The effects of intravenous and oral nifedipine on ex vivo platelet function.

We have studied the possible anti-platelet effects of an intravenous formulation of nifedipine (0.75 mg as a bolus and an infusion of 1.2 mg.h-1 for 2 h), or equivalent volumes of the vehicle alone, or normal saline, in a double-blind crossover fashion in six healthy subjects. The effects of a standard oral formulation (20 mg sustained-release) compared to identical placebo were also studied in twelve other subjects. Platelet function was assessed by the addition of collagen, adenosine diphosphate, or adrenaline to whole blood followed by single platelet counting. Intravenous nifedipine had no effect on aggregation in response to any of the agonists, but oral nifedipine reduced aggregation caused by collagen by approximately 15%, despite similar plasma nifedipine concentrations after both formulations (18.5 ng.ml-1 after intravenous and 21.5 ng.ml-1 after oral administration). The lack of effect of intravenous nifedipine may be due to endothelial irritation caused by the vehicle. Intravenous nifedipine is unlikely to have a useful anti-platelet effect in patients who have acute coronary insufficiency.