Role of phosphate and calcium stores in muscle fatigue

Abstract

Intensive activity of muscles causes a decline in performance, known as fatigue, that is thought to be caused by the effects of metabolic changes on either the contractile machinery or the activation processes. The concentration of inorganic phosphate (P(i)) in the myoplasm ([P(i)](myo)) increases substantially during fatigue and affects both the myofibrillar proteins and the activation processes. It is known that a failure of sarcoplasmic reticulum (SR) Ca(2+) release contributes to fatigue and in this review we consider how raised [P(i)](myo) contributes to this process. Initial evidence came from the observation that increasing [P(i)](myo) causes reduced SR Ca(2+) release in both skinned and intact fibres. In fatigued muscles the store of releasable Ca(2+) in the SR declines mirroring the decline in SR Ca(2+) release. In muscle fibres with inoperative creatine kinase the rise of [P(i)](myo) is absent during fatigue and the failure of SR Ca(2+) release is delayed. These results can all be explained if inorganic phosphate can move from the myoplasm into the SR during fatigue and cause precipitation of CaP(i) within the SR. The relevance of this mechanism in different types of fatigue in humans is considered.

Figure 1. Muscle fatigue is partly caused by failure of SR Ca²⁺ release

A, force production from a mouse single fibre stimulated to give repeated brief tetani at gradually reducing intervals until force had declined to ∼40 % of control. At that time caffeine (10 mm) was applied, which reversed much of the decline of force. B, [Ca²⁺]_myo records of selected tetani from experiments similar to A; (i) is the first tetanus, (ii) is at the end of the early decline of force, (iii) is a fatigued tetanus just before the addition of caffeine, and (iv) is in the presence of caffeine. A is taken from Lännergren & Westerblad (1991). B is modified from data in Westerblad & Allen (1991).

Figure 2. Increased [P_i]_myo reduces SR Ca²⁺ release

A, force records from skinned fibres with intact SR. Caffeine was used to release SR Ca²⁺ producing the contractures shown; thus the size of the contracture is an indication of the Ca²⁺ available for release in the SR. In the middle record, the muscle was exposed to 50 mm P_i for 20 s, the P_i was then washed off and caffeine applied. Adapted from Fryer et al. (1995) with permission. B, schematic diagram of Ca²⁺ and P_i movements across the SR membrane and binding sites within the SR. Under control conditions [P_i]_myo =[P_i]_SR = 3 mm and [Ca²⁺]_SR = 1 mm. Thus [Ca²⁺]_SR×[P_i]_SR = 3 mm² and because this is below the solubility product of CaP_i (which is 6 mm²) none of this product is present. Ca²⁺ in the SR, however, binds rapidly and reversibly to calsequestrin (CS) so that there is a large pool of CaCS which buffers [Ca²⁺]_SR. When the SR Ca²⁺ release channel opens, a large flux of Ca²⁺ into the myoplasm occurs because [Ca²⁺]_SR is high and [Ca²⁺]_SR is maintained high by the buffering of CaCS. In fatigue [P_i]_myo is 30 mm and P_i enters the SR via anion channels. Once [P_i]_SR exceeds 6 mm, the product of [Ca²⁺]_SR×[P_i]_SR exceeds the solubility product of CaP_i and precipitation of CaP_i starts to occur slowly in the SR. As a consequence [Ca²⁺]_SR and CaCS fall and when the SR Ca²⁺ release channels are open the flux is smaller both because [Ca²⁺]_SR is lower and the buffering of [Ca²⁺]_SR by CaCS is reduced. Dissociation of CaP_i is assumed to be too slow to contribute to Ca²⁺ release. Heavy arrows indicate changes of key concentrations during fatigue.

Figure 3. SR Ca²⁺ stores decline during fatigue and recovery is prevented by inhibition of oxidative metabolism

[Ca²⁺]_myo recorded from a single cane toad muscle fibre. The first record shows a single short tetanus followed by ∼10 s application of 4-chloro-m-cresol (4-CmC, indicated by bar above record). This drug opens SR Ca²⁺ release channels and the large rise in [Ca²⁺]_myo represents the amount of rapidly releasable SR Ca²⁺. Similar results are obtained with caffeine. The fibre was then rested for 20 min and then fatigued with repeated brief tetani until the tetanic force (not shown) was reduced to 40 %. 4-CmC was then reapplied and the amount of rapidly releasable SR Ca²⁺ was reduced compared to control. The fibre was then allowed to rest for 20 min in the presence of 2 mm cyanide, which inhibits oxidative metabolism, and a tetanus and 4-CmC application repeated. Neither tetanic [Ca²⁺]_myo, the rapidly releasable SR Ca²⁺ store, nor the force (not shown) recovered. The fibre was then rested for 20 min in cyanide-free solution and partial recovery of all three parameters occurred. Adapted from Kabbara & Allen (1999).

Figure 4. The decline of tetanic [Ca²⁺]_myo is delayed in muscle fibres which fatigue without an increase in [P_i]_myo

A, left: tetanic [Ca²⁺]_myo records obtained in a wild-type mouse muscle fibre at the start and the end (tetanic force reduced to 30 %; not shown) of fatigue caused by repeated tetani. This fibre was fatigued after 88 tetani and at that time tetanic [Ca²⁺]_myo was markedly reduced. A, right: records from a genetically engineered fibre lacking creatine kinase (CK^−/−). In this fibre 100 tetani had no significant effect on either tetanic [Ca²⁺]_myo or force (not shown). Adapted from Dahlstedt et al. (2000). B, the first two records show tetanic [Ca²⁺]_myo at the start and end of fatigue produced in a mouse muscle fibre under control conditions. In this fibre 42 tetani were required to reduce force to 30 %. The fibre was then allowed to recover for 90 min before 10 μm 2,4-dinitro-1-fluorobenzene (DNFB), which inhibits CK, was applied and the fibre fatigued again. This procedure produced some reduction in the first tetanic [Ca²⁺]_myo (not shown; for discussion of this point see Dahlstedt & Westerblad, 2001). Note that after DNFB exposure, tetanic [Ca²⁺]_myo was not reduced by 42 tetani. Adapted from Dahlstedt & Westerblad (2001).