Postocclusive 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 (<i>slope 1</i>) and the half-time of the reperfusion kinetics ([Formula: see text]) during the 30 s postischemia. The inverse of [Formula: see text], <i>K</i> (velocity constant) was "normalized" for the intensity of the ischemic/hypoxic stimulus, as estimated by [Formula: see text], the highest [Formula: see text] value at the end of ischemia. Plasma nitrite ([Formula: see text]) and nitrate ([Formula: see text]) concentrations (NO bioavailability indexes) were determined by chemiluminescence. <i>Slope 1</i> and [Formula: see text] were slower after BR in both VL (0.08 ± 0.03 vs. 0.06 ± 0.02 µM·s^-1, <i>P</i> = 0.016; and 12 ± 3 vs. 17 ± 2 s, <i>P</i> = 0.002) and RF (0.07 ± 0.02 vs. 0.05 ± 0.02 µM·s^-1, <i>P</i> = 0.008; and 12 ± 3 vs. 18 ± 4 s, <i>P</i> < 0.001). No differences were detected in <i>K</i>/[Formula: see text] after BR (VL: 0.15 ± 0.04 vs. 0.17 ± 0.03 µM·min^-1, <i>P</i> = 0.166; RF: 0.20 ± 0.06 vs. 0.21 ± 0.05 µM·min^-1, <i>P</i> = 0.220). Plasma [Formula: see text] concentration was reduced after BR (87 ± 37 vs. 67 ± 48 nM, <i>P</i> = 0.011), but not plasma [Formula: see text] concentration (43 ± 24 vs. 29 ± 11 µM, <i>P</i> = 0.130). In conclusion, 10-day horizontal BR does not impair microvascular postocclusive reactive hyperemia parameters, despite decreasing plasma [Formula: see text] concentration.<b>NEW & NOTEWORTHY</b> 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.