Intracellular Ca²⁺measurements in subclones of 1321-N1 cells indicate the presence of P2Y receptor responses. Intracellular Ca²⁺ responses in single 1321-N1 G P2X₁ WT (A) and 1321-N1 G4 P2X_1del cells (B-D)¹³  loaded with the fluorescent calcium indicator fluo-3. Similar results were obtained in the 2 cell lines. (A) In the presence of extracellular Ca²⁺, ATP (30 μM) and ADP (30 μM) evoked a transient Ca²⁺ increase. (B) Loss of the response to ADP (100 μM) after only a short period (120 seconds in this experiment) in nominally Ca²⁺-free medium. This lack of response was caused by the rapid depletion of Ca²⁺ from intracellular stores because Thapsigargin (TG) had no effect in Ca²⁺-free medium. (C) Demonstration that TG evoked an expected large increase in Ca²⁺-containing medium after ADP exposure. (D) Adding ADP earlier (60 seconds) after exposure to Ca²⁺-free medium evoked a small, delayed Ca²⁺ increase.