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

Abstract

We have studied the effects of La 3+ on ATP-evoked rises in intracellular calcium levels ([Ca 2+] i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. ATP evoked [Ca 2+] i rises dose-dependently with an EC 50 of 2.5 μM. The trigger for the Ca 2+ signal was a release of Ca 2+ from the inositol-1,4,5-trisphosphate (IP 3)-sensitive stores because the signal was completely blocked by pretreatment with the endoplasmic reticulum (ER) Ca 2+ 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 Ca 2+ signal was reduced by approximately 50% by extracellular Ca 2+ removal, suggesting that ATP induced capacitative Ca 2+ entry. La 3+ inhibited the ATP-evoked Ca 2+ signal dose-dependently when added before or after ATP. Pretreatment of 0.1 mM La 3+ inhibited approximately 90% of the Ca 2+ signal induced by 10 μM ATP. The mechanisms underlying the La 3+ inhibition appear to involve not only block of capacitative Ca 2+ entry but also interference with ATP binding to the ATP receptors.