Extracellular ATP-dependent activation of plasma membrane Ca²⁺ pump in HEK-293 cells

1. It is well known that extracellular ATP (ATP(o)) elevates the intracellular Ca^2- concentration ([Ca²⁺](i)) by inducing Ca²⁺ influx or mobilizing Ca²⁺ from internal stores via activation of purinoceptors in the plasma membrane. This study shows that ATP(o) also activates the plasma membrane Ca²⁺ pumps (PMCPs) to bring the elevated [Ca²⁺](i) back to the resting level in human embryonic kidney-293 (HEK-293) cells. 2. The duration of ATP(o)-induced intracellular Ca²⁺ transients was significantly increased by PMCP blockers, La³⁺ or orthovanadate. In contrast, replacement of extracellular Na⁺ with NMDG⁺, a membrane-impermeable cation, had no significant effect on duration, thus suggesting that Na⁺/Ca²⁺ exchangers do not participate in the ATP(o)-induced Ca²⁺ transient. 3. A rapid and significant decrease in [Ca²⁺](i), which was not dependent on extracellular Na⁺, was induced by ATP(o) in cells pretreated with thapsigargin (TG). This decrease was blocked by orthovanadate, indicating that it was caused by PMCPs rather than sarco/endoplasmic reticulum Ca²⁺ pumps (SERCPs). 4. UTP and ATPγS also caused a decrease in [Ca²⁺](i) in cells pretreated with TG, although they were less effective than ATP. The effect of UTP implies the involvement of both P2Y₁ anti P2Y₂ receptors, while the effect of ATPγS implies no significant role of ectophosphorylation and agonist hydrolysis in the agonist-induced [Ca²⁺](i) decreases. 5. These results point to a role of PMCPs in shaping the Ca²⁺ signal and in restoring the resting [Ca²⁺](i) level to maintain intracellular Ca²⁺ homeostasis after agonist stimulation.