8-phenyltheophylline: a potent P1-purinoceptor antagonist.

8-Phenyltheophylline was more potent than theophylline in antagonizing the inhibitory effects of adenosine in guinea-pig driven left atrium, rabbit basilar artery and electrically stimulated guinea-pig ileum preparations. In guinea-pig atrium and ileum, the antagonism of adenosine responses by the methylxanthines is of a competitive nature, but in rabbit basilar artery it does not appear to be so. In all three preparations, the effects of theophylline and 8-phenyltheophylline could be reversed by washing. It is concluded that 8-phenyltheophylline is a more potent P1-purinoceptor antagonist than theophylline.