A<3. 37 7. ALTERATIONS IN HUMAN BARORECEPTOR REFLEX REGULATION OF BLOOD PRESSURE FOLLOWING 15 DAYS OF SIMULATED MICROGRAVITY EXPOSURE

Prolonged exposure to microgravity is known to invoke physiological changes which predispose individuals to orthostatic intolerance upon re-adaptation to the earth's gravitational field. Attenuated baroreflex responsiveness has been implicated in contributing to this inability to withstand orthostatic stress. To test this hypothesis, eight individuals were exposed to 15 days of simulated microgravity exposure using the 6 ° head-down bed rest model. Prior to, and after the simulated microgravity exposure, the following were assessed: a) aortic baroreflex function; b) carotid baroreflex function; c) cardiopulmonary baroreflex function; and d) the degree of interaction between the cardiopulmonary and carotid baroreflexes. Fifteen days simulated microgravity exposure increased the gain of the aortic-cardiac baroreflex, expressed as the change in heart rate for a given change in blood pressure (0.45±0.07 to 0.84±0.18 Abpm/AmmHg; p<0.05). Similarly, the gain of the cardiopulmonary baroreflex increased post-head down bed rest (1.510.4 to 2.4±0.6 PRU/mmHg p<0.05) when assessed by comparing the changes i