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 postocclusive 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 postischemia. 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 (NO⁻₂) and nitrate (NO⁻₃) concentrations (NO bioavailability indexes) were determined by chemiluminescence. Slope 1 and 𝑡_1/2 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 NO⁻2 concentration was reduced after BR (87 ± 37 vs. 67 ± 48 nM, P = 0.011), but not plasma NO⁻₃ 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 NO⁻₂ concentration.

NEW & NOTEWORTHY
Short-term (10 days) horizontal bed rest does not affect skeletal muscle microvascular function when assessed via near-infrared spectroscopy and postocclusion reactive hyperemia. However, both skeletal muscle resting oxygen consumption and nitric oxide bioavailability decrease following 10-days bed rest. Future studies should investigate the time course of changes in skeletal muscle microvascular function following longer period of microgravity/inactivity.