POST-SPACEFLIGHT ORTHOSTATIC INTOLERANCE, POSSIBLE RELATIONSHIP TO MICROGRAVITY-INDUCED PLASTICITY IN THE VESTIBULAR SYSTEM

Even after short spaceflights, most astronauts experience at least som e postflight reduction of orthostatic tolerance; this problem is sever e in some subjects. The mechanisms leading to postflight orthostatic i ntolerance are not well-established, but have traditionally been thoug ht to include the following: changes in leg hemodynamics, alterations in baroreceptor reflex gain, decreases in exercise tolerance and aerob ic fitness, hypovolemia, and altered sensitivity of P-adrenergic recep tors in the periphery. Recent studies have demonstrated that signals f rom vestibular otolith organs play an important role in regulating blo od pressure during changes in posture in a I-g environment. Because sp aceflight results in plastic changes in the vestibular otolith organs and in the processing of inputs from otolith receptors, it is possible that another contributing factor to postflight orthostatic hypotensio n is alterations in the gain of vestibular influences on cardiovascula r control. Preliminary data support this hypothesis, although controll ed studies will be required to determine the relationship between chan ges in the vestibular system and orthostatic hypotension following exp osure to microgravity. (C) 1998 Elsevier Science B.V. All rights reser ved.