Dual effect of adenosine on vascular smooth muscle [(3)H]-thymidine DNA labeling: receptor-mediated modulation of DNA synthesis and inhibition of thymidine uptake.

This study examined the contribution of cAMP signaling to the modulation of vascular smooth muscle cell (VSMC) proliferation by adenosine. At a concentration of 1 mM, adenosine inhibited [(3)H]-thymidine uptake, measured as the initial rate of isotope influx, by 10-fold. Diminution of [(3)H]-thymidine uptake by adenosine was independent of the presence of A(1)- and A(2)-receptor antagonists, indicating that adenosine competes with thymidine for plasma membrane transporter-binding sites. Considering these results, in order to estimate [(3)H]-thymidine DNA labeling, VSMCs were preincubated with adenosine for 48 h, and adenosine was then omitted during the subsequent 2 h of incubation in [(3)H]-thymidine-containing medium. In serum-depleted VSMCs, preincubation with 100 microM or 1,000 microM adenosine augmented DNA synthesis by approximately 6- and 3-fold, respectively, whereas the increment of DNA synthesis triggered by serum was decreased in the presence of adenosine by 20-30%. Both cAMP production and inhibition of DNA synthesis by adenosine in serum-supplied cells were independent of the presence of the A(1)-antagonist 1,2-dipropyl-8-cyclopentylxanthine (DPCPX), but were abolished by the A(2)-antagonist 1,3-dimethyl-7-propylxanthine (DMPX). In contrast, the activation of DNA synthesis in serum-depleted cells by adenosine was decreased in the presence of DPCPX and DMPX by approximately 30 and 40%, respectively. Both in serum-supplied and -depleted VSMCs, dose-dependent elevation of cAMP production with an adenylate cyclase activator, forskolin, reduced DNA synthesis by up to 40-60%. Thus, our results show that in addition to suppressing thymidine uptake, adenosine depresses the DNA synthesis triggered by serum-derived growth factors and stimulates DNA synthesis in serum-depleted cells. These data also suggest that the inhibition of DNA synthesis is mediated by cAMP production where the activation of DNA synthesis is independent of cAMP signaling.