Serotonin-induced contractile and structural changes in fast and slow skeletal muscles in mice

The effects of serotonin on the contractile properties of the soleus and extensor digitorum longus muscles in mice were studied after chronic intraperitoneal administration with and without a recovery period, and compared to contractile studies of the same muscles performed after acute administration of serotonin. A curarized in vitro preparation of both muscles was used throughout for the contractile studies. In addition, structural changes were studied in the chronic preparations. Chronic serotonin administration produced histologic changes in type 1 and 2A fibers and a reduction in twitch and tetanic tension and the rate of twitch and tetanic tension development limited to the slow (aerobic) soleus muscle. Acute in vitro administration of serotonin altered the contractile properties of both soleus and extensor digitorum longus muscles with the latter being more involved. The effects of chronic serotonin administration appear to be primarily related to ischemia while the effects of acute administration appear to be mediated by direct, nonvascular action on skeletal muscle.