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A cDNA was isolated from a human monocyte library that encodes the P2X₇ receptor; the predicted protein is 80% identical to the rat receptor. Whole cell recordings were made from human embryonic kidney cells transfected with the human cDNA and from human macrophages. Brief applications (1-3 s) of ATP and 2′,3′-(4-benzoyl)-benzoyl-ATP elicited cation-selective currents. When compared with the rat P2X₇ receptor, these effects required higher concentrations of agonists, were more potentiated by removal of extracellular magnesium ions, and reversed more rapidly on agonist removal. Longer applications of agonists permeabilized the cells, as evidenced by uptake of the propidium dye YO-PRO1, but this was less marked than for cells expressing the rat P2X₇ receptor. Expression of chimeric molecules indicated that some of the differences between the rat and human receptor could be reversed by …