Miniature End-Plate Potentials are Composed of Subunits

Abstract

Miniature end-plate potential (MEPP) amplitude histograms from frog and mouse muscle cells show a number of peaks that are integral multiples of the smallest peak (SMEPP). Kriebel, M.E. and Gross, C.E. (J. Gen. Physiol., 64, 85–103, 1974) found that in unstressed frog cells, SMEPP composed only 2–5% of the total but formed a distinct peak 1/7 that of the major peak. They also showed that repetitive nerve stimulation, heat and/or calcium challenges and nerve degeneration increased the percentage of SMEPPs and decreased the mean MEPP amplitude. MEPP distributions from mouse show SMEPPs to be 1/12 that of the mean amplitude. Colchicine reduced the mean 3–5-fold at which time MEPP distributions showed multiple peaks (Kriebel, M.E., Llados, F. and Matteson, D.R., J. Physiol. (Lond.), 262, 553–581, 1976). Botulinum toxin abolished larger MEPPs after the generation of as few as 200 MEPPs at which time SMEPPs and small multiple MEPPs were generated, but at very low frequencies, In comparison, black widow spider venom (Kriebel, M.E. and Stolper, D.R., Am. J. Physiol., 229, 1321–1329, 1975) or LnClz challenges also left mainly SMEPPs but after depletion of vesicles. The number of MEPPs in the smaller peaks follows Poisson statistics which suggests a “drag effect”. The variance in time-to-peak of MEPPs composing the 2nd and 3rd peaks was found to be larger than that of SMEPPs and larger MEPPs which also suggests a “drag effect”. The number of MEPPs in the larger peaks appears to be binomially distributed. These observations suggest a subunit synchronizing mechanism. Since the intervals between the smaller peaks (drag mechanism) are the same as those between the larger peaks (synchronizing mechanism), both release mechanisms utilize the same subunit.