Immunological characterization of P2X₁receptor expression in myeloid cell lines versus human blood cells. HL60 cells or THP1 cells were untreated or were treated with 100 nmol/L PMA (HL-60 PMA or THP1 PMA), 0.5 mmol/L dibutyryl cAMP (HL-60 cAMP), or 1,000 U/mL IFN-γ plus 100 ng/mL LPS (THP1 I-L) as described under Materials and Methods. BAC1.2F5 murine macrophages (BAC1) were cultured without differentiating agents. Total cell membranes were isolated from these cell lines and from human peripheral blood cell fractions including total WBCs plus platelets (WC/platelets), neutrophils, and mononuclear leukocytes. Equal amounts of membrane protein (25 μg) were loaded in each lane and probed using the P2X₁R antiserum (<<<, 95 kD; <<, 60 kD). The cross-reactive band at about 75 kD is a serum-derived contaminant, the intensity of which can be reduced by repeated washing of cells before lysis. Similar patterns of P2X₁ receptor expression were observed in four other experiments.