Post-occlusive muscle reoxygenation kinetics and nitric oxide bioavailability following 10-day bed rest

Bed rest (BR) studies have demonstrated the detrimental effects of microgravity and physical inactivity on cardiovascular function, including the negative consequences on microvascular function and O₂ extraction. However, whether a short period of BR affects microvascular responsiveness and its molecular mediator, nitric oxide (NO), remain unclear. Nine male volunteers (23±5 yr) completed 10-day horizontal BR. Microvascular post-occlusive reactive hyperemia in the vastus lateralis (VL) and rectus femoris (RF) muscles were assessed by Near-Infrared Spectroscopy during a vascular occlusion test. We calculated the rate of muscle deoxygenation during the first minute of occlusion (Slope 1) and the half-time of the reperfusion kinetics (t_1/2) during the 30 s post-ischemia. The inverse of t_1/2, K (velocity constant) was “normalized” for the intensity of the ischemic/hypoxic stimulus, as estimated by Δ[deoxy(Hb+Mb)]_peak, the highest Δ[deoxy(Hb+Mb)] value at the end of ischemia. Plasma nitrite (NO₂^-) and nitrate (NO₃^-) concentrations (NO bioavailability indexes) were determined by chemiluminescence. Slope 1 and were slower after BR in both VL (0.08±0.03 vs. 0.06±0.02 µM·s^-1 p=0.016, and 12±3 vs. 17±2 s, p=0.002) and RF (0.07±0.02 vs. 0.05±0.02 µM·s^-1 p=0.008, and 12±3 vs. 18±4 s, p<0.001). No differences were detected in K/Δ[deoxy(Hb+Mb)]_peak after BR (VL: 0.15±0.04 vs. 0.17±0.03 µM·min^-1, p=0.166; RF: 0.20±0.06 vs. 0.21±0.05 µM·min^-1, p=0.220). Plasma NO₂^- concentration was reduced after BR (87±37 vs. 67±48 nM, p=0.011), but not plasma NO₃^- concentration (43±24 vs. 29±11 µM, p=0.130).In conclusion, 10-day horizontal BR does not impair microvascular postocclusive reactive hyperemia parameters, despite decreasing plasma NO₂^- concentration.