Adenosine analogues stimulate cyclic AMP-accumulation in cultured neuroblastoma and glioma cells

The effect of three stable adenosine analogues, L-phenylisopropyl-adenosine (L-PIA), 2-chloroadenosine, and adenosine 5'-ethylcarboxamide (NECA), on cyclic AMP accumulation was studied in five different cell lines derived from the nervous system. In N18-neuroblastoma cells, with cholinergic properties, all three analogues caused an increased accumulation of cyclic AMP with the following relative order of potency: NECA greater than 2-chloroadenosine greater than L-PIA. The half maximal effect of NECA was obtained at close to 10(-8) M concentration. In the two other neuroblastoma cell lines, 41A3 with cholinergic and NIE115 with adrenergic properties, the two analogues NECA and PIA had similar effects. In glioma C6 and 138 MG cells NECA was also found to be more potent that PIA in elevating cyclic AMP levels. However, the absolute potency of NECA in these cell lines was almost 100 times lower. Phosphodiesterase (PDE) activity in crude homogenates of the five cell lines showed essentially similar Km and Vmax, with the exception that the three neuroblastoma cell lines showed biphasic, the glial cell lines monophasic Eadie-Hofstee plots. Theophylline was equally potent as an inhibitor of PDE in all cell lines, but the non-xanthine, inhibitor rolipram, was more potent against neuroblastoma than glial cell PDE. These results indicate that all five cell lines have adenosine receptors of the A2-subtype. However, the apparent affinity of the adenosine analogues to these receptors was markedly different between the neuroblastoma and glial cell lines. The absolute potency of adenosine analogues may be a poor criterion to classify adenosine receptors, into A1 and A2 subtypes, especially when intact cells are used.