Effects of nitrendipine and tiapamil on 45Ca2+ influx and on platelet aggregation

The effects of the dihydropyridine derivative nitrendipine and of the phenylalkylamine derivative tiapamil on 45Ca2+ influx was determined in platelets in vitro and on platelet aggregation ex vivo. Thrombin-stimulated 45Ca2+ influx was inhibited by 10 mumol/l nitrendipine and 100 mumol/l tiapamil. ADP- and adrenaline-induced platelet aggregation were inhibited in normotensive volunteers following short-term administration of nitrendipine (20 mg b.i.d.) but not after tiapamil (225 mg t.i.d.). Therefore, mechanisms other than the inhibition of Ca2+ influx should be considered to be responsible for inhibition of platelet aggregation by nitrendipine ex vivo.