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 μM. 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 μM 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 μM 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.