Effects of protein kinase A activation on endothelin- and ATP-induced signal transduction

C₆ glioma cells possess endothelin ET_A receptor and P₂ purinoceptor coupled to two signaling pathways, i.e. phosphoinositide turnover and inhibition of adenylyl cyclase. In this study, the effects of raising cyclic AMP levels on the inositol phospholipid hydrolysis and adenylyl cyclase inhibition caused by endothelin-1 and ATP in C₆ glioma cells were examined. Pretreatment with cAMP generating agents (forskolin, isoproterenol and cholera toxin) or dibutyryl cAMP for 10 min-3 h did not affect the inositol phosphate accumulation caused by endothelin and ATP. Long-term (8–24 h) pretreatment with isoproterenol, forskolin, cholera toxin or dibutyryl cAMP resulted in a 40–50% inhibition of endothelin- and ATP-stimulated inositol phosphate accumulation, whereas the EC₅₀ values of endothelin and ATP were not affected. Consistent with the effects on endothelin and ATP, NaF-induced inositol phosphate formation was also inhibited by cAMP generating agents to a similar extent. Permeabilized cells from 24 h isoproterenol-or forskolin-pretreated C₆ cells also showed a diminished Ca²⁺-sensitivity of phosphoinositide-specific phospholipase C and also attenuated the potentiation response caused by GTPγS. The inhibitory effects on adenylyl cyclase by endothelin, ATP and 2-methylthio-ATP were unaffected by 24 h pretreatment with isoproterenol or forskolin. Long-term treatment with dibutyryl cGMP did not affect the two signaling pathways caused by ATP and endothelin. It is concluded that the phosphoinositide turnover, but not the adenylyl cyclase inhibition caused by endothelin and ATP in C₆ cells, was inhibited by protein kinase A-dependent pathway. The decreased phospholipase C activity is responsible for the inhibitory effect of protein kinase A-dependent pathway on agonist-induced phosphoinositide turnover in C₆ glioma cells.