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

Abstract

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 CO(2) 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 Ca(2+) 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 on isolated muscles.

Figure 1. Effect of 20 mm lactic acid on tetanic force in rat soleus muscles exposed to a [K⁺]_o of 11 mm

[K⁺]_o was increased to 11 mm at time 30 min. ▪, controls, n = 2; •, lactic acid added together with the increase in [K⁺]_o, n = 6; ○, lactic acid added after 90 min at 11 mm K⁺, n = 6. Muscles were stimulated tetanically every 10 min using 30 Hz pulse trains of 1.5 s duration and a pulse duration of 0.2 ms. Data are means and s.e.m.

Figure 2. Effect on force and intracellular pH of 20 mm lactic acid, 20 mm propionic acid or an increase in CO₂ from 5 to 23 %

A, representative traces showing changes in pH_i in muscles exposed to lactic acid, propionic acid or 23 % CO₂ as indicated by the bars. B, tetanic force in muscles incubated at 12.5 mm K⁺ for 70 min (▪) and after a further 50 min incubation in buffer containing both 12.5 mm K⁺ and the indicated acids (). Muscles were stimulated tetanically every 10 min using 30 Hz pulse trains of 2 s duration and a pulse duration of 1 ms. Data show means and s.e.m. from 6-12 muscles. * Force before and after indicated addition is significantly different (P < 0.001).

Figure 3. Effect of high [K⁺]_o and lactic acid on M-wave and contractile properties

After determination of control values, [K⁺]_o was increased to 10 mm K⁺ and after 100 min 20 mm lactic acid was introduced, as indicated. Muscle contractions were elicited via the motor nerve using a suction electrode. A, time course of M-wave area and tetanic force. ▪, tetanic force; ▵, M-wave area. Data show means and s.e.m. from 4 muscles. B, representative M-wave traces from a single muscle obtained at the time points indicated by a, b, and c in A.

Figure 4. Effect of lactic acid or increased CO₂ tension on the relation between steady state tetanic force and [K⁺]_o

Muscles were incubated at different [K⁺]_o values until a steady force level was obtained. ▪, control muscles, buffer pH 7.4 (n = 4–6); ○, 20 mm lactic acid added, buffer pH 6.82 (n = 8–10); •, CO₂ increased to 50 %, buffer pH 6.72 (n = 8). Data show means and s.e.m. At all [K⁺]_o values above 8 mm, the relative force of the muscles at low pH was significantly higher than in the control muscles at the corresponding [K⁺]_o (P < 0.05).