Cardiac fibroblasts express the cAMP-adenosine pathway

The extracellular "cAMP-adenosine pathway" refers to the local production o f adenosine mediated by cAMP egress into the extracellular space, conversio n of cAMP to AMP by ectophosphodiesterase, and the metabolism of AMP to ade nosine by ecto-5'-nucleotidase. The goal of this study was to assess whethe r the cAMP-adenosine pathway limits cardiac fibroblast growth. Studies were conducted in ventricular cardiac fibroblasts maintained in 3-dimensional c ultures. Addition of exogenous cAMP to cardiac fibroblasts increased extrac ellular levels of AMP, adenosine, and inosine in a concentration-dependent and time-dependent manner. This effect was attenuated by blockade of total phosphodiesterase activity (3-isobutyl-1-methylxanthine), ectophosphodieste rase activity (high concentration of 1,3-dipropyl-8-p-sulfophenylxanthine), or ecto-5'-nucleotidase (alpha, beta-methylene-adenosine-5'-diphosphate). Treatment with exogenous cAMP inhibited cell growth as assessed by DNA synt hesis (H-3-thymidine incorporation), cell proliferation (cell counts), and protein synthesis (H-3-leucine incorporation). Antagonism of A(2) (KF17837) or A(1)/A(2) (low concentration of 1,3-dipropyl-8-p-sulfophenylxanthine), but not A(1) (8-cyclopentyl-1,3-dipropylxanthine), adenosine receptors bloc ked the growth-inhibitory effects of exogenous cAMP, but not the growth inh ibitory effects of 8-bromo-cAMP (stable cAMP analogue). The growth-inhibito ry effects of exogenous cAMP were enhanced by the combined inhibition of ad enosine deaminase [erythro-9-(2-hydroxy-3-nonyl) adenine] and adenosine kin ase (iodotubercidin). In conclusion, the extracellular cAMP-adenosine pathw ay exists in cardiac fibroblasts and attenuates cell growth. Pharmacologica l augmentation of this pathway could abate pathological cardiac remodeling in heart disease.