HEMODILUTION, CENTRAL BLOOD-VOLUME, AND RENAL RESPONSES AFTER AN ISOTONIC SALINE INFUSION IN HUMANS

To test the hypothesis that hemodilution is a mediator of the renal re sponses to an isotonic saline infusion in the supine position, eight m ales underwent 1) intravenous infusion of 1.5 liter of saline over 21 min (Saline), 2) infusion of 1.5 liter of saline in combination with l ower body negative pressure for 3 h (LBNP+Saline) to maintain central blood volume unchanged, and 3) a control study without infusion or LBN P During the Saline series, central venous pressure (CVP) and left. at rial diameter (LAD) increased by 4.4 +/- 0.6 mmHg and 2.6 +/- 0.4 mm ( P < 0.05), respectively, whereafter they declined toward preinfusion l evels. During LBNP+Saline, CVP and LAD were unchanged. Plasma colloid osmotic pressure remained unchanged during control and showed identica l decreases by 5 mmHg (P < 0.05) in the Saline and LBNP+Saline series. During the 3rd h of LBNP, renal sodium excretion (UNaV) peaked at 296 +/- 55 mu mol/min vs. a higher value of 383 +/- 54 mu mol/min (P < 0. 05) during Saline. The increase in UNaV above that of control during t he 3rd h of LBNP+Saline constituted 48% of that during Saline. Plasma renin activity and plasma aldosterone concentration showed similar pat terns of decrease after saline infusion irrespective of LBNP, whereas plasma norepinephrine was elevated late in the LBNP period compared wi th during Saline and control (P < 0.05). It is concluded that the main tenance of a constant CVP and LAD reduces the natriuresis of acute sal ine loading by about one-half. Thus hemodilution in conjunction with s uppression of renin and aldosterone release (independent of change in CVP and LAD) might account for the remaining natriuresis of infusion.