On the mechanism of relaxation of tracheal muscle by theophylline and other cyclic nucleotide phosphodiesterase inhibitors.

The mechanism of action of theophylline was studied by investigating the relationship between relaxant effect and inhibition of cyclic nucleotide phosphodiesterase (PDE) and by studying interactions with adenosine actions. Guinea pig tracheal smooth muscle cyclic AMP PDE had two apparent Kms′: 0.4 and 70 μM for cyclic AMP. Theophylline and papaverine competetively inhibited the low Km form. Hydrolysis of 2.0 μM cyclic AMP and cyclic GMP was inhibited by several drugs. Some agents (e.g. ZK 62 711, ICI 63, 197, Ro 20-1724, dipyridamol) were considerably more potent as inhibitors of cyclic AMP than of cyclic GMP hydrolysis, while other agents (M & B 22.948 and dilazep) selectively inhibited cyclic GMP breakdown, and some (theophylline, papaverine, IBMX and SQ 20,006) showed little selectivity. There was a weak but significant correlation between inhibition of cyclic AMP phosphodiesterase and relaxation of tracheal smooth muscle in vitro. There was also a correlation between the ratio of IC25 cyclic AMP/IC25 cyclic GMP and the smooth muscle relaxation, indicating that inhibition of cyclic AMP rather than cyclic GMP hydrolysis determined relaxation. However, there was a marked tachyphylaxis to the relaxant effect of the cyclic AMP selective PDE-inhibitors, while the nonselective methylxanthines did not show tachyphylaxis. The effect of theophylline was antagonized by low concentrations of adenosine, which by itself caused a weak tracheal contraction. The effect of PDE-inhibitors can be partly explained by decreased cyclic AMP breakdown but other mechanisms, such as antagonism of endogenous adenosine, may contribute to the observed relaxant action.