Methylxanthines antagonize adenosine but not morphine inhibition in guinea pig ileum

In the electrically stimulated guinea pig ileum longitudinal muscle strip preparation the inhibitory actions of adenosine were antagonized by various concentrations of theophylline (12.5 and 100 micro M) and 8-phenyltheophylline (4 and 20 micro M). However, the inhibitory actions of morphine were not antagonized by the same concentration of theophylline and 8-phenyltheophylline. These results suggest that in the guinea pig ileum longitudinal muscle-strip adenosine is not a mediator in the inhibitory actions of morphine.