Antioxidant properties of calcium antagonists related to membrane biophysical interactions.

The antioxidant activities of representative calcium antagonists, including amlodipine, verapamil, and diltiazem, were measured in hepatic microsomal membranes by the Fe-catalyzed, hydroxyl radical-producing system (dihydroxyfumarate + Fe3+) and assessed by malondialdehyde (MDA) formation. Despite the absence of L-type calcium channels in this membrane preparation, the calcium antagonists showed dose-dependent antioxidant activity. The biophysical mechanism for calcium-antagonist antioxidant activity was evaluated using radioligand binding assays, high-resolution differential scanning calorimetry, and small-angle x-ray diffraction approaches. These analyses demonstrated that calcium-antagonist antioxidant potency correlated directly with the compounds' relative affinity for the membrane lipid bilayer and ability to modulate membrane thermodynamic properties (amlodipine >> verapamil > diltiazem). The charged 1,4-dihydropyridine calcium antagonist, amlodipine, had the highest affinity for the membrane lipid bilayer (Kp>10(4)) and produced the largest changes in membrane thermodynamic properties, including a reduction in thermal phase transition temperature (-11%), enthalpy (-14%), and cooperative unit size (-59%), relative to control phosphatidylcholine liposomes. Electron density profiles generated from x-ray diffraction data demonstrated that amlodipine effected a broad and dose-dependent increase in molecular volume associated with the membrane hydrocarbon core. These data indicate that lipophilic calcium antagonists inhibit lipid peroxidation in cellular membranes as a result of modulating physicochemical properties of the membrane lipid bilayer, independently of calcium channel inhibition. Amlodipine had the most potent antioxidant activity as a result of distinct biophysical interactions with the membrane lipid bilayer. The nonreceptor-mediated antioxidant activity of calcium antagonists may contribute to cytoprotective mechanisms of action in cardiovascular diseases.