Membrane currents evoked by ATP in whole-cell patch-clamp recordings from human blood monocytes versus HEK cells expressing recombinant P2X₁ receptors. Individual human monocytes or HEK cells expressing recombinant rat P2X₁receptors (HEK-P2X1 cells) were patch-clamped as described under Materials and Methods. The membrane potential was held at −40 mV, while the cell was stimulated for indicated time with 100 μmol/L ATP. (A) Representative current record from an HEK–P2X₁ cell. Similar responses were observed in four of eight patch-clamped HEK–P2X₁ cells. Since this stably transfected cell line was not generated from a single cloned cell, the magnitude of P2X₁ receptor expression and function varies considerably between individual cells. (B) Representative current record from a human monocyte. A similar absence of ATP-induced response was observed in nine other patch-clamped monocytes.