SPACEFLIGHT ALTERS AUTONOMIC REGULATION OF ARTERIAL-PRESSURE IN HUMANS

Spaceflight is associated with decreased orthostatic tolerance after l anding. Short-duration spaceflight (4-5 days) impairs one neural mecha nism: the carotid baroreceptor-cardiac reflex. To understand the effec ts of longer-duration spaceflight on baroreflex function, we measured R-R interval power spectra, antecubital vein plasma catecholamine leve ls, carotid baroreceptor-cardiac reflex responses, responses to Valsal va maneuvers, and orthostatic tolerance in 16 astronauts before and af ter shuttle missions lasting 8-14 days. We found the following changes between preflight and landing day: I) orthostatic tolerance decreased ; 2) R-R interval spectral power in the 0.05- to 0.15-Hz band increase d; 3) plasma norepinephrine and epinephrine levels increased; 4) the s lope, range, and operational point of the carotid baroreceptor cardiac reflex response decreased; and 5) blood pressure and heart rate respo nses to Valsalva maneuvers were altered. Autonomic changes persisted f or several days after landing. These results provide further evidence of functionally relevant reductions in parasympathetic and increases i n sympathetic influences on arterial pressure control after spacefligh t.