Abstract
1. The role of muscle mass and function in the regulation of choline acetyltransferase activity at the neuromuscular junction has been investigated in the rat.2. Choline acetyltransferase (ChAc) is located in presynaptic nerve terminals and is a specific enzymatic marker of cholinergic innervation in muscle.3. ChAc activity increased co-ordinately with developmental growth of the soleus muscle. However, another form of muscle growth, work hypertrophy, did not produce an increase in ChAc.4. Growth arrest of muscle by hypophysectomy did not alter the normal development of ChAc activity, and cortisone-induced muscle atrophy did not reduce ChAc activity in the soleus or plantaris.5. Tenotomy-induced muscle atrophy provoked a significant fall in ChAc in the soleus and plantaris.6. The tonic soleus had significantly greater ChAc activity than the phasic plantaris.7. These observations suggest that muscle mass per se does not influence the development and regulation of ChAc in muscle but that the quality of muscle contraction may modulate enzyme activity.