Studies on the renal concentrating mechanism. II. Effect of small acute changes in solute excretion.

The renal concentrating mechanism is considered to be limited by a maximal urine osmolal concentration or a maximal osmotic urine/plasma (U/P) ratio at low rates of solute excretion (1). When solute excretion is increased by the administration of large osmotic loads to hvdropenic subjects urine osmolality decreases (2-4). This decrease has recently been ascribed to the limitation imposed by the existence of a maximal rate of net water reabsorption (termed TmCH2O) at high rates of solute excretion and urine flow (5-7). When no osmotic load is given, solute excretion is low in hydropenic subjects so that the net rate of water reabsorption is much below its maximum. Under these circumstances the existence of a constant maximal "osmotic ceiling" independent of small changes in solute excretion and urine flow has been postulated (1, 7), but such a limit has not yet been clearly demonstrated experimentally. The present study was therefore undertaken to assess the effects of small, acute changes in solute excretion on the osmotic U/P ratio observed in hydropenic normal men. Osmotic U/P ratio was not constant but tended to vary inversely with the concurrent rate of solute excretion.