Are methylxanthine effects due to antagonism of endogenous adenosine

Caffeine, theophylline and other methylxanthines have a large number of effects, some of which are clinically important. The effects of these compounds are commonly ascribed to an inhibitor of cyclic-AMP breakdown. In cases where this mechanism fails to provide an adequate explanation for the observed actions a direct action on calcium metabolism is commonly invoked. In the following review I will present some of the increasing evidence that adenosine may be physiological modulator of several biological functions and that theophylline in low doses antagonizes the actions of this purine nucleoside. It is concluded that in some instances the question posed in the title could have a positive answer.