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1 Laboratoire de Physiologie de l'Environnement (Groupement d'Intérêt Public Exercice), Faculté de Médecine Lyon Grange-Blanche, F-69373 Lyon Cedex 08, France; and 2 Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Changes in autonomic nervous system activity
could be linked to the orthostatic intolerance (OI) that individuals
suffer after a spaceflight or head-down bed rest (HDBR). We examined
this possibility by assessing the sympathetic nervous system activity
during 42 days of HDBR in seven healthy men. Heart rate variability was studied with the use of power spectral analysis, which provided indicators of the sympathetic (SNSi) and parasympathetic (PNSi) nervous
system influences on the heart. Urinary catecholamines and the
spontaneous baroreflex sensitivity were measured. Urinary catecholamines decreased by 21.3%, showing a decrease in SNSi. Heart
rate variability was greatly reduced during 42 days of HDBR with a drop
in PNSi but with no significant changes in SNSi. The baroreflex
sensitivity was greatly reduced (30.7%) on day
42 of HDBR. These results suggest a dissociation
between the catecholamine response and the SNSi of the heart rate. This
dissociation could be the consequence of an increase in 
-adrenergic
receptor density and/or activity induced by a decrease in
catecholamines during HDBR. The subjects who suffered from OI also had
a greater sympathetic response and much lower baroreflex sensitivity
when supine than those who finished the stand test. However, the mean
response of all subjects indicated that the sympathetic activity
(catecholamine excretion) was probably slightly inhibited during HDBR
and could contribute to OI.
orthostatic intolerance; catecholamines; heart rate variability; spontaneous baroreflex
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