EXTRACELLULAR ATP TRIGGERS 2 FUNCTIONALLY DISTINCT CALCIUM SIGNALING PATHWAYS IN PC12 CELLS

We have investigated the effects of extracellular ATP on Ca2+ signalling, and its relationship to secretion in rat pheochromocytoma (PC12) cells. In single cells, extracellular ATP evoked two very distinct subcellular distributions of intracellular calcium concentration ([Ca2+](i)), only one of which could be mimicked by the pyrimidine nucleotide UTP, suggesting the involvement of more than one cell surface receptor in mediating the ATP-induced responses. ATP and UTP were equipotent in activating a receptor leading to inositol phosphate production and the mobilisation of intracellular Ca2+. In some cells (19%) this rise in [Ca2+](i) initiated at a discrete site and then propagated across the cell in the form of a Ca2+ wave. In addition to mobilising intracellular Ca2+ through a 'nucleotide' receptor sensitive to ATP and UTP, the results indicate that ATP also activates divalent cation entry through an independent receptor-operated channel. Firstly, ATP-induced entry of Ca2+ or Mn2+ was independent of Ca2+ mobilisation, as prior treatment of cell populations with UTP abolished the ATP-evoked release of intracellular Ca2+ stores, but left the Ca2+- and Mn2+-entry components uneffected. Secondly, although UTP and ATP were equally effective in generating inositol phosphates, only ATP stimulated divalent cation entry, indicating that ATP-activated influx was independent of phosphoinositide turnover. Thirdly, single cell experiments revealed a subpopulation of cells that responded to ATP with divalent cation entry without mobilising Ca2+ from intracellular stores. Lastly, the dihydropyridine antagonist, nifedipine, reduced the ATP-induced rise in [Ca2+](i) by only 24%, suggesting that Ca2+ entry was largely independent of L-type voltage-operated Ca2+ channels. The Ca2+ signals could also be distinguished at a functional level, Activation of ATP-induced divalent cation influx was absolutely required to evoke transmitter release, because ATP triggered secretion of [H-3]dopamine only in the presence of external Ca2+, and UTP was unable to promote secretion, irrespective of the extracellular [Ca2+]. The results suggest that the same extracellular stimulus can deliver different Ca2+ signals into the same cell by activating different Ca2+ signalling pathways, and that these Ca2+ signals can be functionally distinct.