title: Fluid volume and osmoregulation in humans after a week of head-down bed rest.
abstract: Body fluid homeostasis was investigated during chronic bed rest (BR) and compared with that of acute supine conditions. The hypothesis was tested that 6 degrees head-down BR leads to hypovolemia, which activates antinatriuretic mechanisms so that the renal responses to standardized saline loading are attenuated. Isotonic (20 ml/kg body wt) and hypertonic (2.5%, 7.2 ml/kg body wt) infusions were performed in eight subjects over 20 min following 7 and 10 days, respectively, of BR during constant sodium intake (200 meq/day). BR decreased body weight (83.0 +/- 4.8 to 81.8 +/- 4.4 kg) and increased plasma osmolality (285.9 +/- 0.6 to 288.5 +/- 0.9 mosmol/kgH(2)O, P < 0.05). Plasma ANG II doubled (4.2 +/- 1.2 to 8.8 +/- 1.8 pg/ml), whereas other endocrine variables decreased: plasma atrial natriuretic peptide (42 +/- 3 to 24 +/- 3 pg/ml), urinary urodilatin excretion rate (4.5 +/- 0.3 to 3.2 +/- 0.1 pg/min), and plasma vasopressin (1.7 +/- 0.3 to 0.8 +/- 0.2 pg/ml, P < 0.05). During BR, the natriuretic response to the isotonic saline infusion was augmented (39 +/- 8 vs. 18 +/- 6 meq sodium/350 min), whereas the response to hypertonic saline was unaltered (32 +/- 8 vs. 29 +/- 5 meq/350 min, P < 0.05). In conclusion, BR elicits antinatriuretic endocrine signals, but it does not attenuate the renal natriuretic response to saline stimuli in men; on the contrary, the response to isotonic saline is augmented.
authors.affiliation: Department of Medical Physiology, Panum Institute, University of Copenhagen, DK-2200 Copenhagen, Denmark. mbestle@dadlnet.dk
authors.forename: M H
authors.initials: MH
authors.lastname: Bestle
authors.forename: P
authors.initials: P
authors.lastname: Norsk
authors.forename: P
authors.initials: P
authors.lastname: Bie
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D000804:Angiotensin II / Q000378:metabolism; D009320:Atrial Natriuretic Factor / Q000378:metabolism; D001798:Blood Proteins / Q000378:metabolism; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D007552:Isotonic Solutions / Q000008:administration & dosage; D008297:Male; D009318:Natriuresis / Q000502:physiology; D011188:Potassium / Q000097:blood / Q000652:urine; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage; D012964:Sodium / Q000097:blood / Q000652:urine; D014556:Urine; D014667:Vasopressins / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D001798:Blood Proteins; D007552:Isotonic Solutions; D012462:Saline Solution, Hypertonic; D014667:Vasopressins; D000804:Angiotensin II; D009320:Atrial Natriuretic Factor; D012964:Sodium; D011188:Potassium