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Protective Effects of Lactic Acid on Force Production in Rat Skeletal Muscle


Protective Effects of Lactic Acid on Force Production in Rat Skeletal Muscle

O B Nielsen et al. J Physiol.


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
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
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 pHi 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 (formula image). 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
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
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).

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