Vasomotor sympathetic nerve activity in men during bed rest and on orthostasis after bed rest

Background: Alterations in autonomic function are commonly seen during and after spaceflight, and its ground-based analog, 6 degrees head-dawn bed res t (HDBR). They may include peripheral Vascular regulation, but vasomotor sy mpathetic efferent nerve discharges to peripheral vasculatures have not bee n examined. The aim of our study was to examine changes in vasomotor sympat hetic nerve activity during HDBR and under orthostasis after HDBR. Methods: We performed 6 d of HDBR on six male subjects, and measured muscle sympath etic nerve activity (MSNA) together with plasma norepinephrine concentratio ns in the supine position before HDBR and in 6 degrees head-down position o n the sixth day (HDBR6) of HDBR. We also measured MSNA in head-up tilt (HUT ) test before and after HDBR. Results: On HDBR6, MSNA burst rate was the sa me (17 +/- 4 bursts.min(-1)) as that in supine position before HDBR (15 +/- 2 bursts.min(-1)), but plasma norepinephrine concentrations were decreased to 1.14 +/- 0.10 pmol.ml(-1) compared with the supine value before HDBR (1 .56 +/- 0.20 pmol.ml(-1), p < 0.05). After HDBR, supine MSNA burst rate sig nificantly increased by 58% to 24 +/- 4 bursts min(-1). MSNA increment in r esponse to HUT was similar between before (34 +/- 3 bursts.min(-1).sin HUT- 1) and after (40 +/- 6 bursts min(-1).sin HUT-1) HDBR. Conclusions: Our fin dings suggest that: a) the relationship between MSNA and plasma norepinephr ine concentrations was altered on the sixth day during HDBR; bf the vasomot or sympathetic nerve activity was enhanced alter HDBR; and c) the augmentat ion of vasomotor sympathetic outflow to muscles under orthostasis was prese rved after HDBR.