Fig. 3.
Effects of train-of-four (TOF) stimulation at high release levels (A) and at more normal levels of acetylcholine release (B). Note that the steep rundown of acetylcholine release reflected as end-plate potentials (EPPs) at high release levels when compared with the normal. Traces in A were consecutively recorded. Solutions used for A were the same as figures 1 and 2. The solution used for B, termed physiological Ca2+current solution, contained reduced Ca2+ concentrations (0.7 mm), reduced concentrations of potassium channel blockers (100 μm 3,4,-diaminopyridine and 250 μm tetraethylammonium) and tubocurarine to reduce EPPs below threshold for action potentials.15 Traces in B were the averaged records of four consecutive TOF repeats (TOF ratio = 0.39). The TOF ratio for Ca2+ currents in this experiment was 0.78 (P = 0.03). B also shows the Na+ component of the propagating action potential. For further details see text.