Mechanisms of rise and decay of ADP-evoked calcium signal in MDCK cells.

We have studied the effects of ADP on intracellular calcium levels ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. ADP evoked [Ca2+]i rises dose-dependently by releasing Ca2+ from the inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ pool followed by capacitative Ca2+ entry. The Ca2+ 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 P2u and P2y receptors based on rank order of agonist potency: ATP = UTP > ADP > 2-methylthio-ATP > 2-methylthio-ADP > adenosine > alpha, beta-methylene ATP. Buffering by lysosomes and efflux via plasmalemmal Ca2+ pumps might play important roles in the decay of the Ca2+ signal. The Ca2+ signal was dramatically inhibited by 100 microM LaCl3.