Abstract-Adenosine is a vascular endothelial cell mitogen, but anti-mitogenic for aortic smooth muscle cells and fibroblasts when acting via the A 2B adenosine receptor. However, we show that adenosine increases porcine coronary artery smooth muscle cell (CASMC) number, cellular DNA content, protein synthesis, and PCNA staining. RT-PCR analysis indicates that porcine CASMC express A 1 , A 2A , A 3 , and barely detectable levels of A 2B receptor mRNAs. The mitogenic effect of adenosine is mimicked by NECA, CCPA, and R-PIA, but not by CGS21680 and 2-Cl-IB-MECA, and is inhibited by DPCPX, indicating a prominent role for the A 1 receptor. This interpretation is supported by the finding that adenosine-and CCPA-induced DNA synthesis is significantly inhibited by pertussis toxin, but substantially potentiated by PD81723, an allosteric enhancer of the A 1 receptor. When a cDNA encoding the porcine A 1 receptor was cloned and expressed in COS-1 cells, A 1 receptor pharmacology is confirmed. Anti-sense oligonucleotides to the cloned sequence dramatically suppress the mitogenic effect of adenosine and CCPA. Conversely, over-expression of the cloned A 1 receptor in CASMC increases adenosine-and CCPA-induced DNA synthesis. Furthermore, stimulation with adenosine or CCPA of intact coronary arteries in an organ culture model of vascular disease increases cellular DNA synthesis, which was abolished by DPCPX. We conclude that adenosine acts as a novel mitogen in porcine CASMC that express the A 1 adenosine receptor, possibly contributing to the development of coronary artery disease. Key Words: adenosine receptors Ⅲ coronary artery smooth muscle cells Ⅲ proliferation Ⅲ molecular cloning Ⅲ porcine T he diverse cellular actions of adenosine are mediated by a family of adenosine receptors (ARs), of which 4 subtypes (A 1 R, A 2A R, A 2B R, and A 3 R) have been cloned and pharmacologically characterized. 1,2 In general, the A 1 R and A 3 R are coupled with G i/o proteins, whose activation causes a decrease of intracellular cAMP. In contrast, activation of the G s -coupled A 2A R and A 2B R increases intracellular cAMP. 1,2 Thus, the end biological action of adenosine in a particular organ or cell population may depend on the relative expression level and signaling efficiency of the individual AR subtypes. 3 Adenosine, like many other vasodilators, historically has been thought to act as an inhibitor of the proliferation of vascular smooth muscle cells (VSMCs). This contention was supported by recent studies in cultured aortic VSMCs, showing that adenosine was antimitogenic through its activation of the A 2B R. 4 -8 To the best of our knowledge, however, current studies of the long-term effects of adenosine on VSMC proliferation have been limited to aortic smooth muscle.Given the heterogeneity of VSMCs 9 and the diversity of the expression profile of ARs in cells from different blood vessels, 10 it remains to be shown whether the antiproliferative action of adenosine can be extended to other VSMCs, particularly coronary artery smo...