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The potency of 8-phenyltheophylline as an antagonist at A1 adenosine receptors in guinea-pig atria and at A2 adenosine receptors in the guinea-pig aorta has been investigated. 8-Phenyltheophylline was an apparently competitive antagonist of the negative chronotropic effect of adenosine, 2-chloroadenosine, L-N6-phenyl-isopropyl adenosine (L-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) on atria and of the relaxant effect of adenosine, 2-chloroadenosine and NECA on the aorta. The pA2 values for 8-phenyltheophylline ranged from 6.4 to 6.6 and were not significantly different, irrespective of the agonist or tissue used. These results indicate that 8-phenyltheophylline is a relatively potent antagonist at adenosine receptors but does not exhibit selectivity for either of the putative sub-types in isolated tissues.

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