Cellular release of and response to ATP as key determinants of the set-point of signal transduction pathways

The determinants of "basal" activity of signaling pathways regulating cellu lar responses are poorly defined. One possibility is that cells release fac tors to establish the set-point of such pathways. Here we show that treatme nt of Madin-Darby canine kidney cells with the nucleotidase apyrase decreas es basal arachidonic acid release and cAMP production 30-40% and that inhib itors of P2Y receptor action also affect basal and forskolin-stimulated cAM P accumulation. Changing medium prominently increases extracellular levels of ATP in Madin-Darby canine kidney, COS-7, and HEK-293 cells. Mechanical s timulation of ATP release likely occurs in virtually every experimental pro tocol with cultured cells, implicating such release and P2Y receptor activa tion as critical in establishing the set-point for signal transduction path ways.