The ability of neuromuscular junctions in old animals to maintain tetanic output was tested in phasic and tonic limb muscles and the physiologic mechanism of maintenance was elucidated by analysis of the turnover of a false transmitter during prolonged tetani. Transmitter release during and after tetani was compared in limb muscles of young (8-9 month) and old (ZS-30 month) male CBF-1 mice. Amplitudes of end-plate potentials (epp's) in curarized preparations and of spontaneous miniature

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... e potentials (mepp's) were measured in viCro at 30°C in soleus and extensor digitorum longus (edl) muscles. In both young and old soleus muscles, epp amplitude was maintained at about 45% of resting level during the latter part of trains of 1200 stimuli at 10 Hz but recovered to about 90% control within a few seconds after stimulation ceased. In edl muscles of young mice, epp amplitudes during a 20 Hz train of 1200 impulses steadily declined to about 20% of control and gradually recovered over 2 min after the tetanus. In old edl muscles, tetanic decay of the epp's was greater and recovery slower than in young muscles, but absolute epp amplitudes were invariably greater. During trains of 6000 impulses at 10 Hz, plateau epp amplitude decayed to 40-50% in young soleus muscle and 30-40% control in old muscle, but recovery was similar and absolute epp amplitudes were greater in old soleus muscle. A false transmitter precursor, homocholine (HoCh), was used to investigate the mechanism of this prolonged output, and, therefore, the use of HoCh in this system was first validated. Radiolabeled HoCh was taken up and concentrated at the end-plate region, and label was released upon nerve stimulation just as in the case of radiolabeled choline. Unlabeled HoCh partially reduced uptake of labeled choline. Acetyl-HoCh but not HoCh produced postsynaptic depolarization. In the presence of 50 PM HoCh, there was substantial rundown of epp amplitude in young soleus muscle during a 10 Hz/ 6000 pulse tetanus and recovery was incomplete. Amplitudes of mepp's in similarly tetanized noncurarized preparations showed a similar percentage of depression from pretetanic values. At the end of the tetanus, epp's were 20% of control in young muscle and 13% of control in old muscle. The maximum tetanic amplitude reduction (compared to that without HoCh) was 80%, and this was assumed to represent complete replacement of ACh by acetyl-HoCh in released quanta. When replacement of ACh by HoCh was formulated as an exponential, the decay rate con-