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 intracellul ar Ca2+ concentration ([Ca2+](i)) by inducing Ca2+ influx or mobilizing Ca2 + from internal stores via activation of purinoceptors in the plasma membra ne. This study shows that ATP, also activates the plasma membrane Ca2+ pump s (PMCPs) to bring the elevated [Ca2+](i) back to the resting level in huma n embryonic kidney-293 (HEK-293) cells. 2 The duration of ATP(o)-induced intracellular Ca2+ transients was signific antly increased by PMCP blockers, La3+ or orthovanadate. In contrast, repla cement of extracellular Na+. with NMDG(+), a membrane-impermeable cation, h ad no significant effect on duration, thus suggesting that Na+/Ca2+ exchang ers 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 thapsiga rgin (TG). This decrease was blocked by orthovanadate, indicating that it w as caused by PMCPs rather than sarco/endoplasmic reticulum Ca-2+ pumps (SER CPs). 4 UTP and ATP gamma S also caused a decrease in [Ca2+](i) in cells pretreat ed with TG, although they were less effective than ATP. The effect of UTP i mplies the involvement of both P2Y(1) and P2Y(2) receptors, while the effec t of ATP gamma S implies no significant role of ectophosphorylation and ago nist 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+ homeos tasis after agonist stimulation.