Effects of thyroid hormone on alpha-actin and myosin heavy chain gene expression in cardiac and skeletal muscles of the rat: measurement of mRNA content using synthetic oligonucleotide probes.

Effects of thyroid hormone on alpha-actin and myosin heavy chain gene expression were compared in ventricle, soleus, and extensor digitorum longus muscles of hypothyroid rats. Changes in mRNA content were analyzed using synthetic oligonucleotide probes complementary to the unique 3' untranslated regions of four striated myosin heavy chain mRNAs and cardiac and skeletal muscle alpha-actin mRNAs. The results indicate that daily treatment with 3,5,3'-triiodo-L-thyronine (2 micrograms/100 g body weight) increased alpha-myosin heavy chain mRNA content in heart muscle by 500% and decreased beta-myosin heavy chain mRNA by 65% within 48 hours. beta-mRNA in extensor digitorum longus was decreased by 60% at 48 hours while in soleus, beta-mRNA levels were not affected by 9 weeks of treatment. Fast IIa mRNA was present in small amounts in hypothyroid soleus and increased by 150% and 200% after 7 and 9 weeks of thyroid hormone administration, respectively. Fast IIb mRNA also was found in hypothyroid soleus and a small...