Mechanism of lanthanum inhibition of extracellular ATP-evoked calcium mobilization in MDCK cells

We have studied the effects of La3+ on ATP-evoked rises in intracellular calcium levels ([Ca2+](i)) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. ATP evoked [Ca2+](i) rises dose-dependently with an EC50 of 2.5 mu M The trigger for the Ca2+ signal was a release of Ca2+ from the inositol-1,4,5-trisphosphate (IP3)-sensitive stores because the signal was completely blocked by pretreatment with the endoplasmic reticulum (ER) Ca2+ pump inhibitor thapsigargin (TG) or the phospholipase C (PLC) inhibitor U73122. Both the peak height and area under the curve of 10 mu M ATP-evoked Ca2+ signal was reduced by approximately 50% by extracellular Ca2+ removal, suggesting that ATP induced capacitative Ca2+ entry. La3+ inhibited the ATP-evoked Ca2+ signal dose-dependently when added before or after ATP. Pretreatment of 0.1 mM La3+ inhibited approximately 90% of the Ca2+ signal induced by 10 mu M ATP. The mechanisms underlying the La3+ inhibition appear to involve not only block of capacitative Ca2+ entry but also interference with ATP binding to the ATP receptors.