Effects of fatigue on depolarization- and caffeine-induced contractures of skinned fibres

Transient impairments in single muscle fibre contractile function after prolonged cycling in elite endurance athletes

AIM

Prolonged muscle activity impairs whole-muscle performance and function. However, little is known about the effects of prolonged muscle activity on the contractile function of human single muscle fibres. The purpose of this study was to investigate the effects of prolonged exercise and subsequent recovery on the contractile function of single muscle fibres obtained from elite athletes.

METHODS

Nine male triathletes (26 ± 1 years, 68 ± 1 mL O2  min(-1) kg(-1) , training volume 16 ± 1 h week(-1) ) performed 4 h of cycling exercise (at 73% of HRmax ) followed by 24 h of recovery. Biopsies from vastus lateralis were obtained before and following 4 h exercise and following 24 h recovery. Measurements comprised maximal Ca(2+) -activated specific force and Ca(2+) sensitivity of slow type I and fast type II single muscle fibres, as well as cycling peak power output.

RESULTS

Following cycling exercise, specific force was reduced to a similar extent in slow and fast fibres (-15 and -18%, respectively), while Ca(2+) sensitivity decreased in fast fibres only. Single fibre-specific force was fully restored in both fibre types after 24 h recovery. Cycling peak power output was reduced by 4-9% following cycling exercise and fully restored following recovery.

CONCLUSION

This is the first study to demonstrate that prolonged cycling exercise transiently impairs specific force in type I and II fibres and decreases Ca(2+) sensitivity in type II fibres only, specifically in elite endurance athletes. Further, the changes in single fibre-specific force induced by exercise and recovery coincided temporally with changes in cycling peak power output.

Alterations in triad ultrastructure following repetitive stimulation and intracellular changes associated with exercise in amphibian skeletal muscle

This study used Rana temporaria sartorius muscles to examine the effect of fatiguing electrical stimulation on the gap between the T-tubular and sarcoplasmic reticular membranes (T-SR distance) and the T-tubule diameter and compared this with corresponding effects on resting fibres exposed to a range of extracellular conditions that each replicate one of the major changes associated with muscular activity: membrane depolarisation, isotonic volume increase, acidification and intracellular lactate accumulation. Following each treatment, muscles were immersed in isotonic fixative solution and processed for transmission electron microscopy (TEM). Mean T-SR distances were estimated from orthogonal intercepts to provide estimates of diffusion distances between T and SR membranes and T-tubule diameter was estimated by measuring its shortest axis in the sampled J-SR complexes. Measurements from muscles fatigued by low frequency intermittent stimulation showed significant (P << 0.05) reversible increases in both T-SR distance and T-tubule diameter from 15.97 ± 0.37 nm to 20.15 ± 0.56 nm and from 15.44 ± 0.60 nm to 22.26 ± 0.84 nm (n = 40, 30) respectively. Exposure to increasing concentrations of extracellular [K⁺] in the absence of Cl⁻ to produce membrane depolarisation without accompanying cell swelling reduced T-SR distance and increased T-tubule diameter, whilst comparable increases in [K⁺]_e in the presence of Cl⁻ suggested that isotonic cell swelling has the opposite effect. Acidification alone, produced by NH₄Cl addition and withdrawal, also decreased T-SR distance and T-tubule diameter. A similar reduction in T-SR distance occurred following exposure to extracellular Na-lactate where such acidification was accompanied by elevations of intracellular lactate, but these conditions produced a significant swelling of T-tubules attributable to movement of lactate from the cell into the T-tubules. This study thus confirms previous reports of significant increases in T-SR distance and T-tubule diameter following stimulation. However, of membrane depolarisation, isotonic cell swelling, intracellular acidification and lactate accumulation, only isotonic cell swelling increases T-SR distance whilst membrane depolarisation and intracellular lactate likely contribute to the observed increases in T-tubule diameter.