Postural Induced Changes in Plasma Volume Inversely Influences Plasma Nitrite Concentration in Humans

Abstract

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 0.05). 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.