Sympathetic outflow to muscle in humans during short periods of microgravity produced by parabolic flight

We have investigated the changes in muscle sympathetic nerve activity (MSNA ) from the tibial nerve during brief, periods of microgravity (mu G) for si milar to 20 s produced by parabolic flight. MSNA was recorded microneurogra phically from 13 quietly seated human subjects with their knee joints exten ded in a jet aircraft simultaneously with the electrocardiogram, the blood pressure wave (measured with a Finapres), the respiration curve, and the th oracic fluid volume (measured by impedance plethysmography). During quiet a nd seated parabolic flight, MSNA was activated in hypergravity and was supp ressed in mu G phasically. At the entry to hypergravity at 2 G just before mu G, the thoracic fluid volume was reduced by 3.2 +/- 3%, and the arterial blood pressure was lowered transiently and then gradually elevated from 89 .5 +/- 1.7 to 100.2 +/- 1.7 mmHg, which caused the enhancement of MSNA by 9 1.4 +/- 14.2%. At the entry to mu G, the thoracic fluid volume was increase d by 3.4%, which lowered the mean blood pressure to 77.9 +/- 2.3 mmHg and s uppressed the MSNA by 17.2%. However, this suppression lasted only similar to 10 s, followed by an enhancement of MSNA that continued for several seco nds. We conclude that MSNA is suppressed and then enhanced during mu G prod uced by parabolic flight. These changes in MSNA are in response not only to intrathoracic fluid volume changes but also to arterial blood pressure cha nges, both of which are caused by body fluid shifts induced by parabolic fl ight, and these changes are quite phasic and transient.