Micropuncture studies of sodium transport in the remnant kidney of the dog. The effect of graded volume expansion

Chemical evidence for the tradeoff-in-the-nephron hypothesis to explain secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]_DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca⁺⁺]_DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (E_P/C_cr), a surrogate for [P]_DCT. In the present investigation, we estimated [P]_DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]_DCT could lower [Ca⁺⁺]_DCT and raise [PTH].

Methods and findings

We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m² (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m² (mean 86.2 ± 10.2). In each subject, total [Ca]_DCT and [P]_DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca⁺⁺]_DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca₃(PO₄)₂ (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]_DCT, and [Ca⁺⁺]_DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]_DCT was found to be the sole determinant of [Ca⁺⁺]_DCT, and precipitation of Ca₃(PO₄)₂ (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]_DCT was the principal determinant of [Ca⁺⁺]_DCT, [P]_DCT was a minor determinant, and precipitation of Ca₃(PO₄)₂ (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]_DCT and inversely with [Ca⁺⁺]_DCT, and a reduced [Ca⁺⁺]_DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca⁺⁺]_DCT and to eGFR resembled each other closely.

Conclusions

As [P]_DCT increases, chemical speciation calculations predict reduction of [Ca⁺⁺]_DCT through precipitation of Ca₃(PO₄)₂ (am., s.). [PTH] appears to rise unequivocally if [Ca⁺⁺]_DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.

Blunted natriuretic response to saline loading in sheep with hypertensive kidney disease following radiofrequency catheter-based renal denervation

Renal sympathetic nerves contribute to renal excretory function during volume expansion. We hypothesized that intact renal innervation is required for excretion of a fluid/electrolyte load in hypertensive chronic kidney disease (CKD) and normotensive healthy settings. Blood pressure, kidney hemodynamic and excretory response to 180 min of isotonic saline loading (0.13 ml/kg/min) were examined in female normotensive (control) and hypertensive CKD sheep at 2 and 11 months after sham (control-intact, CKD-intact) or radiofrequency catheter-based RDN (control-RDN, CKD-RDN) procedure. Basal blood pressure was ~ 7 to 9 mmHg lower at 2, and 11 months in CKD-RDN compared with CKD-intact sheep. Saline loading did not alter glomerular filtration rate in any group. At 2 months, in response to saline loading, total urine and sodium excretion were ~ 40 to 50% less, in control-RDN and CKD-RDN than intact groups. At 11 months, the natriuretic and diuretic response to saline loading were similar between control-intact, control-RDN and CKD-intact groups but sodium excretion was ~ 42% less in CKD-RDN compared with CKD-intact at this time-point. These findings indicate that chronic withdrawal of basal renal sympathetic activity impairs fluid/electrolyte excretion during volume expansion. Clinically, a reduced ability to excrete a saline load following RDN may contribute to disturbances in body fluid balance in hypertensive CKD.

Fibroblast growth factor 23 is associated with fractional excretion of sodium in patients with chronic kidney disease

BACKGROUND

Recent studies suggest that the phosphaturic hormone fibroblast growth factor 23 (FGF23) is involved in regulation of renal sodium excretion and blood pressure. There is evidence of both direct effects via regulation of the sodium-chloride symporter (NCC) in the distal tubule, and indirect effects through interactions with the renin-angiotensin-aldosterone system. However, clinical data on the association between FGF23 and renal sodium regulation is lacking. Herein, we investigated the associations of FGF23 with renal sodium handling and blood pressure in non-dialysis CKD patients.

METHODS

This was a cross-sectional study encompassing 180 CKD patients Stage 1-5, undergoing renal biopsy. Plasma intact FGF23, 24-h urinary sodium excretion, fractional excretion of sodium (FENa) and blood pressure were measured at baseline. The association between FGF23 and renal sodium handling was explored by multivariate regression analysis.

RESULTS

The median age was 52.8 years, 60.6% were men and the median estimated glomerular filtration rate (eGFR) was 50.6 mL/min/1.73 m2. In univariate analysis, FGF23 was positively associated with FENa (Spearman's rho = 0.47; P < 0.001) and systolic blood pressure (rho = 0.17, P < 0.05), but not with plasma sodium, 24-h urinary sodium excretion or mean arterial blood pressure. The association between FGF23 and FENa remained significant after adjustment for potential confounders (multivariable adjusted β coefficient 0.60, P < 0.001). This association was stronger among the 107 individuals with eGFR <60 mL/min/1.73 m2 (β = 0.47, P = 0.04) and in the 73 individuals on any diuretics (β = 0.88, P < 0.001). Adjustment for measured GFR instead of eGFR did not alter the relationship.

CONCLUSIONS

FGF23 is independently associated with increased FENa in non-dialysis CKD patients. These data do not support the notion that FGF23 causes clinically significant sodium retention. Further studies are warranted to explore the mechanism underlying this association.

Changes in Total Sodium Intake Do Not Lead to Proportionate Changes in Total Sodium Removal in CAPD Patients

Background

Dietary salt and fluid restriction is important in controlling fluid balance in patients on continuous ambulatory peritoneal dialysis (CAPD). However, it is often difficult to monitor patients’ dietary total sodium intake (TSI). Usually, total sodium removal (TSR), the sum of urinary sodium removal (USR) and dialysate sodium removal (DSR), is suggested to represent TSI. In the present study, we investigated the reliability of using TSR as a surrogate to TSI in CAPD patients.

Methods

40 clinically stable CAPD patients were closely followed for 3 months. Their TSI, USR, DSR, and fluid status were measured twice: at baseline and at the end of this study respectively. Fluid status was evaluated by bioimpedance analysis. Patients with increased sodium intake (group ISI) or decreased sodium intake (group DSI) (both >0.5 g/day or >21.74 mmol/day elemental sodium) were included in this study.

Results

There were 15 patients in group ISI and 9 patients in group DSI. During the follow-up, although TSI increased in group ISI and decreased in group DSI ( p < 0.05), there were no significant changes in USR, DSR, or TSR in either group. No relationship was found between TSI and TSR. Changes in weight, blood pressure, urine volume, ultra-filtration, and small solute removal (Kt/V and creatinine clearance) were not statistically significant between the two groups. Fluid status deteriorated in group ISI and improved in group DSI ( p < 0.05).

Conclusions

Our study suggests that changes in total sodium intake do not lead to proportionate changes in total sodium removal in CAPD patients. Therefore, TSR (the sum of USR and DSR) should be used cautiously to monitor TSI in this patient population.

Mechanisms of PKC-Dependent Na+K+ATPase Phosphorylation in the Rat Kidney with Chronic Renal Failure

The present work was designed to study Na+ K+ ATPase alpha1-subunit phosphorylation in rats with chronic renal failure (CRF) in comparison with normal rats. Na+ K+ ATPase alpha1-subunit phosphorylation degree was measured by binding the McK-1 antibody to dephosphorylated Ser-23 in microdissected medullary thick ascending limb of Henle (mTAL) segments. In addition, the total Na+ K+ ATPase alpha1-subunit expression and activity were also measured in the outer renal medulla homogenates and membranes. CRF rats showed a higher Na+ K+ ATPase activity, as compared with control rats (18.95 +/- 2.4 vs. 11.21 +/- 1.5 micromol Pi/mg prot/h, p < 0.05), accompanied by a higher total Na+ K+ ATPase expression (0.54 +/- 0.04 vs. 0.27 +/- 0.02 normalized arbitrary units (NU), p < 0.05). When McK-1 antibody was used, a higher immunosignal in mTAL of CRF rats was observed, as compared with controls (6.3 +/- 0.35 vs. 4.1 +/- 0.33 NU, p < 0.05). The ratio Na+ K+ ATPase alpha1-subunit phosphorylation/total Na+ K+ ATPase alpha1-subunit expression per microg protein showed a non-significant difference between CRF and control rats in microdissected mTAL segments (2.11 +/- 0.12 vs. 2.26 +/- 0.18 NU, p = NS). The PKC inhibitor RO-318220 10(-6) M increased immunosignal (lower phosphorylation degree) in mTAL of CRF rats to 128.43 +/- 7.08% (p < 0.05) but did not alter McK1 binding in control rats. Both phorbol 12-myristate 13-acetate (PMA) 10(-6) M and dopamine 10(-6) M decreased immunosignal in CRF rats, corresponding to a higher Na+ K+ ATPase alpha1-subunit phosphorylation degree at Ser-23 (55.26 +/- 11.17% and 53.27 +/- 7.12% compared with basal, p < 0.05). In mTAL of CRF rats, the calcineurin inhibitor FK-506 10(-6) M did not modify phosphorylation degree at Ser-23 of Na+ K+ ATPase alpha1-subunit (100.21 +/- 3.00% compared with basal CRF). In control rats, FK 506 10(-6) M decreased the immunosignal, which corresponds to a higher Na+ K+ ATPase alpha1-subunit phosphorylation degree at Ser-23. The data suggest that the regulation of basal Na+ K+ ATPase alpha1-subunit phosphorylation degree at Ser-23 in mTAL segments of CRF rats was primarily dependent on PKC activation rather than calcineurin dependent mechanisms.

Treatment of edematous disorders with diuretics

Generalized edema results from alterations in renal sodium homeostasis that ultimately result in an expansion of extracellular fluid volume and accumulation of interstitial fluid. The common edematous disorders include congestive heart failure, cirrhosis, nephrotic syndrome, and renal insufficiency. The abnormalities of sodium homeostasis contributing to edema formation in each condition are discussed. Management of volume homeostasis, with an emphasis on the role of diuretic therapy, is reviewed.

Altered expression of Na transporters NHE-3, NaPi-II, Na-K-ATPase, BSC-1, and TSC in CRF rat kidneys

In chronic renal failure (CRF), reduction in renal mass leads to an increase in the filtration rates of the remaining nephrons and increased excretion of sodium per nephron. To address the mechanisms involved in the increased sodium excretion, we determined the total kidney levels and the densities per nephron of the major renal NaCl transporters in rats with experimental CRF. Two weeks after 5/6 nephrectomy (reducing the total number of nephrons to approximately 24 +/- 8%), the rats were azotemic and displayed increased Na excretion. Semiquantitative immunoblotting revealed significant reduction in the total kidney levels of the proximal tubule Na transporters NHE-3 (48% of control), NaPi-II (13%), and Na-K-ATPase (30%). However, the densities per nephron of NHE-3, NaPi-II, and Na-K-ATPase were not significantly altered in remnant kidneys, despite the extensive hypertrophy of remaining nephrons. Immunocytochemistry confirmed the reduction in NHE-3 and Na-K-ATPase labeling densities in the proximal tubule. In contrast, there was no significant reduction in the total kidney levels of the thick ascending limb and distal convoluted tubule NaCl transporters BSC-1 and TSC, respectively. This corresponded to a 3.6 and 2.5-fold increase in densities per nephron, respectively (confirmed by immunocytochemistry). In conclusion, in this rat CRF model: 1) increased fractional sodium excretion is associated with altered expression of proximal tubule Na transporter expression (NHE-3, NaPi-II, and Na-K-ATPase), consistent with glomerulotubular imbalance in the face of increased single-nephron glomerular filtration rate; and 2) compensatory increases in BSC-1 and TSC expression per nephron occur in distal segments.

Role of urinary arginine vasopressin in the sodium excretion in patients with chronic renal failure

Patients with chronic renal failure show almost equal levels of sodium excreted in the urine as healthy subjects through an increase of the fractional excretion sodium (FE(Na)). The mechanisms of this adaptation, however, are unknown. Recently, urinary arginine vasopressin (AVP) has been shown to inhibit the antidiuretic action of plasma AVP in the collecting ducts of rabbits and rats. In this article, the roles of plasma and urinary AVP are examined with other hormones in the sodium excretion of 57 patients with chronic renal disease. The fractional excretion of AVP, plasma atrial natriuretic peptide (ANP) and endothelin-1 (ET-1), urinary ET-1, and FE(ET-1) correlated with the decrease of creatinine clearance (Ccr). Multiple and stepwise regression analyses showed that FE(AVP) is the major dependent determinant for FE(Na) (adjusted r2 = 0.78). These results suggest that the increase of AVP excretion per remaining nephron could be a cause of the increase of FE(Na) in patients with renal failure. Although plasma AVP works as an antidiuretic hormone, urinary AVP serves as an intrinsic diuretic, especially in patients with chronic renal failure.

Tubular sodium handling and tubuloglomerular feedback in compensatory renal hypertrophy

Tubular sodium handling and tubuloglomerular feedback (TGF) activity were assessed in established compensatory renal hypertrophy in Sprague Dawley rats. Hyperfiltration at the level of the single nephron was confirmed 4-6 weeks following a reduction in renal mass. TGF activity, determined as the difference between late proximal and early distal measurements of single-nephron glomerular filtration rate (SNGFR), was significantly increased in compensatory renal hypertrophy, being 7.8 +/- 1.0 vs 23.3 +/- 1.9 vs 25.5 +/- 2.6 nl/min (P for analysis of variance less than 0.05) following sham operation, unilateral nephrectomy, and 1 1/3 nephrectomy, respectively. Enhanced net tubular Na transport was also observed, with total Na reabsorption up to the late proximal site being 1.8 +/- 0.2 vs 2.7 +/- 0.1 vs 3.1 +/- 0.3 nmol/min (P less than 0.05), and to the early distal site being 3.4 +/- 0.5 vs 5.8 +/- 0.6 vs 7.9 +/- 0.8 nmol/min (P less than 0.05) in the three animal groups respectively. Comparison of proximal tubular length demonstrated a 71.9 +/- 8.1% increase in uninephrectomised vs sham-operated animals. This increase was proportionately greater than the increase in proximal Na reabsorption (50.0 +/- 4.0%) observed in the corresponding animal groups. Concurrent electron microprobe experiments in uninephrectomised and sham-operated animals demonstrated that the proximal tubular intracellular Na concentration was significantly lower following uninephrectomy (16.8 +/- 0.6 vs 18.9 +/- 0.5 mmol/kg wet weight, P less than 0.01), in association with evidence of reduced basolateral Na/K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubular handling of bicarbonate in dogs with experimental renal failure

In this study, micropuncture experiments were performed to examine the segmental reabsorption of bicarbonate and chloride in the normal dog kidney (stage 1) and in the remnant kidney (stage 3) of dogs with experimental renal failure. The protocol consisted of three phases: (1) 3% extracellular fluid volume expansion, (2) infusion of Ringer's solution containing 90 mM of sodium bicarbonate, and (3) infusion of 150 mM of sodium bicarbonate in Ringer's solution. In the animals with remnant stage 3 kidneys, the ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was increased compared to dogs with stage 1 kidneys at both the proximal and distal sampling sites. These data suggest that bicarbonate reabsorption was elevated in both the distal and the proximal tubules in experimental chronic renal failure. In contrast to the reabsorption of bicarbonate, chloride reabsorption was depressed in stage 3 kidneys at the late proximal puncture site. This resulted in greater delivery to the distal nephron. The distal segments reabsorbed a constant fraction of delivered chloride, resulting in an increase in fractional chloride excretion in chronic renal failure compared to that in normal animals.

Sodium homeostasis in dogs with chronic renal insufficiency

Volume homeostasis in the fasting rate and 24-hr sodium balance are maintained in chronic renal insufficiency as a result of adaptations in the residual nephrons. This study evaluates the limitations to these adaptations and the dynamics of sodium excretion (UNaV) after an acute challenge with 100 mEq of sodium chloride in normal dogs (GFR 50 ml/min) and in dogs with one remnant kidney and moderate chronic renal insufficiency (GFR 15 ml/min). When food was administered with the sodium challenge, no or minimal changes in serum protein and hematocrit values occurred, and the natriuretic responses were small and equivalent in normal and remnant dogs. On the other hand, when the sodium challenge was given without food, the natriuretic response was large in normal dogs and markedly blunted in remnant. Within 5 hr of the sodium challenge, the various groups of normal dogs excreted 40 to 63% of the sodium load, but the remnant animals eliminated only 12 to 22% (P less than 0.001). The blunted natriuresis in remnant dogs was associated with a prolonged hemodilution of circulating proteins, indicating a longer lasting expansion of the intravascular volume. The blunted response was independent of sodium diet, of the administration route (p.o. vs. i.v.) or strength (isotonic vs. hypertonic) of the sodium load, and appears to result from an inability of the remnant kidney to rapidly excrete a sodium load. Thus, administration of sodium to dogs with chronic renal insufficiency leads to prolonged sodium retention, prolonged extracellular fluid (ECF) volume expansion, and requires a prolonged excretory cycle to restore 24-hr balance.

Functional profile of the isolated uremic nephron. Evidence of proximal tubular "memory" in experimental renal disease

In experimental models of glomerular and nonglomerular renal disease, single nephron filtration rate and proximal tubular reabsorption of fluid decrease or increase in parallel in the same nephron. To assess whether intrinsic adaptations in proximal tubular function, i.e., changes that are independent of the peritubular or humoral milieu, contribute to this phenomenon, segments of rabbit late superficial proximal convoluted tubules (PCT) were studied by in vitro perfusion. PCT were obtained from normal kidneys, from remnant kidneys, and from kidneys embolized with microspheres. Single nephron filtration rates are increased in the remnant and decreased in the embolized kidneys. Whereas the embolized-kidney rabbits were nonazotemic (the contralateral kidney was left in situ), the remnant-kidney animals were uremic. In order to study a nonazotemic model of increased single nephron filtration rate, PCT were also obtained from uninephrectomized rabbits. Significant compensatory hypertrophy occurred in the PCT of the remnant kidney. Net fluid reabsorption (Jv) per unit length was increased by approximately 60%; Jv per unit luminal surface area was the same as in the normal PCT. Transepithelial potential difference (PD) was significantly greater than normal. This was associated with a reversal of the normal permselective properties (P(Cl) > P(Na)) of the late superficial PCT so that P(Na) exceeded P(Cl). The changes could not be ascribed to some undetermined effect of the uremic state in vivo, since increases in tubule size, Jv per unit length, and PD also occurred in PCT from nonazotemic uninephrectomized rabbits. In contrast, Jv, per unit length or per unit luminal surface area, was decreased by approximately 50% in PCT from embolized kidneys and PD was also reduced. In these tubules, the normal permselective properties were also reversed. Tubule size, however, was not significantly different from normal. The increases or decreases in Jv that occurred in the different disease models were not dependent on tubular fluid flow rate or the uremic milieu in vitro. These studies indicate that intrinsic proximal tubular function is modified by the disease state in vivo and that the "memory" of this adaptation is expressed in the in vitro situation. The changes in Jv observed in vitro parallel the increases or decreases in single nephron filtration rates that occur in vivo. Compensatory hypertrophy, with an attendant increase in luminal surface area, could explain the increased Jv per millimeter in the remnant kidneys, but the adaptation observed in the embolized kidneys cannot be ascribed to changes in tubule size.

Medullary collecting duct function in the remnant kidney before and after volume expansion

The role of the medullary collecting ducts (CD) in the regulation of water and electrolyte excretion by the remnant kidney has not been determined. Medullary CD function was therefore studied by the microcatheterization technique during hydropenia and after volume expansion with an isotonic saline load in rats with sham-operated normal kidney (stage I), remnant kidney (stage II), remnant kidney after contralateral nephrectomy (stage III), and sham-operated normal kidney after contralateral nephrectomy (stage-III control). Sodium, potassium, water, and solute reabsorption along the medullary CD were not altered in stage II during hydropenia when compared to normal control (stage I), but water reabsorption proximal to the medullary CD was decreased. In stage III, where the uremia was mild (BUN, 41 mg/dl), the fractions of the filtered load of sodium (72%) and water (62%) which were reabsorbed along the medullary CD were reduced in comparison to stage II (94% and 83%, respectively), stage I, or stage-III control. The fraction of filtered potassium entering the medullary CD was increased to 53% in stage III, compared to 16% in stage II, and 10 or 11% in stage I or stage-III control. No change in the fraction of filtered potassium remaining along the collecting ducts was observed. After volume expansion, there was no significant change in the fraction of filtered sodium and water remaining along the medullary CD in stage I, II, or III. The greater fractional excretion in stage III resulted from decreased reabsorption in more proximal nephron segments. The results indicate that (a) during hydropenia, fractional reabsorption of sodium and water is decreased along the medullary CD of the stage-III remnant kidney in the presence of mild uremia, (b) the increased fractional excretion of potassium in the remnant kidney with uremia is not determined by altered potassium handling in the medullary CD but occurs proximal to this nephron segment, and (c) extracellular fluid volume expansion with isotonic saline results in similar inhibition of fractional sodium and water reabsorption in the collecting ducts of both remnant and normal kidneys.

Functional profile of the isolated uremic nephron. Role of compensatory hypertrophy in the control of fluid reabsorption by the proximal straight tubule

An in vitro approach to the study of single nephron function in uremia has been employed in evaluating the control of fluid reabsorption by the renal superficial proximal straight tubule (PST). Isolated segments of PSTs from the remnant kidneys of uremic rabbits (stage III) were perfused in vitro and their rate of fluid reabsorption compared with normal PSTs and with PSTs derived from the remnant kidneys of nonuremic rabbits (stage II). All segments were exposed to a peritubular bathing medium of both normal and uremic rabbit serum thereby permitting a differentiation to be made between adaptations in function which are intrinsic to the tubular epithelium and those which are dependent upon a uremic milieu.Compared with normal and stage II PSTs, there was significant hypertrophy of the stage III tubules as evidenced by an increase in length and internal diameter, and a twofold increase in the dry weight per unit length. Fluid reabsorption per unit length of tubule was 70% greater in stage III than in normal and stage II PSTs, and was closely correlated with the increase in dry weight. Substitutions between normal and uremic rabbit serum in the peritubular bathing medium did not affect fluid reabsorption significantly in any of the three groups of PSTs. Perfusion of the tubules with an ultrafiltrate of normal vs. uremic serum likewise failed to influence the rate of net fluid reabsorption. It has previously been observed that net fluid secretion may occur in nonperfused or stop-flow perfused normal rabbit PSTs exposed to human uremic serum. Additional studies were thus performed on normal and stage III PSTs to evaluate whether net secretion occurs in the presence of rabbit uremic serum. No evidence for net secretion was found. These studies demonstrate that fluid reabsorption is greatly increased in the superficial PST of the uremic remnant kidney and that this functional adaptation is closely correlated with compensatory hypertrophy of the segment. Humoral factors in the peritubular environment do not appear to be important mediators of the enhanced fluid reabsorption.

Acute functional adaptation to nephron loss: micropuncture studies

The renal and proximal tubule response to contralateral kidney exclusion was studied in a variety of circumstances. Recollection micropuncture studies were performed to assess the response to contralateral kidney clamping in the normal or a remnant kidney of the dog. Acute clamping of the contralateral kidney for a normal and unilateral remnant kidney resulted in marked reduction in proximal TF/P inulin ratios in the experimental kidney reflecting a 15 percent reduction in fluid reabsorption. Mean fractional excretion of sodium, potassium and water increased significantly in remnant kidney dogs but no significant change was observed in normal dogs except for potassium excretion. The marked reduction in proximal reabsorption occurred as soon as 5-15 minutes after contralateral kidney clamping and was compensated by distal reabsorption. Acute obstruction of the contralateral ureter results in a similar markedly reduced proximal tubular reabsorption. The reduction in proximal reabsorption induced by contralateral clamping occurred in the presence of reduced perfusion pressure and volume expansion and to some extent with renal denervation. When prostaglandin E(2) or acetycholine were infused prior to contralateral kidney clamping, proximal reabsorption remained at control levels and the contralateral clamping response was blocked. Similar blockade occurred after treatment with indomethacin. Acute reduction in nephron mass causes a marked depression of proximal tubular sodium and fluid absorption not obviously accounted for by hemodynamicphysical factors and humoral factors may be involved. The level of distal reabsorption to increased proximal delivery following contralateral clamping, determines the net urinary excretion.

Effects of nephrectomy on renal salt and water transport in the remaining kidney

Fluid, sodium, and potassium transport was studied in proximal and distal tubules in rats in which one kidney had been removed two weeks after a suprarenal aortic clamp had been placed to prevent adaptive changes in glomerular filtration rate (GFR) in the experimental kidney. Free-flow micropuncture techniques were used and tubular fluid (TF) samples analyzed for inulin, sodium and potassium. In addition, peritubular total protein concentrations and luminal and peritubular hydrostatic pressures were measured. The following changes were observed 15 hr after unilateral nephrectomy: (1) a significant increase in single nephron GFR; (2) unchanged absolute proximal tubular reabsorption rates of fluid and sodium; (3) increased delivery of fluid into distal tubules; (4) increased distal tubular reabsorption of sodium, but of insufficient magnitude to prevent natriuresis; and (5) an augmentation of distal tubular potassium secretion. Reduction of single nephron GRF to control levels by aortic clamping abolished the natriuresis following nephrectomy.

Enhanced end-organ responsiveness of the uremic kidney to the natriuretic factor

In chronic renal disease, the addition of a fixed quantity of Na to the extracellular fluid (ECF) will evoke a natriuretic response per nephron which is inversely proportional to the glomerular filtration rate (GFR). One factor that could contribute to this "magnification" phenomenon is an increased sensitivity of residual nephrons to physiologic natriuretic forces. The present studies were designed to examine this possibility. Natriuretic urine fractions from uremic patients, infused into one renal artery of normal rats, produced a small but significant unilateral natriuresis. Infusion of the same fractions in identical amount into remnant kidneys of stage II nonuremic rats (i.e., rats with a contralateral normal kidney in situ) produced a natriuresis in the remnant kidney only which was equivalent to that observed in the normal kidneys. The i.v. infusion of natriuretic fractions into stage II rats produced comparable increments in the fractional excretion of sodium (FENa) bilaterally. However, when the natriuretic fractions were infused into remnant kidneys of stage III rats (no contralateral kidney), deltaFENa was significantly greater than in the foregoing groups. Because stage III rats have increased control values for FENa, baseline FENa was increased to an equivalent level in normal rats by unilateral renal denervation. Natriuretic factor was administered into the ipsilateral renal artery. Although the natriuretic response was increased, it was significantly less than in the stage III remnant kidneys. The data support the view that the uremic state per se is associated with an enhanced responsiveness of the residual nephrons to the natriuretic factor found in the urine (and blood) of uremic patients.

Acute effect of nephrotoxic serum on renal sodium transport in the dog

In order to study the tubular mechanism for salt retention in acute proliferative glomerulonephritis, clearance and micropuncture experiments were performed on nine volume-expanded dogs before and after injection of sheep anti-dog glomerular basement membrane (GBM) antibodies (nephrotoxic sera [NTS]). Histologic sections obtained two hours after NTS injection following completion of the functional studies demonstrated infiltration of glomerular tufts by polymorphonuclear leukocytes, swelling of both glomerular endothelial and mesangial cells and the mesangial matrix, and linear anti-GBM antibody deposits along the glomerular capillaries. There was an almost immediate reduction in glomerular filtration rate (GFR), filtration fraction, urine flow and absolute and fractional sodium excretion; arterial blood pressure and renal plasma flow were not significantly altered. Micropuncture studies revealed no change in fractional sodium reabsorption in the proximal tubule despite a fall in single nephron GFR. A proportional reduction in single nephron and kidney GFR coupled with an unchanged intrarenal distribution of blood flow following NTS suggests a relatively uniform effect on both superficial and deep nephrons. In contrast to the well-preserved glomerulotubular balance in the proximal tubule, fractional distal sodium reabsorption was significantly increased as reflected by the decreased fractional urinary sodium excretion. We conclude that salt retention and low fractional urinary sodium excretion observed in acute glomerulonephritis could be primarily due to reduced distal delivery of sodium which leads to increased fractional sodium reabsorption in the nephron segments distal to the proximal convoluted tubule.

The effect on total renal and tubular function and plasma renin of a moderate isotonic saline load in rats anesthetized with amytal and inactin

The renal effects of i.v. saline loading equal to 1% body weight (b.wt.) were studied in 2 groups of rats: group I was anesthetized with Amytal (15 mg/100 g b.wt. plus supplementary doses), group II with Inactin (12.5-15.0 mg/100 g b.wt.). In group I the saline load caused an increase in urine flow (+92%), solute excretion (+67%), inulin clearance (CIN) (+24%), PAH clearance (+31%) and absolute proximal reabsorption rate (+27%). Proximal fractional reabsorption and filtration fraction (FF) remained unchanged, while plasma oncotic pressure (COP) decreased by 10%. Plasma renin fell and there was an inverse relationship between renin concentration and proximal reabsorption rate. In group II urine flow and solute excretion increased after saline (+85% and 110%, respectively); CIN and absolute proximal reabsorption rate was lower than in group I and failed to increase after saline. Proximal fractional reabsorption was also lower and decreased after saline. COP as well as renin decreased as in group I, but no relationship between renin concentration and proximal reabsorption rate could be demonstrated. The data indicate that Inactin depresses both resting proximal tubular reabsorptive capacity and the tubular response to a physiological volume expansion. The results are compatible with the hypothesis that the renin-angiotensin system is significantly involved in regulation of proximal tubular function, while they are incompatible with the idea that peritubular COP plays any major role in this adjustment.

Effect of sodium intake on single nephron glomerular filtration rate and sodium reabsorption in experimental uremia

Sodium balance, clearance and micropuncture studies were performed on three groups of uremic rats in which renal mass was reduced experimentally by approximately 85%. All animals received a sodium-free synthetic diet to which a measured amount of NaCl was added. Sodium intake was 3 mEq/day in one group, 1 mEq/day in a second group and 0.13 mEq/day in the third. In the latter, the Na intake was reduced (from an initial level of 1 mEq/day) as renal mass was reduced in proportion to the estimated reduction in renal mass in an effort to obviate the requirement for an increased natriuresis/nephron. Clearance and micropuncture studies also were performed in a group of normal rats maintained on 1 mEq/day of Na. All three groups of uremic rats on the standard diet maintained external Na balance. Single nephron glomerular filtration rate (SNGFR) in superficial nephrons was increased in all three groups of uremic rats and seemed to be independent of the Na intake; fractional fluid reabsorption was decreased in the proximal tubules in all three groups of uremic rats. Furthermore, absolute proximal Na reabsorption was markedly increased; and calculated values for distal reabsorption were markedly increased in all groups of uremic rats. The data suggest that the increase in SNGFR and the decrease in tubular fluid to plasma (TF/P) inulin ratios in superficial proximal tubules correlate poorly with the dictates for an increase in sodium excretion rate per residual nephron. These data also have implications regarding the operation of the control system in the regulation of external Na balance in uremia.

Micropuncture studies of phosphate transport in the proximal tubule of the dog. The relationship to sodium reabsorption

Micropuncture studies were performed in the dog to examine the relationship between sodium and phosphate transport in the proximal tubule. In hydropenic, thyroparathyroidectomized animals, administration of parathyroid extract, saline, or acetazolamide resulted in a fall in proximal tubule fluid-to-plasma (TF/P) inulin ratio as well as a rise in tubule fluid-to-plasma ultrafilterable (TF/UF) phosphate ratio. A correlation was found between the changes in fractional reabsorption of sodium and phosphate but the phosphate changes were generally greater than those of sodium. Also, a high distal phosphate delivery in the face of low fractional excretion of phosphate in the urine in thyroparathyroidectomized dogs suggests significant phosphate reabsorption in the distal nephron. On the other hand, calcium chloride infusion to saline-loaded, normal dogs to suppress endogenous parathyroid hormone reduced proximal TF/UF phosphate without change in TF/P inulin, while both parameters remained unchanged in saline-loaded, thyroparathyroidectomized dogs after calcium infusion. An increase in proximal TF/UF phosphate associated with unchanged TF/P inulin was also demonstrated by administration of highly purified parathyroid hormone to saline-loaded, thyroparathyroidectomized dogs. It was concluded that although proximal tubule phosphate transport is generally closely related to that of sodium, the two can dissociate under certain experimental conditions, especially under the influence of parathyroid hormone. These observations also indicate that the effect of parathyroid hormone on proximal tubule phosphate transport is not solely dependent upon its effect on sodium transport.