Inhibition of LDL oxidation and oxidized LDL-induced cytotoxicity by dihydropyridine calcium antagonists

Purpose. The antioxidant activity of dihydropyridine calcium channel antagonists was evaluated based on LDL oxidation kinetics, oxidative cell injury associated with reactive species generation, and increases in free intracellular calcium (Ca2+) levels. Interactions with ascorbic acid were studied under conditions representative of LDL oxidation in plasma and tissue. Methods. Analysis of antioxidant activity utilized measurements of one-electron oxidation potentials and scavenging of peroxy radical-mediated oxidation. LDL antioxidant potency was determined spectrophotometrically using copper-mediated oxidation kinetics in the absence and presence of 100 mu M ascorbic acid. Prevention of oxidant-induced endothelial cell injury was determined from the formation of reactive oxygen species generation and increases in intracellular free calcium concentrations following addition of oxidized LDL or linoleic acid hydroperoxide. Results. Felodipine and amlodipine effectively inhibit peroxyl radical-mediated oxidation in lipoproteins and cells that is markedly enhanced in the presence of ascorbic acid. In the presence of ascorbic acid, inhibition of LDL oxidation is over four times greater than in LDL treated without antioxidants, and oxidized LDL and linoleic acid hydroperoxide-induced reactive oxygen species formation is effectively suppressed in cells. Inhibition of intracellular calcium increases was achieved using nM concentrations of felodipine or amlodipine. Conclusions. The additive effect for ascorbic acid and the calcium channel antagonist is postulated to involve a combination of peroxide-degrading and peroxyl radical scavenging reactions, demonstrating the importance of lipid peroxides during LDL oxidation and oxidized LDL-induced cytotoxicity. Cytoprotection is associated with inhibition of oxidant-induced increases in intracellular free calcium. Both the cytoprotective and LDL antioxidant activity for these compounds is manifested at concentrations approaching the therapeutic levels found in plasma.