Measurement of reabsorption by single segments of the human nephron

A widely diffused method to measure the reabsorption by single segments of the nephron in humans is based on water diuresis (WD), assuming that the peak urine flow rate approximates proximal delivery (PD). However, back-diffusion of an unknown volume of solute-free water (CH2O-BD) from lumen to the interstitium even in the absence of antidiuretic hormone (ADH) introduces a significant error in the estimate of PD. We herein describe an improved method that more precisely estimates the rates of segmental reabsorption. It is based on the assumption that the volume back diffusing during WD will not be subtracted when the hypertonicity of the interstitium is dissipated during the action of furosemide (F); hence, it will be estimated by the difference in urine flow rates measured before (V) and at the peak effect of F (Vf), such that CH2O-BD = Vf - V. Therefore, Vf (the urine flow rate during F) is assumed to directly measure PD of filtrate to the distal tubule (DT) during maximal WD. Adding CH2O-BD to the free water excreted during baseline WD (CH2O) yields the total volume (CH2O-T) of solute-free water formed: CH2O-T = CH2O + CH2O-BD. Therefore, the solute-free water generated by the ascending limb of Henle's loop (CH2O-HL) can be separated from that formed by the convoluted DT (CH2O-DT), since the latter is the only free water formed and excreted during F. Therefore CH2O-DT = CH2Of. Subtracting CH2Of from the total yields the free water formed by Henle's loop: CH2O-HL = CH2O-T - CH2Of. A suitable modification of this method allows the estimation of CH2O-HL even during maximal antidiuresis (AD). The present paper describes the theory, discusses its assumptions, the experimental validation by micropuncture experiments in rats, the results in humans, in health and disease, by simple experiments carried out at the bedside. The data show that this new method represents a significant improvement in accuracy with respect to former techniques to estimate segmental reabsorption, and that it can be suitably used to disclose the pathophysiologic derangements of the nephron in a number of diseases.     

Evidence for a proximal and distal nephron action of atrial natriuretic factor in man

The intrarenal mechanism of action of atrial natriuretic (ANF) remains uncertain. Animal work suggests that part of the natriuretic effect of ANF may be due to inhibition of proximal and distal nephron sodium reabsorption and we now present evidence for similar effects of ANF in man. Six sodium replete normal male volunteers were studied during maximal water diuresis. Lithium clearance was used to assess segmental nephron function. ANF infusion caused a significant increase in fractional lithium clearance (FELi) compared to placebo infusion. A similar change in fractional distal delivery, a conventional marker of proximal tubular outflow, also occurred during ANF infusion independently corroborating the increase in FELi. These findings suggest that ANF inhibits whole-kidney fractional proximal tubular sodium reabsorption in man. Evidence is also presented to show that ANF depresses distal nephron fractional sodium reabsorption as evaluated both by the lithium method and by estimation of solute-free water clearance.     

Distal nephron function in Bartter's syndrome: abnormal conductance to chloride in the cortical collecting tubule?

Five patients with the clinical patterns of Bartter's syndrome underwent a series of clearance studies in order to characterize the underlying tubule defect. Free water generation during maximal water diuresis (CH2O), expressed as percentage of the distal delivery (CH2O + CCl), was lower in the patients (72.5 +/- 3.2%) than in controls (84.4 +/- 5.5, p < 0.0001). During maximal water diuresis and furosemide administration (40 mg i.v. as bolus), NaCl reabsorption along the diluting nephron segments could be separated into 2 components, that occurring in the loop of Henle (DRNaHL) and that occurring in tubule segments beyond the macula densa (DRNaDT): DRNaHL was normal, while DRNaDT was reduced (3.1 +/- 0.8 vs. 6.2 +/- 2.5 ml/min in controls, p < 0.015). Thus, according to this furosemide protocol, our patients had normal solute reabsorption in the loop of Henle but reduced NaCl reabsorption in tubule segments beyond the macula densa. During 0.9% saline infusion (2 liters in 2 h, after stimulation of distal Na reabsorption with fludrocortisone) fractional excretion (FE) of K showed a linear rise with the increase of FECl-FEK, however, was much higher in the patients than in controls for every FECl level. In contrast, the infusion of Na2SO4, after fludrocortisone administration, induced similar FEK increases in patients and in controls. Thus, in these patients Na reabsorption in the distal nephron (possibly the cortical collecting tubule) was associated with the generation of a higher than normal electric potential gradient in the presence of Cl but not of another poorly reabsorbable anion, such as SO4(2-). These observations indicate that, in our patients, Henle's loop function is normal, while the collecting tubule function is abnormal. We suggest that NaCl wasting and enhanced tubular secretion of H+ and K in our patients might result from an abnormally low conductance to Cl in distal nephron site(s) where Na reabsorption is electrogenic, possibly the cortical collecting tubule. A larger than normal transtubular electric gradient would be generated by Na reabsorption, causing: (1) a direct stimulation of tubular secretion of K and H+ (leading to hypokalemia and alkalosis) and (2) inhibition of the reabsorption of Na ('trapped' into the tubular lumen by electric forces), with consequent extracellular volume contraction, hyperreninemia and hyperaldosteronism.     

Micropuncture and clearance measurements of segmental reabsorption by the rat nephron

BACKGROUND.: We wanted to verify whether the calculations of segmental tubularreabsorption obtained during water diuresis were supported bydirect micropuncture measurements. METHODS.: Experiments were performed on 18 rats during baseline waterdiuresis (B) and after the administration of frusemide (F),10 mg/kg, by whole-kidney clearance measurements and micropuncturecollections from early distal (ED) and last proximal (LP) tubularsegments. RESULTS.: GFR was 957±79 in B, 1053±77 µl/min in F,P>0±13. SNGFR was 38±1 in 166 and 38±1nl/min in 165 tubules respectively, P>0.77. In LP collectionsthe percentage reabsorption was 71±2 in B and 76±2%during F (P> 0.07)in 99 and 95 samples respectively. Theabsolute proximal reabsorption was not changed by F (27.6±1.5versus 27.7±1·3 nl/min, P>0.96). The data weresuperimposable when the analysis was restricted to paired data.The difference between ED and LP resorption was 17±3during B and fell significantly (P<0.008) to 5±3%during F. The percentage of GFR excreted during F, measuredby clearance techniques, and the percentage delivery of filtratebeyond the proximal tubule, measured independently by micropuncture,were not different (27±2 versus 24±2%, P>0.10),while they were significantly correlated (P<0.04). The calculationsof segmental Na reabsorption along the different nephron segmentsby clearance techniques were not statistically different fromand were significantly correlated with the reabsorptions measureddirectly by micropuncture. CONCLUSIONS.: The present experiments validate the calculations of reabsorptionby techniques applicable to human studies of clinical physiology.     

Effects of the adenosine A1 receptor inhibitor FK 838 on proximal tubular fluid output in rats.

BACKGROUND: Adenosine A(1) receptor blockade has been suggested as a treatment in conditions with sodium and fluid retention because it increases urinary Na(+) excretion and increases proximal tubular fluid output. In the present study, we examine the time course for the renal responses to adenosine A(1) receptor blockade in order to investigate whether the effects may be prolonged and not just temporary. METHODS: The acute effects of the adenosine A(1) receptor inhibitor FK 838 on segmental tubular Na(+) handling were examined by a renal clearance technique in conscious chronically instrumented rats. Lithium clearance (C(Li)) was used as a clearance marker of proximal tubular fluid output. RESULTS: Acute adenosine A(1) receptor inhibition did not affect the glomerular filtration rate (GFR) significantly. In contrast, the inhibition led to significant increases in C(Li) (from 290+/-28 to 431+/-28 microl/min/100 g), fractional Li(+) excretion (FE(Li)) (from 33+/-2 to 47+/-3%) and fractional Na(+) excretion (FE(Na)) (from 0.44+/-0.07 to 2.03+/-0.42%). Sodium excretion, expressed as a fraction of proximal tubular fluid output (C(Na)/C(Li)), rose from 1.3+/-0.2 to 4.2+/-0.4%, suggesting that the natriuretic effect was supported by inhibition of distal nephron Na(+) reabsorption. All values returned to baseline values during the clearance study and thereby indicated that neither proximal tubular fluid output nor urinary sodium excretion remained elevated for a prolonged time. CONCLUSION: It is concluded that in conscious unstressed rats, acute adenosine A(1) receptor inhibition by FK 838 led to a significant natriuresis that was caused by inhibition of proximal tubular Na(+) reabsorption, possibly with a contribution from inhibition of distal nephron Na(+) reabsorption. The increased proximal tubular fluid output and the increased urinary Na(+) excretion returned to baseline values during the clearance study, indicating that none of these effects of adenosine A(1) blockade were long lasting.     

Intrarenal sodium handling during chronic spironolactone treatment

Intrarenal sodium handling was studied in 8 patients with essential hypertension before spironolactone treatment (200 mg/day), on the 4th day of treatment, and after 3 months of treatment. The results were compared with similar studies with chlorthalidone (50 mg/day). Renal sodium handling was assessed by simultaneous determination of the glomerular filtration rate, sodium, and potassium excretion, and maximal free-water clearance (CH2O). We took CH2O as an index of 'distal' sodium chloride reabsorption from which the proximal sodium reabsorption was calculated. During the first days of spironolactone treatment a natriuresis and increase in urinary flow rate was found. It resulted in a decrease of the extracellular fluid volume amounting to 0.9 liters and a 2.5-fold increase in the plasma renin activity. Potassium excretion showed a small but significant rise. After 3 months, virtually the same degree of volume depletion was found, which was comparable to that obtained after 3 months of chlorthalidone treatment. CH2O, as a fraction of glomerular filtration rate, decreased by 24% both after 3 days and 3 months, whereas proximal sodium reabsorption increased from 87.8 to 90.4% of the filtered load. CH2O, corrected for the 'distal' delivery of sodium, decreased from 85.3 to 80.7%. These changes were nearly the same as those found after 3 months of chlorthalidone treatment. It is concluded from these calculated values that spironolactone inhibits sodium chloride reabsorption in the diluting segment of the nephron and that the resulting increase in sodium delivery or urinary flow to the potassium excretory site partly counteracts the blocking effect of spironolactone on this part of the nephron, thus increasing potassium excretion during the acute administration of this drug.     

Renal handling of urea in subjects with persistent azotemia and normal renal function

Fourteen subjects with persistent azotemia and normal glomerular filtration rate were studied by renal clearances and hormonal determinations to establish the nephron site of altered urea transport and the mechanism(s) responsible for their azotemia. During constant alimentary protein, urea nitrogen appearance was normal and urea clearance was much lower than in 10 age-matched control subjects (23.3 +/- 2.1 ml/min and 49.6 +/- 2.6 ml/min per 1.73 m2, P less than 0.001). Inulin and para-aminohippurate clearances, blood volume and plasma concentration of antidiuretic hormone were within normal limits. During maximal antidiuresis, in spite of greater urea filtered load, the urinary excretion of urea was less, and both the maximum urinary osmolality and the free-water reabsorption relative to osmolar clearance per unit of GFR were greater than in control subjects. After sustained water diuresis, the plasma urea concentration markedly decreased to near normal levels in azotemic subjects. The basal urinary excretion of prostaglandins E2 was significantly reduced in azotemic subjects and was directly correlated with fractional urea clearance (r = 0.857, P less than 0.001). An additional group of control subjects (N = 8) showed a marked reduction of fractional clearance of urea after inhibition of prostaglandin synthesis (P less than 0.01). These data suggest that azotemia is due to increased tubular reabsorption of urea in the distal part of nephron, presumably because of increased back diffusion in the papillary collecting duct, accounting for the enhanced maximum urinary osmolality and free-water reabsorption. Renal prostaglandin E2 may participate in the pathogenesis of azotemia by altering recycling of urea in the medulla.     

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.     

Abnormal circadian rhythm of diuresis or nocturnal polyuria in a subgroup of children with enuresis and hypercalciuria is related to increased sodium retention during daytime

PURPOSE: In a subgroup of children with enuresis an increase in nighttime water and solute excretion has been documented. To investigate if modifications in renal function are involved in nocturnal enuresis, we assessed circadian variation in natriuresis and tubular sodium handling in polyuric hypercalciuric children. MATERIALS AND METHODS: A total of 10 children with proved hypercalciuria and nocturnal polyuria and 10 age matched controls were included in the study. A 24-hour urine collection was performed in 8 sampling periods for measurement of urinary sodium excretion. Segmental tubular sodium transport was investigated during a daytime oral water load test and calculated according to standardized clearance methodology. RESULTS: The children with enuresis showed a marked increase in the fractional excretion of sodium during the night (0.93% +/- 0.36%), while daytime sodium excretion was decreased (0.84% +/- 0.23%). Analysis of segmental tubular sodium transport revealed decreased delivery of sodium to distal tubule (C(H2O) + C(Na) = 10.7 ml/100 ml glomerular filtration rate), indicating increased proximal tubular sodium reabsorption but also stimulation of distal sodium reabsorption as demonstrated by increased fractional distal sodium reabsorption (92.9% +/- 2.2%, controls 90.5% +/- 2.9%). Increased distal reabsorption was associated with increased fractional potassium excretion (17.5% +/- 2.7%, controls 13.6% +/- 6.4%), indicating increased distal tubular sodium/potassium exchange. CONCLUSIONS: No intrinsic defect in renal tubular sodium transport was found, but during the day increased sodium reabsorption in proximal and distal tubules was observed, suggesting extrarenal factors to be involved in altered circadian variation in solute and water excretion by the kidney.     

Amiloride-sensitive and amiloride-insensitive kaliuresis in advanced chronic kidney disease

BACKGROUND: Salt delivery to the distal nephron and sodium reabsorption in this segment are considered critical factors that modulate kaliuresis in chronic kidney disease (CKD). Amiloride, a drug that blocks Na(+) reabsorption in the distal nephron, can help to assess the role of Na+ transport in this segment on the kaliuresis of CKD patients. METHODS: A bolus of amiloride (1 mg/kg body weight) followed by an intravenous infusion (1 mg/kg body weight per hour) was administered to 6 normal subjects and 10 patients with CKD undergoing water diuresis. Serum and urine electrolytes were measured. Glomerular filtration rate (GFR) was measured with clearance of (125)I-iodothalamate. RESULTS: Normal subjects and CKD patients had a control fractional excretion of potassium (FE(K)(+)) of 26% +/- 11% and 126% +/- 28%, respectively; the corresponding FE(Na)(+) was 2.3% +/- 0.8% and 15% +/- 3%. In response to amiloride, FE(Na)(+)increased significantly to 3.5% +/- 0.6% and 20% +/- 3% in normal and CKD subjects, respectively, and FE(K)(+) decreased significantly to 6.5% +/- 0.6% and 39% +/- 8%, respectively. Amiloride-sensitive and amiloride-insensitive kaliuresis in normal subjects were 71.4% and 28.6%, respectively; the corresponding values for CKD patients were 73% and 27%, respectively. Urine output correlated positively with kaliuresis in CKD. CONCLUSIONS: The very high FEK+ observed in CKD occurs in the absence of hyperkalemia and is largely amiloride-sensitive; therefore maintenance of potassium balance by the kidney in CKD is mostly dependent on sodium reabsorption through channels along the distal nephron. The high urinary flow of CKD further promotes potassium excretion.     

Small intra- and large inter-individual variability in lithium clearance in humans

We report the inter- and intra-individual variability in fractional lithium clearance (CLi), an alleged quantitative index of Na and water delivery from the proximal tubules, in humans (N = 91). The inter-individual variability was large, the variation coefficients at various Na excretion rates ranging between 11% and 19%. The intra-individual variability was small, the relative intra-individual standard deviation for duplicate measurements (N = 33) being 5%. These observations suggest large inter-individual differences in proximal tubular Na reabsorption. To confirm this, we also studied the inter- and intra-individual variability in the maximum urine flow during water diuresis (Vmax), an index of Na delivery to the diluting segment. They were found to be almost identical to the inter- and intra-individual variability in CLi, and fractional CLi and Vmax correlated strongly (r = 0.83, P less than 0.001). In addition, the inter-individual variability in the fractional clearance of uric acid (CUA), a directional marker of Na reabsorption in the proximal tubules, was large, but the intra-individual variability small. The correlation between fractional CUA and CLi, however, was relatively weak (r = 0.40, P less than 0.01). Although our results do not prove the exact, quantitative validity of the lithium clearance concept, we conclude that the variability in CLi reflects large inter-individual differences in Na handling in the proximal segments of the nephron. Our observations also have implications for the use of the lithium clearance method. The large inter-individual variability in CLi makes the method less suitable to detect subtle differences in CLi in small, unpaired groups of subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

A hypothesis linking sodium and lithium reabsorption in the distal nephron.

BACKGROUND: A hypothesis is proposed linking Na(+) and Li(+) reabsorption in the distal nephron. The handling of these two ions in the distal nephron is related because they share the same apical membrane entry mechanism: the amiloride-sensitive Na(+) channel (ENaC). However, the two ions exit the cell through different transport mechanisms: Na(+) via the Na(+)-K(+)-ATPase and Li(+) via the Na(+)/H(+) exchanger. Studies in rats have shown that under normal circumstances hardly any Li(+) is reabsorbed in the distal nephron, so that the urinary excretion of Li(+), expressed as a fraction of the delivery to the early distal tubule (FE(Li dist)), amounts to approximately 0.97. In contrast, during severe dietary Na(+) restriction, FE(Li dist) decreases to 0.50-0.60. Our hypothesis is that the absence of distal Li(+) reabsorption during intake of a normal diet can be explained by a negative driving force for Li(+) entrance across the apical membrane in those segments in which ENaC is active. METHOD: We propose a model that incorporates this concept. RESULTS: The model indicates that the lowering of FE(Li dist) during dietary Na(+) restriction can be explained by activation of apical ENaC in extra sub-segments further downstream. In these extra sub-segments the driving force for Li(+) reabsorption is positive, leading to significant Li(+) reabsorption. During dietary K(+) restriction, FE(Li dist) is reduced to 0.35-0.55. The model shows that this reduction in FE(Li dist) can be explained by hyperpolarization of the apical membrane in ENaC-containing sub-segments, which is known to occur in this condition. CONCLUSION: We conclude that the model may improve current understanding of both Na(+) and Li(+) handling in the distal nephron.     

Indomethacin increases renal lithium reabsorption in man

We examined the effect of semi-acute indomethacin (4 x 50 mg orally during the preceding 27 h) on renal function parameters, including fractional lithium reabsorption (FRLi) in seven healthy subjects during a 200 mmol and a 40 mmol sodium diet. Studies were carried out during maximal water diuresis. During the sodium-rich diet, indomethacin raised minimal urine osmolality from 61 +/- 1 to 72 +/- 2 mosm/kg (P less than 0.05), and during the sodium-restricted diet from 55 +/- 3 to 93 +/- 6 mosm/kg (P less than 0.01). Indomethacin reduced maximal free water clearance only during the low-sodium diet, and had no consistent effect on inulin clearance and the fractional excretions of sodium, phosphate and uric acid. Nevertheless, FRLi rose substantially, from 71 +/- 2% to 75 +/- 2% (P less than 0.01) and from 75 +/- 2% to 81 +/- 2% (P less than 0.01) during high- and low-sodium diets respectively. During either diet, indomethacin caused a significant reduction of 24-h urine PGE2 excretion. Since indomethacin is not supposed to influence proximal tubular sodium reabsorption, the rise in lithium reabsorption occurred beyond this nephron level, presumably in the loop of Henle. Clearly, the FRLi cannot be used as a quantitative marker of proximal tubular sodium reabsorption in humans in all conditions.     

Does defective chloride reabsorption at the loop of Henle play a major role in the pathogenesis of Bartter's syndrome?

In a patient with typical features of Bartter's syndrome, intrarenal reabsorption of sodium and water at different sites of the nephron, during maximal water diuresis, was studied twice in the course of the disease. During the first study, chloride reabsorption at the loop of Henle and renal diluting ability were not impaired. Chloride reabsorption at the loop of Henle was 0.72 (normal values 0.66-0.90), minimal urine osmolality 71 mosm/kg (normal values less than 75 mosm/kg) and chloride fractional clearance 0.44 (normal values 0.3-6.8). An impaired chloride reabsorption at the loop of Henle was found 6 months later. Chloride reabsorption at the loop of Henle was 0.28, minimal urine osmolality 197 mosm/kg and chloride fractional clearance 8.23. These data suggest that the defect in chloride reabsorption at the loop of Henle is not a pathophysiological abnormality of the syndrome because it appeared at a later time than the other symptoms.     

Pseudo-Bartter's syndrome from surreptitious diuretic intake: differential diagnosis with true Bartter's syndrome.

Five patients with pseudo-Bartter's syndrome from surreptitious diuretic abuse were compared with six patients with true Bartter's syndrome, diagnosed as a normotensive, hyperreninaemic, hypokalaemic metabolic alkalosis with normal urine chloride excretion, low CH2O/(CH2O+CCl) ratio during maximal water diuresis and negative urine screen for diuretics. The latter was positive for frusemide in four and for hydrochlorothiazide in the remaining pseudo-Bartter's patients. The two groups of patients did not differ as for plasma Na+, Cl-, K+, HCO3-, renin, and aldosterone, while uric acid and Mg2+ were greater in pseudo-Bartter's patients. Daily and fasting urine Na+, Cl- and K+ excretion were less in pseudo-Bartter's patients; however, there was substantial overlap of values between the two groups. Fractional distal solute reabsorption during maximal water diuresis was low in the six patients with Bartter's syndrome and in two pseudo-Bartter's patients; thus, this parameter could not be taken as a specific diagnostic marker of Bartter's syndrome. Frusemide administration, 40 mg i.v., induced a brisk increase of urine flow (11.7-21.8 ml/min), UOsm (148-186 mOsm/kg H2O) and FENa (14.6-24%) in Bartter's syndrome, but not pseudo-Bartter's patients; in all pseudo-Bartter's patients frusemide-induced changes of UOsm (13-97) and FENa (-0.5 to 10.2) were markedly less than in Bartter's syndrome patients. Frusemide resistance in pseudo-Bartter's patients was most probably related to diuretic-induced ECF volume contraction and increased proximal tubule solute reabsorption; in fact fractional lithium clearance (FELi, a marker of post-proximal solute delivery) was low in pseudo-Bartter's, but not in Bartter's syndrome patients.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nephron responses to converting enzyme inhibition in non-clipped kidney of Goldblatt hypertensive rat at normotensive pressures

To evaluate the effects of angiotensin converting enzyme inhibition (SQ 20881, CEI) on superficial nephron function of the non-clipped kidney in Goldblatt hypertensive rats in the absence of alterations in renal arterial pressure, control renal arterial pressure (RAP) was reduced first to the range generally obtained during CEI (124 +/- 4 mm Hg). RAP was maintained during the CEI period by adjustment of a suprarenal aortic clamp. At the reduced RAP, whole kidney and single nephron glomerular filtration rates (GFR) were reduced from the hypertensive levels and were lower than the measurements in normotensive control rats. During CEI, whole kidney GFR and single nephron GFR increased by 55 and 42%, respectively. There were decreases in absolute as well as fractional proximal reabsorption rates. In the intermediate nephron segment, fractional reabsorption was decreased, but absolute fluid reabsorption increased in proportion to the increased delivery rate. Proximal tubule and peritubular capillary hydrostatic pressures increased significantly during CEI also. These results indicate that an increased activity of the renin-angiotensin system occurring in Goldblatt hypertensive rats subjected to aortic constriction exerts effects to lower GFR and increase proximal reabsorption rate. The concomitant superficial nephron and whole kidney GFR responses to CEI when arterial pressure was maintained suggests that the pre-existing levels of angiotensin exerted similar influences on the total nephron population.     

Evaluation of the renal mechanisms for urate homeostasis in uremic patients by probenecid and pyrazinamide test

The tubular transport of urate was studied in 47 uremic patients and in 20 normal subjects using probenecid and pyrazinamide tests. There was a marked increase in urate excretion per nephron as the renal function deteriorated. Presecretory reabsorption of urate per nephron, which was almost complete in normal subjects, showed a diminution with increasing severity of chronic renal failure. Until the creatinine clearance had decreased to less than 10 ml/min, the secreted urate per nephron remained almost constant, while in the end stage of renal failure it was markedly decreased. With the progression of renal disease, the postsecretory reabsorption of urate per nephron diminished. In patients with a creatinine clearance less than 10 ml/min, it was 4 times lower than in normal subjects. These findings indicate that urate secretion does not contribute to the increase of urate excretion per nephron at any level of renal failure, whereas the impairment of both reabsorptive components accounts for the augmented urate excretion per nephron in uremic patients.     

Renal tubular sodium and water excretion in antibiotic-induced nephrotoxicity. Renal function in antibiotic nephrotoxicity

Clearance techniques were used to evaluate renal tubular sodium and water excretion in 4 patients with antibiotic-induced acute renal failure (ARF). Creatinine clearances and maximal urine flow rates of patients with ARF (22.6 and 5.23 ml/min, respectively) were significantly lower than control values during hypotonic volume expansion (125.5 and 13.71 ml/min, respectively, both p less than 0.01). During the period of maximal hydration, fractional sodium excretion (CNa/Ccr) and maximal urine osmolality (11.4% and 171 mosm/kg H2O, respectively) were increased compared to controls (1.04% and 53 mosm/kg H2O, respectively, both p less than 0.05). The increased CNa/Ccr observed in patients with ARF was consistent with reduced proximal sodium reabsorption as reflected by increased (CH2O + CNa)/Ccr and reduced fractional distal sodium reabsorption as indicated by decreased CH2O/(CH2O + CNa). The reduction in proximal and distal sodium reabsorption cannot be explained on the basis of an osmotic effect of urea as fractional clearances of BUN (CBUN/Ccr) were similar in patients with ARF and controls.     

Renal hemodynamic and tubular response to furosemide in man during normal and restricted sodium intake

To investigate the factors determining the natriuretic response to furosemide (F) during Na restriction, we performed clearance studies in 7 healthy humans on a daily Na intake of 200 and 20 mmol. The maximum urine flow during water loading (Vmax) and simultaneous F administration was used as index of tubular fluid output from the proximal tubules. The F-induced natriuresis was only moderately reduced during Na restriction (Na excretion on low vs. normal Na intake: 4.28 +/- 0.25 vs. 4.94 +/- 0.25 mmol/min; p less than 0.05). The diminished natriuresis was mainly due to a significant fall in Na delivery to Henle's loop of 0.51 +/- 0.10 mmol/min which was either caused by a decrease in filtered Na load or a rise in fractional proximal reabsorption. Fractional distal Na reabsorption was less suppressible by F during Na restriction, but this contributed relatively little (0.15 +/- 0.11 mmol/min) to the total reduction in Na excretion (0.66 +/- 0.10 mmol/min). The F-induced increases in uric acid, phosphate, and bicarbonate excretion suggest an additional proximal site of action of F. This was confirmed by a rise in lithium clearance (CLi), another alleged index of tubular fluid delivery from the proximal tubules. However, the magnitude of the rise in CLi to values markedly exceeding Vmax suggest that CLi overestimates tubular fluid delivery to Henle's loop during F administration.     

Expression of concentrative nucleoside transporters SLC28 (CNT1, CNT2, and CNT3) along the rat nephron: effect of diabetes

BACKGROUND: The renal reabsorption of natural nucleosides and a variety of nucleoside-derived drugs relies on the function of the apically located, Na(+)-dependent, concentrative nucleoside transporters CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3). The aims of this study were to determine the segmental localization of the three SLC28 family members and to establish whether streptozotocin-induced diabetes alters their expression. METHODS: SLC28 expression was measured by real-time polymerase chain reaction (PCR) on microdissected sections of rat nephrons. Diabetes was induced by streptozotocin treatment and the biochemical profiles of control, diabetic, and insulin-treated rats were established. The effect of diabetes on SLC28 expression was assessed in those segments that significantly express SLC28 genes. RESULTS: CNT1-3 mRNAs were expressed in the proximal tubule and glomerulus. In addition, CNT2 and CNT3 mRNAs were expressed in the outer medullary and cortical collecting duct, respectively. Diabetes reduced expression of the three CNTs in almost all nephron segments, and this effect was not prevented by an insulin treatment that normalized all blood and urine parameters. Diabetes increased CNT1 and CNT3 expression in the glomerulus and insulin treatment decreased it. CONCLUSION: The relative distribution of SLC28 gene expression suggests a role for the proximal tubule in renal nucleoside clearance and an accessory role for CNT2 and CNT3, in adenosine-mediated regulation of collecting duct functions. Diabetes probably may impair nucleoside clearance independently of insulin.