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Reduced sarcoplasmic reticulum content of releasable Ca 2+ in rat soleus muscle fibres after eccentric contractions

2007; Wiley; Volume: 191; Issue: 3 Linguagem: Inglês

10.1111/j.1748-1716.2007.01732.x

1748-1716

Jens Steen Nielsen, Kent Sahlin, Niels Ørtenblad,

Muscle Physiology and Disorders

Abstract Aim: The purpose was to evaluate the effects of fatiguing eccentric contractions (EC) on calcium (Ca 2+ ) handling properties in mammalian type I muscles. We hypothesized that EC reduces both endogenous sarcoplasmic reticulum (SR) content of releasable Ca 2+ (eSRCa 2+ ) and myofibrillar Ca 2+ sensitivity. Methods: Isolated rat soleus muscles performed 30 EC bouts. Single fibres were isolated from the muscle and after mechanical removal of sarcolemma used to measure eSRCa 2+ , rate of SR Ca 2+ loading and myofibrillar Ca 2+ sensitivity. Results: Following EC maximal force in whole muscle was reduced by 30% and 16/100 Hz force ratio by 33%. The eSRCa 2+ in fibres from non‐stimulated muscles was 45 ± 5% of the maximal loading capacity. After EC, eSRCa 2+ per fibre CSA decreased by 38% ( P = 0.05), and the maximal capacity of SR Ca 2+ loading was depressed by 32%. There were no effects of EC on either myofibrillar Ca 2+ sensitivity, maximal Ca 2+ activated force per cross‐sectional area and rate of SR Ca 2+ loading, or in SR vesicle Ca 2+ uptake and release. Conclusions: We conclude that EC reduces endogenous SR content of releasable Ca 2+ but that myofibrillar Ca 2+ sensitivity and SR vesicle Ca 2+ kinetics remain unchanged. The present data suggest that the long‐lasting fatigue induced by EC, which was more pronounced at low frequencies (low frequency fatigue), is caused by reduced Ca 2+ release occurring secondary to reduced SR content of releasable Ca 2+ .