Effects of Clenbuterol, a β2-adrenergic Agonist, on the Myofiber Diameter, Fiber Type, and Expressions of Insulin-like Growth Factors in the Adult Mouse Masseter Muscle

Abstract

To determine whether clenbuterol, a β2-adrenergic agonist, affects the mass and fiber type of the mouse masseter muscle by altering the expressions of insulin-like growth factors (IGFs), their receptors (IGFRs), and their binding proteins (IGFBPs), we analyzed changes in the myofiber diameter, the expressions of myosin heavy chain (MHC) mRNAs, the markers for muscle fiber type, and the expressions of IGF, IGFR, and IGFBP mRNAs. In addition, to identify a possible contribution of muscle satellite cells in the change of the mouse masseter induced by clenbuterol, we analyzed the expressions of the myoD family (myf5, myoD, myogenin, and MRF4) and myocyte nuclear factor (MNF) mRNAs, and performed immunolocalization for proliferating cell nuclear antigen (PCNA), because they are all markers for activated and quiescent satellite cells. Clenbuterol (40 μg/ml) was orally administered to 6-month-old mice via their drinking water for 2 weeks. The relative amounts of mRNAs were analyzed by competitive polymerase chain r action in combination with reverse-transcription. The administration of clenbuterol increased the myofiber diameter by 26% (p<0.001), but it did not significantly change the amounts of MHC mRNAs, suggesting that clenbuterol induced hypertrophy but did not alter the fiber type. The administration of clenbuterol increased the amount of mRNA for IGF-I by 219% (p<0.05), but it decreased that for IGFBP3 by 21% (p<0.001). The amounts of mRNAs for all genes except for IGF-I and IGFBP3, and the immunolocalization for PCNA were not significantly changed by clenbuterol. These results suggest that clenbuterol induces hypertrophy in the mouse masseter muscle and that IGF-I and IGFBP3 are involved in the clenbuterol-induced hypertrophy, but the satellite cells might not be involved.