Effects of isoprenaline, levodopa and a phosphodiesterase inhibitor (ICI 63,197) on cyclic AMP levels and contractions of soleus muscles in anesthetized cats.

1. Cyclic AMP levels have been determined in the soleus muscles of anaesthetized cats in the absence of drugs, and during depression of incomplete tetanic contractions produced by (-)-isoprenaline, ICI 63,197 (a phosphodiesterase inhibitor) or levodopa. 2. Cyclic AMP levels were elevated at the peak of tension depression produced by isoprenaline. Effects of isoprenaline on cyclic AMP and on contractions were dose dependent and statistically significantly related one to the other. Both effects were blocked by propranolol. 3. ICI 63,197 and levodopa produced isoprenaline-like effects on contractions but times to peak effect and recovery were longer. Cyclic AMP levels estimated during the depressant action were elevated. 4. The results support the involvement of cyclic AMP in the depressant effect of beta-adrenoreceptor agonists on slow-contracting mammalian skeletal muscle.