Sustained transmitter output by increased transmitter turnover in limb muscles of old mice

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Abstract
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 (28–30 month) male CBF-1 mice. Amplitudes of end- plate potentials (epp's) in curarized preparations and of spontaneous miniature end-plate potentials (mepp's) were measured in vitro at 30 degrees 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 1,200 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 1,200 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 6,000 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.(ABSTRACT TRUNCATED AT 250 WORDS)