Further studies on segmental sodium transport in the rat kidney during expansion of the extracellular fluid volume

Alterations in regional kidney oxygenation during expansion of extracellular fluid volume in conscious healthy sheep

Expansion of extracellular fluid volume with crystalloid solutions is a common medical intervention, but its effects on renal cortical and medullary oxygenation are poorly understood. Therefore, we instrumented sheep under general anesthesia to enable continuous measurement of systemic and renal hemodynamics, global renal oxygen delivery and consumption, and intrarenal tissue perfusion and oxygen tension (Po₂) in conscious animals ( n = 7). The effects of three sequential intermittent infusions of 500 ml of compound sodium lactate solution, administered at hourly intervals, were determined. Volume expansion induced transient increases in mean arterial pressure (+7 ± 2%), central venous pressure (+50 ± 19%), and cardiac output (+15 ± 3%). There were sustained increases in renal medullary tissue Po₂ (+35 ± 10%) despite increases in global renal oxygen consumption (+66 ± 18%) and renal oxygen extraction (+64 ± 8%). Volume expansion did not significantly alter renal blood flow, renal oxygen delivery, or medullary perfusion. The sustained increase in medullary Po₂ was paralleled by increased bladder urine Po₂ (34 ± 4%). Cortical perfusion and Po₂ did not change significantly. Our findings indicate that extracellular fluid volume expansion can increase renal medullary oxygenation, providing a potential mechanistic basis for its use as prophylaxis against iatrogenic acute kidney injury. They also indicate that continuous measurement of bladder urine Po₂ could be used to monitor the effects of volume expansion on medullary oxygenation. However, the mechanisms mediating increased medullary oxygenation during volume expansion remain to be determined.

Integrated control of Na transport along the nephron

The kidney filters vast quantities of Na at the glomerulus but excretes a very small fraction of this Na in the final urine. Although almost every nephron segment participates in the reabsorption of Na in the normal kidney, the proximal segments (from the glomerulus to the macula densa) and the distal segments (past the macula densa) play different roles. The proximal tubule and the thick ascending limb of the loop of Henle interact with the filtration apparatus to deliver Na to the distal nephron at a rather constant rate. This involves regulation of both filtration and reabsorption through the processes of glomerulotubular balance and tubuloglomerular feedback. The more distal segments, including the distal convoluted tubule (DCT), connecting tubule, and collecting duct, regulate Na reabsorption to match the excretion with dietary intake. The relative amounts of Na reabsorbed in the DCT, which mainly reabsorbs NaCl, and by more downstream segments that exchange Na for K are variable, allowing the simultaneous regulation of both Na and K excretion.

Upregulation of histidine decarboxylase expression in superficial cortical nephrons during pregnancy in mice and women

Mechanisms regulating pregnancy-induced changes in renal function are incompletely understood. Few candidate genes have been identified and data suggest that alternate mechanisms remain to be elucidated. Our objective was to screen thousands of genes expressed in kidneys from mice throughout gestation to identify possible key regulators of renal function during pregnancy. Mouse complementary DNA microarrays were used to screen for differences in expression during pregnancy in C57BL/6 mice. Interesting candidate genes whose expression varied with pregnancy were further analyzed by reverse transcription-PCR and Northern blot. Expression was localized by in situ hybridization and immunohistochemistry. Follow-up immunohistochemical analyses in archival human kidney sections from the fetus, non-pregnant, and pregnant women were also performed. Histidine decarboxylase (HDC), the enzyme that synthesizes histamine, was markedly upregulated in the mouse kidney during pregnancy. HDC expression localized to proximal tubule cells of fetal and adult mice. Females showed strong expression in the juxtamedullary zone before pregnancy and upregulation in the superficial cortical zone (SCZ) by mid-gestation. Histamine colocalized with HDC. Male mice showed only low HDC expression. Similar expression patterns were observed in human kidneys. Our results show that HDC expression and histamine production are increased in the SCZ during pregnancy. If histamine acts as a vasodilator, we speculate that increasing production in the SCZ may increase renal blood flow to this zone and recruit superficial cortical nephrons during pregnancy.

Compensation of proximal tubule malabsorption in AQP1-deficient mice without TGF-mediated reduction of GFR

AIM

By crossing aquaporin 1 (AQP1)-/- and adenosine 1 receptor (A1AR)-/- mice, we generated an animal model that combines a proximal tubular absorption defect with absence of tubuloglomerular feedback (TGF) regulation of glomerular filtration rate (GFR). The aim of studies in these animals was to determine whether a TGF-induced reduction of GFR is a prerequisite for preventing potentially fatal fluid losses.

METHODS AND RESULTS

In contrast to AQP1 deficient mice, AQP1/A1AR-/- mice were found to have a normal GFR. TGF responses were abolished in these animals, in contrast to AQP1-/- mice in which TGF responses of single nephron glomerular filtration rate (SNGFR) were left-shifted. Proximal tubule fluid absorption in AQP1/A1AR-/- mice was reduced to levels previously reported for AQP1-/- mice. However, SNGFR was significantly higher in AQP1/A1AR-/- than AQP1-/- mice (10.6 +/- 0.8 nL min(-1) vs. 5.9 +/- 0.7 nL min(-1)). As a consequence of the normal GFR and the reduced proximal reabsorption distal fluid delivery was markedly higher in the double knockout compared with normal or AQP1-/- mice (5.5 +/- 0.5 nL min(-1) vs. 2.35 +/- 0.3 nL min(-1) in AQP1-/-). Despite the approximate doubling of distal fluid and Cl delivery, AQP1/A1AR-/- mice have a normal salt excretion, normal arterial blood pressure, and only a small increase in plasma renin concentration.

CONCLUSION

The ability to compensate for proximal tubule malabsorption without a TGF-induced reduction of GFR attests to a remarkable adaptability of distal tubule transport mechanisms.

Return of the secretory kidney

The evolution of the kidney has had a major role in the emigration of vertebrates from the sea onto dry land. The mammalian kidney has conserved to a remarkable extent many of the molecular and functional elements of primordial apocrine kidneys that regulate fluid balance and eliminate potentially toxic endogenous and xenobiotic molecules in the urine entirely by transepithelial secretion. However, these occult secretory processes in the proximal tubules and collecting ducts of mammalian kidneys have remained underappreciated in the last half of the twentieth century as investigators focused, to a large extent, on the mechanisms of glomerular filtration and tubule sodium chloride and fluid reabsorption. On the basis of evidence reviewed in this paper, we propose that transepithelial salt and fluid secretion mechanisms enable mammalian renal tubules to finely regulate extracellular fluid volume and composition day to day and maintain urine formation during the cessation of glomerular filtration.

Sodium transport deficiency and sodium balance in gene-targeted mice

Animals with induced or natural null mutations in renal NaCl and water transporter genes provide a powerful tool to study the physiological mechanisms that enable the kidney to optimize the match between glomerular filtration rate and tubular reabsorption. Deficiencies in the Na/H exchanger NHE3 and in the water channel aquaporin 1 (AQP1) cause reductions in proximal fluid absorption which are accompanied by proportionate decrements in glomerular filtration rate (GFR). Compensation of the transport defect by a reduction in filtered load is so efficient that clinically symptomatic Na losses are not observed in either NHE3 or AQP1 deficient animals. On the other hand, severe syndromes of salt wasting are caused by loss of function of the Na,K,2Cl-cotransporter (NKCC2) in the thick ascending limb, or of the epithelial Na channel (ENaC) the collecting duct indicating that the severity of Na dysregulation is unrelated to the basal absorption of NaCl in a given nephron segment. In these states, the increased delivery of Na to downstream segments is not monitored by a sensor linked to the site of filtrate formation. In the absence of adaptations in the filtered load intrarenal compensation of a circumscribed NaCl malabsorption by adjustment of NaCl transport in other nephron segments is sometimes insufficient, particularly in the immature kidney of the newborn.

Sodium handling by deep nephrons and the terminal collecting duct in glomerulonephritis

The present study was designed to characterize the effects of anti-glomerular basement membrane (anti-GBM) glomerulonephritis (GN) on sodium handling by surface nephrons, deep nephrons and the terminal collecting duct segment. Studies were performed in rats during hydropenia and volume expansion. In hydropenia, the glomerular filtration rate (GFR) and sodium excretion tended to be lower in rats with GN than in controls. However, the major differences between the control and GN animals were seen in volume expansion. In the volume expanded groups fractional excretion of sodium was greater in controls (3.20 +/- 0.51%) than in GN (1.20 +/- 0.36%, P less than 0.01). Despite this, delivery to end proximal sites was similar in the two groups in absolute terms and higher in the expanded GN group compared to the expanded controls. Absolute sodium delivery to the bend of the loop of Henle in the expanded GN rats was decreased in absolute terms but increased in fractional terms compared to expanded controls. However, fractional delivery of sodium to the base of the terminal collecting duct was less in GN (3.71 +/- 1.39%) than in controls (7.19 +/- 0.96%, P less than 0.002). In both groups, fractional delivery between tip of the collecting duct fell compared to base (P less than 0.05) but delivery to the tip was again greater in controls (5.49 +/- 1.08%) than in GN (1.51 +/- 0.47%). In GN 62.6 +/- 5.0% of delivered sodium was reabsorbed between collecting duct sites, nearly twofold that of controls (28.8 +/- 9.4%, P less than 0.01). Thus, fractional sodium reabsorption in the collecting duct was enhanced by GN.

Renal medullary circulation: hormonal control

It is now becoming apparent that the medullary circulation in the kidney can be regulated separately from overall renal blood flow. This characteristic of the medullary circulation plays an important role in the kidney's ability to excrete a dilute or concentrated urine in concert with changes in water and sodium transport in the distal nephron secondary to the action of vasopressin, prostaglandins, the renal nerves, and other hormones without significant other renal hemodynamic changes. There is strong evidence that renal autocoids such as angiotensin II and prostaglandins uniquely affect regional blood flow in the inner medulla because of the special structure and organization of the microvasculature in this region. There is also evidence that this regional blood flow is in part regulated by circulating hormones, such as vasopressin and atrial natriuretic peptide, which are released in response to changes in extracellular fluid volume or osmolality. In addition, data are emerging to suggest that the kallikrein-kinin system, acetylcholine, the renal nerves and adenosine participate in this regulation. In addition to the role of the medullary circulation in the urinary concentrating operation, there are data to suggest that the medullary circulation either directly (by changes in physical forces) or indirectly (by regulating medullary toxicity) may influence sodium excretion in a variety of conditions. In this regard, activation of the renin-angiotensin system locally reduces blood flow in the papilla which may be necessary before sodium retention is fully expressed in salt retaining states. Future research looking at the microvasculature of the medulla and papilla and those factors that control the contractility of these vessels are necessary before a clearer picture emerges. Nevertheless, from the data already available it seems reasonable to suggest that the medullary circulation may be as important to kidney function during physiological and pathophysiological states as is the cortical circulation.

Regulation of papillary plasma flow by angiotensin II

We examined in anesthetized dogs the effects of left (L) intrarenal artery infusion of angiotensin II (AII) on renal hemodynamics, urinary concentration and Na excretion, and papillary plasma flow (PPF) (measured by the albumin accumulation technique) in both kidneys. Following AII infusion (0.5 ng/kg/min) into the L renal artery, urinary Na excretion decreased and osmolality increased slightly ipsilaterally, whereas Na excretion did not change significantly and osmolality decreased in the right (R) kidney. PPF was significantly lower in the L compared to the R kidney. When saline loading was superimposed on L intrarenal AII infusion, there was a blunted natriuretic response ipsilaterally with a significantly smaller decrease in urine osmolality compared with the R kidney. PPF increased significantly in the R, but not in the L kidney. Finally, AII blockade with saralasin prior to AII infusion and saline loading prevented the differences between the two kidneys, including PPF. In all groups GFR and renal blood flow did not differ between the two kidneys before or after AII. These data suggest that AII regulates regional blood flow in the medulla, and that the exogenously administered AII induces papillary ischemia, which serves to preserve medullary hypertonicity, preventing an increase in PPF during saline loading, and possibly contributing to the diminished natriuretic response.

Interaction of nonsteroidal anti-inflammatory drugs with antihypertensive and diuretic agents. Control of vascular reactivity by endogenous prostanoids

Indomethacin and some other nonsteroidal anti-inflammatory drugs partially antagonize the blood pressure lowering effect of drugs used to treat hypertension. They can also produce a mild elevation of blood pressure in normotensive individuals. The elevated arterial pressure caused by these agents is associated with increases in the vascular resistance of mainly the renal and splanchnic beds. This may be due to direct inhibition of the synthesis of vasodilator prostanoids, or it may be due to indirect potentiation of the action of the sympathetic nervous system or of angiotensin II. Nonsteroidal anti-inflammatory drugs also cause renal retention of sodium and this probably contributes to their hypertensive effects. In humans, the sodium retention may involve increased reabsorption in the proximal tubule. Although a direct tubular action is possible, these drugs may change proximal sodium reabsorption by their vascular effects. However, the exact mechanism is not understood. These interactions are clinically significant and may complicate the treatment of common diseases.

Additive effects of atrial natriuretic polypeptide and of renal vasodilating agents in the anesthetized dog

In order to examine the contribution of an increase in renal papillary plasma flow to the mechanism of natriuresis by atrial natriuretic polypeptide (ANP), we compared the natriuretic effects of ANP administered into the renal artery of the dog together with secretin or acetylcholine (ACh). At an equivalent renal vasodilating dose, ACh increased urinary excretion of sodium (UNaV) to 212 +/- 36% of the control associated with a decrease in urine osmolality (62 +/- 6%), whereas secretin did not change UNaV (113 +/- 12%) or urine osmolality (101 +/- 14%). This result was compatible with the view that ACh causes natriuresis mainly by increasing papillary plasma flow. Combined administration of ANP with secretin caused a marked increase in UNaV to 407 +/- 55%, in association with a decrease in urine osmolality to 55 +/- 9%, suggesting that ANP may cause natriuresis by a mechanism similar to that of ACh. Combined administration of ANP with ACh further increased UNaV to 323 +/- 67% and decreased urine osmolality to 50 +/- 6%. These observations suggest that ANP and ACh share common but not identical mechanisms of natriuretic action since ANP caused additional natriuresis during ACh infusion. These findings, however, do not necessarily exclude the possibility that ANP also inhibits renal sodium reabsorption by a direct action.

Distal blockade in experimental glomerulonephritis: the role of the diluting segment in sodium retention

In the light of accumulating evidence implicating the diluting segment as the site of final regulation of sodium excretion by the nephron, we produced in this experiment distal blockade in anti-glomerular basement membrane (GBM) glomerulonephritic (GN) rats by the administration of furosemide and polythiazide. This allowed to dissociate the sodium reabsorption that occurs in the proximal tubule from the one that occurs more distally and permitted an appreciation of the rôle played by the diluting segment in the sodium retention of anti GBM GN. In a previous experiment we showed that GN conscious or anaesthetized rats presented an increase in Na tubular reabsorption and failed to raise their fractional and absolute excretion of sodium as normal one did after rapid volume expansion. In this study distal blockade corrected almost completely the difference in sodium excretion that existed between GN and normal groups before the administration of diuretics, pointing to the important rôle played by the diluting segment in the sodium retention of experimental GN.

Haemodynamic, hormonal, and diuretic effects of felodipine in healthy normotensive volunteers

Felodipine and placebo were infused in a double-blind, crossover study in 10 healthy normotensive volunteers. Compared with placebo, felodipine caused a significant decrease in diastolic blood pressure and forearm vascular resistance, while there was no change in systolic blood pressure. The rises in heart rate, plasma renin activity and plasma noradrenaline (norepinephrine) concentration further demonstrated the vasodilating activity of felodipine. Plasma aldosterone, adrenaline (epinephrine) and antidiuretic hormone concentrations were similar after a 90-minute infusion of felodipine or placebo. The response of plasma aldosterone levels to exogenous adenocorticotrophic hormone showed evidence of a slight blunting during felodipine infusion. Felodipine had a marked diuretic effect, probably secondary to an increase in natriuresis, which might be due to a direct tubular effect of the drug.

Segmental analysis of renal glucose transport in young female rats

Free-flow micropuncture studies were performed on twenty-seven young female Sprague-Dawley rats before and after 10% extracellular volume expansion to evaluate glucose reabsorption at the accessible sites of both surface and papillary nephrons. In the distal nephron segments no significant glucose reabsorption was observed for the distal tubule and papillary collecting duct but significant difference in fractional glucose delivery was demonstrated between the bend of the Henle's loop and early distal tubule and between the late distal tubule and the base of the collecting duct. Comparison of the fractional glucose delivery within the same nephron group for both superficial and juxtamedullary nephrons indicated that glucose reabsorption occurred at some sites beyond the bend of the Henle's loop. Volume expansion inhibited glucose reabsorption in the proximal convoluted tubule, enhanced it in the segment between the late proximal and early distal tubules, but had no effect on glucose transport at further distal sites. It is concluded that, in addition to the proximal tubule, the ascending loop of Henle or cortical collecting tubule may play a role in maintaining glucose-free urine under physiological conditions.

Role of renal papillae in the regulation of sodium excretion during acute elevation of renal perfusion pressure in the rat

We studied the role of renal papillae in the mechanism of increased sodium excretion during acute increase in mean arterial pressure (MAP). Sodium excretion increased dramatically in normal rats after acute increase in MAP by epinephrine (E) infusion (0.4 micrograms/min/100g). Glomerular filtration rate (GFR), renal blood flow (RBF), and papillary plasma flow (PPF) remained unchanged after the E administration. To define the role of the medulla in the mechanism of pressure-induced natriuresis, experiments were performed in a group of rats 8 to 12 days after the development of papillary necrosis induced by bromoethylamine hydrobromide. Urinary sodium and fractional sodium excretions were 2.00 +/- 0.34 microEq/min and 2.37 +/- 0.53% (n = 7), respectively, in papillary necrosis rats infused with saline. Administration of E to papillary necrosis rats, however, failed to increase both urinary sodium (2.89 +/- 0.61 microEq/min) and fractional sodium (FENa, 2.82 +/- 0.63%, n = 6) excretions despite a marked increase in MAP (129 vs 150 mm Hg, p less than 0.01). The RBF increased slightly after E infusion (4.42 vs 3.24 ml/min/100 g, p less than 0.05), but the GFR was not different between the control (0.39 +/- 0.05 ml/min/100g, n = 7) and the E-treated rats (0.43 +/- 0.06, n = 6). Failure to increase sodium excretion during acute increase in MAP was not due to the decreased GFR, since control rats with bilateral partial nephrectomy were able to increase sodium excretion from 1.92 +/- 0.33 to 7.76 +/- 1.63 microEq/min (p less than 0.01) after E infusion. These findings, therefore, suggest that renal papillae play a major role in the mechanism of natriuresis during acute increase in MAP.

The permeability of collecting ducts to 22Na+ and 36Cl- in rat isolated papillae

1. The diffusional permeability of collecting ducts to 22Na+ and 36Cl- was measured in rat papillae in vitro. 2. The permeability of the collecting duct to 36Cl- was 0.72 (s.e.m. = 0.01; n = 356) microns/sec which was significantly higher than the value of 0.51 (s.e.m. = 0.01; n = 356) microns/sec measured for 22Na+. 3. Collecting ducts in papillae taken from rats on a high sodium intake had a 22Na+ permeability of 0.63 (s.e.m. = 0.04; n = 53) microns/sec which was significantly higher than the value on a normal salt intake (0.50, s.e.m. = 0.04; n = 46 microns/sec). 4. When papillae from normal rats were studied in plasma taken from salt loaded rats, the 22Na+ permeability of 0.59 (s.e.m. = 0.04; n = 18) microns/sec was significantly higher than when incubated in plasma from normal rats (0.44, s.e.m. = 0.05; n = 12) microns/sec. 5. An extract of urine with natriuretic activity had no effect on 22Na+ permeability when tested in this system. 6. Adrenalectomy, PGE2, indomethacin and antidiuretic hormone had no significant effect on 22Na+ and 36Cl- permeability. 7. A substance exists in plasma from salt loaded animals that increases the permeability of collecting ducts to sodium. This effect could explain the component of the natriuresis that follows saline infusion which is independent of changes in glomerular filtration rate, aldosterone, or proximal tubule reabsorption.

Prostaglandin synthesis inhibition during volume expansion: collecting duct function

The aim of this study was to examine the possible role of renal prostaglandins in the response of the inner medullary collecting duct (IMCD) to acute volume expansion. Collecting duct microcatheterization and clearance studies were carried out in anesthetized rats, volume-expanded with isotonic Ringer's solution. In volume-expanded control animals, there was no significant sodium or chloride reabsorption between the beginning and end (papillary tip) of the IMCD. Administration of indomethacin or meclofenamate prior to and during volume expansion in two other groups of rats resulted in significant water, sodium, and chloride reabsorption along the IMCD and markedly blunted the diuretic, natriuretic and chloriuretic response to volume expansion. Because delivery to the beginning of the duct was not significantly decreased, enhanced reabsorption in the IMCD largely accounted for the decrease in natriuresis and chloriuresis. Inner medullary tissue fluid chloride concentration increased after inhibition of prostaglandin synthesis. The results indicate that renal prostaglandins, perhaps by directly decreasing sodium chloride reabsorption, have an important role in the decrease in collecting duct reabsorption of sodium and chloride observed with acute volume expansion.

Effect of medullary tonicity on urinary sodium excretion in the rat

In previous reports from this laboratory we have suggested that a reduction in medullary tonicity decreases the thin ascending loop of Henle sodium reabsorption and is in part responsible for the magnitude for the natriuresis accompanying 10% body weight Ringer loading. According to this postulate, one would expect that the medullary washout associated with water diuresis would also result in a natriuresis, but this does not occur. It is possible, however, that increased delivery from the proximal tubule is necessary to demonstrate an effect of medullary tonicity on urinary sodium excretion. Micropuncture studies were designed to test that possibility by increasing distal delivery by 2% Ringer loading in animals with and without reduced medullary tonicity. In an initial series of experiments the alpha-adrenergic agonist clonidine was used to induce a water diuresis. When given alone, this agent caused a marked decrease in urine osmolality and an increase in urine flow rate but had no effect on proximal reabsorption in either superficial or juxtamedullary nephrons, and did not alter urinary sodium excretion. Volume expansion with 2% body weight Ringer solution resulted in a significant fall in proximal reabsorption and a trivial increment in sodium excretion. When this same degree of volume expansion was conferred on animals undergoing a water diuresis, a marked increase in absolute and fractional sodium excretion occurred. In a second group of studies medullary tonicity was reduced in the left kidney only by removal of the left ureter 1 h before micropuncture. When these animals were infused with 2% body weight Ringer solution, proximal reabsorption was decreased in juxtamedullary nephrons, and a marked increase in sodium excretion was observed only from the left kidney. Finally, the effect of water diuresis on fractional sodium delivery to the early and late distal tubule of superficial nephrons during 2% Ringer loading was evaluated. Delivery to both of these sites was comparable after 2% Ringer loading alone and during 2% Ringer loading plus water diuresis. From these data, we conclude that medullary tonicity does influence renal sodium handling but that this effect is manifest in the final urine only under conditions in which proximal reabsorption is decreased. The data also suggest that this effect is limited to juxtamedullary nephrons and is probably localized to the thin ascending limb of the loop of Henle.

Studies on the mechanism of sodium excretion during drug-induced vasodilatation in the dog

The administration of vasodilating agents such as bradykinin and acetylcholine cause an increase in urinary sodium excretion. Yet the mechanisms involved in this natriuretic effect are not clear. Recent studies with another renal vasodilator, secretin have shown this drug also causes a profound increase in renal blood flow but without major changes in sodium excretion. To attempt to delineate the basis of this difference in sodium excretion with these drugs, the renal functional effects of secretin and bradykinin were compared at an equivalent vasodilating dose. Bradykinin increased renal blood flow from 222 to 342 ml/min, urine volume from 0.2 to 1.2 ml/min, and urine sodium excretion from 28 to 115 mueq/min. Urine osmolality fell from 1,230 to 401 mosmol/kg. Secretin caused a comparable increase in renal blood flow (216 to 325 ml/min) while changes in urine flow, sodium excretion, and urine osmolality were significantly less. In further studies papillary plasma flow was estimated using the albumin accumulation technique. Control papillary plasma flow was 29 ml/min per 100 g. Bradykinin increased urinary sodium excretion 108 mueq/min and decreased urinary osmolality from 1,254 to 516 mosmol/kg in association with a rise in papillary plasma flow to 62 ml/min per 100 g. Urine sodium excretion, urinary osmolality, and urine flow rate, as well as papillary plasma flow rate (32 ml/min per 100 g) were unchanged from control when secretin was administered. Studies with acetylcholine were qualitatively similar to those of bradykinin. Renal blood flow increased from 150 to 248 ml/min, urinary sodium excretion increased from 20 to 243 mueq/min, urinary osmolality decreased from 1,237 to 411 mosmol/kg and papillary plasma flow increased from 39 to 52 ml/min per 100 g. It is suggested that the natriuretic effect of some vasodilators is due, at least in part, to alterations in medullary hemodynamics, as evidenced by the increase in papillary plasma flow seen with bradykinin and acetylcholine, but not secretin.

Renal function studies in an experimental model of papillary necrosis in the rat

Twenty four hours after i.v. injection of bromoethylamine-hydrobromide (BEA) in rats, a uniform papillary necrosis is observed. The present study investigates the renal functional and the papillary haemodynamics in response to acute volume expansion (12% of body weight) in this model. Renal function studies were performed in hydropenic and volume expanded sham- or BEA-injected rats. In hydropenic normal animals a GFR of 1.97 +/- 0.14 ml/min, an urinary osmolarity (UOsm) of 1 011 +/- 94.5 mOsm/kg and a fractional sodium excretion (FENa) of 0.18 +/- 0.026% were obtained. In contrast, BEA-treated hydropenic animals showed a lower GFR (1.16 +/- 0.14 ml/min), UOsm (469 +/- 30.31 mOsm/kg) and a higher FENa (0.37 +/- 0.06%). In volume expansion a similar UOsm and FENa were obtained in both groups. The papillary plasma flow (PPF) was measured in each of the experimental groups by the albumin accumulation technique. The mean value in hydropenic normal animals was 50.65 +/- 2.12 m 100 g-1 min-1 and increased to 66.02 +/- 2.00 ml 100 g-1 min-1 after volume expansion (P less than 0.001). In BEA rats the PPF was 58.86 +/- 2.33 ml 100 g-1 min-1 in hydropenia (P less than 0.01 vs. control animals) and remained unchanged after volume expansion. Thus, during hydropenia, BEA-induced papillary necrosis results with a salt wasting state and an urinary concentration defect. After volume expansion no disturbance in sodium excretion capacity was observed. These results are compatible with the nephron-heterogeneity concept in the regulation of sodium excretion. The histological lesions cannot be explained by a decreased renal papillary plasma flow.

Body fluid homeostasis in man. A contemporary overview

In the steady state, urinary excretion of sodium is closely matched to dietary salt intake. Given rigorous defense of extracellular fluid osmolality, it is the quantity of sodium in the extracellular fluid that determines the volume of this compartment. Changes in extracellular fluid volume are detected by volume sensors located in the intrathoracic vascular bed, kidney and other organs. These mechanoreceptors gauge the adequacy of intravascular volume, relative to capacitance, at various sites within the circulation. The perception of a change in the normal relationship between intravascular volume and circulatory capacity evokes a host of renal effector mechanisms that lead ultimately to physiologically appropriate changes in urinary sodium excretion. These effector mechanisms involve physical adjustments in the glomerular filtration rate, renal microvascular hemodynamics and peritubular capillary Starling forces, tubule fluid composition, flow rate and transtubular ion gradients. Neural and humoral pathways are also involved and, among the latter, angiotensin II, aldosterone, prostaglandins and kinins have been studied extensively. The continuous interaction between these sensor and effector mechanisms serves to ensure near-constancy of the extracellular fluid volume, a condition essential for optimal circulatory performance.

Collecting duct sodium reabsorption in deoxycorticosterone-treated rats

In vitro studies of isolated, perfused, cortical collecting tubules have demonstrated that prior chronic deoxycorticosterone acetate (DOCA) treatment increases sodium reabsorption in this nephron segment, yet sodium balance in vivo is maintained. To evaluate the effect of chronic DOCA treatment on collecting duct sodium reabsorption in vivo, we compared fractional sodium delivery (FD(Na)%) out of the superficial late distal tubule with the fraction of sodium remaining at the base and the tip of the papillary collecting duct during extracellular fluid volume expansion in untreated, salt-treated, and DOCA-salt-treated rats. In untreated rats, FD(Na)% to the distal tubule was 6.5+/-1.0%, and to the base was 8.7+/-1.6% (Delta2.2+/-0.9%, P < 0.05). FD(Na)% to the tip was 4.9+/-1.1%, significantly less than FD(Na)% to the base (Delta3.7+/-1.1%, P < 0.01). In salt-treated rats, FD(Na)% to the distal tubule was 8.3+/-0.8%, and to the base was 10.4+/-1.1%. FD(Na)% to the tip was 5.9+/-0.6%, significantly less than FD(Na)% to the base (Delta 4.6+/-1.0%, P < 0.005). In DOCA-salt-treated rats, FD(Na)% to the distal tubule was 16.1+/-2.6% and to the base was 9.5+/-1.9% (Delta 6.6+/-1.7%, P < 0.005). FD(Na)% to the tip was 5.9+/-1.2%, also significantly less than FD(Na)% to the base (Delta 3.6+/-1.1%, P < 0.01). We conclude that (a) in DOCA-salt-treated rats, sodium delivery to the end of the superficial distal tubule is greater than in untreated or salt-treated rats; (b) in DOCA-salt-treated rats, sodium delivery to the end of the superficial distal tubule is greater than to the base of the papillary collecting duct, suggesting stimulation of sodium reabsorption in the cortical and(or) outer medullary collecting duct; and (c) sodium reabsorption by the papillary collecting duct is unaffected by chronic DOCA-salt treatment in the volume-expanded rat.