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

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.

A kinetic characterization of (Na+, K+)-ATPase activity in the gills of the pelagic seabob shrimp Xiphopenaeus kroyeri (Decapoda, Penaeidae)

We characterize the kinetic properties of a gill (Na(+), K(+))-ATPase from the pelagic marine seabob Xiphopenaeus kroyeri. Sucrose density gradient centrifugation revealed membrane fractions distributed mainly into a heavy fraction showing considerable (Na(+), K(+))-ATPase activity, but also containing mitochondrial F0F1- and Na(+)- and V-ATPases. Western blot analysis identified a single immunoreactive band against the (Na(+), K(+))-ATPase α-subunit with an Mr of ≈ 110 kDa. The α-subunit was immunolocalized to the intralamellar septum of the gill lamellae. The (Na(+), K(+))-ATPase hydrolyzed ATP obeying Michaelis-Menten kinetics with VM = 109.5 ± 3.2 nmol Pi min(-1) mg(-1) and KM = 0.03 ± 0.003 mmol L(-1). Mg(2+) (VM = 109.8 ± 2.1 nmol Pi min(-1 )mg(-1), K0.5 = 0.60 ± 0.03 mmol L(-1)), Na(+) (VM = 117.6 ± 3.5 nmol Pi min(-1 ) mg(-1), K0.5 = 5.36 ± 0.14 mmol L(-1)), K(+) (VM = 112.9 ± 1.4 nmol Pi min(-1 )mg(-1), K0.5 = 1.32 ± 0.08 mmol L(-1)), and NH4 (+) (VM = 200.8 ± 7.1 nmol Pi min(-1 )mg(-1), K0.5 = 2.70 ± 0.04 mmol L(-1)) stimulated (Na(+), K(+))-ATPase activity following site-site interactions. K(+) plus NH4 (+) does not synergistically stimulate (Na(+), K(+))-ATPase activity, although each ion modulates affinity of the other. The enzyme exhibits a single site for K(+) binding that can be occupied by NH4 (+), stimulating the enzyme. Ouabain (KI = 84.0 ± 2.1 µmol L(-1)) and orthovanadate (KI = 0.157 ± 0.001 µmol L(-1)) inhibited total ATPase activity by ≈ 50 and ≈ 44 %, respectively. Ouabain inhibition increases ≈ 80 % in the presence of NH4 (+) with a threefold lower KI, suggesting that NH4 (+) is likely transported as a K(+) congener.

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.

Gill (Na+,K+)-ATPase in diadromous, freshwater palaemonid shrimps: Species-specific kinetic characteristics and α-subunit expression

To better comprehend physiological adaptation to dilute media and the molecular mechanisms underlying ammonia excretion in palaemonid shrimps, we characterized the (Na+,K+)-ATPase from Macrobrachium amazonicum gills, disclosing high- (K(0.5) = 4.2+/-0.2 micromol L(-1); V = 33.9+/-1.9 U mg(-1)) and low-affinity (K(0.5) = 0.144+/-0.010 mmol L(-1); V = 232.9+/-15.3 U mg(-1)) ATP hydrolyzing sites. Stimulation by Na+ (K(0.5) = 5.5+/-0.3 mmol L(-1); V = 275.1+/-15.1 U mg(-1)), Mg2+ (K(0.5) = 0.79+/-0.06 mmol L(-1); V = 261.9+/-18.3 U mg(-1)), K+ (K(M) = 0.88+/-0.04 mmol L(-1); V = 271.8+/-10.9 U mg(-1)) and NH4(+) (K(M) = 5.0+/-0.2 mmol L(-1); V = 385.9+/-15.8 U mg(-1)) obeys single saturation curves, activity being stimulated synergistically by NH4(+) and K+. There is a single K+ binding site, NH4(+) binding to a second, exclusive site, stimulating activity by 33%, modulating K+ affinity. (Na+,K+)-ATPase activity constitutes approximately 80% of total ATPase activity (K(Iouabain) = 147.5+/-8.9 micromol L(-1)); Na+-, K+-, Ca2+-, V- and F(o)F(1)-ATPases are also present. M. amazonicum microsomal fractions possess approximately 2-fold less (Na+,K+)-ATPase alpha-subunit than M. olfersi, consistent with a 2.6-fold lower specific activity. These differences in (Na+, K+)-ATPase stimulation by ATP and ions, and specific activities of other ATPases, suggest the presence of distinct biochemical adaptations to life in fresh water in these related species.

Effect of hyper or hypo-osmotic conditions on neutral amino acid uptake and oxidation in tissues of the crab Chasmagnathus granulata

We investigated the transport of (14)C-methylaminoisobutyric acid ((14)C-MeAIB) and (14)C-alanine oxidation in hepatopancreas and jaw muscle of Chasmagnathus granulata submitted to 24, 72, and 144 h of hypo- or hyperosmotic stress. While (14)C-MeAIB uptake increased in jaw muscle and hepatopancreas from crabs submitted to hyperosmotic stress, it did not change in tissues from animals submitted to hypo-osmotic stress. Incubation of jaw muscle and hepatopancreas from control groups with 1 mM ouabain did not decrease (14)C-MeAIB uptake. However, ouabain prevented (14)C-MeAIB uptake in hepatopancreas at 24 h of hyperosmotic stress. In contrast, in jaw muscle from crabs submitted to the same conditions, (14)C-MeAIB uptake was not prevented by ouabain in the incubation medium. Jaw muscle from the control group produced four times more (14)CO(2) from (14)C-alanine than the hepatopancreas. During hypo-osmotic stress, amino acid oxidation does not seem to be one of the pathways implicated in the decrease of the amino acid pools in hepatopancreas and jaw muscle. In contrast, during hyperosmotic stress the reduction in (14)C-alanine oxidation appears to be one of the mechanisms involved in the increase of the amino acid pool in the hepatopancreas.

Normal expression and inflammation-induced changes of Na and Na/K ATPase activity in spinal dorsal horn of the rat

We tested whether that peripheral inflammation induces changes in the spinal dorsal horn ATPase activity. Adult Sprague-Dawley rats were anesthetized (thiobarbital), the left hind paw (inflammation group; n = 15) was immersed in water at 60 degrees C for 60s, which induced a local inflammation. A control group (n = 12) was tested with water at room temperature. After 60 min of peripheral inflammation left (LDH) or right lumbar dorsal horn (RDH) were processed for total, Na/K, Na and remanent ATPase activities (nM P(i) (mgprotein)(-1) min(-1)). In control animals isoenzymatic activities were: Na (31.2%); Na/K (20.6%) and remanent (48.2%) from total ATPase activity. No LDH-RDH asymmetry was found. The inflammation group presented an ipsilateral increase of total ATPase activity in LDH (X+/-S.E.M.; 4798.9+/-601) over the RDH (3982.2+/-451; Delta+817; P<0.05). This is due to an increase in Na ATPase activity (1609.3+/-297) over RDH (1164.2+/-166; Delta+445; P<0.05). ATPase activities were increased in LDH from inflamed over the control group as follows: total (4798.9+/-601; Delta+840; P<0.05), Na/K (1298.1+/-301; Delta+483; P<0.05) and Na (1609.3+/-297; Delta+373; P<0.05). These increased ATPase activities, induced in a short time, can be considered a functional marker of nociceptive neuronal activity.

Backdoor phosphorylation of basolateral plasma membranes of small intestinal epithelial cells: characterization of a furosemide-induced phosphoprotein related to the second sodium pump

Enterocyte has two different Na+-stimulated ATPases, the ouabain-sensitive Na+/K+ ATPase and a furosemide-inhibitable Na+ ATPase. To identify the polypeptide associated with the Na+-ATPase, 32Pi phosphorylation into basolateral membranes of enterocyte was investigated. Both, ouabain and furosemide induced Mg2+-dependent, vanadate-sensitive 32Pi incorporation into a 100kDa polypeptide. K(m) for Pi was 17.7+/-1.82 microM and 16.8+/-0.69 microM for ouabain-induced and furosemide-induced phosphorylation, respectively. K(m) for furosemide was 1.3+/-0.21 mM. Furosemide-induced 32Pi incorporation was sensitive to alkaline pH and hydroxylamine suggesting an acyl-phosphate bond. Na+ and K+ inhibited 32Pi incorporation induced by ouabain. In contrast, Na+ stimulated furosemide-induced phosphorylation with a K(m) of 16.5+/-5.59 mM while K+ had no effect. Purified Na+/K+ ATPase only presented ouabain-induced phosphoprotein, indicating that furosemide-induced phosphorylation is not related to this enzyme and appears to correspond to a new member of P-type ATPases associated with the second Na+ pump.

Ouabain-insensitive Na(+)-ATPase activity of Malpighian tubules from Rhodnius prolixus

In the present paper, we show the existence of a furosemide-sensitive Na(+)-stimulated, Mg(2+)-dependent ATPase activity in cell lysates of Malpighian tubular cells from Rhodnius prolixus, which could be the biochemical expression of the Na(+)-pump. The main characteristics of this activity are: (1) K0.5 for Na+ = 1.49 +/- 0.18 mM, (2) Vmax = 2.8 +/- 0.1 nmol inorganic orthophosphate (Pi).mg prot-1.min-1, (3) it is fully abolished by 2 mM furosemide, (4)it is insensitive to ouabain concentrations up to 10(-2) M, (5) it is sensitive to the presence of vanadate in the incubation medium indicating it to be a P-type ATPase, and (6) it is stimulated by nanomolar concentrations of Ca2+ in the incubation medium.

Effects of heat stress on Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of broiler chickens vital organs

Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of brain, heart, kidney, and small and large intestinal mucosa of broiler chickens exposed to heat stress (41 degrees C, 65% relative humidity for 6 h) and thermoneutral (25 degrees C, 65% relative humidity) conditions were determined. Brain and kidneys were found to have significantly higher Na+,K(+)-ATPase activities than those of heart and intestinal mucosa. Mg(2+)-activated ATPase and Na(+)-ATPase activities in the intestinal mucosa were higher than those of brain, kidneys, and heart under thermoneutral conditions. While there was a significant inhibition of total ATPase, Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activity of small and large intestinal mucosa of broiler chickens exposed to heat stress, the inhibitory effect was limited to total ATPase and Na+,K(+)-ATPase enzymes in kidneys. Heat stress produced a significant increase only in Mg(2+)-activated ATPase activity of the heart, without a remarkable change in all forms of ATPase activity in the brain. Heat stress significantly decreased the ratio of Na+,K(+)-ATPase to Mg(2+)-ATPase in the heart, kidneys, and small and large intestinal mucosa. The percentage of Na(+)-ATPase in Na+,K(+)-ATPase of brain, heart, and kidneys did not significantly change during heat stress, but the ratio in small and large intestinal mucosa increased significantly during heat stress. The severe disturbances in both serum electrolytes and acid-base balance observed in previous heat stress studies could partly be mediated by direct or indirect effects of heat stress on Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of kidneys, and small and large intestinal mucosa of broiler chickens.

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.