Catecholamine levels were higher when the astronauts were standing than when they were lying supine, both before and after flight. Postflight catecholamine levels were higher than preflight levels in both the supine and the standing positions. From before the flight to landing day, supine plasma norepinephrine and epinephrine levels and standing norepinephrine and epinephrine levels increased by 34%, 65%, 65% and 91%, respectively. Standing catecholamine levels were reported for only 16 astronauts. Epinephrine levels had returned to preflight levels 3 days after landing, but norepinephrine levels remained elevated. Supine heart rate, supine systolic blood pressure, standing heart rate and standing diastolic blood pressure also increased between the preflight period and landing day. However, increases in peripheral vascular resistance per unit increase in circulating norepinephrine were reduced; sympathetic nervous system activity and peripheral resistance apparently became uncoupled.
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