[Mechanism of increase in Ca2+ level in platelet cytoplasm induced by aggregating factors]

Using the fluorescent dye, quin-2, it was shown that addition of ADP, thrombin and the so-called platelet activation factor (PAF) to a human platelet suspension results in a 5-20 increase in the concentration of free Ca2+ (Ca2+in) in the cytoplasm. No ADP and PAF-stimulated increase in Ca2+in occurs, when the concentration of extracellular Ca2+ sharply diminishes prior to addition of these agents; the value of the thrombin-stimulated increase in Ca2+in decreases by one order of magnitude under these conditions. Verapamil (10(-4) M) completely blocks the effect of PAF on Ca2+in concentration and decreases 2-3 times the response to ADP; in the case of thrombin verapamil increases the time necessary for the full response without changing its maximum value. Depolarization of the platelet plasma membrane after removal of the K+ diffusion potential does not affect Ca2+ content in the cytoplasm. No significant effect of ADP, thrombin or PAF on the electric potential of the platelet membrane followed by the intensity of diS-C3-(5) fluorescence was found. The elevation of the cAMP content in platelets after addition of prostaglandin E1, forskolin or papaverine leads to a removal of the PAF and ADP effects on the Ca2+in level. The data obtained suggest that the increase in Ca2+ content in platelet cytoplasm caused by the above-mentioned aggregation factors is due to the opening of the receptor-sensitive channels.