Vol. 290, Issue 2, 817-824, August 1999

Amplification of the Cyclic AMP Response to Forskolin in Pheochromocytoma PC12 Cells through Adenosine A_2A Purinoceptors¹

Chiara Florio, Fabiana Frausin, Rodolfo Vertua and Rosa Maria Gaion

Department of Biomedical Sciences, Faculty of Pharmacy, Trieste, Italy

In this study, we present evidence on the ability of endogenous adenosine to modulate adenylyl cyclase activity in intact PC12 cells. The adenosine receptor antagonists PD 115199, xanthine amine congener, 8-cyclopentyl-1,3-dipropylxanthine, 8-(p-sulfophenyl)theophylline, and 3,7-dimethyl-1-propargylxanthine inhibited 10 µM forskolin-induced cyclic AMP (cAMP) accumulation, with IC₅₀ values of 2.76 ± 1.16 nM, 17.4 ± 1.08 nM, 443 ± 1.03 nM, 2.00 ± 1.01 µM, and 2.25 ± 1.05 µM, respectively. Inhibition by 2.5 nM PD 115199 was only partially reversed by increasing forskolin concentrations up to 100 µM. The addition of PD 115199 with or 60 min after forskolin caused a comparable inhibition of forskolin effect over the next hour. Both exogenous adenosine (0.1 µM) and its precursor, AMP (10 and 100 µM), significantly enhanced forskolin-induced cAMP accumulation, whereas inosine was ineffective. Forskolin activity was also potentiated by the hydrolysis-resistant adenosine receptor agonists 5'-N-ethylcarboxamido adenosine and CGS 21680 (8.9- and 12.2-fold increase, respectively). Adenosine deaminase (1 U/ml) and 8-SPT (25 µM), which nearly abolished the response to 1 µM adenosine, also reduced cAMP accumulation caused by AMP (78 and 54%, respectively). These results demonstrate that in PC12 cells, activation of adenylyl cyclase by forskolin is highly dependent on the occupancy of A_2A adenosine receptors and that AMP potentially contributes to the amplification of forskolin response.