Small mode miniature end plate potentials are increased and evoked in fatigued preparations and in high Mg2+ saline

Amplitude histograms of miniature end-plate potentials (MEPPs) from small sartorius fibers (6-20 micron diam.) of very small frogs were found to have multiple peaks. All MEPPs had the same time characteristics which demonstrates that they arose at the same junction. The amplitude of the smallest mode (s-MEPPs) was one-seventh that of the major mode (m-MEPPs), and in the unstressed preparation s-MEPPs composed only a few per cent of the total. After repetitive nerve stimulation (1 min at 20 Hz) the percentage of s-MEPPs was initially increased. Successive periods of stimulation gradually reduced the end-plate potential (EPP) (above 90% failures) to the same size as s-MEPPs. A 12.5 MM Mg2+ and half normal Ca2+ saline was also found to increase the percentage of s-MEPPs and reduce EPPs so that profiles of EPP amplitude histograms showed a class of small EPPs the same size as s-MEPPs. The data demonstrate that quanta the size of s-MEPPs were released with nerve stimulation in both the Mg2+ blocked and fatigued preparation.