Agonist-induced desensitization of ATP receptor-mediated phosphoinositide turnover in C6 glioma cells: Comparison with the negative-feedback regulation by protein kinase C

In C6 glioma cells, ATP increased 3H-inositol phosphate (IP) accumulation in a dose-dependent manner. Preincubation of cells with ATP (100 μM or 1 mM) resulted in a time-dependent loss of the ability of ATP to stimulate phosphoinositide (PI) hydrolysis. The agonist-induced desensitization of ATP-stimulated PI hydrolysis developed rapidly, and appeared to be independent on the activation of protein kinase C (PKC). Thus, PKC inhibitors (staurosporine, H-7 and polymyxin B), depletion of PKC and diacylglycerol (DG) kinase inhibitors (R59002, R59949) had no effect on the homologous desensitization. ATP-induced PI breakdown was inhibited by a 10 min pretreatment with the PKC activator, phorbol 12-myristate 13-acetate (PMA) or octylindolactam V, with a comparable IC50 of 5 nM, but was unaffected by the biologically inactive 4-α-phorbol 12,13-didecanoate (4α-PDD). The inhibition caused by PMA and octylindolactam V was completely prevented by staurosporine (1 μM) and partially prevented by H-7 (300 μM), H-8 (300 μM) and polymyxin B (300 μg/ml). In addition, PKC activator-induced inhibition was unchanged after ATP pretreatment, but disappeared after PKC depletion. The IP formation elicited by NaF was inhibited by PMA and octylindolactam V with a comparable IC50 value of 7.5 nM while was unchanged after ATP pretreatment. These results indicate that ATP receptors are present in the C6 glioma cells, and that these receptors are coupled to PI turnover and undergo homologous desensitization. The agonist-induced desensitization, unlike the negative-feedback regulation caused by PMA and octyl-indolactam V, does not seem to involve PKC activation. ? 1993.