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

Abstract

Effects of thyroid hormone on α-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 α-actin mRNAs. The results indicate that daily treatment with 3,5,3'-triiodo-L-thyronine (2 μg/100 g body weight) increased α-myosin heavy chain mRNA content in heart muscle by 500% and decreased β-myosin heavy chain mRNA by 65% within 48 hours. β-mRNA in extensor digitorum longus was decreased by 60% at 48 hours while in soleus, β-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 increase (60%) was observed after 1 day of treatment. In extensor digitorum longus, Fast IIb mRNA increased by 200% and Fast IIa mRNA decreased by 50% after 1 week of treatment. When larger daily doses of thyroid hormone (15 μg/100 g body weight) were administered, similar changes in mRNA levels were observed, except that β-mRNA content of soleus muscle was decreased slightly (25%). Expression of the cardiac form of α-actin was induced transiently in ventricle, but the skeletal form of α-actin mRNA in soleus and extensor digitorum longus did not change significantly after thyroid hormone treatment. Skeletal α-actin mRNA was not found in ventricle, and cardiac α-actin mRNA was not detected in skeletal muscles. Thus, cardiac α-actin mRNA levels are transiently stimulated in ventricle by thyroid hormone treatment while all of the myosin heavy chain genes expressed in adult striated muscles are affected in a complex, tissue-specific manner.