Spontaneous subminature end‐plate potentials in mouse diaphragm muscle: evidence for synchronous release.

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Abstract
Miniature end-plate potentials (MEPP) were recorded from small muscle cells of mouse diaphragms. MEPP amplitude histograms showed successive peaks which were integral multiples of the smallest peak. The smallest potentials (subMEPP) averaged 0.3-0.6 mV and the mean of the larger MEPP averaged 3-7 mV, depending on the muscle cell diameter. There was a positive correlation between time-to-peak and MEPP amplitude. Time-to-peak of the subMEPP fell slightly below the regression line through the larger MEPP. Sometimes MEPP amplitude distributions changed spontaneously such that the mean of the major mode MEPP decreased 2-fold during which time the mean of the subMEPP did not change. Spontaneous decreases were most pronounced during low frequencies of release (10/min) achieved at 32.degree. C. Small changes in temperature (2.degree. C steps in the range 32-40.degree. C) greatly altered the number of peaks of MEPP amplitude histograms without noticeably changing the position of the subMEPP peak. At 32.degree. C subMEPP composed 5-20% of the histograms and the amplitude of the major mode peak was 12-15 times that of the subMEPP. Over-all bell-shaped distributions were obtained at 37.degree. C which showed up to 8 peaks with the major peak at the 4th-6th peak. Temperatures slightly above 37.degree. C gave a flat distribution with the mean amplitude at the 3rd peak. MEPP amplitude histograms were initially skewed (mostly small MEPP) after a 40.degree. C heat challenge. Two to 8 times the normal concentration of Ca2+ in the saline reversibly increased the MEPP frequency and also decreased the mean of the major mode MEPP (2-9 times) without noticeably changing the mean of the subMEPP. Botulinum toxin A, 105 .times. i.p. LD50/ml, almost abolished MEPP in 30-90 min. The relative proportion of subMEPP increased and the mean of the major mode MEPP usually did not change during the initial decrease in frequency. Major mode MEPP essentially ceased after 200-1000 were generated and remaining MEPP of some cells showed skewed distributions with 3 small peaks that were integral multiples of the subMEPP peak. Smaller MEPP were more resistant to block than the larger MEPP and, although frequencies were low, small MEPP were recorded after 4 h of botulinum toxin incubation. Colchicine (5 .times. 10-4 M) within minutes reduced the major mode MEPP by half (mean of major peak reduced to 6th or 7th peak). Additional colchicine (10-3 M) reduced the major mode MEPP amplitude to a 5th of that of control (mean of major mode MEPP at the third peak) with no change in position of the subMEPP peak. MEPP amplitudes slowly recovered to half control values after washing. The multiple peak distributions and the changes in the number of peaks that were integral multiples of the smallest peak indicated that most spontaneous MEPP were composed of subunits and that the release of a single subunit generated the subMEPP.