Protective effects of lactic acid on force production in rat skeletal muscle

1. During strenuous exercise lactic acid accumulates producing a reduction in muscle pH. In addition, exercise causes a loss of muscle K+ leading to an increased concentration of extracellular K+ ([K+](o)). Individually, reduced pH and increased [K+](o) have both been suggested to contribute to muscle fatigue. 2. To study the combined effect of these changes on muscle function, isolated rat soleus muscles were incubated at a [K+](o) of 11 mM, which reduced tetanic force by 75%. Subsequent addition of 20 mm lactic acid led, however, to an almost, complete force recovery. A similar recovery was observed if pH was reduced by adding propionic acid or increasing the CO2, tension. 3. The recovery of force was associated with a recovery of muscle excitability as assessed from compound action potentials. In contrast, acidification had no effect on the membrane potential or the Ca2+ handling of the muscles. 4. It is concluded that acidification counteracts the depressing effects of elevated [K+](o) on muscle excitability and force. Since intense exercise is associated with increased [K+](o) this indicates that, in contrast to the often suggested role for acidosis as a cause of muscle fatigue, acidosis may protect against fatigue. Moreover, it suggests that elevated [K+](o) is of less importance for fatigue than indicated by previous studies oil isolated muscles.