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Reduced sarcoplasmic reticulum content of releasable Ca 2+ in rat soleus muscle fibres after eccentric contractions
Author(s) -
Nielsen J. S.,
Sahlin K.,
Ørtenblad N.
Publication year - 2007
Publication title -
acta physiologica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.591
H-Index - 116
eISSN - 1748-1716
pISSN - 1748-1708
DOI - 10.1111/j.1748-1716.2007.01732.x
Subject(s) - myofibril , sarcolemma , endoplasmic reticulum , calcium , chemistry , soleus muscle , skeletal muscle , vesicle , biophysics , sarcoplasm , medicine , endocrinology , biochemistry , biology , membrane , organic chemistry
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+ .