Loading of human platelets with quin2 considerably increases platelet activating factor (PAF)-induced 45Ca2+ uptake. 45Ca2+ binding in the absence of agonists remains unchanged. The data show that PAF stimulates calcium ion influx into platelets, since quin2 provides substantial additional buffer capacity for Ca2+ in the cytoplasm. PAF and prostaglandin endoperoxide H2 analogue, U46619, induce Ba2+ and Mn2+ entry. The entrance of these cations and the rise in [Ca2+]i are both blocked by cAMP elevation and activation of protein kinase C by phorbol ester which indicates a single mechanism for agonist-induced transport of all three cations into platelets. It is assumed that agonists stimulate Ca2+ entry through the receptor-operated channels.
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