Effects of oral sodium nitrate on forearm blood flow, oxygenation and exercise performance during acute exposure to hypobaric hypoxia (4300 m).

A reduction in oxygen transport contributes to impaired exercise capacity at high altitude. Since blood flow is mediated, in part, by nitric oxide (NO), we hypothesized that sodium nitrate provided before forearm grip exercise performed at a simulated altitude of 4300 m (hypobaric hypoxia (HH)) would increase forearm blood flow and oxygenation, and decrease the decrement in grip performance. In a double-blind, randomized crossover study, 10 healthy subjects (9 males and 1 female) performed continuous (CGrip) and repeated rhythmic (RGrip) isometric forearm exercise until task failure in normobaric normoxia (NN), 2.5 h following consumption of placebo and sodium nitrate (15 mmol) in HH, and then again post-HH at sea-level pressure. Measurements included forearm blood flow (FBF) and anterior forearm tissue oxygenation (StO2), mean arterial blood pressure (MAP), arterial blood O2 saturation (SpO2), plasma NO reaction products (NOx) and nitrite, and exhaled NO (PENO). Compared to baseline testing in NN, performing CGrip and RGrip exercise in HH resulted in significant reductions in forearm blood flow, SaO2 and StO2, responses that were accompanied by significant performance decrements (∼10%) in both CGrip and RGrip exercise. In spite of a 10-fold increase in plasma NOx levels and a significant decrease in MAP during CGrip exercise following nitrate consumption, there were no significant main effects of treatment (placebo vs. sodium nitrate) for forearm blood flow, SpO2, StO2, or grip performance. PENO remained unchanged between NN, HH and post-HH conditions with placebo, but increased (∼24%) following nitrate supplementation in HH and post-HH. These data do not support a benefit in consuming a single dose of supplemental nitrate on forearm blood flow and isometric exercise in healthy adults at a simulated altitude of 4300 m.