ROLE OF LIPID-PEROXIDATION AND THE GLUTATHIONE-DEPENDENT ANTIOXIDANT SYSTEM IN THE IMPAIRMENT OF ENDOTHELIUM-DEPENDENT RELAXATIONS WITH AGE

1 Age-related changes in the blood prooxidant-antioxidant state, as we ll as its influence on the relaxant responses to acetylcholine (ACh) w ere studied in the tail artery from 6-, 24- and 30-month-old Sprague-D awley (SD) rats. 2 Malondialdehyde (MDA) plasma levels increased 2 and 3 times in 24- and 30-month-old rats, respectively, when compared wit h 6-month-old rats (0.43+/-0.09 mu M). This increase was accompanied b y an induction of 6-phosphogluconate dehydrogenase (6PG-DH) and glutat hione reductase (GR) activities in red blood cells from 24-month-old r ats. In 30-month-old rats, a further induction of these enzymatic acti vities, as well as glucose-6-phosphate dehydrogenase (G6P-DH) and glut athione peroxidase (GPx) activities was observed. 3 No differences wit h age were found in the concentration-response curves to ACh in isolat ed tail artery segments from 6- and 24-month-old rats precontracted wi th 0.3 mu M 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)x10(-7)M and 18.1 (8.9-30.1)x10(-7)M for 6- and 30-m onth-old rats, respectively. Moreover, a decrease in maximum ACh relax ation (10 mu M) 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 w ith MDA (0.5, 1 or 10 mu M) caused a reduction of ACh-induced relaxati ons that was different in the three ages. Thus, the reduction of ACh-i nduced relaxations became significant with 0.5 mu M MDA in 6-, with 1 mu M MDA in 24-, and with 10 mu M MDA in 30-month-old rats. In additio n, MDA did not cause a shift in the concentration-response curve to AC h, 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 con clude that: (1) ageing produces an increase in lipid peroxidation proc ess, as indicated by the increase in MDA plasma levels, that is accomp anied by an induction of lipid peroxide detoxification enzymes; (2) th e changes in prooxidant-antioxidant equilibrium with age contribute, a t 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.