Effect of Extracellular Volume Expansion upon Sodium Reabsorption in the Distal Nephron of Dogs

Micropuncture studies have disclosed that extracellular fluid (ECF) volume expansion inhibits sodium reabsorption in the proximal tubule. The diuresis that ensues represents only a portion of the increment in sodium and water escaping proximal reabsorption, since a large and variable fraction of the increment is reabsorbed distally. In certain experimental models proximal reabsorption may be depressed by ECF volume expansion, yet only a negligible amount of sodium appears in the final urine. This suggests that saline diuresis is the consequence of depressed distal sodium reabsorption. Previous clearance and micropuncture studies have not conclusively proven this. Eight dogs were studied repeatedly: in some studies glomerular filtration rate and distal delivery were increased markedly without sodium administration; in others comparably high distal sodium loads were achieved by progressive 1/2 isotonic saline infusion. C(H2O) at high distal sodium loads was depressed by expansion of the ECF volume with hypotonic saline. The difference in free water formation between dogs which did and did not receive hypotonic saline was accounted for by the difference in sodium excretion. In one dog hypotonic saline expansion failed to depress free water formation; likewise the level of natriuresis in this dog was severely attenuated. The results of these experiments provide strong evidence that the natriuresis that occurs following ECF volume expansion with saline is a consequence of alteration in function of the distal nephron.     

Renal effects of lithium administration in rats: alterations in water and electrolyte metabolism and the response to vasopressin and cyclic-adenosine monophosphate during prolonged administration.

Lithium (Li+) chloride, 2 to 3 mEq. per kilogram of body weight, was administered intraperitoneally to normal Wistar rats daily for 4 to 66 days. This resulted in a marked reduction in urine osmolality (Uosm.) and increase in the excretion of water, Na+, K+, uric acid, and phosphate. The excretion of uric acid and potassium was a direct function of UNaV. The magnitude of depression in urine osmolality was significantly related to the rate of excretion of lithium in the urine, suggesting that the change in water reabsorption is dependent on the presence of the ion in the luminal side of the tubule. During 2 per cent saline diuresis, Li+-treated rats achieved less fractional free water reabsorption (TcH2O/GFR times 100) at any level of fractional osmolar clearance (Cosm./GFR times 100) than normal rats. On the other hand, during 0.225 per cent saline diuresis, fractional free water clearance (CH2O/GFR times 100) was normal over a wide range of fractional urine flow (V/GFR times 100), indicating intact function of the ascending limb of the loop of Henle. The intravenous infusion of vasopressin (VP) or dibutyryl cyclic-adenosine monophosphate (dcAMP) to Li+-treated rats resulted in a modest rise in Uosm. and a reduction in V/GFR times 100 and CH2O/GFR times 100. Although the response to VP appeared earlier than that to dibutyryl cyclic-AMP, the magnitude of the changes in Uosm., V/GFR times 100, and CH2O/GFR times 100 was eventually the same with both substances. Comparison between normal and Li+-treated rats revealed that the response to both VP and dibutyryl cyclic-AMP was blunted, albeit to a greater extent in the former. Inhibition by Li+ of adenylate cyclase will only partially explain the present data. Impairment in the release of endogenous VP or a block distal to the formation of cyclic-AMP must have played a role. In view of a normal diluting capacity and the increase in the excretion of phosphate and uric acid, it is suggested that Li+, when administered chronically in the present doses, inhibits proximal tubular reabsorption.     

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.     

Renal water and sodium handling in compensated liver cirrhosis: mechanism of the impaired natriuresis after saline loading

The present study was attempted to evaluate sodium and water balance in compensated liver cirrhosis. Renal sodium and water handling was studied in six cirrhotic patients without ascites and/or oedema and in six controls before and after saline loading. Fractional sodium reabsorption at the various nephron sites (proximal, diluting and distal) was evaluated by means of clearance techniques during maximal water diuresis and hypotonic saline infusion. Compensated cirrhotic patients showed a normal baseline sodium and water balance but a blunted natriuretic response when saline loaded (urinary sodium excretion after saline load = 338 +/- 290 compared to 933 +/- 504 mumol min-1 of controls; P less than 0.05). The impaired natriuresis was found to be related to an increased reabsorption of sodium in the proximal tubule (proximal fractional sodium reabsorption = 88.4 +/- 3.8 compared to 81.7 +/- 4.3% of controls; P less than 0.05). These findings confirm the hypothesis that renal sodium handling abnormalities might precede ascites formation. Additional studies, however, are necessary to further define renal factors mediating the increased reabsorption of sodium in compensated liver cirrhosis.     

Increased sodium reabsorption in the proximal and distal tubule of caval dogs

The effects of water diuresis, hypotonic NaCl, and hypotonic mannitol diuresis on renal sodium and water excretion were examined in normal dogs and in dogs with chronic constriction of the thoracic inferior vena cava and ascites (caval dogs). During all three diuretic states, the capacity to excrete solute-free water relative to the supply of sodium to the water clearing segment of the nephron was significantly greater in the caval dog. This finding was most evident during hypotonic NaCl diuresis but was also striking during hypotonic mannitol diuresis despite the more unfavorable gradient for sodium reabsorption at the distal tubule produced by this agent in caval dogs. In addition, fractional distal sodium load was significantly smaller in caval dogs during water diuresis and could not be increased as readily as in normal dogs by hypotonic NaCl or mannitol infusion. The data indicate that fractional sodium reabsorption is increased at the water clearing segment and the proximal tubule in caval dogs.     

Functional Characteristics of the Diluting Segment of the Dog Nephron and the Effect of Extracellular Volume Expansion on its Reabsorptive Capacity

The functional characteristics of the ascending limb of Henle's loop were examined during hypotonic saline infusion by measuring solutefree water clearance (C(H2O)) at varying rates of solute delivery. The influence of expansion of extracellular volume was studied by comparing C(H2O) during hypotonic saline diuresis in normal dogs with dogs whose extracellular volume had been expanded acutely by saline infusions or chronically by the administration of deoxycorticosterone acetate and salt.In normal animals hypotonic saline infusions greatly increased urine flow (V) and C(H2O) without appreciably augmenting osmolar clearance (C(osm)). C(H2O) was, therefore, analyzed as a function of V, rather than C(osm), since V was the best estimate of delivery of filtrate to the diluting segment. C(H2O) increased as a linear function of V without any evidence of saturation.The validity of interpreting increases in C(H2O) and V as indications of increased sodium reabsorption and delivery was reinforced by tissue studies that disclosed a rise in papillary osmolality with rising urine flows. The observed increase in C(H2O) and V could not, therefore, be due to a decrease in back diffusion of solute-free water as a result of a diminished osmotic driving force, but probably represented increased formation consequent to augmented delivery as a result of decreased fractional reabsorption in the proximal tubule.In animals whose extracellular volume was acutely or chronically overexpanded before the infusion of hypotonic saline, sodium excretion was greater, and C(H2O) less, at any given V. Although the curve relating C(H2O) to V was flatter than in the control group, no tubular maximum was observed. The diminished C(H2O) in this group was interpreted to mean that massive expansion of extracellular volume inhibits sodium reabsorption in the ascending limb of Henle's loop.     

Effect of Chronic Bile Duct Obstruction on Renal Handling of Salt and Water

Renal sodium reabsorption and the concentrating and diluting abilities of the kidney were evaluated in the same trained mongrel dogs before and after chronic common bile duct ligation (BDL). Glomerular filtration rate (GFR) and C(PAH) were not altered by BDL. The natriuretic response to a standardized infusion of 0.45% solution of NaCl was markedly blunted by BDL (P < 0.01); calculated distal sodium delivery was significantly less in experiments after BDL than in control studies. Furthermore, the fractional reabsorption of sodium at the diluting segment for any given rate of distal delivery was enhanced by BDL. Similarly, C(H2O)/100 ml GFR for a given sodium delivery was higher after BDL than control values. Maximal urinary concentration (Uosm-max) was lower after BDL, and the mean Uosm-max for the whole group of animals was 60% of the control value (P < 0.001). Mean maximal T(H2O)/100 ml GFR after BDL was not different from control values; however, T(c) (H2O)/100 ml GFR for a given Cosm/100 ml GFR was lower after BDL in three dogs only. The sodium content of the inner part of renal medulla after BDL was significantly lower than the values obtained in control animals. The excretion of an oral water load in the conscious state was impaired after BDL; although all animals excreted hypotonic urine, urinary osmolality was usually higher after BDL than in control studies. Maximal urinary concentration and the excretion of an oral water load were not affected by sham operation.These studies demonstrate that chronic, common bile duct ligation is associated with (a) enhanced sodium reabsorption both in the proximal and diluting segments of the nephron, (b) a defect in attaining maximal urinary concentration, (c) diminished sodium content in the renal papilla, and (d) impaired excretion of a water load. The results suggest that decreased distal delivery of sodium may underlie the abnormality in the concentrating mechanism and in the inability to normally excrete a water load. In addition, antidiuretic activity despite adequate hydration, may contribute to the impaired water diuresis. Chronic, common bile duct ligation appears to provide a readily available and reproducible model for the study of liver-kidney functional interrelationship.     

Tubular handling of sodium and phosphate in non-ascitic liver cirrhosis

The renal response to a maximal water load was evaluated in eight cirrhotic patients free of ascites and without previous evidence of ascites and in seven controls. Fractional sodium reabsorption in the proximal and diluting segment was estimated by clearance methods during hypotonic diuresis. Since phosphate excretion has been proposed as a proximal marker in liver cirrhosis, sodium reabsorption in the proximal tubule was compared with phosphate fractional excretion. In spite of a normal sodium balance during the pre-study period, non-ascitic cirrhotics showed a blunted proximal natriuretic response to maximal water load. In fact sodium excretion during hypotonic diuresis was reduced (p less than 0.05) and proximal sodium reabsorption increased (p less than 0.005) in cirrhotics. Fractional phosphate excretion was not impaired in our patients, and no correlation was found between phosphate excretion and proximal sodium reabsorption, as evaluated by clearance methods. This study demonstrates that an increased reabsorption of sodium in the proximal tubule is responsible for the impaired response to maximal water load in non-ascitic cirrhotics. Abnormalities in tubular handling of phosphate may account for the dissociation between proximal sodium reabsorption and phosphate excretion during hypotonic diuresis in these patients.     

Effect of acetazolamide on sodium reabsorption in the diluting segment in man.

The purpose of this study is to compare the effects of extracellular volume expansion (ECVE) and acetazolamide (ACTZ) on the transport of sodium in the diluting segment in man. All studies were performed at the peak of sustained oral water diuresis (Uosm. less than 75 mOsm. per kilogram). Distal delivery was estimated by the rate of urine flow and distal sodium reabsorption was approximated by the generation of free water. Free water clearance for given rates of distal delivery expressed as V(range 10 to 35 ml. per minute) was significantly lower and the clearance of sodium higher in the ACTZ group as compared with subjects with ECVE. However when CH2O was plotted against distal delivery expressed as CC1 + CH2O, no difference was detected between ACTZ and ECVE studies. We interpret these findings as suggesting that ACTZ does not have a direct inhibitory influence on the NaCl pump at the diluting site. Rather, the lowering of CH2O/V following ACTZ is due to the flooding of the diluting site with the poorly reabsorbable bicarbonate ion. Thus, the proximal action of ACTZ, which initiates urinary bicarbonate loss results secondarily in an apparent interference with the diluting mechanism in man.     

Effect of saline infusions on intrarenal distribution of glomerular filtrate and proximal reabsorption in the dog

The effect of acute extracellular volume expansion with saline on the intrarenal distribution of glomerular filtrate, was studied in dogs utilizing micropuncture techniques in which samples were obtained by both recollection and from new tubules.Recollection was examined in seven dogs during continuous hydropenia and in five dogs during continuous saline diuresis. Recollection was associated with an increase in nephron flow rate of 8% during hydropenia and 27% during saline diuresis. In addition, during continuous saline diuresis, shortened transit times and lowered intratubular pressures were recorded in previously punctured tubules. Despite increased tubular flow, fractional reabsorption was unchanged.Nephron glomerular filtration rates (gfr) were measured during hydropenia and then after acute volume expansion in 10 dogs. In the repunctured tubules gfr rose 38% more than total glomerular filtration rate (GFR). In contrast, when new tubules were punctured during volume expansion, nephron gfr and total GFR changed proportionately. The disproportionate rise in nephron gfr after volume expansion noted with the recollection technique appears to be artifactual when contrasted to micropuncture of new tubules. With acute volume expansion, fractional reabsorption decreased 15% in recollected samples and 16% in newly sampled tubules. Increased nephron gfr cannot account for the fall in fractional reabsorption. It is concluded that in dogs, saline diuresis is not associated with redistribution of filtrate from deep to superficial nephrons, and that the fall in proximal fractional reabsorption is caused by diminished absolute reabsorption.     

Urine concentration and dilution in hypokalemic and hypercalcemic dogs

The urine-concentrating mechanism was studied in chronic hypokalemia (seven dogs given a low K(+), high NaCl diet plus injections of deoxycorticosterone acetate [DOCA]) and chronic hypercalcemia (seven dogs given vitamin D). In the potassium-depleted dogs, muscle, serum, and urine K(+) fell markedly, but glomerular filtration rate (GFR) and body weight varied little. Maximum urine osmolality fell in all dogs (mean decrease = 45%); however, solute-free water reabsorption (T(CH2O)) at high rates of solute excretion remained normal in three of four dogs. Free water excretion (C(H2O)) increased normally or supranormally as a function of increasing Na(+) delivery to Henle's loop in six dogs so tested.Hypercalcemia of several weeks duration caused a decrease in both GFR (mean 36%) as well as in maximum urine osmolality (mean 57%). Maximum T(CH2O) was not invariably depressed; in fact, when the values were adjusted for the reduced number of functioning nephrons (T(CH2O)/C(In)), four of seven studies were normal. C(H20)/C(In) increased normally (or supranormally) with increasing fractional Na delivery to Henle's loop in four of five dogs.I conclude that the lowered maximum urine osmolality in these hypokalemic and hypercalcemic dogs was not related to abnormal water reabsorption from the collecting ducts. Although not specifically measured in this study, it is very likely that solute accumulation in the renal medulla was reduced. This probably was not caused by abnormal delivery of sodium to, nor reabsorption of sodium from Henle's loop. It is likely that a more subtle defect exists in the countercurrent mechanisms for establishing a steep concentration gradient in the renal medulla. In the few hypercalcemic dogs in whom GFR was very low, I believe that injury to, and blockage of medullary tubules could account for most of the reduction in maximum U(Osm). Although not specifically ruled out, there is no evidence here to suggest that high serum Ca(+) or low serum K(+) per se causes a defect in sodium and water reabsorption in the mammalian nephron.     

Chlorides in the Kidney,The Blood and the Urine in Experimental Nephritis

In 4 dogs, free water clearance (C_H2O) increased with solute excretion during loading with hypotonic saline until an osmolal clearance of 5–6 ml per 100 ml filtrate. C_H2O was similar after ureteroperitoneostomy of one kidney and 3–4 weeks after nephrectomy of the peritoneostomized kidney. C_H2O from a single excreting kidney was significantly higher than C_H2O of each kidney in the control period, in spite of higher osmolal clearances in control experiments. The increase in C_H2O probably reflects an increase in sodium reabsorption in the distal nephron, as two other indices of distal sodium reabsorption—the natriuretic effect of chlorothiazide and maximal reabsorption of solute-free water (T^cC_H2O)—increased per kidney after nephrectomy in studies on 4 other dogs. It is concluded that sodium reabsorption in the diluting segment (including the medullary part of the ascending limb of Henle's loop), and thereby the capacity to excrete free water and reabsorb solute-free water, are increased after nephrectomy and ureteroperitoneostomy. This increased capacity is not simply a consequence of increased delivery of solutes from the proximal tubules and is not dependent on tubular hypertrophy and hyperplasia.     

Mechanisms of the antinatriuretic action of physiological doses of angiotensin II in man

1. Angiotensin II (ANG II; 1 ng min-1 kg-1) or 5% (w/v) D-glucose (placebo) was infused in six normal male volunteers, pretreated with 500 mg of lithium carbonate, who were undergoing maximal water diuresis. 2. This dose of ANG II caused a circulating increment within the physiological range (27 +/- 4 to 48 +/- 9 pmol/l). 3. Compared with placebo, ANG II caused a significant fall in urinary sodium excretion (113 +/- 13 to 82 +/- 10 mumol/min). This antinatriuretic effect occurred without a fall in creatinine clearance (107 +/- 3 versus 113 +/- 3 ml/min). 4. ANG II caused a significant fall in fractional lithium clearance (28 +/- 2 to 23 +/- 2%). This may indicate a proximal tubular effect of ANG II. 5. ANG II also reduced fractional distal delivery [(sodium clearance plus free water clearance) divided by creatinine clearance], another measure of proximal tubular outflow. A parallel change in these two separate markers of proximal function supports an action of ANG II at this nephron segment. 6. Furthermore, the antinatriuretic effect of ANG II was unlikely to be due to stimulation of aldosterone secretion because (a) the fall in sodium excretion was temporally dissociated from the rise in aldosterone secretion, (b) potassium excretion also tended to fall during ANG II infusion and (c) aldosterone has a distal nephron effect, while, in this study, proximal nephron fractional reabsorption of sodium increased and distal nephron fractional reabsorption of sodium was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Renal tubular reabsorption of sodium and water during infusion of low-dose dopamine in normal man

1. Using the renal clearance of lithium (CLi) as an index of proximal tubular outflow of sodium and water, together with simultaneous measurements of effective renal plasma flow, glomerular filtration rate (GFR) and sodium clearance (CNa), renal function and the tubular segmental reabsorption rates of sodium and water during dopamine infusion (3 micrograms min-1 kg-1) were estimated in 12 normal volunteers. 2. CNa increased by 128% (P less than 0.001). Effective renal plasma flow and GFR increased by 43% (P less than 0.001) and 9% (P less than 0.01), respectively. CLi increased in all subjects by, on average, 44% (P less than 0.001). Fractional proximal reabsorption [1-(CLi/GFR)] decreased by 13% after dopamine infusion (P less than 0.001), and estimated absolute proximal reabsorption rate (GFR-CLi) decreased by 8% (P less than 0.01). Absolute distal sodium reabsorption rate [(CLi-CNa) x PNa, where PNa is plasma sodium concentration] increased (P less than 0.001), and fractional distal sodium reabsorption [(CLi-CNa)/CLi] decreased (P less than 0.001). 3. It is concluded that natriuresis during low-dose dopamine infusion is caused by an increased outflow of sodium from the proximal tubules that is not fully compensated for in the distal tubules.     

Effect of hypotonic expansion on sodium, water, and urea excretion in late pregnancy: the influence of posture on these results

Mineralocorticoid-treated, normotensive third trimester subjects positioned in lateral recumbency were studied before and during the infusion of 300 mEq of hypotonic saline. Urinary sodium excretion increased in all subjects from a mean value of 199 to 416 μEq/min. In 12 maximally hydrated subjects free water clearance (C_H2O) and urine flow (V) increased from means of 7.54 and 9.50 to 11.6 and 14.5 ml/100 ml of glomerular filtrate (GFR) Also the ratio of urea to inulin clearance (C_urea/C_inulin) increased from 0.59 to 0.64. The changes in the renal handling of water and urea suggest that fractional sodium reabsorption decreased at proximal nephron sites.     

The effect of the plasma bicarbonate level on proximal tubule sodium reabsorption in NH4Cl-loaded dogs.

In an attempt to examine the effects of mild and severe chronic metabolic acidosis on proximal tubule sodium reabsorption, 6 dogs were given 10 mEq. per kilogram per day and 5 dogs were given 20mEq. per kilogram per day of ammonium chloride for 3 days and compared to 12 normal dogs during a steady-state water diuresis and following the administration of ethacrynic acid (EA) intravenously (2 mg. per kilogram) utilizing standard clearance methodology, In the severely acidotic group (pH decrease is greater tthan 0.2) plasma pH was 7.08 +/- 0.06 and plasma bicarbonate was 6.3 +/- 1.0 Eq. per liter compared to a pH of 7.33 +/-0.02 and bicarbonate of 13.4 +/- 0.7 in mild acidosis (pH decrease is less than 0.2). During a steady-state water diuresis urine flow was 14.2 +/- 0.9 in severely acidotic compared to 10.5 +/-0.7 ml. per minute per 100 ml. glomerular filtration rate (GFR) in normal dogs (p is less than 0.01). Following EA sodium clearance increased 38.4 +/- 3.5 in severely acidotic dogs and 27.6 +/- 2.0 ml. per minute per 100 ml. GFR in normal dogs (p is less than 0.02). In mild acidosis, steady-state fractional urine flow and the increase in fractional sodium clearance following EA were not significantly different than normal dogs. We conclude that chronic metabolic acidosis leads to an increase in distal solute load and enhanced natriuretic effect of EA secondary to a decrease in proximal tubule sodium reabsorption which may be dependent upon the degree of reduction in the plasma bicarbonate level.     

The syndrome of hypertension and hyperkalemia with normal GFR (Gordon's syndrome): is there increased proximal sodium reabsorption?

The syndrome of hypertension and hyperkalemia, hyperchloremic acidosis with normal glomerular filtration rate (Gordon's syndrome) is characterised by volume expansion, suppressed renin and reduced mineralocorticoid-induced renal clearance of potassium. The clinical and biochemical defects are aggravated by high salt diet and corrected by low salt diet, leading to the hypothesis of excessive sodium reabsorption in the nephron proximal to where aldosterone acts. In this study, we used lithium clearance as a marker of proximal sodium reabsorption in three patients with Gordon's syndrome, in order to further localise the site in the nephron of defective sodium handling. Fractional excretion of lithium was decreased, and absolute and fractional proximal reabsorption of sodium was increased compared to normal controls. In addition, absolute distal reabsorption of sodium was decreased, consistent with decreased mineralocorticoid activity. Fractional excretion of potassium was markedly decreased and did not rise with increased distal delivery of sodium during saline infusion. However, after severe dietary sodium restriction had elevated plasma aldosterone (lowering plasma potassium levels to normal), fractional excretion of potassium was raised by saline infusion. Reduced lithium clearance in patients with Gordon's syndrome supports the hypothesis of increased proximal sodium reabsorption in this condition.     

Renal tubular effects of chronic phosphate depletion.

The effects of chronic phosphate depletion on renal tubular function were evaluated by micropuncture and free water clearance studies in the dog. Proximal tubular punctures demonstrated that chronic hypophosphatemia led to a reduction in ratio of tubular fluid to plasma inulin in late superficial tubular from 1.59+/-0.08 in control animals to 1.29+/-0.06 in phosphate-depleted dogs, with proportional inhibition of calcium and sodium reabsorption. The chronic decrease in proximal tubular fluid reabsorption was confirmed by the analysis of sustained water diuresis in conscious, phosphate-depleted dogs, before and after repletion of body PO4 stores, and in control animals. Urine flow rate/100 ml glomerular filtration rate (V/GFR) was significantly higher in PO4 DEPLETION THAN CONTROL (15.8+/-1.1 VS. 10.7+/-0.82). In addition, acetazolamide infusion did not increase V/GFR in phosphate-depleted dogs (15.8+/-1.1 vs. 17.16+/-0.9), supporting the conclusion that inhibition of proximal tubular fluid reabsorption was responsible for the elevated urine flow rate. PO4 repletion over 5 days reduced V/GFR to 9.2+/-0.7 despite no change in urine osmolality and no change in GFR, further suggesting a specific reversible alteration in proximal tubular reabsorption in phosphate depletion. Although hypercalciuria was a constant finding in phosphate depletion (fractional excretion of calcium of 2.04+/-0.4% vs. 0.47+/-0.13% in controls), the enhanced distal delivery of calcium was not a crucial factor; acute phosphate infusion reduced urinary calcium excretion to control values without affecting the reduced proximal tubular reabsorption in either intact or thyroparathyroidectomized phosphate-depleted dogs the change in distal nephron calcium reabsorption was independent of parathyroid hormone (PTH) levels since infusion of PTH failed to alter urinary calcium excretion. We conclude that chronic phosphate depletion leads to a reversible, sustained inhibition in proximal tubular reabsorptive fuction as well as a specific decrease in distal nephron calcium reabsorption. This latter reabsorptive defect is sensitive to phosplate infusion but not corrected by PTH.     

Mechanism of Impaired Water Excretion in the Hypothyroid Rat

The ability to excrete an oral water load and the renal diluting mechanism were studied in hypothyroid rats and in age-matched euthyroid controls. Hypothyroid animals excreted a significantly smaller fraction of a 50-ml/kg oral water load than controls, demonstrating the same limited ability to excrete free water as thyroid-deficient man. During hypotonic (0.45%) saline infusion, absolute sodium delivery to the diluting segment and free water clearance were markedly lower in hypothyroid rats. However, both fractional distal sodium delivery and fractional free water clearance were similar in hypothyroid and control animals, suggesting that the reduced absolute free water formation in hypothyroid rats was due to decreased net distal delivery. In support of this hypothesis was the observation that fractional distal sodium reabsorption was equal or higher in thyroid-deficient rats, which indicates that the sodium reabsorptive capacity of the diluting segment was preserved in these animals. The results cannot be attributed to incomplete suppression of antidiuretic hormone (ADH) since they were identical in diabetes insipidus rats, nor to different rates of non-ADH-dependent backflux of filtrate since tissue osmolality and solute concentrations in the cortex, medulla, and papilla were similar in hypothyroid and control rats of both Sprague-Dawley and Brattleboro strains.The functional integrity of the diluting segment in hypothyroid rats was further demonstrated in experiments in which distal delivery was increased by contralateral nephrectomy or by administration of carbonic anhydrase inhibitors which decrease proximal sodium reabsorption. In both studies, fractional free water clearance increased markedly reaching levels significantly greater than in euthyroid controls.These results demonstrate that the impaired ability of the hypothyroid rat to excrete a water load is not due to incomplete suppression of ADH or decreased reabsorptive capacity of the diluting segment but results from decreased filtrate delivery to this site secondary to reduced GFR.