Ultrastructural remodelling of slow skeletal muscle fibres in creatine kinase deficient mice: a quantitative study

Creatine kinase content, isoform distribution, and participation in energy transfer are muscle type specific. We analysed ultrastructural changes in slow muscle fibres of soleus due to invalidation of creatine kinase (CK) to reveal a difference in the remodelling strategy in comparison with fast muscle fibres of gastrocnemius published previously. We have employed the stereological method of vertical sections and electron microscopy of soleus muscles of wild type (WT) and CK-/- mice. The mitochondrial volume density was 1.4× higher but that of sarcoplasmic reticulum (SR) was almost 5× lower in slow CK-/- muscles fibres than in WT fibres. The volume density of terminal cisterns and of t-tubules was also lower in CK-/- than in WT fibres. The analysis of organelle environment revealed increased neighbourhood of mitochondria and A-bands that resulted from the decreased volume density of SR, from relocation of mitochondria along myofibrils, and from intrusion of mitochondria to myofibrils. These processes direct ATP supply closer to the contractile machinery. The decreased interaction between mitochondria and SR suggests reduced dependence of calcium uptake on oxidative ATP production. In conclusion, the architecture of skeletal muscle cells is under control of a cellular program that optimizes energy utilization specifically for a given muscle type.