Quantification of thyroid hormone receptor isoforms, 9‐cis retinoic acid receptor γ, and nuclear receptor co‐repressor by reverse‐transcriptase PCR in maturing and adult skeletal muscles of rat

Quantitative competitive reverse‐transcriptase polymerase chain reaction (qcRT‐PCR) was established for determining absolute molecule numbers of the thyroid hormone receptor (T3R) isoforms T3Rα1, T3Rα2, T3Rβ1, and the 9‐cis retinoic acid receptor γ (RXRγ) in developing and adult fast‐twitch extensor digitorum longus (EDL) and slow‐twitch soleus (SOL) muscles of rat. Expression levels of the nuclear receptor co‐repressor (NCoR) were measured in the same muscles because responses to thyroid hormones during muscle maturation might not only depend on the expression levels of the various receptors but might also be modulated by changes in the expression of NCoR. The qcRT‐PCR method was based on the addition of known amounts of homologous competitor RNAs to the reverse transcriptase (RT) reaction. We show that all nuclear receptors under study were expressed in fast and slow muscles. Transcript numbers of T3Rβ1, which was the most abundant isoform, were higher in SOL than in EDL during all developmental stages. The mRNAs for T3Rα1, T3Rα2, RXRγ and the NCoR displayed molecule numbers in similar ranges, but were differentially expressed. T3Rα1 mRNA increased in SOL during postnatal development, while T3Rα2 mRNA initially decreased, then increased to adult levels. Conversely, pronounced decreases were observed for T3Rα1 (10‐fold) and T3Rα2 (28‐fold) mRNAs in the EDL muscle during postnatal maturation. RXRγ mRNA was 10‐fold downregulated during EDL maturation, but unaltered in maturing SOL. NCoR transcript number displayed only minor changes in both muscles.