Enhancement of the response to purinergic agonists in P2Y(1) transfected 1321N1 cells by antagonists suramin and PPADS

1 We have previously shown that both suramin and pyridoxal-phosphate-6-azophenyl-2', 4' disulphonic acid (PPADS) act as antagonists at transfected P2Y(1) receptors. Here we show that under certain experimental conditions these two P2 antagonists can enhance the response to agonists acting at these receptors. 2 The expression of either P2Y(1) or P2Y(2) receptors in 1321N1 human astrocytoma cells results, on a change of medium, in an elevation of basal (no added agonist) accumulation of [H-3]-inositol(poly)phosphates([H-3]-InsP(x)) compared to cells not expressing these receptors. This elevation is much greater in P2Y(1) transfectants than in P2Y(2) transfectants. 3 Both PPADS and suramin reduced this basal level of [H-3]-InsP(x) accumulation in the P2Y(1) expressing cells. 4 When a protocol was used which required changing the culture medium, antagonists were added at a concentration which reduced the basal accumulation by about 50%, there was a significant stimulation in response to increasing concentrations of 2-methylthioadenosine 5'-triphosphate (2MeSATP), in the absence of antagonists there was no significant effect of the agonist. 5 However, when 2MeSATP was added in the absence of a change of medium and with no antagonist present, there was a several fold increase in [3H]-InsP(x) accumulation. These results show that a release of endogenous agonist activity (possibly ATP/ADP) from the P2Y(1) expressing cells can create conditions in which a response to an agonist such as 2MeSATP can only be seen in the presence of a competitive antagonist.