Functional characteristics of the diluting segment of the dog nephron and the effect of extracellular volume expansion on its reabsorptive capacity

Mechanisms of renal tubular defects in old age

The mechanisms of renal tubular dysfunction in old age have been examined in twenty-eight clinically healthy elderly subjects without infection, and in fourteen subjects of similar age with laboratory evidence of intrarenal infection. The data were compared with those from thirteen clinically healthy young subjects. Studied were: proximal tubular (Tm(PAH)) and distal tubular (CH2O) activity, minimal and maximal osmolal U/P ratios, maximal osmolal excretion in hydropenia, and GFR levels under standard hydration and under water-loading. The reduction of GFR in old age is evident particularly in men under conditions of standard hydration: it is accentuated in the presence of renal infection. Proximal tubular activity is also significantly lower in elderly men, especially if they have chronic bacteriuria. The reduction is closely related to GFR levels, with identical Tm(PAH):C(in) ratios in all groups. This supports the intact nephron hypothesis for this part of the nephron. Distal tubular activity is depressed in old age in both sexes proportionately more than proximal tubular activity or the GFR. The lower CH2O: GFR ratios imply a selective distal tubular damage. Maximal osmolal U/P ratios in hydropenia are significantly higher in the young (mean 367) than in either the elderly non-infected (mean 279) or the elderly infected subjects (mean 212). Conversely, minimal U/P ratios in water-loading are lower in the young (mean 0.247) than in either elderly group (means 0.418 and 0.668). Osmolal excretion in hydropenia is not different between the groups, but urine flows in water-loading clearly separate them. The data indicate that simple functions of the distal-collecting tubule (e.g. the CH2O), are less affected in old age than are functions involving several medullary structures (as is the maximal U(osm) or U/P ratio). They suggest that the main impairment of the distal tubular cell involves the failure to achieve a proper osmotic gradient between tubular fluid and blood, rather than an inability to excrete or re-absorb an adequate amount of solute. Finally, it appears that renal infection aggravates the larger glomerular and proximal tubular deficits observed in non-infected men: it depresses distal tubular function equally in both sexes.

Evaluation of lithium clearance as a marker of proximal tubule sodium handling

Estimations of proximal tubule sodium reabsorption with the FELi method come closer to direct measurements than any other indirect method. There is little doubt that most lithium reabsorption takes place in the proximal tubules, very likely in proportion to the reabsorption of sodium and water. It is also likely that changes in proximal tubule sodium reabsorption due to changes in volume status are paralleled by changes in proximal tubule lithium reabsorption, at least in the superficial nephrons. Nonetheless, changes in FELi probably do not purely reflect changes in proximal reabsorption, since lithium is also handled beyond the proximal tubules. Acknowledged problems are lithium reabsorption in Henle's loop and in the late distal and collecting tubules. The latter occurs in the rat and the dog, but not or much less in men. Sodium restriction enhances this lithium transport considerably. It is as yet uncertain whether other conditions, such as increased vasopressin activity or lowering of renal perfusion pressure, also influence this transport. Amiloride appears to prevent this reabsorption of lithium. Therefore, this drug can be used in lithium clearance studies whenever unwanted "distal" lithium reabsorption is expected. Lithium reabsorption in Henle's loop forms a greater problem as it cannot be prevented by any drug without influencing proximal tubule reabsorption. It is estimated that about 7% of the filtered lithium (one-tenth of total lithium reabsorption) is normally taken up here, preferentially in deep nephrons. In view of studies with furosemide, this reabsorption probably varies with sodium intake, but the proportion of this variation to that of proximal tubule lithium reabsorption is obscure. This remains an uncertain factor in any circumstance where the lithium clearance method is used. In some conditions the change in FELi may be so large relative to the expected changes in proximal reabsorption, that use of FELi as marker of end-proximal solute delivery seems unjustified. Disproportionately large suppression is likely during mineralo-corticoid-induced volume expansion, and stimulation during prostaglandin synthesis inhibition and vasopressin. Based on observations in these conditions the potential range of lithium reabsorption in the loop of Henle would be 0 to 15% of filtered load. In this review attention was paid mainly to the validity of lithium clearance as a pure "proximal marker". Many of our interpretations suffer from incomplete certainty with respect to the renal effects of tested maneuvers, a problem which is acknowledged.(ABSTRACT TRUNCATED AT 400 WORDS)

Site of the action of a synthetic atrial natriuretic peptide evaluated in humans

The renal site of the natriuretic effect of human, atrial natriuretic peptide (hANP) was studied using clearance techniques in eight salt-loaded normal volunteers undergoing maximal water diuresis. Lithium was used as a marker of proximal sodium reabsorption. According to a two-way, single blind, crossover design, hANP (Met12-(3-28)-eicosahexapeptide, (2 micrograms/min) or its vehicle (Ve) were infused for two hours, followed by a two-hour recovery period. Blood pressure, heart rate and insulin clearance remained unchanged. During hANP infusion, the filtration fraction increased slightly from 19.6 to 24.3% (P less than 0.001), fractional water excretion rose transiently at the beginning of the infusion. Fractional excretion of sodium increased markedly from 2.2% to 7.4% (P less than 0.001) but remained unchanged with Ve. ANP increased fractional excretion of lithium slightly from 46 to 58% (P less than 0.01), while it remained stable at 47% during Ve. The distal tubular rejection fraction of sodium calculated from sodium and lithium clearances rose markedly from 4.7 to 13% (P less than 0.001) and returned to 6.2% at the end of the recovery period. Thus, under salt loading and water diuresis conditions, hANP infusion did not alter GFR, but reduced proximal reabsorption of sodium, and markedly enhanced the fraction of sodium escaping distal tubular reabsorption, suggesting that hANP-induced natriuresis is due, for an important part, to inhibition of sodium reabsorption in the distal nephron.

Renal lithium clearance as a measure of the delivery of water and sodium from the proximal tubule in humans

The delivery of tubular fluid from the proximal straight tubule to the thin descending limb of the loop of Henle was measured in nine volunteers by the lithium clearance method and by the water diuresis method. Lithium clearance and urine flow during water diuresis varied in proportion to each other with a high degree of correlation (r = 0.93, p less than 0.001). The proportionality between lithium clearance and urine flow was unaffected by variations in fractional sodium excretion. The results support the assumption that lithium clearance can be used as a measure of the delivery of tubular fluid from the proximal tubule in humans with sodium intakes within the normal range.

Insights into intrarenal sites and mechanisms of action of diuretic agents

Effective diuresis requires both sufficient glomerular filtrate and adequate delivery of the diuretic drug to the lumen of the renal tubule. Diuretics will not "force open" the kidney. Diuretics that work primarily in the proximal tubule include osmotic diuretics (e.g., mannitol), diuretics that interfere with the adenyl cyclase system (e.g., xanthines), and those which inhibit carbonic anhydrase (e.g., acetazolamide). Some thiazide and thiazide-like diuretics have a secondary site of action in the proximal tubule based on either carbonic anhydrase inhibition or other mechanisms, such as inhibition of sodium phosphate reabsorption. The diuretics that work primarily in the medullary diluting segment of the loop of Henle, furosemide and ethacrynic acid, block the active reabsorption of chloride and interfere with the tubular reabsorption of free water. The exact mechanism remains unknown. These diuretics tend to have a "high ceiling," to be potent and rapidly acting, and to have a short duration of effect. They are excellent for the treatment of severe fluid overload or pulmonary edema but are not ideal for the treatment of uncomplicated hypertension. Furosemide is a sulfonamide derivative; ethacrynic acid can be used in patients who are allergic to sulfa drugs. Diuretics that work primarily in the cortical diluting segment include the thiazides and thiazide-like drugs. They inhibit sodium transport by an undetermined mechanism. Most of them seem to reach a dose-response plateau beyond which little additional effect is gained by increasing the dose. Most of them appear to lose efficacy as the glomerular filtration rate decreases, except for metolazone and indapamide. The thiazides are most commonly used to treat hypertension. Diuretics that work primarily in the distal tubule and collecting tubule include the aldosterone inhibitor spironolactone and two drugs that impair tubular reabsorption of sodium by direct action, triamterene and amiloride. These drugs are primarily used for their potassium-sparing effect.

The role of chloride transport in the thick ascending limb in the pathogenesis of Bartter's syndrome

Fractional chloride reabsorption in the thick ascending limb of the loop of Henle, measured by clearance techniques, is subnormal in patients with Bartter's syndrome. This defect is a marker for the syndrome and, presumably, is the cause of the supranormal tubular secretion of potassium that characterizes the disorder. The potassium depletion that results from excessive potassium excretion is probably the stimulus for the increased synthesis of prostacyclin by blood vessels and prostaglandin E2 by kidneys that occurs in Bartter's syndrome. The overproduction of prostaglandins mediates hyperreninemia, supranormal plasma bradykinin, supranormal plasma norepinephrine and vascular resistance to the pressor effects of angiotensin II and norepinephrine; treatment with a prostaglandin synthetase inhibitor corrects these abnormalities. Increases in angiotensin II and in norepinephrine appear to be compensatory changes, occurring in response to vasodilatation induced by vascular prostaglandins to maintain blood pressure. The hyperreninemia also stimulates production of aldosterone with aggravation of potassium loss.

Treatment of the syndrome of inappropriate secretion of antidiuretic hormone by urea

Recent data have shown the role of urea in the urinary concentrating mechanism. We studied the effects of exogenous urea administration in hyponatremia associated with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In 20 patients with SIADH, we observed a positive correlation between serum sodium and blood urea levels (r = 0.65; p less than 0.01). In one patient with an oat cell carcinoma and SIADH-induced hyponatremia, we observed the same positive correlation (r = 0.80; p less than 0.01) but also a negative one between the excreted fraction of filtered sodium and urinary urea (r = -0.67; p less than 0.001). The short-term administration of low doses of urea (4 to 10 g) resulted in correcting the "salt-losing" tendency of this patient. Longer term administration of high doses of urea (30 g/day) was attempted with the same patient as well as with a healthy volunteer subject with Pitressin-induced SIADH. in both patients, urea treatment lowered urinary sodium excretion as long as hyponatremia was significant (less than 130 meq/liter). Urea treatment also induced a persistent osmotic diuresis, allowing a normal daily intake of water despite SIADH. This was clearly shown during the long-term treatment of a third patient with SIADH who was taking 30 g urea/day during 11 weeks. It is concluded that urea is a good alternative in the treatment of patients with SIADH who presented with persistent hyponatremia despite the restriction of water intake.

The clinical physiology of water metabolism. Part II: Renal mechanisms for urinary concentration; diabetes insipidus

The renal reabsorption of water independent of solute is the result of the coordinated function of the collecting duct and the ascending limb of the loop of Henle. The unique juxtaposition of the ascending and descending portions of the loop of Henle and of the vasa recta permits the function of a counter-current multiplier system in which water is removed from the tubular lumen and reabsorbed into the circulation. The driving force for reabsorption is the osmotic gradient in the renal medulla which is dependent, in part, on chloride (followed by sodium) pumping from the thick ascending loop of Henle. Urea trapping is also thought to play an important role in the generation of a hypertonic medullary interstitium. Arginine vasopressin (AVP) acts by binding to receptors on the cell membrane and activating adenylate cyclase. This, inturn, results in the intracellular accumulation of cyclic adenosine monophosphate (AMP) which in some fashion abruptly increases the water permeability of the luminal membrane of cells in the collecting duct. As a consequence, water flows along an osmotic gradient out of the tubular lumen into the medullary interstitium. Diabetes insipidus is the clinical condition associated with either a deficiency of or a resistance to AVP. Central diabetes insipidus is due to diminished release of AVP following damage to either the neurosecretory nuclei or the pituitary stalk. Possible causes include idiopathic, familial, trauma, tumor, infection or vascular lesions. Patients present with polyuria, usually beginning over a period of a few days. The diagnosis is made by showing that urinary concentration is impaired after water restriction but that there is a good response to exogenous vasopressin therapy. Nephrogenic diabetes insipidus can be identified by a patient's lack of response to AVP. Nephrogenic diabetes insipidus is caused by a familial defect, although milder forms can be acquired as a result of various forms of renal disease. Central diabetes insipidus is eminently responsive to replacement therapy, particularly with dDAVP, a long lasting analogue of AVP. Nephrogenic diabetes insipidus is best treated with a combination of thiazide diuretics as well as a diet low in sodium and protein.

Effect of acute metabolic acidemia on renal electrolyte transport in man

The effect of acute NH4C1-induced metabolic acidemia on renal electrolyte excretion was examined in nine healthy subjects during steady state water diuresis. Following oral NH4C1, venous pH and bicarbonate concentration declined significantly (p less than 0.01) while inulin and PAH clearances remained unchanged. Mean sodium excretion (UNaV) increased from 142 +/- 16 mueq/min (mean +/- SEM) to 310 +/- 49 mueq/min (p less than 0.01) at 8 hr without change in plasma aldosterone or renin levels. Urine flow remained unchanged while CH2O/(CH2O + CCl) declined significantly, suggesting that acute metabolic acidemia inhibits sodium transport in the distal nephron. Similar results were observed in two subjects with central diabetes insipidus. Three subjects restudied following the ingestion of an equivalent amount of chloride administered as NaCl, failed to demonstrate a significant rise in UNaV. UKV fell acutely from 91 +/- 13 to 45 +/- 5 mueq/min (p less than 0.001) despite an increase in serum potassium concentration. No change in plasma insulin was observed. UCaV rose from 66 +/- 15 to 143 +/- 18 microgram/min and fractional excretion of calcium increased from 0.55 +/- 0.13 to 1.24 +/- 0.21% (p less than 0.001). Total serum calcium fell slightly, but ionized calcium rose from 3.99 +/- 0.05 to 4.30 +/- 0.03 mg/dl (p less than 0.001). No change in nephrogenous cyclic (cAMP) excretion was observed. In conclusion, acute metabolic acidemia in man (1) inhibits sodium reabsorption in the distal nephron independent of changes in plasma aldosterone concentration, filtered chloride load, or volume expansion; (2) inhibits potassium excretion despite a rise in serum potassium concentration; and (3) inhibits tubular calcium reabsorption independetn of changes in parathyroid hormone (as reflected by urinary cAMP).

Characteristics of transcellular NaCl reabsorption in the kidney

To examine the characteristics of transcellular, energy-requiring NaCl reabsorption, increased delivery of tubular fluid of different bicarbonate and chloride composition to the outer medulla was achieved by infusion of acetazolamide (30 mg/kg body wt) or 0.9% NaCl in anaesthetized dogs. As an index of energy-requiring NaCltransport, cortical and outer medullary metabolism were determined by the heat production technique. Outer medullary metabolism was correlated to sodium excretion but not to chloride excretion. A rise in sodium excretion up to 20-25% of the filtered load during hydropenia was associated with a 70-80% increase in outer medullary metabolism. Further increments in sodium excretion induced by increasing systemic blood pressure and thereby increasing glomerular filtration rate or by infusing 2.9% NaCl did not significantly increase either reabsorption of sodium or cortical and outer medullary metabolism. By infusion of furosemide (2mg/kg body wt) sodium reabsorption and outer medullary heat production could be reduced below control values. These experiments show that sodium rather than chloride determine transcellular NaCl reabsorption. The maximal capacity of this reabsorption system is approached first at sodium excretion rates beyond the physiological range. Calculations based on clearance studies and heat production measurements, before and after furosemide infusion, indicate that transcellular NaCl reabsorption accounts for more than half of the NaCl reabsorption in the kidney.

Renal functional adaptation in the remnant kidney in patients with renal agenesis and in patients nephrectomized in childhood

The renal response to volume expansion was determined in four patients with renal hypertrophy due to unilateral renal agenesis (URA) and in four patients with renal hypertrophy due to nephrectomy (Nz). Four healthy controls were also studied. The studies were performed during water diuresis and following i.v. infusion of isotonic saline solution. Conventional clearance techniques were used. GFR and PAH clearence were increased to about the same extent in Nz and in URA. Fractional Na+ excretion was highest in the Nz group and lowest in the control group. It was higher in the Nz group than in the URA group. Fractional water excretion (V/GFR) and free water clearance (CH2O) were also determined and the results indicate that the high fractional excretion of Na+ from the hypertrophied kidney can be attributed to reduced fractional reabsorption of filtrated Na+ both in the proximal and the distal tubules. The fractional Na+ reabsorption in the distal tubule appears to be higher in URA than in Nz. It is concluded that glomerular tubular balance for Na+ is more similar to that found in healthy controls if the stimulus to hypertrophy occurs prenatally than if it occurs postnatally.

Influence of volume expansion on NaC1 reabsorption in the diluting segments of the nephron: a study using clearance methods

Whether volume expansion influences NaC1 reabsorption by the diluting segment of the nephron remains a matter of controversy. In the present studies this question has been examined in normal unanesthetized dogs, undergoing maximal water diuresis. Free water clearance (CH2O/GFR) has been used as the index of NaC1 reabsorption in the diluting segment. Three expressions have been employed for "distal delivery" of NaC1: a) V/GFR, designated as the "volume term"; b) (CNa/GFR + CH2O/GFR), the "sodium term;" and c) (CC1/GFR + CH2O/GFR), the "chloride term". The validity of these terms is discussed. Three techniques were used to increase distal delivery: 1) the administration of acetazolamide to dogs in which extracellular fluid (ECF) volume was not expanded (grop 1); 2) "moderate" volume expansion (group 2); and 3) "marked" volume expansion (group 3). CH2O/GFR increased progressively with rising values for "distal delivery" regardless of which term was used to calculate the latter. With all three delivery terms, differences in distal NaC1 reabsorption emerged between the two volume-expanded groups, though only with the "chloride" term did substantial differences also emerge between the nonexpanded group 1 dogs and both volume-expanded groups. In group 1, values for CH2O/GFR increased in close to a linear fashion up to distal delivery values equal to 24% of the volume of glomerular filtrate. However, at high rates of distal delivery the rate of rise of CH2O/GFR was less in group 2 than in group 1 and the depression of values was even greater in group 3. Within the limits of the techniques used, the data suggest that volume expansion inhibits fractional NaC1 reabsorption in the diluting segment of the nephron in a dose-related fashion. The "chloride" term was found to be superior to the "volume" and "sodium" terms in revealing these changes.

The effect of insulin on renal handling of sodium, potassium, calcium, and phosphate in man

The effects of insulin on the renal handling of sodium, potassium, calcium, and phosphate were studied in man while maintaining the blood glucose concentration at the fasting level by negative feedback servocontrol of a variable glucose infusion. In studies on six water-loaded normal subjects in a steady state of water diuresis, insulin was administered i.v. to raise the plasma insulin concentration to between 98 and 193 muU/ml and infused at a constant rate of 2 mU/kg body weight per min over a total period of 120 min. The blood glucose concentration was not significantly altered, and there was no change in the filtered load of glucose; glomerular filtration rate (CIN) and renal plasma flow (CPAH) were unchanged. Urinary sodium excretion (UNaV) decreased from 401 plus or minus 46 (SEM) to 213 plus or minus 18 mueq/min during insulin administration, the change becoming significant (P smaller than 0.02) within the 30-60 min collection period. Free water clearance (CH2O) increased from 10.6 plus or minus 0.6 to 13 plus or minus 0.5 ml/min (P smaller than 0.025); osmolar clearance decreased and urine flow was unchanged. There was no change in plasma aldosterone concentration, which was low throughout the studies, and a slight reduction was observed in plasma glucagon concentration. Urinary potassium (UKV) and phosphate (UPV) excretion were also both decreased during insulin administration; UKV decreased from 66 plus or minus 9 to 21 plus or minus 1 mueq/min (P smaller than 0.005), and tupv decreased from 504 plus or minus 93 to 230 plus or minus 43 mug/min (P smaller than 0.01). The change in UKV was associated with a significant reduction in plasma potassium concentration. There was also a statistically significant but small reduction in plasma phosphate concentration which was not considered sufficient alone to account for the large reduction in UPV. Urinary calcium excretion (UCaV) increased from 126 plus or minus 24 to 200 plus or minus 17 mug/min (P smaller than 0.01). These studies demonstrate a reduction in UNaV associated with insulin administration that occurs in the absence of changes in the filtered load of glucose, glomerular filtration rate, renal blood flow, and plasma aldosterone concentration. The effect of insulin on CH2O suggests that insulin's effect on sodium excretion is due to enhancement of sodium reabsorption in the diluting segment of the distal nephron.

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.

Acute effect of prednisolone on renal handling of sodium

The effect of prednisolone on renal handling of sodium (Na) was studied in rats under three experimental conditions: 1) hydropenia, 2) water diuresis, and 3) distal tubular blockade (DTB). Prednisolone, 0.25 mg/100 g per hr, was infused directly into left renal artery and urine was collected separately from each kidney. Predominantly unilateral increases in urine flow (V) and Na excretion were noticed in all experiments during prednisolone infusion. In the hydropenic rats the maximal increments on the infused side were, for V (mean ± SD), from 9.3 ± 1.5 to 21.4 ± 0.8 μl/min (P < 0.001); for C_Na/C_In, from 0.28 ± 0.11 to 2.97 ± 0.71 % (P < 0.005); and for TH2Oc/CIn, from 2.93 ± 2.26 to 5.32 ± 1.92% (P < 0.05). In the rats with water diuresis, the maximal increases were, for V/C_In, from 5.87 ± 1.97 to 10.1 ± 6.0% (P < 0.005); for C_H2O/C_In, from 4.09 ± 0.68 to 6.00 ± 0.44% (P < 0.0005); and for C_Na/C_In, from 0.22 ± 0.07 to 0.70 ± 0.38% (P < 0.01). In DTB-rats the maximal increases were for V from 48.6 ± 9.0 to 72.7 ± 14.1 μl/min (P < 0.0005) and for C_Na/C_In from 9.42 ± 2.97 to 20.23 ± 7.34% (P < 0.005). In the contralateral kidney these changes were less pronounced. These observations suggest that prednisolone depresses directly Na reabsorption. The association of natriuresis with augmented TH2Oc/CIn and C_H2O/C_In during hydropenia and water diuresis, respectively, and the increases in V and C_Na/C_In during DTB, all are consistent with inhibition of Na reabsorption in the proximal tubule.

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

Proximal and distal tubule micropuncture studies were performed to examine the response to graded extracellular volume (ECV) expansion in 10 normal dogs (stage I), 11 dogs with a unilateral remnant kidney (stage II), and 7 dogs with a remnant kidney after removal of the contralateral kidney (stage III). Before ECV expansion in stage III, there was a suggestive reduction in proximal tubule as well as loop fractional reabsorption of sodium. After ECV expansion to 3% body weight proximal tubule reabsorption was depressed in all groups of animals, while little further inhibition was observed in this segment with additional expansion to 10% body weight. In contrast, the fraction of filtered sodium remaining in the distal tubule rose progressively in all three groups after graded ECV expansion, suggesting that the graded natriuretic response found in the final urine was largely due to a similar response in the loop of Henle rather than that in the proximal tubule. The distal tubule response of the remnant kidney in both stages II and III was greater than that in stage I. These data indicate that although enhanced sodium excretion per nephron in chronic renal failure may be related to uremia, its exaggerated response to ECV expansion is due, at least in part, to certain as yet unidentified intrarenal factors consequent to reduction in functioning renal mass.

Renal sodium-potassium-activated adenosine triphosphatase and sodium reabsorption

The role of renal Na(+),K(+)-ATPase in sodium reabsorption was further examined in dogs in which digoxin, a specific inhibitor of the enzyme system, was infused into one renal artery in doses ranging from 0.4 to 0.9 mug/kg/min (low dose) and from 1.0 to 4.0 mug/kg/min (high dose). A significant natriuresis occurred with both dose ranges which was accompanied by inhibition of Na(+),K(+)-ATPase of cortex and medulla in the infused kidney. Despite over 90% enzyme inhibition in many experiments, at least 80% of the filtered sodium continued to be reabsorbed. The per cent change in enzyme activity correlated with the rate of digoxin administration and the total dose administered but not with changes in sodium excretion. Changes in medullary Na(+),K(+)-ATPase activity, however, bore a direct relationship to alterations in fractional solute free water reabsorption (T(c) (H2O)). Inhibition of cortical enzyme activity alone was not associated with natriuresis, suggesting that medullary enzyme activity must be depressed for increased sodium excretion to occur during digoxin infusion. In high-dose experiments, significant inhibition of cortical and medullary enzyme in the contralateral control kidney was also observed, but natriuresis did not occur. In these experiments the rate at which digoxin reached the control kidney rose progressively but never equaled the rates in the directly infused kidney with either dose. Nevertheless, it is clear that under certain circumstances enzyme inhibition of either cortex or medulla need not be accompanied by natriuresis. We conclude that the major role of renal Na(+),K(+)-ATPase is in sodium reabsorption in the medulla (ascending limb of Henle's loop) and that it has a relatively small role in proximal sodium reabsorption. The kidney can rely on other sodium reabsorptive mechanisms depending on the rate of enzyme inhibition, so that natriuresis may not occur at all if depression in activity occurs "slowly." The nature of these mechanisms is not clear.

Tubular reabsorption of sodium during acute and chronic volume expansion in man

Renal hemodynamics and tubular fractional sodium reabsorption (FSR) were evaluated by clearance techniques during acute and chronic extracellular volume expansion in man. (1 - V/GFR) x 100 was used as an index of proximal and (C(H2O)/V) x 100 as an estimate of distal fractional reabsorption. After acute loading with isotonic saline 37 ml/kg body wt, proximal FSR decreased by 4.8% and distal FSR decreased by 4.4%. After comparable chronic expansion by mineralocorticoids ("escape"), proximal FSR also decreased by 3.9%, but distal reabsorption was not altered.In separate studies, subjects were progressively infused with saline to 57 (E(1)) and to 80 (E(2)) ml/kg body wt, and appeared to divide into "excreters" (maximum U(Na)V > 1000 muEq/min) and "nonexcreters" (maximum U(Na)V < 550 muEq/min). In the excreters, GFR rose, proximal FSR decreased by 7.1% after E(1) and only 0.9% further after E(2). Distal FSR fell by 14.8% after E(1) and by an additional 4.9% after E(2). In the nonexcreters, GFR was stable and proximal FSR did not fall significantly after E(1) or E(2). Distal FSR decreased 4.5% after E(1) and 1.3% further after E(2). It is concluded that both acute and chronic extracellular expansion decrease proximal FSR in man, but only acute loading depresses distal FSR. Ability of some men to excrete sodium rapidly after acute infusion is related to larger increases in GFR and greater decreases in both proximal and distal FSR than occur in men in whom natriuresis is more limited.

Renal effects of adenosine 3',5'-cyclic monophosphate and dibutyryl adenosine 3',5'-cyclic monophosphate. Evidence for a role for adenosine 3',5'-cyclic monophosphate in the regulation of proximal tubular sodium reabsorption

Stable water diuresis was produced in anesthetized, hydrocortisone-treated hypophysectomized dogs by infusion of 2.5% dextrose. Infusion of adenosine 3',5'-cyclic monophosphate (cyclic AMP) in the left renal artery decreased ipsilaterally glomerular filtration rate (GFR), cortical and non-cortical renal plasma flow, and tended to increase urine flow (V) and free-water clearance (C(H2O)) despite a decrease in mean arterial pressure. Infusion of dibutyryl adenosine 3',5'-cyclic monophosphate (dibutyryl cyclic AMP) in the left renal artery increased V and C(H2O) significantly (P<0.01) bilaterally with essentially no change in GFR, in total renal plasma flow or its cortical and non-cortical components. For each kidney the magnitude of the change in V was similar to the magnitude of the change in C(H2O) and the change in sodium excretion was trivial. Cyclic AMP probably produced its effects on renal hemodynamics and mean arterial pressure wholly or in part through the action of metabolites such as 5'-AMP and adenosine on the renal and systemic vasculature. The absence of an effect of dibutyryl cyclic AMP on renal hemodynamics and its bilateral effect may be explained by the resistance of this nucleotide derivative to metabolism. Dibutyryl cyclic AMP appears to decrease by direct cellular effect(s) proximal tubular sodium reabsorption but does not prevent virtually complete reabsorption of the increased load of sodium in the distal nephron. This effect on the kidney is qualitatively and quantitatively similar to the effect of renal arterial infusion of isoproterenol. The results suggest that synthesis of cyclic AMP in proximal renal tubule cells in response to stimulation of beta adrenergic or other receptors can mediate a decrease in the proximal tubular reabsorption of sodium.

Depression of proximal tubular sodium reabsorption in the dog in response to renal beta adrenergic stimulation by isoproterenol

Water diuresis was produced in anesthetized hypophysectomized, cortisone-treated dogs by infusion of 2.5% dextrose. Alpha adrenergic blockade of the left kidney produced by infusion of phenoxybenzamine in the left renal artery was associated with a significantly (P < 0.05) greater rate of urine flow (V) and free water excretion (C(H2O)) in the left kidney than in the right despite similar glomerular filtration rates (GFR) (17 +/- 1.3 ml/min, left; 18 +/-0.9 ml/min, right). Sodium excretion (U(Na)V) was similar in the two kidneys (3 and 5 muEq/min). When beta adrenergic stimulation of the left kidney was superimposed on alpha blockade by the addition of isoproterenol to the left renal artery infusate, GFR remained unchanged and similar in the two kidneys, as V and C(H2O) increased significantly (P < 0.01) in the left kidney but not in the right. When isoproterenol was discontinued, V and C(H2O) returned towards control in the left kidney and remained unchanged in the right. The ratios of the left kidney to the right during control, isoproterenol, and postcontrol were 1.22, 1.65, and 1.35, respectively, for V and 1.36, 1.90, and 1.44, respectively, for C(H2O). Sodium excretion remained unchanged and similar in the two kidneys throughout the study. The results indicate that blockade of alpha adrenergic activity inhibits the increased proximal tubular sodium reabsorption which anesthesia induces in the dog. Beta adrenergic stimulation appears to decrease proximal tubular sodium reabsorption but does not prevent virtually complete reabsorption of the increased quantity of delivered sodium by the ascending limb of the loop of Henle and the distal tubule. These changes in sodium reabsorption presumably are not associated with changes in colloid osmotic pressure or hydrostatic pressure in the peritubular capillary inasmuch as cortical and non-cortical plasma flow, filtration fraction, and mean arterial pressure in the left kidney were unchanged. Thus, isoproterenol probably produced its effects through a direct action on the renal tubule, possibly through the mediation of the adenyl cyclase system.

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

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

Effects of hematocrit on renal hemodynamics and sodium excretion in hydropenic and volume-expanded dogs

The effects of hematocrit on renal hemodynamics and sodium excretion were studied in anesthetized dogs during both hydropenia and volume expansion. The hematocrit was decreased by isovolemic exchange with the animal's own previously harvested plasma and increased by isovolemic exchange with fresh, washed red blood cells. Renal perfusion pressure was maintained constant throughout the experiments by the adjustment of a suprarenal aortic clamp. During hydropenia, a decrease in hematocrit was associated with an increase in sodium and potassium excretion and solutefree water reabsorption. These changes were accompained by an increase in renal plasma flow and renal blood flow and a decrease in renal vascular resistance. Glomerular filtration rate was unchanged and filtration fraction was significantly decreased as hematocrit was lowered. Increasing hematocrit during hydropenia had the opposite effects on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics. In another group of animals, hematocrit was lowered during volume expansion with either saline or plasma, then returned to the control level by isovolemic exchange with washed red blood cells. This increase in hematocrit during volume expansion had a similar effect on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics as during hydropenia. These results therefore suggest that acute changes in hematocrit may significantly affect sodium excretion and renal hemodynamics during both hydropenia and volume expansion. The changes in solute-free water reabsorption and potassium excretion suggest that the alterations in hematocrit may affect primarily the reabsorption of sodium in the proximal tubule. The concommitant effects of hematocrit on renal vascular resistance and filtration fraction may mediate this change in sodium reabsorption by altering hydrostatic and oncotic pressures in the peritubular circulation.

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

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

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

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

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

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

The importance of aerobic metabolism in the renal concentrating process

The extent to which the concentrating function of the kidney depends on oxidative processes was investigated by infusing cyanide into one renal artery of dogs undergoing mild mannitol diuresis while receiving an infusion of vasopressin. This produced an abrupt fall in concentrating capacity (T(c) (H2O)) that was reversed when the cyanide infusion was stopped. The change could not be accounted for by the accompanying solute diuresis, since it was not reproduced by increasing the rate of mannitol infusion. The reduction in T(c) (H2O) induced by cyanide did not result from increased delivery of dilute urine to the collecting ducts, since free water clearance (C(H2O)), studied in other dogs during water diuresis, was unchanged or decreased by cyanide. Cyanide produced renal vasodilatation, as did intraarterial acetylcholine, but in contrast to the striking reduction in concentrating capacity evoked by cyanide, T(c) (H2O) was not significantly changed by acetylcholine. The data indicate that concentrating ability is closely tied to oxidative metabolism in the kidney, and it is suggested that the region where this is critically important is the red medulla and the thick ascending limb of Henle's loop.

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. The differences in the pattern of free water clearance and tubular sodium transport between normal and caval dogs could not be easily explained by alterations in renal hemodynamics or aldosterone secretion. It is suggested that in the caval dog an alteration occurs in other factors which might influence renal tubular sodium transport, such as intrarenal hemodynamics, renal interstitial volume or pressure, or a natriuretic hormone, leading to increased tubular sodium reabsorption.

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.