Effects of Clenbuterol on Daily Activity in Rat Masseter Muscle

Objectives: We have previously reported that clenbuterol (CB), a beta-2-agonist, induces both hypertrophy and a slow-to-fast fiber-type transition in masseter muscle of rats. Although the CB-induced hypertrophy has been shown to be promoted through Akt/mTOR signaling pathway, the mechanism underlying the slow-to-fast transition is less clear. In the present study we examined the hypothesis that the slow-to-fast transition would be caused by changes in neuromuscular activity, which are well known to be a factor influencing fiber-type transitions. Methods: Daily electromyograms (EMGs) in masseter muscle of freely moving rats (n = 6) were recorded with a radio-telemetry system. Two 24-h periods of EMG (1 and 2 weeks after the implantation of the transmitter) were recorded, and served as control. Subsequently, the CB treatment (via the drinking water containing 30 mg/L) was initiated. Again, two 24-h periods of EMG (1 and 2 weeks after the initiation of the CB treatment) were recorded. Results: The CB treatment increased the duty times (i.e., the total duration of daily muscle activity) by about 1.5, 2.5, 4 and 5 times at activity levels exceeding 5, 20, 50 and 80% of the day's peak activity, respectively. This indicates the greater effects of CB on the duty time at the higher (50 and 80%) activity levels as compared with those at the lower (5 and 20%) activity levels. Conclusion: These results suggest that the CB treatment changes masseter muscle towards a faster phenotype by an increase in the duty time at the higher (50 and 80%) activity levels.