PREVENTING HEMODILUTION ABOLISHES NATRIURESIS OF WATER IMMERSION IN HUMANS

The hypothesis was tested that hemodilution is one of the determinants of the water immersion (WI)-induced natriuresis. Eight males were sub jected to 3 h of 1) WE to the midchest (Chest), 2)WI to the neck combi ned with thigh cuff-induced (80 mmHg) venous stasis (Neck + stasis), a nd 3) a seated time control (n = 6). Central venous pressure and left atrial diameter increased to the same extent during Chest and Neck + s tasis (P < 0.05), whereas renal sodium excretion only increased during Chest from 77 +/- 7 to 225 +/- 13 mu mol/min (P < 0.05). During Chest , plasma colloid osmotic pressure (COP) decreased from 27.7 +/- 0.7 to 25.1 +/- 0.7 mmHg (P < 0.05), and plasma volume (PV) increased from 3 ,263 +/- 129 to 3,581 +/- 159 ml (P < 0.05), whereas these variables r emained unchanged during Neck + stasis. Plasma norepinephrine concentr ation decreased similarly during Chest and Neck + stasis by 45 +/- 7 a nd 34 +/- 4%, respectively (P < 0.05), whereas plasma renin activity d ecreased only during Chest (P < 0.05). In conclusion, during WI in hum ans 1) hemodilution (decrease in COP and increase in PV) is a pivotal stimulus for the natriuresis and 2) central blood volume expansion wit hout hemodilution does not augment renal sodium output.