Analysis of thyroid hormone effects on myosin heavy chain gene expression in cardiac and soleus muscles using a novel dot-blot mRNA assay.

Effects of thyroid hormone on myosin heavy chain (MHC) gene expression were compared in ventricle and soleus muscle of hypothyroid rats, since in this condition, both muscle types express predominately beta-MHC mRNA. Changes in MHC mRNAs were analyzed using synthetic oligonucleotide probes complementary to the 3' untranslated region of the MHC mRNAs. The results indicated that daily treatment with 3,5,3'-triiodo-L-thyronine (T3) at a dose of 2 micrograms/100 g body weight increased alpha-MHC mRNA content in heart muscle by 600% and decreased beta-MHC mRNA by 70% within 48 h. In soleus muscle, beta-MHC mRNA levels were not affected by 9 wks of treatment, however, Fast IIa-MHC mRNA was increased by 150% at 7 wks and 300% after 9 wks of T3 administration. Thus, regulation of MHC gene expression by thyroid hormone is both gene and tissue specific.