Replacement of tyr62 by trp in the designer protein milk bundle-1 results in significant improvement of conformational stability

Protein design is currently used for the creation of new proteins with desirable traits. In our lab, we focus on the synthesis of proteins with high essential amino acid content, having potential application in animal nutrition. One of the limitations we face in this endeavor is the achievement of stable proteins in spite of a highly biased amino acid content. We report here the synthesis and characterization of MB-1Trp, a protein with a tailored content in selected essential amino acids. The protein is a Tyr62-Trp mutant of the parent molecule MB-1 described earlier. The new protein is largely helical as per design, is well folded, and has a melting temperature of 55 degrees C. Its resistance to proteolytic degradation compares to that of cytochrome c, a protein of similar size. Design strategy used for MB-1Trp is discussed with regards to its applicability toward the creation of efficient nutritional proteins.

Benefit of prestorage leukocyte depletion of single-donor platelet concentrates

BACKGROUND AND OBJECTIVES

The presence of contaminating white blood cells (WBCs) in platelet concentrates is associated with transfusion reactions and may adversely alter the quality of platelets during storage. Leukocyte depletion by filtration of platelets has been increasingly used to avoid these complications. However, the best time for filtration and the benefits of filtering single-donor platelet concentrates (thrombapheresis, TH) have yet to be clearly defined.

METHODS

In a randomized study of 202 TH collected with an Autopheresis C system, we determined whether prestorage filtration (preSF) of WBCs from TH as compared with poststorage (bedside) filtration (postSF) resulted in a better product. Levels of cytokines and C3a accumulating in the medium during storage, platelet activation state, in vivo platelet recovery, and transfusion reactions were compared in pre- and poststorage products.

RESULTS

As compared to preSF, significantly more postSF TH had detectable levels of tumor necrosis factor-alpha (TNF-alpha; 47 vs. 15%; p<0.0001) and interleukin 6 (13 vs. 3%; p = 0.02), lower pH (p<0.0001) and decreased levels of C3a (910 vs. 2,000 pg/ml; p<0. 0001). Furthermore, platelet activation was increased in postSF TH (p = 0.022). PostSF TH tended to plug the bedside filter (27% of postSF TH delivered) from day 3 onward. There was also a significant difference in platelet recovery, postSF TH showing a lower corrected count increment (CCI; p = 0.0055) when taking into account the postSF TH that plugged filters (CCI = 0), but no difference when plugged TH were excluded. A correlation could be established between TNF-alpha levels and poor in vivo recovery (p<0.0001). Febrile nonhemolytic transfusion reactions were low in both groups (4 and 9%).

CONCLUSION

These results indicate a benefit of preSF TH as compared with postSF TH based on the following parameters: decrease in cytokine levels, less platelet activation, maintenance of higher pH, and more efficient use of stored platelets (27% of postSF TH were lost because of plugging of filters). These results apply particularly to the Autopheresis C systems with its high initial WBC content.

Quality evaluation of plateletpheresis using the new AMICUS (Baxter) cell separator: evolution of CD 62 expression

The purpose of this study was to evaluate the new AMICUS (Baxter-Fenwal Division) cell separator in terms of donor safety, efficiency, and quality of the product obtained. One hundred eighty-three single-donor plateletpheresis procedures were performed, using a collection of 4-4.5 x 10(11) platelets as endpoint. During the first part of the study, the mean volume processed was 3,225 ml and the mean procedure duration 69.5 min. During the second part, after a software change, the mean volume and mean procedure time were 3,071 ml and 68.3 min, respectively. According to local policy, every collection bag was separated into two therapeutic units each containing a mean of 1.87 (1.83) x 10(11) platelets. The white blood cell (WBC) contamination per therapeutic unit was less than 5 x 10(6) in 91% of phereses performed in part one of the study and in 98% of phereses performed in part two. During the recommended 5 days storage, sequential in vitro analyses were performed in 27 units, showing limited platelet activation according to CD62 expression and morphological changes on electron microscopy (EM). Furthermore, there was a correlation between CD62 expression and the degree of WBC contamination (P = 0.03). In conclusion, platelet collection with the new Amicus allows for high platelet yields of adequate quality as judged by WBC content, CD62 expression, and electron microscopic morphological changes.

A repression-derepression mechanism regulating the transcription of human immunodeficiency virus type 1 in primary T cells

BACKGROUND

Despite some controversy regarding the preferential infection and replication of human immunodeficiency virus type 1 (HIV-1), it appears that primary T lymphocytes, in their quiescent state, are nonpermissive for viral expression and propagation. Massive activation of viral gene expression occurs only when the host lymphocyte is activated. These observations prompted us to investigate the transcriptional regulation of HIV-1 in resting or activated T cells that were isolated from cord blood or adult peripheral blood.

MATERIALS AND METHODS

To this end, we employed cellular purification and phenotyping techniques, in vitro protein-DNA binding studies, functional transactivation assays using proteins isolated from cord blood or adult peripheral blood T lymphocytes, and transfection experiments in primary T cells.

RESULTS

We showed that transcription from the HIV-1 long terminal repeat is repressed in resting naive T lymphocytes; whereas, mitogenically stimulated CD4+ cells form an activator that derepresses transcription. Negative and positive regulation act through a repressor-activator target sequence (RATS), which shares homology with the interleukin-2 (IL-2) purine-rich response element, through the adjacent binding site of the nuclear factor of activated T cells (NFAT), and weakly, through the KB region.

CONCLUSIONS

This regulation exerted by cellular transcription factors can account for several important features of HIV-1 expression in primary CD4+ cells. Tight repression in resting naive T helper cells may be a main cause of viral latency and transcriptional activation accounts for massive viral production in activated T lymphocytes.

Serial microanalysis of renal transcriptomes

Large-scale gene expression studies can now be routinely performed on macroamounts of cells, but it is unclear to which extent current methods are valuable for analyzing complex tissues. In the present study, we used the method of serial analysis of gene expression (SAGE) for quantitative mRNA profiling in the mouse kidney. We first performed SAGE at the whole-kidney level by sequencing 12,000 mRNA tags. Most abundant tags corresponded to transcripts widely distributed or enriched in the predominant kidney epithelial cells (proximal tubular cells), whereas transcripts specific for minor cell types were barely evidenced. To better explore such cells, we set up a SAGE adaptation for downsized extracts, enabling a 1, 000-fold reduction of the amount of starting material. The potential of this approach was evaluated by studying gene expression in microdissected kidney tubules (50,000 cells). Specific gene expression profiles were obtained, and known markers (e.g., uromodulin in the thick ascending limb of Henle's loop and aquaporin-2 in the collecting duct) were found appropriately enriched. In addition, several enriched tags had no databank match, suggesting that they correspond to unknown or poorly characterized transcripts with specific tissue distribution. It is concluded that SAGE adaptation for downsized extracts makes possible large-scale quantitative gene expression measurements in small biological samples and will help to study the tissue expression and function of genes not evidenced with other high-throughput methods.

[Renal K-ATPases: structure, function and dysfunction]

Na,K-ATPase and H,K-ATPase consist of two transmembrane proteins, the larger of which (catalytic subunit) exchanges extracellular K+ against intracellular Na+ or proton, at the expense of ATP hydrolysis. Cloning of four isoforms of Na,K-ATPase and two isoforms of H,K-ATPase has provided a molecular basis to the functional heterogeneity of these ATPases. Besides its house keeping functions, renal Na,K-ATPase energizes most solute and water transports along the whole nephron. For this purpose, it utilizes about 80% of renal metabolic energy. H,K-ATPase, which is restricted to the renal collecting duct, has a more limited role: it energizes K+ reabsorption during hypokalemia and, along with H-ATPase, participates to acid/base homeostasis. Dysregulation of tubular Na,K-ATPase and H,K-ATPase are involved in physiopathological alterations. For examples, results are presented which show the relationships that exist between a) Na+ retention during experimental nephrotic syndrome and stimulation of collecting duct Na,K-ATPase, and b) kaliuretic effect of loop diuretics and inhibition of collecting duct H,K-ATPase.

Collecting duct adaptation to potassium depletion

Kidneys are the main effectors of the maintenance of potassium balance, under both normal and altered conditions of dietary potassium uptake. The collecting duct system plays a major role in this control of potassium homeostasis because most of the filtered potassium is reabsorbed between the glomerulus and the distal convoluted tubule: under normal physiological conditions or in response to potassium loading, collecting ducts adjust their rate of secretion of potassium into urine so as it matches the dietary daily intake, whereas in response to restriction of potassium ingestion, this secretion process is mostly curtailed, and a reabsorptive mechanism appears. In this short review, we analyzed the cellular and molecular mechanisms underlying transepithelial transport of potassium in the collecting duct and their adaptation in response to potassium depletion. A special emphasis is given on the axial and cellular heterogeneity of the collecting duct with regard to potassium transport and its adaptation. We also discuss the factors controlling duct hypertrophy and hyperplasia during potassium depletion and their possible relationship with the control of potassium conservation.

Effect of cAMP on the activity and the phosphorylation of Na+,K(+)-ATPase in rat thick ascending limb of Henle

BACKGROUND

In rat kidney medullary thick ascending limb of Henle's loop (MTAL), activation of protein kinase A (PKA) was previously reported to inhibit Na+,K(+)-ATPase activity. This is paradoxical with the known stimulatory effect of cAMP on sodium reabsorption. Because this inhibition was mediated by phospholipase A2 (PLA2) activation, a pathway stimulated by hypoxia, we evaluated the influence of oxygen supply on cAMP action on Na+,K(+)-ATPase in MTAL.

METHODS

Ouabain-sensitive 86Rb uptake and Na+,K(+)-ATPase activity were measured in isolated MTALs. Cellular ATP content and the phosphorylation level of Na+,K(+)-ATPase were determined in suspensions of outer medullary tubules. Experiments were carried out under nonoxygenated or oxygenated conditions in the absence or presence of PKA activators.

RESULTS

cAMP analogues or forskolin associated with 3-isobutyl-1-methylxanthine (IBMX) inhibited ouabain-sensitive 86Rb uptake in nonoxygenated MTALs. In contrast, when oxygen supply was increased, cAMP stimulated ouabain-sensitive 86Rb uptake and Na+,K(+)-ATPase activity. Improved oxygen supply was associated with increased intracellular ATP content. The phosphorylation level of the Na+,K(+)-ATPase alpha subunit was increased by cAMP analogues or forskolin associated with IBMX in oxygenated as well as in nonoxygenated tubules. Under nonoxygenated conditions, the inhibition of Na+,K(+)-ATPase was dissociated from its cAMP-dependent phosphorylation, whereas under oxygenated conditions, the stimulatory effect of cAMP analogues on ouabain-sensitive 86Rb uptake was linearly related and cosaturated with the level of phosphorylation of the Na+,K(+)-ATPase alpha subunit.

CONCLUSION

In oxygenated MTALs, PKA-mediated stimulation of Na+,K(+)-ATPase likely participates in the cAMP-dependent stimulation of sodium reabsorption. Under nonoxygenated conditions, this stimulatory pathway is likely overridden by the PLA2-mediated inhibitory pathway, a possible adaptation to protect the cells against hypoxic damage.

Regulation of Na+, K(+)-ATPase in the rat outer medullary collecting duct during potassium depletion

Because in outer medullary collecting ducts (OMCD) of K(+)-depleted rats, K+ secretion is abolished, whereas Na+, K(+)-ATPase, which energizes this secretion, is markedly stimulated, it has been proposed that Na+, K(+)-ATPase was mislocated to the apical cell membrane and energized K+ reabsorption. This hypothesis has been supported by paradoxical effects of ouabain in K(+)-depleted compared with normal rats. However, we have recently shown that ouabain inhibits not only Na+, K(+)-ATPase but also apical H+, K(+)-ATPase in the OMCD of K(+)-depleted rats. Therefore, this study was designed to evaluate whether previous observations were accounted for by Na+, K(+)-ATPase or by ouabain-sensitive H+, K(+)-ATPase. Na+, K(+)-ATPase was distinguished from H+, K(+)-ATPase by its insensitivity to Sch-28080. Results indicate that the hydrolytic and transport activities of Na+, K(+)-ATPase, the number of its functional units, and the expression of mRNA of its alpha 1 and beta 1 subunits were increased threefold or more in the OMCD of rats fed a K(+)-depleted diet for 2 wk. By immunofluorescence, Na+, K(+)-ATPase staining was strongly increased in K(+)-depleted rats but remained localized to the basolateral pole of OMCD principal cells. In conclusion, K+ depletion is associated with marked induction of functional Na+, K+ pumps at the basolateral pole of rat OMCD. Therefore, reduced K+ secretion might result from inhibition of apical K+ conductances and stimulation of basolateral K+ recycling. It is proposed that increased Na+, K(+)-ATPase participates in the increased Na+ reabsorption prevailing in collecting ducts of K(+)-depleted rats.

Cold- and ouabain-resistance of renal Na,K-ATPase in cold-exposed and hibernating jerboas (Jaculus orientalis)

The temperature dependence and the ouabain sensitivity of Na,K-ATPase was examined in the nephron of normal, cold-exposed, and hibernating jerboas. The transport and hydrolytic activity of renal Na,K-ATPase displayed similar temperature dependence in rats and normal jerboas. Cold-resistance of Na,K-ATPase appeared in cold-exposed jerboas and further increased during hibernation. Three subpopulations of Na,K-ATPase displaying very high (Ki approximately 10(-13) M), high (Ki approximately 10(-9) M) and low sensitivity to ouabain (Ki approximately 10(-6) M) were detected in the thick ascending limb and collecting duct of jerboas. In thick ascending limbs, the subpopulation of very high sensitivity to ouabain disappeared in cold-exposed animals, which accounted for the previously reported decrease in Na,K-ATPase activity. In collecting ducts of cold-exposed animals, the subpopulation of very high sensitivity to ouabain also disappeared, but the resulting decrease in activity was overbalanced by the appearance of the subpopulation of high sensitivity.

H+, K(+)-ATPASE in the kidney: localization and function in the nephron

In the past few years, many studies aimed at characterizing the localisation, the molecular nature, and the pharmacological and functional properties of renal H+, K(+)-ATPases. These studies included: (1) molecular cloning (by homology with other known P-type K(+)-ATPases) and expression of H+, K(+)-ATPase alpha-subunits from amphibian and mammalian epithelia; (2) kinetic and pharmacological characterization of K(+)-ATPase activities different from Na+, K(+)-ATPase in single segments of rat nephron, and (3) analysis of K+ and/or H+ transport in isolated collecting ducts from rat or rabbit kidney. The present paper summarizes these data with a special emphasis on those which demonstrate the existence along the mammalian nephron of several forms of H+, K(+)-ATPase displaying distinct biochemical, pharmacological and functional properties.

Re-evaluation of the expression of the gastric H,K-ATPase alpha subunit along the rat nephron

Under normal conditions, the rat collecting duct displays an H, K-ATPase activity with kinetic and pharmacological properties very close to those of the gastric H,K-ATPase. However, whether the collecting duct H,K-ATPase and the gastric enzyme are identical remains controversial. Therefore, we re-evaluated the expression of the mRNAs encoding the gastric H,K-ATPase alpha subunit in the rat nephron. For this purpose, gastric H,K-ATPase mRNAs were quantitated by RT-PCR at the level of microdissected nephron segments using known amounts of gastric H,K-ATPase cRNA as external standards. Results indicate that gastric H,K-ATPase mRNAs are undetectable (<1 copy per cell) in the glomerulus and along the proximal tubule, thick ascending limb of Henle's loop and collecting duct, although a faint expression ( approximately 400 copies per micro;g total RNA) is measurable in whole-kidney preparations. Gastric H,K-ATPase mRNA is also absent along the nephron of K-depleted rats and of rats with chronic metabolic acidosis and alkalosis. Taken with other data from the literature, these results suggest that the collecting duct of normal rats might express an H,K-ATPase similar, but not identical, to the gastric isoform.

Na,K-ATPase: a molecular target for Leptospira interrogans endotoxin

On the basis of our report that a glycolipoprotein fraction (GLP) extracted from Leptospira interrogans contains a potent inhibitor of renal Na,K-ATPase, we proposed that GLP-induced inhibition of Na,K-ATPase might be the primary cellular defect in the physiopathology of leptospirosis. The present study was designed to test this hypothesis by determining whether or not 1). GLP inhibits all the isoforms of Na,K-ATPase which are expressed in the tissues affected by leptospirosis, 2) Na,K-ATPase from leptospirosis-resistant species, such as the rat, is sensitive to GLP, 3) GLP inhibits Na,K-ATPase from intact cells, and 4) GLP inhibits ouabain-sensitive H,K-ATPase. The results indicate that in the rabbit, a leptospirosis-sensitive species, GLP inhibits with similar efficiency (apparent IC50: 120-220 micrograms protein GLP/ml) all isoforms of Na,K-ATPase known to be expressed in target tissues for the disease. Na,K-ATPase from rat kidney displays a sensitivity to GLP similar to that of the rabbit kidney enzyme (apparent IC50: 25-80 and 50-150 micrograms protein GLP/ml for rat and rabbit, respectively), indicating that resistance to the disease does not result from the resistance of Na,K-ATPase to GLP. GLP also reduces ouabain-sensitive rubidium uptake in rat thick ascending limbs (pmol mm-1 min-1 +/- SEM; control: 23.8 +/- 1.8; GLP, 88 micrograms protein/ml: 8.2 +/- 0.9), demonstrating that it is active in intact cells. Finally, GLP had no demonstrable effect on renal H,K-ATPase activity, even on the ouabain-sensitive form, indicating that the active principle of GLP is more specific for Na,K-ATPase than ouabain itself. Although the hypothesis remains to be demonstrated in vivo, the present findings are compatible with the putative role of GLP-induced inhibition of Na,K-ATPase as an initial mechanism in the physiopathology of leptospirosis.

K depletion modifies the properties of Sch-28080-sensitive K-ATPase in rat collecting duct

Two distinct Sch-28080-sensitive K-adenosine triphosphatases (K-ATPases) were previously described in the rat nephron: a ouabain-resistant K-ATPase (type I) present in collecting ducts (CD) and a ouabain-sensitive from (type II) located in proximal tubules (PT) and thick ascending limbs (TAL). In K-depleted rats, K-ATPase activity is increased in CD, whereas it is reduced in PT and TAL. Because expression of colonic H-K-ATPase is restricted to the CD of K-depleted rats, we hypothesized that K-ATPase from the CD of K-depleted rats might be different from types I and II. Indeed, type III K-ATPase displays higher sensitivities to ouabain and to Sch-28080 than type II, a lower sensitivity to Sch-28080 than type I, and, conversely to types I and II, it can be stimulated by Na+. Pharmacological differences between types II and III K-ATPases were confirmed by [3H]ouabain binding experiments. Thus the rat kidney expresses three K-ATPases that differ by their pharmacological and kinetic properties, their distribution profile along the nephron and their behavior during K depletion.

Quantitative RT-PCR analysis of mRNAs encoding a colonic putative H, K-ATPase alpha subunit along the rat nephron: effect of K+ depletion

The rat nephron displays two ouabain-sensitive K-ATPases: one, which is present in proximal tubules and thick ascending limbs of normal rats, is specifically activated by K+ and is down-regulated by K+ depletion, whereas the other one appears in collecting ducts of K+-depleted rats and is activated by either Na+ or K+. To determine which of these two ATPases is similar to colonic-type H,K-ATPase, we quantitated by reverse transcriptase-polymerase chain reaction (RT-PCR) the mRNAs encoding the colonic H,K-ATPase alpha subunit in microdissected nephron segments. In normal rats, statistically significant amounts of colonic H,K-ATPase mRNAs were detected exclusively in cortical thick ascending limbs and cortical collecting ducts (200-500 copies/mm). Because these levels of expression were low (1-1.2 copies/target cell), they probably have no physiological relevance. In rats fed a K+-depleted diet for 2 weeks, expression of colonic H,K-ATPase was markedly enhanced in cortical and medullary collecting ducts (5000-12,000 copies/mm or 30-40 copies per cell), whereas it remained low in all other nephron segments. Thus, colonic H,K-ATPase alpha subunit is specifically expressed in cortical and outer medullary collecting ducts of K+-depleted rats where it likely accounts for the ouabain-sensitive K-ATPase activity.

Na-K-ATPase along rat nephron after subtotal nephrectomy: effect of enalapril

Tubular overwork is thought to be a promoter of the tubular hypertrophy and renal failure that occur in response to renal mass reduction. Because Na-K-adenosinetriphosphatase (Na-K-ATPase) is an index of tubular work, we evaluated the effects of subtotal nephrectomy and of enalapril therapy, which delays the evolution of renal lesions, on tubular hypertrophy and Na-K-ATPase activity along the rat nephron. Within 6 wk, 70% reduction of renal mass engendered hypertrophy of the proximal convoluted tubule (PCT), thick ascending limb (TAL), and collecting duct (CD), as well as parallel increments in Na-K-ATPase activity per millimeter tubule length (Na-K-ATPase activity per unit surface area was not modified by subtotal nephrectomy). Chronic enalapril therapy prevented part of the hypertrophy (but not Na-K-ATPase stimulation) of the PCT and the whole stimulation of Na-K-ATPase (but not hypertrophy) in the CD, whereas it had no effect on the TAL. Enalapril effect on Na-K-ATPase in CD might result from reduced bradykinin metabolism, as the reduction in urinary excretion of bradykinin observed in subtotally nephrectomized rats was prevented by enalapril therapy.

Regulation of renal Na+,K(+)-ATPase in rat thick ascending limb during K+ depletion: evidence for modulation of Na+ affinity

1. NaCl reabsorption along the loop of Henle is reduced in K(+)-depleted rats. Because Na+,K(+)-ATPase energizes this transport and because K+ depletion is known to induce an upregulation of Na+,K(+)-ATPase in most tissues, the regulation of this enzyme was investigated at the level of single thick ascending limbs of the loop of Henle freshly microdissected from rats fed either a normal (control rats) or a low-K+ diet (LK rats). 2. Within 2 weeks of K+ depletion, Na+,K(+)-ATPase activity and [3H]ouabain binding were increased by 30-50% in the medullary portion of the thick ascending limb (MTAL). 3. Despite this increase in the number of Na+,K(+)-ATPase units, the transport capacity of the Na+,K+ pump, determined by ouabain-sensitive Rb+ uptake in the presence of an extracellular concentration of Rb+ mimicking the kalaemia determined in control (4.0 mM Rb+) and LK rats (2.3 mM Rb+), was reduced in MTAL from LK rats. 4. Inhibition of the Na+,K+ pump was not accounted for by changes in either extracellular K+ or intracellular Na+ concentrations, but by a decrease in the pump affinity for Na+. 5. Because this change in the apparent affinity of the Na+,K+ pump for Na+ was detectable in intact but not in permeabilized MTAL cells, it is probably induced by a rapidly reversible cytosolic factor.

Effects of cold exposure and hibernation on renal Na,K-ATPase of the jerboa Jaculus orientalis

Changes in activity and abundance of renal Na,K-ATPase were evaluated during cold exposure and hibernation of the jerboa Jaculus orientalis by measuring the hydrolytic activity, the number of units and the transport activity of Na,K-ATPase in isolated nephron segments. As compared to controls, jerboas exposed to cold (6 degrees C) for 4-5 weeks displayed mild diuresis, decreased urinary osmolality and increased kaliuresis. In cold-exposed jerboas, Na,K-ATPase hydrolytic activity was reduced in the medullary thick ascending limb and enhanced in the cortical and outer medullary collecting duct, whereas it was not altered in other nephron segments. The number of Na,K-ATPase units and the activity of Na,K-pump, determined by [3H]-ouabain binding and by ouabain-sensitive rubidium uptake respectively, changed in parallel with the hydrolytic activity in the medullary thick ascending limb and cortical collecting duct. The maximal rate of activity (Vmax) of Na,K-ATPase was not modified further during hibernation. Thus, cold exposure, but not the onset of hibernation, induces segment-specific changes in the abundance and activity of Na,K-ATPase units which are likely to be related to the entry into hibernation, but not to the maintenance of some renal functions during deep hibernation.

Presence of two isoforms of Na, K-ATPase with different pharmacological and immunological properties in the rat kidney

Previous studies have demonstrated the presence of two populations of Na,K-ATPase with distinct kinetic, pharmacological and immunological characteristics along the rabbit nephron, indicating that the proximal segments of the nephron express exclusively the alpha 1 isoform of the catalytic subunit, whereas the collecting duct expresses an alpha 3-like isoform. Because pharmacological studies have shown the existence of two populations of Na,K-ATPase with different sensitivities to ouabain in the rat cortical collecting duct, which may result from the presence in the same nephron segment of the two isoforms demonstrated in the different segments of the rabbit nephron, the present study was undertaken to characterize the properties of Na,K-ATPase along the rat nephron. Results indicate that each segment of the rat nephron contains two subpopulations of Na,K-ATPase: a component highly sensitive to ouabain (IC50 approximately 5.10(-6) M) which is recognized by an anti-alpha 3 antibody and another moiety of lower affinity for ouabain (IC50 approximately 5.10(-4) M) which is recognized by an anti-alpha 1 antibody. Whether these two subpopulations correspond to different isoforms of the alpha subunit of Na,K-ATPase (alpha 1 and alpha 3-like) remains to be determined.

Ouabain-sensitive and -insensitive K-ATPases in rat nephron: effect of K depletion

Because a ouabain-sensitive H-K-adenosinetriphosphatase (H-K-ATPase) has been identified recently in the amphibian bladder, we evaluated whether such an ATPase might exist also in the mammalian kidney, along with the ouabain-insensitive H-K-ATPase previously described in the collecting duct. For this purpose, we searched for an Na-independent, K-stimulated, ouabain- and Sch-28080-inhibitable ATPase activity in single segments of rat nephron. Ouabain-sensitive K-stimulated ATPase activity was detected in the absence of Na+ in rat proximal convoluted and straight tubules and in medullary and cortical thick ascending limbs of Henle's loop but not in collecting ducts. This K-ATPase differs from Na-K-ATPase by 1) its absence of requirement for Na, 2) its sensitivity to Sch-28080, 3) its higher sensitivity to ouabain, and 4) its absence in the collecting duct. It differs from the collecting duct H-K-ATPase by 1) its distribution along the nephron, 2) its sensitivity to ouabain, and 3) its lower sensitivity to Sch-28080. Furthermore, in rats fed a K-depleted diet for 2 wk, ouabain-sensitive K-ATPase activity was markedly reduced in both proximal tubules and thick ascending limbs, whereas collecting duct H-K-ATPase was upregulated.

Protein kinase C-dependent stimulation of Na(+)-K(+)-ATP epsilon in rat proximal convoluted tubules

In rat proximal convoluted tubule (PCT), activation of protein kinase C (PKC) by phorbol 12,13-dibutyrate (PDBu) was previously reported to inhibit Na(+)-K(+)-ATPase, a paradoxical finding in view of the known stimulatory effect of PKC on Na+ reabsorption. Because this inhibition occurs via phospholipase A2 activation, a pathway stimulated by hypoxia, we evaluated the influence of oxygen supply on PKC action on Na(+)-K(+)-ATPase. Results confirmed that PDBu inhibited PCT Na(+)-K(+)-ATPase activity under usual conditions. In contrast, when oxygen supply was increased, PDBu had no effect on Na(+)-K(+)-ATPase hydrolytic activity, but it dose-dependently stimulated ouabain-sensitive 86Rb+ uptake. This latter effect, which was abolished by PKC inhibitors, resulted from an increment of the Na+ sensitivity of Na(+)-K(+)-ATPase. Thus, in oxygenated rat PCTs, activation of PKC primarily stimulated Na(+)-K(+)-ATPase. This likely contributes to increase solute reabsorption. Inhibition of Na(+)-K(+)-ATPase was observed only under hypoxic conditions. It may represent an adaptation to protect PCTs against deleterious effects of hypoxia.

Inhibition of Na,K-ATPase by an endotoxin extracted from Leptospira interrogans: a possible mechanism for the physiopathology of leptospirosis

Clinical manifestations of leptospirosis include disorders of the electrolytical balance which might be related to inhibition of Na,K-ATPase. Although the physiopathological cellular mechanism of leptospirosis remains unknown, a bacterial endotoxin has been incriminated. Therefore, we evaluated whether a glycolipoprotein fraction extracted from Leptospira interrogans and known to be cytotoxic might inhibit Na,K-ATPase. This glycolipoprotein fraction (GLP) inhibited Na,K-ATPase activity in rabbit kidney epithelial cells as well as Na,K-ATPase purified from rabbit kidney medulla. Inhibition was dose-dependent, and at maximum it almost abolished Na,K-ATPase activity whereas it had no effect on other enzymes. The GLP did not change the apparent affinity of Na,K-ATPase for potassium whereas it increased that for sodium, revealing a mechanism of inhibition different from that of ouabain. Finally, the inhibitory principle present in the GLP preparation was thermostable and was curtailed by the presence of albumin. In conclusion, a glycolipoproteic fraction extracted from Leptospira interrogans contains a specific inhibitor of Na,K-ATPase. This glycolipoproteic fraction which is present in diseased tissues might induce, through this inhibitor, cellular dysfunctions responsible for the symptoms, in particular those associated with electrolytical disorders such as disturbances of renal electrolyte handling, cardiac arrhythmia or diarrhoea.