Exogenous and Endogenous Adenosine Inhibits Fetal Calf Serum–Induced Growth of Rat Cardiac Fibroblasts

Abstract

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-à-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 ([ ³ H]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.