Plasma sodium-osmotic dissociation and hormonal interaction with drinking-induced hypervolemia at 2800 m altitude
Greenleaf, J.E.; Hinghofer-Szalkay, H.; Rössler, A.; Farrell, P.A.; Loomis, J.L.; Fedele, M.J.; West, J.; Cowell, S.A.; Greenleaf, J.E.
Aviation Space and Environmental Medicine 72(6): 522-528
2001
ISSN/ISBN: 0095-6562 PMID: 11396557 Document Number: 529826
Purpose: To study hormonal factors that may account for the dissociation between beverage-induced plasma sodium p(Na+) and osmotic p(Osm) concentrations that appear to refute the high theoretical correlation between p(Na+) and p(Osm). Methods: Ten men (24 +- SD 3 yr of age) sat reclining (head up) for 12 h in a chamber (21-23degreeC dry bulb, 25-33% relative humidity) at 2800 m (9184 ft, 539 mm Hg) altitude (ALT), and at 321 m (1053 ft, 732 mm Hg) on the ground (GND). During 1000-1030 hours they consumed 3 fluids (12 ml cntdot kg-1, X = 948 ml cntdot d-1) with large differences in sodium and osmotic contents: AstroAde (AA) with 185 mEq cntdot L-1 Na+ and 283 mOsm cntdot kg-1, Performance 1TM (Shaklee) (P1) with 22 mEq cntdot L-1 Na+ and 365 mOsm cntdot kg-1, or H2O at ALT; and only H2O on the GND. Results: After drinking: plasma volume (PV) increased at 1200 hours by 8.3% (p < 0.05) with AA but was not significantly (NS) changed in the other sessions (XDELTA = +0.9%, range -0.9 to 2.8%); p(Na+) and p(Osm) were unchanged. Urinary rates and free-water clearances were attenuated with AA and P1 vs. those with H2O. Correlations between and among p(Na+) and p(Osm) suggest that the pNa+ ion is more tightly controlled than pOsm; and that there was no clear hormonal response that could account for this dissociation from theoretical considerations. Conclusions: There is significant dissociation between plasma sodium and osmotic concentrations after fluid intake. Induction and maintenance of hypervolemia requires increased (near isotonic) drink Na+ osmols rather than increased non-ionic osmols.