Role of lipid peroxidation and the glutathione-dependent antioxidant system in the impairment of endothelium-dependent relaxations with age

Abstract

1. Age-related changes in the blood prooxidant-antioxidant state, as well as its influence on the relaxant responses to acetylcholine (ACh) were studied in the tail artery from 6-, 24- and 30-month-old Sprague-Dawley (SD) rats.

2. Malondialdehyde (MDA) plasma levels increased 2 and 3 times in 24- and 30-month-old rats, respectively, when compared with 6-month-old rats (0.43±0.09 μM). This increase was accompanied by an induction of 6-phosphogluconate dehydrogenase (6PG-DH) and glutathione reductase (GR) activities in red blood cells from 24-month-old rats. In 30-month-old rats, a further induction of these enzymatic activities, as well as glucose-6-phosphate dehydrogenase (G6P-DH) and glutathione peroxidase (GPx) activities was observed.

3. No differences with age were found in the concentration-response curves to ACh in isolated tail artery segments from 6- and 24-month-old rats precontracted with 0.3 μM noradrenaline (NA). However, a decrease in sensitivity to ACh-induced relaxation was observed in 30-month-old rats; EC₃₀ values were 3.5 (1.3–8.0)×10⁻⁷ M and 18.1 (8.9–30.1)×10⁻⁷ M for 6- and 30-month-old rats, respectively. Moreover, a decrease in maximum ACh relaxation (10 μM) was found in 30-month-old rats in comparison with that obtained in 6-month-old rats (58.5±3.9% and 42.5±3.4% of previous NA contraction, respectively).

4. Incubation of tail artery segments with MDA (0.5, 1 or 10 μM) caused a reduction of ACh-induced relaxations that was different in the three ages. Thus, the reduction of ACh-induced relaxations became significant with 0.5 μM MDA in 6-, with 1 μM MDA in 24-, and with 10 μM MDA in 30-month-old rats. In addition, MDA did not cause a shift in the concentration-response curve to ACh, but a decrease in the maximum response.

5. Superoxide dismutase (SOD; 150 u ml⁻¹, a superoxide anion scavenger) reversed the inhibitory effect of MDA on ACh-induced relaxations at all ages studied.

6. We conclude that: (1) ageing produces an increase in lipid peroxidation process, as indicated by the increase in MDA plasma levels, that is accompanied by an induction of lipid peroxide detoxification enzymes; (2) the changes in prooxidant-antioxidant equilibrium with age contribute, at least partially, to the impairment of the relaxant responses evoked by ACh; and (3) the effect of MDA appears to be mediated by superoxide anion at all ages studied.

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