Cyclooxygenase-pathway participates in the regulation of regional cerebral blood flow in response to neuronal activation under normo- and hypercapnia

The present study was designed to investigate whether cyclooxygenase products are involved in the regulation of the regional cerebral blood flow, evoked by somatosensory activation (evoked rCBF) under normo- and hypercapnia. Indomethacin (IMC) was used as cyclooxygenase inhibitor. It was applied intravenously (i.v., 10 mg/kg/h) in two experimental protocols-before hypercapnia (i) and after hypercapnia (ii). Somatosensory activation was induced by electrical hind paw stimulation (5 Hz frequency, 5 s duration, 1.5 mA). The evoked rCBF-response was measured in alpha -chloralose anesthetized rats using laser-Doppler flowmetry. IMC abolished completely the effect of hypercapnia on the baseline level of CBF. The drug reduced significantly evoked rCBF-response also. The inhibitory effect of IMC on evoked rCBF-response is better expressed under normocapnia (approximately 70%) than that under hypercapnia (approximately 40%). After IMC application, the normalized evoked rCBF curves peaked earlier as compared to that before its application (P<0.05), although the rise time of 0.5 s was nearly constant regardless of stimulus frequency. In conclusion, the results suggest a participation of IMC-sensitive and cyclooxygenase-dependent mechanisms in the regulation of evoked rCBF, induced by somatosensory stimulation.

Relationships between serum levels of autoantibodies against oxidized low density lipoproteins, lipid-soluble antioxidants and apolipoprotein B in patients with coronary heart disease

High affinity IgG autoantibodies against oxidized low density lipoproteins (oxLDLs), apolipoprotein B and lipid-soluble antioxidants--alpha-tocopherol and beta-carotene, were tested in patients with coronary heart disease. Correlation relationships between these parameters were analysed. Fifty one patients with coronary heart disease (37 males/14 females) defined as Q-wave myocardial infarction and/or stenosis of more than 50%, and 51 healthy blood donors (34 males/17 females) as controls participated in this study. LDLs were isolated by density gradient ultracentrifugation and oxidized with Cu2+. OxLDLs or native LDLs (nLDLs) were used as antigens in enzyme immunoassay (ELISA) to detect IgG autoantibodies in the serum. The contents of alpha-tocopherol and beta-carotene were measured by HPLC. Apolipoprotein B was determined by immunoturbidimetry. Correlation analysis of the parameters was carried out by Spearmann's test. Alpha-tocopherol was decreased significantly in the serum of patients with coronary heart disease (2.96+/-1.63 nmol/mg serum protein vs 6.23+/-2.28 nmol/mg serum protein in Control group) (p < 0.01). Also, the serum level of beta-carotene was decreased in patients with coronary heart disease (174.0+/-95.7 pmol/mg serum protein vs 313.2+/-141.5 pmol/mg serum protein in Control group) (p < 0.01), while apolipoprotein B was increased significantly (1.20+/-0.34 g/l in patients with coronary heart disease vs 0.86+/-0.23 g/l in Control group) (p < 0.001). In a previous study we established that the mean serum level of IgG autoantibodies against oxLDLs (expressed in optical density units) was about 2.5 times higher in patients with coronary heart disease as compared to control subjects (p < 0.001). A good positive linear correlation was observed between alpha-tocopherol and apolipoprotein B levels in Control group (r = 0.78, p < 0.001), as well as in the group of patients with coronary heart disease (r = 0.42, p < 0.001). Poor nonsignificant correlations were established between all another measured parameters. In conclusion, the lipid-soluble antioxidants--alpha-tocopherol and beta-carotene, are not informative with respect to the susceptibility of the serum to oxidative modifications and as to the extent of the subsequent humoral immune response. Presumably, the reduction of the correlation coefficient between apolipoprotein-B and alpha-tocopherol in patients with coronary heart disease in comparison with control subjects could provide indirect information on modifications of apolipoprotein-B and on a decrease of its susceptibility to interact with this major lipid-soluble antioxidant in atherogenesis.

Oxidation of low density lipoproteins leads to disturbance of their binding with alpha-tocopherol

The dynamics of binding of exogenous alpha-tocopherol (alpha-T) added to native or oxidatively modified LDLs (LDLs or oxLDLs) were investigated. Venous blood from 31 clinically healthy blood donors (15 males and 16 females) was used. LDLs were isolated by density gradient ultracentrifugation. LDLs were oxidized in vitro by CuSO4. LDLs or oxLDLs were enriched with exogenous alpha-T (initial concentrations: 0; 10; 20; 50; or 100 nmol per mg protein). The contents of alpha-T in LDLs or in oxLDLs were measured by HPLC. Lag-phase of LDL oxidation before or after saturation with alpha-T was recorded. Correlation analysis of the lag-phase of LDL oxidation and alpha-T content in LDLs was carried out by the method of Esterbauer et al. The experimental results demonstrated that: (i) alpha-T was incorporated into native LDLs to a higher extent as compared to oxLDLs. (ii) A saturation of LDLs and oxLDLs with alpha-T was observed. (iii) A positive correlation was observed between the duration of the lag-phase of LDL oxidation in vitro and the content of alpha-T in LDLs. (iv) Based on LDL saturation with alpha-T, the persons could be classified in two groups: LDLs from group I of 26 persons were found to incorporate exogenous alpha-T to the extent of 1.8 to 3 times its initial concentration; LDLs from group II of 5 persons incorporated little or no exogenous alpha-T. In the first group, oxidation of LDLs lead to a considerable decrease in alpha-T dependent variable k and to a moderate reduction of alpha-T-independent variable alpha in the equation of Esterbauer et al.: lag-phase = k.[alpha-tocopherol]+alpha. In the second group, oxidation of LDLs lead to insignificant changes in k, as well as in a. (v) According to the levels of k and a the native LDLs from the second group of 5 persons were very close to oxLDLs from the first group of 26 persons. Presumably, native LDLs from the second group of persons were initially oxidatively modified, and probably this will be a risk group in relation to atherogenic disorders.

Mechanisms of stabilization of biomembranes by alpha-tocopherol. The role of the hydrocarbon chain in the inhibition of lipid peroxidation

The effects of alpha-tocopherol and its homologues with different chain lengths (6-hydroxy-chromanes: C1, C6, C11) on lipid peroxidation in natural membranes (liver microsomes and mitochondria, brain synaptosomes) and liposomes were studied. It was shown that the antioxidant activity of alpha-tocopherol homologues decreased in the order: C1 greater than C6 greater than C11 greater than alpha-tocopherol (C16). Using fluorescent measurements, the possible reasons underlying these differences were investigated: (i) the distribution between the aqueous media and nonpolar phase of the membrane, which predetermines the binding of alpha-tocopherol homologues to membranes; (ii) the incorporation of alpha-tocopherol homologues into lipid bilayer; (iii) non-uniform distribution (formation of the clusters) of tocopherol homologues in the lipid bilayer; and (iv) transbilayer mobility of alpha-tocopherol homologues and accessibility of the inhibitors for radical-generating centres under enzymically and non-enzymically induced lipid peroxidation. It was demonstrated that: (i) binding of C1 with membranes was less efficient than that of longer-chain homologues (C6, C11, C16); (ii) the level of incorporation of alpha-tocopherol homologues into membranes decreased in a succession alpha-tocopherol C11 greater than C6 greater than C1; (iii) all alpha-tocopherol homologues existed in the lipid bilayer not only in a monomeric form but also associated in clusters thus decreasing the efficiency of radical scavenging; (iv) the short-chain alpha-tocopherol homologue, C1, exhibited a high transbilayer mobility whereas the long-chain one, C16, underwent no transbilayer migration within tens of minutes. The inhibiting effect of alpha-tocopherol esters and C1-acetate was predetermined by their hydrolysis in biomembranes; a strong correlation exists between the rate of the ester hydrolysis and their antioxidant activity in the membrane. In liposomes, in which the esterase activity was absent, alpha-tocopherol esters and C1-acetate exhibited very low lipid peroxidation inhibition.

Intermembrane transfer and antioxidant action of alpha-tocopherol in liposomes

Intermembrane transfer and exchange of tocopherol are not well understood. To study this we tested the ability of alpha-tocopherol containing unilamellar donor liposomes to inhibit the accumulation of lipid peroxidation products in acceptor liposomes. With molar ratios of alpha-tocopherol:phospholipids from 1:100 to 1:1000 in donor liposomes prepared by sonication of lipid dispersions, alpha-tocopherol was incorporated into both monolayers and was homogenously distributed in monomeric form without forming clusters in the liposomes. Concentrations of alpha-tocopherol which completely prevented the peroxidation of lipids were chosen for donor liposomes. Hence inhibition of lipid peroxidation in mixtures of donor and acceptor liposomes was determined by the antioxidant effect of alpha-tocopherol in acceptor liposomes which resulted from intermembrane transfer and exchange of alpha-tocopherol. Evidence was obtained that this was not due to fusion of donor with acceptor liposomes. The efficiency of the "intermembrane" antioxidant action of tocopherol was more pronounced when donor liposomes contained unsaturated phospholipids, indicating that the presence of unsaturated fatty acids in the outer monolayer phospholipids facilitates intermembrane tocopherol exchange.

[Intermembrane transport and antioxidant action of alpha-tocopherol in liposomes]

Studies were made of the ability of alpha-tocopherol, incorporated into unilamellar liposomes from saturated or unsaturated phospholipids (donor liposomes) to inhibit the accumulation of lipid peroxidation (LPO) products in unilamellar liposomes from rat cerebral cortex lipids (acceptor liposomes) in the presence of LPO inducer (Fe + ascorbate). With the molar alpha-tocopherol: phospholipids rations from 1:1000 to 1:100 in donor liposomes, obtained through sonication of lipid dispersions, alpha-tocopherol was incorporated into both monolayers of liposomes and was distributed in monomeric form without forming clusters. Based on the dependencies of LPO inhibition on the alpha-tocopherol concentrations, we chose the ones that completely prevented the accumulation of LPO products in donor liposomes. Under these conditions LPO inhibition in mixtures of donor and acceptors liposomes was fully determined by the antioxidant effect of alpha-tocopherol in acceptor liposomes due to its intermembrane transfer. The efficiency of the "intermembrane" antioxidant action of alpha-tocopherol increased in the course of preincubation of donor and acceptor liposomes (up to 60 min) and this increase was more pronounced when the donor liposomes contained unsaturated phospholipids. Evidence was obtained that the intermembrane transfer of alpha-tocopherol did not result from the fusion of donor and acceptor liposomes during preincubation.

Lipid peroxidation activation and cytochrome P-450 decrease in rat liver endoplasmic reticulum under oxidative stress

Iron loading was associated with development of oxidative stress, viz, decrease in tocopherol content and an increase in amount of lipid peroxidation products but only slight, if any, decrease in cytochrome P-450 content. Combinations of iron loading with other stress-inducing treatments (exhaustive physical exercise and hyperoxia) caused marked decreases in cytochrome P-450 content. Thus, a combination of factors favoring development of oxidative stress, but insufficient to exert a damaging effect on the cytochrome P-450-dependent detoxification system when acting alone, may become quite potent when acting in concert.

Oxidative stress leads to inhibition of calcium transport by sarcoplasmic reticulum in skeletal muscle

Iron administration results in the development of oxidative stress in skeletal muscles, as evidenced by increases in amounts of lipid oxidation fluorescent end products, decreases in vitamin E concentration, and inhibition of calcium transport by sarcoplasmic reticulum. Exhaustive physical loading or hyperoxia, or their combination, does not lead to apparent modification in calcium transport by sarcoplasmic reticulum in skeletal muscle homogenates. However, physical loading or hyperoxia does in fact induce oxidative stress since they magnify the effect of iron loading on the inhibition of calcium transport.

The role of secondary messengers in the regulation of lipid peroxidation in rat liver microsomes

The effect of phorbol-12-myristate-13-acetate (PMA), an activator of protein kinase C (PK-C) on lipid peroxidation (LPO) in rat liver homogenates and microsomes was studied. PMA (10(-10) to 10(-6) M) produced a concentration-dependent inhibition of LPO, which was greatly decreased by polymyxin B (PxB) (an inhibitor of PK-C). The non-active analogue of PMA, 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD) exerted no inhibitory effect. The adenylate cyclase activator, forskolin (FK) (10(-6) M) abolished the inhibitory effect of PMA on LPO. PMA and FK did not inhibit LPO in liposomes. It is suggested that LPO in biomembranes could be regulated by PK-C, whose inhibitory effect might be prevented by cAMP-dependent protein kinases.

[Effectiveness of lipid peroxidation inhibition in biomembranes by antioxidants with and without hydrocarbon chains]

The efficacy of lipid peroxidation inhibition by the natural antioxidant alpha-tocopherol and 2,2,5,7,8-pentamethyl-6-hydroxy-chromane (PMC), a derivative without hydrocarbon tail, as well as by the synthetic antioxidant 4-methyl-2,6-diterbutyl phenol (BHT) and its phospholipid derivative was studied in the membranes of rat liver microsomes and mitochondria. The presence of hydrocarbon tail in the antioxidant molecule determines the decrease of antioxidant efficiency in biomembranes. PMC and BHT exert a destructive effect on biomembranes, leading to an increase in their permeability to ions. This evidence suggests that the presence of hydrocarbon tail in the molecules of natural antioxidants provides not only for a relatively high antioxidant efficiency but also for a structural stability of biomembranes.