The ouabain-insensitive sodium pump

Ceramide-1-Phosphate as a Potential Regulator of the Second Sodium Pump from Kidney Proximal Tubules by Triggering Distinct Protein Kinase Pathways in a Hierarchic Way

Kidney proximal tubules are a key segment in the reabsorption of solutes and water from the glomerular ultrafiltrate, an essential process for maintaining homeostasis in body fluid compartments. The abundant content of Na⁺ in the extracellular fluid determines its importance in the regulation of extracellular fluid volume, which is particularly important for different physiological processes including blood pressure control. Basolateral membranes of proximal tubule cells have the classic Na⁺ + K⁺-ATPase and the ouabain-insensitive, K⁺-insensitive, and furosemide-sensitive Na⁺-ATPase, which participate in the active Na⁺ reabsorption. Here, we show that nanomolar concentrations of ceramide-1 phosphate (C1P), a bioactive sphingolipid derived in biological membranes from different metabolic pathways, promotes a strong inhibitory effect on the Na⁺-ATPase activity (C1P₅₀ ≈ 10 nM), leading to a 72% inhibition of the second sodium pump in the basolateral membranes. Ceramide-1-phosphate directly modulates protein kinase A and protein kinase C, which are known to be involved in the modulation of ion transporters including the renal Na⁺-ATPase. Conversely, we did not observe any effect on the Na⁺ + K⁺-ATPase even at a broad C1P concentration range. The significant effect of ceramide-1-phosphate revealed a new potent physiological and pathophysiological modulator for the Na⁺-ATPase, participating in the regulatory network involving glycero- and sphingolipids present in the basolateral membranes of kidney tubule cells.

Two sodium pumps in the hepatopancreas of the intertidal euryhaline crab Neohelice granulata: biochemical characteristics and differential modulation after feeding

No study has been done on the existence, biochemical characteristics, and modulation of K+-independent ouabain-insensitive Na+ ATPase activity (the second sodium pump) in the digestive tract of intertidal euryhaline crabs and moreover on the coexistence and modulation under distinct physiological and (or) environmental conditions of different sodium pumps. We determined the occurrence, characteristics, and responses at different times (0, 1, 24, 48, and 120 h) after feeding upon distinct salinities of Na+ ATPase activity and Na+/K+ ATPase in the hepatopancreas of Neohelice granulata (Dana, 1851), which is a model species. The stimulation by Na+ under total inhibition of Na+/K+ ATPase activity revealed the occurrence of Na+ ATPase activity that was totally inhibited by 2 mmol·L–1 furosemide, exhibits Michaelis–Menten kinetics for ATP (apparent Km = 0.52 ± 0.16 mmol·L–1), and highest activity at around pH 7.4. In crabs acclimated to 35 psu (osmoconforming conditions), Na+ ATPase activity was highly increased (about 15-fold) (532 ± 58 nmol Pi·mg protein−1·min−1) in the hepatopancreas 48 h after feeding. In 10 psu (hyper-regulating conditions), Na+ ATPase activity decreased in the hepatopancreas 24 h after feeding (7 ± 9 nmol Pi·mg protein−1·min−1) and recovered initial values after 48 h (24 ± 35 nmol Pi·mg protein−1·min−1). Unlike Na+ ATPase, Na+/K+ ATPase activity did not change after feeding at any salinity, suggesting the specific modulation of the second sodium pump and its role in postprandial adjustments in the hepatopancreas.

A Kinetic Characterization of the Gill (Na+, K+)-ATPase from the Semi-terrestrial Mangrove Crab Cardisoma guanhumi Latreille, 1825 (Decapoda, Brachyura)

We provide a kinetic characterization of (Na⁺, K⁺)-ATPase activity in a posterior gill microsomal fraction from the semi-terrestrial mangrove crab Cardisoma guanhumi. Sucrose density gradient centrifugation reveals two distinct membrane fractions showing considerable (Na⁺, K⁺)-ATPase activity, but also containing other microsomal ATPases. The (Na⁺, K⁺)-ATPase, notably immuno-localized to the apical region of the epithelial pillar cells, and throughout the pillar cell bodies, has an M _r of around 110 kDa and hydrolyzes ATP with V _M = 146.8 ± 6.3 nmol Pi min^-1 mg protein^-1 and K _M = 0.05 ± 0.003 mmol L^-1 obeying Michaelis-Menten kinetics. While stimulation by Na⁺ (V _M = 139.4 ± 6.9 nmol Pi min^-1 mg protein^-1, K _M = 4.50 ± 0.22 mmol L^-1) also follows Michaelis-Menten kinetics, modulation of (Na⁺, K⁺)-ATPase activity by MgATP (V _M = 136.8 ± 6.5 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.27 ± 0.04 mmol L^-1), K⁺ (V _M = 140.2 ± 7.0 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.17 ± 0.008 mmol L^-1), and NH₄⁺ (V _M = 149.1 ± 7.4 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.60 ± 0.03 mmol L^-1) shows cooperative kinetics. Ouabain (K _I = 52.0 ± 2.6 µmol L^-1) and orthovanadate (K _I = 1.0 ± 0.05 µmol L^-1) inhibit total ATPase activity by around 75%. At low Mg²⁺ concentrations, ATP is an allosteric modulator of the enzyme. This is the first study to provide a kinetic characterization of the gill (Na⁺, K⁺)-ATPase in C. guanhumi, and will be useful in better comprehending the biochemical underpinnings of osmoregulatory ability in a semi-terrestrial mangrove crab.

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Kinetic analysis of gill (Na⁺,K⁺)-ATPase activity in selected ontogenetic stages of the Amazon River shrimp, Macrobrachium amazonicum (Decapoda, Palaemonidae): interactions at ATP- and cation-binding sites

We investigated modulation by ATP, Mg²⁺, Na⁺, K⁺ and NH₄⁺ and inhibition by ouabain of (Na⁺,K⁺)-ATPase activity in microsomal homogenates of whole zoeae I and decapodid III (formerly zoea IX) and whole-body and gill homogenates of juvenile and adult Amazon River shrimps, Macrobrachium amazonicum. (Na⁺,K⁺)-ATPase-specific activity was increased twofold in decapodid III compared to zoea I, juveniles and adults, suggesting an important role in this ontogenetic stage. The apparent affinity for ATP (K(M) = 0.09 ± 0.01 mmol L⁻¹) of the decapodid III (Na⁺,K⁺)-ATPase, about twofold greater than the other stages, further highlights this relevance. Modulation of (Na⁺,K⁺-ATPase activity by K⁺ also revealed a threefold greater affinity for K⁺ (K₀.₅ = 0.91 ± 0.04 mmol L⁻¹) in decapodid III than in other stages; NH₄⁺ had no modulatory effect. The affinity for Na⁺ (K₀.₅ = 13.2 ± 0.6 mmol L⁻¹) of zoea I (Na⁺,K⁺)-ATPase was fourfold less than other stages. Modulation by Na⁺, Mg²⁺ and NH₄⁺ obeyed cooperative kinetics, while K⁺ modulation exhibited Michaelis-Menten behavior. Rates of maximal Mg²⁺ stimulation of ouabain-insensitive ATPase activity differed in each ontogenetic stage, suggesting that Mg²⁺-stimulated ATPases other than (Na⁺,K⁺)-ATPase are present. Ouabain inhibition suggests that, among the various ATPase activities present in the different stages, Na⁺-ATPase may be involved in the ontogeny of osmoregulation in larval M. amazonicum. The NH₄⁺-stimulated, ouabain-insensitive ATPase activity seen in zoea I and decapodid III may reflect a stage-specific means of ammonia excretion since functional gills are absent in the early larval stages.

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.

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 (I(50) approximately 100nM). 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 (PKCzeta) isoform. When PKCzeta 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 PKCzeta.

Response of Na(+)-dependent ATPase activities to the contaminant ammonia nitrogen in Tapes philippinarum: possible atpase involvement in ammonium transport

In vivo and in vitro experiments elicited different responses to ammonia nitrogen (ammonia-N) of gill and mantle Na,K-ATPase and ouabain-insensitive Na-ATPase activities in the Philippine clam Tapes philippinarum. Short-term (120 h) exposed clams to sublethal ammonia-N (NH(3)+NH (4) (+) ) concentrations (1.5 and 3.0 mg/L ammonia-N) showed enhanced gill and mantle ouabain-insensitive ATPase activity and decreased mantle Na,K-ATPase activity with respect to unexposed clams, while gill Na,K-ATPase was unaffected. In vitro experiments showed that NH (4) (+) could efficiently replace Na(+) in ouabain-insensitive ATPase activation and K(+), but not Na(+), in Na, K-ATPase activation. Simple saturation kinetics was constantly followed with similar K (0.5) values to that of the substituted cation. The same maximal ouabain-insensitive ATPase activation was obtained at 80 mM Na(+) or NH (4) (+) in the gills and at 50 mM Na(+) or NH (4) (+ ) in the mantle and that of Na,K-ATPase at 10 mM K(+) or NH (4) (+) in the presence of 100 mM Na(+) in both tissues. The two coexistent ATPase activities maintained their typical response to ouabain also when stimulated by NH (4) (+) : when activated by Na(+)+K(+) or by Na(+)+NH (4) (+) the ATPase activity was completely suppressed by 10(-3 )M ouabain, whereas the Na(+)- or NH (4) (+) -stimulated ATPase activity was unaffected by up to 10(-2 )M ouabain. The whole of the data suggests a possible involvement of the two ATPase activities in NH (4) (+) transmembrane transport.

Leishmania amazonensis: characterization of an ouabain-insensitive Na+-ATPase activity

We characterized ouabain-insensitive Na+-ATPase activity present in the plasma membrane of Leishmania amazonensis and investigated its possible role in the growth of the parasite. An increase in Na+ concentration in the presence of 1mM ouabain, increased the ATPase activity with a V(max) of 154.1+/-13.5nmol Pi x h(-1) x mg(-1) and a K0.5 of 28.9+/-7.7mM. Furosemide and sodium orthovanadate inhibited the Na+-stimulated ATPase activity with an IC(50) of 270microM and 0.10microM, respectively. Furosemide inhibited the growth of L. amazonensis after 48h incubation, with maximal effect after 96h. The IC50 for furosemide was 840. On the other hand, ouabain (1mM) did not change the growth of the parasite. Taken together, these results show that L. amazonensis expresses a P-type, ouabain-insensitive Na+-ATPase that could be involved with the growth of the parasite.

Regulation of renal ouabain-resistant Na+-ATPase by leptin, nitric oxide, reactive oxygen species, and cyclic nucleotides: implications for obesity-associated hypertension

This study examined the effect of leptin on renal ouabain-resistant Na(+)-ATPase, which drives the reabsorption of about 10% of sodium transported in the proximal tubule. Chronic leptin administration (0.25 mg/kg s.c. twice daily for seven days) increased Na(+)-ATPase activity by 62.9%. This effect was prevented by the coadministration of superoxide dismutase mimetic, tempol, or the NADPH oxidase inhibitor, apocynin (2 mM in the drinking water). Acutely administered NO donors decreased Na(+)-ATPase activity. This effect was abolished by soluble guanylate cyclase inhibitor, ODQ, but not by protein kinase G inhibitors. Exogenous cGMP reduced Na(+)-ATPase activity, but its synthetic analogues, 8-bromo-cGMP and 8-pCPT-cGMP, were ineffective. The inhibitory effect of NO donors and cGMP was abolished by EHNA, an inhibitor of cGMP-stimulated phosphodiesterase (PDE2). Exogenous cAMP analogue and dibutyryl-cAMP increased Na(+)-ATPase activity and abolished the inhibitory effect of cGMP. Finally, the administration of superoxide-generating mixture (xanthine oxidase+hypoxanthine) increased Na(+)-ATPase activity. The results suggest that nitric oxide decreases renal Na(+)-ATPase activity by stimulating cGMP, which in turn activates PDE2 and decreases cAMP concentration. Increased production of reactive oxygen species may lead to the elevation of Na(+)-ATPase activity by scavenging NO and limiting its inhibitory effect. Chronic hyperleptinemia is associated with increased Na(+)-ATPase activity due to excessive oxidative stress.

Response to alkyltins of two Na+-dependent ATPase activities in Tapes philippinarum and Mytilus galloprovincialis

Organotin effects on the Na-dependent ATPases involved in ionic regulation of aquatic animals are poorly known, in spite of the largely documented contamination of seafood, especially bivalve molluscs. This study deals with the in vitro effect of TBT on the Na,K-ATPase and the ouabain-insensitive Na-ATPase in gill and mantle microsomes from the cultured bivalve molluscs Tapes philippinarum and Mytilus galloprovincialis. In the mussel also MBT, DBT and TeET were tested. While in both species the Na-ATPase showed an overall refractoriness to organotins, the Na,K-ATPase was progressively inhibited by increasing TBT concentrations (0-34 microM). In both species the Na,K-ATPase activity was more strongly inhibited in the gills than in the mantle. At the maximal TBT concentration tested (34.4 microM), while gill Na,K-ATPase activity was abolished, mantle enzyme activity was, respectively, reduced to 20% in T. philippinarum and to 50% in M. galloprovincialis. Mussel Na,K-ATPase was differently susceptive to the organotins tested and in both tissues showed an inhibition efficiency order TBT>DBT>>MBT=TeET (no effect), tentatively related to the different organotin polarity and to a possible interaction with membrane-bound enzyme complexes. The different response of the two ATPases to organotins is consistent with the known different susceptivity of the two enzyme activities to environmental contaminants, assay conditions and endogenous factors.

Decoupling the Na+–K+–ATPase in vivo: A possible new role in the gills of freshwater fishes

The literature suggests that when Na(+)-K(+)-ATPase has reduced access to its glycosphingolipid cofactor sulfogalactosyl ceramide (SGC), it is converted to a Na(+) uniporter. We recently showed that such segregation can occur within a single membrane when Na(+)-K(+)-ATPase is excluded from membrane microdomains or 'lipid rafts' enriched in SGC (D. Lingwood, G. Harauz, J.S. Ballantyne, J. Biol. Chem. 280, 36545-36550). Specifically we demonstrated that Na(+)-K(+)-ATPase localizes to SGC-enriched rafts in the gill basolateral membrane (BLM) of rainbow trout exposed to seawater (SW) but not freshwater (FW). We therefore proposed that since the freshwater gill Na(+)-K(+)-ATPase was separated from BLM SGC it should also transport Na(+) only, suggesting a new role for the pump in this epithelium. In this paper we discuss the biochemical evidence for SGC-based modulation of transport stoichiometry and highlight how a unique asparagine-lysine substitution in the FW pump isoform and FW gill transport energetics gear the Na(+)-K(+)-ATPase to perform Na(+) uniport.

Regulation of fish gill Na(+)-K(+)-ATPase by selective sulfatide-enriched raft partitioning during seawater adaptation

Na(+)-K(+)-ATPase is arguably the most important enzyme in the animal cell plasma membrane, but the role of the membrane in its regulation is poorly understood. We investigated the relationship between Na(+)-K(+)-ATPase and membrane microdomains or "lipid rafts" enriched in sulfatide (sulfogalactosylceramide/SGC), a glycosphingolipid implicated as a cofactor for this enzyme, in the basolateral membrane of rainbow trout gill epithelium. Our studies demonstrated that when trout adapt to seawater (33 ppt), Na(+)-K(+)-ATPase relocates to these structures. Arylsulfatase-induced desulfation of basolateral membrane SGC prevented this relocation and significantly reduced Na(+)-K(+)-ATPase activity in seawater but not freshwater trout. We contend that Na(+)-K(+)-ATPase partitions into SGC-enriched rafts to help facilitate the up-regulation of its activity during seawater adaptation. We also suggest that differential partitioning of Na(+)-K(+)-ATPase between these novel SGC-enriched regulatory platforms results in two distinct, physiological Na(+) transport modes. In addition, we extend the working definition of cholesterol-dependent raft integrity to structural dependence on the sulfate moiety of SGC in this membrane.

Isolation and characterization of enterocytes along the intestinal tract of the gilthead seabream (Sparus aurata L.)

Epithelial cells were successfully isolated along the intestine of the gilthead seabream using a dissociation method based on intracellular-like solutions. Biochemical and physiological tests revealed highly viable cells from all intestinal segments. Image analysis was used to identify cell types in the epithelial preparations which were highly enriched in enterocytes (>95%) over mucous cells. Several digestive hydrolases were determined in the isolated cells. Maltase (M), sucrase (S), leucine aminopeptidase (LA), 5'nucleotidase (5'N), but not gamma-glutamyl transferase (gamma-GT) or alkaline phosphatase (AP) activities were found to be enriched in the epithelial preparations versus the corresponding intestinal homogenates. Comparison of digestive hydrolases revealed the existence of a clear heterogeneity in their expression pattern in the enterocytes, along the intestine. Na(+)-K(+)-ATPase, Na(+)-ATPase and Cl(-)-ATPase activities were also determined in the membrane fraction of isolated cells. Analyses of enzymatic profiles revealed a clear asymmetry in the distribution of all Mg(2+)-dependent ATPases; that is, maximal Na(+)-K(+)- and Na(+)-ATPase activities were observed in the enterocytes from pyloric caeca, while Cl(-)-ATPase activity was about twice as high in the enterocytes from anterior and posterior intestines compared with pyloric caeca. This is the first report demonstrating the existence of heterogeneous metabolic and enzymatic profiles in different enterocyte populations from euryhaline teleosts.

Modulation of gill Na+,K+-ATPase activity by ammonium ions: Putative coupling of nitrogen excretion and ion uptake in the freshwater shrimp Macrobrachium olfersii

The effect of NH4+ ions on (Na+,K+)-ATPase hydrolytic activity was examined in a gill microsomal fraction from M. olfersii. In the absence of NH4+ ions, K+ ions stimulated ATP hydrolysis, exhibiting cooperative kinetics (nH=0.8), to a maximal specific activity of V=556.1+/-22.2 nmol.min(-1).mg(-1) with K(0.5)=2.4+/-0.1 mmol.L(-1). No further stimulation by K+ ions was observed in the presence of 50 mmol.L(-1) NH4+ ions. ATP hydrolysis was also stimulated by NH4+ ions obeying Michaelian kinetics to a maximal specific activity of V=744.8+/-22.3 nmol.min(-1).mg(-1) and KM=8.4+/-0.2 mmol.L(-1). In the presence of 10 mmol.L(-1) K+ ions, ATP hydrolysis was synergistically stimulated by NH4+ ions to V=689.8+/-13.8 nmol.min(-1).mg(-1) and K(0.5)=6.6+/-0.1 mmol.L(-1), suggesting that NH4+ ions bind to different sites than K+ ions. PNPP hydrolysis was also stimulated cooperatively by K+ or NH4+ ions to maximal values of V= 235.5+/-11.8 nmol.min(-1).mg(-1) and V=234.8+/-7.0 nmol.min(-1).mg(-1), respectively. In contrast to ATP hydrolysis, K(+)-phosphatase activity was not synergistically stimulated by NH4+ and K+ ions. These data suggest that at high NH4+ ion concentrations, the (Na+, K+)-ATPase exposes a new site; the subsequent binding of NH4+ ions stimulates ATP hydrolysis to rates higher than those for K+ ions alone. This is the first demonstration that (Na+, K+)-ATPase activity in a freshwater shrimp gill is modulated by ammonium ions, independently of K+ ions, an effect that may constitute a fine-tuning mechanism of physiological relevance to osmoregulatory and excretory processes in palaemonid shrimps.

Characterization of an ectonucleoside triphosphate diphosphohydrolase 1 activity in alkaline phosphatase-depleted rat osseous plate membranes: possible functional involvement in the calcification process

An ectonucleoside triphosphate diphosphohydrolase 1 (NTPDase1) activity present in alkaline phosphatase-depleted rat osseous plate membranes, obtained 14 days after implantation of demineralized bone particles in the subcutaneous tissue of Wistar rats, was characterized. At pH 7.5, NTPDase1 hydrolyzed nucleotide triphosphates at rates 2.4-fold higher than those of nucleotide diphosphates, while the hydrolysis of nucleotide monophosphates and non-nucleotide phosphates was negligible. NTPDase 1 hydrolyzed ATP and ADP following Michaelis-Menten kinetics with V=1278.7+/-38.4 nmol Pi/min/mg and K(M)=83.3+/-2.5 microM and V=473.9+/-18.9 nmol Pi/min/mg and K(M)=150.6+/-6.0 microM, respectively, but in the absence of magnesium and calcium ions, ATP or ADP hydrolysis was negligible. The stimulation of the NTPDase1 by calcium (V=1084.7+/-32.5 nmol Pi/min/mg; and K(M)=377.8+/-11.3 microM) and magnesium (V=1367.2+/-41.0 nmol Pi/min/mg and K(M)=595.3+/-17.8 microM) ions suggested that each ion could replace the other during the catalytic cycle of the enzyme. Oligomycin, ouabain, bafilomycin A(1), theophylline, thapsigargin, ethacrynic acid, P(1),P(5)-(adenosine-5')-pentaphosphate and omeprazole had negligible effects on the hydrolysis of ATP and ADP by NTPDase1. However, suramin and sodium azide were effective inhibitors of ATP and ADP hydrolysis. To our knowledge this is the first report suggesting the presence of NTPDase1 in rat osseous plate membranes. Considering that the ectonucleoside triphosphate diphosphohydrolase family of enzymes participates in many regulatory functions, such as response to hormones, growth control, and cell differentiation, the present observations raise interesting questions about the participation of this activity in the calcification process.

Modulation by ammonium ions of gill microsomal (Na+,K+)-ATPase in the swimming crab Callinectes danae: a possible mechanism for regulation of ammonia excretion

The modulation by Na(+), K(+), NH(4)(+) and ATP of the (Na(+),K(+))-ATPase in a microsomal fraction from Callinectes danae gills was analyzed. ATP was hydrolyzed at high-affinity binding sites at a maximal rate of V=35.4+/-2.1 Umg(-1) and K(0.5)=54.0+/-3.6 nM, obeying cooperative kinetics (n(H)=3.6). At low-affinity sites, the enzyme hydrolyzed ATP obeying Michaelis-Menten kinetics with K(M)=55.0+/-3.0 microM and V=271.5+/-17.2 Umg(-1). This is the first demonstration of a crustacean (Na(+),K(+))-ATPase with two ATP hydrolyzing sites. Stimulation by sodium (K(0.5)=5.80+/-0.30 mM), magnesium (K(0.5)=0.48+/-0.02 mM) and potassium ions (K(0.5)=1.61+/-0.06 mM) exhibited site-site interactions, while that by ammonium ions obeyed Michaelis-Menten kinetics (K(M)=4.61+/-0.27 mM). Ouabain (K(I)=147.2+/-7.microM) and orthovanadate (K(I)=11.2+/-0.6 microM) completely inhibited ATPase activity, indicating the absence of contaminating ATPase and/or neutral phosphatase activities. Ammonium and potassium ions synergistically stimulated the enzyme, increasing specific activities up to 90%, suggesting that these ions bind to different sites on the molecule. The presence of each ion modulates enzyme stimulation by the other. The modulation of (Na(+),K(+))-ATPase activity by ammonium ions, and the excretion of NH(4)(+) in benthic crabs are discussed.

Nitrophenylphosphate as a tool to characterize gill Na(+), K(+)-ATPase activity in hyperregulating Crustacea

The kinetic properties of a gill Na(+), K(+)-ATPase from the freshwater shrimp Macrobrachium olfersii were studied using p-nitrophenylphosphate (PNPP) as a substrate. Sucrose gradient centrifugation of the microsomal fraction revealed a single protein fraction that hydrolyzed PNPP. The Na(+), K(+)-ATPase hydrolyzed PNPP (K(+)-phosphatase activity) obeying Michaelis-Menten kinetics with K(M)=1.72+/-0.06 mmol l(-1) and V(max)=259.1+/-11.6 U mg(-1). ATP was a competitive inhibitor of K(+)-phosphatase activity with a K(i)=50.1+/-2.5 micromol l(-1). A cooperative effect for the stimulation of the enzyme by potassium (K(0.5)=3.62+/-0.18 mmol l(-1); n(H)=1.5) and magnesium ions (K(0.5)=0.61+/-0.02 mmol l(-1), n(H)=1.3) was found. Sodium ions had no effect on K(+)-phosphatase activity up to 1.0 mmol l(-1), but above 80 mmol l(-1) inhibited the original activity by approximately 75%. In the range of 0-10 mmol l(-1), sodium ions did not affect stimulation of the K(+)-phosphatase activity by potassium ions. Ouabain (K(i)=762.4+/-26.7 micromol l(-1)) and orthovanadate (K(i)=0.25+/-0.01 micromol l(-1)) completely inhibited the K(+)-phosphatase activity, while thapsigargin, oligomycin, sodium azide and bafilomycin were without effect. These data demonstrate that the activity measured corresponds to that of the K(+)-phosphatase activity of the Na(+), K(+)-ATPase alone and suggest that the use of PNPP as a substrate to characterize K(+)-phosphatase activity may be a useful technique in comparative osmoregulatory studies of Na(+), K(+)-ATPase activities in crustacean gill tissues, and for consistent comparisons with well known mechanistic properties of the vertebrate enzyme.

Response of rainbow trout gill Na+-ATPpase to T(3) and NaCl administration

The effect of the administration of commercial diets supplemented with 9 mg kg(-1) 3,5,3'-triiodo-l-thyronine (T(3)) or 10% (w/w) NaCl was evaluated on the ouabain-insensitive Na+-ATPase activity in rainbow trout gill microsomes. The trial, carried out following the seasonal trend from March to mid-May, included a treatment phase in freshwater and a subsequent transfer to brackish water (22 per thousand salinity) where trout were not treated. pH dependence, apparent Km values for Mg(2+) and Na+, and Hill coefficients evaluated throughout the trial for Na+-ATPase were generally not affected by the treatments and habitat change. In comparison with the control group, in both treated groups, Na+-ATPase activity was lower during the freshwater phase and higher after brackish-water transfer. As compared with untreated trout, gill (Na++K+)-ATPase activity during the freshwater phase was stimulated by NaCl treatment and also by T(3) treatment after transfer to brackish water. The results indicate that NaCl and T(3) administration act differently on the two ATPase activities involved in Na+ regulation and suggest a prevalent role of Na+-ATPase activity in hypoosmotic conditions.

Characterization of (Na+, K+)-ATPase in gill microsomes of the freshwater shrimp Macrobrachium olfersii

To better understand the adaptive strategies that led to freshwater invasion by hyper-regulating Crustacea, we prepared a microsomal (Na+, K+)-ATPase by differential centrifugation of a gill homogenate from the freshwater shrimp Macrobrachium olfersii. Sucrose gradient centrifugation revealed a light fraction containing most of the (Na+, K+)-ATPase activity, contaminated with other ATPases, and a heavy fraction containing negligible (Na+, K+)-ATPase activity. Western blotting showed that M. olfersii gill contains a single alpha-subunit isoform of about 110 kDa. The (Na+, K+)-ATPase hydrolyzed ATP with Michaelis Menten kinetics with K5, = 165+/-5 microM and Vmax = 686.1+/-24.7 U mg(-1). Stimulation by potassium (K0.5 = 2.4+/-0.1 mM) and magnesium ions (K0.5 = 0.76+/-0.03 mM) also obeyed Michaelis-Menten kinetics, while that by sodium ions (K0.5 = 6.0+/-0.2 mM) exhibited site site interactions (n = 1.6). Ouabain (K0.5 = 61.6+/-2.8 microM) and vanadate (K0.5 = 3.2+/-0.1 microM) inhibited up to 70% of the total ATPase activity, while thapsigargin and ethacrynic acid did not affect activity. The remaining 30% activity was inhibited by oligomycin, sodium azide and bafilomycin A. These data suggest that the (Na+, K+)-ATPase corresponds to about 70% of the total ATPase activity; the remaining 30%, i.e. the ouabain-insensitive ATPase activity, apparently correspond to F0F1- and V-ATPases, but not Ca-stimulated and Na- or K-stimulated ATPases. The data confirm the recent invasion of the freshwater biotope by M. olfersii and suggest that (Na+, K+)-ATPase activity may be regulated by the Na+ concentration of the external medium.

beta-adrenergic agonists stimulate Na+-K+-Cl- cotransport by inducing intracellular Ca2+ liberation in crypt cells

Epinephrine and beta-adrenergic agonists (beta1 and beta2 for isoproterenol, beta1 for dobutamine, beta2 for salbutamol) stimulated K+ (or 86Rb) influx mediated by the Na+-K+-2Cl- cotransporter and the Na+-K+ pump in isolated colonic crypt cells. Preincubation with bumetanide abolished the epinephrine effect on the Na+-K+ pump, suggesting that the primary effect is on the cotransporter. Maximal effect was obtained with 1 microM epinephrine with an EC50 of 91.6 +/- 9.98 nM. Epinephrine-induced K+ transport was blocked by propranolol with an IC50 of 134 +/- 28.2 nM. alpha-Adrenergic drugs did not modify K+ transport mechanisms. Neither Ba2+ nor tetraethylammonium nor DIDS modified the adrenergic enhancement on the cotransporter. In addition, epinephrine did not affect K+ efflux. Dibutyryl cAMP did not alter K+ transport. Reduction of extracellular Ca2+ to 30 nM did not influence the response to epinephrine. However, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM abolished epinephrine-induced K+ transport. Ionomycin increased Na+-K+-2Cl- cotransport activity. Moreover, epinephrine increased intracellular Ca2+ concentration in a process inhibited by propranolol. In conclusion, epinephrine stimulated the Na+-K+-2Cl- cotransporter in a process mediated by beta1- and beta2-receptors and modulated by intracellular Ca2+ liberation.

The ionic composition of the hydrothermal vent tube worm Riftia pachyptila: evidence for the elimination of SO2-4SO and H+ and for a Cl-/HCO-3HCO shift

Riftia pachyptila is one of the most specialized invertebrate hosts of chemoautotrophic symbionts. Crucial to the functioning of this symbiosis is how these worms cope with fluctuating ion concentrations. Internal sulfate levels in R. pachyptila appear comparable with other benthic marine invertebrates, despite the production of sulfate internally by means of the bacterial oxidation of hydrogen sulfide, suggesting that these worms are able to eliminate sulfate effectively. Internal chloride levels appear comparable; however, coelomic fluid chloride levels decrease significantly as the amount of coelomic fluid bicarbonate increases, demonstrating a 1:1 stoichiometry. We believe this shift in chloride, out of the body fluids, is needed to compensate for changes in electrochemical balance caused by the large increase (up to and greater than 60 mmol L-1) in negatively charged bicarbonate. Riftia pachyptila fits the general pattern of monovalent ion concentrations that is seen in other benthic marine invertebrates, with a high [Na+] : [K+] ratio extracellularly and low [Na+] : [K+] ratio intracellularly. Extracellular pH values of 7.38+/-0.03 and 7.37+/-0. 04 for coelomic fluid and vascular blood, respectively, as well as intracellular pH values of 7.37+/-0.04 and 7.04+/-0.05 for plume and trophosome tissue, respectively, were measured. On the basis of significant decreases in extracellular pH and, in some cases, Na+ and K+, in worms exposed to carbonyl cyanide m-chlorophenylhydrazone, sodium vanadate, and N-ethylmaleimide, we suggest that high concentrations of H+-ATPases, perhaps Na+/H+- or K+/H+-ATPases, are involved in H+ elimination in these animals.

Human ATP1AL1 gene encodes a ouabain-sensitive H-K-ATPase

The cDNA for ATP1AL1, the fifth member of the human Na-K-adenosinetriphosphatase (ATPase)/H-K-ATPase gene family, was recently cloned (A. V. Grishin, V. E. Sverdlov, M. B. Kostina, and N. N. Modyanov. FEBS Lett. 349: 144-150, 1994). The encoded protein (ATP1AL1) has all the primary structural features common to the catalytic alpha-subunit of ion-transporting P-type ATPases and is similar (63-64% identity) to the Na-K-ATPase alpha-subunit isoforms and the gastric H-K-ATPase alpha-subunit. In this study, ATP1AL1 was expressed in Xenopus laevis oocytes in combination with the beta-subunit of rabbit gastric H-K-ATPase. The functional properties of the stable alpha/beta-complex were studied by 86Rb+ uptake and demonstrated that ATP1AL1 is a novel human K(+)-dependent ATPase [apparent half-constant activation/(K1/2) for K+ approximately 375 microM)]. ATP1AL1-mediated inward K+ transport was inhibited by ouabain (inhibition constant approximately 13 microM) and was found to be inhibited by high concentrations of SCH-28080 (approximately 70% at 500 microM). ATP1AL1 expression resulted in the alkalinization of the oocytes' cytoplasm and ouabain-sensitive proton extrusion, as measured with pH-sensitive microelectrodes. These data argue that ATP1AL1 is the catalytic alpha-subunit of a human nongastric P-type ATPase capable of exchanging extracellular potassium for intracellular protons.

Ouabain-insensitive, Na(+)-stimulated ATPase activity in rabbit cardiac sarcolemma

The rabbit cardiac sarcolemma shows an ouabain, Na,K-stimulated ATPase activity and an ouabain-insensitive, Na-stimulated ATPase activity. The Na-ATPase has the following characteristics: (i) It is also stimulated by other monovalent cations. (ii) It is inhibited by 2 mM Furosemide and by 2 mM ethacrynic acid. (iii) It reaches maximal values (Vmax) at around 20 mM Na+. (iv) The apparent Km is around 5 mM. Except for the monovalent cation stimulation, the main characteristics of this ATPase are very similar to those of the ouabain-insensitive, Na-stimulated ATPase of mammalian kidneys.

Differential modulation of hepatic cytochrome P-450 enzymes in rat and Syrian hamster by 4'-trifluoromethyl-2,3,4,5-tetrachlorobiphenyl

The effects of a single injection (40 mg/kg) of 4'-trifluoromethyl-2,3,4,5-tetrachlorobiphenyl (CF3) on hepatic cytochrome P-450 monooxygenases were assessed in rat and syrian hamster. The CF3 treatment significantly increased the total amount of cytochrome P-450 in both species. In rats, CF3 treatment caused marked increases in ethoxyresorufin O-deethylase (EROD), arylhydrocarbon hydroxylase (AHH), and testosterone 7 alpha-hydroxylase activities but significantly reduced the activities of benzphetamine N-demethylase (BzND), erythromycin N-demethylase (ErND), testosterone 6 beta, 16 alpha, and 16 beta-hydroxylase, and formation of androstenedione. Administration of CF3 to hamsters strongly induced the activities of EROD, AHH, BzND, testosterone 15 alpha, and 16 alpha-hydroxylases, and androstenedione production, whereas ErND, testosterone 6 beta, and 7 alpha-hydroxylases were decreased. Administration of CF3 to rats induced the CYP1A family proteins and CYP2A1, while CF3 reduced the level of CYP2B1, and, to a lesser extent, of CYP6 beta 2. In hamsters, CF3 treatment significantly induced the CYP1A2, CYP2A1, CYP2A8, and CYP2B1 isozymes, whereas the CYP6 beta 2 level was decreased. The ability of hepatic microsomes to activate aflatoxin B1 and benzo(a)pyrene was elevated by CF3 treatment in hamsters, while activation of aflatoxin B1 was decreased in microsomes from CF3-treated rats. These results showed differences in the CF3-induced pattern of rat and hamster cytochrome P-450 monooxygenases.

Lipid composition and microsomal ATPase activities in gills and kidneys of warm- and cold-acclimated sea bass (Dicentrarchus labrax L.)

The response to cold of gill and kidney membrane lipid composition and microsomal (Na(+)+K(+))-ATPase, Na(+)-ATPase and Mg(2+)-ATPase activities in reared sea bass (Dicentrarchus labrax L.) was investigated. Fish acclimation was carried out according to the seasonal cycle from August to March. No cold-promoted increase in fatty acid unsaturation was shown in gill and kidney polar lipids and in total lipids of mitochondria and microsomes. In both tissues the (Na(+)+K(+))-ATPase exhibited positive compensation for cold acclimation whereas the Na(+)-ATPase displayed negative compensation. The Mg(2+)-ATPase showed no compensation in the gills and positive compensation in the kidneys. During cold acclimation the break in the Arrhenius plot of the (Na(+)+K(+))-ATPase decreased, whereas breaks of both the Na(+)-ATPase and the Mg(2+)-ATPase activities remained unchanged. The results indicate that the sea bass does not adopt membrane unsaturation as a cold-facing strategy. The cold-promoted enhancement of (Na(+)+K(+))-ATPase activity in osmoregulatory tissues may be advantageous to maintain efficient osmoregulation under thermodynamically unfavourable conditions.

Partial characterization of the inhibitory effect of lipid peroxidation on the ouabain-insensitive Na-ATPase of rat kidney cortex plasma membranes

The present work evaluates the effect of lipid peroxidation on the ouabain-insensitive Na-ATPase of basolateral plasma membranes from rat kidney proximal tubular cells as an indirect way to study the lipid dependence of this enzyme. An inverse relationship between lipid peroxidation and Na-ATPase activity was found. This effect was due neither to a change in the optimal Km of the system for Na+ nor for the substrate Mg:ATP, nor the optimal pH value of the medium. The optimal temperature value, however, was shifted toward a higher value. There was also an increase of the apparent energy of activation in the region of temperatures above the transition point (20 degrees C) with increase in lipid peroxidation. Peroxidized membranes incubated with phosphatidylcholine from soybean restored their Na-ATPase activity. On the other hand, the Na-ATPase activity was sensitive to oleoly lysophosphatidylcholine. These results suggest that lipid peroxidation might be affecting the Na-ATPase activity through either an increase of peroxidized phospholipids, which might change the membrane fluidity of the lipid microenvironment of the ATPase molecules, or through a direct effect of lysophospholipids released during the lipid peroxidation.

Effect of ethanol on the Na(+)- and the Na+,K(+)-ATPase activities of basolateral plasma membranes of kidney proximal tubular cells

The Na(+)- and the Na+,K(+)-ATPase activities of basolateral plasma membranes from rat kidney proximal tubular cells were affected differentially by ethanol. Moreover, at concentrations of ethanol that can be reached in vivo in the blood plasma (50 mM) there was a significant effect on the Na(+)-ATPase activity and practically no effect on the Na+,K(+)-ATPase activity.

Ouabain-insensitive Na-ATPase activity in homogenates from different animal tissues

1. Two Na(+)-stimulated ATPase activities were determined in gill homogenates from squid, shrimp and teleost fish; in kidney slice homogenates from teleost fish, bullfrog, toad, iguana, chicken, duck, rat, pig and cow, as well as in homogenates from rat small intestinal cells, brain cortex and liver slices. The two Na(+)-stimulated ATPase activities, the Na- and the Na,K-ATPase, showed a different behavior toward K+ and ouabain. 2. The ouabain-insensitive, K(+)-independent, Na-ATPase activity for all the studied homogenates was completely inhibited by 2 mM furosemide. 3. An increase in cell volume of the kidney, brain cortex and liver slice preparations, as well as of the rat small intestinal cells, produced a concomitant increase of the ouabain-insensitive Na-ATPase.