Carnitine has been used to enhance human exercise performance. To test
the hypothesis that carnitine can directly modify skeletal muscle fun
ction, fatigue of isolated rat skeletal muscle strips was studied in v
itro. Carnitine (10 mM) did not modify the initial force of soleus con
traction. The time over which force declined by 50% during repetitive
electrical stimulation of the soleus muscle (fiber type I) was prolong
ed 25% in the presence of 10 mM carnitine. In contrast, carnitine had
no effect on the fatigue of extensor digitorum longus muscle strips (f
iber type II). The beneficial effect of carnitine on soleus muscle str
ips was not observed if the routine 30-min preincubation in the presen
ce 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% i
ncrease 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 i
n a decreased initial force of contraction and a delay in reaching max
imal contractile force. Thus, carnitine can directly improve the fatig
ue characteristics of muscles enriched in type I fibers.