Effects of increasing the frequency of twitches and of isoprenaline on maximal twitches and cyclic AMP levels in slow‐ and fast‐contracting cat skeletal muscles

Increasing the frequency of twitches and treatment with isoprenaline have been compared for effects on twitch tension, tension-time integral and cyclic adenosine 3', 5'-monophosphate (cyclic AMP) levels in the slow-contracting soleus muscle of cats, anaesthetized with chloralose and pentobarbitone. The effect of change in frequency of contractions on cyclic AMP in the fast-contracting extensor digitorum longus muscle was also examined. Although both isoprenaline and increasing the frequency of contractions depressed twitch tension in the soleus, only isoprenaline enhanced cyclic AMP levels. The effects of isoprenaline were independent of the existing frequency of contractions of the muscle. Increasing the frequency of contractions enhanced twitches in the extensor digitorum longus muscle but did not change cyclic AMP levels. It is concluded that cyclic AMP may mediate effects of beta-adrenoceptor agonists but not those caused by increasing the frequency of contractions on slow- or fast-contracting skeletal muscles.