Endogenous cyclic AMP-adenosine pathway regulates cardiac fibroblast growth

Our previous studies show that cardiac fibroblasts express the extracellula r "cAMP-adenosine pathway," that is, the generation of adenosine from extra celluar cAMP. The goal of this study was to assess whether activation of th e cAMP-adenosine pathway by stimulation of endogenous cAMP synthesis regula tes cardiac fibroblast growth. Cardiac fibroblasts in 3D cultures were used as the model system. Treatment of cardiac fibroblasts with forskolin, isop roterenol, or norepinephrine increased cAMP production and extracellular le vels of adenosine, and these effects were prevented by inhibition of adenyl yl cyclase (2',5'-dideoxyadenosine). Treatment with forskolin, isoprotereno l, or norepinephrine for 24 hours inhibited DNA synthesis (H-3-thymidine in corporation), and this effect was enhanced by combined inhibition of adenos ine deaminase (erythro-9-[2-hydroxy-3-nonyl] adenine) plus adenosine kinase (iodotubercidin). Inhibition of adenylyl cyclase or adenosine receptors (1 ,3-dipropyl-8-p-sulfophenylxanthine or KF17837) prevented the effects of fo rskolin, isoproterenol, and norepinephrine on DNA synthesis. Forskolin also inhibited protein synthesis (H-3-leucine incorporation) and cell prolifera tion, and these effects were blocked by adenosine receptor antagonism. Trea tment of cardiac fibroblasts with norepinephrine for > 48 hours but not < 4 8 hours increased DNA synthesis, protein synthesis, and cell number. Howeve r, blockade of adenylyl cyclase or antagonism of adenosine receptors caused norepinephrine to induce proliferation in < 48 hours. Our findings indicat e that the endogenous cAMP-adenosine pathway regulates cardiac fibroblast g rowth.