Skeletal muscles can be injured by a myriad of insults that can 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). In the present study we investigated the contribution of β-AR signalling to early muscle regeneration, to determine the relative contribution of individual β-AR subtypes to 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). Muscle function was determined by assessing the contractile properties of the tibialis anterior (TA) muscle in situ (Gehrig et al., 2010). Briefly, mice were anaesthetised (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 immobilised. At the conclusion of the experiment the mice were killed by cardiac excision while still anaesthetised deeply.
When muscle function was examined in uninjured TA muscles, both β2-AR KO mice and β1/β2-AR KO mice produced significantly less force than their respective controls (p < 0.05), however, TA muscles from β1-AR KO mice showed no significant deficit in force production. To determine the relative contribution of the individual β-AR subtypes to early muscle regeneration, mice were anaesthetised (ketamine 80 mg/kg and xylazine 10 mg/kg; i.p.) and the TA muscle of the right hindlimb was injected with the myotoxin, Notexin (1 μg/ml, i.m.) to cause complete muscle fibre degeneration. Mice were allowed to recover for 7, 10 or 14 days, after which TA function was assessed in situ. β1/β2-AR KO mice produced significantly less force than their controls at 7 days post-injury (p < 0.05) but force production had increased to similar levels as control at 10 and 14 days post-injury. Muscles from β2-AR KO mice showed a similar pattern of force production during regeneration with significantly less force at 7 days but similar force production at 10 and 14 days post-injury, while muscles from β1-AR KO mice did not exhibit force deficits at any stage during regeneration.
These results suggest that the β2-adrenoceptor is the dominant β-AR subtype involved in early muscle fibre regeneration. Selective stimulation of β2-adrenoceptors may therefore be a therapeutic strategy to improve the rate, extent and efficacy of the regenerative process, and may have important implications for 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.