CLENBUTEROL INCREASES THE EXPRESSION OF MYOGENIN BUT NOT MYOD IN IMMOBILIZED RAT MUSCLES

Immobilization of one hindlimb of young rats in plantar flexion for 3 days led to changes in the plantaris muscles. These comprised a loss o f muscle mass and a reduction in protein and RNA content, but no chang e in the transcript levels of the myogenic regulatory factors myogenin and myoD. Dietary administration of the beta-adrenoceptor agonist cle nbuterol (2 mg/kg diet), which has been shown to ameliorate muscle was ting in a wide range of atrophic conditions, also limited muscle wasti ng in terms of weight, protein, and RNA in the immobilized plantaris m uscles. In addition, drug treatment in immobilized plantaris muscles w as associated with a marked increase in the steady-state levels of mRN A for myogenin (similar to 360% increase over control) but not myoD. T hese data provide the first evidence for independent changes in these two myogenic regulatory factors in immobilized muscle and suggest that the action of clenbuterol on these factors may depend on the mechanis tic basis for the atrophic response.