DIFFERENCES BETWEEN SALINE AND MANNITOL DIURESIS IN HYDROPENIC MAN

Comparison of the effects of 7.2% hypertonic saline and 20% mannitol on whole blood coagulation and platelet function in dogs with suspected intracranial hypertension - a pilot study

BACKGROUND

Hyperosmolar therapy with either mannitol or hypertonic saline (HTS) is commonly used in the treatment of intracranial hypertension (ICH). In vitro data indicate that both mannitol and HTS affect coagulation and platelet function in dogs. The aim of this study was to compare the effects of 20% mannitol and 7.2% HTS on whole blood coagulation using rotational thromboelastometry (ROTEM®) and platelet function using a platelet function analyzer (PFA®) in dogs with suspected ICH. Thirty client-owned dogs with suspected ICH needing osmotherapy were randomized to receive either 20% mannitol (5 ml/kg IV over 15 min) or 7.2% HTS (4 ml/kg IV over 5 min). ROTEM® (EXTEM® and FIBTEM® assays) and PFA® analyses (collagen/ADP cartridges) were performed before (T0), as well as 5 (T5), 60 (T60) and 120 (T120) minutes after administration of HTS or mannitol. Data at T5, T60 and T120 were analyzed as a percentage of values at T0 for comparison between groups, and as absolute values for comparison between time points, respectively.

RESULTS

No significant difference was found between the groups for the percentage change of any parameter at any time point except for FIBTEM® clotting time. Within each group, no significant difference was found between time points for any parameter except for FIBTEM® clotting time in the HTS group, and EXTEM® and FIBTEM® maximum clot firmness in the mannitol group. Median ROTEM® values lay within institutional reference intervals in both groups at all time points, whereas median PFA® values were above the reference intervals at T5 (both groups) and T60 (HTS group).

CONCLUSIONS

Using currently recommended doses, mannitol and HTS do not differ in their effects on whole blood coagulation and platelet function in dogs with suspected ICH. Moreover, no relevant impairment of whole blood coagulation was found following treatment with either solution, whereas a short-lived impairment of platelet function was found after both solutions.

Mannitol increases renal blood flow and maintains filtration fraction and oxygenation in postoperative acute kidney injury: a prospective interventional study

Introduction

Acute kidney injury (AKI), which is a major complication after cardiovascular surgery, is associated with significant morbidity and mortality. Diuretic agents are frequently used to improve urine output and to facilitate fluid management in these patients. Mannitol, an osmotic diuretic, is used in the perioperative setting in the belief that it exerts reno-protective properties. In a recent study on uncomplicated postcardiac-surgery patients with normal renal function, mannitol increased glomerular filtration rate (GFR), possibly by a deswelling effect on tubular cells. Furthermore, experimental studies have previously shown that renal ischemia causes an endothelial cell injury and dysfunction followed by endothelial cell edema. We studied the effects of mannitol on renal blood flow (RBF), glomerular filtration rate (GFR), renal oxygen consumption (RVO₂), and extraction (RO₂Ex) in early, ischemic AKI after cardiac surgery.

Methods

Eleven patients with AKI were studied during propofol sedation and mechanical ventilation 2 to 6 days after complicated cardiac surgery. All patients had severe heart failure treated with one (100%) or two (73%) inotropic agents and intraaortic balloon pump (36%). Systemic hemodynamics were measured with a pulmonary artery catheter. RBF and renal filtration fraction (FF) were measured by the renal vein thermo-dilution technique and by renal extraction of chromium-51-ethylenediaminetetraacetic acid (⁵¹Cr-EDTA), respectively. GFR was calculated as the product of FF and renal plasma flow RBF × (1-hematocrit). RVO₂ and RO₂Ex were calculated from arterial and renal vein blood samples according to standard formulae. After control measurements, a bolus dose of mannitol, 225 mg/kg, was given, followed by an infusion at a rate of 75 mg/kg/h for two 30-minute periods.

Results

Mannitol did not affect cardiac index or cardiac filling pressures. Mannitol increased urine flow by 61% (P < 0.001). This was accompanied by a 12% increase in RBF (P < 0.05) and a 13% decrease in renal vascular resistance (P < 0.05). Mannitol increased the RBF/cardiac output (CO) relation (P = 0.040). Mannitol caused no significant changes in RO₂Ext or renal FF.

Conclusions

Mannitol treatment of postoperative AKI induces a renal vasodilation and redistributes systemic blood flow to the kidneys. Mannitol does not affect filtration fraction or renal oxygenation, suggestive of balanced increases in perfusion/filtration and oxygen demand/supply.

Effects of mannitol alone and mannitol plus furosemide on renal oxygen consumption, blood flow and glomerular filtration after cardiac surgery

OBJECTIVE

Imbalance of the renal medullary oxygen supply/demand relationship can cause hypoxic medullary damage and ischaemic acute renal failure (ARF). The use of mannitol for prophylaxis/treatment of clinical ischaemic ARF is controversial and the effect of mannitol on renal oxygenation in man has not yet been investigated. We evaluated the effects of mannitol on renal oxygen consumption (RVO(2))(,) renal blood flow (RBF) and glomerular filtration rate (GFR) in postoperative patients.

DESIGN

Prospective interventional study.

SETTING

University hospital cardiothoracic ICU.

PATIENTS

Ten uncomplicated mechanically ventilated and sedated postcardiac surgery patients with preoperatively normal renal function.

INTERVENTIONS

Mannitol infusion (225 mg/kg + 75 mg/kg/h) and combined mannitol and furosemide infusion (0.25 mg/kg + 0.25 mg/kg/h).

MEASUREMENTS AND RESULTS

Systemic haemodynamics were evaluated by a pulmonary artery catheter. RBF and GFR were measured by the renal vein thermodilution technique and by renal extraction of (51)Cr-EDTA, respectively. Mannitol increased urine flow (60%), GFR (20%) and filtration fraction (FF) (20%) with no change in RBF. This was accompanied by an increase in renal sodium reabsorption (18%), RVO(2) (19%) and renal oxygen extraction (21%). When combined with mannitol, furosemide normalised sodium reabsorption, RVO(2), renal oxygen extraction with no change in RBF, while GFR and FF were still elevated compared to control.

CONCLUSIONS

In patients with normal renal function, mannitol increases GFR, which increases tubular sodium load, sodium reabsorption and RVO(2) after cardiac surgery. The lack of effect on RBF, indicates that mannitol impairs the renal oxygen supply/demand relationship. Furosemide normalised renal oxygenation when combined with mannitol.

The renal kallikrein-kinin system: its role as a safety valve for excess sodium intake, and its attenuation as a possible etiologic factor in salt-sensitive hypertension

The distal tubules of the kidney express the full set of the components of the kallikrein-kinin system, which works independently from the plasma kallikrein-kinin system. Studies on the role of the renal kallikrein-kinin system, using congenitally kininogen-deficient Brown-Norway Katholiek rats and also bradykinin B2 receptor knockout mice, revealed that this system starts to function and to induce natriuresis and diuresis when sodium accumulates in the body as a result of excess sodium intake or aldosterone release, for example, by angiotensin II. Thus, it can be hypothesized that the system works as a safety valve for sodium accumulation. The large numbers of studies on hypertensive animal models and on essential hypertensive patients, particularly those with salt sensitivity, indicate a tendency toward the reduced excretion of urinary kallikrein, although this reduction is modified by potassium intake and impaired renal function. We hypothesize that the reduced excretion of the renal kallikrein may be attributable to a genetic defect of factor(s) in renal kallikrein secretion process and may cause salt-sensitive hypertension after salt intake.

Antagonism of V2-receptor effect of antidiuretic hormone by atrial natriuretic peptide in man

Human alpha-atrial natriuretic peptide (h-alpha ANP) makes the urine of dehydrated volunteers hypotonic to plasma despite high circulating concentrations of antidiuretic hormone. Urinary dilution with h-alpha ANP also occurs in subjects receiving indomethacin. Therefore, h-alpha ANP antagonises effects of antidiuretic hormone on distal tubular V2-receptors in man, probably without involving prostaglandins.

Renal mechanisms of human alpha-atrial natriuretic peptide in man

1. Eight normal volunteers were studied on two separate days after being dehydrated overnight. Each volunteer received a background intravenous infusion of arginine vasopressin (5.5 X 10(-7) i.u. kg-1 min-1) on both days and also received an intravenous infusion of human alpha-atrial natriuretic peptide (15 pmol kg-1 min-1) plus carrier on one day and carrier alone on the other. The ensuing changes in blood pressure, in the excretion of urinary solutes, and in the excretion of solute-free water were recorded. 2. The infusion of atrial peptide had a small hypotensive effect, and increased the rate of excretion of sodium but not of potassium. There were no significant changes of urinary osmolality or of creatinine clearance. 3. The infusion of atrial peptide increased the rate of solute-free water reabsorption and did so in direct proportion to its effect of increasing sodium excretion. 4. A further six normal, dehydrated volunteers were studied on each of two days after taking 500 mg of lithium carbonate on the previous evening. On one day, they received an intravenous infusion of human alpha-atrial natriuretic peptide (15 pmol kg-1 min-1) plus carrier and on the other day they received carrier alone. The excretion of urinary electrolytes and the creatinine clearance were recorded. 5. The infusion of atrial peptide produced significant increases in the rates of excretion of both sodium and lithium, but there were no such changes of creatinine clearance. 6. Another six normal volunteers were studied on each of two days. On each day they drank 2 l of water over 30 min and then water to replace their urinary losses. They also received loading doses and maintenance infusions of inulin and sodium para-aminohippurate. Once a full water diuresis had become established, each subject received an infusion of human alpha-atrial natriuretic peptide (15 pmol kg-1 min-1) plus carrier on one day and carrier alone on the other, exactly as before. The excretion of sodium and solute-free water, and the clearances of inulin and para-amino-hippurate were recorded. 7. The infusion of atrial peptide increased the rates of excretion of both sodium and solute-free water. It also increased the clearance of inulin, but not that of para-aminohippurate. 8. These results suggest that, in our volunteers, infusion of human alpha-atrial natriuretic peptide increases sodium excretion mainly by increasing the delivery of sodium along the renal tubule from sites upstream of the loop of Henle.(ABSTRACT TRUNCATED AT 400 WORDS)

Relevance of plasma, glandular and urinary kallikrein in renal hypertrophy in streptozotocin-diabetic rats

The relevance of plasma, glandular and renal kallikrein as an intrarenal hemodynamic regulator, in renal hypertrophy, in 1-5 weeks streptozotocin diabetic rats has been investigated. The fasting plasma glandular kallikrein level significantly decreased with increasing duration of diabetes (p less than 0.05). Glandular kallikrein correlated negatively with kidney weight (r = 0.76, p = 0.05). The 24 hour urinary kallikrein excretion significantly increased with increasing duration of diabetes (p less than 0.05), but this level was not correlated with glucose level, nor with kidney weight. Aprotinin (a kallikrein inhibitor) injected (10 X 10(3) KIU/kg) twice daily for 2 weeks in diabetic rats, significantly decreased plasma glucose levels by 28%, 24 hour urinary kallikrein by 37% (p less than 0.05) and kidney weight by 6%. These results suggest that plasma, glandular and renal kallikrein did not play an important role in the renal hypertrophy observed in streptozotocin diabetic rats.

The variable hyponatremic response to hyperglycemia

Hyperglycemia may lower the plasma sodium concentration. Theoretical analyses have suggested that elevations in glucose concentration produce an invariant hyponatremic response. We propose, however, that change in plasma sodium concentration in response to hyperglycemia is variable and depends on (1) the distribution of total body water and solute, (2) the relationship between the gain of extracellular glucose and the loss of intracellular solute and (3) the intake and loss of solute and water. These factors are incorporated into a formulation of the relationship between the plasma sodium and glucose concentrations.

Renal response to graded intravenous hypertonic NaCl infusion in healthy and hypertensive subjects:dose-related impairment in distal NaCl reabsorption

The effects of graded acute intravenous hypertonic NaCl loads on the baseline relationship between osmolal clearance and free water reabsorption established during high NaCl dietary intake and on the fractional excretion of various ions were investigated in 15 healthy subjects and in 12 "normal renin" essential hypertensive patients. No significant influence on the baseline relationship could be demonstrated after a moderate NaCl load in the healthy subjects, while free water reabsorption was depressed by the same intervention in the hypertensive patients. High NaCl loads induced depression of free water reabsorption in a dose-related fashion in both groups. No difference was found in phosphaturia between the groups after the same NaCl load as well as in the healthy persons after different NaCl loads, supporting the contention that the observed differences in free water reabsorption were not due to changes in the proximal neprhon. It was concluded that: (1) impaired NaCl reabsorption in Henle's loop (depression of free water reabsorption) may also occur in response to acute NaCl loadings in healthy subjects, and (2) "exaggerated natriuresis" is the consequence of a normal renal response (impaired NaCl transport in Henle's loop) to a certain degree of volume expansion reset abnormally to a lower level in hypertensive patients.

Mannitol osmolar clearance in diabetes insipidus of children

A modified technique of amnnitol-induced diuresis is described, in order to assess renal concentrating ability in infants and children. The infusion of 10% mannitol in 0.9% saline avoided the hypertonic saline overload and the fluid restriction period, both badly tolerated by infants and small children. In a control group of children aged from two months to seven years, the values of T(H2O) plotted against C(OSM) allowed to calculate the adjustment curve y=0.80x0.75, r=0.98 (p is less than 0.0001). In six patients with pituitary diabetes insipidus (PDI), the test was used in order to quantify the degree of ADH deficiency and evaluate the carbamazepine and clofibrate effect, in the renal concentrating mechanism. The test was tolerated perfectly in every case, obtaining qualitative and quantitative data and avoiding the hyponatremia and hypokalemia produced by the mannitol.

Effect of cold exposure of free water reabsorption in hydropenic man

With clearance technique, changes in tubular reabsorption of solute free water (TcH2O), were studied in a group of 9 healthy hydropenic volunteers during a cold stress of 1 h at +15 degrees C. Increases were found in TcH2O and in fractional excretion of sodium and osmoles during cold. Since TcH2O was used as an index of sodium reabsorption in the ascending limb of Henle's loop, the findings indicated that under the conditions used in this study, the concentrating ability of the kidney was not negatively influenced by cold, despite a significant increase in arterial pressure. It is suggested that the reabsorption by the ascending limb of the loop was increased secondary to a reduced proximal tubular reabsorption of sodium.

Urine concentration during solute diuresis in potassium-depleted rabbits. Evidence for a defect in tubular sodium transport

1. The relationship between osmolal clearance (C(osm)) and the reabsorption of solute-free water by the kidney (T(H2O) (c)) was examined during 10% mannitol and 2.3% saline diuresis in normal and potassium-depleted rabbits.2. In normal rabbits at osmolal clearances close to 3.0 ml./min, T(H2O) (c) during mannitol diuresis was 0.87 +/- 0.06 ml./min and during saline diuresis 1.19 +/- 0.07 ml./min. The mean difference in T(H2O) (c) of 0.32 +/- 0.05 ml./min was highly significant (P < 0.001).3. In one group of potassium-depleted rabbits with a reduction in maximal urinary concentration, T(H2O) (c) during both mannitol and saline diureses was reduced significantly below normal and the increment in T(H2O) (c) normally seen during saline diuresis was abolished.4. In a second group of potassium-depleted rabbits maximal urinary concentration (1253 +/- 88 m-osmole/kg H(2)O) was not significantly different from that in normal rabbits (1272 +/- 116 m-osmole/kg H(2)O). In these animals, T(H2O) (c) at osmolal clearances close to 3.0 ml./min was not significantly different during mannitol diuresis (0.83 +/- 0.07 ml./min) from that in normal animals, whereas it was reduced significantly during saline diuresis (0.89 +/- 0.07 ml./min, P < 0.001) and the difference in T(H2O) (c) normally seen between mannitol and saline diuresis was abolished.5. The inability to increase T(H2O) (c) during saline diuresis above that achieved during mannitol appears to be the earliest manifestation of the concentrating defect associated with potassium depletion. It probably results from an impairment of sodium transport by the ascending limb of the loop of Henle. This is supported by the fact that potassium-depleted rabbits excreted a greater percentage of the filtered load of sodium than did normal controls.

Effects of increased sodium delivery on distal tubular sodium reabsorption with and without volume expansion in man

The separate effects of volume expansion and of increased delivery of sodium on sodium reabsorption in the diluting segment of the distal nephron were studied in man. In six normal subjects during a sustained water diuresis, sodium delivery to the distal nephron was increased without volume expansion by the administration of acetazolamide. In these subjects, free water clearance rose linearly as a function of urine flow. In five patients with complete, central diabetes insipidus, distal sodium delivery was increased by the infusion of hypertonic saline during a sustained water diuresis. In four of these five patients, changes in free water clearance were also observed during hypertonic saline diuresis in the presence of distal blockade of sodium reabsorption with chlorothiazide. At high rates of distal delivery the following observations were made: (a) free water clearance was lower and fractional sodium excretion higher during saline diuresis compared to acetazolamide diuresis; (b) although free water clearance was moderately reduced by chlorothiazide at low rates of urine flow, there was no difference in free water clearance between saline loading alone and saline plus chlorothiazide at high rates of urine flow; and (c) during saline loading free water clearance rose without evidence of a limit when increased distal delivery was accompanied by spontaneous increases in glomerular filtration rate, but tended toward a limit when glomerular filtration rate remained constant. The data indicate that during acute volume expansion with saline, there is a decrease in the fraction of delivered sodium reabsorbed in the distal nephron when compared to the response of the distal nephron to comparable increases in distal sodium delivery in the absence of volume expansion.

Effects of 0-9 per cent saline infusion on urinary and renal tissue composition in the hydropaenic, normal and hydrated conscious rat

1. Changes in water and solute outputs of hydropaenic, normal and hydrated conscious rats were determined during intravenous infusion (0.2 ml./min) of isotonic (0.9%) saline for 4 hr; renal tissue composition was determined before, and after 1 or 2 hr, infusion.2. In normal and hydrated rats increased excretion of water and sodium was such that urinary output matched intravenous input from about 2 hr. In hydropaenic rats, the diuretic and natriuretic response was much reduced; a retention of infused saline, equivalent to 15% body weight, occurred over 4 hr.3. A considerable increase in urea output and clearance, and a smaller increase in potassium and ammonium outputs, occurred in all groups.4. The corticomedullary osmolal gradients characteristic of non-diuretic rats were largely dissipated during saline infusion: by 1 hr in normal and hydrated rats, and by 2 hr in the hydropaenic group.5. These changes were ascribable mainly to an increase in tissue water content in all segments, particularly in the hydropaenic group; and to a profound decrease in urea content in all groups.6. Changes in tissue sodium content were smaller, and differed between segments and between the differently hydrated groups. A decrease in papillary content occurred in hydropaenic and normal groups and an increase in cortical and outer medullary content occurred in all groups.7. After 2 hr saline infusion, incomplete papillary-urinary osmotic equilibration was evident in all groups.8. These changes in medullary osmolality and in papillary-urinary osmotic equilibration preceded the maximal diuresis, and must contribute to the diuresis induced by saline infusion, as in water and osmotic diureses.

Mechanism of exaggerated natriuresis in hypertensive man: impaired sodium transport in the loop of Henle

To evaluate the effects of saline loading on distal sodium reabsorption in hypertensive man, studies were performed during both water deprivation and water diuresis in eight hypertensive subjects, and the results were compared to data obtained from similar studies in normal subjects. All hypertensive patients exhibited an enhanced excretion of filtered sodium (C(Na)/C(In)) at any level of distal delivery of sodium compared to normal controls. Free water reabsorption (T(c) (H2O)) during hypertonic saline loading was quantitatively abnormal in the hypertensives at high levels of osmolar clearance (C(Osm)), and also the curve of T(c) (H2O) vs. C(Osm) leveled off above a C(Osm) of 18 ml/min per 1.73 m(2) in the hypertensive group in contrast to the normal controls in whom T(c) (H2O) showed no evidence of achieving an upper limit. Sodium depletion exaggerated the abnormality in T(c) (H2O) in hypertensives, and resulted in a positive free water clearance (C(H2O)) during hydropenia. During hypotonic saline loading in water diuresis, changes in free water clearance per 100 ml of glomerular filtrate (C(H2O)/C(In)) were less at any given increment in urine flow per 100 ml of glomerular filtrate (V/C(In)) in the hypertensives compared to normal controls (P < 0.001). This abnormality in C(H2O)/C(In) in the hypertensives in conjunction with the defect in T(c) (H2O) observed during hydropenia indicates that sodium reabsorption in the loop of Henle was abnormal at any given rate of distal delivery of sodium in hypertension. Furthermore, these abnormalities in T(c) (H2O) and C(H2O) coincided temporally with the development of the exaggerated natriuresis. Although the distal defect in sodium transport, in large part, accounted for the augmented natriuresis in hypertension, evidence was present also for enhanced rejection of sodium in the proximal tubule during saline loading in the hypertensives. Additional studies utilizing acetazolamide which increases distal delivery of sodium without extracellular fluid volume expansion showed only minimal abnormalities in C(H2O) in the hypertensive group, indicating that the defect in sodium transport in the loop of Henle in hypertensives is mainly an abnormal response to extracellular fluid expansion rather than an intrinsic defect in the loop to handle increased tubular loads of sodium. It is possible that the abnormality in sodium reabsorption in the loop of Henle is due to the transmission of the abnormally elevated blood pressure of the hypertensives to the medullary vasa recta during saline loading.

The effects of infusion of water on renal hemodynamics and the tubular reabsorption of sodium

Anesthetized dogs receiving an infusion of chlorothiazide and ethacrynic acid were given 600-ml infusions of distilled water or dilute dextrose solutions. The absolute rate of tubular sodium reabsorption was depressed, and the glomerular filtration rate was increased during the water loading, despite the associated decreases in plasma sodium concentration and decreases in the filtered load of sodium. The extent to which fractional sodium reabsorption decreased and the excretion of sodium increased was inversely related to the degree to which the filtered load of sodium was depressed as a result of the decreased plasma sodium concentration. We conclude that, in the presence of the diuretic blockade of distal tubular sodium reabsorption, infusion of water depresses proximal tubular reabsorption of sodium and that these changes are qualitatively similar to those previously observed during infusions of saline. Similar depression of tubular reabsorption of sodium and increased excretion of sodium occurred during water loading in the absence of diuretics in dogs undergoing saline diuresis, which presumably provided a high rate of distal sodium reabsorption before water loading. We suggest that volume expansion with water depresses proximal tubular reabsorption of sodium in a manner qualitatively similar to infusions of saline and that the extent to which sodium excretion is increased during water loading is dependent upon 1) the absolute extent to which proximal reabsorption is depressed, 2) the extent to which the filtered load of sodium is maintained in the presence of a falling concentration of sodium in plasma, and 3) the extent to which increased distal reabsorption compensates for the depressed proximal reabsorption of sodium. Mechanisms are suggested whereby the previously reported inverse relationship between plasma concentration of sodium and over-all tubular reabsorption of sodium may be only apparent, and could be the result of physiologic "glomerulotubular balance" during the specific experimental maneuvers.

Effects of hypotonic saline loading in hydrated dog: evidence for a saline-induced limit on distal tubular sodium transport

We performed studies on dogs under hydrated conditions, utilizing the rate of free water formation (C(H2O)) as an index of the rate of distal tubular sodium transport. Since C(H2O) could be progressively increased with no evidence of a maximal rate during loading with hypotonic (2.5%) mannitol, it was concluded that there is no limit on distal tubular sodium transport during mannitol loading. In contrast, during hypotonic (0.45%) saline loading C(H2O) rose initially, but as urine flow (V) exceeded 25% of the filtered load C(H2O) attained maximal levels (up to 20% of the filtered load) and remained stable as V increased to 50% of the filtered load. It was concluded that saline loading progressively inhibits proximal sodium reabsorption. Initially, the distal tubule absorbes a large fraction of the proximal rejectate and sodium excretion rises slightly. Eventually, an alteration in distal sodium transport appears which culminates in a maximal rate or transport limit. This distal transport limit provoked by saline loading could not be characterized by a classical Tm as seen with glucose and does not seem to be consequent to high rates of flow through the distal tubule. Regardless of the precise nature of this limit, the major increment in sodium excretion develops during saline loading only after saline alters the capacity of the distal tubule to transport sodium.

Nature of the renal concentrating defect in sickle cell disease

Free water reabsorption (T(c) (H2O)) measured during 10% mannitol diuresis and subsequently during 3% saline diuresis was compared in patients with sickle cell anemia and in normal subjects. During mannitol infusion, T(c) (H2O) progressively rose with increasing osmolar clearance (C(osm)) and reached a maximal level in both groups studied. During hypertonic saline diuresis, T(c) (H2O) progressively rose in the normal subjects and exceeded the maximal levels attained during mannitol diuresis, with no evidence of a maximal T(c) (H2O) level appearing. In contrast, none of the saline curves significantly exceeded the mannitol curves in the sickle cell patients but tended to parallel the mannitol curves at comparable rates of solute clearance. Since T(c) (H2O) is an index of both solute (sodium) transport from the loop of Henle and solute accumulation in the hypertonic medullary interstitium, tubular sodium handling was examined in both sickle cell patients and control subjects alike. No difference in the tubular transport of sodium could be demonstrated either under conditions of sodium loading or under conditions in which the tubular sodium load was low (water diuresis). These data support the conclusion that the defect in urinary concentration in sickle cell patients is caused by a limitation in maintaining a high concentration of solute in the medullary interstitium, thus limiting the rate of T(c) (H2O) from the collecting duct.