Mercedes Santiso

Lipid Peroxidation In Vivo Induced by Reversible Global Ischemia in Rat Brain

Abstract: It has been hypothesized that ischemia, followed by reperfusion, facilitates peroxidative free‐radical chain processes in brain. To resolve this question, rats were subjected to reversible global ischemia. From coronal sections of brains frozen in situ, small (ca. 2 mg) amounts of tissue were sampled from neocortex, hippocampus, and thalamus of both cerebral hemispheres of four groups of rats exposed to 30 min cerebral ischemia followed by 0, 30, 60, and 240 min of reperfusion, and from a control group subjected to the same operative procedures, except for the induction of ischemia. Heptane‐solubilized total lipid extracts from these samples were analyzed spectroscopically in the 190–330 nm range for content of isolated (nonconjugated) double bonds and of conjugated diene structures; the latter are formed from isolated double bonds during peroxidation of unsaturated fatty acids. Spectra derived from tissue regions of rats subjected to ischemia, or ischemia followed by reperfusion, were compared to averaged, region‐specific control spectra and were normalized to the original content of isolated double bonds in the peroxidized samples. The resultant difference spectra were analyzed in terms of ratios of conjugated diene concentration to the concentration of isolated double bonds originally at risk in the specific tissue zones considered. The peak representing conjugated diene formation was centered at 238 ± 1 nm and was usually well resolved when the molar ratio [conjugated diene]/[isolated double bonds], expressed as a percentage ([CD]/[IDB]), was greater than 0.25%. The incidence of resolvable conjugated diene peaks was much greater in the reperfused groups (18 of 124 samples total; 0.25% [CD]/[IDB] 1.34%), compared to the nonreperfused group (1 of 38 samples; [CD]/[IDB] = 0.62%). No specific regional susceptibility to conjugated diene formation was observed. It also was shown by computer averaging over all the tissue zones sampled that, if one had used large samples consisting of pooled brain tissue for analysis of lipid peroxides and/or their reaction products, the conjugated diene peak would have been attenuated below distinctly resolvable levels. These results constitute the first demonstration of lipid peroxidation induced by reversible global ischemia. The observation that the effect is highly focal and not generalized throughout the brain suggests that special conditions of reperfusibility and reoxygenation must be satisfied for lipid peroxidation to be detectable in an ischemic milieu.

Polyphosphoinositides as a probable source of brain free fatty acids accumulated at the onset of ischemia

The quantitative relationship between phos‐phoinositides and free fatty acids (FFAs) in brain ischemia was studied by measuring contents of individual fatty acids in phosphatidylinositol 4,5‐bisphosphate (PIP2), phosphatidylinositol 4‐phosphate (PIP), phosphatidylinositol (PI), phosphatidic acid (PA), diacylglycerol (DAG), and the FFA pool. Various periods of complete ischemia (1, 3, 10, and 30 min) were produced by decapitation. Ischemia of 1‐3 min caused rapid decreases in PIP2 and PIP content together with preferential production of stearic and arachidonic acids in the DAG and FFA pools. The decrement in levels of these fatty acid residues in polyphosphoinositides was sufficient to account for their increment in levels in the enlarged DAG and FFA pools. After 10 min of ischemia, levels of PIP2, PIP, and DAG approached plateau values, but levels of all FFAs continued to increase. The increases in content of DAG and FFAs at later ischemic periods could not be accounted for by the decreases in content of PIP2 and PIP. PI and PA levels showed only transient and subtle changes. These results indicate that, at the onset of ischemia, phosphodiesteric cleavage of PIP2 and PIP and subsequent deacylation by lipases are primarily responsible for the preferential increase in levels of free stearic and arachidonic acids and that, later, hydrolysis of other phospholipids plays a major role in the continuous accumulation of FFAs.

Cerebral phosphoinositide, triacylglycerol, and energy metabolism in reversible ischemia: origin and fate of free fatty acids.

Abstract: Levels of phosphatidylinositol 4,5‐bisphosphate (PIP2), phosphatidylinositol 4‐phosphate (PIP), phosphatidylinositol (PI), phosphatidic acid, diacylglycerol (DAG), triacylglycerol (TAG), and free fatty acids (FFAs), as well as their fatty acid composition, were determined in rat forebrain during ischemia and postischemic recirculation. Cerebral energy state and electroencephalograms (EEGs) were also studied. Fifteen minutes of ischemia resulted in a decrease in PIP2 and PIP contents but not in PI content, concurrent with an enlargement of the FFA and DAG pools. The latter were enriched in stearate and arachidonate. Prolongation of ischemia did not produce further changes in content of any of the inositol phosphalipids, but the increase in levels of FFAs and DAG continued. At the end of 45 min of ischemia, levels of both PIP2 and PIP decreased by 45–50%, and the total phosphoinositide content (PIP2+ PIP + PI) decreased by 21%, whereas levels of FFAs and DAG increased to 14– and 3.6‐fold of control levels, respectively. During ischemia, the TAG‐palmitate level decreased, but the TAG‐arachidonate level increased; the tissue energy state deteriorated severely; and the EEG was suppressed. A 30‐min recirculation period after 15 or 45 min of ischemia led to increases in PIP2, PIP, and total phosphoinositide contents, whereas levels of FFAs and DAG promptly decreased toward control values. The TAG‐arachidonate level peaked and the TAG‐palmitate level returned to a low control value during early recirculation. The ischemic changes in tissue lipids were completely reversed within 3 h of recirculation after both periods of ischemia. Adenylates were fully phosphorylated with as little as 30 min of reflow. The EEG activity partially recovered during reflow after 15 min of ischemia, whereas it remained depressed after prolonged ischemia. Thus, phosphodiesteric cleavage of PIP2 and PIP followed by deacylation of DAG is likely to contribute to the production of FFAs in early ischemia. Deacylation of undetermined lipids plays a role for the increment in levels of FFAs in the later period of ischemia. The rapid postischemic increase in levels of PIP2 and PIP indicates active synthesis not only from existing PI, but probably also by means of accumulated FFAs and DAG. These results indicate that the impaired resynthesis of inositol phospholipids cannot be a cause of the poor EEG activity after prolonged ischemia. Degradation and resynthesis of polyphosphoinositides and formation of TAG‐arachidonate may be important for modulation of free arachidonic acid levels in the brain during temporary ischemia.

Postischemic Cerebral Lipid Peroxidation In Vitro: Modification by Dietary Vitamin E

Abstract: Using an in vitro system, we studied the effect of postischemic reoxygenation on cerebral lipid peroxidation in relation to the dietary intake of vitamin E (VE) in rats. Homogenates prepared from VE‐deficient, ‐normal, and ‐supplemented brains, which were previously rendered ischemic for 30 min by decapitation, were incubated under air or nitrogen gas for 60 min. The extent of peroxidation in brain tissue was estimated by a thiobarbituric acid (TBA) test and by diene conjugation in total lipid extracts. The brain levels of α‐tocopherol and of total and free fatty acids (FAs) were also determined. Aerobic incubation increased TBA reactants in all dietary groups; the effect was largest in the VE‐deficient group, intermediate in the VE‐normal group, and smallest in the VE‐supplemented group. In contrast, nitrogen incubation did not alter the basal levels of TBA reactants except for a small rise associated with VE deficiency. Conjugated dienes changed in parallel with TBA reactants. α‐Tocopherol decreased after aerobic incubation and also, to a lesser degree, after nitrogen incubation in each dietary group. Only in the reoxygenated samples of the VE‐deficient group was there a significant fall in total polyunsaturated FAs. The levels of free FAs continuously increased throughout ischemia and subsequent incubation. However, the level of free polyunsaturated FAs was similar after aerobic and nitrogen incubation in each dietary group, and was not affected by VE. Thus, cerebral reoxygenation after ischemia propagates peroxidative reactions within esterified polyunsaturated FAs. The modification by VE of reoxygenation‐induced lipid peroxidation suggests free radical mediation.

Free Fatty Acids and Energy Metabolites in Ischemic Cerebral Cortex with Noradrenaline Depletion

Abstract: We tested whether cerebral noradrenaline (NA) may play a central role in mediating the increased production of free fatty acids (FFAs) during cerebral ischemia. Levels of FFAs, cyclic AMP, and NA, as well as ATP, ADP, and AMP, were measured in cerebral cortex during decapitation ischemia in rats 2 weeks after unilateral locus ceruleus lesion. Comparisons were made between the results obtained from the contralateral cortex with normal NA content and the NA‐depleted ipsilateral cortex. Although NA depletion was associated with a diminished transient rise of cyclic AMP in response to ischemia, it failed to influence the magnitude of FFA increase or the decline of energy state within the 15‐min period of ischemia. A more than twofold increase of total FFAs (sum of palmitic, stearic, oleic, arachidonic, and docosahexaenoic acids) was observed in both hemispheres at 1 min after decapitation, when energy failure became manifest. The increased production of FFAs continued throughout the 15 min of ischemia, with a preferential rise in the levels of stearic and arachidonic acids. There was an inverse correlation between FFA levels and total adenylate pool. The results do not support a major role for NA and cyclic AMP in increasing cortical FFAs during complete ischemia. Instead, they are consistent with the view that impaired oxidative phosphorylation activates deacylating enzymes. Disturbance of reacylation due to energy depletion is probably another factor contributing to the continuous increase of FFAs during prolonged ischemia.

Brain Lipid Peroxidation Induced by Postischemic Reoxygenation in vitro: Effect of Vitamin E:

The aerobic incubation of brain afer a period of ischemia induced lipid peroxidation. The effect was greatest in vitamin E—deficient rats, intermediate in vitamin E—normal rats, and least in animals supplemented with vitamin E. In contrast, nitrogen incubation following ischemia produced a small effect only in the vitamin E—deficient animals. It appears that reoxygenation is required for lipid peroxides to accumulate in the brain. However, a trace of oxygen remaining during extreme ischemic hypoxia may be sufficient to cause slow propagation of free radical reactions when the vitamin E level is low.

Concurrent measurement of (Na+,K+)-ATPase activity and lipid peroxides in rat brain following reversible global ischemia

Lipid peroxides, quantitated as lipid conjugated dienes, and (Na+,K+)-ATPase activity were assayed concurrently in brains of control rats and in three groups subjected to 30 min of reversible forebrain ischemia followed by 0, 1, and 4 hr of recirculation. Multiple small samples were taken from lateral, dorsolateral and medial cortex, hippocampus, thalamus and striatum following in situ freezing. (Na+,K+)-ATPase activity was elevated in hippocampus, dorsolateral and lateral cortex (P<0.10) and in thalamus (P<0.05) following 30 min ischemia. ATPase activity in medial cortex continued to increase during the first 1 hr of recirculation (P<0.10). Following 4 hr of recirculation, decreased enzyme activities were observed in all of these regions (lateral cortex and hippocampus,P<0.10). No changes in ATPase activity were observed in samples from striatum. Of the regional samples assayed for lipid peroxide content, the incidence of conjugated dienes as a function of recirculation time was 6% (0 hr), 23% (1 hr), and 17% (4 hr). For these samples, plots of normalized ATPase activity vs. tissue conjugated diene concentration revealed that normalized ATPase activity varied with recirculation time, but was independent of the magnitude of the lipid peroxidative process (expressed in terms of tissue conjugated diene concentration). These results suggest that disturbances in membrane structure and function presumed to arise from lipid peroxidation are not responsible for the behavior of the ATPase under the current in vivo conditions.

Cerebral phosphoinositide, triacylglycerol and energy metabolism during severe hypoxia and recovery

The cerebral concentrations of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidic acid (PA), triacylglycerol (TAG) and free fatty acids (FFA), as well as cerebral metabolites, were measured in rats subjected to 10 min of hypoxia and subsequent recovery of 7 or 30 min duration. The experiments were carried out with control of physiological variables. Hypoxia (paO2 values of about 15 mm Hg) caused a decrease in PI, whereas PIP and PIP2 did not change significantly. A two-fold increase of total FFA was noted, mainly comprising stearic and arachidonic acids. TAG-arachidonate tended to increase, but the other species in TAG decreased. Adenosine triphosphate (ATP) and energy charge (EC) decreased slightly and there was a marked lactate accumulation. PA did not change throughout the experiment. With recovery of 7 min duration, PI decreased further and total FFA continued to increase. TAG-arachidonate increased significantly. ATP remained depressed but EC recovered to the control range. Both tissue and plasma glucose increased. Tissue lactate remained elevated and systemic acidosis occurred. After a recovery period of 30 min, all lipids normalized and the energy state returned toward control. The data suggest that the phosphoinositide alterations during hypoxia are metabolically linked to changes in FFA and the lipid changes are accompanied by alterations in cerebral energy and carbohydrate metabolism. The selective increase in TAG-arachidonate may represent an incorporation of arachidonic acid into TAG, which may serve to reduce the free arachidonic acid level in the brain.

Cerebral phosphoinositide and energy metabolism during and after insulin-induced hypoglycemia

Abstract: During and after insulin‐induced hypoglycemia, changes in levels of cerebral phosphatidylinositol (PI), phosphatidylinositol 4‐phosphate (PIP), phosphatidylinositol 4,5‐bisphosphate (PIP2), phosphatidic acid (PA), triacylglycerol (TAG), diacylglycerol (DAG), and free fatty acids (FFAs) as well as the cerebral energy state were studied in relation to the EEG. In hypoglycemic rats with an EEG pattern of quasiperiodic sharp or slow sharp waves, which preceded the development of an isoelectric EEG, PIP2 levels increased significantly, together with a slight decrease in PI content. Levels of the other lipids did not change during this period. The cerebral energy state was affected only slightly in spite of profound decreases in plasma and tissue glucose levels. With 30 min of an isoelectric EEG, levels of all phos‐phoinositides and PA decreased significantly; total FFA and DAG contents increased seven‐and twofold, respectively; the TAG‐palmitate level decreased, and that of TAG‐arachidonate increased. Plasma and tissue glucose were nearly depleted, and the cerebral energy state deteriorated severely. The increment in fatty acids in the DAG and FFA pools was less than their loss from phosphoinositides and PA, an observation suggesting vascular washout or oxidation of a portion of the FFAs produced. Following 90 min of glucose infusion, PIP and PA levels recovered to control values; however, the PIP2 content exceeded control levels, and that of PI remained below control levels. DAG and FFA contents returned to normal. The data suggest that PIP2 synthesis is increased or PIP2 degradation is blocked because of disturbed neurotransmission during the preisoelectric and recovery periods, at a time when the tissue energy state is minimally affected, and that all phosphoinositides are degraded during the period of isoelectric EEG, probably owing to energy shortage.

Cerebral phosphoinositide, triacylglycerol and energy metabolism during sustained seizures induced by bicuculline

In ventilated rats, levels of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), diacylglycerol (DAG), triacylglycerol (TAG), free fatty acids (FFA) and phosphatidic acid, as well as their fatty acid contents, were measured in forebrain tissue after 1, 20 and 60 min of seizures induced by bicuculline. Cerebral energy state was also measured. PI decreased progressively throughout 60 min of seizures, whereas the levels of PIP and PIP2 did not change. DAG increased modestly and persistently. FFA increased markedly during the early seizure period, but decreased later. Following an initial drop, TAG rose above control. Phosphatidic acid did not change. The levels of ATP and energy charge potential decreased slightly and lactate accumulated. Stearic acid (18:0) and arachidonic acid (20:4) primarily accounted for the changes in the levels of the lipids. At the onset of seizures, the decrease of 18.0 and 20:4 in PI occurred in parallel with an enrichment of these fatty acids in FFA and DAG. Despite the fact that the losses of 18:0 and 20:4 from PI were quantitatively similar to each other at all times examined, the increase in free 18:0 was much larger than the increase in free 20:4 at 20 min of seizures. Concurrently there was a rise of 20:4 in TAG. As the FFA levels declined thereafter, 20:4 and docosahexaenoate (22:6) in TAG continued to increase. The results are consistent with the view that seizure activity stimulates the hydrolytic breakdown of brain phosphoinositides--the pathway catalyzed by phosphodiesterase of the phospholipase C type followed by lipases, and probably the pathway catabolized by phospholipases A as well. Preferential incorporation of polyunsaturated fatty acids into TAG-acyl residues may represent a mechanism to reduce the level of their free forms when the latter are produced in large amounts.