Renal and hormonal responses to isotonic saline infusion after 3 days' head-down tilt vs. supine and seated positions.
OBJECTIVE
The study aimed to determine whether prolonged exposure to simulated microgravity produces a level of thoracic volume receptor loading similar to that seen in the upright position or immediately after lying down.


METHODS
We used a cross-over design to compare responses to a saline infusion in eight healthy subjects during a 4-day, -6 degree head-down tilt (HDT) and in the acute seated and acute supine positions.


RESULTS
The first 24 h of HDT were associated with greater urinary excretion of water and sodium (UV, UNaV) than seated and acute supine [cumulative UV, 3035 +/- 219, 2311 +/- 156 (P < 0.05), and 2448 +/- 182 mL (P < 0.05), respectively; cumulative UNaV, 256 +/- 19, 180 +/- 11 (P < 0.05), and 189 +/- 15 mmol (P < 0.05), respectively]. Haemoglobin and haematocrit were increased after 24 h and plasma volume decreased after 48 h of HDT (P < 0.05). With prolongation of HDT, UV and UNaV returned near the baseline values, and plasma atrial natriuretic factor (ANF) and renin values returned to acute seated levels; in acute supine, ANF values were higher and renin lower than in the two other positions. After a 30-min infusion of 20 mL kg(-1) isotonic saline on the fourth HDT day or during acute seated or acute supine, sodium excretion within 4 h was similar during HDT and acute seated (83 +/- 6 and 84 +/- 9 mmol, respectively) and greater during supine (104 +/- 8 mmol, P < 0.05). The renin decrease was greater in HDT and seated than in supine. The plasma ANF increase was greater during HDT than during supine; during seated, plasma ANF was unchanged.


CONCLUSIONS
These data suggest that, after 4 days of HDT, thoracic volume receptor loading returns to the same level as in the seated position, leading to blunted responses to volume expansion as compared with the acute supine position.
Mauran
P
P
Department de Physioloogie de la Faculté de Médecine de Reims, American Memorial Hospital, France.
Sediame
S
S
Pavy-Le Traon
A
A
Maillet
A
A
Carayon
A
A
Barthelemy
C
C
Weerts
G
G
Guell
A
A
Adnot
S
S
D000328:Adult; D000450:Aldosterone / Q000097:blood; D009320:Atrial Natriuretic Factor / Q000097:blood; D001794:Blood Pressure / Q000187:drug effects; D018592:Cross-Over Studies; D018475:Head-Down Tilt / Q000502:physiology*; D006339:Heart Rate / Q000187:drug effects; D006400:Hematocrit; D006454:Hemoglobins / Q000032:analysis; D006801:Humans; D007552:Isotonic Solutions; D007668:Kidney / Q000187:drug effects* / Q000502:physiology; D008297:Male; D010953:Plasma Volume / Q000502:physiology; D012964:Sodium / Q000652:urine; D012965:Sodium Chloride / Q000494:pharmacology*; D016683:Supine Position / Q000502:physiology*; D014554:Urination / Q000187:drug effects / Q000502:physiology; D018474:Weightlessness Simulation
D006454:Hemoglobins; D007552:Isotonic Solutions; D012965:Sodium Chloride; D000450:Aldosterone; D009320:Atrial Natriuretic Factor; D012964:Sodium