Background Because proliferation of cardiac fibroblasts participates in cardiac hypertrophy/remodeling associated with hypertension and myocardial infarction, it is important to elucidate factors regulating cardiac fibroblast proliferation. Adenosine, a nucleoside abundantly produced by cardiac cells, is antimitogenic vis-g-vis vascular smooth muscle cells; however, the effect of adenosine on cardiac fibroblast proliferation is unknown. The objective of this study was to characterize the effects of exogenous and endogenous (cardiac fibroblast-derived) adenosine on cardiac fibroblast proliferation. Methods and Results Growth-arrested cardiac fibroblasts were stimulated with 2.5% FCS in the presence and absence of adenosine, 2-chloroadenosine (stable adenosine analogue), or modulators of adenosine levels, including (1) erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA; adenosine deaminase inhibitor); (2) dipyridamole (adenosine transport blocker); and (3) iodotubericidin (adenosine kinase inhibitor). All of these agents inhibited, in a concentration-dependent manner, FCS-induced cardiac fibroblast proliferation as assessed by DNA synthesis ([H-3]thymidine incorporation) and cell counting. EHNA, dipyridamole, and iodotubericidin increased extracellular levels of adenosine by 2.3- to 5.6-fold when added separately to cardiac fibroblasts, and EHNA+ iodotubericidin or EHNA + iodotubericidin + dipyridamole increased extracellular adenosine levels by >690-fold. Both KF17837 (selective A(2) antagonist) and DPSPX (nonselective A(2) antagonist) but not DPCPX (selective A(1) antagonist) blocked the antimitogenic effects of 2-chloroadenosine, EHNA, and dipyridamole on DNA synthesis, suggesting the involvement of A(2A) and/or A(2B) but excluding the participation of A(1) receptors. The lack of effect of CGS21680 (selective A(2A) agonist) excluded involvement of A(2A) receptors and suggested a major role for A(2B) receptors. This conclusion was confirmed by the rank order potencies of four adenosine analogues. Conclusions Cardiac fibroblasts synthesize adenosine,and exogenous and cardiac fibroblast-derived adenosine inhibits cardiac fibroblast proliferation via activation of A(2B) receptors. Cardiac fibroblast-derived adenosine may regulate cardiac hypertrophy and/or remodeling by modulating cardiac fibroblast proliferation.
