P2-PURINERGIC RECEPTORS IN HUMAN BREAST-TUMOR CELLS - COUPLING OF INTRACELLULAR CALCIUM SIGNALING TO ANION SECRETION

ATP increases intracellular Ca++ ([Ca++]i) by activating different P2- purinoreceptors. Because ATP increases Cl- secretion in cystic fibrosi s (CF)-affected epithelia, the current study was designed to establish the link between these two events. Studies were done in epithelial, h uman MCF-7 breast tumor cells in which the presence of mRNA transcript s encoding CF transmembrane conductance regulator was initially establ ished. Changes in [Ca++]i were measured in single cells by fluorescenc e microscopy: anion transport was measured by I-125 efflux. ATP stimul ated concentration-dependent increases in [Ca++]i and I-125 efflux fro m MCF-7 cells. The relative order of agonist potency of various select ive P2-purinoreceptor agonists in increasing [Ca++]i and I-125 efflux was: UTP greater-than-or-equal-to ATP > ADP = AMP; 2-chloro-ATP, 2-met hylthio-ATP and AMP-phencyclidine were considerably less potent than A TP. The Ca++ ionophore ionomycin increased both intracellular [Ca++]i and I-125 secretion. Exposing cells to the intracellular chelator ethy lene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA)-ac etoxymethylester decreased (AM) decreased ATP- and ionomycin-stimulate d I-125 efflux. Extracellular EGTA did not alter the Ca++ response to ATP, but inhibited the response to ionomycin. The chelator inhibited b oth ATP- and ionomycin-induced I-125 secretion. Exposure of cells to n ifedipine did not affect the responsiveness of MCF-7 cells to ATP. The anion transport antagonist 4,4'-diisothiocyananatostilbene-2,2'disulf onic acid partially inhibited ATP- and cationophore-stimulated increas es in [Ca++]i and I-125 secretion. The data suggest that activation Of P2 receptors in MCF-7 cells leads to an increase in anion transport a s a result of the ability of ATP to increase [Ca++]i; moreover, anion channel antagonists may produce their inhibitory effect on I-125 secre tion, in part, by blocking agonist-induced intracellular Ca++ signalin g.