We have examined the mechanism of action of adenosine, a naturally occurring nucleoside that has profound effects on lymphocyte function. Adenosine (0.01 µM to 10 µM) increased lymphocyte cAMP levels in a dose-dependent fashion with a maximal (10 µM) increase of about 4-fold, whereas adenine, guanosine, and inosine had no effect on lymphocyte cAMP levels at concentrations of 100 µM. Adenosine appears to act on the cell surface since 1) 2-chloroadenosine, a poorly metabolized adenosine analogue, was as active as adenosine and 2) dipyridamole, which markedly inhibited [3H]-adenosine uptake by human lymphocytes, did not affect adenosine-induced accumulation of cAMP.

The specificity of the adenosine effect was established by showing that the methylxanthine derivatives, theophylline and 3-isobutyl-1-methylxanthine (IBMX), specifically block the accumulation of cAMP in lymphocytes induced by adenosine. Theophylline is a competitive inhibitor of the effect of adenosine, with an estimated dissociation constant of theophylline-receptor complex of about 6.3 × 10-7 M. The results suggest that adenosine increases the intracellular cAMP content of lymphocytes as a result of its interaction with a specific membrane receptor which results in the activation of adenylate cyclase.

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