Gender-related differences in the regulatory influence of thyroid hormone on the expression of myosin isoforms in young and old rats

The effects of 4 weeks of thyroid hormone (3,5,3'-triiodorhyionine. T3) treatment on the expression of myosin heavy chain (MyHC) isoforms were examined in young (3-6 months) and old (20-24 months) female rats. and compared with those in age-matched male rats (Larsson er al. 1995). In control rats. ageing was associated with a type IIA to I MyHC isoform switching in the slow-twitch soleus and a type IIB to IIX MyHC isoform switching in the fast-twitch extensor digitorum longus muscle (EDL). Gender- and muscle-specific differences were observed in the regulation oi MyHC isoforms by T3. In the soleus. dramatic downregulation of the type I and upregulation oi the type IIA MyHC isoform were observed in both females and males. but upregulation oi the IIX MyHC isoform was observed only inmate rats. In EDL. T3 treatment had no significant influence on the MyHC isoform composition in the males irrespective of the age of the animal. In the females, on the other hand. T3 treatment resulted in a significant MyHC transformation from IIA to IIB. probably via IIX myosin. in spire oi the fact that type IIA mRNA has been reported to be downregulated in both females and males. It is concluded that the regulation of MyHC isoforms by thyroid hormone differs between females and males. presumably as a result oi a gender-related difference in the translational or post-translational regulation of MyHC synthesis.