Extracellular ATP-dependent activation of plasma membrane Ca(2+) pump in HEK-293 cells

1. It is well known that extracellular ATP (ATP(o)) elevates the intracellular Ca(2+) concentration ([Ca(2+)](i)) by inducing Ca(2+) influx or mobilizing Ca(2+) from internal stores via activation of purinoceptors in the plasma membrane. This study shows that ATP(o) also activates the plasma membrane Ca(2+) pumps (PMCPs) to bring the elevated [Ca(2+)](i) back to the resting level in human embryonic kidney-293 (HEK-293) cells. 2. The duration of ATP(o)-induced intracellular Ca(2+) transients was significantly increased by PMCP blockers, La(3+) or orthovanadate. In contrast, replacement of extracellular Na(+) with NMDG(+), a membrane-impermeable cation, had no significant effect on duration, thus suggesting that Na(+)/Ca(2+) exchangers do not participate in the ATP(o)-induced Ca(2+) transient. 3. A rapid and significant decrease in [Ca(2+)](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 Ca(2+) pumps (SERCPs). 4. UTP and ATPgammaS also caused a decrease in [Ca(2+)](i) in cells pretreated with TG, although they were less effective than ATP. The effect of UTP implies the involvement of both P2Y(1) and P2Y(2) receptors, while the effect of ATPgammaS implies no significant role of ectophosphorylation and agonist hydrolysis in the agonist-induced [Ca(2+)](i) decreases. 5. These results point to a role of PMCPs in shaping the Ca(2+) signal and in restoring the resting [Ca(2+)](i) level to maintain intracellular Ca(2+) homeostasis after agonist stimulation.