Effect of Prehydration Upon Renal Excretion of Sodium in Man

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Abstract
The induction of maximal water diuresis (which itself does not immediately influence Na excretion) leaves a trace effect that materially increases the renal excretion of Na 8-13 hrs. later. At this time, when a test dose (3 1.) of isotonic saline is administered intraven., both the Na and water are copiously excreted. The increase in Na excretion is due in great measure to decreased tubular re-absorption rather than to increased filtration. Such prehydrated subjects excrete in 100 min. some 20% of the administered Na and 40% of the administered water, whereas the corresponding figures in unhydrated subjects are 5 and 20%. The 8- to 13-hour prehistory of the subject with respect to hydration is therefore important in influencing the excretion of salt and water after isotonic saline admn.