DUAL EFFECTS OF ATP ON PHOSPHATIDYLINOSITOL BREAKDOWN IN RAT HEPATOCYTE MEMBRANES

The mechanisms whereby adenosine-5'-triphosphate (ATP) regulates the inositol phospholipid-signalling system were studied in rat hepatocytes. Intact hepatocytes respond to extracellular ATP, adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S), ADP and weakly to guanosine-5'-triphosphate (GTP), but not to other purine nucleotides (GDP or AMP). This is consistent with the idea that a P2 purinergic receptor is coupled to the phosphatidylinositol metabolism in these cells. Partially purified plasma membranes prepared from myo-[H-3]inositol prelabelled hepatocytes exhibit a phosphatidylinositol-4,5-bisphosphate phospholipase C activity sensitive to ATP, ATP-gamma-S and guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S). Moreover the GTP-gamma-S effect is greatly enhanced by ATP and ATP-gamma-S. These potentiating effects differ according to the adenylnucleotide considered. ATP produces (1) an increase in the GTP-gamma-S-PLC sensitivity, (2) a potentiation of the phospholipase C (PLC) response induced by maximal dose of GTP-gamma-S, and (3) an increase in the inositol lipids pools. At variance, ATP-gamma-S, a nonhydrolysable analogue of ATP, only increases the PLC-sensitivity towards GTP-gamma-S. These results may signify that ATP stimulates inositol phosphate accumulation via at least two distinct mechanisms (i) a direct activation of a P2 purinergic receptor coupled to a PLC via a GTP binding protein and (ii) a stimulation of the phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinases which increased the pool of phospholipase C substrates.