Rate of Ca⁺⁺ influx in P_2X1delreceptors stably transfected in 1321 astrocytoma cells.

In experiment 1, 3 P_2X1del cell lines (DL 1, DL 2, DL 5) labeled with Fura-2 responded rapidly to 30 μM ADP (column 1, mean ± SD) but were completely unresponsive to a second exposure to 30 μM ATP (column 2). Initial exposure to 30 μM ATP in experiment 2 resulted in a significantly reduced rate of Ca⁺⁺ influx (column 3) compared to column 1, but a secondary exposure to 30 μM ADP continued to activate, albeit at a significantly reduced level, the P_2X1del transfected cells (column 4). The initial activation by ATP decreased a secondary activation by ADP but influenced only the maximal extent of influx without significantly affecting the time required for maximal activation. In experiment 3, the inability of α,β-methylene-ATP (100 μM) to activate the P_2X1del-tranfected cells is shown in column 5. In addition, both α,β-methylene-ATP (100 μM, column 5) and β,γ-methylene-ATP (100 μM, data not shown) were ineffective at blocking influx induced by a secondary addition of 30 μM ADP (column 6). The rate of Ca⁺⁺ influx is expressed as the maximal peak of Ca⁺⁺ influx divided by the time required for maximum activation.