Carnitine delays rat skeletal muscle fatigue in vitro

Carnitine has been used to enhance human exercise performance. To test the hypothesis that carnitine can directly modify skeletal muscle function, fatigue of isolated rat skeletal muscle strips was studied in vitro. Carnitine (10 mM) did not modify the initial force of soleus contraction. The time over which force declined by 50% during repetitive electrical stimulation of the soleus muscle (fiber type I) was prolonged 25% in the presence of 10 mM carnitine. In contrast, carnitine had no effect on the fatigue of extensor digitorum longus muscle strips (fiber type II). The beneficial effect of carnitine on soleus muscle strips was not observed if the routine 30-min preincubation in the presence of carnitine was decreased to 5 min; it was associated with a five- to sixfold increase in muscle total carnitine content and a 50-150% increase in muscle long-chain acylcarnitine content. Carnitine did not consistently modify lactate accumulation or glycogen depletion during the fatigue protocol. Incubation with propionyl-L-carnitine resulted in a decreased initial force of contraction and a delay in reaching maximal contractile force. Thus, carnitine can directly improve the fatigue characteristics of muscles enriched in type I fibers.