PSU-PURINERGIC RECEPTOR ACTIVATION MEDIATES INHIBITION OF CAMP ACCUMULATION IN CULTURED RENAL MESANGIAL CELLS

Extracellular ATP has been reported to exert mitogenic and contractile effects on cultured renal mesangial cells (MCs). Since it is possible that these actions involve changes in the cAMP second messenger syste m, we examined the effect of extracellular nucleotides on the accumula tion of cAMP in rat MCs. ATP, UTP and adenosine 5'O-(3-thio)triphospha te (ATP gamma S) (100 mu M) had no significant effects on baseline cAM P levels, but inhibited forskolin-stimulated accumulation of cAMP by 2 1-75% in the presence of the phosphodiesterase inhibitor 3-isobutyl-1- methylxanthine (IBMX). Maximal inhibitory effects were observed at 100 mu M of ATP gamma S with a threshold dose of 1 mu M. ATP gamma S, ATP and UTP were the most potent inhibitors indicating stimulation of the P2u receptor. The P2x agonists adenosine 5'-(alpha,beta-methylene) tr iphosphate and adenosine 5'-(beta,gamma-methylene) triphosphate, and t he P2y agonist 2-methylthio-ATP did not affect cAMP accumulation. Trea tment with the P2 receptor antagonist suramin (200 mu M) reduced the i nhibition by 58%. The inhibitory effects of the nucleotides were signi ficantly attenuated by preincubation with pertussis toxin (10-100 ng/m l). Inhibition of phospholipase C and protein kinase C did not prevent the inhibitory effect of the nucleotides. Inhibitors of forskolin-sti mulated cAMP accumulation had different effects on DNA synthesis in cu ltured MCs as measured by H-3-thymidine uptake at 48 h: ATP, ATP gamma S and the inhibitor of adenylyl cyclase, SQ 22536, stimulated DNA syn thesis in MCs, while UTP showed no significant mitogenic effect. Agent s which increased baseline levels of intracellular cAMP (forskolin, IB MX, dibutyryl-cAMP) significantly diminished DNA synthesis in MCs. The results indicate that the P2u-purinergic receptor mediates inhibition of forskolin-induced cAMP accumulation which is likely due to inhibit ion of adenylyl cyclase. This effect appears to be partially mediated by PTX-sensitive G proteins. While the increase in cAMP accumulation i s anti-mitogenic, inhibition of cAMP accumulation by P2u receptors is not correlated with MC growth control. Thus, additional mechanisms oth er than inhibition of cAMP accumulation by P2u receptors are likely to be involved in the mitogenesis of extracellular ATP.