Mechanisms of Rise and Decay of ADP-Evoked Calcium Signal in MDCK Cells

We have studied the effects of ADP on intracellular calcium levels ([Ca ²⁺ ]j) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. ADP evoked [Ca ²⁺ ] _i rises dose-dependently by releasing Ca ²⁺ from the inositol 1,4,5-trisphosphate (IP ₃ )-dependent Ca ²⁺ pool followed by capacitative Ca ²⁺ entry. The Ca ²⁺ signal consisted of a peak and a gradual decline which reached a plateau in the case of 0.1-1 mM ADP; a plateau was not seen in the response to 10 mM ADP. ADP acted by stimulating P _2u and P _2y receptors based on rank order of agonist potency: ATP = UTP > ADP > 2-methylthio-ATP > 2-methylthio-ADP > adenosine > α,β-methylene ATP. Buffering by lysosomes and efflux via plasmalemmal Ca ²⁺ pumps might play important roles in the decay of the Ca ²⁺ signal. The Ca ²⁺ signal was dramatically inhibited by 100 μM LaCl ₃ .