CYCLIC-AMP ADENOSINE PATHWAY INHIBITS VASCULAR SMOOTH-MUSCLE CELL-GROWTH

In this study we determined whether cAMP is metabolized to adenosine i n vascular smooth muscle cells and whether cAMP-derived adenosine modu lates vascular smooth muscle cell growth. Confluent smooth muscle cell s were exposed to cAMP (0.01 to 30 mu mol/L) in the presence and absen ce of 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/L; an inhibitor of bot h extracellular and intracellular phosphodiesterase), alpha,beta-methy leneadenosine 5'-diphosphate (AMP-CP, 100 mu mol/L; an ecto-5'-nucleot idase inhibitor), and 1,3-dipropyl-8-p-sulfophenyl-xanthine (DPSPX, 10 0 mu mol/L; a xanthine that can inhibit extracellular phosphodiesteras e) for 0 to 60 minutes. Medium was then sampled and assayed for AMP, a denosine, and inosine. cAMP increased the amount of AMP, adenosine, an d inosine in the medium in a time- and concentration-dependent manner. The conversion of cAMP to adenosine and inosine was inhibited by bloc kade of phosphodiesterase with IBMX, of ecto-phosphodiesterase with DP SPX, and of ecto-5'-nucleotidase with AMP-CP. To evaluate the physiolo gical relevance of cAMP-derived adenosine in vascular smooth muscle ce ll proliferation, we studied the inhibitory effects of cAMP (10(-4) mo l/L) and 8-bromo-cAMP (10(-4) mol/L) on fetal calf serum-induced DNA s ynthesis ([H-3]thymidine incorporation) in the presence and absence of erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA, an inhibitor of adenosin e deaminase), dipyridamole (a blocker of adenosine transport), KF17837 (a selective A(2) adenosine receptor antagonist), and DPSPX (a nonsel ective adenosine receptor antagonist). cAMP inhibited DNA synthesis, a nd both EHNA and dipyridamole enhanced this effect. Both KF17837 and D PSPX Significantly reduced the inhibitory effects of cAMP on DNA synth esis; however, they did not reduce the inhibitory effects of 8-bromo-c AMP on DNA synthesis. These results indicate that vascular smooth musc le cells metabolize cAMP to adenosine via the sequential action of ect o-phosphodiesterase and ecto-5'-nucleotidase and provide the first evi dence that cAMP-derived adenosine can inhibit vascular smooth muscle c ell growth. Hence, this cAMP-adenosine pathway may importantly contrib ute to the regulation of vascular biology.