Lactate and force production in skeletal muscle

Lactic acid accumulation is generally believed to be involved in muscle fatigue. However, one study reported that in rat soleus muscle ( in vitro ), with force depressed by high external K + concentrations a subsequent incubation with lactic acid restores force and thereby protects against fatigue. However, incubation with 20 m m lactic acid reduces the pH gradient across the sarcolemma, whereas the gradient is increased during muscle activity. Furthermore, unlike active muscle the Na + –K + pump is not activated. We therefore hypothesized that lactic acid does not protect against fatigue in active muscle. Three incubation solutions were used: 20 m m Na‐lactate (which acidifies internal pH), 12 m m Na‐lactate +8 m m lactic acid (which mimics the pH changes during muscle activity), and 20 m m lactic acid (which acidifies external pH more than internal pH). All three solutions improved force in K + ‐depressed rat soleus muscle. The pH regulation associated with lactate incubation accelerated the Na + –K + pump. To study whether the protective effect of lactate/lactic acid is a general mechanism, we stimulated muscles to fatigue with and without pre‐incubation. None of the incubation solutions improved force development in repetitively stimulated muscle (Na‐lactate had a negative effect). It is concluded that although lactate/lactic acid incubation regains force in K + ‐depressed resting muscle, a similar incubation has no or a negative effect on force development in active muscle. It is suggested that the difference between the two situations is that lactate/lactic acid removes the negative consequences of an unusual large depolarization in the K + ‐treated passive muscle, whereas the depolarization is less pronounced in active muscle.