Muscle phenotype is determined by combined effects of intrinsic genetic and extrinsic factors like innervation, hormonal levels and mechanical factors or muscle activity. We have been studying the effect of altered thyroid hormone levels on the expression of myosin heavy chain (MyHC) isoforms in control and regenerating soleus and extensor
digitorum longus muscles of euthyroid, hypothyroid or hyperthyroid female inbred Lewis rats. The fiber type composition has been determined according to the mATPase activity and immunocytochemical staining of MyHC isoforms, the content of MyHC isoforms has been determined by SDS-PAGE, the mRNA levels have been measured by RT-PCR and the ultrastructural transformation has been analyzed by electron-microscopy. Our results indicate that although the innervation plays a decisive role in the determination of muscle phenotype, levels of thyroid hormones contribute to the extent of muscle phenotype transformation.
Skeletal muscles of small rodents contain four main fiber types, namely type 1, 2A, 2X/D and 2B fibers containing myosin heavy chain (MyHC) 1, 2a, 2x/d and 2b isoforms. Each of these MyHC isoforms is the product of a distinct gene and their expression is believed to be primarily transcriptionally controlled. In most rat muscles, messenger RNA (mRNA) transcripts for MyHC1, 2a, 2x/d and 2b and their corresponding protein products were found with the exception of the soleus muscle, where typically only MyHC1 and 2a transcripts and protein isoforms were demonstrated under normal conditions. Here we show the expression of all four MyHC1, 2a, 2x/d and 2b mRNA transcripts in the soleus muscle under normal conditions in euthyroid, as well as in experimental hypothyroid and hyperthyroid (with the exception of 2b MyHC transcript) 7-month-old female inbred Lewis rats. This is not matched, however, by the appearance of corresponding four isoforms, as we have found that 2x/d and 2b protein isoforms are not present at levels detectable by SDS-PAGE. We also show that the chronic hypothyroid and hyperthyroid status affects the expression of MyHC isoforms both at the mRNA and protein levels.
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.