Bed rest immobilization with various oral sodium supply: plasma hormones and body fluids

To answer the question if plasma hormone concentrations (plasma renin activity--PRA, vasopressin--pAVP, and aldosterone concentration) due to antiorthostatic immobilization (8 days -6 degree head-down tilt bed rest--HDBR) are altered by oral salt load, we provided constant sodium supply during 4 days ambulatory conditions followed by 8 days HDBR in 10 normotensive men.

A low' (LS: 143 10 mM) and high' (HS: 434+/-17 mM Na+/d excreted) sodium treatment were provided in randomized order, separated > or = 1 mo. Before and at the end of HDBR, hemodynamic variables and thoracic impedance were determined, and blood was taken for aldosterone and PRA, venous hematocrit, and plasma mass density. Extracellular fluid volume and pAVP were determined every second day. Whole body electrical impedance spectroscopy was employed to assess changes in extracellular volume, hormone determinations were done with radioimmunoassay, mass density measurements with the mechanical oscillator technique.

Extracellular volume decreased with HDBR (LS: -4.0%, p=0.002; HS: -5.8%, p=0.018) without significant difference between salt treatments. Resting hormone levels were not altered by HDBR, but pAVP was lower (5.5+/-0.1 pg/ml) in HS than in LS (7.2+/-0.3 pg/ml) as was plasma aldosterone (HS: 69+/-7 pg/ml, LS: 180+/-24 pg/ml). On the other hand, HDBR reduced extracellular volume by ?5% irrespective of dietary sodium supply.

Our data support the hypothesis that hormonal activities are more affected by oral salt load than by simulated short-term space flight, and suggest that the reduction of extracellular fluid volume due to head down bed rest is not influenced by moderate changes of dietary sodium supply.