Extracellular
adenosine triphosphate (
ATP) and
adenosine diphosphate (
ADP) activate multiple types of P2-nucleotide receptors expressed in platelets or leukocytes. Electrophysiological and biochemical studies have indicated expression of the
P2X1 receptor, an
ATP-gated
cation channel, in human and rat platelets, rat basophilic
leukemia (RBL) cells, and
phorbol myristate acetate (PMA)-differentiated HL-60 myeloid cells. Although these findings suggest that P2X1 receptors are present in both blood leukocytes and blood platelets, the relative levels of
P2X1 receptor expression and function in human blood leukocytes and platelets have not been directly characterized. On the basis of both immunoblot analysis and functional assays of
P2X1 receptor-mediated ionic fluxes, we report that there is significant expression of P2X1 receptors in human platelets, but not in neutrophils, monocytes, or blood lymphocytes. Thus, unlike platelets and myeloid progenitor cell lines, fully differentiated human blood leukocytes do not express functionally significant numbers of P2X1 receptors, suggesting the downregulation of
P2X1 receptor gene expression during the differentiation of phagocytic leukocytes. By contrast,
P2X1 receptor expression is strongly maintained during megakaryocytic differentiation and platelet release. Immunoblot analysis indicated that the platelet
P2X1 receptor migrates as an approximately 60-kD
protein during SDS-electrophoresis under reducing or nonreducing conditions. Treatment of platelet membranes with
endoglycosidase-F causes the
P2X1 receptor band to migrate as a 46-kD
protein, verifying the highly glycosylated nature of the mature receptor
protein. Additional studies of
nucleotide-induced changes in Ca2+ influx/mobilization demonstrated that the platelet P2X1 receptors are pharmacologically distinct from the well-characterized
ADP receptors of these cells. This finding suggests a unique role for these
ATP-gated
ion channels during hemostasis or
thrombosis.