Chronic administration of β₂-adrenoreceptor agonists induces a conversion of skeletal muscle phenotype from slow to fast twitch fibres (Zeman et al. 1988). An understanding of the mechanism of β₂-agonist-induced fibre type switching could provide understanding as to how to prevent muscle remodelling during disease. Muscle fibre type may be regulated by the myogenic transcription factor MyoD since it has previously been reported that administration of the β₂-agonist clenbuterol, together with thyroid hormone (T₃), leads to an increase in MyoD mRNA expression and fast type IIA fibres (Hughes et al. 1993). However, MyoD expression is already higher in fast fibres (Hughes et al. 1993; Sakuma et al. 1999) so upregulation of MyoD may simply be a consequence of the increased number of fast fibres. Therefore, we examined the effects of β₂-agonist administration on muscle fibre type and MyoD transcription factor expression in order to determine whether changes in MyoD protein expression precede changes in fibre type.

Four-week-old, male Wistar rats were administered 900 µg (kg body mass)^-1 of β₂-agonist pro-drug BRL-47672 (Sillence et al. 1995) or saline control (n = 10) by subcutaneous injection for either 1 day, 4 weeks or 8 weeks. Each rat was then terminally anaesthetised and the soleus muscle rapidly excised and snap-frozen in liquid nitrogen. The expression of myosin heavy chain (MHC) by SDS PAGE and MyoD protein by immunoblotting, using a polyclonal MyoD antibody (Santa Cruz), was measured.

Over the 8 weeks of saline treatment MHCI expression increased and MHCIIA decreased in the control group (Table 1). Conversely, in the β₂-agonist treatment group MHCI expression declined and MHCIIA increased. After 4 and 8 weeks of treatment there was significantly higher expression of MHCIIA and lower expression of MHCI in the β₂-agonist treatment group relative to the control. Analysis of the immunoblots revealed that MyoD protein expression was significantly higher in the β₂-agonist treated group compared with control after 1 day and 4 weeks of dosing (Table 1). In conclusion, the elevated expression of MyoD protein occurred prior to any change in MHC expression, suggesting that β₂-agonist-induced changes in muscle phenotype are mediated, at least in part, by prior upregulation of the transcription factor MyoD.

All work was carried out in accordance with UK legislation.