EXTRACELLULAR ATP STIMULATES ADENYLYL-CYCLASE AND PHOSPHOLIPASE-C THROUGH DISTINCT PURINOCEPTORS IN NG108-15 CELLS

In neuroblastoma x glioma hybrid NG108-15 cells, ATP induced a concent ration-dependent increase in the intracellular Ca2+ concentration ([Ca 2+](i)), accompanied by inositol phosphate formation. Under the same c onditions, we found a marked increase in cAMP levels produced by ATP a t concentrations similar to those required to increase [Ca2+](i). The Ca2+ ionophore A23187 or bradykinin, which evoked inositol phosphate f ormation and increases in [Ca2+](i), did not increase, and instead sli ghtly decreased, cAMP content, indicating that ATP-induced cAMP accumu lation was not due to activation of Ca2+- sensitive adenylyl cyclase. The effect of ATP on cAMP production was not dependent on generation o f adenosine caused by ATP hydrolysis. Among several P-2 purinoceptor a gonists, adenosine-5'-O-(3-thio)triphosphate, 5'-adenylylimidodiphosph ate, and adenosine-5'-O-(2-thio)diphosphate evoked both cAMP accumulat ion and Ca2+ mobilization. In contrast, beta,gamma-methylene-ATP selec tively elicited cAMP accumulation, whereas 2-methylthio-ATP and UTP in duced only Ca2+ mobilization, without affecting cAMP levels. The poten t P-2x purinoceptor agonist alpha,beta-methylene-ATP did not induce cA MP accumulation or Ca2+ mobilization, The cAMP accumulation induced by ATP was not affected by the P-2 receptor antagonist suramin but was i nhibited by P-1 receptor antagonists such as 8-(p-sulfophenyl)theophyl line, 3-isobutyl-1-methylxanthine, and xanthine amine congener. Howeve r, the ATP-induced increase in [Ca2+](i) was not affected by suramin o r xanthine amine congener. Taken together, these results indicate that ATP activates two distinct purinoceptors that are coupled to differen t signal transduction systems, one being adenylyl cyclase and the othe r phospholipase C, in NG108-15 cells. Furthermore, pharmacological pro files of the adenylyl cyclase-coupled receptor were quite different fr om those of any known purinoceptor subtypes, especially in the unusual sensitivity of the receptor to P-1 and P-2 receptor agonists acid ant agonists. It is therefore suggested that ATP-induced cAMP accumulation may be mediated by a novel subtype of purinoceptor in NG108-15 cells.