Defect In Sodium Transport Research Articles

Preserved Na+/H+ exchanger isoform 3 expression and localization, but decreased NHE3 function indicate regulatory sodium transport defect in ulcerative colitis†

A major causative factor of diarrhea in ulcerative colitis (UC) patients is the loss of Na(+) absorptive capacity of the inflamed colonic mucosa. Potential contributing mechanisms include reduced driving force for active transport, and impaired expression, mislocalization, or defective transport function of Na(+) absorptive proteins. We therefore studied the expression, brush border membrane (BBM) localization, and transport capacity of the major intestinal Na(+) absorptive protein, the Na(+)/H(+) exchanger isoform 3 (NHE3) in biopsies from UC patients. In UC and control biopsies, inflammation was graded histologically, NHE3, tumor necrosis factor alpha (TNF-alpha), villin, as well as other housekeeping genes were analyzed by quantitative real-time polymerase chain reaction (PCR), BBM localization of NHE3 determined by immunohistochemistry, and confocal microscopy. Na(+) absorptive capacity was assessed by (22)Na(+) isotope fluxes and NHE3 transport activity measured microfluorometrically in BCECF-loaded surface colonocytes within isolated crypts. In mildly, moderately, and severely inflamed sigmoid colon of UC patients, neither NHE3 mRNA expression nor the abundance of NHE3 in the BBM was significantly altered compared to other structural components of the BBM. However, Na(+) absorption was strongly reduced by approximately 80% and acid-activated NHE3 transport activity was significantly decreased in the surface cells of sigmoid colonic crypts even in moderately inflamed mucosa. In the colonic mucosa of patients with active UC, NHE3 transport capacity was found significantly decreased despite correct NHE3 location and abundance in the brush border, independent of current treatment. These findings suggest functional NHE3 transport as a novel factor for inflammatory diarrhea in UC patients.

High altitude impairs nasal transepithelial sodium transport in HAPE-prone subjects.

High-altitude pulmonary oedema (HAPE) occurs in predisposed individuals at altitudes >2,500 m. Defective alveolar fluid clearance secondary to a constitutive impairment of the respiratory transepithelial sodium transport contributes to its pathogenesis. Hypoxia impairs the transepithelial sodium transport in alveolar epithelial type II cells in vitro. If this impairment is also present in vivo, high-altitude exposure could aggravate the constitutive defect in sodium transport in HAPE-prone subjects, and thereby further facilitate pulmonary oedema. Therefore, the aim of the current study was to measure the nasal potential difference (PD) in 21 HAPE-prone and 29 HAPE-resistant subjects at low altitude and 30 h after arrival at high altitude (4,559 m). High-altitude exposure significantly decreased the mean +/- SD nasal PD in HAPE-prone (18.0 +/- 6.2 versus 12.5 +/- 6.8 mV) but not in HAPE-resistant subjects (25.6 +/- 9.4 versus 22.9 +/- 9.2 mV). This altitude-induced decrease was not associated with an altered amiloride-sensitive fraction, but was associated with a significantly lower amiloride-insensitive fraction of the nasal PD. These findings provide evidence in vivo that an environmental factor may impair respiratory transepithelial sodium transport in humans. They are consistent with the concept that in high-altitude pulmonary oedema-susceptible subjects, the combination of a constitutive and an acquired defect in this transport mechanism facilitates the development of pulmonary oedema during high-altitude exposure.

Impairment of blood volume restitution after large hemorrhage: a mathematical model.

A mathematical model tests possible mechanisms for the progressive failure of blood volume restitution seen after larger hemorrhages ( > 26%) with increasing changes in plasma osmolality. After 10% hemorrhage, the model requires a decrease in net hydrostatic capillary pressure, the release of solute into the extracellular space, and the release of Na+ and K+ from a bound pool in equilibrium with the interstitium to match the experimental data. The solute and released cations expand the interstitium to drive the restitution of volume and protein from 3 to 24 h. After 30% hemorrhage, the best prediction of the average experimental responses occurs when the Na(+)-K(+)-adenosinetriphosphatase (ATPase) in the cell membrane is inhibited by 38.7% from 0.8 to 3 h, and the proportionality between capillary pressure and blood volume is reduced by 68% from its value for 10% hemorrhage. When the change in plasma osmolality is doubled after 30% hemorrhage, an increase in the inhibition of the ATPase to 85% and extension of its duration to 24 h are necessary to match experimental findings. The associated defect in sodium transport may occur after large hemorrhage so that sodium and water move into cells. This response may oppose osmotically driven expansion of the interstitium and thus account for the failure of restitution.

Transport defects of rabbit medullary thick ascending limb cells in obstructive nephropathy.

To characterize the sodium transport defect responsible for salt wasting in obstructive nephropathy, the major sodium transporters in the medullary thick ascending limb (mTAL), the apical Na-K-2Cl cotransporter and the basolateral Na-K-ATPase, were studied in fresh suspensions of mTAL cells and outer medulla plasma membranes prepared from obstructed and untreated kidneys. Oxygen consumption (QO2) studies in intact cells revealed marked reductions in the inhibitory effects of both furosemide and ouabain on QO2 in cells from obstructed, as compared with control animals, indicating a reduction in activities of both the Na-K-2Cl cotransporter and the Na-K-ATPase. Saturable [3H]bumetanide binding was reduced in membranes isolated from obstructed kidneys, but the Kd for [3H]bumetanide was unchanged, indicating a decrease in the number of functional luminal Na-K-2Cl cotransporters in obstructed mTAL. Ouabain sensitive Na-K-ATPase activity in plasma membranes was also reduced, and immunoblots using specific monoclonal antibodies directed against the alpha and beta subunits of rabbit Na-K-ATPase showed decreased amounts of both subunits in outer medullas of obstructed kidney. A significant decrease in [3H]bumetanide binding was detected after 4 h of ureteral obstruction, whereas Na-K-ATPase activity at this time was still not different from control. We conclude that ureteral obstruction reduces the amounts of both luminal Na-K-2Cl cotransporter and basolateral Na-K-ATPase in mTAL of obstructed kidney and that these reductions contribute to the salt wasting observed after release of obstruction.

Erythrocyte sodium transport at twenty-year follow-up of childhood hypertension.

Twelve-hour urinary sodium excretion, the number of sodium pump sites (Bmax) and the Na/K flux ratio in erythrocytes were measured in 13 previously diagnosed hypertensive subjects. At the time of the study, six subjects were still hypertensive and showed a significantly lower Na/K flux ratio in erythrocytes than the remaining seven subjects who were normotensive. Bmax was also lower in the hypertensive group compared to the normotensive group, although this was not statistically significant. Urinary Na excretion did not show any significant difference between the two groups. These findings suggest that a cell membrane sodium transport defect may have a role in the development of essential hypertension in adult life.

Sodium transport defect of ouabain-resistant renal Na,K-ATPase

The murine renal Na,K-ATPase is resistant to cardiac glycosides. It is not yet known however whether altered active transport is associated with the drug-resistance. To investigate this problem Na,K-ATPases were purified from the outer medulla of both rat and rabbit kidneys and reconstituted identically into liposomes. The Na-stimulation of the Na,K-ATPase activity before reconstitution and of the Na-transport after reconstitution was measured. A Na-defect inherent in the ouabain-resistant rat Na,K-ATPase was discovered indicating a link between the cardiac glycoside sensitivity and the Na-transport.

Reversal by verapamil of defect in sodium transport in leucocytes in essential hypertension.

The effect of treatment with verapamil on cell sodium transport was studied in the leucocytes of patients with essential hypertension. Previously described abnormalities of sodium efflux rate constant and intracellular sodium content were confirmed, the component of the sodium efflux rate constant sensitive to ouabain being lower and the intracellular sodium content higher in the patients compared with controls. Verapamil reversed these abnormalities and reduced blood pressure.

Reversal by Verapamil of the Sodium Transport Defect in Leucocytes in Essential Hypertension

Reversal by Verapamil of the Sodium Transport Defect in Leucocytes in Essential Hypertension

Bartter's Syndrome: Physiological and Pharmacological Studies

Six siblings with Bartter's syndrome were studied. Increased urinary immunoreactive prostaglandin E (iPgE) was corrected by administration of the prostaglandin synthetase inhibitors, indomethacin, ibuprofen and meclofenamate. In addition, plasma potassium rose, plasma renin activity and angiotensin resistance decreased, and the exaggerated natriuresis following saline loading was abolished. Increased urinary iPgE also became normal following the phospholipase inhibitor, mepacrine, but the other abnormalities remained unaltered. The kallikrein inhibitor, aprotinin, did not alter urinary iPgE, plasma potassium or electrolyte balance. During hypotonic saline infusion, proximal tubular potassium or electrolyte balance. During hypotonic saline infusion, proximal tubular sodium reabsorption was normal or increased. Free water clearance and the percentage of distally delivered sodium which was reabsorbed were, however, significantly decreased. The results suggest that neither the increased renal PgE production nor the hyperbradykininemia seen in Bartter's syndrome play a major role in its pathogenesis, or manifestations, and that the effects of the prostaglandin synthetase inhibitors on the syndrome are non-specific. The results and relevant literature are analysed in an attempt to identify the initial defect in the interrelated sequence of events. The data are compatible with an intrarenal defect in sodium transport, leading to increased sodium delivery to the distal tubule, with secondary hyperreninemia, hypokalemia and elevated iPgE excretion.

Zinc and the sodium pump in uremia

A defect in sodium transport is known to exist in the leucocytes of uremic patients. Recent work has shown changes in cation flux rates associated with alterations of extracellular zinc concentration. As plasma zinc is known to be low in uremia, the possibility that this might be the cause of the defect in membrane transport was investigated. Plasma zinc was shown to be lower in uremic patients than in normal controls but this was not matched by alterations in leucocyte zinc content. Leucocytes from normal subjects and from patients with uremia have similar increases in the sodium efflux rate constant when exposed to elevated extracellular zinc concentrations. However, the abnormality in leucocyte sodium transport in uremia was not completely corrected by elevation of the extracellular zinc, when compared with normal cells in the same zinc concentrations. Although extracellular zinc is a factor that must be controlled in studies of cellular membrane transport, a low plasma zinc is not the explanation for the defect of sodium transport seen in uremia.

The renal functional defect of postobstructive nephyropathy. The effects of bilateral ureteral obstruction in the rat.

This study was designed to examine the pathogenesis of the excretory defect produced by bilateral ureteral obstruction in the rat. After release of obstruction of 24 hr duration glomerular filtration rate was reduced to 20% of normal. Free flow proximal tubular pressure was normal, excluding residual obstruction as a cause of depressed filtration, and indicating that an intrarenal hemodynamic abnormality was primarily responsible for the excretory defect. Total renal blood flow and cortical distribution of flow were normal. Clearance and micropuncture studies indicated the presence of marked heterogeneity of nephron function with residual excretory function residing primarily in vasodilated nephrons in which decreased postglomerular arteriolar resistance effected a reduction in glomerular filtration pressure. Heterogeneity of nephron function was evidenced by a wide scatter of values for single nephron filtration rate and from direct intratubular injection of dye which revealed that at least 28% of surface nephrons were either nonfiltering or had filtration rates too low to measure. The observed decrease in Hippuran extraction and increased ratio of Hippuran to inulin clearance ratio is characteristic of the vasodilated kidney. Further evidence of the vasodilated nature of residual functioning nephrons was demonstrated by the failure of intrarenal papaverine infusion to increase filtration rate in this lesion. The hemodynamic defect produced by bilateral obstruction is contrasted with that seen after release of unilateral ureteral ligation in which depression of filtration rate appears to result primarily from preglomerular vasoconstriction. This difference raises the possibility that a vasodilating substance accumulates during total suppression of renal excretory function. Diuresis and natriuresis were constant features of the postobstructive lesion. The present data support previously published studies which localize the defect in sodium transport to the distal nephron and indicate that this defect is a consequence of increased intraluminal pressure and tubular dilatation induced by bilateral ureteral ligation.

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.

Red-cell transport defect in patients with cystic fibrosis and in their parents.

The ouabain-sensitive and the ethacrynic acid-sensitive sodium efflux from erythrocytes of patients with cystic fibrosis are both decreased. Furthermore, the ouabain-sensitive adenosine triphosphatase activity is diminished in the red blood cell ghosts of these patients. Perhaps of greater significance is the fact that ethacrynic acid-sensitive sodium efflux is clearly diminished in the erythrocytes of the asymptomatic parents of these sick children. This defect in sodium transport may be valuable for detecting the heterozygous carrier state.

Quantitative Microdetermination of Enzymes in Sweat Gland: V. Ouabain-Sensitive Sodium-Potassium Activated Adenosinetriphosphatase

Extract: The excessive concentration of sodium chloride in sweat in cystic fibrosis may be due to failure of a ‘sodium pump’ mechanism. In view of a possible relationship between electrolyte transport and ouabain sensitive ATPase, determination of this enzyme activity was undertaken in isolated sweat gland tissue from cystic fibrosis and control subjects. In each case, determinations were made with optimal Na+, K+ and Mg++ in the substrate mixture, with or without ouabain. With activities expressed as moles phosphate liberated per kg dry sweat gland tissue per hour, seven cystic fibrosis patients gave a mean ouabain sensitive ATPase activity of 2.0 ± 1.1 (SD). Seven controls gave 2.9 ± 1.6. This difference was not significant statistically (t = 1.1). Speculation: Contrary to what might have been suspected from reported results on a relationship between ouabain sensitive ATPase and sodium transport in various sites, namely, crab nerve tissue, avian salt gland, human and sheep erythrocytes and various other tissues, the present study involving skin biopsies from cystic fibrosis patients and controls fails to show this enzyme to be deficient in relation to the defect in sodium transport in the sweat gland in cystic fibrosis.

Individual nephron function in experimental bilateral pyelonephritis. II. Distal tubular sodium and water reabsorption and the concentrating defect.

Abstract The renal concentrating defect in rats with nonobstructive bilateral pyelonephritis was studied by combined clearance and micropuncture methods. Control groups consisted of normal rats and those in which approximately 65 to 70 per cent of renal mass had been ablated by bilateral subtotal nephrectomy. The residual functioning renal mass and solute load per nephron were estimated to be of about the same order of magnitude in the pyelonephritic and partially nephrectomized animals. It was found, however, that both U max and t c h 2 o were diminished to a greater extent in the rats with intrinsic renal disease. Measurements of osmolality, sodium, and inulin concentrations in the distal tubular fluid of surviving cortical nephrons failed to reveal any specific defect in sodium transport by the thick ascending limb of Henle's loop, or any change in the permeability characteristics which could account for the severity of the concentrating defect in the pyelonephritic animals. The data suggest, rather, that at comparable rates of total solute excretion in the urine, individual surviving loops of Henle in the pyelonephritic rats were actually transporting more sodium into the medullary interstitium than were the loops in the normal controls. The concentrating defect in the pyelonephritic animals thus could not be accounted for by the reduction in renal mass, by the adaptational increases in glomerular filtration rate per surviving nephron, or by any detectable defect in salt or water transfer in individual cortical nephrons. The observations localize the disturbance to the renal medulla. It is suggested that disruption of the architecture of the renal medulla may be an important factor contributing to the concentrating defect.