Adaptive effects of the β₂-agonist clenbuterol on expression of β₂-adrenoceptor mRNA in rat fast-twitch fiber-rich muscles

Administration of the β₂-agonist clenbuterol has been shown to reduce the expression of β₂-adrenoceptor (AR) mRNA in fast-twitch fiber-rich (extensor digitorum longus, EDL) muscle without changing that in slow-twitch fiber-rich (soleus, SOL) muscle in rats. However, the regulatory mechanism for muscle fiber type-dependent down-regulation of the expression of β₂-AR mRNA induced by clenbuterol is still unclear. Therefore, mRNA expression of transcriptional and post-transcriptional regulatory factors for β₂-AR mRNA levels in fast-twitch fiber-rich (EDL and plantaris, PLA) and slow-twitch fiber-rich (SOL) muscles in clenbuterol-administered (1.0 mg/kg body weight/day for 10 days, subcutaneous) rats was studied by real-time reverse transcription-polymerase chain reaction. Administration of clenbuterol significantly reduced expression of β₂-AR mRNA in EDL and PLA muscles without changing that in SOL muscle. Administration of clenbuterol also significantly reduced the mRNA expression of transcriptional regulatory factor (glucocorticoid receptor) and mRNA stabilizing factor (Hu antigen R) in EDL and PLA muscles without changing those in SOL muscle. These results suggest that muscle fiber type-dependent effects of clenbuterol on expression of β₂-AR mRNA are closely related to the down-regulation of mRNA expression of transcriptional and post-transcriptional regulatory factors for β₂-AR mRNA levels.