Activation of receptor-operated cation channels via P-2X1 not P-2T purinoceptors in human platelets

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 mu M) and the non-hydrolyzable ATP analogue alpha beta-meATP (alpha,beta-methyleneadenosine 5'-triphosphate, 10 mu M) 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 ATP gamma S (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 mu M) 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 P-2X1 receptors in other tissues. We conclude that human platelets possess a P-2X1 purinoceptor, which mediates a rapid phase of ADP- or ATP-evoked Ca2+ entry via a cation channel, whereas one or more separate ADP-selective P-2 purinoceptors evoke release of calcium from intracellular stores.