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[FIGURE DASH]5.2 ± 3.8 %, P<0.01; BR Δ[FIGURE DASH]4.0 ± 3.5%, P=0.02), sitting (CON Δ[FIGURE DASH]10.1 ± 3.7 %; BR Δ[FIGURE DASH]6.4 ± 3.6 %, both P<0.001) and following exercise (CON Δ[FIGURE DASH]18.1 ± 5 %; BR Δ[FIGURE DASH]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.