The effects of increased nitric oxide (NO) activity and/or NO precursor levels following nitrate (NO3−) supplementation on venous vascular control remain poorly understood. We investigated the effect of acute NO3− supplementation on the venous vascular response of a single conduit vein at rest and during sympathoexcitation (e.g., static exercise and muscle metaboreflex). Participants were 15 healthy young adults who consumed either beetroot juice (BRJ, 140 mL; ∼8 mmol NO3−) or a control beverage (prune juice; CON, 166 mL; <0.01 mmol NO3−) following a random crossover study design. Two hours after consuming the allocated beverage, each participant rested for 4 min and then performed a continuous isometric forearm exercise (forearm exercise) using the right arm at 45 % of their maximal voluntary constriction for 1.5 min, followed by a 2-min recovery period with arterial occlusion of the exercising arm to activate the muscle metaboreflex only. Mean arterial pressure (MAP), heart rate (HR), and the cross-sectional area of the superficial vein in the left non-exercising arm (CSAvein) were measured. BRJ intake increased the plasma NO3− concentration (p < 0.05). All resting parameters were similar with CON and BRJ. MAP and HR increased and the CSAvein decreased with exercise (p < 0.05), and these changes were maintained during recovery, except for HR. The increase in MAP was lower in the BRJ group than in the CON group (p < 0.05), although the magnitude of the CSAvein decrease did not differ between the groups. These findings suggest that, in a single conduit vein, the increasing NO precursors by BRJ intake does not alter either the venous vascular tone at rest or the sympathetic venoconstriction during forearm exercise or during the activation of muscle metaboreceptors.
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