The influence of thyroid hormone on myosin isoform composition and shortening velocity of single skeletal muscle fibres with special reference to ageing and gender

This review summarizes the effects of altered thyroid hormone levels on the expression of myosin isoforms and contractility in single muscle fibres fr om fast- and slow-twitch muscles from young and old male and female rats. T he differences between male and female hyperthyroid soleus muscles are sugg ested to be related to an interaction of thyroid hormones and sex hormones in the regulation of myosin gene expression. Additionally, the mismatch bet ween the protein and mRNA levels of MyHCs between male and female hyperthyr oid extensor digitorum longus (EDL) muscles raises the possibility of a gen der-related difference in post-transcriptional, translational or post-trans lational regulation of MyHC isoforms by T3.