Search

Aim: Dietary nitrate (NO3−) supplementation increases nitric oxide bioavailability and can enhance exercise performance. We investigated the distribution and metabolic fate of ingested NO3− at rest and during exercise with a focus on skeletal muscle. Methods: In a randomized, crossover study, 10 healthy volunteers consumed 12.8 mmol 15N‐labeled potassium nitrate (K15NO3; NIT) or potassium chloride placebo (PLA). Muscle biopsies were taken at baseline, at 1‐ and 3‐h post‐supplement ingestion, and immediately following the completion of 60 maximal intermittent contractions of the knee extensors. Muscle, plasma, saliva, and urine samples were analyzed using chemiluminescence to determine absolute [NO3−] and [NO2−], and by mass spectrometry to determine the proportion of NO3− and NO2− that was 15N‐labeled. Results: Neither muscle [NO3−] nor [NO2−] were altered by PLA. Following NIT, muscle [NO3−] (but not [NO2−]) was elevated at 1‐h (from ~35 to 147 nmol/g, p < 0.001) and 3‐h, with almost all of the increase being 15N‐labeled. There was a significant reduction in 15N‐labeled muscle [NO3−] from pre‐ to post‐exercise. Relative to PLA, mean muscle torque production was ~7% greater during the first 18 contractions following NIT. This improvement in torque was correlated with the pre‐exercise 15N‐labeled muscle [NO3−] and the magnitude of decline in 15N‐labeled muscle [NO3−] during exercise (r = 0.66 and r = 0.62, respectively; p < 0.01). Conclusion: This study shows, for the first time, that skeletal muscle rapidly takes up dietary NO3−, the elevated muscle [NO3−] following NO3− ingestion declines during exercise, and muscle NO3− dynamics are associated with enhanced torque production during maximal intermittent muscle contractions. [ABSTRACT FROM AUTHOR]

Dietary nitrate (NO 3 −) supplementation can increase nitric oxide (NO) bioavailability, reduce blood pressure (BP) and improve muscle contractile function in humans. Plasma nitrite concentration (plasma [NO 2 −]) is the most oft-used biomarker of NO bioavailability. However, it is unclear which of several NO biomarkers (NO 3 −, NO 2 −, S-nitrosothiols (RSNOs)) in plasma, whole blood (WB), red blood cells (RBC) and skeletal muscle correlate with the physiological effects of acute and chronic dietary NO 3 − supplementation. Using a randomized, double-blind, crossover design, 12 participants (9 males) consumed NO 3 −-rich beetroot juice (BR) (∼12.8 mmol NO 3 −) and NO 3 −-depleted placebo beetroot juice (PL) acutely and then chronically (for two weeks). Biological samples were collected, resting BP was assessed, and 10 maximal voluntary isometric contractions of the knee extensors were performed at 2.5–3.5 h following supplement ingestion on day 1 and day 14. Diastolic BP was significantly lower in BR (−2 ± 3 mmHg, P = 0.03) compared to PL following acute supplementation, while the absolute rate of torque development (RTD) was significantly greater in BR at 0–30 ms (39 ± 57 N m s−1, P = 0.03) and 0–50 ms (79 ± 99 N m s−1, P = 0.02) compared to PL following two weeks supplementation. Greater WB [RSNOs] rather than plasma [NO 2 −] was correlated with lower diastolic BP (r = −0.68, P = 0.02) in BR compared to PL following acute supplementation, while greater skeletal muscle [NO 3 −] was correlated with greater RTD at 0–30 ms (r = 0.64 , P= 0.03) in BR compared to PL following chronic supplementation. We conclude that [RSNOs] in blood, and [NO 3 −] in skeletal muscle, are relevant biomarkers of NO bioavailability which are related to the reduction of BP and the enhanced muscle contractile function following dietary NO 3 − ingestion in humans. • It is unclear which NO biomarkers in blood and muscle correlate with the physiological effects of acute and chronic dietary nitrate supplementation. • In a crossover design, participants consumed NO 3 −-rich beetroot juice (BR) and NO 3 −-depleted placebo (PL) acutely and then for two weeks. • Greater whole blood [RSNOs] was correlated with lower diastolic BP with BR, while greater skeletal muscle [NO 3 −] was correlated with greater rate of muscle torque development with BR. • RSNOs in blood, and NO 3 − in skeletal muscle, are relevant biomarkers of NO bioavailability which are related to the physiological effects of dietary NO 3 − ingestion in humans. [ABSTRACT FROM AUTHOR]

Key points: Nitric oxide (NO), a potent vasodilator and a regulator of many physiological processes, is produced in mammals both enzymatically and by reduction of nitrite and nitrate ions.We have previously reported that, in rodents, skeletal muscle serves as a nitrate reservoir, with nitrate levels greatly exceeding those in blood or other internal organs, and with nitrate being reduced to NO during exercise.In the current study, we show that nitrate concentration is substantially greater in skeletal muscle than in blood and is elevated further by dietary nitrate ingestion in human volunteers. We also show that high‐intensity exercise results in a reduction in the skeletal muscle nitrate store following supplementation, likely as a consequence of its reduction to nitrite and NO.We also report the presence of sialin, a nitrate transporter, and xanthine oxidoreductase in human skeletal muscle, indicating that muscle has the necessary apparatus for nitrate transport, storage and metabolism. Rodent skeletal muscle contains a large store of nitrate that can be augmented by the consumption of dietary nitrate. This muscle nitrate reservoir has been found to be an important source of nitrite and nitric oxide (NO) via its reduction by tissue xanthine oxidoreductase. To explore if this pathway is also active in human skeletal muscle during exercise, and if it is sensitive to local nitrate availability, we assessed exercise‐induced changes in muscle nitrate and nitrite concentrations in young healthy humans, under baseline conditions and following dietary nitrate consumption. We found that baseline nitrate and nitrite concentrations were far higher in muscle than in plasma (∼4‐fold and ∼29‐fold, respectively), and that the consumption of a single bolus of dietary nitrate (12.8 mmol) significantly elevated nitrate concentration in both plasma (∼19‐fold) and muscle (∼5‐fold). Consistent with these observations, and with previous suggestions of active muscle nitrate transport, we present western blot data to show significant expression of the active nitrate/nitrite transporter sialin in human skeletal muscle. Furthermore, we report an exercise‐induced reduction in human muscle nitrate concentration (by ∼39%), but only in the presence of an increased muscle nitrate store. Our results indicate that human skeletal muscle nitrate stores are sensitive to dietary nitrate intake and may contribute to NO generation during exercise. Together, these findings suggest that skeletal muscle plays an important role in the transport, storage and metabolism of nitrate in humans. Key points: Nitric oxide (NO), a potent vasodilator and a regulator of many physiological processes, is produced in mammals both enzymatically and by reduction of nitrite and nitrate ions.We have previously reported that, in rodents, skeletal muscle serves as a nitrate reservoir, with nitrate levels greatly exceeding those in blood or other internal organs, and with nitrate being reduced to NO during exercise.In the current study, we show that nitrate concentration is substantially greater in skeletal muscle than in blood and is elevated further by dietary nitrate ingestion in human volunteers. We also show that high‐intensity exercise results in a reduction in the skeletal muscle nitrate store following supplementation, likely as a consequence of its reduction to nitrite and NO.We also report the presence of sialin, a nitrate transporter, and xanthine oxidoreductase in human skeletal muscle, indicating that muscle has the necessary apparatus for nitrate transport, storage and metabolism. [ABSTRACT FROM AUTHOR]

Dietary nitrate (NO 3 −) ingestion can be beneficial for health and exercise performance. Recently, based on animal and limited human studies, a skeletal muscle NO 3 − reservoir has been suggested to be important in whole body nitric oxide (NO) homeostasis. The purpose of this study was to determine the time course of changes in human skeletal muscle NO 3 − concentration ([NO 3 −]) following the ingestion of dietary NO 3 −. Sixteen participants were allocated to either an experimental group (NIT: n = 11) which consumed a bolus of ∼1300 mg (12.8 mmol) potassium nitrate (KNO 3), or a placebo group (PLA: n = 5) which consumed a bolus of potassium chloride (KCl). Biological samples (muscle (vastus lateralis), blood, saliva and urine) were collected shortly before NIT or PLA ingestion and at intervals over the course of the subsequent 24 h. At baseline, no differences were observed for muscle [NO 3 −] and [NO 2 −] between NIT and PLA (P > 0.05). In PLA, there were no changes in muscle [NO 3 −] or [NO 2 −] over time. In NIT, muscle [NO 3 −] was significantly elevated above baseline (54 ± 29 nmol/g) at 0.5 h, reached a peak at 3 h (181 ± 128 nmol/g), and was not different to baseline from 9 h onwards (P > 0.05). Muscle [NO 2 −] did not change significantly over time. Following ingestion of a bolus of dietary NO 3 −, skeletal muscle [NO 3 −] increases rapidly, reaches a peak at ∼3 h and subsequently declines towards baseline values. Following dietary NO 3 − ingestion, human m. vastus lateralis [NO 3 −] expressed a slightly delayed pharmacokinetic profile compared to plasma [NO 3 −]. • Nitrate in skeletal muscle may be important in whole-body nitric oxide homeostasis. • We measured changes in human muscle nitrate after dietary nitrate ingestion. • Muscle nitrate levels increase at 0.5 h, peak at 3 h and return to baseline by 9 h. • Muscle nitrate pharmacokinetics are broadly similar to those of plasma nitrate. [ABSTRACT FROM AUTHOR]