Skeletal muscles can be injured by myriad insults that compromise their functional capacity. Regenerative processes are often slow and incomplete, and so developing novel therapeutic strategies to enhance muscle regeneration represents an important research area. We have shown previously that the β-adrenoceptor (AR) signalling pathway plays an important role in skeletal muscle regeneration after injury (Beitzel et al., 2004; 2007), and that transgenic mice lacking both β1- and β2-ARs have delayed regeneration following myotoxic injury (Sheorey et al., 2008, Church et al., 2010). In the present study we further investigated the relative contribution of individual β-AR subtypes to the early stages (up to 7 days) of muscle repair after injury.
Mice (8-9 weeks) lacking β1-adrenoceptors (β1-AR KO), β2-adrenoceptors (β2-AR KO), or both subtypes of β-adrenoceptors (β1/β2-AR KO), were obtained from The Jackson Laboratory (Bar Harbour, ME, USA). Littermate wildtype mice were used as controls for the β1-AR KO and β2-AR KO mice, while control mice for the β1/β2-AR KO mice were from a C57BL/6 background, as employed previously (Sheorey et al., 2008). Mice were anaesthetized (ketamine 80mg/kg and xylazine 10mg/kg; i.p.) such that they were unresponsive to tail or toe pinch, and the tibialis anterior (TA) muscle of the right hindlimb was injected with the myotoxin, Notexin (1μg/ml, i.m.) to cause complete muscle fibre degeneration. Mice were allowed to recover for 7 days, after which TA muscle function was assessed in situ as reported previously (Gehrig et al., 2010). Briefly, mice were anaesthetized (60 mg/kg, sodium pentobarbital, i.p.), the right TA muscle was surgically exposed, and the distal tendon was attached to the lever arm of a force transducer, with the knee and foot immobilized. At the conclusion of the experiment mice were killed by cardiac excision while still anaesthetized deeply.
As reported previously (Church et al., 2010), when muscle function was examined in uninjured TA muscles, both β2-AR KO mice and β1/β2-AR KO mice produced significantly less force (Po) than controls (P < 0.05), and TA muscles from β1-AR KO mice showed no significant deficit in force production. At 7 days post-injury, the regenerating TA muscles of β1/β2-AR KO mice produced significantly less force than those of controls (P < 0.05) but neither β1-AR KO nor β2-AR KO mice showed any delayed restoration of force producing capacity. When force production was normalized to cross-sectional area (sPo), or to that of uninjured controls (to account for altered force production between the strains), a similar result was obtained, with the β1/β2-AR KO mice displaying delayed restoration of function while mice lacking either β1- or β2-adrenoceptors alone did not.
These results suggest that while the absence of both β1- and β2-adrenoceptors can delay muscle regeneration after injury, the absence of either subtype alone does not. This apparent redundancy in the β-AR signalling pathway in skeletal muscle is unexpected, and may have important implications for the use of β-AR agonists to enhance regeneration after injury, as well as in the treatment of other conditions where muscle wasting and weakness are indicated.
Beitzel F, Gregorevic P, Ryall JG, Plant DR, Sillence MN & Lynch GS. (2004) Journal of Applied Physiology 96, 1385-1392.
Beitzel F, Sillence MN & Lynch GS. (2007) American Journal of Physiology, Endocrinology and Metabolism 293, E932-940.
Gehrig SM, Koopman R, Naim T, Tjoakarfa C & Lynch GS. (2010) American Journal of Pathology 176, 29-33.
Sheorey R, Ryall JG, Church JE & Lynch GS. (2008) Proceedings of the Australian Physiological and Pharmacological Society 39, 76P.
Church JE, Trieu J, Moore P, Gregorevic P & Lynch GS (2010). Proceedings of the Australian Physiological and Pharmacological Society 41, 162P.