Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+ release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Intracellular acidification of skeletal muscles is commonly thought to contribute to muscle fatigue. However, intracellular acidosis also acts to preserve muscle excitability when muscles become depolarized, which occurs with working muscles. Here, we show that this process may be mediated by decreased chloride permeability, which enables action potentials to still be propagated along the internal network of tubules in a muscle fiber (the T system) despite muscle depolarization. These results implicate chloride ion channels in muscle function and emphasize that intracellular acidosis of muscle has protective effects during muscle fatigue.

Whilst calsequestrin (CSQ) is widely recognized as the primary Ca2+ buffer in the sarcoplasmic reticulum (SR) in skeletal muscle fibres, its total buffering capacity and importance have come into question. This study quantified the absolute amount of CSQ isoform 1 (CSQ1, the primary isoform) present in rat extensor digitorum longus (EDL) and soleus fibres, and related this to their endogenous and maximal SR Ca 2+ content. Using Western blotting, the entire constituents of minute samples of muscle homogenates or segments of individual muscle fibres were compared with known amounts of purified CSQ1. The fidelity of the analysis was proven by examining the relative signal intensity when mixing muscle samples and purified CSQ1. The CSQ1 contents of EDL fibres, almost exclusively type II fibres, and soleus type I fibres [SOL (I)] were, respectively, 36 ± 2 and 10 ± 1 μmol (l fibre volume) −1 , quantitatively accounting for the maximal SR Ca 2+ content of each. Soleus type II [SOL (II)] fibres (∼20% of soleus fibres) had an intermediate amount of CSQ1. Every SOL (I) fibre examined also contained some CSQ isoform 2 (CSQ2), which was absent in every EDL and other type II fibre except for trace amounts in one case. Every EDL and other type II fibre had a high density of SERCA1, the fast-twitch muscle sarco(endo)plasmic reticulum Ca 2+ -ATPase isoform, whereas there was virtually no SERCA1 in any SOL (I) fibre. Maximal SR Ca 2+ content measured in skinned fibres increased with CSQ1 content, and the ratio of endogenous to maximal Ca 2+ content was inversely correlated with CSQ1 content. The relative SR Ca 2+ content that could be maintained in resting cytoplasmic conditions was found to be much lower in EDL fibres than in SOL (I) fibres (∼20 versus >60%). Leakage of Ca2+ from the SR in EDL fibres could be substantially reduced with a SR Ca 2+ pump blocker and increased by adding creatine to buffer cytoplasmic [ADP] at a higher level, both results indicating that at least part of the Ca 2+ leakage occurred through SERCA. It is concluded that CSQ1 plays an important role in EDL muscle fibres by providing a large total pool of releasable Ca 2+ in the SR whilst maintaining free [Ca 2+ ] in the SR at sufficiently low levels that Ca 2+ leakage through the high density of SERCA1 pumps does not metabolically compromise muscle function.

1. The effects on normal excitation‐contraction (E‐C) coupling of two important intracellular ions, H+ and Mg2+, were examined in skinned fibres from the extensor digitorum longus muscle of rat. 2. A single depolarization (2‐3 s duration) in the presence of 1 mM Mg2+ (pH 7.1, 23 degrees C) released most of the available Ca2+ in the sarcoplasmic reticulum (SR), but a similar depolarization in the presence of 10 mM Mg2+ was unable to release almost any Ca2+. Thus, raised [Mg2+] potently inhibits depolarization‐induced Ca2+ release in mammalian muscle. 3. Depolarization at pH 6.2 (1 mM Mg2+, 23 degrees C) induced a large force response, which was on average 78 +/‐ 2%, n = 6, of the depolarization‐induced response at pH 7.1; this reduction resulted from a corresponding reduction in maximum Ca(2+)‐activated force at pH 6.2. Similar results were obtained at 37 degrees C. Also, a single depolarization at pH 6.2 caused almost complete depletion of the releasable Ca2+ in the SR. Thus, low pH does not prevent depolarization‐induced Ca2+ release in mammalian muscle. 4. Lowering the free [Mg2+] from 1 mM to 15 microM caused massive release of Ca2+, and depletion of the SR, at both pH 7.1 and 6.2, indicating that over this pH range, H+ did not readily substitute for Mg2+ at its inhibitory site on the Ca2+ release channel.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Raising the intracellular [Ca2+] for 10 s at 23 degrees C abolished depolarization‐induced force responses in mechanically skinned muscle fibres of toad and rat (half‐maximal effect at 10 and 23 microM, respectively), without affecting the ability of caffeine or low [Mg2+] to open the ryanodine receptor (RyR)/Ca2+ release channels. Thus, excitation‐contraction coupling was lost, even though the Ca2+ release channels were still functional. Coupling could not be restored in the duration of an experiment (up to 1 h). 2. The Ca(2+)‐dependent uncoupling had a Q10 > 3.5, and was three times slower at pH 5.8 than at pH 7.1. Sr2+ caused similar uncoupling at twenty times higher concentration, but Mg2+, even at 10 mM, was ineffective. Uncoupling was not noticeably affected by removal of ATP or application of protein kinase or phosphatase inhibitors. 3. Confocal laser scanning microscopy showed that the transverse tubular system was sealed in its entirety in mechanically skinned fibres and that its integrity was maintained in uncoupled fibres. Electron microscopy revealed distorted or severed triad junctions and Z‐line aberrations in uncoupled fibres. 4. Only when uncoupling was induced at a relatively slow rate (e.g. over 60 s with 2.5 microM Ca2+) could it be prevented by the protease inhibitor leupeptin (1 mM). Immunostaining of Western blots showed no evidence of proteolysis of the RyR, the alpha 1‐subunit of dihydropyridine receptor (DHPR) or triadin in uncoupled fibres. 5. Fibres which, whilst intact, were stimulated repeatedly by potassium depolarization with simultaneous application of 30 mM caffeine showed reduced responsiveness after skinning to depolarization but not to caffeine. Rapid release of endogenous Ca2+, or raised [Ca2+] under conditions which minimized the loss of endogenous diffusible myoplasmic molecules from the skinned fibre, caused complete uncoupling. Taken together, these results suggest that Ca(2+)‐dependent uncoupling can also occur in intact fibres. 6. This Ca(2+)‐dependent loss of depolarization‐induced Ca2+ release may play an important feedback role in muscle by stopping Ca2+ release in localized areas where it is excessive and may be responsible for long‐lasting muscle fatigue after severe exercise, as well as contributing to muscle weakness in various dystrophies.