Calmodulin increases Ca-dependent inhibition of the Na,K-ATPase in human red blood cells

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

Mechanisms of Sodium Transport in Plants-Progresses and Challenges

Understanding the mechanisms of sodium (Na⁺) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na⁺ accumulation and assure the maintenance of low Na⁺ concentration in the cytosol and, hence, plant tolerance to salt stress. Na⁺ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na⁺ is compartmentalized into vacuoles by Na⁺/H⁺ exchangers (NHXs). Na⁺ efflux from the plant roots is mediated by the activity of Na⁺/H⁺ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na⁺, K⁺-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na⁺, K⁺-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na⁺ influx, compartmentalization, and efflux in higher plants in response to salt stress.

A novel fluorescence imaging approach to monitor salt stress-induced modulation of ouabain-sensitive ATPase activity in sunflower seedling roots

Seedlings exposed to salt stress are expected to show modulation of intracellular accumulation of sodium ions through a variety of mechanisms. Using a new methodology, this work demonstrates ouabain (OU)-sensitive ATPase activity in the roots of sunflower seedlings subjected to salt stress (120 mM NaCl). 9-Anthroylouabain (a derivative of ouabain known to inhibit Na(+), K(+) -ATPase activity in animal systems, EC 3.6.3.9) has been used as a probe to analyze OU-sensitive ATPase activity in sunflower (Helianthus annuus) seedling roots by spectrofluorometric estimation and localization of its spatial distribution using confocal laser scanning microscopy. Salt stress for 48 h leads to a significant induction of OU-sensitive ATPase activity in the meristematic region of the seedling roots. Calcium ions (10 mM) significantly inhibit enzyme activity and a parallel accumulation of sodium ions in the cytosol of the columella cells, epidermis and in the cells of the meristematic region of the roots is evident. As a rapid response to NaCl stress, the activity of OU-sensitive ATPase gets localized in the nuclear membrane of root protoplasts and it gets inhibited after treatment with calcium ions. Nuclear membrane localization of the OU-sensitive ATPase activity highlights a possible mechanism to efflux sodium ions from the nucleus. Thus, a correlation between OU-sensitive ATPase activity, its modulation by calcium ions and accumulation of sodium ions in various regions of the seedling roots, has been demonstrated using a novel approach in a plant system.

Fatigue depresses maximal in vitro skeletal muscle Na(+)-K(+)-ATPase activity in untrained and trained individuals

This study investigated whether fatiguing dynamic exercise depresses maximal in vitro Na(+)-K(+)-ATPase activity and whether any depression is attenuated with chronic training. Eight untrained (UT), eight resistance-trained (RT), and eight endurance-trained (ET) subjects performed a quadriceps fatigue test, comprising 50 maximal isokinetic contractions (180 degrees /s, 0.5 Hz). Muscle biopsies (vastus lateralis) were taken before and immediately after exercise and were analyzed for maximal in vitro Na(+)-K(+)-ATPase (K(+)-stimulated 3-O-methylfluoroscein phosphatase) activity. Resting samples were analyzed for [(3)H]ouabain binding site content, which was 16.6 and 18.3% higher (P < 0.05) in ET than RT and UT, respectively (UT 311 +/- 41, RT 302 +/- 52, ET 357 +/- 29 pmol/g wet wt). 3-O-methylfluoroscein phosphatase activity was depressed at fatigue by -13.8 +/- 4.1% (P < 0.05), with no differences between groups (UT -13 +/- 4, RT -9 +/- 6, ET -22 +/- 6%). During incremental exercise, ET had a lower ratio of rise in plasma K(+) concentration to work than UT (P < 0.05) and tended (P = 0.09) to be lower than RT (UT 18.5 +/- 2.3, RT 16.2 +/- 2.2, ET 11.8 +/- 0.4 nmol. l(-1). J(-1)). In conclusion, maximal in vitro Na(+)-K(+)-ATPase activity was depressed with fatigue, regardless of training state, suggesting that this may be an important determinant of fatigue.

Na pump isoforms in human erythroid progenitor cells and mature erythrocytes

This study is aimed at identifying the Na pump isoform composition of human erythroid precursor cells and mature human erythrocytes. We used purified and synchronously growing human erythroid progenitor cells cultured for 7-14 days. RNA was extracted from the progenitor cells on different days and analyzed by RT-PCR. The results showed that only the alpha1, alpha3, beta2, and beta3 subunit isoforms and the gamma modulator were present. Northern analysis of the erythroid progenitor cells again showed that beta2 but not beta1 or alpha2 isoforms were present. The erythroid cells display a unique beta subunit expression profile (called beta-profiling) in that they contain the message for the beta2 isoform but not beta1, whereas leukocytes and platelets are known to have the message for the beta1 but not for the beta2 isoform. This finding is taken to indicate that our preparations are essentially purely erythroid and free from white cell contamination. Western analysis of these cultured progenitor cells confirmed the presence of alpha1, alpha3, (no alpha2), beta2, beta3, and gamma together now with clear evidence that beta1 protein was also present at all stages. Western analysis of the Na pump from mature human erythrocyte ghosts, purified by ouabain column chromatography, has also shown that alpha1, alpha3, beta1, beta2, beta3, and gamma are present. Thus, the Na pump isoform composition of human erythroid precursor cells and mature erythrocytes contains the alpha1 and alpha3 isoforms of the alpha subunit, the beta1, beta2, and beta3 isoforms of the beta subunit, and the gamma modulator.

A calnaktin-like inhibitor of Na,K-ATPase in rat brain: regulation of alpha 1 and alpha 2 isozymes

This study was designed to determine if a Ca(2+)-dependent, calnaktin-like inhibitor of Na,K-ATPase existed in rat brain and to compare the inhibition of different Na,K-ATPase isozymes in brain, heart and kidney. Based on the size and characteristics of human red blood cell calnaktin, a soluble protein fraction was obtained from rat brain and subjected to ultrafiltration and gel filtration to restrict the proteins to an appropriate molecular range of 6-50 kDa (6/50 fraction) for a crude calnaktin preparation. The 6/50 fraction was reconstituted with semipurified rat brain Na,K-ATPase and resulted in Ca(2+)-dependent inhibition of Na,K-ATPase activity. A 6/50 fraction was also prepared from rat heart ventricles, and, in its presence, Ca(2+)-dependent inhibition of cardiac Na,K-ATPase activity was observed. With brain preparations, the threshold for inhibition was approximately 100 nM free Ca2+, and inhibition was half maximal at 3-10 microM free Ca2+. Different isozymes of Na,K-ATPase were examined using differential sensitivity to ouabain and differential tissue distribution in brain, heart and kidney. The alpha 1 activity was inhibited in all three tissues. The alpha 2 activity of heart and the alpha 2 and/or alpha 3 activity of brain were also inhibited by the brain 6/50 fraction. In synaptosomal preparations from rat forebrain, resting intracellular (intrasynaptosomal) free Ca2+ was close to the threshold for calnaktin-like inhibition. The results are consistent with the presence of a calnaktin-like inhibitor of Na,K-ATPase in rat brain and indicate that calnaktin could be a widespread regulator of the alpha 1 isozyme. In addition, this study provides the first evidence that calnaktin also inhibits the alpha 2 activity of heart and the alpha 2 and/or alpha 3 isozymes of brain.

Species variability of erythrocyte transport ATPases in mammals

Na+, K(+)-ATPase, and Ca(2+)-ATPase in whole erythrocytes from five species of mammals (rat, mouse, guinea pig, golden hamster, rabbit) after cell treatment with Tween 20 (7.5 mg/ml) varied over a wide range: from 3.0 +/- 0.9 mumol Pi/hr/ml cells in rabbit to 27.3 +/- 4.9 mumol Pi/hr/ml cells in mouse for Na+, K(+)-ATPase and from 8.0 +/- 1.6 mumol Pi/hr/ml cells in hamster to 47.2 +/- 4.9 mumol Pi/hr/ml cells in mouse for Ca(2+)-ATPase. Differences were less pronounced in red cell ghosts. Fatty acid and phospholipid compositions of erythrocyte membranes were similar for all species. Nevertheless, the activity of Ca(2+)-ATPase in ghosts significantly correlated (r = -0.884) with the ratio of PC + SM/PE + PS in red cell membranes. Rabbit membranes had the lowest content of arachidonate. Rat hemolysate activated Na+, K(+)-ATPase in the ghosts from the animals of any species investigated, whereas the enzyme activation by the homohemolysate was characteristic only of the rat, mouse, and guinea pig ghosts. The data obtained suggest that there are differences in both activity and intracellular control of transport ATPase in erythrocytes of different mammals.

Phosphorylation of cardiac Na+-K+ ATPase by Ca2+/calmodulin dependent protein kinase

Na+-K+ ATPase is known to be involved in the transport of sodium and potassium across the cell membrane. We describe here a novel mechanism for the regulation of cardiac Na+-K+ ATPase through phosphorylation by a Ca2+/calmodulin-dependent protein kinase (CaM kinase) present in the sarcolemmal membrane. Incubation of cardiac sarcolemma in the presence of Ca2+ and calmodulin resulted in phosphorylation of a 110 kDa protein, identified as the alpha-subunit of Na+-K+ ATPase. The compound W-7, a potent inhibitor of calmodulin, caused significant inhibition of the CaM kinase-mediated phosphorylation while ouabain, a potent inhibitor of Na+-K+ ATPase, had no effect. Furthermore, phosphorylation of the sarcolemmal membrane with Ca2+/calmodulin caused significant reduction in the activity of Na+-K+ ATPase. These results suggest that phosphorylation of the alpha-subunit of Na+-K+ ATPase by an endogenous CaM kinase may lead to an inhibition of its catalytic activity.

The effects of calcium and calcium channel blockers on sodium pump

The effects of 10 mM Ca2+ and Ca2+ channel blockers verapamil, diltiazem and flunarizine on the ouabain-sensitive electrogenic Na+, K+ pump activity of mouse diaphragm muscle fibres enriched with Na+ were compared with the changes in cytosolic [Ca2+]. The electrogenic Na+ pump activity produced by adding K+ to muscles previously bathed for 4 h in a K(+)-free, 2-mM [Ca2+] solution increased the resting membrane potential by about 18 mV. This hyperpolarization was completely inhibited after 10 min incubation in 10 mM Ca2+. Verapamil 10(-5) M, 10(-5) M diltiazem and 10(-7) M flunarizine effectively prevented the effect of elevated [Ca2+]. At these concentrations, these drugs did not affect the K(+)-induced hyperpolarization. In mouse diaphragm, the basal cytosolic [Ca2+] measured by the fluorescent indicator 1-[2-(5-carboxyoxazol-2-yl)-6- aminobenzofuran-5-oxy]2-(2'-amino-5'-methylphenoxy)ethane-N,N,N',N '- tetraacetic acid acetoxymethyl ester (fura-2/AM) was 261 +/- 6 nM. After 4 h in a Liley K(+)-free, 2 mM [Ca2+] solution, the cytosolic [Ca2+] increased to 314 +/- 28 nM. Increase in [Ca2+] from 2 to 10 mM caused a twofold increase of cytosolic [Ca2+] to 637 +/- 26 nM. This rise was, like the Ca(2+)-induced inhibition of electrogenic pump, prevented by 10(-5) M verapamil, 10(-5) M diltiazem and 10(-7) M flunarizine. The results suggest that substances which block Ca2+ entry into the cell prevent the Ca(2+)-induced inhibition of the Na+ pump.

Binding and elution of EGTA to anion exchange columns: implications for study of (Ca+Mg)-ATPase inhibitors

EGTA bound to DEAE-Sephadex and DEAE-cellulose at low ionic strength in the presence and absence of applied protein. It remained bound when the column was washed at low ionic strength, but was eluted as the ionic strength was increased. The amount of EGTA recovered at high ionic strength was 60 to 90% of that applied to the column. At the peak of its elution, the concentration of EGTA in the eluted fractions was 25 mM, over 10-fold higher than the concentration of EGTA applied to the column. Eluted fractions containing EGTA inhibited the (Ca+Mg)-ATPase by two mechanisms: (1) by chelating the Ca and (2) by affecting activity even when the free Ca was held constant. We suggest that at least some of the inhibitory effects previously attributed to a cytoplasmic inhibitor of the (Ca+Mg)-ATPase may in fact be due to contaminating amounts of EGTA.