Abstract
We have investigated the purinoceptor subtypes responsible for calcium signaling in human platelets, which previous studies have shown to involve both Ca2+ influx via receptor-operated cation channels and release of Ca2+ from intracellular stores. Fura-2 measurements of [Ca2+]i in stirred platelet suspensions showed that both ADP (40 microM) and the non-hydrolyzable ATP analogue alphabeta-meATP (alpha, beta-methyleneadenosine 5-triphosphate, 10 microM) activated a rapid Ca2+ influx whereas only ADP mobilized Ca2+ from internal stores. In "nystatin" whole-cell patch clamp recordings, ATP, ADP, and the non-hydrolyzable ATP analogues, alpha, beta-meATP and ATPgammaS (adenosine 5 -O-(3-thiotriphosphate), all activated a cation channel permeable to both monovalent and divalent cations with a single-channel conductance of 11 picosiemens in NaCl saline. The current response to ATP (40 microM) was activated within 20 ms and desensitized with a time constant of 47-107 ms in the continued presence of agonist, which are characteristics of P2X1 receptors in other tissues. We conclude that human platelets possess a P2X1 purinoceptor, which mediates a rapid phase of ADP- or ATP-evoked Ca2+ entry via a cation channel, whereas one or more separate ADP-selective P2 purinoceptors evoke release of calcium from intracellular stores.
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