Studies on the Renal Concentrating Mechanism. Ii. Effect of Small Acute Changes in Solute Excretion*

Abstract

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. METHODSThe subjects were four paid volunteer male medical students who were selected for their ability to void small volumes of urine so that catheterization could be avoided

Studies on the Renal Concentrating Mechanism. Iii. Effect of Heavy Exercise

“…Plasma osmolality usually increased after exercise and remained increased thereafter, the largest increment being 8 The effect of exercise on the concentrating ability was more difficult to evaluate during an osmotic diuresis. From previous studies (13,15), an increase in osmotic U/P ratio would be expected when osmolal clearance decreases. Such an increase in osmotic U/P ratio was observed during exercise only in the experiments with large mannitol loads.…”

“…The effect could not be prevented by the administration of vasopressin and is therefore considered not to be the consequence of a decrease in endogenous antidiuretic hormone production. It is difficult to ascribe the decrease in osmotic U/P ratio to the decrease in total solute excretion which occurs with heavy exercise, since in most other circumstances an inverse relationship between U/P ratio and solute excretion is observed (13). On the other hand a marked decrease in sodium excretion might be an important factor, since there is evidence that active sodium transport may be the initial process leading to the formation of a concentrated urine (16,17).…”

“…The medical students were selected for their ability to pass the "bladder emptying test" described in the preceding paper (13 Chemical methods and calculations are given in the preceding report (13). Glomerular filtration rate was estimated by the clearances of creatinine and mannitol.…”

Renal Concentrating Mechanisms in Newborn Infants. Effect of Dietary Protein and Water Content, Role of Urea, and Responsiveness to Anti-Diuretic Hormone *

“…Despite this presumably greater volume of isotonic fluid delivered to the concentrating site and the increased solute load, urine osmolality and urine U/P osmolal ratio were increased, suggesting (33) an enhanced rate of reabsorption of water in the collecting tubule (TCH2o). Addition of urea to the diet, in place of protein, resulted in similar findings, indicating further that the increased concentrating performance observed following high protein feeding most likely resulted simply from the provision of additional urea for excretion.…”

Studies on the Renal Concentrating Mechanism. Iv. Osmotic Diuresis*

“…Although concentrating ability appears to be limited by a maximal osmotic urine/plasma (U/P) ratio, small increases in solute excretion have been shown to lead to a decrease in osmotic U/P ratio even when urine flow remains less than 2 ml. per minute (3). At high flows, when there is a marked decrease in osmotic U/P ratio, there is evidence that the concentrating mechanism may be limited by a constant maximal rate of net water reabsorption (TCmH20) (4).…”