Synaptosomal Na, K-ATPase during forebrain ischemia in Mongolian gerbils

Impact of Ginkgo Biloba Extract EGb 761 on ischemia/reperfusion - induced oxidative stress products formation in rat forebrain

Dysbalance in reactive oxygen/nitrogen species is involved in the pathogenesis of cerebral ischemia/reperfusion injury (IRI). Ginkgo biloba extract (Egb 761) pre-treatment was used to observe potential antioxidant/neuroprotective effect after global ischemia/reperfusion. Egb 761 significantly decreased the level of lipoperoxidation (LPO) in rat forebrain total membrane fraction (homogenate) induced by in vitro oxidative stress (Fe(2+)+H(2)O(2)). In animals subjected to four-vessel global ischemia for 15 min and 2-24 h reperfusion the EGb pretreatment slightly decreased LPO in forebrain homogenate. However, as detected in EGb treated group, the LPO-induced lysine conjugates are attenuated in comparison to non-treated IRI animals. EGb significantly improved parameters which indicate forebrain protein oxidative damage after IRI. The intensity of tryptophane fluorescence was increased by the 18.2% comparing to non-treated IRI group and bityrosine fluorescence was significantly decreased in ischemic (21%) and 24 h reperfused (15.9%) group in comparison non-treated IRI group. In addition, the level of total free SH- groups in pre-treated animals was significantly higher comparing to non-treated animals. Our results indicate that extract of EGb 761 has potent antioxidant activity and could play a role to attenuate the IRI-induced oxidative protein modification and lipoperoxidation in the neuroprotective process.

The influence of MK-801 on the hippocampal free arachidonic acid level and Na+,K+-ATPase activity in global cerebral ischemia-exposed rats

The influence of 20 min global cerebral ischemia on the free arachidonic acid (FAA) level and Na+,K+-ATPase activity in the rat hippocampus at different time points after ischemia was examined. In addition, the effect of MK-801 on mentioned parameters was studied. Animals were exposed to 20 min global cerebral ischemia and were sacrificed immediately, 0.5, 1, 2, 6, 24, 48, 72, and 168 h after ischemic procedure. The level of the FAA and the Na+,K+-ATPase activity was measured during all reperfusion periods examined. Various doses of MK-801 (0.3, 1.0, 3.0, and 5.0 mg/kg) had been injected 30 min before ischemic procedure started. It was found that 20 min global cerebral ischemia induces a statistically significant increase of the FAA level immediately after ischemia and during the first 0.5 h of reperfusion. After a transient decrease, the level of FAA level increased again after 24 and 168 h of recirculation. Treatment with 3.0 mg/kg of MK-801 significantly prevented the FAA accumulation immediately and 0.5 h after ischemic insult while application of 5.0 mg/kg of MK-801 exerted a protective effect during the first 24 h. Global cerebral ischemia induces the significant decline in the Na+,K+-ATPase activity in the hippocampus starting from 1 to 168 h of reperfusion. Maximal inhibition was obtained 24 h after the ischemic damage. Application of 3.0 mg/kg of MK-801 exerted statistically significant protection during the first 24 h while the treatment with 5.0 mg/kg of MK-801 prevented fall in enzymatic activity during all reperfusion periods examined. Our results suggest that, in spite of different and complex pathophysiological mechanisms involved in the increase of FAA level and the decrease of the Na+,K+-ATPase activity, blockade of NMDA receptor subtype provides a very important strategy for the treatment of the postischemic excitotoxicity.

Carnosine protects rats under global ischemia

Rat brain subjected to 45-min global ischemia is characterized by decreased activity of K-p-nitrophenyl phosphatase and monoamine oxidase B and a disordering of the membrane bilayer by reactive oxygen species attack, the latter being monitored by the fluorescence of the membrane fluorescent probe, 1-anilino, 8-naphtalene sulphonate (ANS). Ischemic injury resulted in 67% mortality of the animals. In the group of animals pre-treated with the neuropeptide carnosine the mortality was only 30%. At the same time, carnosine protected both the activity of the above-mentioned enzymes and the brain membrane disordering, which was also tested by ANS fluorescence. The conclusion was made that carnosine protects the brain against oxidative injury and thereby increases the survival of the animals.

Preconditioning prevents the inhibition of Na+,K+-ATPase activity after brain ischemia

Application of single transient forebrain ischemia (ISC) in adult Wistar rats, lasting 2 or 10 min, caused inhibition of Na+,K+-ATPase activity in cytoplasmic membrane fractions of hippocampus and cerebral cortex immediately after the event. In the 2-min ISC group followed by 60 min of reperfusion, the enzyme inhibition was maintained in the cortex, while there was an increase in hippocampal enzyme activity; both effects were over 1 day after the event. However, in the 10-min ISC group enzyme inhibition had been maintained for 7 days in both cerebral structures. Interestingly, ischemic preconditioning (2-min plus 10-min ISC, with a 24-hour interval in between) prevented the inhibitory effect of ischemia/reperfusion on Na+,K+-ATPase activity observed either after a single insult of 2 min or 10 min ischemia. We suggest that the maintenance of Na+,K+-ATPase activity afforded by preconditioning be related to cellular neuroprotection.

Ischemia-induced inhibition of active calcium transport into gerbil brain microsomes: effect of anesthetics and models of ischemia

The excessive increase in intracellular Ca2+ concentration is associated with events linking cerebral blood flow reduction to neuronal cell damage. We have investigated the possible effect of ischemia and ischemia-reperfusion injury on endoplasmic reticulum (ER) Ca2+ transport. Two different models of ischemia as well as two different anesthetics were used. 5 min and 15 min of global forebrain ischemia caused significant depression of the rate of microsomal Ca2+ accumulation in pentobarbital anesthetised gerbils. The Ca2+ uptake activity recovered partially after 1 hour of reperfusion. Unlike pentobarbital anesthetised gerbils, no significant changes were detected in the active microsomal Ca(2+)-transport after 10 min of global forebrain ischemia in gerbil forebrain and hippocampus under halothane anesthesia. In addition, using the model of decapitation ischemia, we observed significant changes of the Ca2+ uptake in both halothane and pentobarbital anesthetised gerbils. These findings indicate that ischemic insult alters the brain microsomal Ca2+ transport which is not due to inhibition of the Ca(2+)-ATPase activity. However, the effect of ischemia on this transport system is dependent on the model of ischemia and on the type of anesthetics.

Membrane ion transport systems during oxidative stress in rodent brain: protective effect of stobadine and other antioxidants

The effect of oxidative stress in vitro induced by radical generating systems (RGS) (Fe2+-EDTA and Fe2+-EDTA plus H2O2) on synaptosomal and microsomal ion transport systems as well as on the membrane fluidity was investigated. Oxidative insult reduced Na+, K+-ATPase activity by 50.7% and Na+-dependent Ca2+ uptake measured in choline media by 46.7%. Membrane fluidity was also significantly reduced as observed with the fluorescent probe. Stobadine (ST) prevented the decrease in membrane fluidity and Na+-dependent Ca2+ uptake, however Na+, K+-ATPase activity was only partially protected, indicating a more complex mechanism of inhibition. Incubation of microsomes with RGS led to the loss of ability of membranes to sequester Ca2+, as well as to the decrease of Ca2+-ATPase activity and to the increase of Ca2+ permeability to 125.1%. The relative potency of the two RGS to decrease membrane fluidity correlated well with the system's potencies to induce lipid peroxidation. The extent of protection against depression of Ca2+ uptake values and Ca2+-ATPase activity by membrane soluble antioxidants (U-74500A, U-83836E, t-butylated hydroxytoluene-BHT and ST) was dependent on the experimental conditions and on the dose and nature of antioxidant used. ST seems to be at least as affective as BHT and 21-aminosteroids, and more potent than tocopherol acetate. Water soluble glutathione had no significant effect on the RGS induced inhibition of Ca2+-ATPase activity. Combination of ST with glutathione enhanced ST antioxidant efficacy, so drug combination might be beneficial therapeutically.

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.

Na/K-ATPase under oxidative stress: molecular mechanisms of injury

1. The authors compare oxidative injury to brain and kidney Na/K-ATPase using in vitro and in vivo approaches. The substrate dependence of dog kidney Na/K-ATPase was examined both before and after partial hydrogen peroxide modification. A computer simulation model was used for calculating kinetic parameters. 2. The substrate dependence curve for the unmodified endogenous enzyme displayed a typical curve with an intermediate plateau, adequately described by the sum of hyperbolic and sigmoidal components. 3. The modified enzyme demonstrated a dependent curve that closely approximates normal hyperbola. The estimated ATP K(m) value for the endogenous enzyme was about 85 microM; the Kh was equal to 800 microM. The maximal number of protomers interacting was 8. Following oxidative modification, the enzyme substrate dependence curve did not show a significant change in the maximal protomer rate Vm, while the K(m) was increased slightly and interprotomer interaction was abolished. 4. Na/K-ATPase from an ischemic gerbil brain showed a 22% decrease in specific activity. The maximal rate of ATP hydrolysis by an enzyme protomer changed slightly. but the sigmoidal component, characterizing the enzyme's ability to form oligomers was abolished completely. The K(m) value was almost unchanged, but the Hill coefficient fell to 1. These data show that Na/K-ATPase molecules isolated from the ischemic brain have lost the ability to interact with one another. 5. We suggest that the most important consequence of oxidative modification is Na/K-ATPase oligomeric structure formation and subsequent hydrolysis rate suppression.

Iron-induced inhibition of Na+, K(+)-ATPase and Na+/Ca2+ exchanger in synaptosomes: protection by the pyridoindole stobadine

The effect of oxidative stress, induced by Fe(2+)-EDTA system, on Na+,K(+)-ATPase, Na+/CA2+ exchanger and membrane fluidity of synaptosomes was investigated. Synaptosomes isolated from gerbil whole forebrain were incubated in the presence of 200 microM FeSO4-EDTA per mg of protein at 37 degrees C for 30 min. The oxidative insult reduced Na+,K(+)-ATPase activity by 50.7 +/- 5.0% and Na+/Ca2+ exchanger activity measured in potassium and choline media by 47.1 +/- 7.2% and 46.7 +/- 8.6%, respectively. Membrane fluidity was also significantly reduced as observed with the 1,6-diphenyl-1,3,5-hexatriene probe. Stobadine, a pyridoindole derivative, prevented the decrease in membrane fluidity and in Na+/Ca2+ exchanger activity. The Na+,K(+)-ATPase activity was only partially protected by this lipid antioxidant, indicating a more complex mechanism of inhibition of this protein. The results of the present study suggest that the Na+/Ca2+ exchanger and the Na+,K(+)-ATPase are involved in oxidation stress-mediated disturbances of intracellular ion homeostasis and may contribute to cell injury.

Changes in ouabain affinity of Na+, K+-ATPase during focal cerebral ischaemia in the mouse

We investigated the effect of focal cerebral ischaemia on the activity and the affinity of the ouabain sites of Na+,K+-ATPase in the mouse. The Na+,K+-ATPase activity was decreased by 38% as early as 30 min following ischaemia. In the sham group, the dose-response curves for ouabain disclosed three inhibitory states which contribute, respectively, 24.9 +/- 6.7%, 39.1 +/- 7.5% and 36.0% of the total activity (low affinity, LA; high affinity, HA and very high affinity, VHA, respectively). Their computed IC50 values are, respectively: 1.3 X 10(-3) M, 4.5 X 10(-6) M and 2.9 X 10(-9) M. Surprisingly, in ischaemic cortices, only two sites for ouabain were detected. The first site exhibits a LA (IC50 = 2.0 X 10[-4] M) but its relative contribution to the total activity (46.1 +/- 5.2%) is twice that noted for the LA site in non-ischaemic tissues. The second site presents an affinity intermediate between those of HA and VHA sites of the sham group (IC50 = 1.7 X 10[-7] M) and contributes 53.9% to the total activity. Loss in the specific activity of the second site explains that of the total activity. The most likely explanation in the presence of only two ouabain sites of Na+,K+-ATPase following ischaemia may be a change in ouabain affinity of alpha2 and/or alpha3 isoforms, as the presence of all three alpha isoforms has been observed by Western blotting. These results suggest that ischaemia induces intrinsic modifications in Na+,K+-ATPase which result from perturbations in membrane integrity and/or association of the alpha isoforms of this enzyme.