Role of β-adrenergic signaling in masseter muscle

In skeletal muscle, the major isoform of β-adrenergic receptor (β-AR) is β 2 -AR and the minor isoform is β 1 -AR, which is opposite to the situation in cardiac muscle. Despite extensive studies in cardiac muscle, the physiological roles of the β-AR subtypes in skeletal muscle are not fully understood. Therefore, in this work, we compared the effects of chronic β 1 - or β 2 -AR activation with a specific β 1 -AR agonist, dobutamine (DOB), or a specific β 2 -AR agonist, clenbuterol (CB), on masseter and cardiac muscles in mice. In cardiac muscle, chronic β 1 -AR stimulation induced cardiac hypertrophy, fibrosis and myocyte apoptosis, whereas chronic β 2 -AR stimulation induced cardiac hypertrophy without histological abnormalities. In masseter muscle, however, chronic β 1 -AR stimulation did not induce muscle hypertrophy, but did induce fibrosis and apoptosis concomitantly with increased levels of p44/42 MAPK (ERK1/2) (Thr-202/Tyr-204), calmodulin kinase II (Thr-286) and mammalian target of rapamycin (mTOR) (Ser-2481) phosphorylation. On the other hand, chronic β 2 -AR stimulation in masseter muscle induced muscle hypertrophy without histological abnormalities, as in the case of cardiac muscle, concomitantly with phosphorylation of Akt (Ser-473) and mTOR (Ser-2448) and increased expression of microtubule-associated protein light chain 3-II, an autophagosome marker. These results suggest that the β 1 -AR pathway is deleterious and the β 2 -AR is protective in masseter muscle. These data should be helpful in developing pharmacological approaches for the treatment of skeletal muscle wasting and weakness.