Endocrine responses to 7 days of head-down bed rest and orthostatic tests in men and women

The objective of this study was to investigate plasma volume (PV), total bo dy water, hormones and hydroelectrolyte responses in eight males (25-40 yea rs) and eight females (25-31 years) during 7 days of exposure to simulated microgravity (-6 degrees head-down bed rest, HDBR). Bed rest is a model tha t has commonly been used to simulate spaceflight. Heart rate (HR), blood pr essure (BP) and vasoactive hormone responses were studied before and after HDBR during a 10-min stand test. No change in total body water and body mas s was noted in either sex. The decrease in PV was similar in both men (9.1 +/- 1.4%) and women (9.4 +/- 0.8%). Urinary normetanephrine (NMN) was decre ased during HDBR in both sexes. Urinary metanephrine (MN) and plasma catech olamines were unchanged. Daily urinary excretion of urea, an indirect index of protein breakdown, was increased only in the female subjects during HDB R. Plasma active renin (AR) and aldosterone were increased in both sexes, b ut urinary atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) were unchanged throughout the study. Also, the hormonal responses to 7 days of HDBR were comparable between men and women. Moreover, the results show similar cardiovascular and endocrine responses to standing after HDBR. Howe ver, the orthostatic intolerance following HDBR was associated with a blunt ed increase in noradrenaline (NA) only in the women during the stand test. It is concluded that: (i) 7 days of physical inactivity achieved during HDB R resulted in a reduced sympathetic activity in both sexes and alterations in protein metabolism in women and (ii) standing after HDBR resulted in an attenuated release of noradrenaline in women.