ALTERED THERMOREGULATORY RESPONSES AFTER 15 DAYS OF HEAD-DOWN TILT

To determine whether extended exposure to a simulation of microgravity alters thermoregulatory reflex control of skin blood flow, six adult males (mean age 40 +/- 2 yr) were exposed to 15 days of 6 degrees head -down tilt (HDT). On an ambulatory control day before HDT exposure and on HDT day 15, the core temperature of each subject was increased by 0.5-1.0 degrees C by whole body heating with a water-perfused suit. Me an skin temperature, oral temperature (T-or), mean arterial pressure, and forearm blood flow were measured throughout the protocol. Forearm vascular conductance (FVC) was calculated from the ratio of forearm bl ood flow to mean arterial pressure. After HDT exposure, the T-or thres hold at which reflex thermally induced increases in FVC began was elev ated (36.87 +/- 0.06 to 37.00 +/- 0.09 degrees C; P = 0.043), whereas the slope of the T-or-FVC relationship after this threshold was reduce d (13.7 +/- 2.3 to 9.5 +/- 1.1 FVC units/degrees C; P = 0.038). Moreov er, normothermic FVC and FVC at the highest common T-or between pre- a nd post-HDT trials were reduced after HDT (normothermic: 4.2 +/- 0.5 t o 3.0 +/- 0.4 ml.100 ml(-1).min(-1).100 mmHg(-1), P = 0.04; hypertherm ic: 12.4 +/- 1.0 to 7.8 +/- 0.7 ml.100 ml(-1).min(-1).100 mmHg(-1), P < 0.001). These data suggest that HDT exposure reduces thermoregulator y responses to heat stress. The mechanisms resulting in such an impair ed thermoregulatory response are unknown but are likely related to the relative dehydration that accompanies this exposure.