Elevation of cytosolic ionized calcium plays a critical role in human platelet activation. We have evaluated three well-characterized calcium antagonists for their ability to prevent thrombin-induced calcium mobilization in Fura 2 AM-loaded platelets and also their ability to inhibit platelet-vessel wall interactions. Thrombin (0.2 U/ml) caused significant elevation of cytosolic calcium (basal 84 +/- 18, activated 546 +/- 76 nM; n = 3). Verapamil, diltiazem, and nifedipine (100 microM) did not exert any inhibitory effect on thrombin-mediated calcium elevation. Untreated platelets perfused through a Baumgartner chamber containing a rabbit aorta preparation reacted with exposed and denuded subendothelium. The percentage of the total area covered by control platelet thrombi was 39.6 +/- 3.4. Diltiazem and Nifedipine significantly reduced the percentage of area covered by platelet thrombi, but the drugs were not as effective as aspirin (8.2 +/- 1.4). Calcium antagonists studied did not inhibit thrombin-stimulated elevation of cytosolic calcium in blood platelets. Although these drugs have been shown to prevent in vitro platelet aggregation and offer some protection against risks for atherosclerosis and thrombosis, they failed to significantly inhibit platelet-vessel wall interactions leading to formation of spread platelets and aggregates.