Structure-function relationships of calcium antagonists

Human low density lipoprotein (LDL) incubated with active Ca2+ antagonists from three different chemical groups, 1,4-dihydropyridines that are of reduced activity as Ca2+ antagonists, vitamin E, and probucol, was more resistant than control to copper- or human monocyte-induced oxidation, as assessed by thiobarbituric acid reactive substance (TEARS) content, degradation by J774 macrophages, and relative electrophoretic mobility on agarose gel. In the copper-induced oxidation system, the drugs tested reduced the TEARS levels of LDL in a concentration-dependent manner. The order of potency was vitamin E > felodipine > 2-chloro analog of nifedipine > nifedipine > amlodipine, nitrendipine, verapamil > diltiazem. In agreement with the results of the TEARS assay, felodipine (25 mu M) was also the most effective calcium antagonist in the degradation assay, inducing a significant (P < 0.05) 97 +/- 2% reduction in the amount of oxidized [I-125]LDL degraded by J774 macrophages compared with nifedipine and its 4-nitro analog, amlodipine, and verapamil. The relative mobility of oxidized LDL on agarose gel was reduced significantly (P < 0.05) by felodipine (50 mu M) and amlodipine (25 and 50 mu M) when compared with control, and was similar to that of native LDL, suggesting an effect of these drugs on the net negative charge of oxidized LDL. In the cell-induced oxidation system, both nifedipine and felodipine (25 mu M) induced significant (P < 0.05) reductions in the TEARS content of LDL (96 +/- 2 and 65 +/- 9%, respectively) compared with amlodipine, verapamil and the 4-nitro analog of nifedipine. However, in this oxidation system nifedipine was a more effective antioxidant than felodipine. Analysis of the structure-function relationships for the effect of 1,4-dihydropyridines on the oxidative modification of LDL suggests an important role for the 2-substitution of the phenyl ring, and an essential role for the dihydropyridine ring. This study clearly shows that Ca2+ antagonists from different chemical groups have a concentration-dependent effect as antioxidants against LDL oxidation. However, the order of potency of the drugs depends on the oxidation system and the assay used to measure the antioxidant effect. Our data suggest that such a protective effect of Ca2+ antagonists against LDL oxidation could play a role in the antiatherosclerotic effect of these drugs.