SIMULATED MICROGRAVITY INCREASES CUTANEOUS BLOOD-FLOW IN THE HEAD ANDLEG OF HUMAN

Background: The cutaneous microcirculation vasodilates during acute 6 degrees head-down tilt (HDT, simulated microgravity) relative to uprig ht conditions, more in the lower body than in the upper body. Hypothes is: We expected that relative magnitudes of and differences between up per and lower body cutaneous blood flow elevation would be sustained d uring initial acclimation to simulated microgravity. Methods: We measu red cutaneous microvascular blood flow with laser-Doppler flowmetry at the leg (over the distal tibia) and cheek (over the zygomatic arch) o f eight healthy men before, during, and after 24 h of HDT. Results wer e calculated as a percentage of baseline value (100% measured during p re-tilt uptight sitting). Results: Cutaneous blood flow in the cheek i ncreased significantly to 165 +/- 37% (mean +/- SE, p<0.05) at 9-12 h HDT, then returned to near baseline values by 24 h HDT (114 +/- 29%, N SD), despite increased local arterial pressure. Microvascular flow in the leg remained significantly elevated above baseline throughout 24 h HDT (427 +/- 85% at 3 h HDT and 215 +/- 142% at 24 h HDT, p<0.05). Du ring the 6-h uptight sitting recovery period, cheek and leg blood flow levels returned to near pre-tilt baseline values. Conclusions: Becaus e hydrostatic effects of HDT increase local arterial pressure at the c arotid sinus, baroreflex-mediated withdrawal of sympathetic tone proba bly contributed to increased microvascular flows at the head and leg d uring HDT. In the leg, baroreflex effects combined with minimal stimul ation of local veno-arteriolar and myogenic autoregulatory vasoconstri ction to elicit relatively larger and more sustained increases in cuta neous flow during HDT. In the cheek, delayed myogenic vasoconstriction and/or humoral effects apparently compensated for flow elevation by 2 4 h of HDT. Therefore, localized vascular adaptations to gravity proba bly explain differences in acclimation of lower and upper body blood f low to HDT and actual microgravity.