THE PHOSPHOLIPASE-C ACTIVATING P-2U PURINOCEPTOR ALSO INHIBITS CYCLICAMP FORMATION IN DDT1 MF-2 SMOOTH-MUSCLE CELLS

The P-2U purinoceptor mediated effect on cellular cAMP was investigate d in DDT1 MF-2 smooth muscle cells. Stimulation of these receptors by ATP or UTP caused a pronounced decrease of about 50% in cellular cAMP levels in forskolin or isoprenaline pretreated cells. This action of t he nucleotides was concentration dependent with an IC50 of 9.4 +/- 0.2 mu M and 29.0 +/- 0.5 mu M for UTP and ATP, respectively and was inhi bited by the P-2-purinoceptor antagonist suramin. The cAMP level appea red to be modified by intracellular Ca2+, represented by an initial de cline in cAMP. Neither inactivation of protein kinase C by staurospori ne nor elevated cytoplasmic Ca2+ concentrations interfered with the su stained decrease in cAMP levels induced by ATP or UTP, showing that th is effect is not mediated via the phospholipase C pathway known to be activated after P-2U purinoceptor stimulation in DDT1 MF-2 cells. Pert ussis toxin inhibited the action of these nucleotides on the cellular cAMP level. It can be concluded that the P-2U purinoceptor in DDT1 MF- 2 cells is coupled to different G-proteins, activating phospholipase C and inhibiting adenylyl cyclase activity.