Effect of head-down bed rest on the neuroendocrine response to orthostatic stress in physically fit XXXX .

@entity1 @entity1369 @entity1425 @entity916 @entity1063
The role of neuroendocrine responsiveness in the development of orthostatic intolerance after bed rest was studied in physically fit subjects. Head-down bed-rest (HDBR, -6 degrees, 4 days) was performed in 15 @entity1 after 6 weeks of aerobic training. The standing test was performed before, after training and on day 4 of the HDBR. Orthostatic intolerance was observed in one subject before and after training. The blood pressure response after training was enhanced (mean BP increments 18+/-2 vs. 13+/- 2 mm Hg, p<0.05, means +/- S.E.M.), although @entity1063 response was diminished (1.38+/-0.18 vs. 2.76+/-0.25 mol.l(-1), p<0.01). Orthostatic intolerance after HDBR was observed in 10 subjects, the BP response was blunted, and @entity1063 as well as plasma @entity916 activity (PRA) responses were augmented (NA 3.10+/-0.33 mol.l(-1), p<0.001; PRA 2.98+/-1.12 vs. 0.85+/-0.15 ng.ml(-1), p<0.05). Plasma @entity1063 , @entity1425 and @entity1369 responses in orthostatic intolerant subjects were similar to the tolerant group. We conclude that six weeks of training attenuated the sympathetic response to standing and had no effect on the orthostatic tolerance. In orthostatic intolerance the BP response induced by subsequent HDBR was absent despite an enhanced sympathetic response.
