Skeletal muscle fatigue can be defined as the inability to produce a desired amount of force. Fatigue can not only limit athletic performance and rehabilitation, but it can affect one's ability to perform every day activity as well. Despite extensive investigation of muscle fatigue, little is known about the exact mechanisms that result in decreased muscle performance. It likely involves several factors that are themselves dependent upon activation patterns and intensity. The process of excitation-contraction (EC) coupling is of particular importance with respect to regulation of force production. The release of calcium (Ca2+) from the sarcoplasmic reticulum (SR), which is stimulated by the depolarization of the sarcolemma, causes muscle contraction. The SR Ca2+-adenosine triphosphatase (ATPase) drives the translocation of two Ca2+ ions into the SR, utilizing the energy derived from the hydrolysis of one adenosine triphosphate (ATP) molecule. The process of SR Ca2+ uptake causes muscle relaxation. It has been proposed that both glycogen and glycolytic enzymes are associated with the SR membrane (SR-glycogenolytic complex). Interestingly, glycogen phosphorylase, an enzyme involved in glycogen breakdown, seems to be associated with the SR-glycogenolytic complex through its binding to glycogen. The presence of the SR-glycogenolytic system may serve to locally regenerate ATP utilized by the SR Ca2+-ATPase.
The purpose of the present study was to investigate the effects of prolonged muscle contraction on glycogen concentration, glycogen phosphorylase content and activity, and maximum Ca2+ uptake rate associated with the SR. Tetanic contractions, elicited once per second for 15 minutes, significantly reduced glycogen associated with SR to 5.1% of control from 401.17 ± 79.81 to 20.46 ± 2.16 mg/mg SR protein (p≤0.05). The optical density of glycogen phosphorylase from SDS-PAGE was significantly reduced to 21.2% of control (p≤0.05). Activity of glycogen phosphorylase, in the direction of glycogen breakdown, was significantly reduced to 4.1% of control (p≤0.05). Pyridoxal 5'-phosphate (PLP) concentration, a quantitative indicator of glycogen phosphorylase content, was significantly reduced to 3.3% of control (£ 0.05). Maximum SR Ca2+ uptake rates were significantly reduced to 80.8% of control (£ 0.05). These data suggest reduced glycogen and glycogen phosphorylase may be involved, either directly or indirectly, in a mechanism that causes decreased SR Ca2+ uptake normally found in fatigue.
