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Postural induced changes in plasma volume inversely influences plasma nitrite concentration in humans

Liddle, Luke and Monaghan, Chris and McIlvenna, Luke C and Burleigh, Mia C. and Muggeridge, David J. and Easton, Chris (2017) Postural induced changes in plasma volume inversely influences plasma nitrite concentration in humans. Medicine and Science in Sports and Exercise. p. 1002. ISSN 0195-9131

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Moving from a supine to a standing position typically reduces plasma volume (PV) and while this increases the concentration of some molecules in the blood, the effect on plasma nitrate [NO3-] and nitrite [NO2-] has not been reported. PURPOSE: To determine the change (Δ) in PV, [NO3-] and [NO2-] while lying supine, sitting, standing, and following short-duration exercise. METHODS: Fourteen participants (9 male, age 27 ± 4 yr, body mass 71 ± 11 kg) completed two trials. The first was conducted with no dietary intervention (control; CON) and the second was preceded by ingestion of 3 x 70 ml of NO3--rich beetroot juice the day before and 2 x 70 ml two hours before the trial (BR; total of ~31 mmol NO3-). Both trials comprised 30 min lying supine followed by 2 min of standing, 2 min of sitting, and then 5 min of cycling at 60% of the age-predicted maximal heart rate. Repeated blood samples were collected to allow measurements of haemoglobin and haematocrit in whole blood and plasma [NO3-] and [NO2-] by chemiluminescence. The ΔPV was calculated using the Dill and Costill formula. RESULTS: Following the supine phase, PV increased from baseline in both trials (CON Δ12.6 ± 10.3 %; BR Δ12.5 ± 7 %, both P<0.01) and then decreased upon standing (CON Δ‒5.2 ± 3.8 %, P<0.01; BR Δ‒4.0 ± 3.5%, P=0.02), sitting (CON Δ‒10.1 ± 3.7 %; BR Δ‒6.4 ± 3.6 %, both P<0.001) and following exercise (CON Δ‒18.1 ± 5 %; BR Δ‒15.5 ± 3.4 %, both P<0.001). Plasma [NO2-] levels at baseline were 120 ± 49 nM and 357 ± 129 nM in CON and BR, respectively. Plasma [NO2-] decreased from baseline after lying supine in both trials (CON 77 ± 30 nM; BR 231 ± 92 nM, both P<0.05) before increasing during standing (CON 109 ± 42 nM; BR 297 ± 105 nM, both P<0.001) and sitting (CON 131 ± 43 nM; BR 385 ± 125 nM, both P<0.002). Plasma [NO2-] remained elevated following exercise in the CON trial (125 ± 61 nM, P<0.05) but was not different to the 30 min supine value in the BR trial. There were no statistical differences in [NO3-] between measurement points in either condition (all P>0.05). CONCLUSIONS: Plasma [NO2-] changes in the opposite direction to PV during changes in posture, both in the presence and absence of prior dietary NO3- supplementation. Given that [NO2-] offers the best approximation of nitric oxide bioavailability, researchers must be cognisant of these outcomes when designing and interpreting dietary NO3- research.