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

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Abstract
The effects of 4 weeks of thyroid hormone (3,5,3′-triiodothyronine, T3) treatment on the myosin isoform composition and maximum velocity of unloaded shortening (V0) of single soleus muscle fibres of young (3–6 months) and old (20–24 months) female (149 fibres) and male (200 fibres) rats were studied. Gender-related differences in the up-regulation of fast myosin heavy chain (MyHC) and myosin light chain (MyLC) isoforms were observed. In the female hyperthyroid rats, pure type I fibres and fibres co-expressing type I and type IIA MyHC (type I/IIA fibres) predominated. Some fibres expressed an α cardiac-like MyHC isoform either purely (α cardiac-like fibre type) or in co-expression with IIA MyHC (α cardiac-like/IIA fibre type). In the male hyperthyroid rats, on the other hand, all fibres were either type I/IIA or type I/IIAX. The relative quantities of fast MyLC isoforms in type I/IIA and type I/IIAX fibres was higher in female than in male hyperthyroid rats. V0 was similar in male and female control rats, and decreased with age in both genders (P3 treatment, the average V0 increased (P V0 of the pooled fibres was higher (P3 treatment on myosin isoform composition and V0 in soleus fibres. These differences are presumably related to an interaction of thyroid and sex hormones in the regulation of myosin gene expression.

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