The influence of theophylline on the motility and the cyclic 3',5'-AMP content in the pregnant and non-pregnant rat uterus in vitro.

The tension developed by muscle strips obtained from pregnant and non-pregnant rat uteri and uterine cyclic 3',5'-AMP was measured in the presence of increasing amounts of theophylline. In the pregnant uterus the initial tension was elevated by 112% and the tissue level of cAMP by 59% as compared to the normal organ. 1 mM theophylline relaxed the normal uterus by 81% and the pregnant uterus by 36% without any change in the cAMP concentrations. Higher amounts of theophylline (3-10 mM) able to relax the pregnant uterus completely also increased uterine cAMP in a dose dependent manner. It seems likely that the effect of high amounts of theophylline is mediated by cAMP, which, however, is not involved in the action of low (therapeutic) doses of the drug.