Slow to fast muscle transformation following heterochronous isotransplantation is influenced by host thyroid hormone status.

We studied the effect of regeneration, altered innervation and thyroid hormone (TH) levels on fiber type transitions in slow soleus (SOL) muscles grafted (GRAFT) into host extensor digitorum longus (EDLh) muscles of euthyroid (EU), hyperthyroid (HT) and hypothyroid (HY) Lewis strain rats. SOL muscles were excised from 3-week to 4-week-old inbred Lewis rats and intramuscularly transplanted into EDLh muscles of 2-month-old female rats of the same strain. The proportions of type 1, 2A, 2X and 2B fibers of GRAFT were determined by immunohistochemistry and compared with those of EDLh muscle and EDL and SOL muscles of the unoperated contralateral hind limb.

Fibre type composition of soleus and extensor digitorum longus muscles in normal female inbred Lewis rats.

We have analysed the fibre type composition of soleus and extensor digitorum longus (EDL) muscles of normal female 4-6-month-old inbred Lewis rats. This rat strain is used in our ongoing study of the effects of thyroid hormone on myosin heavy chain (MyHC) isoform expression. On the basis of the mATPase reaction, soleus muscles contained 96.