12±0.05) compared to 0 μM NaNO 2 administration (0.09±0.04, P =0.03). These findings, involving both in vivo and in vitro administration approaches, albeit in the presence of relatively high oxygen concentrations, suggest that neither NO 3 - nor NO 2 - improves mitochondrial respiration, at least in young healthy adults.