Muscle sympathetic nerve activity (MSNA) after 120 days of 6 degrees head-down bed rest (HDBR)

The alterations in the sympathetic nervous system are commonly seen after spaceflight and its ground-based simulation model, 6 degrees head-down bed rest (HDBR). Several studies have examined the effect of HDBR for a few days or a weeks on the vasomotor sympathetic tone at rest. However, it remains unclear how a long-lasting (>50 days) HDBR affects resting vasomotor sympathetic nervous activity. Consequently, we measured muscle sympathetic nerve activity (MSNA) in six healthy male volunteers (range, 26-42 years) in a resting supine position before and after 60 and 120 days of HDBR. MSNA was measured directly from the tibial nerve using a microneurographic technique. Resting supine MSNA significantly increased after 60 (28.2 +/- 2.7 bursts/min) and 120 (29.4 +/- 4.2 bursts/min) days of HDBR compared with the pre-HDBR level (19.0 +/- 2.3 bursts/min). Heart rate was significantly higher after 60 (73 +/- 2 beats/min) and 120 (72 +/- 3 beats/min) days of HDBR than before (65 +/- 2 beats/min). In contrast, mean blood pressure remained unchanged during and after 120 days of HDBR. In conclusion, the resting vasomotor sympathetic tone increased after long-lasting simulated microgravity.