Failure of Calcium Channel Blockade to Reduce Platelet-Mediated Cyclic Flow Variations in Dogs with Coronary Stenosis and Endothelial Injury

Experimental canine coronary artery stenosis associated with endothelial injury results in a typical pattern of coronary flow characterized by gradual decreases in blood flow to almost zero values followed by abrupt restorations to original levels. Cyclic flow variations (CFVs) are the consequence of recurrent platelet aggregation at the site of the stenosis and subsequent dislodgement of the thrombus. The present study was designed to test the efficacy of diltiazem, nifedipine, and verapamil in inhibiting in vivo platelet aggregation as compared with that of aspirin and ketanserin, two potent reference compounds effective in this model. Except for aspirin, compounds were given as a slow intravenous infusion (i.v.) for 60 min to avoid hemodynamic changes. Diltiazem (0.01 mg/kg/min), nifedipine (3 micrograms/kg/min), and verapamil (0.01 mg/kg/min) were totally inactive against CFVs. A higher dose of verapamil (0.02 mg/kg/min) abolished CFVs in 3 of 4 dogs, but serious side effects were observed [atrioventricular (AV) block and death of 2 animals]. Aspirin (10 mg/kg bolus) caused complete inhibition of CFVs in 4 of 4 dogs, and ketanserin (0.01 mg/kg/min) abolished CFVs in 4 of 5 dogs. These data suggest that calcium channel blockade alone in contrast to cyclooxygenase inhibition or 5-HT2 antagonism cannot inhibit thrombus formation in this model.