THE RISE TIMES OF MINIATURE END-PLATE CURRENTS SUGGEST THAT ACETYLCHOLINE MAY BE RELEASED OVER A PERIOD OF TIME

Models of miniature endplate currents predict 20-80% rise times of 100 mu s or less. These predictions are substantially less than most of t he rise times recorded in the literature. New measurements were made o f rise times at the frog neuromuscular junction using extracellular re cording. The mean 20-80% rise time was 250 mu s. Rise times were varia ble; at 20 degrees C, 95% of them fell in a range from 140 to 460 mu s . The most questionable assumption in the models is that the acetylcho line (ACh) is released instantaneously. Modifying the model, so that A Ch diffuses from the vesicle through a pore, lengthens the rise time t o observed levels. It has been proposed that ACh is released from the vesicle in exchange for Na+. However, the rise times of miniature endp late currents recorded in solutions in which the Na+ is replaced by su crose are in the normal range. The Q(10) for the rise of miniature end plate currents is similar to 2, which is consistent with the models an d with temperature effects on pore formation in mast cells.