We have studied the effect of chronic thyroid status alterations on the myosin heavy chain (MyHC) isoform composition (by SDS-PAGE) and on MyHC mRNA levels (by RT-PCR) in the fast extensor digitorum longus (EDL)
muscle of 7-month-old inbred Lewis strain female rats and compared this with corresponding results of the previously studied slow soleus muscle. Our findings show that in the EDL muscle, all four types 1, 2a, 2x/d and 2b of MyHC mRNA transcripts and protein isoforms are present in euthyroid, hypothyroid and hyperthyroid rats, i.e. after chronic
treatment with methimazole and T3, respectively. This is in contrast with the soleus, where only MyHC1 and 2a protein isoforms are expressed under similar conditions. Except for 2x/d MyHC mRNA transcripts in the EDL muscles, there was always significant difference between hypothyroid and hyperthyroid rats both at mRNA and protein levels. From our results we can conclude that extended alteration of the thyroid status leads to typical changes in the expression of MyHC mRNA transcripts and MyHC protein isoforms in the fast EDL and the slow soleus muscles. These changes
correspond to those described after shorter periods of altered thyroid status. The characteristic phenotype differences between soleus and EDL muscles remain, however, preserved even after 7 months of thyroid hormone status alteration.