Gender- and age-related differences in the regulatory influence of thyroidhormone on the contractility and myosin composition of single rat soleus muscle fibres

The effects of 4 weeks of thyroid hormone (3,5,3'-triiodothyronine, T-3) tr eatment on the myosin isoform composition and maximum velocity of unloaded shortening (V-0) of single soleus muscle fibres of young (3-6 months) and o ld (20-24 months) female (149 fibres) and male (200 fibres) rats were studi ed. Gender-related differences in the up-regulation of fast myosin heavy ch ain (MyHC) and myosin light chain (MyLC) isoforms were observed. In the fem ale hyperthyroid rats, pure type I fibres and fibres co-expressing type I a nd type IIA MyHC (type I/IIA fibres) predominated. Some fibres expressed an alpha cardiac-like MyHC isoform either purely (alpha cardiac-like fibre ty pe) or in co-expression with IIA MyHC (alpha cardiac-like/IIA fibre type). In the male hyperthyroid rats, on the other hand, all fibres were either ty pe I/IIA or type I/IIAX. The relative quantities of fast MyLC isoforms in t ype I/IIA and type I/IIAX fibres was higher in female than in male hyperthy roid rats. V-0 was similar in male and female control rats, and decreased w ith age in both genders (P<0.001). After T-3 treatment, the average V-0 inc reased (P<0.001) in females with a concomitant up-regulation of fast MyHC a nd fast MyLC isoforms irrespective of age. The average V-0 of the pooled fi bres was higher (P<0.001) in female than in male hyperthyroid rats at both ages. In conclusion, gender- and age-related differences were observed in t he regulatory influence of 4 weeks' T-3 treatment on myosin isoform composi tion and V-0 in soleus fibres. These differences are presumably related to an interaction of thyroid and sex hormones in the regulation of myosin gene expression.