Effects of contractile activity and hypothyroidism on nuclear hormone receptor mRNA isoforms in rat skeletal muscle

Absolute molecule numbers of thyroid hormone receptor isoforms T3R alpha 1, T3R alpha 2, T3R beta 1, and the 9-cis retinoic acid receptor gamma were m easured in adult fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of rat by competitive reverse transcriptase (RT)-PCR. The nuclear hormone receptor corepressor (NCoR) mRNA was quantified by noncompetitive R T-PCR in the same muscles. T3R beta 1 mRNA was the most abundant isoform in both muscle types. All nuclear hormone receptor (NHR) mRNAs were found at lower molecule numbers in fast than in slow muscle. No differences existed with regard to NCoR mRNA. With die exception of T3R alpha 1 in the EDL, hyp othyroidism led to decreases in NHR mRNAs, especially in SOL, but did not s ignificantly affect the level of NCoR mRNA. Enhanced neuromuscular activity of the fast EDL muscle, as induced by chronic low-frequency stimulation, t ransiently increased NHR mRNAs,but decreased NCoR mRNA. These chronic-low-f requency-stimulation-induced changes were attenuated by hypothyroidism.