Difference in phospholipid dependence between two isozymes of brain (Na+ + K+)-ATPase

Diet deficient in alpha-linolenic acid alters fatty acid composition and enzymatic properties of Na+, K+-ATPase isoenzymes of brain membranes in the adult rat

The effects of dietary (n-6)/(n-3) polyunsaturated fatty acid balance on fatty acid composition, ouabain inhibition, and Na(+) dependence of Na(+), K(+)-ATPase isoenzymes of whole brain membranes were studied in 60-day-old rats fed over two generations a diet either devoid of alpha-linolenic acid [18:3(n-3)] (sunflower oil diet) or rich in 18:3(n-3) (soybean oil diet). In the brain membranes, the sunflower oil diet led to a dramatic decrease in docosahexaenoic acid [22:6(n-3)] membrane content. The activities of Na(+), K(+)-ATPase isoenzymes were discriminated on the basis of their differential affinities for ouabain and their sensitivity to sodium concentration. The ouabain titration curve of Na(+), K(+)-ATPase activity displayed three inhibitory processes with markedly different affinity [i.e., low (alpha1), high (alpha2), and very high (alpha3)] for brain membranes of rats fed the sunflower oil diet, whereas the brain membranes of rats fed the soybean oil diet exhibited only two inhibitory processes, low (alpha1) and high (alpha2' = alpha2 + alpha3). Regardless of the diet, on the basis of the Na(+) dependence of Na(+), K(+)-ATPase activity, three isoenzymes were found: alpha1 form displaying an affinity 1.5- to 2-fold higher that of than alpha2 and 3-fold higher that of alpha3. In rats fed the sunflower oil diet, alpha2 isoenzyme exhibited higher affinity for sodium (Ka = 8.8 mmol/L) than that of rats fed the soybean oil diet (Ka = 11.7 mmol/L). These results suggest that the membrane lipid environment modulates the functional properties of Na(+), K(+)-ATPase isoenzymes of high ouabain affinity (alpha2).

Focal cerebral ischaemia induces a decrease in activity and a shift in ouabain affinity of Na+, K+-ATPase isoforms without modifications in mRNA and protein expression

In a mouse model of focal cerebral ischaemia, we observed after 1 h of ischaemia, that the total Na+, K+-ATPase activity was decreased by 39.4%, and then did not vary significantly up to 6 h post-occlusion. In the sham group, the dose-response curves for ouabain disclosed three inhibitory sites of low (LA), high (HA) and very high (VHA) affinity. In ischaemic animals, we detected the presence of only two inhibitory sites for ouabain. After 1 h of permanent occlusion, the first site exhibited a low affinity while the second site presented an affinity intermediate between those of HA and VHA sites, which evolved after 3 h and 6 h of occlusion towards that of the VHA site. The presence of only two ouabain sites for Na+, K+-ATPase after ischaemia could result from a change in ouabain affinity of both HA and VHA sites (alpha2 and alpha3 isoforms, respectively) to form a unique component. Irrespective of the duration of ischaemia, the smaller activity of this second site accounted entirely for the loss in total activity. Surprisingly, no modifications in protein and mRNA expression of any alpha or beta isoforms of the enzyme were observed, thus suggesting that ischaemia could induce intrinsic modifications of the Na+, K+-ATPase.

Effect of fish oil diet on fatty acid composition of phospholipids of brain membranes and on kinetic properties of Na+,K(+)-ATPase isoenzymes of weaned and adult rats

The influence of dietary (n-3) fatty acids (such as eicosapentaenoic and docosahexaenoic acids) as found in fish oil on Na+ sensitivity and ouabain affinity of Na+,K(+)-ATPase isoenzymes (alpha 1, alpha 2, alpha 3) was studied in whole brain membranes from weaned and adult rats fed diets for two generations. The long chain (n-3) fatty acids supplied by fish oil decreased the fatty acids of the (n-6) series compared with the standard diet, resulting in a decrease in the (n-6)/(n-3) molar ratio in both 21- and 60-day-old rats. On the basis of ouabain titration, three inhibitory processes with markedly different affinities were associated with isoenzymes, i.e., low affinity (alpha 1), high affinity (alpha 2), and very high affinity (alpha 3). It appears that the fish oil diet, in part via the modification of membrane fatty acid composition, altered the proportion and ouabain affinity of isoenzymes. Na+ sensitivity is the best criterion of physiologic change induced by fish oil diet. We calculated the Na+ activation for each isoenzyme and found one Na+ sensitivity and two Na+ sensitivities per isoenzyme in weanling and adult rats fed different diets, respectively. In contrast to alpha 2 and alpha 3, alpha 1 appears insensitive to membrane change induced by fish oil diet. Fish oil diet, which is known to confer cardioprotection, induced significant modulation of Na+,K(+)-ATPase isoenzymes at the brain level.

Effect of dietary alpha-linolenic acid on functional characteristic of Na+/K(+)-ATPase isoenzymes in whole brain membranes of weaned rats

The influence of dietary fatty acids on Na+ sensitivity and ouabain affinity of Na+/K(+)-ATPase isoenzymes of whole brain membranes were studied in weaned rats fed for two generations with diets either devoid of alpha-linolenic acid (sunflower oil diet) or rich in alpha-linolenic acid (soya oil diet). The (n--3) deficiency induced by the sunflower oil diet led to an increase in the (n--6)/(n--3) molar ratio in whole brain membranes. Na+/K(+)-ATPase isoenzymes were discriminated on the basis of their differential affinities for ouabain. In rats fed sunflower oil diet, the ouabain titration displayed three inhibitory processes with markedly different affinities: low affinity (alpha 1); high affinity (alpha 2); and very high affinity (alpha 3). Membranes of rats fed soya oil diet exhibited only two inhibitory processes, i.e., low affinity (likely alpha 1+ alpha 2) and high affinity (likely alpha 2+ alpha 3) with the low affinity form intermediate between the sunflower alpha 1 and alpha 2 forms, and the high affinity form intermediate between the sunflower alpha 2 and alpha 3 forms. In fact, the Na+ response shows that the three isoenzymes have different Na+ sensitivities. Regardless of the diet, alpha 1 has a similar Na+ sensitivity (less than 1 mM), whilst alpha 2 and alpha 3 are more sensitive in soya oil membranes compared to sunflower oil membranes (5.1 vs. 7.2 mM and about 11 vs. 22.5 mM, respectively). Thus, sodium appears to be a better criterion of heterogeneity than ouabain.

Glucose and oxygen deprivation induces a Ca(2+)-mediated decrease in (Na(+)+K+)-ATPase activity in rat brain slices

Exposure of rat brain cortical slices to a medium lacking in glucose, oxygen or both glucose and oxygen, resulted in a decrease of the tissue ATP content and a reduction of (Na(+)+K+)-ATPase activity in membranes prepared from the slices. These treatments also inhibited partial reactions of (Na(+)+K+)-ATPase such as Na(+)-dependent phosphorylation and K(+)-stimulated phosphatase, as well as specific binding of [3H]ouabain in membranes prepared from the slices. Glucose deprivation and hypoxia decreased (Na(+)+K+)-ATPase activity in the absence of extracellular Ca2+, but the effects were blocked by 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetomethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. Metabolic inhibitors mimicked the effects of glucose deprivation and hypoxia. The effect of glucose-free hypoxia was dependent on extracellular Ca2+. It was blocked by Mg2+ at high concentration, bepridil or amiloride, but not by voltage-sensitive Ca2+ channel antagonists and glutamate receptor antagonists. None of the drugs tested here, except for dithiothreitol, affected the inhibitory effect of glucose-free hypoxia on the enzyme activity. In contrast to brain (Na(+)+K+)-ATPase, the kidney enzyme was insensitive to glucose and oxygen deprivation and metabolic inhibitors which depleted the tissue ATP.

The effect of micromolar Ca2+ on the activities of the different Na+/K(+)-ATPase isozymes in the rat myometrium

In the present work we show the existence of two Na+/K(+)-ATPase isozymes in rat myometrial microsomes and suggest that they have different Ca2+ sensitivities. The catalytic subunits (alpha 1, alpha 2) of Na+/K(+)-ATPase were labelled by fluorescein-isothiocyanate and separated by SDS gel electrophoresis. The two isozyme Ca2(+)-sensitivities were studied by comparing the kinetics of Ca2+, strophantidin, ouabain and N-ethylmaleimide inhibitions. Our results indicate that the activity of the high ouabain-sensitive part (alpha 2 type) of Na+/K(+)-ATPase enzyme could only be inhibited by micromolar Ca2+. Furthermore, treatment of the microsomal preparation with 1mM N-ethylmaleimide selectively inactivated the high Ca2+ sensitive isoform of myometrial Na+/K(+)-ATPase.

Modulatory effect of two cardioglycosides on reconstituted Na+/K(+)-ATPase in proteoliposomes

1. Na,K-ATPase was extracted from Cavia cobaya kidneys, solubilized with nonionic detergent C12E8 (octaethyleneglycol dodecyl monoether) in mixed lipid-detergent-protein micelles. The Na,K-ATPase specific activity was 30-35 IU/mg protein. 2. The enzyme was reconstituted in vesicles, made of phosphatidylethanolamine and cholesterol: an enhancement of +60% in specific activity was obtained. 3. Two different vesicle-types were carried out: open liposomes (partially organized membranes) and closed liposomes. 4. Proteoliposomes were employed for measuring the modulatory effect of two cardioglycosides: ouabain and digoxin. 5. Inhibition of the Na,K-ATPase activity revealed apparent Ki of 1.25 microM for ouabain and 0.25 microM for digoxin in open liposomes, and apparent Ki of 0.75 microM for ouabain and of 1.75 microM for digoxin in closed liposomes. 6. Maximum enhancement of enzymatic activity was found at concentrations of 5-0.5 nM for ouabain and 5-1 nM for digoxin in open liposomes, and 25-1 nM for both digoxin and ouabain in closed liposomes.

Sodium affinity of brain Na(+)-K(+)-ATPase is dependent on isozyme and environment of the pump

The sodium affinities for the two forms of the Na(+)-K(+)-ATPase in brain were characterized. To mimic physiological conditions, synaptosomes, which are pinched off presynaptic nerve termini, were used. Examination of the pump in vitro was performed by preparing synaptic plasma membranes (SPMs). It was first shown that synaptosomes contain the two forms of the Na(+)-K(+)-ATPase, alpha 1 and alpha 2, and that these forms have markedly different affinities for the inhibitory cardiac glycoside ouabain. The apparent dissociation constant (K0.5) of alpha 1 for sodium changed from 12 to 9 mM when going from synaptosomes to membranes. For alpha 2, however, a shift from 36 to 12.5 mM was evident. The conclusion is that in vivo alpha 2 exists as a low sodium affinity species but can be altered to a high-affinity form simply by vesicle disruption. By comparison, the Na(+)-K(+)-ATPase from the mouse fibroblast cell line, 3T3-F442A cells, expressed only the alpha 1-isozyme, as shown by immunoblotting and by measurement of its ouabain and sodium affinities. The physiological relevance of these observations is also presented.

Difference between neuronal and nonneuronal (Na+ + K+)-ATPases in their conformational equilibrium

Several experiments were carried out to study the difference between two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase in the conformational equilibrium. Rat brain (Na+ + K+)-ATPase was much more thermolabile than the kidney enzyme. Both enzymes were protected from heat inactivation not only by Na+ and K+, but also by choline in varying degrees, though there was a difference between the two enzymes in the protection by the ligands. The brain enzyme was partially protected from N-ethylmaleimide (NEM) inactivation by both Na+ and K+, but the effects of the ligands on NEM inactivation of the kidney enzyme were more complex. Though ligands differentially affected the thermostability and NEM sensitivity of the two enzymes, the effects were not simply related to the conformational states. The sensitivity of phosphoenzyme (EP) formed in the presence of ATP, Na+, and Mg2+ to ADP or K+ and K+-p-nitrophenyl phosphatase (pNPPase) was then studied as a probe of the differences in the conformational equilibrium between the two isozymes. The EP of the brain enzyme was partially sensitive to ADP, while those of the heart and kidney enzymes were not. At physiological Na+ concentrations the percentages of E1P formed by the brain and kidney enzymes were determined to be about 40-50 and 10-20% of the total EP, respectively. The hydrolytic activity of pNPP in the presence of Li+, a selective activator at catalytic sites of the reaction, was much higher in the kidney enzyme than in the brain enzyme. The inhibition of K+-stimulated pNPPase by ATP and Na+ was greater in the latter enzyme than in the former. These results suggest that neuronal and nonneuronal (Na+ + K+)-ATPases differ in their conformational equilibrium: the E1 or E1P may be more stable in the alpha(+) than in the alpha during the turnover, and conversely the E2 or E2P may be more stable in the latter than in the former.

Phosphorylation of two isozymes of (Na+ + K+)-ATPase by inorganic phosphate

The phosphorylation of two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase by 32Pi was studied under equilibrium conditions in various enzyme preparations from rat medulla oblongata, rat cerebral cortex, rat cerebellum, rat kidney, guinea pig kidney, and rabbit kidney. In ouabain-sensitive (Na+ + K+)-ATPases such as the brain, guinea pig kidney, and rabbit kidney enzymes, ouabain stimulated the Mg2+-dependent phosphorylation at lower concentrations, while a higher concentration was required for the stimulation of rat kidney (Na+ + K+)-ATPase, an ouabain-insensitive enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that two isozymes of the brain (Na+ + K+)-ATPase were also phosphorylated by 32Pi in the presence of ouabain. The properties of the phosphorylation were compared between the medullar oblongata (referred to as alpha(+] and the kidney (referred to as alpha) (Na+ + K+)-ATPases. The steady-state level of phosphorylation was achieved faster in the kidney enzymes than in the medulla oblongata enzyme. Phosphorylation without ouabain was greater in the kidney enzymes than in the brain enzymes. Furthermore, the former enzymes were inhibited by K+ much more than the latter. These findings suggest that the two isozymes of (Na+ + K+)-ATPase differ in their conformational changes during enzyme turnover.

Acute changes in myo-inositol uptake and 22Na+ flux in murine neuroblastoma cells (N1E-115) following insulin

myo-Inositol uptake was investigated in a murine neuroblastoma clone (N1E-115) to determine the effect of altered Na+,K+-ATPase activity. The Na+ ionophore monensin, and veratridine, an alkaloid affecting voltage-dependent Na+ entry, increased acute 22Na+ uptake and 22Na+ efflux from pre-loaded cells, concomitant with enhanced myo-inositol uptake. This effect was also seen following insulin. Insulin-stimulated myo-inositol uptake was inhibited by amiloride, ouabain and pyrithiamine. Amiloride inhibition suggests that activation of Na+/H+ exchange preceding Na+,K+-ATPase activation is involved in insulin stimulation of myo-inositol uptake. Pyrithiamine inhibition is an indication of prior activation of the Na+,K+-ATPase alpha + catalytic subunit by insulin. The results provide evidence that insulin contributes to the maintenance of Na+,K+-ATPase in neuronal tissue.