title: Sleep restriction does not affect orthostatic tolerance in the simulated microgravity environment.
abstract: Orthostatic intolerance (OI) is a major problem following spaceflight, and, during flight, astronauts also experience sleep restriction. We hypothesized that sleep restriction will compound the risk and severity of OI following simulated microgravity and exaggerate the renal, cardioendocrine, and cardiovascular adaptive responses to it. Nineteen healthy men were equilibrated on a constant diet, after which they underwent a tilt-stand test. They then completed 14-16 days of simulated microgravity [head-down tilt bed rest (HDTB)], followed by repeat tilt-stand test. During HDTB, 11 subjects were assigned to an 8-h sleep protocol (non-sleep restricted), and 8 were assigned to a sleep-restricted protocol with 6 h of sleep per night. During various phases, the following were performed: 24-h urine collections, hormonal measurements, and cardiovascular system identification. Development of presyncope or syncope defined OI. There was a significant decrease in time free of OI (P = 0.02) and an increase in OI occurrence (P = 0.06) after HDTB among all subjects. However, the increase in OI occurrence did not differ significantly between the two groups (P = 0.60). The two groups also experienced similar physiological changes with HDTB (initial increase in sodium excretion; increased excretion of potassium at the end of HDTB; increase in plasma renin activity secretion without a change in serum or urine aldosterone). No significant change in autonomic function or catecholamines was noted. Simulated microgravity leads to increased OI, and sleep restriction does not additively worsen OI in simulated microgravity. Furthermore, conditions of sleep restriction and nonsleep restriction are similar with respect to renal, cardioendocrine, and cardiovascular responses to simulated microgravity.
authors.affiliation: Division of Endocrinology, Hypertension and Diabetes, Brigham and Women's Hospital, 221 Longwood Ave., Boston, MA 02115, USA.
authors.forename: S Marlene
authors.initials: SM
authors.lastname: Grenon
authors.forename: Shelley
authors.initials: S
authors.lastname: Hurwitz
authors.forename: Natalie
authors.initials: N
authors.lastname: Sheynberg
authors.forename: Xinshu
authors.initials: X
authors.lastname: Xiao
authors.forename: Brad
authors.initials: B
authors.lastname: Judson
authors.forename: Craig D
authors.initials: CD
authors.lastname: Ramsdell
authors.forename: Christine
authors.initials: C
authors.lastname: Kim
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cohen
authors.forename: Gordon H
authors.initials: GH
authors.lastname: Williams
mesh_terms: D000328:Adult; D001510:Bed Rest; D002319:Cardiovascular System / Q000503:physiopathology; D004702:Endocrine Glands / Q000503:physiopathology; D018475:Head-Down Tilt; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology* / Q000652:urine; D007668:Kidney / Q000503:physiopathology; D008297:Male; D011188:Potassium / Q000652:urine; D012083:Renin / Q000652:urine; D012892:Sleep Deprivation; D012964:Sodium / Q000652:urine; D018474:Weightlessness Simulation
chemical_list: D012964:Sodium; D012083:Renin; D011188:Potassium