Changes in body posture alter plasma nitrite but not nitrate concentration in humans

PURPOSE: This study evaluated the change (Δ) in plasma volume (PV), nitrate [NO₃^-], and nitrite [NO₂^-] concentration following changes in posture in the presence and absence of elevated plasma [NO₃^-] and [NO₂^-]. 
METHODS: Fourteen healthy participants completed two trials that were preceded by either supplementation with NO₃^--rich beetroot juice (BR; total of ~31 mmol NO₃^-) or no supplementation (CON). Both trials comprised 30 min of lying supine followed by 2 min of standing, 2 min of sitting and 5 min of sub-maximal cycling. Measurements of plasma [NO₃^-] and [NO₂^-] were made by gas-phase chemiluminescence and ΔPV was estimated using the Dill and Costill method. 
RESULTS: Plasma [NO₂^-] decreased from baseline (CON: 120 ± 49 nM, BR: 357 ± 129 nM) after lying supine for 30 min (CON 77 ± 30 nM; BR 231 ± 92 nM, both P<0.01) before increasing during standing (CON 109 ± 42 nM; BR 297 ± 105 nM, both P<0.01) and sitting (CON 131 ± 43 nM; BR 385 ± 125 nM, both P<0.01). Plasma [NO₂^-] remained elevated following exercise only in CON (125 ± 61 nM P=0.02). Plasma [NO₃^-] was not different between measurement points in either condition (P>0.05). PV increased from baseline during the supine phase before decreasing upon standing, sitting, and exercise in both trials (all P<0.05). 
CONCLUSIONS: Changing body posture causes rapid and consistent alterations in plasma [NO₂^-]. Researchers should therefore carefully consider the effect of posture when measuring this variable.