Abstract
[Ca 2+] i increase is necessary in physiological platelet activity, particularly aggregation and release. The increase of [Ca 2+] i observed during platelet activation depends in part on Ca 2+ influx from the extracellular medium. The participation of voltage-operated Ca 2+ channels as a pathway for Ca 2+ entry is controversial. In the present study we have attempted to reinvestigate this problem by measuring aggregation and [Ca 2+] i changes in platelets activated by ADP or thrombin and incubated with organic or inorganic blockers of calcium channels. The main findings of the present paper can be summarized as follows: (i) Ni 2+, Co 2+ and Mn 2+, well known inorganic blockers of Ca 2+ channels, inhibited platelet aggregation induced by ADP or thrombin in a dose-dependent manner, Ni 2+ being the most effective agent. (ii) Thrombin induced a rise in free [Ca 2+] i in platelets incubated both in 1 mmol/l Ca 2+-containing medium and in nominally Ca 2+-free medium; the rise of free [Ca 2+] i was in the first case up to 370 ± 31nmol/l and in the second case up to 242 ± 26nmol/l, indicating that this observed difference was due to Ca 2+ entry from the extracellular medium. Co 2+ and Ni 2+ abolished that difference by inhibiting Ca 2+ influx. (iii) Nisoldipine, nitrendipine and nimodipine (10–50 nmol/l) inhibited in a dose-dependent manner platelet aggregation induced by either ADP or thrombin in platelets incubated in normal-Ca 2+ normal-K + medium, also, aggregation was inhibited to a similar extent in platelets incubated in normal-Ca 2+ high-K + medium. (iv) Nisoldipine — the most effective dihydropyridine to inhibit platelet aggregation — also inhibited Ca 2+ influx in platelets incubated in normal-Ca 2+ medium, either in normal-K + or high-K + media. Our data support the existence of voltage-operated, dihydropyridine-sensitive calcium channels (L-type) and a physiological role for them in platelet function.
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