Evidence that an acute increase in glomerular filtration has little effect on sodium excretion in the dog unless extracellular volume is expanded

Increased ureteral back pressure enhances renal tubular sodium reabsorption

Moderate increases of ureteral back pressure usually cause decreases of glomerular filtration rate and even greater decreases of sodium excretion. It has been assumed previously that increased ureteral back pressure does not enhance renal tubular sodium reabsorption directly and that the decreases of sodium excretion are caused by the decreases of glomerular filtration rate. In the experiments reported here, the effect of increased ureteral back pressure on urinary sodium excretion was studied in dogs in which changes of filtration rate were minimized by infusing saline while ureteral back-pressure was increased. When ureteral back pressure was increased on one side by 10-23 cm of water, the inulin clearance of the experimental kidney decreased by only 3-12% in 21 experiments, did not change significantly (+/-2%) in eight experiments, and increased by 3-8% in seven experiments. The sodium excretion of the experimental kidney decreased in all experiments regardless of whether its inulin clearance increased, decreased, or was unchanged from control values. When the inulin clearance of the experimental kidney increased or remained unchanged during increased ureteral back pressure, its reabsorption of sodium increased more than could be accounted for by the increase of filtered sodium. When the inulin clearance of the experimental kidney decreased during increased ureteral back pressure, its reabsorption of sodium decreased less than could be accounted for by the decrease of filtered sodium.Therefore, the effect of increased ureteral back pressure to decrease urinary sodium excretion is caused in part by increased tubular reabsorption of sodium.

On the influence of extracellular fluid volume expansion and of uremia on bicarbonate reabsorption in man

The patterns of bicarbonate reabsorption during increasing plasma concentrations were studied in subjects with a range of glomerular filtration rates (GFR) from 170 to 2 ml/min. In a group of five subjects with GFR values above 30 ml/min, paired bicarbonate titration studies were performed first under conditions which minimized extracellular fluid (ECF) volume expansion, and second under conditions which were conducive to exaggerated expansion of ECF volume. In patients with GFR values below 30 ml/min, a single protocol was employed. Studies also were performed on two patients with far advanced renal disease who were nephrotic and exhibited a sodium-retaining state. When ECF volume expansion was minimized in the nonuremic subjects, values for bicarbonate reabsorption were well in excess of the usually accepted Tm level and over the range of plasma bicarbonate concentrations employed, no evidence of a Tm phenomenon was observed. A similar pattern emerged in the two nephrotic patients despite the presence of uremia. However, with both exaggerated expansion of ECF volume (GFR greater than 30) and in patients with advanced renal disease in the absence of exaggerated ECF volume expansion a tendency towards saturation kinetics for bicarbonate reabsorption was demonstrable. In comparing the minimized with the exaggerated expansion studies, evidence emerged for a decrease in both bicarbonate reabsorption per unit of GFR and the absolute rate of bicarbonate reabsorption. When ECF volume expansion was exaggerated in uremic patients after stable rates of bicarbonate reabsorption had been achieved, a decrease in reabsorption per unit of GFR and in absolute bicarbonate reabsorption occurred. The possible relationship of the factors controlling sodium excretion to the observed patterns of bicarbonate reabsorption is considered in the text.

The influence of hypertonic saline infusions upon the fractional reabsorption of urate and other ions in normal and hypertensive man

Renal hemodynamics and the excretion rates of six ions were studied in a group of 24 normotensive and hypertensive subjects during expansion of extracellular fluid volume (ECF) with intravenous infusions of hypertonic saline. In response to the 2.5% saline infusions arterial blood pressure did not change from control values, but glomerular filtration rate increased, and renal vascular resistance decreased.

Accompanying these hemodynamic alterations the urinary excretion rates of sodium, calcium, magnesium, potassium, chloride, and urate increased significantly. Net fractional reabsorption of the six ions fell significantly below control values. The induced changes in net Ca, Mg, K, Cl, and urate reabsorption in all the subjects were directly and significantly related to the simultaneous depression of Na reabsorption.

The data indicate that increased excretion and net tubular rejection of urate accompany depression of tubular sodium reabsorption during hypertonic saline infusions in normal and hypertensive man. The nonspecificity of depressed fractional ion reabsorption during the infusions is compatible with the hypothesis that physical forces which alter fluid uptake by peritubular capillaries determine to a significant extent the natriuretic response of the human nephron to hypertonic saline infusions. Alternatively, if a natriuretic hormone elicited by ECF volume expansion in man accounts for the depressed fractional Na reabsorption, the data imply that directly or indirectly this substance inhibits net fractional reabsorption of a variety of other ions.

Change of tubular reabsorption of sodium and water after renal denervation in the dog

1. Renal function was compared in dogs before and after denervation, with normal or reduced glomerular filtration rate (GFR). GFR was reduced by one of two means, aortic clamping or injection of plastic microspheres into one renal artery.2. The data showed that the tubular rejection fraction of denervated kidneys increased at a time when the same value for the control kidney decreased, indicating that denervation diuresis is not simply a result of increased filtered load of Na and water.3. Changes in excretion after aortic clamping are not due solely to reduced filtered load. A reduction in renal blood pressure itself appears to have a direct effect on Na transport since Na excretion was significantly decreased before there were any changes in GFR, renal plasma flow and urine volume.4. When GFR in a denervated kidney was reduced by microsphere injection it was demonstrated that a significant natriuresis (U(Na) V) and diuresis (V) occurred when. GFR was reduced by as much as 40%.5. These experiments support the hypothesis that denervation diuresis does not result solely from increased filtered load, but entails altered Na transport.

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.

Studies on the control of sodium excretion in experimental uremia

A study of the mechanisms governing the high rate of sodium excretion per nephron characteristic of patients with chronic renal disease has been made in dogs. A "remnant kidney" was produced by 85% infarction of the left kidney while the right kidney was left intact. A bladder-splitting procedure allowed simultaneous measurement of glomerular filtration rate and the rate of sodium excretion by each kidney. The animals were fed a constant known amount of sodium chloride and 0.1 mg of 9 alpha-fluorohydrocortisone twice daily throughout the study. In a group of dogs fed 3 or 5 g of salt per day, sodium excretion by the remnant kidney averaged 6.5 muEq/min while the intact kidney was present and 53.7 muEq/min when the animals became uremic after the intact kidney was removed. The increased sodium excretion per nephron by the remnant organ often occurred within 18 hr after contralateral nephrectomy and persisted despite experimentally induced acute reductions in the glomerular filtration rate to below prenephrectomy levels. A second group of animals studied in the same manner but receiving 1 g of salt per day or less failed to develop a natriuresis after contralateral nephrectomy despite high grade uremia. Thus an increased impermeable solute load per nephron was not a regulatory factor in the production of the natriuresis. The increased rate of sodium excretion per nephron in uremia resembles that after saline loading in that it may occur without an increase in glomerular filtration rate or a reduction in mineralocorticoid stimulation. It follows that an additional factor or factors must be involved in the genesis of the natriuresis. In contrast to the natriuresis that is seen in normal animals subjected to saline loading, these uremic animals were found not to have a detectable increase in extracellular fluid volume or blood volume in the presence of high fractional sodium excretion rates. Sodium excretion in response to a small salt load by the remnant organ in uremia was 30% greater than the response of both kidneys in the preuremic state despite a markedly reduced total GFR. These data are consistent with the view that the volume control mechanism becomes more responsive in uremia.

Some determinants of the effects of VAL-5-angiotensin II amide on glomerular filtration rate and sodium excretion in dogs

In 12 dogs anesthetized with chloralose, angiotensin (angiotensin II amide) given intravenously increased the glomerular filtration rate (GFR) of an ischemic kidney while simultaneously having little effect on the GFR of the contralateral kidney. In the ischemic kidney, in 14 of 30 observations, increments of GFR greater than 100% of mean control GFR (9 ml/min) occurred in response to angiotensin. The magnitude of the increase in GFR produced by angiotensin was independent of dose (range 0.005-0.050 mug/kg per min), the degree of accompanying pressor response, and alterations in renal blood flow (RBF) (electromagnetic flow-meter). In the ischemic kidney, increments of GFR could be produced by sub-pressor doses of angiotensin. Dissociations between increments of GFR and sodium excretion occurred. Equivalent increments of GFR in the ischemic kidney in dogs receiving either 5% glucose in water or 10% mannitol in 0.3% saline were associated with natriuresis only in the latter group: a) as an initial response of the contralateral kidney to renal arterial constriction (RAC) in spite of a concomitant reduction in RBF and an unchanged GFR; b) in the ischemic kidney on giving angiotensin. The natriuresis produced by angiotensin was independent of the magnitude of elevations in blood pressure, altered filtration fraction, and was associated with a further reduction in RBF. After release of RAC in the dogs receiving mannitol, an antinatriuresis was again observed in response to angiotensin. The presence of unilateral renal ischemia allowed the demonstration of a differential action of angiotensin on the GFR of an ischemic and nonischemic kidney. The natriuresis in response to angiotensin requires, in addition to mannitol, the participation of undefined factors invoked by unilateral renal ischemia.

Renal sodium reabsorption after acute renal denervation in the rabbit

1. Separate effects on the functions of left and right kidneys were examined after left-sided renal handling and acute denervation. Studies were done on pentobarbital-anaesthetized rabbits using clearance techniques to evaluate renal haemodynamics and water and electrolyte excretion.2. When compared with its counterpart, the handled kidney exhibited some decrease in function for at least 20 min. Recovery of most functions occurred in 40-60 min.3. The effects of denervation on renal functions were observed when the effects of handling had subsided. Renal plasma flow (R.P.F.) and glomerular filtration rate (G.F.R.) were not significantly changed, whereas the decrease in Na, K and water excretion, which was usually observed for no ascertained reason in the innervated kidney, was prevented.4. The magnitude of the denervation natriuresis was greater in these rabbits than in dogs studied previously; other functions studied were comparable in the two species.5. The results from thirty-five experiments are interpreted to indicate that denervation decreases Na reabsorption independently of G.F.R., perhaps by a direct effect on tubular transport, but more probably by a redistribution of filtration to nephrons of lesser reabsorptive capacity.

Studies on the characteristics of the control system governing sodium excretion in uremic man

Sodium excretion was studied in a group of patients with chronic renal disease, (a) on constant salt intakes of varying amounts with and without mineralocorticoid hormone administration and, (b) after acute extracellular fluid volume expansion. The lower the steady-state glomerular filtration rate (GFR), the greater was the fraction of filtered sodium excreted on both a 3.5 and 7.0 g salt diet; and the lower the GFR, the greater was the change in fractional excretion in the transition from the 3.5 to the 7.0 g salt diet. This regulatory capacity did not appear to be influenced by mineralocorticoid hormone administration. After acute expansion of extracellular fluid (ECF) volume, the increment in sodium excretion exceeded the concomitant increment in filtered sodium in six of nine studies and in the remaining three studies, the increment in excretion averaged 59% of the Delta filtered load (i.e., only 41% of the increase in filtered sodium was reabsorbed). During saline loading, the decrease in fractional reabsorption of sodium tended to vary inversely with the steady-state GFR, although all patients received approximately the same loading volume. When an edema-forming stimulus was applied during saline infusion, the natriuretic response was aborted and the lag time was relatively short. When GFR and the filtered load of sodium were increased without volume expansion, the Delta sodium excretion averaged only 19% of the Delta filtered load; moreover, changes in fractional sodium reabsorption were considerably smaller than those observed during saline loading. The data implicate the presence of a factor other than GFR and mineralocorticoid changes in the modulation of sodium excretion in uremic man.