Ceramide-activated protein kinases A and C zeta inhibit kidney proximal tubule cell Na-ATPase

The basolateral membranes of kidney proximal tubule cells have (Na⁺+K⁺)-ATPase and Na⁺-ATPase activities, involved in Na⁺ reabsorption. We showed that ceramide (Cer) modulates protein kinase A (PKA) and protein kinase C (PKC), which are involved in regulating ion transporters. Here we show that ceramide, promotes 60% inhibition of Na⁺-ATPase activity (I₅₀ ≈ 100 nM). This effect was completely reversed by inhibiting PKA but did not involve the classic PKC signaling pathway. In these membranes we found the Cer-activated atypical PKC zeta (PKCζ) isoform. When PKCζ is inhibited, Cer ceases to inhibit the Na⁺-ATPase, allowing the cAMP/PKA signaling pathway to recover its stimulatory effect on the pump. There were no effects on the (Na⁺+K⁺)-ATPase. These results reveal Cer as a potent physiological modulator of the Na⁺-ATPase, participating in a regulatory network in kidney cells and counteracting the stimulatory effect of PKA via PKCζ.     

Short-Term Regulation of the Proximal Tubule Na+,K+-ATPase: Increased/Decreased Na+,K+-ATPase Activity Mediated by Protein Kinase C Isoforms

In different species and tissues, a great variety of hormones modulate Na⁺,K⁺-ATPase activity in a short-term fashion. Such regulation involves the activation of distinct intracellular signaling networks that are often hormone- and tissue-specific. This minireview focuses on our own experimental observations obtained by studying the regulation of the rodent proximal tubule Na⁺,K⁺-ATPase. We discuss evidence that hormones responsible for regulating kidney proximal tubule sodium reabsorption may not affect the intrinsic catalytic activity of the Na⁺,K⁺-ATPase, but rather the number of active units within the plasma membrane due to shuttling Na⁺,K⁺-ATPase molecules between intracellular compartments and the plasma membrane. These processes are mediated by different isoforms of protein kinase C and depend largely on variations in intracellular sodium concentrations.     

Histological and functional renal alterations caused by Bothrops alternatus snake venom: Expression and activity of Na/K-ATPase

Background

Acute renal failure is a serious complication of human envenoming by Bothrops snakes. The ion pump Na⁺/K⁺-ATPase has an important role in renal tubule function, where it modulates sodium reabsorption and homeostasis of the extracellular compartment. Here, we investigated the morphological and functional renal alterations and changes in Na⁺/K⁺-ATPase expression and activity in rats injected with Bothrops alternatus snake venom.

Methods

Male Wistar rats were injected with venom (0.8 mg/kg, i.v.) and renal function was assessed 6, 24, 48 and 72 h and 7 days post-venom. The rats were then killed and renal Na⁺/K⁺-ATPase activity was assayed based on phosphate release from ATP; gene and protein expressions were assessed by real time PCR and immunofluorescence microscopy, respectively.

Results

Venom caused lobulation of the capillary tufts, dilation of Bowman's capsular space, F-actin disruption in Bowman's capsule and renal tubule brush border, and deposition of collagen around glomeruli and proximal tubules that persisted seven days after envenoming. Enhanced sodium and potassium excretion, reduced proximal sodium reabsorption, and proteinuria were observed 6 h post-venom, followed by a transient decrease in the glomerular filtration rate. Gene and protein expressions of the Na⁺/K⁺-ATPase α₁ subunit were increased 6 h post-venom, whereas Na⁺/K⁺-ATPase activity increased 6 h and 24 h post-venom.

Conclusions

Bothrops alternatus venom caused marked morphological and functional renal alterations with enhanced Na⁺/K⁺-ATPase expression and activity in the early phase of renal damage.

General significance

Enhanced Na⁺/K⁺-ATPase activity in the early hours after envenoming may attenuate the renal dysfunction associated with venom-induced damage.     

Cross-reactivity of an antiserum to the α-subunit of the Na+, K+-ATPase of toad (Bufo marinus) kidney with basal and apical membranes of transporting epithelia of the rat

Summary An antibody to the 96 kD -subunit of the Na⁺, K⁺ -ATPase from Bufo marinus has been used in immunostaining rat kidney and salivary glands. Intense staining was observed on basolateral membranes of distal tubules of the kidney and striated ducts of the three major salivary glands. Less intense staining was seen on the basolateral membranes of parotid acinar cells, but no staining was seen on the acinar cells of submandibular or sublingual glands. These sites of staining have been shown, by other methods, to posses substantial Na⁺, K⁺ -ATPase, indicating that the antibody recognizes antigenic determinants of the sodium pump highly conserved in the course of evolution. In addition, staining with this antibody was observed at the apical region of cells of the proximal straight tubule and of the papillary collecting duct in the kidney. Absorption studies suggest that the apical antigenic determinants are the same or closely related to each other but are distinct from basolateral antigenic determinants.     

Localization of Na+, K+-ATPase to the inside of the basolateral cell membranes of epithelial cells of proximal and distal tubules in rabbit kidney

Summary Cysteine-sensitive alkaline phosphatase and/or ouabain-sensitive Na⁺, K⁺-ATPase were studied by ultrastructure cytochemistry in epithelial cells of proximal and distal kidney tubules. Alkaline phosphatase reactivity was confined to the surface of the microvillous luminal cell membrane of proximal tubule cells, whereas distal tubules and collecting ducts were unreactive. The Na⁺, K⁺-ATPase reactivity was localized evenly along the cytoplasmic side of the basolateral cell membrane of cells of proximal and distal tubules and in collecting ducts. In the proximal tubules, where the activity was strongest, the Na⁺, K⁺-ATPase deposits were also found in the 10–50 nm gap between the cell membrane and the cisternae of tubulo-cisternal endoplasmic reticulum (TER) underlying a major part of the basolateral cell membrane. The restriction of Na⁺, K⁺-ATPase sites, which are involved in extrusion of Na⁺ from the cell, to a narrow cytoplasmic compartment located between the cell membrane and the cisternae of TER, is consistent with a transport role for the TER.     

The Detailed Localization Pattern of Na+/K+/2Cl? Cotransporter Type 2 and Its Related Ion Transport System in the Rat Endolymphatic Sac

The endolymphatic sac (ES) is a part of the membranous labyrinth. ES is believed to perform endolymph absorption, which is dependent on several ion transporters, including Na⁺/K⁺/2Cl⁻ cotransporter type 2 (NKCC-2) and Na⁺/K⁺-ATPase. NKCC-2 is typically recognized as a kidney-specific ion transporter expressed in the apical membrane of the absorptive epithelium. NKCC-2 expression has been confirmed only in the rat and human ES other than the kidney, but the detailed localization features of NKCC-2 have not been investigated in the ES. Thus, we evaluated the specific site expressing NKCC-2 by immunohistochemical assessment. NKCC-2 expression was most frequently seen in the intermediate portion of the ES, where NKCC-2 is believed to play an important role in endolymph absorption. In addition, NKCC-2 expression was also observed on the apical membranes of ES epithelial cells, and Na⁺/K⁺-ATPase coexpression was observed on the basolateral membranes of ES epithelial cells. These results suggest that NKCC-2 performs an important role in endolymph absorption and that NKCC-2 in apical membranes and Na⁺/K⁺-ATPase in basolateral membranes work coordinately in the ES in a manner similar to that in renal tubules. (J Histochem Cytochem 58:759–763, 2010)     

Spatial,Cellular, and Intracellular Localization of Na+/K+-ATPase in the Sterically Disposed Renal Tubules of Japanese Eel

The kidney plays a crucial role in the regulation of water and ion balances in both freshwater and seawater fishes. However, the complicated structures of the kidney hamper comprehensive understanding of renal functions. In this study, to investigate the structure of sterically disposed renal tubules, we examined spatial, cellular, and intracellular localization of Na⁺/K⁺-ATPase in the kidney of the Japanese eel. The renal tubule was composed of the first (PT-I) and second (PT-II) segments of the proximal tubule and the distal tubule (DT), followed by the collecting ducts (CDs). Light microscopic immunocytochemistry detected Na⁺/K⁺-ATPase along the renal tubules and CD; however, the subcellular distribution of the Na⁺/K⁺-ATPase immunoreaction varied among different segments. Electron microscopic immunocytochemistry further revealed that Na⁺/K⁺-ATPase was distributed on the basal infoldings of PT-I, PT-II, and DT cells. Three-dimensional analyses showed that the renal tubules meandered in a random pattern through lymphoid tissues, and then merged into the CD, which was aligned linearly. Among the different segments, the DT and CD cells showed more-intense Na⁺/K⁺-ATPase immunoreaction in freshwater eel than in seawater-acclimated eel, confirming that the DT and CD segments are important in freshwater adaptation, or hyperosmoregulation. (J Histochem Cytochem 58:707–719, 2010)     

Immunocytochemical localization of Na+/K+-ATPase and V-H+-ATPase in the salivary glands of the cockroach,Periplaneta americana

The acinous salivary glands of the cockroach (Periplaneta americana) consist of four morphologically different cell types with different functions: the peripheral cells are thought to produce the fluid component of the primary saliva, the central cells secrete the proteinaceous components, the inner acinar duct cells stabilize the acini and secrete a cuticular, intima, whereas the distal duct cells modify the primary saliva via the transport of water and electrolytes. Because there is no direct information available on the distribution of ion transporting enzymes in the salivary glands, we have mapped the distribution of two key transport enzymes, the Na⁺/K⁺-ATPase (sodium pump) and a vacuolar-type H⁺-ATPase, by immunocytochemical techniques. In the peripheral cells, the Na⁺/K⁺-ATPase is localized to the highly infolded apical membrane surface. The distal duct cells show large numbers of sodium pumps localized to the basolateral part of their plasma membrane, whereas their highly folded apical membranes have a vacuolar-type H⁺-ATPase. Our immunocytochemical data are supported by conventional electron microscopy, which shows electrondense 10-nm particles (portasomes) on the cytoplasmic surface of the infoldings of the apical membranes of the distal duct cells. The apically localized Na⁺/K⁺-ATPase in the peripheral cells is probably directly involved in the formation of the Na⁺-rich primary saliva. The latter is modified by the distal duct cells by transport mechanisms energized by the proton motive force of the apically localized V-H⁺-ATPase.     

Nitration as a Mechanism of Na+, K+-ATPase Modification During Hypoxia in the Cerebral Cortex of the Guinea Pig Fetus

Previous studies have shown that hypoxia induces nitric oxide synthase-mediated generation of nitric oxide free radicals leading to peroxynitrite production. The present study tests the hypothesis that hypoxia results in NO-mediated modification of Na⁺, K⁺-ATPase in the fetal brain. Studies were conducted in guinea pig fetuses of 58-days gestation. The mothers were exposed to FiO₂ of 0.07% for 1 hour. Brain tissue hypoxia in the fetus was confirmed biochemically by decreased ATP and phosphocreatine levels. P₂ membrane fractions were prepared from normoxic and hypoxic fetuses and divided into untreated and treated groups. The membranes were treated with 0.5 mM peroxynitrite at pH 7.6. The Na⁺, K⁺-ATPase activity was determined at 37°C for five minutes in a medium containing 100 mM NaCl, 20 mM KCl, 6.0 mM MgCl₂, 50 mM Tris HCl buffer pH 7.4, 3.0 mM ATP with or without 10 mM ouabain. Ouabain sensitive activity was referred to as Na⁺, K⁺-ATPase activity. Following peroxynitrite exposure, the activity of Na⁺, K⁺-ATPase in guinea pig brain was reduced by 36% in normoxic membranes and further 29% in hypoxic membranes. Enzyme kinetics was determined at varying concentrations of ATP (0.5 mM-2.0 mM). The results indicate that peroxynitrite treatment alters the affinity of the active site of Na⁺, K⁺-ATPase for ATP and decreases the Vmax by 35% in hypoxic membranes. When compared to untreated normoxic membranes Vmax decreases by 35.6% in treated normoxic membranes and further to 52% in treated hypoxic membranes. The data show that peroxynitrite treatment induces modification of Na⁺, K⁺-ATPase. The results demonstrate that peroxynitrite decreased activity of Na⁺, K⁺-ATPase enzyme by altering the active sites as well as the microenvironment of the enzyme. We propose that nitric oxide synthase-mediated formation of peroxynitrite during hypoxia is a potential mechanism of hypoxia-induced decrease in Na⁺, K⁺-ATPase activity.     

Assessment of a strontium capture technique for cytochemical localization of potassium-dependent phosphatase in Malpighian tubules of Locusta

Abstract A strontium capture method, using p-nitrophenyl phosphate as substrate, was used to determine the subcellular localization of (Na⁺ + K⁺)-ATPase activity in Malpighian tubules of Locusta migratoria L. Ultrastructural studies revealed that (Na⁺ + K⁺)-ATPase activity was restricted to the basolateral plasma membranes with little evidence of activity associated with the apical microvilli. In contrast, alkaline phosphatase activity was specifically associated with the apical cell membrane. Biochemical assays of fixed and non-fixed tubule homogenates were used to evaluate the p-nitrophenyl phosphate-strontium procedure for localization of the phosphatase component of (Na⁺ + K⁺)-ATPase. No significant potassium-dependent, ouabain-sensitive p-nitrophenyl phosphatase activity was demonstrated in homogenates under conditions necessary for the cytochemical procedure, viz fixation, pH 9.0 and the presence of strontium. The significance of the biochemical results are discussed in relation to the validity of such cytochemical techniques for (Na⁺ + K⁺)-ATPase localization.     

Distinct Na+/K+/2Cl- cotransporter localization in kidneys and gills of two euryhaline species, rainbow trout and killifish

The kidney is an organ playing an important role in ion regulation in both freshwater (FW) and seawater (SW) fish. The mechanisms of ion regulation in the fish kidney are less well studied than that of their gills, especially at the level of transporter proteins. We have found striking differences in the pattern of Na⁺/K⁺/2Cl^- cotransporter (NKCC) expression between species. In the killifish kidney, NKCC is apically localized in the distal and collecting tubules and basolaterally localized in the proximal tubules. However, in the SW killifish gill, NKCC is basolaterally co-localized with Na⁺/K⁺-ATPase, whereas in FW, NKCC immunoreactivity is primarily apical, although still colocalized within the same mitochondria-rich cell with basolateral Na⁺/K⁺-ATPase. Rainbow trout kidney has NKCC only in the apical membrane of the distal and collecting tubules in both environments, with no signal being detected in the proximal tubule. On the other hand, in the trout gill, NKCC is found basolaterally in both FW and SW environments. An important observation is that, in the gills of rainbow trout, the trailing edge of the filament possesses mostly Na⁺/K⁺-ATPase-positive but NKCC-negative mitochondria-rich cells, whereas in the region between and at the roots of the gill lamellae, most mitochondria-rich cells exhibit both Na⁺/K⁺-ATPase- and NKCC-positive immunoreactivity. These results suggest that the differential localization of transporters between the two species represents differences in function between these two euryhaline fishes with different life histories and strategies. Funding for this research was provided by NSERC Discovery Grants to G.G.G. and W.S.M., an Alberta Ingenuity Fund PDF, and a fellowship from the NSERC Research Capacity Development Grant to F.K.     

Studies on the subcellular distribution of (Na+ + K+)-ATPase,K+-stimulated ATPase and HCO3−-stimulated ATPase activities in malpighian tubules of Locusta migratoria L.

The present study confirms previous reports of the presence of (Na⁺ + K⁺)-ATPase and anion-stimulated ATPase activity in Malpighian tubules of Locusta. In addition, the presence of a K⁺-stimulated, ouabain-insensitive ATPase activity has been identified in microsomal fractions. Differential and sucrose density-gradient centrifugation of homogenates has been used to separate membrane fractions which are rich in mitochondria, apical membranes and basolateral membranes; as indicated by the presence of succinate dehydrogenase and the presence or absence of non-specific alkaline phosphatase activity, respectively. Relatively high specific (Na⁺ + K⁺)-ATPase activity was associated with the basolateral membrane-rich fractions with only low levels of this activity being associated with the apical membrane-rich preparation. K⁺-stimulated ATPase activity was also associated, predominantly, with the basolateral membrane-rich fractions. However, comparison of the distribution of this activity with that of the (Na⁺ + K⁺)-ATPase suggests that the two enzymes did not co-separate. The possibility that the K⁺-stimulated ATPase was not associated with the basolateral plasma membrane is discussed.Anion-stimulated ATPase activity was found in the apical and basolateral membrane-rich fractions and in the fraction contaning mainly mitochondria. Nevertheless, the fact that this bicarbonate-stimulated activity did not co-separate with succinate dehydrogenase activity suggests that it was not exclusively mitochondrial in origin. These results are consistent with physiological studies indicating a basolateral (Na⁺ + K⁺)-ATPase but do not support the K⁺-stimulated ATPase as a candidate for the apical electrogenic pump. The possible role of the bicarbonate-stimulated ATPase activity in ion transport across both the basolateral and apical cell membranes is discussed.     

Angiotensin II stimulates renal proximal tubule Na-ATPase activity through the activation of protein kinase C

Recently, our group described an AT₁-mediated direct stimulatory effect of angiotensin II (Ang II) on the Na⁺-ATPase activity of proximal tubules basolateral membranes (BLM) [Am. J. Physiol. 248 (1985) F621]. Data in the present report suggest the participation of a protein kinase C (PKC) in the molecular mechanism of Ang II-mediated stimulation of the Na⁺-ATPase activity due to the following observations: (i) the stimulation of protein phosphorylation in BLM, induced by Ang II, is mimicked by the PKC activator TPA, and is completely reversed by the specific PKC inhibitor, calphostin C; (ii) the Na⁺-ATPase activity is stimulated by Ang II and TPA in the same magnitude, being these effects abolished by the use of the PKC inhibitors, calphostin C and sphingosine; (iii) the Na⁺-ATPase activity is activated by catalytic subunit of PKC (PKC-M), in a similar and nonadditive manner to Ang II; and (iv) Ang II stimulates the phosphorylation of MARCKS, a specific substrate for PKC.     

Evidence against parallel operation of sodium/calcium antiport and ATP-driven calcium transport in plasma membrane vesicles from kidney tubule cells

The present study aimed to clarify the existence of a Na⁺/Ca²⁺ antiport device in kidney tubular epithelial cells discussed in the literature to represent the predominant mechanistic device for Ca²⁺ reabsorption in the kidney. (1) Inside-out oriented plasma membrane vesicles from tubular epithelial cells of guinea-pig kidney showed an ATP-driven Ca²⁺ transport machinery similar to that known to reside in the plasma membrane of numerous cell types. It was not affected by digitalis compounds which otherwise are well-documented inhibitors of Ca²⁺ reabsorption. (2) The vesicle preparation contained high, digitalis-sensitive (Na⁺+K⁺-ATPase activities indicating its origin from the basolateral portion of plasma membrane. (3) The operation of Na⁺/Ca²⁺ antiport device was excluded by the findings that steep Ca²⁺ gradients formed by ATP-dependent Ca²⁺ accumulation in the vesicles were not discharged by extravesicular Na⁺, and did not drive ⁴⁵Ca²⁺ uptake into the vesicles via a Ca²⁺-⁴⁵Ca²⁺ exchange. (4) The ATP-dependent Ca²⁺ uptake into the vesicles became increasingly depressed with time by extravesicular Na⁺. This was not due to an impairment of the Ca²⁺ pump itself, but caused by Na⁺/Ca²⁺ competition for binding sites on the intravesicular membrane surface shown to be important for high Ca²⁺ accumulation in the vesicles. (5) Earlier observations on Na⁺-induced release of Ca²⁺ from vesicles pre-equilibrated with Ca²⁺, seemingly favoring the existence of a Na⁺/Ca²⁺ antiporter in the basolateral plasma membrane, were likewise explained by the occurrence of Na⁺/Ca²⁺ competition for binding sites. The weight of our findings disfavors the transcellular pathway of Ca²⁺ reabsorption through tubule epithelium essentially depending on the operation of a Na⁺/Ca²⁺ antiport device.     

Isolation of basolateral and brush-border membranes from the rabbit kidney cortex: Vesicle integrity and membrane sidedness of the basolateral fraction

A rapid and reproducible method has been developed for the simultaneous isolation of basolateral and brush-border membranes from the rabbit renal cortex. The basolateral membrane preparation was enriched 25-fold in (Na⁺ + K⁺)-ATPase and the brush-border membrane fraction was enriched 12-fold in alkaline phosphatase, whereas the amount of cross-contamination was low. Contamination of these preparations by mitochondria and lysosomes was minimal as indicated by the low specific activities of enzyme markers, i.e., succinate dehydrogenase and acid phosphatase. The basolateral fraction consisted of 35–50% sealed vesicles, as demonstrated by detergent (sodium dodecyl sulfate) activation of (Na⁺ + K⁺)-ATPase activity and [³H]ouabain binding. The sidedness of the basolateral membranes was estimated from the latency of ouabain-sensitive (Na⁺ + K⁺)-ATPase activity assayed in the presence of gramicidin, which renders the vesicles permeable to Na⁺ and K⁺. These studies suggest that nearly 90% of the vesicles are in a right-side-out orientation.     

Identification and reconstitution of a Na/K/Cl cotransporter and K channel from luminal membranes of renal red outer medulla

Electrophysiological studies on renal thick ascending limb segments indicate the involvement of a luminal Na⁺/K⁺/Cl⁻ cotransport system and a K⁺ channel in transepithelial salt transport. Sodium reabsorption across this segment is blocked by the diuretics furosemide and bumetanide. The object of our study has been to identify in intact membranes and reconstitute into phospholipid vesicles the Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channel, as an essential first step towards purification of the proteins involved and characterization of their roles in the regulation of transepithelial salt transport. Measurements of ⁸⁶Rb⁺ uptake into membrane vesicles against large opposing KCl gradients greatly magnify the ratio of specific compared to non-specific isotope flux pathways. Using this sensitive procedure, it has proved possible to demonstrate in crude microsomal vesicle preparations from rabbit renal outer medulla two ⁸⁶Rb⁺ fluxes. (A) A furosemide-inhibited ⁸⁶Rb⁺ flux in the absence of Na⁺ (K⁺-K⁺ exchange). This flux is stimulated by an inward Na⁺ gradient (Na⁺/K⁺ cotransport) and is inhibited also by bumetanide. (B) A Ba²⁺-inhibited ⁸⁶Rb⁺ flux, through the K⁺ channel. Luminal membranes containing the Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channels, and basolateral membranes containing the Na⁺/K⁺ pumps were separated from the bulk of contaminant protein by metrizamide density gradient centrifugation. The Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channel were reconstituted in a functional state by solubilizing both luminal membranes and soybean phospholipid with octyl glucoside, and then removing detergent on a Sephadex column.     

Neurotensin effect on Na+, K+-ATPase is CNS area- and membrane-dependent and involves high affinity NT1 receptor

We have previously shown that peptide neurotensin inhibits cerebral cortex synaptosomal membrane Na⁺, K⁺-ATPase, an effect fully prevented by blockade of neurotensin NT₁ receptor by antagonist SR 48692. The work was extended to analyze neurotensin effect on Na⁺, K⁺-ATPase activity present in other synaptosomal membranes and in CNS myelin and mitochondrial fractions. Results indicated that, besides inhibiting cerebral cortex synaptosomal membrane Na⁺, K⁺-ATPase, neurotensin likewise decreased enzyme activity in homologous striatal membranes as well as in a commercial preparation obtained from porcine cerebral cortex. However, the peptide failed to alter either Na⁺, K⁺-ATPase activity in cerebellar synaptosomal and myelin membranes or ATPase activity in mitochondrial preparations. Whenever an effect was recorded with the peptide, it was blocked by antagonist SR 48692, indicating the involvement of the high affinity neurotensin receptor (NT₁), as well as supporting the contention that, through inhibition of ion transport at synaptic membrane level, neurotensin plays a regulatory role in neurotransmission.     

Inhibition of Rat Brain Na+, K+-ATPase Activity Induced by Homocysteine Is Probably Mediated by Oxidative Stress

The objective of the present study was to investigate the effects of preincubation of hippocampus homogenates in the presence of homocysteine or methionine on Na⁺, K⁺-ATPase and Mg²⁺-ATPase activities in synaptic membranes of rats. Homocysteine significantly inhibited Na⁺, K⁺-ATPase activity, whereas methionine had no effect. Mg²⁺-ATPase activity was not altered by the metabolites. We also evaluated the effect of incubating glutathione, cysteine, dithiothreitol, trolox, superoxide dismutase and GM1 ganglioside alone or incubation with homocysteine on Na⁺, K⁺-ATPase activity. Tested compounds did not alter Na⁺, K⁺-ATPase and Mg²⁺-ATPase activities, but except for trolox, prevented the inhibitory effect of homocysteine on Na⁺, K⁺-ATPase activity. These results suggest that inhibition of this enzyme activity by homocysteine is possibly mediated by free radicals and may contribute to the neurological dysfunction found in homocystinuric patients.     

The interaction between calcium and the activation of Na+, K+-ATPase by noradrenaline

Summary The interaction of noradrenaline, various cation chelators and calcium on Na⁺, K⁺-ATPase from rat cerebral cortex plasma membranes was studied. It was shown that chelation of inhibitory cations by EGTA, EDTA and dipyridyl activated Na⁺, K⁺-ATPase to the same extent as noradrenaline but at higher concentrations; increasing concentrations of EGTA depressed the activation by noradrenaline; calcium in the form of a calcium-EGTA buffer depressed Na⁺, K⁺-ATPase at physiological concentrations; the inhibition of Na⁺, K⁺-ATPase by calcium is dependent on the magnesium concentration in the assay and the inhibition by calcium was partially reversed by noradrenaline.     

Cholesterol affects physical properties and (Na+,K+)-ATPase in basolateral membranes of renal and intestinal epithelia from thermally acclimated rainbow trout

Previous work has shown that cholesterol levels are modulated in plasma membranes from some but not all tissues of poikilotherms over the course of temperature change. To gain a better understanding of tissue and membrane domain-specific cholesterol function during thermal adaptation we examined effects of cholesterol on membrane physical properties and (Na⁺,K⁺)-ATPase in native and cholesterol-enriched basolateral membranes from kidney and intestine of thermally acclimated trout (Oncorhynchus mykiss). Membrane order (as indicated by fluorescence depolarization studies) is increased, whereas its thermal sensitivity is decreased by elevated cholesterol levels in mem branes with relatively low endogenous amounts of cholesterol (intestinal membranes and renal membranes from cold-acclimated fish). Thermal sensitivities of membrane order in kidney are 1.5-fold higher in native compared with cholesterol-enriched basolateral membranes. For renal plasma membranes, (Na⁺,K⁺)- ATPase activity is lowest near the transition between native and surpraphysiological cholesterol levels. Endogenous cholesterol levels (relative to phospholipid contents) in intestinal basolateral membranes from cold-acclimated fish vary more than 1.5-fold; membranes with cholesterol/phospholipid molar ratios of 0.3 have activities of (Na⁺,K⁺)-ATPase that are twofold lower than native membranes having a ratio of 0.2. These results suggests that maintenance of cholesterol levels in intestinal basolateral membranes during thermal acclimation may ensure sufficient activity of (Na⁺,K⁺)-ATPase. Membrane function in kidney, with its high native cholesterol content, is less likely to be affected by temperature change. Accepted: 21 January 1997