Proximal tubular injury attenuates outer medullary hypoxic damage: studies in perfused rat kidneys

The straight segment (S3) of the proximal tubule is predominantly damaged during renal ischemia-reflow, whereas medullary thick ascending limbs (mTALs) are principally affected in other models of hypoxic acute tubular necrosis (ATN). Since the latter injury pattern largely depends on the extent of reabsorptive activity during hypoxic stress, we hypothesized that proximal tubular damage might attenuate downstream mTAL injury by means of diminished distal solute delivery for reabsorption. In isolated rat kidneys perfused for 90 min with oxygenated Krebs-Henseleit solution, mTAL necrosis developed in 75 +/- 3% of tubules in the mid-inner stripe of the outer medulla. By contrast, S3 segments in the outer stripe were minimally affected, with tubular fragmentation involving some 5 +/- 2% of tubules. In kidneys subjected in vivo to proximal tubular injury and subsequently used for isolated perfusion studies, the injury pattern was inverted: following 20 and 30 min ischemia and reflow for 24 h, S3 fragmentation rose to 18 +/- 16% and 72 +/- 13%, while mTAL damage was reduced to 33 +/- 10 and 24 +/- 8%, respectively. In kidneys subjected in vivo to D-serine S3 necrosis rose to 100%, while mTAL damage fell to 1 +/- 1% (p < 0.001). Substantial S3 tubular collapse (involving approximately 30% of tubules) and inner stripe interstitial hemorrhage were also noted, exclusively in kidneys subjected to ischemia-reflow. Proximal tubular necrosis alone or in combination with collapse inversely correlated with mTAL necrosis (R = -0.51 and -0.72, respectively, p < 0.003). This cogent inverse association might imply that disruption of the proximal nephron attenuates downstream mTAL necrosis by a reduction of distal tubular reabsorptive workload.     

Multiple patterns of 11 beta-hydroxysteroid dehydrogenase catalytic activity along the mammalian nephron.

The enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) is thought to be a protective enzyme of the mineralocorticoid receptor (MR). We have previously demonstrated (Bonvalet et al, J Clin Invest 86:832-837, 1990) that 11 beta-OHSD is colocalized with MR along the rabbit nephron. In the present study, we examined whether 11 beta-OHSD is similarly located along the nephron of other mammals. Various tubular segments were microdissected from the mouse, rat, and rabbit nephron, and incubated for two hours at 37 degrees C in the presence of 11 nM [3H]-corticosterone (B). Thereafter, the respective amounts of B and [3H]-11dehydrocorticosterone (A) in the incubation solution were measured by HPLC. In the rabbit, the mouse and the rat, about 520 pmol/10 mm of B were transformed into A in tubular segments possessing MR, that is, the distal parts of the nephron (distal and collecting tubule). Differences appeared in the aldosterone-insensitive proximal tubule; in both the initial and final parts of this segment, 11 beta-OHSD activity was low (26 pmol/10 mm) in the rabbit and the mouse, and relatively high in the rat (328 pmol/10 mm). In the cortical part of the loop of Henle, where the presence of MR is still under discussion, 11 beta-OHSD activity was low in the mouse (70 pmol/10 mm), high in the rat (533 pmol/10 mm) and intermediate in the rabbit (227 pmol/10 mm). The comparison of these results with previous data obtained with immunohistochemical methods suggests that the proximal and distal nephron might express different isoforms of 11 beta-OHSD.     

The renal segmental distribution of claudins changes with development

BACKGROUND: Permeability properties of mammalian nephron are tuned during postnatal maturation. The transepithelial electrical resistance (TER) and complexity of tight junctions (TJs) vary along the different tubular segments, suggesting that the molecules constituting this structure change. We studied the differential expression of occludin and several claudins in isolated renal tubules from newborn and adult rabbits. METHODS: Isolated renal tubules from newborn and adult rabbits were processed for occludin, claudin-1 and claudin-2 immunofluorescence, and Western blot detection of claudin-1 and -2. Claudin-5 was detected in whole kidney frozen sections. RT-PCR from isolated tubules was performed for claudins-1 to -8. RESULTS: Immunofluorescence revealed that occludin, claudin-1 and -2 were present at the cell boundaries at the neonatal stage of development. Claudin-1 was detected in the tighter segments of the nephron (distal and collecting duct), while claudin-2 was found in the leaky portions (proximal). Claudin 5 was found in the kidney vasculature. PCR amplification revealed the presence of claudins-1 to -4 in tubules of newborns. In adults, claudins-1, -2 and -4 were present in proximal, Henle's loop and collecting segments; claudin-3 was in proximal and collecting tubules, while claudins-5 and -6 were absent from all tubular portions. Claudin-7 was restricted to proximal tubules, while claudin-8 was present in proximal and Henle's segments. CONCLUSIONS: The pattern of occludin distribution is present from the neonatal age. Claudins-7 and -8 are up-regulated after birth. Each tubular segment expresses a peculiar set of claudins that might be responsible for the permeability properties of their TJs.     

Congenital murine polycystic kidney disease

In the current study, the ontogeny of tubular cyst formation was studied in the CPK mouse, a murine strain with autosomal recessive polycystic kidney disease. Utilizing the technique of intact nephron microdissection in addition to standard light and transmission electron microscopy, the earliest morphologic alterations in CPK kidneys were localized in fetal tissue at 17 days of gestation to the distal portion of developing proximal tubules. During disease progression, from birth to 21 days of postnatal age, there was a shift in the site of cystic nephron involvement from proximal tubule to collecting tubules without involvement of other nephron segments. Cysts were enlarged tubular segments which remained in continuity with other portions of the nephron and were not associated with abnormalities in the overall pattern of nephron growth or differentiation. Analysis suggested that alterations in transtubular transport in abnormally shortened proximal tubular segments of juxtamedullary nephrons may have pathogenic importance in the early stages of cyst formation, and that epithelial hyperplasia and cytoskeletal alterations may have a role in progressive proximal tubular cystic enlargement. Cellular hyperplasia of epithelial walls of normally formed tubules was a prominent feature of cyst formation and progressive enlargement in collecting tubules. Such data form the basis for future studies into specific pathophysiological processes which may be operative in specific nephron segments during different stages of cyst formation in the CPK mouse.A preliminary report of this work was presented at the Annual Meeting of the Society for Pediatric Research, Washington DC, USA, May 1986, and has appeared in abstract form (Pediatr Res 20: 446A, 1986)     

Angiotensin stimulates bicarbonate transport and Na+/K+ ATPase in rat proximal straight tubules.

The effect of angiotensin on HCO3- absorption, fluid absorption, and Na+/K+ ATPase activity in isolated rat proximal straight tubules was investigated. During the control period, tubules absorbed fluid at 0.66 +/- 0.12 nL/mm.min and bicarbonate at 60.2 +/- 10.7 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, tubules absorbed fluid at 0.93 +/- 0.19 nL/mm.min and bicarbonate at 77.4 +/- 15.2 pmol/mm.min, indicating stimulation of both parameters. Time controls showed no significant change in the rate of bicarbonate or fluid absorption. To determine whether this stimulation was due to an increase in the maximum rate of transport, tubules were perfused at greater than or equal to 20 nL/mm.min. During the control period, tubules absorbed bicarbonate at 82.5 +/- 13.0 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, these same tubules absorbed bicarbonate at 75.9 +/- 11.9 pmol/mm.min. Thus, angiotensin did not alter the maximum rate of transport. Angiotensin also had no effect on bicarbonate permeability, which was 1.1 +/- 0.2 x 10(-4) cm/s before treatment and 1.3 +/- 0.3 x 10(-4) cm/s afterward. Finally, the effect of angiotensin on Na+/K+ ATPase activity was measured in paired experiments. Na+/K+ ATPase activity of control tubules was 36 +/- 6 pmol of ADP/mm.min; after angiotensin treatment, it was 47 +/- 6 pmol ADP/mm.min. From these data it was concluded that: (1) angiotensin stimulates bicarbonate absorption in the rat proximal straight tubule; (2) this stimulation is the result of a change in Km rather than of an increase in the maximum rate of transport or permeability; and (3) angiotensin directly stimulates Na+/K+ ATPase activity in the proximal nephron.     

Proximal tubule morphology after single nephron obstruction in the rat kidney

This study examined the effects on proximal tubule morphology of blocking single nephrons with paraffin wax for one day, one week, or one month in the rat. Proximal tubule lumens were blocked with a short column of wax using micropuncture. Chronically blocked and control (normal) tubules were fixed by either intravascular or intraluminal perfusion of glutaraldehyde solution. Proximal tubule segments downstream to the wax block were examined by light and transmission electron microscopy. Intraluminal Alcian blue dye, serial sectioning, and nephron microdissection techniques were used to identify nephrons. One day after obstruction, all proximal tubule cells downstream to the block were injured. Some recovery was seen. S1 and S2 segments showed more severe damage than S3 segments. Alcian blue, which normally is excluded from cells, entered the cytoplasm of some damaged S1-S2 cells. After one week of obstruction, the tubule appeared to have reconstituted itself, but cells were less differentiated than normal. One month after obstruction, blocked tubules were atrophied. Tubule cells were simplified and were surrounded by a thickened basement membrane. The results suggest that prolonged proximal tubule blockade produces injury and atrophy of the proximal tubule probably due to ischemia and interruption of normal reabsorptive activity.     

Intraluminal ATP concentrations in rat renal tubules

It is becoming increasingly recognized that stimulation of apical P2 receptors can influence solute transport in the nephron, but, to date, no information is available on endogenous intraluminal nucleotide concentrations in vivo. This study measured intraluminal ATP concentrations in the renal tubules of anesthetized rats. Proximal tubular concentrations were found to be in the range of 100 to 300 nmol/L, with no significant variation along the S2 segment, whereas concentrations in the early distal tubule were markedly lower. Using collections of varying duration, the half-life of ATP in collected proximal tubular fluid was found to be 3.4 min, indicating significant breakdown by soluble nucleotidases. For assessment of whether proximal tubular ATP was filtered or secreted, experiments were performed in Munich-Wistar rats. The ATP concentration in midproximal tubules (142 +/- 23 nmol/L) was more than four-fold higher than in Bowman's space (32 +/- 7 nmol/L; P < 0.001), whereas fractional water reabsorption between the two sites was modest. In experiments that were designed to determine the effects of (patho)physiologic disturbances on intraluminal ATP, rats were either volume expanded or subjected to hypotensive hemorrhage. Neither maneuver affected proximal tubular luminal ATP concentrations significantly; rapid degradation of secreted ATP by ecto- and soluble nucleotidases is a possible explanation. It is concluded that the proximal tubule secretes ATP into the lumen, where it may have an autocrine/paracrine regulatory role.     

中国实验用小型猪肾小管的发育及各节段标志物的表达

目的:明确解剖和生理上与人非常接近的中国实验用小型猪肾脏发育过程中肾小管的形态学变化和肾小管各节段的特异性标志物。方法:采用高碘酸-希夫(PAS)染色和免疫荧光染色技术,系统观察中国实验用小型猪妊娠28～112d(E28d～E112d)和出生后1d、7d、14d、21d(P1d～21d),共17个不同时间点猪肾小管的发育及肾小管特异性标志物雪莲花凝集素(LTL)、水通道蛋白1(AQP1)、钙结合蛋白(calbindin)-D28k在肾小管不同节段的表达。结果:(1)中国实验用小型猪E28d可见后肾间充质和输尿管芽,即后肾已经开始发育;但这时还没有肾小管。E35d可见不同节段的肾小管,即肾小管已开始发育。从E35d～P14d(E112d仔猪出生),肾皮质均有生肾区存在,即不断有新的肾单位发生;P21d生肾区消失,即不再有新的肾单位产生。(2)①LTL在E28d表达在输尿管芽,E35d开始在近端小管表达,以刷状缘表达最为明显;表达由弱到强,由点状到线状。②AQP1在E28d未见表达,E35d开始表达;AQP1表达在近端小管和髓袢的降支细段,主要表达在细胞膜,尤其在管腔侧的表达更为明显。③Calbindin-D28k在E28d表达在输尿管芽,E35d开始表达在远端小管和集合管;Calbindin-D28k主要表达在细胞质,随着肾小管发育,表达逐渐增强。(3)发现集合管来源于输尿管芽,发源于输尿管芽的集合管从被膜下的生肾区一直延伸到肾髓质。结论:中国实验用小型猪妊娠35d可以见到不同节段肾小管。LTL、AQP1、Calbindin-D28k可以分别作为猪近端小管、髓袢、远端小管和集合管的标志物。     

Alterations of gluconeogenesis by ischemic renal injury in rats.

This study was designed to determine changes in one metabolic function, gluconeogenesis (GLG), after ischemic renal injury. Tubule suspensions were prepared by collagenase treatment of SD rat kidneys on 1, 3, and 7 days after left renal artery and vein occlusion for 0-90 min and incubated in Krebs-Henseleit buffer with or without 2 mM pyruvate or malate aerobically. Glucose contents were assayed photometrically. On days 1 and 3 after ischemia for longer than 60 min, serum creatinine levels rose significantly. The tendency of increase of GLG was observed on days 1 and 3 after 10-60 min of ischemia. GLG increased significantly on day 1 after 30-min ischemia. On the other hand, GLG decreased significantly on day 1 after 90-min treatment. Morphologic damage was limited to the corticomedullary region on days 1 and 3 after ischemic times of 30 and 60 min. These results suggest that renal GLG is stimulated to supply energy for ATP decrease by ischemia and for further regeneration in extraproximal segments along the nephron.     

Glomerular and proximal tubular morphology after single nephron obstruction

This study examined the effect of blocking proximal tubule lumens on glomerular and early proximal tubular morphology. Single nephrons in the rat kidney were blocked with wax by micropuncture. After one day, one week, or one month of obstruction, the kidneys were fixed with glutaraldehyde by intravascular perfusion, and nephron structure was examined by light and electron microscopy. Following obstruction, glomerular changes developed more slowly than tubular changes. After one day, the only change noted in some glomeruli was the presence of inflammatory cells. The only tubule change upstream to the block was a focal loss of apical microvilli. This is in contrast to the severe damage previously reported (Evan, Tanner: Kidney Int 30: 818-827, 1986) in downstream proximal tubule segments at this time. After one month of obstruction, glomerular size was decreased and the glomerular filtration membrane was abnormal. Tubular cell size was decreased, apical microvilli were lost, basolateral interdigitations were reduced, and mitochondria were fewer and abnormally oriented. Interstitial fibrosis was present. Changes in nephron structure develop slowly after obstruction, perhaps because continued filtration and reabsorption maintain nephron integrity. Eventually, blocked nephrons atrophy, probably because of reduced blood flow, disuse, and inflammatory responses.     

Contribution of the distal tubule to potassium excretion in experimental glomerulonephritis

The renal adaptations that maintain potassium homeostasis in diffuse forms of glomerular disease are not well defined. Thus, handling of potassium by superficial nephron segments was examined in a rat model of antiglomerular basement membrane nephritis. Sampling the same nephron successively from the end and beginning of the distal tubule and the end of the proximal tubule allowed a segmental analysis. Despite a 40% reduction in GFR, potassium excretion in the glomerulonephritis animals was normal due to an increase in FEK. The proximal tubule and loop segment did not contribute to the enhanced FEK seen in these animals. In contrast, potassium entry along the distal tubule was significantly greater in the experimental group averaging 13.7 +/- 4.3 pmol/min compared to 1.2 +/- 1.7 pmol/min in controls (P less than 0.01). Multiple linear regression analysis showed that distal tubule potassium entry at any level of flow was enhanced in glomerulonephritis compared to controls (P less than 0.0001). Plasma aldosterone levels were similar in both groups of animals. Thus, the adaptation to potassium excretion seen in glomerulonephritis is partly achieved by the distal tubule through flow-rate independent mechanisms and appears to be independent of plasma aldosterone levels.     

Decreased sensitivity of distal nephron and collecting duct to parathyroid hormone in pseudohypoparathyroidism type I

Parathyroid hormone (PTH) transiently increases urinary excretion of the lysosomal enzyme, N-acetyl-beta-D-glucosaminidase, which is distributed mainly in proximal tubules. The response is reduced in pseudohypoparathyroidism (PHP) type I, which is characterized by target-organ resistance to PTH. Evidenced by normal calcium resorption, distal tubule sensitivity to PTH has been believed to be normal in this disorder. This hypothesis was tested through a search for another marker of distal nephron sensitivity to PTH. In the human kidney, cathepsin D was expressed predominantly in distal segments of the nephron, cortical and medullary thick ascending limbs of Henle's loop, distal convoluted tubules, and connecting tubules and in cortical collecting ducts and medullary collecting ducts. PTH infusion transiently increased cathepsin D excretion in normal subjects. The cathepsin D response to PTH was reduced in the patients with PHP type I. The decrease in cathepsin D response in PHP type I indicates a resistance to PTH in the distal nephron (cortical thick ascending limbs of Henle's loop, distal convoluted tubules, and connecting tubules) and cortical collecting ducts. These observations suggest that the preservation of renal tubular sensitivity to PTH in this disorder may be confined to PTH-dependent calcium resorption in distal tubules.     

BK channels in the kidney

PURPOSE OF REVIEW: Large, BK (calcium-activated potassium) channels are now regarded as relevant players in many aspects of renal physiology, including potassium secretion. This review will highlight recent discoveries regarding the function and localization of BK in the kidney. RECENT FINDINGS: Patch clamp electrophysiology has revealed BK in cultured podocytes, glomerular mesangial cells, and in several tubule segments including principal cells (connecting tubules/principal cells), and intercalated cells of connecting tubules and cortical collecting ducts. Flow-induced potassium secretion is mediated by BK in the distal nephron and may be partly the result of shear stress-induced increases in cell calcium concentrations. ROMK-/- and wild-type mice on a high potassium diet exhibit BK-mediated potassium secretion, and studies of BK-alpha-/- and BK-beta1-/- mice suggest that flow-induced potassium secretion is mediated by BK-alpha/beta1, which is specifically localized in the apical membrane of the connecting tubule of the mouse and connecting tubule plus initial cortical collecting duct of the rabbit. SUMMARY: BK channels, located in glomerular cells and in many nephron segments, especially mediate potassium secretion in the combined condition of potassium adaptation and high flow. Understanding the molecular makeup of BK in specific renal cells and the dietary and physiological conditions for their expression can yield improved potassium-sparing compounds.     

Effects of aldosterone on NEM-sensitive ATPase in rabbit nephron segments

Aldosterone (aldo) treatment of animals stimulates the rate of H+ secretion in the collecting duct, a process which may involve an H+-ATPase sensitive to inhibition by NEM (N-ethylmaleimide). Therefore, we determined NEM-sensitive ATPase activity in distal nephron segments from three groups of adrenalectomized (adx) rabbits maintained on different doses of aldo (in an osmotic minipump) for seven days. Group 1 was given 1.5 micrograms aldo/100 g body wt/day, whereas groups 2 and 3 were maintained on 5 micrograms and 50 micrograms of aldo/100 g body wt/day, respectively. Aldo concentrations in the plasma of groups 1, 2 and 3 were 10.4 +/- 0.8, 70 +/- 7 and 408 +/- 133 ng/dl, respectively. There was a significant increase in NEM-sensitive ATPase activity in connecting tubule (CNT) and cortical, outer and inner medullary duct segments (CCD, OMCD and IMCD) but not in cortical thick ascending limb (CTAL) and distal convoluted tubule (DCT) in group 2 as compared to group 1. A further increase in plasma concentration of aldo (group 3) did not produce any more increase in NEM-sensitive ATPase activity in the CNT, CCD, OMCD and IMCD, but did increase the enzyme activity in the DCT. These results are consistent with the hypothesis that aldo increases H+ secretion in the connecting tubule and collecting duct segments by increasing the activity of NEM-sensitive H+-ATPase activity in these segments.     

Proximal tubule dopamine histofluorescence in renal slices incubated with L-dopa

Renal cortex slices were incubated with amine precursors L-dopa, or L-5-HTP. Localization of synthesized amines, dopamine or serotonin, was examined by means of histofluorescence methods. Formaldehyde-induced fluorescence was present in the proximal convoluted tubule and not in the pars recta or other segments of the tubules after incubation in the presence of 10(-3) to 10(-7) M L-dopa. Tubule-induced fluorescence was not seen in the presence of an inhibitor of dopa-decarboxylase or in the absence of sodium. It was independent of innervation. It is concluded that dopamine and serotonin are accumulated and likely formed within proximal convoluted tubular cells.     

Endothelin induces diuresis and natriuresis in the rat by acting on proximal tubular cells through a mechanism mediated by lipoxygenase products.

Besides being a potent renal vasoconstrictor, endothelin causes diuresis and natriuresis. At which site along the nephron and how endothelin alters water and sodium handling in the tubule remain to be clarified. It was found that endothelin (75 pmol) given as an i.v. infusion in vivo to rats caused diuresis and urinary sodium excretion to double but did not affect glomerular filtration rate and renal plasma flow. On raising the dose of endothelin to 150 pmol, a further increase in diuresis and natriuresis was found, whereas glomerular filtration rate fell 33% and renal plasma flow fell 36%; 300 pmol of endothelin reduced glomerular filtration rate by 73% and renal plasma flow by 77% but did not significantly affect diuresis and absolute sodium excretion. It did, however, increase fractional sodium excretion eightfold. Lithium clearance studies of changes in tubular handling of water and sodium indicated that infusion of 150 pmol of endothelin to rats caused a reduction in absolute (pre, 84.7 +/- 5.9; post, 47.9 +/- 6.1 microEq/min/100 g) and fractional (pre, 85.7 +/- 3.0; post, 64.7 +/- 6.4%) proximal reabsorption of sodium. Endothelin infusion (150 pmol) was not associated with any significant change in plasma atrial natriuretic peptide levels, which on average remained comparable to those in rats given the vehicle alone (49.7 +/- 8.4 versus 46.3 +/- 5.6 pg/mL). In the isolated perfused rat kidney preparation, exposure to 150 pmol of endothelin significantly increased fractional sodium excretion over preinjection values (pre, 2.2 +/- 0.2; post, 7.3 +/- 1.0%) despite a marked decrease in glomerular filtration rate and renal perfusate flow. Additional in vivo experiments showed that oral administration of the specific 5-lipoxygenase inhibitor L-651,392 to rats prevented the increase in urine flow rate (pre, 5.7 +/- 0.1; post, 6.6 +/- 0.8 microL/min), and in absolute (pre, 0.33 +/- 0.04; post, 0.37 +/- 0.05 microEq/min) and fractional (pre, 0.10 +/- 0.02; post, 0.11 +/- 0.03%) sodium excretion caused by bolus i.v. infusion of endothelin (150 pmol). Similarly, a specific leukotriene C4/D4 receptor antagonist, L-649,923, also prevented the diuretic and natriuretic effect of 150 pmol of endothelin i.v. infusion. These findings show that (1) endothelin has a diuretic and natriuretic effect that is independent of its action on renal hemodynamics; (2) this effect depends on a direct action on the proximal tubules; (3) atrial natriuretic peptide does not appear to be involved in this effect; and (4) the diuretic and natriuretic responses to endothelin are mediated by 5-lipoxygenase products.     

Superficial nephron obstruction and medullary congestion after ischemic injury: effect of protective treatments

90 min of renal artery occlusion in previously unilaterally nephrectomized rats produce acute renal failure (ARF) (plasma creatinine at 48 h after ischemia: 636 +/- 44 vs. 133 +/- 9 mumol/l in controls). Between 1 and 48 h after releasing the occlusion, two populations of superficial nephrons could be observed, one with dilated tubules and elevated proximal tubular pressures (PTP: 39 +/- 1 vs. 12 +/- 1 mm Hg in controls) and the other with collapsed tubules and decreased PTP (9 +/- 1 mm Hg). Proximal tubular passage time (PPT) could not be determined with the Lissamine green technique. Seven methods of pretreatment were tested, 5 of which provided partial functional protection (DOCA/NaCl/NaCl, furosemide infusion, inosine bolus, mannitol bolus and the combination of the last two). Neither renal renin levels nor urinary NaCl excretion were consistently correlated with protection. Functionally protected rats consistently showed no PTP increase and normal PPT in the tubules at the kidney surface. However, plasma creatinine at 48 h differed markedly within the 5 protected groups, ranging from 168 +/- 18 to 398 +/- 35 mumol/l. Extensive medullary congestion was seen at 1-6 h after ischemia only in those rats with obstructed, high pressure nephrons at the kidney surface. To conclude: (1) Functional protection from ischemic ARF, both with and without an accompanying increase in solute excretion, was achieved by the abolition of tubular obstruction. (2) Despite similar degrees of restoration of superficial nephron function, the persisting impairment of whole kidney function differed markedly between the protected groups. (3) Impairment of deeper nephron function must therefore play a major role, perhaps through persisting obstruction in the long loops of Henle. (4) High pressure nephrons may compromise medullary venous outflow in the outer zone of the outer medulla.     

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

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