Effects of synthesized glucocorticoid, dexamethasone (DEX, dose = 1.0 mg/kg body weight/day for 10 days) on the expressions of β 2-adrenoceptor (AR) and glucocorticoid receptor (GR) were studied in fast-twitch (extensor digitorum longus (EDL)) and slow-twitch fiber-rich (soleus(SOL)) muscles of rats. DEX decreased the expression of β 2-AR mRNA in SOL muscle without changing that in EDL muscle. The expression of β 2-AR protein in EDL and SOL muscles was not affected by DEX. DEX- induced decreased action of the expression of GR mRNA was much greater in SOL muscle than in EDL muscle. However, there were no differences in the expression of GR protein in EDL and SOL muscles. DEX also decreased mRNA expression of cAMP response element binding protein (CREB, transcription factor of β 2-AR mRNA) in SOL muscle, whereas increased that in EDL muscle. Further, DEX tended to increase mRNA expressions of post-transcription factors of β 2-AR mRNA in EDL muscle without changing those in SOL muscle. These results demonstrated that the expressions of β 2-AR and GR are regulated at mRNA levels but not protein levels by DEX. Further, these results also suggest that DEX-induced decrease in the expression of β 2-AR mRNA in slow-twitch fiber-rich SOL muscle is associated with the transcriptional regulations.
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